*Article* **Stabilizing Halogen-Bonded Complex between Metallic Anion and Iodide**

**Fei Ying <sup>1</sup> , Xu Yuan 2,3, Xinxing Zhang 2,3,\* and Jing Xie 1,\***


**Abstract:** Halogen bonds (XBs) between metal anions and halides have seldom been reported because metal anions are reactive for XB donors. The pyramidal-shaped Mn(CO)<sup>5</sup> − anion is a candidate metallic XB acceptor with a ligand-protected metal core that maintains the negative charge and an open site to accept XB donors. Herein, Mn(CO)<sup>5</sup> − is prepared by electrospray ionization, and its reaction with CH<sup>3</sup> I in gas phase is studied using mass spectrometry and density functional theory (DFT) calculation. The product observed experimentally at m/z = 337 is assigned as [IMn(CO)<sup>4</sup> (OCCH<sup>3</sup> )]−, which is formed by successive nucleophilic substitution and reductive elimination, instead of the halogen-bonded complex (XC) CH3−I···Mn(CO)<sup>5</sup> <sup>−</sup>, because the I···Mn interaction is weak within XC and it could be a transient species. Inspiringly, DFT calculations predict that replacing CH<sup>3</sup> I with CF<sup>3</sup> I can strengthen the halogen bonding within the XC due to the electro-withdrawing ability of F. More importantly, in so doing, the nucleophilic substitution barrier can be raised significantly, ~30 kcal/mol, thus leaving the system trapping within the XC region. In brief, the combination of a passivating metal core and the introduction of an electro-withdrawing group to the halide can enable strong halogen bonding between metallic anion and iodide.

**Keywords:** halogen bond; metallic anion; nucleophilic substitution reaction; quantum chemistry calculation; reductive elimination

#### **1. Introduction**

The halogen bond (XB) is a type of non-covalent interaction that has attracted the interests of experimentalists and theoretical chemists in recent years [1–9]. According to the International Union of Pure and Applied Chemistry (IUPAC), "a halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity" [10]. This definition states unambiguously that the halogen atom serves as an electrophile and interacts with a nucleophilic moiety. Typically, an XB is denoted as R−X···Y with three dots representing the bond, and X is a halogen atom (i.e., XB donor) that has an electrophilic region on its electrostatic potential surface, and Y is an XB acceptor. For the XB donor molecule (i.e., R−X molecule), the electrophilic (or positive) region on X, named as "σ-hole" [11,12], is induced by the R−X bond, which leaves an anisotropic distribution of electrons. The σ-hole magnitude, which represents the XB strength given by the same XB acceptor, scales with the polarizability of the halogen atom, that is, F < Cl < Br < I. Hence, changing the X atom can tune the XB's strength, and there are other methods as well, including modifying the R-functional group and the electro-withdrawing ability of Y.

**Citation:** Ying, F.; Yuan, X.; Zhang, X.; Xie, J. Stabilizing Halogen-Bonded Complex between Metallic Anion and Iodide. *Molecules* **2022**, *27*, 8069. https://doi.org/10.3390/ molecules27228069

Academic Editors: Qingzhong Li, Steve Scheiner and Zhiwu Yu

Received: 29 October 2022 Accepted: 18 November 2022 Published: 21 November 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The nature and tunability of XB make it useful in different fields spanning from material sciences to biomolecular recognition and drug design [13–18]. are other methods as well, including modifying the R-functional group and the electrowithdrawing ability of Y. The nature and tunability of XB make it useful in different fields spanning from material sciences to biomolecular recognition and drug design [13–18].

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The common XB acceptors are nucleophiles, such as N, O, S, P, or halogen atoms/anions; metal anions are rarely seen. This is because atomic metal anions are usually too reactive towards organohalogens. For example, the reaction between Au−/Ag−/Cu− anions and CH3I in gas phase give rise to a Grignard reagent-like product [CH3−M−I]−, where a covalent M−I bond is formed [19,20]. This structure is calculated to be ~2.0–3.0 eV more stable than the XB complex [CH3−I···M]<sup>−</sup> [20,21]. To achieve the goal of forming metallic acceptor-containing halogen bonds, one of our authors proposed two strategies: one is to utilize a metal cluster anion with a high electron detachment energy; the other is to design a ligand-passivated/protected metal core that can maintain the negative charge [22]. The goal of this work is to check the feasibility of the second strategy experimentally. Hence, herein, we prepared a Mn(CO)<sup>5</sup> − anionic compound by electrospray ionization and investigated its reactivity with CH3I. The common XB acceptors are nucleophiles, such as N, O, S, P, or halogen atoms/anions; metal anions are rarely seen. This is because atomic metal anions are usually too reactive towards organohalogens. For example, the reaction between Au−/Ag−/Cu− anions and CH3I in gas phase give rise to a Grignard reagent-like product [CH3−M−I]−, where a covalent M−I bond is formed [19,20]. This structure is calculated to be ~2.0–3.0 eV more stable than the XB complex [CH3−I···M]− [20,21]. To achieve the goal of forming metallic acceptor-containing halogen bonds, one of our authors proposed two strategies: one is to utilize a metal cluster anion with a high electron detachment energy; the other is to design a ligand-passivated/protected metal core that can maintain the negative charge [22]. The goal of this work is to check the feasibility of the second strategy experimentally. Hence, herein, we prepared a Mn(CO)5− anionic compound by electrospray ionization and investigated its reactivity with CH3I.

To test whether Mn(CO)<sup>5</sup> − anion is a suitable candidate to form a halogen-bonded complex, we first investigated the properties of Mn(CO)<sup>5</sup> − anion by density functional theory (DFT) calculation using M06-2X method [23] with aug-cc-pVTZ basis set [24–26]. As shown in Figure 1a, the structure of Mn(CO)<sup>5</sup> − anion has a pyramidal shape, with one CO ligand in the horizontal direction and the other four CO ligands almost in the same plane (see Figure S1 for an illustration), leaving the left an open site to accept a XB donor. A Mulliken charge analysis [27] (Figure 1a) indicated that the Mn-atom core is the most negative, with a charge of −0.84 e, and this is clearly displayed in the electrostatic potential map (Figure 1b). In addition, the HOMO of Mn(CO)<sup>5</sup> − anion (Figure 1c) comprising C p orbital and Mn *d<sup>x</sup>* <sup>2</sup> orbital has electrons evenly delocalized on four C atoms, thus stabilizing the compound. In brief, Mn(CO)<sup>5</sup> − anion fulfills the two criteria of the second strategy: the metal core is negatively charged and has at least one open site to accept the XB. To test whether Mn(CO)5− anion is a suitable candidate to form a halogen-bonded complex, we first investigated the properties of Mn(CO)5− anion by density functional theory (DFT) calculation using M06-2X method [23] with aug-cc-pVTZ basis set [24–26]. As shown in Figure 1a, the structure of Mn(CO)5− anion has a pyramidal shape, with one CO ligand in the horizontal direction and the other four CO ligands almost in the same plane (see Figure S1 for an illustration), leaving the left an open site to accept a XB donor. A Mulliken charge analysis [27] (Figure 1a) indicated that the Mn-atom core is the most negative, with a charge of −0.84 e, and this is clearly displayed in the electrostatic potential map (Figure 1b). In addition, the HOMO of Mn(CO)5− anion (Figure 1c) comprising C p orbital and Mn dz2 orbital has electrons evenly delocalized on four C atoms, thus stabilizing the compound. In brief, Mn(CO)5− anion fulfills the two criteria of the second strategy: the metal core is negatively charged and has at least one open site to accept the XB.

**Figure 1.** Calculated (**a**) structure and Mulliken charge, (**b**) electrostatic potential map in e/Bohr3, and (**c**) HOMO of Mn(CO)5<sup>−</sup> anion. The M06-2X/aug-cc-pVTZ level of theory is used [23–26]. Color code: C, grey; O, red; Mn, purple. **Figure 1.** Calculated (**a**) structure and Mulliken charge, (**b**) electrostatic potential map in e/Bohr<sup>3</sup> , and (**c**) HOMO of Mn(CO)<sup>5</sup> − anion. The M06-2X/aug-cc-pVTZ level of theory is used [23–26]. Color code: C, grey; O, red; Mn, purple.

In this work, we will first study the reaction between Mn(CO)5− anion and CH3I in gas phase using a linear ion trap mass spectrometer. Then the products and mechanism will be analyzed with the help of DFT calculation. The stability of the halogen-bonded complex is evaluated, and a strategy is proposed to further stabilize it. In this work, we will first study the reaction between Mn(CO)<sup>5</sup> <sup>−</sup> anion and CH3I in gas phase using a linear ion trap mass spectrometer. Then the products and mechanism will be analyzed with the help of DFT calculation. The stability of the halogen-bonded complex is evaluated, and a strategy is proposed to further stabilize it.

#### **2. Methods**

#### **2. Methods**  *2.1. Experimental Methods*

*2.1. Experimental Methods*  Mass spectra were acquired using a linear ion trap mass spectrometer (LTQ-XL, Thermo-Fisher, Waltham, MA, USA). The inlet capillary temperature of the mass spectrometer was maintained at 275 °C. The tube lens voltage on the LTQ-XL was set to be 0 V in order to avoid in-source fragmentation of the fragile species. The applied negative voltage was set at −4000 V in this study in order to trigger the electrospray ionization. A Mass spectra were acquired using a linear ion trap mass spectrometer (LTQ-XL, Thermo-Fisher, Waltham, MA, USA). The inlet capillary temperature of the mass spectrometer was maintained at 275 ◦C. The tube lens voltage on the LTQ-XL was set to be 0 V in order to avoid in-source fragmentation of the fragile species. The applied negative voltage was set at −4000 V in this study in order to trigger the electrospray ionization. A methanol solution of Mn2(CO)<sup>10</sup> was sprayed to generate the Mn(CO)<sup>5</sup> − anion. The collision-induced dissociation (CID) spectrum of the Mn(CO)<sup>5</sup> − anion is presented in Figure S2. Gas-phase

reaction between the Mn(CO)<sup>5</sup> − anion and the neutral CH3I molecule at room temperature was conducted in the linear ion trap by using the collision-induced dissociation (CID) mode that is, the MS<sup>2</sup> mode of the mass spectrometer in order to isolate the Mn(CO)<sup>5</sup> − anion in the trap. The CH3I molecules were introduced to the trap by putting a drop of CH3I into a small stainless-steel reservoir that was connected to the pipeline of the He collision gas. The collision energy was set to be under 10 V in order to trigger the reactions. room temperature was conducted in the linear ion trap by using the collision-induced dissociation (CID) mode that is, the MS2 mode of the mass spectrometer in order to isolate the Mn(CO)5− anion in the trap. The CH3I molecules were introduced to the trap by putting a drop of CH3I into a small stainless-steel reservoir that was connected to the pipeline of the He collision gas. The collision energy was set to be under 10 V in order to trigger the reactions.

methanol solution of Mn2(CO)10 was sprayed to generate the Mn(CO)5− anion. The collision-induced dissociation (CID) spectrum of the Mn(CO)5− anion is presented in Figure S2. Gas-phase reaction between the Mn(CO)5− anion and the neutral CH3I molecule at

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#### *2.2. Computational Methods 2.2. Computational Methods*

Geometry optimizations were performed using M06-2X functional [23], with augcc-pVTZ basis set [24–26] used for H, C, O, F, and Mn atoms, and aug-cc-pVTZ-PP basis set [28,29] used for I atoms. Various configurations were optimized for CH3I−Mn(CO)<sup>3</sup> −, and the most stable structures were used for discussion (see Figure S3 for details). Harmonic vibrational frequencies were calculated to confirm the nature of the stationary points. Intrinsic reaction coordinate (IRC) calculations were performed on transition states to confirm that they connected the correct intermediates. The ground state of Mn(CO)4I − is doublet, and the other metal-involved species are all singlet. The zero-point corrected energy is used in the potential energy profile. Gaussian 16 [30] package was used to perform all the calculations. Geometry optimizations were performed using M06-2X functional [23], with aug-ccpVTZ basis set [24–26] used for H, C, O, F, and Mn atoms, and aug-cc-pVTZ-PP basis set [28,29] used for I atoms. Various configurations were optimized for CH3I−Mn(CO)3−, and the most stable structures were used for discussion (see Figure S3 for details). Harmonic vibrational frequencies were calculated to confirm the nature of the stationary points. Intrinsic reaction coordinate (IRC) calculations were performed on transition states to confirm that they connected the correct intermediates. The ground state of Mn(CO)4I− is doublet, and the other metal-involved species are all singlet. The zero-point corrected energy is used in the potential energy profile. Gaussian 16 [30] package was used to perform all the calculations.

#### **3. Results and Discussion 3. Results and Discussion**

#### *3.1. Mass Spectrometry 3.1. Mass Spectrometry*

A typical mass spectrum showing the reaction products between Mn(CO)<sup>5</sup> − and CH3I is presented in Figure 2a. Three major peaks at m/z 281, 294, and 337 were observed, corresponding to the masses of CH3I-Mn(CO)<sup>3</sup> −, Mn(CO)4I −, and CH3I-Mn(CO)<sup>5</sup> −, among which the latter is the direct product from the reaction between Mn(CO)<sup>5</sup> − and CH3I, but the former two are the collision fragments of the latter. To obtain structural information for the m/z 337 peak, we further isolated it with the MS<sup>3</sup> mode of the mass spectrometer; its CID fragments with a collision energy of 5 V are presented in Figure 2b. If CH3I-Mn(CO)<sup>5</sup> − is a weakly bonded species of Mn(CO)<sup>5</sup> <sup>−</sup> and CH3I, its fragments should predominantly be Mn(CO)<sup>5</sup> <sup>−</sup>. However, two distinct fragments, CH3I-Mn(CO)<sup>4</sup> <sup>−</sup> and MnIO<sup>2</sup> −, were observed, suggesting that the m/z 337 peak is not a weakly bound species. A typical mass spectrum showing the reaction products between Mn(CO)5− and CH3I is presented in Figure 2a. Three major peaks at m/z 281, 294, and 337 were observed, corresponding to the masses of CH3I-Mn(CO)3−, Mn(CO)4I−, and CH3I-Mn(CO)5−, among which the latter is the direct product from the reaction between Mn(CO)5− and CH3I, but the former two are the collision fragments of the latter. To obtain structural information for the m/z 337 peak, we further isolated it with the MS3 mode of the mass spectrometer; its CID fragments with a collision energy of 5 V are presented in Figure 2b. If CH3I-Mn(CO)5− is a weakly bonded species of Mn(CO)5− and CH3I, its fragments should predominantly be Mn(CO)5−. However, two distinct fragments, CH3I-Mn(CO)4− and MnIO2−, were observed, suggesting that the m/z 337 peak is not a weakly bound species.

**Figure 2.** Mass spectrometric results. (**a**) A typical mass spectrum showing the reaction products between Mn(CO)5<sup>−</sup> and CH3I; (**b**) CID mass spectrum of [Mn(CO)5(CH3I)]<sup>−</sup> at m/z 337 taken with the MS3 mode. The nominal applied CID voltage is 5 V. **Figure 2.** Mass spectrometric results. (**a**) A typical mass spectrum showing the reaction products between Mn(CO)<sup>5</sup> − and CH<sup>3</sup> I; (**b**) CID mass spectrum of [Mn(CO)<sup>5</sup> (CH<sup>3</sup> I)]− at m/z 337 taken with the MS<sup>3</sup> mode. The nominal applied CID voltage is 5 V.

#### *3.2. Density Functional Theory Calculation*

3.2.1. Mn(CO)<sup>5</sup> − + CH3I Reaction Mechanism

To identify the structure and understand the formation mechanism of the aforementioned observed products, we performed DFT calculations. Scheme 1 depicts the potential

energy surfaces (PESs) for Mn(CO)<sup>5</sup> <sup>−</sup> + CH3I, and selected structures are displayed in Figure 3. Enthalpy and free energy values at 298.15 K are listed in Table 1. energy surfaces (PESs) for Mn(CO)5− + CH3I, and selected structures are displayed in Figure 3. Enthalpy and free energy values at 298.15 K are listed in Table 1.

ure 3. Enthalpy and free energy values at 298.15 K are listed in Table 1.

To identify the structure and understand the formation mechanism of the aforementioned observed products, we performed DFT calculations. Scheme 1 depicts the potential

To identify the structure and understand the formation mechanism of the aforementioned observed products, we performed DFT calculations. Scheme 1 depicts the potential energy surfaces (PESs) for Mn(CO)5− + CH3I, and selected structures are displayed in Fig-

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3.2.1. Mn(CO)5− + CH3I Reaction Mechanism

*3.2. Density Functional Theory Calculation* 

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*3.2. Density Functional Theory Calculation*  3.2.1. Mn(CO)5− + CH3I Reaction Mechanism

**Scheme 1.** Potential energy profile of Mn(CO)5<sup>−</sup> reacting with CR3I. The captions in black are for R = H, and the captions in blue are for R = F. Zero-point corrected energies (in kcal/mol) are given **Scheme 1.** Potential energy profile of Mn(CO)<sup>5</sup> <sup>−</sup> reacting with CR<sup>3</sup> I. The captions in black are for R = H, and the captions in blue are for R = F. Zero-point corrected energies (in kcal/mol) are given relative to the total energy of isolated R3CI and Mn(CO)<sup>5</sup> −. **Scheme 1.** Potential energy profile of Mn(CO)5<sup>−</sup> reacting with CR3I. The captions in black are for R = H, and the captions in blue are for R = F. Zero-point corrected energies (in kcal/mol) are given relative to the total energy of isolated R3CI and Mn(CO)5<sup>−</sup>.

relative to the total energy of isolated R3CI and Mn(CO)5<sup>−</sup>.

bond distances (Å) in black are for CH3I, and those in blue are for CF3I. **Figure 3.** Structures of selected stationary points on the PES of Mn(CO)5<sup>−</sup> reacting with CH3I. The bond distances (Å) in black are for CH3I, and those in blue are for CF3I. **Figure 3.** Structures of selected stationary points on the PES of Mn(CO)<sup>5</sup> − reacting with CH<sup>3</sup> I. The bond distances (Å) in black are for CH<sup>3</sup> I, and those in blue are for CF<sup>3</sup> I.



Figure 3 depicts that a halogen-bonded complex (XC) CH3−I···Mn(CO)<sup>5</sup> − is formed by an I atom attacking the open site of Mn, and XC is 5.3 kcal/mol lower in energy than the reactants. The I···Mn distance within XC is 3.652 Å, which is 79% of the sum of the van der Walls radii of I (2.36 Å) and Mn (2.24 Å). At the same time, a slightly more stable pre-reaction complex (RC) is formed between a C atom interacting with Mn; it is lower in energy by 2.8 kcal/mol. Of note, additional conformers of RC that are higher in energy are localized: one has a linear I−C−Mn shape, and the other I−C−Mn angle is ~ 90◦ . These two structures are structural isomers of RC, which may appear when CH3I attacks Mn(CO5) − in a different direction. For clarity, they are omitted in Figure 3 and are instead present in Figure S4. After crossing a back-side attack nucleophilic substitution barrier (TS1) of 8.3 kcal/mol, it proceeds to post-reaction complex PC1. We also considered the front-side attack SN2 transition state (TS2); however, it is too high (34.4 kcal/mol) to occur. Within PC1, CH<sup>3</sup> fragment and I fragment are located on the opposite side of Mn with a weak I-C interaction. Relative to the reactants, PC1 is −32.0 kcal/mol in energy. Then PC1 can undergo a reductive elimination barrier (TS3) of 11.8 kcal/mol and ends up with the formation of a C−C bond and the migration of I to bond with Mn. This resulted complex (PC2) is very stable, and is −43.2 kcal/mol relative to the reactants. Because TS1 is almost thermally neutral and TS3 is lower in energy than the reactants, the most stable PC2 can be formed under room temperature (the experimental condition). For this reason, in Figure 2, the signal at 337 m/z was assigned to be PC2, and it agrees with experimental results that this species is quite stable.

The calculated energy for Mn(CO)<sup>5</sup> <sup>−</sup> + CH3I → Mn(CO)4I <sup>−</sup> + COCH<sup>3</sup> reaction is −5.0 kcal/mol, and the calculated energy to form Mn(CO)4I − + CO + CH<sup>3</sup> is 5.7 kcal/mol. Therefore, we believe the experimentally observed Mn(CO)4I − is more likely to be dissociated from PC2 and to generate COCH<sup>3</sup> at the same time, and is less likely to be caused by the collision-induced dissociation that forms CO and CH3.

The calculated energy from generating CH3I−Mn(CO)<sup>3</sup> − + 2CO from reactants is −6.1 kcal/mol downhill. The most stable structure of CH3I−Mn(CO)<sup>3</sup> − has a pseudobipyramidal shape (Figure 3). Analyzing the PES indicates that it may dissociate from PC1 or, provided sufficient energy, dissociate from TS2.

In brief, although there is considerable halogen bonding between CH3I and Mn(CO)<sup>5</sup> −, the passivated Mn center within Mn(CO)<sup>5</sup> <sup>−</sup> is still reactive towards CH3I, thus making the XC a transient species. The observed CH3I-Mn(CO)<sup>5</sup> − signal is PC2, which forms by nucleophilic substitution and the following reductive elimination.

3.2.2. Stabilizing Halogen-Bonded Complex by CF3I

It is known that introducing an electron-withdrawing group, such as F, to the methyl group can increase the σ-hole magnitude, and thus the XB strength. Therefore, we changed CH3I to CF3I, which induces a greater positive region on the I atom, in the hope that

it can stabilize the halogen-bonded complex when interacting with Mn(CO)<sup>5</sup> −. On the other hand, changing CH<sup>3</sup> to a heavier CF<sup>3</sup> group is expected to raise the inversion SN2 barrier [31], thus preventing the SN2 reaction. This may also help trap the system in a halogen-bonded complex well, so we computed the PES of Mn(CO)<sup>5</sup> − + CF3I. that it can stabilize the halogen-bonded complex when interacting with Mn(CO)5−. On the other hand, changing CH3 to a heavier CF3 group is expected to raise the inversion SN2 barrier [31], thus preventing the SN2 reaction. This may also help trap the system in a halogen-bonded complex well, so we computed the PES of Mn(CO)5− + CF3I.

As shown in Scheme 1, the halogen-bonded complex (XC') CF3−I···Mn(CO)<sup>5</sup> − well is 17.5 kcal/mol deep, where the pre-reaction complex RC' is 14.3 kcal/mol higher than it is. Within XC', the I···Mn distance is 3.224 Å, being 0.428 Å shorter than the corresponding value of XC. This is consistent with XC' being more stable than XC. To characterize the interaction between CR3I and Mn(CO5) − within the halogen-bonded complex CR3−I···Mn(CO)<sup>5</sup> −, natural bond orbital (NBO) [32,33] calculations were performed for R = H and F in order to analyze the donor–acceptor charge transfer properties. As shown in Figure 4, taking CF3−I···Mn(CO)<sup>5</sup> <sup>−</sup> as an example, the donor orbital is the Mn−C bonding σ orbital and Mn 3p orbital, and the acceptor orbital is the C–I antibonding σ\* orbital. The same scenario also applies for CH3−I···Mn(CO)<sup>5</sup> −. In comparison, when the halogenbonded complex is composed of a main group nucleophile, such as F<sup>−</sup> and CH3I (i.e., [CH3−I···F]−), the donor NBO is a 2p orbital; when the nucleophile is Cu−/Ag−/Au−, the donor NBO is an s orbital [21,34]. As shown in Scheme 1, the halogen-bonded complex (XC') CF3−I···Mn(CO)5− well is 17.5 kcal/mol deep, where the pre-reaction complex RC' is 14.3 kcal/mol higher than it is. Within XC', the I···Mn distance is 3.224 Å, being 0.428 Å shorter than the corresponding value of XC. This is consistent with XC' being more stable than XC. To characterize the interaction between CR3I and Mn(CO5)− within the halogen-bonded complex CR3−I···Mn(CO)5−, natural bond orbital (NBO) [32,33] calculations were performed for R = H and F in order to analyze the donor–acceptor charge transfer properties. As shown in Figure 4, taking CF3−I···Mn(CO)5− as an example, the donor orbital is the Mn−C bonding σ orbital and Mn 3p orbital, and the acceptor orbital is the C–I antibonding σ\* orbital. The same scenario also applies for CH3−I···Mn(CO)5−. In comparison, when the halogenbonded complex is composed of a main group nucleophile, such as F− and CH3I (i.e., [CH3−I···F]−), the donor NBO is a 2p orbital; when the nucleophile is Cu−/Ag−/Au−, the donor NBO is an s orbital [21,34].

**Figure 4.** Donor and acceptor natural bond orbitals (NBOs) of halogen-bonded complex CF3−I···Mn(CO)5<sup>−</sup> to illustrate the charge transfer interaction between I and Mn. **Figure 4.** Donor and acceptor natural bond orbitals (NBOs) of halogen-bonded complex CF3−I···Mn(CO)<sup>5</sup> − to illustrate the charge transfer interaction between I and Mn.

Additionally, the back-side attack SN2 barrier (TS1′) is largely raised to 28.9 kcal/mol relative to the reactants. Although the front-side attack transition state (TS2′) is lower than TS2, TS2′ is still 26.1 kcal/mol uphill. Of note, different from CH3I, the front-side attack SN2 barrier (TS2′) is lower than the back-side attack SN2 barrier (TS1′) by 2.8 kcal/mol. If the reactants are cooled to room temperature or even lower, they are unlikely to cross these barriers or proceed to nucleophilic substitution and the following reductive elimination. Of note, the PC1′ and PC2′ complexes are even lower than PC1 and PC2, but PC1′ needs to cross a barrier (TS3′) of 37.7 kcal/mol. In another words, if the system crosses Additionally, the back-side attack SN2 barrier (TS10 ) is largely raised to 28.9 kcal/mol relative to the reactants. Although the front-side attack transition state (TS20 ) is lower than TS2, TS20 is still 26.1 kcal/mol uphill. Of note, different from CH3I, the front-side attack SN2 barrier (TS20 ) is lower than the back-side attack SN2 barrier (TS10 ) by 2.8 kcal/mol. If the reactants are cooled to room temperature or even lower, they are unlikely to cross these barriers or proceed to nucleophilic substitution and the following reductive elimination. Of note, the PC10 and PC20 complexes are even lower than PC1 and PC2, but PC10 needs to cross a barrier (TS30 ) of 37.7 kcal/mol. In another words, if the system crosses TS10 , both PC10 and PC20 can be formed and stable.

TS1′, both PC1′ and PC2′ can be formed and stable. To summarize, calculations show that replacing CH3I with CF3I stabilizes the halogen-bonded complex and raises the nucleophilic substitution barrier. This is an effective To summarize, calculations show that replacing CH3I with CF3I stabilizes the halogenbonded complex and raises the nucleophilic substitution barrier. This is an effective strategy to obtain strong halogen bonding between iodide and metallic anions.

strategy to obtain strong halogen bonding between iodide and metallic anions.

#### **4. Conclusions**

**4. Conclusions**  To achieve the goal of constructing a stable halogen-bonded complex between metallic anionic species and halide, we adopted the strategy of passivating the reactive metallic anion by introducing protected ligands. Thus, we designed the Mn(CO)5− anionic compound, and DFT calculation confirms that it maintains a negatively charged core and has an open site to accept halogen bond donors. Next, the Mn(CO)5− species was prepared To achieve the goal of constructing a stable halogen-bonded complex between metallic anionic species and halide, we adopted the strategy of passivating the reactive metallic anion by introducing protected ligands. Thus, we designed the Mn(CO)<sup>5</sup> − anionic compound, and DFT calculation confirms that it maintains a negatively charged core and has an open site to accept halogen bond donors. Next, the Mn(CO)<sup>5</sup> − species was prepared by electrospray ionization and then reacted with CH3I in gas phase using a linear ion

trap mass spectrometer. The major products were CH3I-Mn(CO)<sup>3</sup> <sup>−</sup>, Mn(CO)4I −, and CH3I-Mn(CO)<sup>5</sup> −. DFT calculations suggested that CH3I-Mn(CO)<sup>5</sup> − is a stable species (i.e., [IMn(CO)4(OCCH3)]−) that forms by nucleophilic substitution and reductive elimination. The halogen-bonded complex CH3−I···Mn(CO)<sup>5</sup> − could be a transient species because the interaction between I and Mn is weak.

By substituting CH3I to CF3I, calculations predicted that the resulted halogen-bonded complex CF3−I···Mn(CO)<sup>5</sup> − is stabilized considerably. In addition, the barrier for nucleophilic substitution was greatly raised, allowing the system to trap in the XB complex well, given that the system is cool enough to avoid crossing the SN2 barrier. This work presents an example of stabilizing the halogen bonding between a ligand-protected metal anion and halide with strong electro-withdrawing group. By adopting a similar strategy, it is anticipated that more metallic acceptor-containing XBs will be discovered.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/molecules27228069/s1, Figure S1: Optimized structure of Mn(CO)<sup>5</sup> − anion. Figure S2: CID fragments of the Mn(CO)<sup>5</sup> − anion. Figure S3: Optimized structures of CH<sup>3</sup> I −Mn(CO)<sup>3</sup> −. Figure S4: Additional conformers of RC. Coordinates of all computed structures.

**Author Contributions:** Conceptualization: X.Z. and J.X.; Funding acquisition: X.Z. and J.X.; Investigation: F.Y. and X.Y.; Resources: X.Z. and J.X.; Writing—original draft: F.Y. and X.Y.; Writing—review and editing: X.Z. and J.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** J.X. acknowledges the Beijing Natural Science Foundation (No. 2222028), the National Natural Science Foundation of China (22273004, 21903004) and the Teli Fellowship from Beijing Institute of Technology, China. X.Z. acknowledges the National Natural Science Foundation of China (22174073 & 22003027), the NSF of Tianjin City (21JCJQJC00010), the National Key R&D Program of China (2018YFE0115000), Haihe Laboratory of Sustainable Chemical Transformations, Beijing National Laboratory for Molecular Sciences (BNLMS202106), and the Frontiers Science Center for New Organic Matter at Nankai University (63181206).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data is contained within the article or Supplementary Materials.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Evgenii S. Stoyanov \* , Irina Yu. Bagryanskaya and Irina V. Stoyanova**

N.N. Vorozhtsov Institute of Organic Chemistry, Siberian Branch of Russian Academy of Sciences, 630090 Novosibirsk, Russia

**\*** Correspondence: evgenii@nioch.nsc.ru

**Abstract:** X-ray diffraction analysis and IR spectroscopy were used to study the products of the interaction of vinyl cations C3H<sup>5</sup> <sup>+</sup> and C4H<sup>7</sup> + (Cat<sup>+</sup> ) (as salts of carborane anion CHB11Cl<sup>11</sup> −) with basic molecules of water, alcohols, and acetone that can crystallize from solutions in dichloromethane and C6HF<sup>5</sup> . Interaction with water, as content increased, proceeded via three-stages. (1) adduct Cat<sup>+</sup> ·OH<sup>2</sup> forms in which H2O binds (through the O atom) to the C=C<sup>+</sup> bond of the cation with the same strength as seen in the binding to Na in Na(H2O)<sup>6</sup> + . (2) H<sup>+</sup> is transferred from cation Cat<sup>+</sup> ·OH<sup>2</sup> to a water molecule forming H3O<sup>+</sup> and alcohol molecules (L) having the CH=CHOH entity. The Oatom of alcohols is attached to the H atom of the C=C<sup>+</sup> -H moiety of Cat<sup>+</sup> thereby forming a very strong asymmetric H–bond, (C=)C<sup>+</sup> -H···O. (3) Finally all vinyl cations are converted into alcohol molecule L and H3O<sup>+</sup> cations, yielding proton disolvates L-H<sup>+</sup> -L with a symmetric very strong H-bond. When an acetone molecule (Ac) interacts with Cat<sup>+</sup> , H<sup>+</sup> is transferred to Ac giving rise to a reactive carbene and proton disolvate Ac-H<sup>+</sup> -Ac. Thus, the alleged high reactivity of vinyl cations seems to be an exaggeration.

**Keywords:** vinyl cations; vinyl cation adduct; very strong H-bond; proton disolvate; carborane salt

**Citation:** Stoyanov, E.S.; Bagryanskaya, I.Y.; Stoyanova, I.V. Interaction of Vinyl-Type Carbocations, C3H<sup>5</sup> <sup>+</sup> and C4H<sup>7</sup> <sup>+</sup> with Molecules of Water, Alcohols, and Acetone. *Molecules* **2023**, *28*, 1146. https://doi.org/10.3390/ molecules28031146

Academic Editors: Qingzhong Li, Steve Scheiner and Zhiwu Yu

Received: 6 December 2022 Revised: 11 January 2023 Accepted: 19 January 2023 Published: 23 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

## **1. Introduction**

Carbocations as positively charged particles are strong electrophiles and may (1) react with a nucleophile thus yielding adducts (SN1 reaction), (2) act as a protonating agent turning into highly active species that enter into secondary reactions, or (3) get rearranged into other carbocations [1–3]. These reactions cannot be studied in liquid superacids, where carbocations have so far been mainly investigated by NMR [2], because nucleophiles are inevitably protonated under these conditions. Therefore, experimental studies on carbocation/nucleophile interactions are scarce.

The reactivity of unsaturated carbocations having C=C and C≡C bonds is more difficult to study than that of saturated ones. As a class of reactive intermediates, they have been the subject of extensive theoretical and experimental research in the past five decades [4–12]. Nonetheless, most reactivity studies have been focused on solvolysis reactions where the reactive vinyl cation is intercepted by heteroatom-containing solvent molecules [4,5,13], and these reactions have not been analyzed in detail. It follows from these works that vinyl cations are highly reactive and, therefore, uncontrollable intermediates. This point of view has been refuted by Mayr and coworkers [14], who found that the vinyl cation is even less reactive than diarylcarbenium cations, some of the most stable trisubstituted cations. An overestimation of the reactivity of the vinyl cation, as follows from the research on solvolysis reactions, is also evidenced by high stability of vinyl cation salts in solutions in dichloromethane, from which they can be isolated into a crystalline phase, which has made it possible to study them by X-ray diffraction [15–17]. Recently, the reason for the apparent high reactivity of the C6H5CH<sup>2</sup> <sup>+</sup> benzyl cation toward such a nucleophile as benzene was established: it protonates benzene, turning it into a highly reactive carbene, C6H<sup>5</sup> .. CH, which enters into a secondary reaction with the available carbocation [18].

We are not aware of reports about reactions of vinyl carbocations with the simplest oxygen-containing nucleophiles L. It would be expected that such reactions would proceed by the mechanism of both attachment of the nucleophile and its protonation, with the formation of proton disolvates L-H<sup>+</sup> -L as well, which have been studied by X-ray diffraction and IR spectroscopy (for L = H2O, Et2O, benzophenone, nitrobenzene, tetrahydrofuran, and others) [19,20]. oxygen-containing nucleophiles L. It would be expected that such reactions would proceed by the mechanism of both attachment of the nucleophile and its protonation, with the formation of proton disolvates L-H<sup>+</sup> -L as well, which have been studied by X-ray diffraction and IR spectroscopy (for L = H2O, Et2O, benzophenone, nitrobenzene, tetrahydrofuran, and others) [19,20]. In this work, we examined the interaction of unsaturated vinyl-type carbocations fraction and IR spectroscopy (for L = H2O, Et2O, benzophenone, nitrobenzene, tetrahydrofuran, and others) [19,20]. In this work, we examined the interaction of unsaturated vinyl-type carbocations С3Н<sup>5</sup> <sup>+</sup> and С4Н<sup>7</sup> + (as carborane salts of the CHB11Cl11<sup>−</sup> anion) with the simplest nucleophiles: water, alcohols, and acetone. The reaction products that crystallized were studied by X-ray diffraction and IR spectroscopy. We chose carborane CHB11Cl11<sup>−</sup> as the counter-

nucleophile as benzene was established: it protonates benzene, turning it into a highly reactive carbene, , which enters into a secondary reaction with the available car-

nucleophile as benzene was established: it protonates benzene, turning it into a highly reactive carbene, , which enters into a secondary reaction with the available car-

We are not aware of reports about reactions of vinyl carbocations with the simplest


We are not aware of reports about reactions of vinyl carbocations with the simplest oxygen-containing nucleophiles L. It would be expected that such reactions would proceed by the mechanism of both attachment of the nucleophile and its protonation, with

*Molecules* **2023**, *28*, x FOR PEER REVIEW 2 of 15

the formation of proton disolvates L-H<sup>+</sup>

*Molecules* **2023**, *28*, x FOR PEER REVIEW 2 of 15

In this work, we examined the interaction of unsaturated vinyl-type carbocations C3H<sup>5</sup> <sup>+</sup> and C4H<sup>7</sup> + (as carborane salts of the CHB11Cl<sup>11</sup> − anion) with the simplest nucleophiles: water, alcohols, and acetone. The reaction products that crystallized were studied by X-ray diffraction and IR spectroscopy. We chose carborane CHB11Cl<sup>11</sup> − as the counterion (hereafter denoted as (Cl<sup>11</sup> −), Figure 1) because of its exceptionally high stability at low basicity [21]. С3Н<sup>5</sup> <sup>+</sup> and С4Н<sup>7</sup> + (as carborane salts of the CHB11Cl11<sup>−</sup> anion) with the simplest nucleophiles: water, alcohols, and acetone. The reaction products that crystallized were studied by X-ray diffraction and IR spectroscopy. We chose carborane CHB11Cl11<sup>−</sup> as the counterion (hereafter denoted as (Cl11<sup>−</sup> ), Figure 1) because of its exceptionally high stability at low basicity [21]. ion (hereafter denoted as (Cl11<sup>−</sup> ), Figure 1) because of its exceptionally high stability at low basicity [21].

bocation [18].

bocation [18].

**Figure 1.** Undecachlorocarborane anion CHB11Cl11<sup>−</sup> (atoms: green Cl, brown B, grey С and white **Figure 1.** Undecachlorocarborane anion CHB11Cl<sup>11</sup> − (atoms: green Cl, brown B, grey C and white H). **2. Results**

<sup>+</sup>and С4H<sup>7</sup>

#### H). **2. Results** The bulk amount of salts of cations С3H<sup>5</sup>

**2. Results** The bulk amount of salts of cations С3H<sup>5</sup> <sup>+</sup>and С4H<sup>7</sup> <sup>+</sup> can be easily obtained by adding to the acid H(Cl11) powder such a small amount of 1,2-dichloropropane or 1,2-dichloro-2 methylpropane, respectively, with the sample remaining powdery. The quality of the resulting salts can be controlled by IR spectroscopy (their IR spectra should coincide with the reference spectra of salts [15,16], indicating the absence of impurities). These salts are soluble in pentafluorobenzene only in the presence of small amounts of water. Storage of such solutions for 1 day under ambient conditions led to a release of colorless crystals from it. The solubility of the salts and the yield of crystals increased with an increase in the The bulk amount of salts of cations C3H<sup>5</sup> <sup>+</sup> and C4H<sup>7</sup> + can be easily obtained by adding to the acid H(Cl11) powder such a small amount of 1,2-dichloropropane or 1,2-dichloro-2-methylpropane, respectively, with the sample remaining powdery. The quality of the resulting salts can be controlled by IR spectroscopy (their IR spectra should coincide with the reference spectra of salts [15,16], indicating the absence of impurities). These salts are soluble in pentafluorobenzene only in the presence of small amounts of water. Storage of such solutions for 1 day under ambient conditions led to a release of colorless crystals from it. The solubility of the salts and the yield of crystals increased with an increase in the content of water. X-ray diffraction analysis of crystals isolated from C4H<sup>7</sup> + (Cl<sup>11</sup> −) solutions showed that they are a salt of proton disolvate L−H+−L formed by two alcohol molecules L, representing 1-hydroxy-2-methylpropene (Figure 2). to the acid H(Cl11) powder such a small amount of 1,2-dichloropropane or 1,2-dichloro-2 methylpropane, respectively, with the sample remaining powdery. The quality of the resulting salts can be controlled by IR spectroscopy (their IR spectra should coincide with the reference spectra of salts [15,16], indicating the absence of impurities). These salts are soluble in pentafluorobenzene only in the presence of small amounts of water. Storage of such solutions for 1 day under ambient conditions led to a release of colorless crystals from it. The solubility of the salts and the yield of crystals increased with an increase in the content of water. X-ray diffraction analysis of crystals isolated from С4H<sup>7</sup> + (Cl11<sup>−</sup> ) solutions showed that they are a salt of proton disolvate L−H+−L formed by two alcohol molecules L, representing 1-hydroxy-2-methylpropene (Figure 2).

> + (Cl11<sup>−</sup>

<sup>+</sup> can be easily obtained by adding

(atoms: green Cl, brown B, grey С and white

) solutions

**Figure 2.** X-ray structure of the proton disolvate **I***а.* **Figure 2.** X-ray structure of the proton disolvate I*a*.

**Figure 2.** X-ray structure of the proton disolvate **I***а.* The ОО distance in the OH+O moiety (2.420 Å ) is typical for proton disolvates with very strong H-bonds [19,20]. The position of the bridging proton was determined by means of an electron density difference map (short O-H distance of 1.14 Å ). Bond lengths and bond angles of the cation, which we will designate as **I***a* (Figure 3), are given in Table 1 and Figure 2. Four carbon atoms C1-C4 and an oxygen atom of the alcohol molecule are The ОО distance in the OH+O moiety (2.420 Å ) is typical for proton disolvates with very strong H-bonds [19,20]. The position of the bridging proton was determined by means of an electron density difference map (short O-H distance of 1.14 Å ). Bond lengths and bond angles of the cation, which we will designate as **I***a* (Figure 3), are given in Table 1 and Figure 2. Four carbon atoms C1-C4 and an oxygen atom of the alcohol molecule are The O···O distance in the OH+O moiety (2.420 Å) is typical for proton disolvates with very strong H-bonds [19,20]. The position of the bridging proton was determined by means of an electron density difference map (short O-H distance of 1.14 Å). Bond lengths and bond angles of the cation, which we will designate as I*a* (Figure 3), are given in Table 1 and Figure 2. Four carbon atoms C1-C4 and an oxygen atom of the alcohol molecule are in the same plane, and their CCC and CCO angles are close to 120◦ , i.e., C1 and C2 atoms have sp<sup>2</sup> hybridization and, therefore, are double bonded. The C-O···O angle is 118(3)◦ , which means that the O atom also has sp<sup>2</sup> hybridization and belongs to the alcohol OH group. The C=C double bond length of 1.286 Å was determined as the average of two

isomeric alcohol molecules (Figure S1 in Supplementary Materials). It is shorter than that in molecular C=C(–OH) fragments of enol tautomers (1.362 Å) [22]. The C−O distance of 1.252 Å is also slightly shortened compared to that in a single C–O bond in the same (C=)C–OH fragment (1.333 Å) [22]. isomeric alcohol molecules (Figure S1 in SI). It is shorter than that in molecular C=C(–OH) fragments of enol tautomers (1.362 Å ) [22]. The C−O distance of 1.252 Å is also slightly shortened compared to that in a single C–O bond in the same (C=)C–OH fragment (1.333 Å ) [22].

in the same plane, and their CCC and CCO angles are close to 120, i.e., C1 and C2 atoms have sp<sup>2</sup> hybridization and, therefore, are double bonded. The C-O∙∙∙O angle is 118(3)°, which means that the O atom also has sp<sup>2</sup> hybridization and belongs to the alcohol OH group. The C=C double bond length of 1.286 Å was determined as the average of two

*Molecules* **2023**, *28*, x FOR PEER REVIEW 3 of 15

**Figure 3.** Schematic representation of the structures of the studied carbocation compounds. **Figure 3.** Schematic representation of the structures of the studied carbocation compounds.

**Table 1.** Selected geometric parameters of proton disolvate **I***a* (averaged from two solvate molecules (CH3)2C=CHOH) according to X-ray data. **Table 1.** Selected geometric parameters of proton disolvate Ia (averaged from two solvate molecules (CH<sup>3</sup> )2C=CHOH) according to X-ray data.


C1′–O1′ O1 118(3) The IR spectrum of the crystals is characteristic of proton disolvates: it contains an intense absorption pattern of the OH+O group, consisting of three broad bands at 905, 1297 and 1552 cm−<sup>1</sup> (Figure 4, red). The band at 905 cm−<sup>1</sup> belongs to as(OH+O), and bands 1297 and 1552 cm−<sup>1</sup> to mixed stretching and bending vibrations of the OH+O group [20]. The shape of the band at 905 cm−<sup>1</sup> is distorted by the resonance effect leading to so-called trans-The IR spectrum of the crystals is characteristic of proton disolvates: it contains an intense absorption pattern of the OH+O group, consisting of three broad bands at 905, 1297 and 1552 cm−<sup>1</sup> (Figure 4, red). The band at 905 cm−<sup>1</sup> belongs to νas(OH+O), and bands 1297 and 1552 cm−<sup>1</sup> to mixed stretching and bending vibrations of the OH+O group [20]. The shape of the band at 905 cm−<sup>1</sup> is distorted by the resonance effect leading to so-called transparent windows (Evans holes) [23], which appear as dips in the spectrum at 855 and 640 cm−<sup>1</sup> .

parent windows (Evans holes) [23], which appear as dips in the spectrum at 855 and 640 cm−<sup>1</sup> . The bands of CH stretching vibrations of CH<sup>3</sup> groups of cation **I***a* (Figure 4, inset) are easily interpreted because they are similar to those of CH<sup>3</sup> groups of acetone in the previously studied disolvate Acetone−H+−Acetonе [24], (Table 2). A weak band at 3048 cm<sup>−</sup><sup>1</sup> The bands of CH stretching vibrations of CH<sup>3</sup> groups of cation I*a* (Figure 4, inset) are easily interpreted because they are similar to those of CH<sup>3</sup> groups of acetone in the previously studied disolvate Acetone−H+−Acetone [24], (Table 2). A weak band at 3048 cm−<sup>1</sup> may belong to the stretching vibration of the =C-H bond, the C atom of which is adjacent to the OH+O group.

may belong to the stretching vibration of the =C-H bond, the C atom of which is adjacent to the OH+O group. There are more uncertainties in the interpretation of the C-O and C=C stretching vibrations. It has been found that absorption bands of C−O stretching vibrations of alcohol molecules directly bound to H<sup>+</sup> in proton disolvates are so strongly broadened and weakened in intensity that they are not detectable in IR spectra [24]. Two bands at 1560 and 1683 cm−<sup>1</sup> (Figure 4) are in the expected frequency range of C=C stretch vibrations [15–17] and can be attributed to them. The fact that the position of the bridging proton is determined by X-ray diffraction means that its two-well potential has a high enough potential barrier for the proton to be at the bottom of one of the wells for a sufficiently long time (at the time scale of IR spectroscopy) in order to demonstrate (in an IR spectrum) the non-equivalence of two alcohol molecules in I as two C=C stretching frequencies. One of them at 1560 cm−<sup>1</sup> is close

to the frequency (1555 cm−<sup>1</sup> ) of the cation in contact ion pair (CH3)2C=C+−<sup>H</sup> . . . (Cl<sup>11</sup> −). Therefore, it can be assumed that the frequency 1560 cm−<sup>1</sup> belongs to the isobutylene alcohol molecule, which has a shorter O+−H bond (1.14 Å) and imitates a protonated alcohol molecule solvated by the second L alcohol molecule (CH3)2C=CH−OH−+<sup>H</sup> . . . L, which is less influenced by the positive charge and has an increased C=C stretch frequency: 1683 cm−<sup>1</sup> . The CC and CH stretch frequencies are summarized in Table 2. *Molecules* **2023**, *28*, x FOR PEER REVIEW 4 of 15

**Figure 4.** ATR IR spectra (with ATR correction) of crystals of salts of cations **Ia** (red) and **Ib** (blue). Anion signals are marked with asterisks. **Figure 4.** ATR IR spectra (with ATR correction) of crystals of salts of cations Ia (red) and Ib (blue). Anion signals are marked with asterisks.


**Table 2.** CH and C=C stretch frequencies (in cm<sup>−</sup><sup>1</sup> ) of the vinyl cation in adducts I-IV and in (Ac)2H+. **Table 2.** CH and C=C stretch frequencies (in cm−<sup>1</sup> ) of the vinyl cation in adducts I–IV and in (Ac)2H<sup>+</sup> .

There are more uncertainties in the interpretation of the C-O and C=C stretching vibrations. It has been found that absorption bands of C−O stretching vibrations of alcohol molecules directly bound to H<sup>+</sup> in proton disolvates are so strongly broadened and weakened in intensity that they are not detectable in IR spectra [24]. Two bands at 1560 and 1683 cm−<sup>1</sup> (Figure 4) are in the expected frequency range of C=C stretch vibrations [15–17] It follows from the obtained results that disolvate Ia is generated by the interaction of the C4H<sup>7</sup> + carbocation with water molecules, as shown in Scheme 1. The weak spectrum of the H3O<sup>+</sup> cation [25], which should arise simultaneously, actually manifests itself in the IR spectrum of the viscous phase, which precipitates concurrently with the crystalline phase. of the Н3О<sup>+</sup> cation [25], which should arise simultaneously, actually manifests itself in the IR spectrum of the viscous phase, which precipitates concurrently with the crystalline phase.

at 1560 cm−<sup>1</sup> is close to the frequency (1555 cm−<sup>1</sup> ) of the cation in contact ion pair (CH3)2C=C+−H…(Cl11<sup>−</sup> ). Therefore, it can be assumed that the frequency 1560 cm−<sup>1</sup> belongs to the isobutylene alcohol molecule, which has a shorter O+−H bond (1.14 Å ) and imitates **Scheme 1.** Representation of the formation of cation **Ia** during the interaction of cation С4Н<sup>7</sup> <sup>+</sup> with water molecules. For instance, in the crystal lattice, alcohol molecules C3H5OH with reliably **Scheme 1.** Representation of the formation of cation Ia during the interaction of cation C4H7 <sup>+</sup> with water molecules.

a protonated alcohol molecule solvated by the second L alcohol molecule (CH3)2C=CH−OH−+H…L, which is less influenced by the positive charge and has an in-

partially disordered structure. This property does not allow to see in detail the entire structure of the cation but enables us to determine its similarity with cation **Ia**. The similarity is confirmed by the identity of the IR spectrum of the crystals to that of the salt of disolvate **Ia** (Figure 4), which means that the crystals growing from solutions of

<sup>+</sup>and С4H<sup>7</sup>

benzene is very low, and crystals cannot be grown from them. To increase the solubility of the salts, 1 vol% acetone was added to pentafluorobenzene containing trace amounts of water. This approach helped to obtain a solution with a heightened salt content and a reduced H2O/cation<sup>+</sup> molar ratio. Keeping this solution over hexane vapor for 1 to 2 weeks led to the appearance of crystals. The X-ray diffraction analysis of the crystals obtained

by one molecule of 1-hydroxy-2-methylpropane (Figure 5). The crystal lattice does not have an acetone molecule but contains one solvent molecule, C6HF5, per two salt mole-

С2A atom slightly above or below the plane of three atoms: С1, С3, and С3A (Figures 5a and S1 in SI). The С1-С2A∙∙∙О1 and С5-О1∙∙∙С2A angles are 137° and 121°, respectively (Figure 5b, Table 3), which means sp<sup>2</sup> hybridization of С2A and О1 atoms, i.e., an H atom is attached to each of them. The C∙∙∙O distance is 2.470 Å, which matches the maximum

[19]. Therefore, with a high probability, a bridging proton is located between the C2A and O1 atoms, forming a short, strong, and low-barrier double-well H-bond, although com-

unknown. The short distance (C2A)H∙∙∙O1 of 1.97 Å, also points to the presence of a strong H- bond. The significant difference of the С2A-H∙∙∙O1 angle at 111° from the optimal one at 180° may be partly due to the inaccuracy of determining the localization of the H atom by the calculation method. The question of the presence of a strong H-bond with doublewell proton potential is discussed below when the IR spectra of these crystals are examined.

‧OHС4H<sup>7</sup> cation under consideration is hereafter denoted as **IIa** (Figure 2).

+ 1% acetone indicated that they contain the hydrocarbon cations and C6HF<sup>5</sup> inclusion molecules of the solvents with significantly disordered C and F-atoms. We failed to localize the highly disordered structure of the cation. For this reason, we do not present or discuss these X-ray diffraction data. Nonetheless, they provide some useful information.

X-ray diffraction analysis of crystals obtained from a solution of С3H<sup>5</sup>

X-ray diffraction analysis of crystals—grown from a solution of the C3H<sup>5</sup>

It follows from the obtained results that disolvate **Ia** is generated by the interaction

<sup>+</sup> carbocation with water molecules, as shown in Scheme 1. The weak spectrum

+

) salts revealed that they contain the С4H<sup>7</sup>

<sup>+</sup> cation is disordered over two positions differing in the location of the

. The CC and CH stretch frequencies are summa-


in carefully dehydrated pentafluoro-

+ (Cl11<sup>−</sup>

<sup>+</sup> cation solvated

∙∙∙O moiety in proton

∙∙∙O with a strong H-bond are currently

+ (Cl11<sup>−</sup>

(nitrobenzene)2)

) in C6HF<sup>5</sup>

) salt

) are proton disolvates L2-H<sup>+</sup>

+ (Cl11<sup>−</sup>

pounds containing asymmetric moiety (С)С−H<sup>+</sup>

allowable distance between the O atoms of the symmetric O∙∙∙H<sup>+</sup>

disolvates (2.47 Å for the most unstable proton disolvates obtained: H<sup>+</sup>

The solubility of salts of cations С3H<sup>5</sup>

be referred to as cations **Ib** (Figure 2).

from solutions of the С4H<sup>7</sup>

cules. The С4Н<sup>7</sup>

The С4H<sup>7</sup>

+

rized in Table 2.

of the С4H<sup>7</sup>

С3H<sup>5</sup> + (Cl11<sup>−</sup>

X-ray diffraction analysis of crystals—grown from a solution of the C3H<sup>5</sup> + (Cl<sup>11</sup> −) salt under the same conditions under which crystals of the salt of Ia were obtained—showed partially disordered structure. This property does not allow to see in detail the entire structure of the cation but enables us to determine its similarity with cation Ia. The similarity is confirmed by the identity of the IR spectrum of the crystals to that of the salt of disolvate Ia (Figure 4), which means that the crystals growing from solutions of C3H<sup>5</sup> + (Cl<sup>11</sup> <sup>−</sup>) are proton disolvates L2-H<sup>+</sup> -L2, where L<sup>2</sup> is CH3CH=CHOH. Hereafter, they will be referred to as cations Ib (Figure 2).

The solubility of salts of cations C3H<sup>5</sup> <sup>+</sup> and C4H<sup>7</sup> + in carefully dehydrated pentafluorobenzene is very low, and crystals cannot be grown from them. To increase the solubility of the salts, 1 vol% acetone was added to pentafluorobenzene containing trace amounts of water. This approach helped to obtain a solution with a heightened salt content and a reduced H2O/cation<sup>+</sup> molar ratio. Keeping this solution over hexane vapor for 1 to 2 weeks led to the appearance of crystals. The X-ray diffraction analysis of the crystals obtained from solutions of the C4H<sup>7</sup> + (Cl<sup>11</sup> <sup>−</sup>) salts revealed that they contain the C4H<sup>7</sup> + cation solvated by one molecule of 1-hydroxy-2-methylpropane (Figure 5). The crystal lattice does not have an acetone molecule but contains one solvent molecule, C6HF5, per two salt molecules. The C4H<sup>7</sup> + cation is disordered over two positions differing in the location of the C2A atom slightly above or below the plane of three atoms: C1, C3, and C3A (Figure 5a and Figure S1 in Supplementary Materials). The C1-C2A···O1 and C5-O1···C2A angles are 137◦ and 121◦ , respectively (Figure 5b, Table 3), which means sp<sup>2</sup> hybridization of C2A and O1 atoms, i.e., an H atom is attached to each of them. The C···O distance is 2.470 Å, which matches the maximum allowable distance between the O atoms of the symmetric O···H<sup>+</sup> ···O moiety in proton disolvates (2.47 Å for the most unstable proton disolvates obtained: H<sup>+</sup> (nitrobenzene)2) [19]. Therefore, with a high probability, a bridging proton is located between the C2A and O1 atoms, forming a short, strong, and low-barrier double-well H-bond, although compounds containing asymmetric moiety (C)C−H<sup>+</sup> ···O with a strong H-bond are currently unknown. The short distance (C2A)H···O1 of 1.97 Å, also points to the presence of a strong H- bond. The significant difference of the C2A-H···O1 angle at 111◦ from the optimal one at 180◦ may be partly due to the inaccuracy of determining the localization of the H atom by the calculation method. The question of the presence of a strong H-bond with double-well proton potential is discussed below when the IR spectra of these crystals are examined. The C4H<sup>7</sup> + ·OHC4H<sup>7</sup> cation under consideration is hereafter denoted as IIa (Figure 2). *Molecules* **2023**, *28*, x FOR PEER REVIEW 6 of 15

tomic distances are given in Å .

H<sup>+</sup>

**Figure 5.** The С4H<sup>7</sup> <sup>+</sup> cation at two positions in the crystal lattice of salt (С4H<sup>7</sup> + ‧OHС4H7)(Cl11<sup>−</sup> ) (a) and the structure of the cationic adduct С4H<sup>7</sup> + ‧OHС4H<sup>7</sup> as determined by X-ray crystallography. Intera-**Figure 5.** The C4H<sup>7</sup> + cation at two positions in the crystal lattice of salt (C4H<sup>7</sup> + ·OHC4H<sup>7</sup> )(Cl<sup>11</sup> −) (**a**) and the structure of the cationic adduct C4H<sup>7</sup> + ·OHC4H<sup>7</sup> as (**b**) determined by X-ray crystallography. Interatomic distances are given in Å.

**Table 3.** Selected geometric parameter of cationic adduct **II***a* according to X-ray data. **Bond or Angle Å or º** O1–C5 1.292(9) С5–С8 1.236(9) X-ray diffraction analysis of crystals obtained from a solution of C3H<sup>5</sup> + (Cl<sup>11</sup> −) in C6HF<sup>5</sup> + 1% acetone indicated that they contain the hydrocarbon cations and C6HF<sup>5</sup> inclusion molecules of the solvents with significantly disordered C and F-atoms. We failed to localize the highly disordered structure of the cation. For this reason, we do not present or discuss these X-ray diffraction data. Nonetheless, they provide some useful information. For instance, in the crystal lattice, alcohol molecules C3H5OH with reliably fixed C and O

> С8–СН<sup>3</sup> 1.450(6) С1–С2 1.350(11)

О1–С5–С8 121.1(6) С5–С8–С6 121.6(5) С5–С8–С7 119.7(5) С5–С8–С6 118.7(5) С2–С1–С3 118.3(5) С2–С1–С3а 110.1(5) C2…O1 2.47(1)

fixed C and O atoms were identified, possibly indicating the solvation of the cation with

similar to **IIa,** and designate it as **IIb** (Figure 2). The IR spectra of crystals containing **IIa**

In the IR spectra of cationic adducts **IIa** and **IIb**, specific absorption of a strong С–

∙∙∙O H-bond should be present; however, this type of strong H-bond has not yet been

bonds in proton disolvates L2H<sup>+</sup> with double- well proton potential separated by a low potential barrier that transforms it into flat-bottom potential for vibrational transitions. In these cases, the proton vibrations appear in the IR spectra as an intense and broad absorp-

**Ia** and **Ib** (Figure 4). In proton disolvates with an asymmetric moiety, for example, N−H+…O, the bottom of the double-well potential is asymmetric and the maximum of the

adducts **IIa** and **IIb**, a broad band at 1630 cm−1 is observed, as is absorption in the region of 1200–1500 cm−1, which is not described by a single Gaussian (Figure 6, inset). They cor-

+

effect should specifically influence the absorption of its stretch vibrations. For example, it has been found [24] that if a single C-O bond is attached to a bridged proton its absorption band broadens and decreases in intensity so much that it is undetectable in the IR spec-

cussed below), it still appears in the spectrum as a weak-to-moderately intense broadened

<sup>+</sup> and C4H<sup>7</sup>

<sup>+</sup> cation

∙∙∙N H-

‧C3H5OH,

+

‧C3H5OH.

∙∙∙O or N∙∙∙H<sup>+</sup>

+

[25], as observed in the spectra of cations

is affected by the bridging proton, and this

∙∙∙X<sup>2</sup> fragment, in our case =С−H<sup>+</sup>

, as in proton disolvate Acetone-H<sup>+</sup>

[25]. In the spectra of the analyzed

∙∙∙O.


the C3H5OH molecule in the same way as the C4H7OH molecule solvates the C4H<sup>7</sup>

in adduct **II***a*. That is, we can assume the emergence of cationic adduct C3H<sup>5</sup>

and **IIb** adducts are identical (Figure 6), which confirms that **IIb** is C3H<sup>5</sup>

found. Well-studied strong hydrogen bonds are symmetric O∙∙∙H<sup>+</sup>

tion pattern with a maximum at 850–1000 cm−<sup>1</sup>

respond fairly well to the asymmetric X1‒H<sup>+</sup>

The C=C bond of cations C3H<sup>5</sup>

trum. If the C=O bond is attached to H<sup>+</sup>

broad and intense absorption shifts to 1400–1700 cm−<sup>1</sup>

atoms were identified, possibly indicating the solvation of the cation with the C3H5OH molecule in the same way as the C4H7OH molecule solvates the C4H<sup>7</sup> + cation in adduct II*a*. That is, we can assume the emergence of cationic adduct C3H<sup>5</sup> + ·C3H5OH, similar to IIa, and designate it as IIb (Figure 2). The IR spectra of crystals containing IIa and IIb adducts are identical (Figure 6), which confirms that IIb is C3H<sup>5</sup> + ·C3H5OH.


**Table 3.** Selected geometric parameter of cationic adduct II*a* according to X-ray data.

**Figure 6.** ATR IR spectra (with ATR correction) of crystals of (С4H<sup>7</sup> + ∙ С4H7OH)(Cl11<sup>−</sup> ) (**IIa**, red) and (С3H<sup>5</sup> + ∙ С3H5OH)(Cl11<sup>−</sup> ) (**IIb**, blue). Characteristic absorption bands of the C6HF<sup>5</sup> inclusion molecule are indicated by asterisks, and the strongest absorption bands of the (Cl11<sup>−</sup> ) anion are marked with . The dotted line shows separation of contours of the bands of stretching C=C vibrations (in the inset). **Figure 6.** ATR IR spectra (with ATR correction) of crystals of (C4H<sup>7</sup> + ·· C4H7OH)(Cl<sup>11</sup> −) (IIa, red) and (C3H<sup>5</sup> + ·· C3H5OH)(Cl<sup>11</sup> −) (IIb, blue). Characteristic absorption bands of the C6HF<sup>5</sup> inclusion molecule are indicated by asterisks, and the strongest absorption bands of the (Cl<sup>11</sup> −) anion are marked with . The dotted line shows separation of contours of the bands of stretching C=C vibrations (in the inset).

The C=C band of the alcohol molecule in adducts **IIa** and **IIb** is subject to a weaker influence of the bridging proton. Its C=C stretching vibration is observed at 1704 cm−<sup>1</sup> (Figure 6). It is a single band for adduct **IIb**, whereas in the spectrum of **IIa** it is split into two components at 1715 and 1694 cm−<sup>1</sup> of equal intensity (Figure 6, inset). Obviously, in the crystal lattice of the **II***a* adduct salt, there are two weakly nonequivalent C4H7OH molecules. In the IR spectra of cationic adducts IIa and IIb, specific absorption of a strong C–H<sup>+</sup> ···O H-bond should be present; however, this type of strong H-bond has not yet been found. Well-studied strong hydrogen bonds are symmetric O···H<sup>+</sup> ···O or N···H<sup>+</sup> ···N H-bonds in proton disolvates L2H<sup>+</sup> with double- well proton potential separated by a low potential barrier that transforms it into flat-bottom potential for vibrational transitions. In these cases, the proton vibrations appear in the IR spectra as an intense and broad

In the CH stretch frequency region, the three bands at 2961, 2932 and 2874 cm−<sup>1</sup> can be unambiguously interpreted as vibrations of the CH<sup>3</sup> group (Table 2). The IR spectra of

The band at 3370 cm−<sup>1</sup> may belong to OH groups of the C4H7OH alcohol molecule,

The solution from which crystals of the salt of **II***b* grew was kept in a sealed ampoule, and after a few days a small number of tiny crystals grew from it. It was possible to find one crystal of sufficient size for X-ray analysis. It turned out that this was a salt of the

∙OH2)(Cl11<sup>−</sup>

+

) anion.

) (Figure 7). The C3H<sup>5</sup>

+

∙OH<sup>2</sup> species

accessory and its crystal lattice is destroyed, pentafluorobenzene is released and slowly evaporates when the sample is kept in a glove box atmosphere. Therefore, the intensity of its absorption decreases over time. The most intense C6F5H bands in Figure 6 are marked

which are engaged in weak H bonds with the (Cl11<sup>−</sup>

monohydrate of the propylene cation (С3H<sup>5</sup>

with asterisks.

absorption pattern with a maximum at 850–1000 cm−<sup>1</sup> [25], as observed in the spectra of cations Ia and Ib (Figure 4). In proton disolvates with an asymmetric moiety, for example, <sup>N</sup>−H<sup>+</sup> ···O, the bottom of the double-well potential is asymmetric and the maximum of the broad and intense absorption shifts to 1400–1700 cm−<sup>1</sup> [25]. In the spectra of the analyzed adducts IIa and IIb, a broad band at 1630 cm−<sup>1</sup> is observed, as is absorption in the region of 1200–1500 cm−<sup>1</sup> , which is not described by a single Gaussian (Figure 6, inset). They correspond fairly well to the asymmetric X1−H<sup>+</sup> ···X<sup>2</sup> fragment, in our case =C−H<sup>+</sup> ···O.

The C=C bond of cations C3H<sup>5</sup> <sup>+</sup> and C4H<sup>7</sup> + is affected by the bridging proton, and this effect should specifically influence the absorption of its stretch vibrations. For example, it has been found [24] that if a single C-O bond is attached to a bridged proton its absorption band broadens and decreases in intensity so much that it is undetectable in the IR spectrum. If the C=O bond is attached to H<sup>+</sup> , as in proton disolvate Acetone-H<sup>+</sup> -Acetone (discussed below), it still appears in the spectrum as a weak-to-moderately intense broadened band [24]. Therefore, absorption of the C=C stretch vibration of C3H<sup>5</sup> <sup>+</sup> and C4H<sup>7</sup> + may look like a weak band at 1633 cm−<sup>1</sup> (Figure 6). It is higher in frequency than νC=C at ~1560–1590 cm−<sup>1</sup> in the contact ion pairs (CIPs) formed with the (Cl<sup>11</sup> −) anion in solutions and in a solid phase [15–17]. This means that the interaction of C3H<sup>5</sup> <sup>+</sup> and C4H<sup>7</sup> + cations with an alcohol molecule in II is stronger than the interaction with the (Cl<sup>11</sup> −) anion in the CIP.

The C=C band of the alcohol molecule in adducts IIa and IIb is subject to a weaker influence of the bridging proton. Its C=C stretching vibration is observed at 1704 cm−<sup>1</sup> (Figure 6). It is a single band for adduct IIb, whereas in the spectrum of IIa it is split into two components at 1715 and 1694 cm−<sup>1</sup> of equal intensity (Figure 6, inset). Obviously, in the crystal lattice of the II*a* adduct salt, there are two weakly nonequivalent C4H7OH molecules.

In the CH stretch frequency region, the three bands at 2961, 2932 and 2874 cm−<sup>1</sup> can be unambiguously interpreted as vibrations of the CH<sup>3</sup> group (Table 2). The IR spectra of the crystals also contain absorption bands of the captured pentafluorobenzene molecule. They are easily identified because of the finding that when a crystal is crushed on an ATR accessory and its crystal lattice is destroyed, pentafluorobenzene is released and slowly evaporates when the sample is kept in a glove box atmosphere. Therefore, the intensity of its absorption decreases over time. The most intense C6F5H bands in Figure 6 are marked with asterisks.

The band at 3370 cm−<sup>1</sup> may belong to OH groups of the C4H7OH alcohol molecule, which are engaged in weak H bonds with the (Cl<sup>11</sup> −) anion.

The solution from which crystals of the salt of II*b* grew was kept in a sealed ampoule, and after a few days a small number of tiny crystals grew from it. It was possible to find one crystal of sufficient size for X-ray analysis. It turned out that this was a salt of the monohydrate of the propylene cation (C3H<sup>5</sup> + ·OH2)(Cl<sup>11</sup> <sup>−</sup>) (Figure 7). The C3H<sup>5</sup> + ·OH<sup>2</sup> species has two localizations in the unit cell with slightly different positions of C and O atoms, but they can be distinguished (Figure 7a). Similarly, an anion can be disordered over two positions, as indicated by the presence of electron density peaks in the difference map in the region of the anion. Nonetheless, we failed to localize the second position of the anion. This may be the reason for substantial deterioration of the R<sup>f</sup> factor, but does not interfere with the determination of coordinates of the C and O atoms of the cation with accuracy sufficient to establish the topology of the cation qualitatively and its main geometric parameters (Table 4).

The main feature of the structure of the C3H<sup>5</sup> + ·OH<sup>2</sup> cationic adduct, which is denoted below as III*b* (Figure 2), is as follows. The H2O molecule is attached to the C3H<sup>5</sup> + cation through the O atom in the direction perpendicular to the C=C double bond at a distance of 2.32 Å, which is very close to the average Na···O(H2) distance of 2.333 Å in the first hydration shell of hexahydrate Na(OH2)<sup>6</sup> as determined by X-ray diffraction analysis for 13 structures (retrieval was made according to the Cambridge structural data base, ConQuest 2021.3.0) [26]. Therefore, the nature of the interaction of the H2O molecule with

the C=C<sup>+</sup> bond of the vinyl cation is similar to that of the interaction of water molecules with the alkali metal cation in its first hydration shell. sufficient to establish the topology of the cation qualitatively and its main geometric parameters (Table 4).

has two localizations in the unit cell with slightly different positions of C and O atoms, but they can be distinguished (Figure 7a). Similarly, an anion can be disordered over two positions, as indicated by the presence of electron density peaks in the difference map in the region of the anion. Nonetheless, we failed to localize the second position of the anion. This may be the reason for substantial deterioration of the R<sup>f</sup> factor, but does not interfere with the determination of coordinates of the C and O atoms of the cation with accuracy

*Molecules* **2023**, *28*, x FOR PEER REVIEW 8 of 15

**Figure 7.** X-ray structure of the (C4H<sup>7</sup> + ‧OH2)(Cl11<sup>−</sup> ) (**III***b***)** with two locations of the cation (**a**), and more detailed representation of the C4H<sup>7</sup> + ‧OH<sup>2</sup> adduct (**b**). **Figure 7.** X-ray structure of the (C4H<sup>7</sup> + ·OH<sup>2</sup> )(Cl<sup>11</sup> −) (III*b*) with two locations of the cation (**a**), and more detailed representation of the C4H<sup>7</sup> + ·OH<sup>2</sup> adduct (**b**).

**Table 4.** Selected geometric parameters of cationic adduct **III***b* (averaged over two positions) according to X-ray data. **Table 4.** Selected geometric parameters of cationic adduct III*b* (averaged over two positions) according to X-ray data.


The main feature of the structure of the С3H<sup>5</sup> + ∙OH<sup>2</sup> cationic adduct, which is denoted below as **III***b* (Figure 2), is as follows. The H2O molecule is attached to the С3H<sup>5</sup> <sup>+</sup> cation through the O atom in the direction perpendicular to the C=C double bond at a distance of 2.32 Å , which is very close to the average Na‧‧‧O(H2) distance of 2.333 Å in the first hydration shell of hexahydrate Na(OH2)<sup>6</sup> as determined by X-ray diffraction analysis for 13 structures (retrieval was made according to the Cambridge structural data base, Con-Quest 2021.3.0) [26]. Therefore, the nature of the interaction of the H2O molecule with the C=C<sup>+</sup> bond of the vinyl cation is similar to that of the interaction of water molecules with the alkali metal cation in its first hydration shell. Unfortunately, the insufficient amount of the obtained crystals of (С3H<sup>5</sup> + ∙OH2)(Cl11<sup>−</sup> ) did not allow us to register their IR spectrum. Nevertheless, the spectra of these compounds have been obtained by us earlier, when we characterized crystalline salts С3H<sup>5</sup> + (Cl11<sup>−</sup> ) and С4H<sup>7</sup> + (Cl11<sup>−</sup> ) by X-ray diffraction and IR spectroscopy [15,16]. In the IR spectra of individually selected single crystals of these salts subjected to X-ray analysis, no traces of water absorption were found. On the other hand, during the recording of IR spectra of an aggregate of small crystals that arose simultaneously with larger ones, a weak spectrum of water molecules was observed (Figure 8). Its frequencies νas, ν<sup>s</sup> and δ at 3612, 3544, and 1606 cm−<sup>1</sup> , respectively, are similar to those of monomeric molecules dis-Unfortunately, the insufficient amount of the obtained crystals of (C3H<sup>5</sup> + ·OH2)(Cl<sup>11</sup> −) did not allow us to register their IR spectrum. Nevertheless, the spectra of these compounds have been obtained by us earlier, when we characterized crystalline salts C3H<sup>5</sup> + (Cl<sup>11</sup> −) and C4H<sup>7</sup> + (Cl<sup>11</sup> −) by X-ray diffraction and IR spectroscopy [15,16]. In the IR spectra of individually selected single crystals of these salts subjected to X-ray analysis, no traces of water absorption were found. On the other hand, during the recording of IR spectra of an aggregate of small crystals that arose simultaneously with larger ones, a weak spectrum of water molecules was observed (Figure 8). Its frequencies νas, ν<sup>s</sup> and δ at 3612, 3544, and 1606 cm−<sup>1</sup> , respectively, are similar to those of monomeric molecules dissolved in organic solvents or hydrated alkali metal cations (Table 5). This result was surprising and could not be explained. Now it is clear that the observed bands belong to non–Hbonded water molecules of monohydrates (C4H<sup>7</sup> + ·OH2)(Cl<sup>11</sup> <sup>−</sup>) and (C3H<sup>5</sup> + ·OH2)(Cl<sup>11</sup> −) designated subsequently as IIIa and IIIb (Figure 2), which are formed and co-crystallize with salts of anhydrous vinyl cations. They are typical of the H2O molecules that hydrate the H3O<sup>+</sup> cation in H3O<sup>+</sup> (H2O)3(Cl<sup>11</sup> −) crystals [27], or alkali metal cations (Table 5). Such mostly ionic interaction Cat<sup>+</sup> ···OH<sup>2</sup> leads to a slight decrease in the frequencies of OH stretching, compared to those of the dissolved monomer molecule. In adducts III*a/b*, OH stretching frequencies are somewhat lower than those in hydrates of alkali metal cations and H3O<sup>+</sup> ; hence the strength of bond Cat<sup>+</sup> ···OH<sup>2</sup> in III*a/b* is higher.

solved in organic solvents or hydrated alkali metal cations (Table 5). This result was surprising and could not be explained. Now it is clear that the observed bands belong to non– **Table 5.** IR frequencies of H2O molecules bonded to cations through O-atom.


(a) In diluted solutions in dichloroethane; (b) in the crystal phase studied by X-ray diffraction [27]; (c) in a dichloroethane extract from aqueous solutions of Cs<sup>+</sup> (Cl<sup>11</sup> −).

ionic interaction Cat<sup>+</sup>

hence the strength of bond Cat<sup>+</sup>

**Figure 8.** The IR spectrum of a set of *i*-C4H<sup>7</sup> + (Cl11<sup>−</sup> ) crystals and (*i*-C4H<sup>7</sup> + ‧OH2)(Cl11<sup>−</sup> ) crystals cocrystallized with them. **Figure 8.** The IR spectrum of a set of *i*-C4H<sup>7</sup> + (Cl<sup>11</sup> <sup>−</sup>) crystals and (*i*-C4H<sup>7</sup> + ·OH<sup>2</sup> )(Cl<sup>11</sup> −) crystals cocrystallized with them.

salts of anhydrous vinyl cations. They are typical of the H2O molecules that hydrate the H3O<sup>+</sup> cation in H3O+(H2O)3(Cl11<sup>−</sup>) crystals [27], or alkali metal cations (Table 5). Such mostly

compared to those of the dissolved monomer molecule. In adducts **III***a/b*, OH stretching frequencies are somewhat lower than those in hydrates of alkali metal cations and H3O<sup>+</sup>

‧‧‧OH<sup>2</sup> in **III***a/b* is higher.

‧‧‧OH<sup>2</sup> leads to a slight decrease in the frequencies of OH stretching,

;

(1)

(2)

‧3H2O, i.e.,

**Table 5.** IR frequencies of H2O molecules bonded to cations through O-atom. **νas ν<sup>s</sup> δ** Monomeric H2O (*a*) 3675 3591 1607 H3O+(H2O)3(Cl11<sup>−</sup>) (*b*) 3637 3578 1605 Cs+(H2O)6(Cl11<sup>−</sup>) (c) 3656 3578 1611 *i*-C4H7‧H2O(Cl11<sup>−</sup>) 3612 3544 1606 (*a*) In diluted solutions in dichloroethane; (*b*) in the crystal phase studied by X-ray diffraction [27]; (c) in a dichloroethane extract from aqueous solutions of Cs<sup>+</sup> (Cl11<sup>−</sup> ). As mentioned above, crystals of compounds **IIa** and **IIb** were obtained from saturated solutions of vinyl cation salts in C6HF<sup>5</sup> containing 1 vol% acetone. It could be expected that after an increase in the acetone content, crystals containing acetone could be obtained. Incubation of saturated solutions of the С3H<sup>5</sup> <sup>+</sup>(Cl11<sup>−</sup>) or С4H<sup>7</sup> <sup>+</sup>(Cl11<sup>−</sup>) salts in C6HF<sup>5</sup> As mentioned above, crystals of compounds IIa and IIb were obtained from saturated solutions of vinyl cation salts in C6HF<sup>5</sup> containing 1 vol% acetone. It could be expected that after an increase in the acetone content, crystals containing acetone could be obtained. Incubation of saturated solutions of the C3H<sup>5</sup> + (Cl<sup>11</sup> <sup>−</sup>) or C4H<sup>7</sup> + (Cl<sup>11</sup> −) salts in C6HF<sup>5</sup> + 5% acetone over hexane vapor gave rise to needle-like crystals. X-ray diffraction analysis indicated that this was proton disolvate salt Ac-H<sup>+</sup> -Ac (IV), where Ac is the acetone (Figures 2 and 9a). The same salt was obtained in a different way by reacting the H(Cl11) acid with acetone vapor and its subsequent dissolution in dichloromethane. Slow evaporation of the solution in the glove box led to the formation of crystals. Their X-ray diffraction analysis showed that this was also proton disolvate salt (Ac)2H<sup>+</sup> (Cl<sup>11</sup> −) but with other crystal lattice parameters, that is, it is a different polymorph (Figure 9b). The O···O distance in cations for both polymorphs is almost the same, 2.429 and 2.423 Å. Selected geometric parameters of IV and IV0 are given in Table 6. *Molecules* **2023**, *28*, x FOR PEER REVIEW 10 of 15

parameters of **IV** and **IV** are given in Table 6. The IR spectra of the (Ac)2H<sup>+</sup> cation (Figure S2 in SI) match its known IR spectra [24,28]; interpretation of the spectrum is given in ref. [28]. **Figure 9.** The proton disolvate in two polymorphs of salt (Ac-H<sup>+</sup> -Ac)(Cl11<sup>−</sup> ). In polymorph **IV***b*, it was not possible to localize the bridging proton by X-ray diffraction analysis. **Figure 9.** The proton disolvate in two polymorphs of salt (Ac-H<sup>+</sup> -Ac)(Cl<sup>11</sup> −). In polymorph IV*b*, it was not possible to localize the bridging proton by X-ray diffraction analysis.

**Table 6.** Selected geometric parameters of proton disolvates **IVa** and **IVb** (averaged over two independent fragments (acetone molecules) of the adduct) according to X-ray data. **Table 6.** Selected geometric parameters of proton disolvates IVa and IVb (averaged over two independent fragments (acetone molecules) of the adduct) according to X-ray data.


ons with water molecules. Initially, an H2O molecule is attached to the C=C<sup>+</sup> bond of cation RCH=C+H or R2C=C+H (where R is CH3) via the O- atom (Equation (1), in it and in subse-

The nature of this bond is close to that of bonds formed by water molecules with

With an increase in the concentration of the salt of cationic adduct R2C=C+H···H2O in solutions, the self-association of ion pairs increases (which is typical for salts of carbocations in solutions [29]). This enhances the contact interaction of H2O with the cation and promotes the transfer of a proton to a water molecule with the formation of H3O<sup>+</sup> and

> **. . .**

> > H

H

OCH=CR2

in crystal salts of H3O<sup>+</sup>

+ H3O

+

quent equations, the R2C=C+H cation is used).

R2C=C

. . .

2

H

O

. . .

H

<sup>+</sup>H

disolvate and regenerating cation R2C=C<sup>+</sup> H (Equation (3)),

R

2C

C

With a further increase in the contents of water and of the carbocation salt, the concentration of the resulting adducts **II** and of the H3O+ cation increases. The latter can protonate an alcohol molecule that is more basic than H2O, thereby producing a proton

alkali metal cations during their hydration or with H3O<sup>+</sup>

**3. Discussion**

the bond is strongly ionic.

cationic adduct **II**:

The IR spectra of the (Ac)2H<sup>+</sup> cation (Figure S2 in Supplementary Materials) match its known IR spectra [24,28]; interpretation of the spectrum is given in ref. [28]. С2–С1–С3 120.0(2) С3–С2–С4 120.2(6) O1…O1′ 2.429(3) O1…O2 2.423(9) С2–С1–С3 120.0(2) С3–С2–С4 120.2(6) O1…O1′ 2.429(3) O1…O2 2.423(9)

О1–С1–С2 120.1(2) О1–С2–С3 122.2(5) О1–С1–С3 119.8(2) О1–С2–С4 117.5(5)

О1–С1–С2 120.1(2) О1–С2–С3 122.2(5) О1–С1–С3 119.8(2) О1–С2–С4 117.5(5)

**Table 6.** Selected geometric parameters of proton disolvates **IVa** and **IVb** (averaged over two independent fragments (acetone molecules) of the adduct) according to X-ray data.

**Table 6.** Selected geometric parameters of proton disolvates **IVa** and **IVb** (averaged over two independent fragments (acetone molecules) of the adduct) according to X-ray data.

**IV IV Bond or Angle Å or º Bond or Angle Å or º** O1–C1 1.237(3) О1–С2 1.228(8) С1–СH<sup>3</sup> 1.473(4) C–СН<sup>3</sup> 1.473(8)

**IV IV Bond or Angle Å or º Bond or Angle Å or º** O1–C1 1.237(3) О1–С2 1.228(8) С1–СH<sup>3</sup> 1.473(4) C–СН<sup>3</sup> 1.473(8)

*Molecules* **2023**, *28*, x FOR PEER REVIEW 10 of 15

*Molecules* **2023**, *28*, x FOR PEER REVIEW 10 of 15

**Figure 9.** The proton disolvate in two polymorphs of salt (Ac-H<sup>+</sup>

**Figure 9.** The proton disolvate in two polymorphs of salt (Ac-H<sup>+</sup>

O1–H1 1.28(5) O1′–H1 1.15(5)

O1–H1 1.28(5) O1′–H1 1.15(5)

was not possible to localize the bridging proton by X-ray diffraction analysis.

was not possible to localize the bridging proton by X-ray diffraction analysis.

#### **3. Discussion 3. Discussion 3. Discussion**

2

HOCH=CR2

The results make it possible to establish the sequence of the interaction of vinyl cations with water molecules. Initially, an H2O molecule is attached to the C=C<sup>+</sup> bond of cation RCH=C+H or R2C=C+H (where R is CH3) via the O- atom (Equation (1), in it and in subsequent equations, the R2C=C+H cation is used). The results make it possible to establish the sequence of the interaction of vinyl cations with water molecules. Initially, an H2O molecule is attached to the C=C<sup>+</sup> bond of cation RCH=C+H or R2C=C+H (where R is CH3) via the O- atom (Equation (1), in it and in subsequent equations, the R2C=C+H cation is used). The results make it possible to establish the sequence of the interaction of vinyl cations with water molecules. Initially, an H2O molecule is attached to the C=C<sup>+</sup> bond of cation RCH=C+H or R2C=C+H (where R is CH3) via the O- atom (Equation (1), in it and in subsequent equations, the R2C=C+H cation is used).

$$\begin{array}{ccccc} \text{R}\_2\text{C=C}^+\text{H} & + & \text{H}\_2\text{O} & \longrightarrow & \text{R}\_2\text{C=C}^+\text{H} \\ & & & \begin{array}{c} \text{R}\_2\text{C=C}^+\text{H} \\ \text{H}\_2\text{O} \end{array} \end{array} \tag{1}$$



). In polymorph **IV***b*, it

). In polymorph **IV***b*, it

The nature of this bond is close to that of bonds formed by water molecules with alkali metal cations during their hydration or with H3O<sup>+</sup> in crystal salts of H3O<sup>+</sup> ‧3H2O, i.e., the bond is strongly ionic. The nature of this bond is close to that of bonds formed by water molecules with alkali metal cations during their hydration or with H3O<sup>+</sup> in crystal salts of H3O<sup>+</sup> ·3H2O, i.e., the bond is strongly ionic. The nature of this bond is close to that of bonds formed by water molecules with alkali metal cations during their hydration or with H3O<sup>+</sup> in crystal salts of H3O<sup>+</sup> ‧3H2O, i.e., the bond is strongly ionic.

With an increase in the concentration of the salt of cationic adduct R2C=C+H···H2O in solutions, the self-association of ion pairs increases (which is typical for salts of carbocations in solutions [29]). This enhances the contact interaction of H2O with the cation and promotes the transfer of a proton to a water molecule with the formation of H3O<sup>+</sup> and cationic adduct **II**:With an increase in the concentration of the salt of cationic adduct R2C=C+H···H2O in solutions, the self-association of ion pairs increases (which is typical for salts of carbocations in solutions [29]). This enhances the contact interaction of H2O with the cation and promotes the transfer of a proton to a water molecule with the formation of H3O<sup>+</sup> and cationic adduct II: With an increase in the concentration of the salt of cationic adduct R2C=C+H···H2O in solutions, the self-association of ion pairs increases (which is typical for salts of carbocations in solutions [29]). This enhances the contact interaction of H2O with the cation and promotes the transfer of a proton to a water molecule with the formation of H3O<sup>+</sup> and cationic adduct **II**:

$$2\begin{pmatrix} ^{\text{R}\_2\text{C=C}^+\text{H}} \\ \cdots \\ ^{\text{O}\_2\text{O}} \end{pmatrix} \longrightarrow \begin{array}{} ^{\text{A}}\text{H} \text{ .} ^{\text{C}}\text{H} \text{.} ^{\text{C}}\text{H} \text{.} ^{\text{C}}\text{OH} \text{=CR} \text{ } + \text{ } \text{H}\_3\text{O}^+ \end{array} \tag{2}$$

With a further increase in the contents of water and of the carbocation salt, the concentration of the resulting adducts **II** and of the H3O+ cation increases. The latter can protonate an alcohol molecule that is more basic than H2O, thereby producing a proton disolvate and regenerating cation R2C=C<sup>+</sup> H (Equation (3)), With a further increase in the contents of water and of the carbocation salt, the concentration of the resulting adducts **II** and of the H3O+ cation increases. The latter can protonate an alcohol molecule that is more basic than H2O, thereby producing a proton disolvate and regenerating cation R2C=C<sup>+</sup> H (Equation (3)), With a further increase in the contents of water and of the carbocation salt, the concentration of the resulting adducts II and of the H3O<sup>+</sup> cation increases. The latter can protonate an alcohol molecule that is more basic than H2O, thereby producing a proton disolvate and regenerating cation R2C=C<sup>+</sup> H (Equation (3)), *Molecules* **2023**, *28*, x FOR PEER REVIEW 11 of 15*Molecules* **2023**, *28*, x FOR PEER REVIEW 11 of 15

$$\begin{array}{cccc} \text{2} & \begin{pmatrix} \text{R}\_{2}\text{C=C}^{+}\text{H} \\ \vdots \\ \text{HOCH=CR}\_{2} \end{pmatrix} & \xrightarrow{+\text{H}\_{2}\text{O}} & \begin{array}{c} \text{H} \\ \text{R}\_{2}\text{C=C}\text{O} \\ \text{H} \end{array} \\ \text{-\(O\text{C}=\text{C}\text{O}\rightarrow\text{H}\text{)} & \begin{array}{c} \text{H} \\ \text{H} \end{array} \\ \text{-\(O\text{C}=\text{C}\text{O}\rightarrow\text{H}\text{)} & \begin{array}{c} \text{H} \\ \text{C}\text{O} \rightarrow \text{H} \end{array} \\ \end{array}$$

H

which next interacts with water, closing the cycle. The alcohol molecule of adduct **II** can also be protonated directly by cation **III**, thus bypassing the stage of H3O<sup>+</sup> formation: which next interacts with water, closing the cycle. The alcohol molecule of adduct II can also be protonated directly by cation III, thus by passing the stage of H3O<sup>+</sup> formation: which next interacts with water, closing the cycle. The alcohol molecule of adduct **II** can also be protonated directly by cation **III**, thus bypassing the stage of H3O<sup>+</sup> formation:

H

H

$$\begin{array}{cccc} \text{R}\_2\text{C=C}^+\text{H} & + & \overset{\text{H}}{\underset{\text{C=CR}}{\text{C=CR}}} & \longrightarrow & \text{R}\_2\text{C=C}\overset{\text{H}}{\underset{\text{H}}{\text{C=C}}} + & \overset{\text{H}}{\underset{\text{H}}{\text{C=CR}}} + & \text{R} \end{array} \tag{4}$$

H

+

H

(4)

). This

) of molecules of

). This

+

) of molecules of

+

teraction of vinyl cations with basic molecules of H2O, alcohol, and acetone as their basicity increases. The H2O molecule is attached to the C=C bond of the vinyl cation (Scheme 2, III) with a strength similar to that of an H2O molecule attached to an alkali metal cation or to the H3O<sup>+</sup> cation in H3O<sup>+</sup> (H2O)3. As the basicity of the O atom increases from H2O to alcohol molecule, its interaction with the cation shifts to the H atom, thereby producing a strong H-bond, with a partially covalent character [30], and an increase in the С О distance to 2.47 Å (Scheme 2, **II**). In this case, the asymmetric double-well potential of the bridging proton has a deeper minimum at the C atom. A further increase in the basicity of the oxygen atom (acetone molecule) causes a shift of the minimum of the double-well potential to the O atom with a high probability of proton transfer to the acetone molecule. The loss of a proton by the vinyl cation results in an extremely reactive carbene molecule (with C=C: as the active site), which then reacts with the components of the mixture, forming non-crystallizing products (wax phase). The released protonated acetone adds a second acetone molecule thereby generating proton disolvate **IV**. The studied cationic adducts enable one to trace the change in the nature of the interaction of vinyl cations with basic molecules of H2O, alcohol, and acetone as their basicity increases. The H2O molecule is attached to the C=C bond of the vinyl cation (Scheme 2, III) with a strength similar to that of an H2O molecule attached to an alkali metal cation or to the H3O<sup>+</sup> cation in H3O<sup>+</sup> (H2O)3. As the basicity of the O atom increases from H2O to alcohol molecule, its interaction with the cation shifts to the H atom, thereby producing a strong H-bond, with a partially covalent character [30], and an increase in the С О distance to 2.47 Å (Scheme 2, **II**). In this case, the asymmetric double-well potential of the bridging proton has a deeper minimum at the C atom. A further increase in the basicity of the oxygen atom (acetone molecule) causes a shift of the minimum of the double-well potential to the O atom with a high probability of proton transfer to the acetone molecule. The loss of a proton by the vinyl cation results in an extremely reactive carbene molecule (with C=C: as the active site), which then reacts with the components of the mixture, forming non-crystallizing products (wax phase). The released protonated acetone adds a second The studied cationic adducts enable one to trace the change in the nature of the interaction of vinyl cations with basic molecules of H2O, alcohol, and acetone as their basicity increases. The H2O molecule is attached to the C=C bond of the vinyl cation (Scheme 2, III) with a strength similar to that of an H2O molecule attached to an alkali metal cation or to the H3O<sup>+</sup> cation in H3O<sup>+</sup> (H2O)3. As the basicity of the O atom increases from H2O to alcohol molecule, its interaction with the cation shifts to the H atom, thereby producing a strong H-bond, with a partially covalent character [30], and an increase in the C· · · · ·O distance to 2.47 Å (Scheme 2, II). In this case, the asymmetric double-well potential of the bridging proton has a deeper minimum at the C atom. A further increase in the basicity of the oxygen atom (acetone molecule) causes a shift of the minimum of the double-well potential to the O atom with a high probability of proton transfer to the acetone molecule. The loss of a proton by the vinyl cation results in an extremely reactive carbene molecule (with C=C: as the active site), which then reacts with the components

acetone molecule thereby generating proton disolvate **IV**.

**Scheme 2.** Schematic representation of the charged moiety in adducts **I**-**III**.

posed in ref. [31] and proved for the benzyl carbocation [32].

H2O or alcohol molecule results in the emergence (in the case of C4H<sup>7</sup>

posed in ref. [31] and proved for the benzyl carbocation [32].

**Scheme 2.** Schematic representation of the charged moiety in adducts **I**-**III**.

nificant gain in energy.

nificant gain in energy.

Thus, the interaction of the vinyl cation with nucleophile L proceeds: (1) through its addition to a charged double bond (if L = H2O) or H-bonded to the =C-H group (if L = alcohol molecule), and (2) through the protonation of L with the transition of the vinyl cation to the neutral carbene molecule. The second mechanism of interaction was pro-

The finding that adducts **II** and **III** with strongly ionic Cat+−L interaction exist is sur-

means that vinyl cations behave like rather chemically inert particles, contradicting predictions of quantum chemical calculations. For example, if the crystallographic structures of adducts **II** and **III** are optimized, then the covalent interaction of the cation with the

protonated isobutenyl alcohol and diisobutenyl ether, respectively (Figure 10), with a sig-

For example, if we compare energies of the structures calculated at the UB3LYP/6- 311++G(d,p) level of theory: (1) energy with optimized H atomic coordinates and fixed coordinates of C and O that are equal to the coordinates that follow from the X-Ray data for **IIa**, and (2) energy with fully optimized structure (including the C, O and H atomic coordinates), energies (1) and (2) are related as 69.71 and 0 kcal/mol with the transformation of structure **IIa** into protonated diisobutenyl ether. The environment cannot have a stronger stabilizing effect on the **IIa** structure because the purely ionic interaction of C

prising. They can exist if the basicity of L slightly exceeds that of counterion (Cl11<sup>−</sup>

means that vinyl cations behave like rather chemically inert particles, contradicting predictions of quantum chemical calculations. For example, if the crystallographic structures of adducts **II** and **III** are optimized, then the covalent interaction of the cation with the

protonated isobutenyl alcohol and diisobutenyl ether, respectively (Figure 10), with a sig-

For example, if we compare energies of the structures calculated at the UB3LYP/6- 311++G(d,p) level of theory: (1) energy with optimized H atomic coordinates and fixed coordinates of C and O that are equal to the coordinates that follow from the X-Ray data for **IIa**, and (2) energy with fully optimized structure (including the C, O and H atomic coordinates), energies (1) and (2) are related as 69.71 and 0 kcal/mol with the transformation of structure **IIa** into protonated diisobutenyl ether. The environment cannot have a stronger stabilizing effect on the **IIa** structure because the purely ionic interaction of C

The finding that adducts **II** and **III** with strongly ionic Cat+−L interaction exist is sur-

Thus, the interaction of the vinyl cation with nucleophile L proceeds: (1) through its addition to a charged double bond (if L = H2O) or H-bonded to the =C-H group (if L = alcohol molecule), and (2) through the protonation of L with the transition of the vinyl cation to the neutral carbene molecule. The second mechanism of interaction was pro-

H2O or alcohol molecule results in the emergence (in the case of C4H<sup>7</sup>

prising. They can exist if the basicity of L slightly exceeds that of counterion (Cl11<sup>−</sup>

R2C=C

<sup>+</sup>H

of the mixture, forming non-crystallizing products (wax phase). The released protonated acetone adds a second acetone molecule thereby generating proton disolvate IV. acetone molecule thereby generating proton disolvate **IV**.

**Scheme 2.** Schematic representation of the charged moiety in adducts **I**-**III**. **Scheme 2.** Schematic representation of the charged moiety in adducts I–III.

*Molecules* **2023**, *28*, x FOR PEER REVIEW 11 of 15

R2C=C

H

also be protonated directly by cation **III**, thus bypassing the stage of H3O<sup>+</sup>

H

H

O O

H

which next interacts with water, closing the cycle. The alcohol molecule of adduct **II** can

+

H

The studied cationic adducts enable one to trace the change in the nature of the interaction of vinyl cations with basic molecules of H2O, alcohol, and acetone as their basicity increases. The H2O molecule is attached to the C=C bond of the vinyl cation (Scheme 2, III) with a strength similar to that of an H2O molecule attached to an alkali metal cation

alcohol molecule, its interaction with the cation shifts to the H atom, thereby producing a strong H-bond, with a partially covalent character [30], and an increase in the С О distance to 2.47 Å (Scheme 2, **II**). In this case, the asymmetric double-well potential of the bridging proton has a deeper minimum at the C atom. A further increase in the basicity of the oxygen atom (acetone molecule) causes a shift of the minimum of the double-well potential to the O atom with a high probability of proton transfer to the acetone molecule. The loss of a proton by the vinyl cation results in an extremely reactive carbene molecule (with C=C: as the active site), which then reacts with the components of the mixture, forming non-crystallizing products (wax phase). The released protonated acetone adds a second

H

OC=CR2

2 R2C=C

+

R2C=C

H

O

H

+

H

(H2O)3. As the basicity of the O atom increases from H2O to

+

H

H

OC=CR2

H + H2O (3)

(4)

). This

+ 

formation:

H

C=CR2

+H3O

<sup>+</sup>H

+

R2C=C

**. . .**

H

O

**. . .**

H

R2C=C

2 **.**

**.**

**.**

+

H

HOCH=CR2

or to the H3O<sup>+</sup> cation in H3O<sup>+</sup>

Thus, the interaction of the vinyl cation with nucleophile L proceeds: (1) through its addition to a charged double bond (if L = H2O) or H-bonded to the =C-H group (if L = alcohol molecule), and (2) through the protonation of L with the transition of the vinyl cation to the neutral carbene molecule. The second mechanism of interaction was pro-Thus, the interaction of the vinyl cation with nucleophile L proceeds: (1) through its addition to a charged double bond (if L = H2O) or H-bonded to the =C-H group (if L = alcohol molecule), and (2) through the protonation of L with the transition of the vinyl cation to the neutral carbene molecule. The second mechanism of interaction was proposed in ref. [31] and proved for the benzyl carbocation [32].

posed in ref. [31] and proved for the benzyl carbocation [32]. The finding that adducts **II** and **III** with strongly ionic Cat+−L interaction exist is surprising. They can exist if the basicity of L slightly exceeds that of counterion (Cl11<sup>−</sup> means that vinyl cations behave like rather chemically inert particles, contradicting predictions of quantum chemical calculations. For example, if the crystallographic structures of adducts **II** and **III** are optimized, then the covalent interaction of the cation with the H2O or alcohol molecule results in the emergence (in the case of C4H<sup>7</sup> + ) of molecules of The finding that adducts II and III with strongly ionic Cat+−L interaction exist is surprising. They can exist if the basicity of L slightly exceeds that of counterion (Cl<sup>11</sup> −). This means that vinyl cations behave like rather chemically inert particles, contradicting predictions of quantum chemical calculations. For example, if the crystallographic structures of adducts II and III are optimized, then the covalent interaction of the cation with the H2O or alcohol molecule results in the emergence (in the case of C4H<sup>7</sup> + ) of molecules of protonated isobutenyl alcohol and diisobutenyl ether, respectively (Figure 10), with a significant gain in energy. *Molecules* **2023**, *28*, x FOR PEER REVIEW 12 of 15

coordinates), energies (1) and (2) are related as 69.71 and 0 kcal/mol with the transformation of structure **IIa** into protonated diisobutenyl ether. The environment cannot have a stronger stabilizing effect on the **IIa** structure because the purely ionic interaction of C **Figure 10.** A comparison of two structures: **IIa** with optimized H atomic coordinates and fixed atomic C and O coordinates equal to those, determined from X-ray crystallography and optimized structure for all atomic coordinates for vacuum (at the UB3LYP/6- 311++G(d,p) **Figure 10.** A comparison of two structures: IIa with optimized H atomic coordinates and fixed atomic C and O coordinates equal to those, determined from X-ray crystallography and optimized structure for all atomic coordinates for vacuum (at the UB3LYP/6- 311++G(d,p) level of theory).

level of theory). and O atoms with Cl atoms of the anionic environment is weak (C‧‧‧Cl and O‧‧‧Cl distances exceed the sum of van der Waals atomic radii). Thus, it follows from quantum chemical calculations that adducts **II** and **III** should not exist, which contradicts the experimental findings. Therefore, the use of quantum chemical calculations requires caution in studies on mechanisms of the interaction of vinyl cations with neutral molecules. **4. Materials and Methods** The salts of vinyl cations С3Н<sup>5</sup> <sup>+</sup> and С4Н<sup>7</sup> <sup>+</sup> were obtained as described previously [15– 17]. The pentafluorobenzene (Sigma-Aldrich) used as a solvent was thoroughly dried with molecular sieves and was not purified further. For example, if we compare energies of the structures calculated at the UB3LYP/6- 311++G(d,p) level of theory: (1) energy with optimized H atomic coordinates and fixed coordinates of C and O that are equal to the coordinates that follow from the X-Ray data for IIa, and (2) energy with fully optimized structure (including the C, O and H atomic coordinates), energies (1) and (2) are related as 69.71 and 0 kcal/mol with the transformation of structure IIa into protonated diisobutenyl ether. The environment cannot have a stronger stabilizing effect on the IIa structure because the purely ionic interaction of C and O atoms with Cl atoms of the anionic environment is weak (C···Cl and O···Cl distances exceed the sum of van der Waals atomic radii). Thus, it follows from quantum chemical calculations that adducts II and III should not exist, which contradicts the experimental findings. Therefore, the use of quantum chemical calculations requires caution in studies on mechanisms of the interaction of vinyl cations with neutral molecules.

#### All sample handling was carried out in an atmosphere of argon (H2O, [O2] < 0.5 ppm) in a glove box. ATR IR spectra were recorded on a Shimadzu IRAffinity-1S spectrometer **4. Materials and Methods**

H2O)(Cl11<sup>−</sup>

housed inside the glove box in the 4000−400 cm−<sup>1</sup> frequency range using an ATR accessory with a diamond crystal. The spectra were processed in the GRAMMS/A1 (7.00) software from Thermo Scientific. The salts of vinyl cations C3H<sup>5</sup> <sup>+</sup> and C4H<sup>7</sup> <sup>+</sup> were obtained as described previously [15–17]. The pentafluorobenzene (Sigma-Aldrich, Saint Louis, MO, USA) used as a solvent was thoroughly dried with molecular sieves and was not purified further.

X-ray diffraction data were collected on a Bruker Kappa Apex II CCD diffractometer using φ,ω-scans of narrow (0.5°) frames with Mo Kα radiation (λ = 0.71073 Å) and a graphite monochromator at temperature 200 K. The structures were solved by direct methods All sample handling was carried out in an atmosphere of argon (H2O, [O2] < 0.5 ppm) in a glove box. ATR IR spectra were recorded on a Shimadzu IRAffinity-1S spectrometer housed inside the glove box in the 4000−400 cm−<sup>1</sup> frequency range using an ATR accessory

with the help of SHELXT-2014/5 [33], and refined by the full-matrix least-squares method against all F2 in an anisotropic-isotropic (for H atoms) procedure using SHELXL-2018/3 [33]. Absorption corrections were applied by the empirical multiscan method using SA-

The hydrogen atom positions for OH -groups were located by means of a difference Fourier map. The crystallographic data and details of the refinements for all structures are summarized in Table S1 in SI. We were unable to obtain good single crystals of the (C3H<sup>7</sup>

an X-ray diffraction experiment and localized non-hydrogen atoms, but could not refine the structure to obtain a good R-factor. Crystals of the salt of cationic adduct **II** contain strongly disordered C6HF<sup>5</sup> solvent molecules. We were not able to find their atomic coordinates. The accessible volume of free solvent molecules in these crystals, as determined

occupying this volume could not be modeled as a set of discrete atomic positions. We employed the PLATON/SQUEEZE procedure to calculate the contribution to the diffraction from the solvent region, and thereby produced a series of solvent-free diffraction in-

two anions and two cationic proton disolvates **I**. In Table 1, the geometry of disolvate **I** is given as an average over two solvate molecules (CH3)2С=СHOH. A similar averaging of geometric parameters over two independent positions was performed for adduct **III** and

CCDC 2223519, 2223520, 2223521, 2223522 and 2223523 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The

tensities. The independent part of the unit cell of salt [(CH3)2С=СHOH]2H<sup>+</sup>

) salt (of adduct **III**) for X-ray diffraction analyses. Nonetheless, we conducted

). The highly disordered C6HF<sup>5</sup> molecules

(Cl11<sup>−</sup>

) contains

by routine PLATON analysis, was 15.0% (846 Å <sup>3</sup>

proton disolvates **IVa** and **IVb** (Tables 4 and 6).

with a diamond crystal. The spectra were processed in the GRAMMS/A1 (7.00) software from Thermo Scientific, Waltham, MA, USA.

X-ray diffraction data were collected on a Bruker Kappa Apex II CCD diffractometer using ϕ,ω-scans of narrow (0.5◦ ) frames with Mo Kα radiation (λ = 0.71073 Å) and a graphite monochromator at temperature 200 K. The structures were solved by direct methods with the help of SHELXT-2014/5 [33], and refined by the full-matrix least-squares method against all F2 in an anisotropic-isotropic (for H atoms) procedure using SHELXL-2018/3 [33]. Absorption corrections were applied by the empirical multiscan method using SADABS software [34]. Hydrogen atom positions were calculated using the riding model. The hydrogen atom positions for OH -groups were located by means of a difference Fourier map. The crystallographic data and details of the refinements for all structures are summarized in Table S1 in Supplementary Materials. We were unable to obtain good single crystals of the (C3H<sup>7</sup> + · H2O)(Cl<sup>11</sup> −) salt (of adduct III) for X-ray diffraction analyses. Nonetheless, we conducted an X-ray diffraction experiment and localized non-hydrogen atoms, but could not refine the structure to obtain a good R-factor. Crystals of the salt of cationic adduct II contain strongly disordered C6HF<sup>5</sup> solvent molecules. We were not able to find their atomic coordinates. The accessible volume of free solvent molecules in these crystals, as determined by routine PLATON analysis, was 15.0% (846 Å<sup>3</sup> ). The highly disordered C6HF<sup>5</sup> molecules occupying this volume could not be modeled as a set of discrete atomic positions. We employed the PLATON/SQUEEZE procedure to calculate the contribution to the diffraction from the solvent region, and thereby produced a series of solvent-free diffraction intensities. The independent part of the unit cell of salt [(CH3)2C=CHOH]2H<sup>+</sup> (Cl<sup>11</sup> −) contains two anions and two cationic proton disolvates I. In Table 1, the geometry of disolvate I is given as an average over two solvate molecules (CH3)2C=CHOH. A similar averaging of geometric parameters over two independent positions was performed for adduct III and proton disolvates IVa and IVb (Tables 4 and 6).

CCDC 2223519, 2223520, 2223521, 2223522 and 2223523 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center at http://www.ccdc.cam.ac.uk/data\_request/cif (accessed on 9 January 2023).

The obtained crystal structures were analyzed for short contacts between non-bonded atoms using PLATON [35,36] and MERCURY software packages [37].

#### **5. Conclusions**

Vinyl cations C3H<sup>5</sup> <sup>+</sup> and C4H<sup>7</sup> + (Cat<sup>+</sup> ) in solutions of their salts in dichloromethane and C6HF<sup>5</sup> interact with O-containing nucleophiles as follows.

An H2O molecule attaches to the C=C bond of Cat<sup>+</sup> in a similar manner to the hydration of alkali metal cations, thereby yielding Cat<sup>+</sup> ·H2O adducts with a strongly ionic bond. Its strength only slightly exceeds the strength of the interaction of the (Cl<sup>11</sup> −) anion with the vinyl cation in contact ion pairs Cat<sup>+</sup> (Cl<sup>11</sup> −).

With an increase in the content of Cat<sup>+</sup> ·H2O adducts in solutions, the adduct selfassociates and interacts with a transfer of a proton to one water molecule and attachment of the second one to the C=C bond, thus forming H3O<sup>+</sup> and an alcohol molecule, respectively.

The alcohol molecule interacts predominantly with the H atom of the C=C+−H moiety of the vinyl cation, thereby producing a proton disolvate with strong asymmetric H-bond =C−+H···O having double-well proton potential with a deeper minimum near the C atom.

A further increase in the water content in the solutions leads to complete conversion of vinyl cations into alcohol molecules with the formation of symmetric proton disolvates LH+L containing strong and partially covalent O−H+−O hydrogen bonds [30] and H3O<sup>+</sup> cations.

The interaction of the vinyl cations with acetone molecules, which are more basic than H2O or alcohol molecules, causes the formation of only symmetrical proton disolvates, LH+L, in the absence of acetone-containing cationic adducts. The vinyl cation is converted into carbene containing a highly reactive C=C: moiety.

Summing up, we can say that the interaction of the vinyl cation with base L proceeds through two mechanisms: via the formation of adducts (SN1 reaction), and via the mechanism where vinyl cation acts as a protonating agent. When the basicity of L is close to that of a single water molecule, L attaches to the double C=C bond thereby producing an adduct. As the basicity of L increases, the interaction with the C=C+−H moiety of the vinyl cation strengthens and is shifted to the H atom, thus forming a solvate having a strong asymmetrical =C−+H···O hydrogen bond. Further strengthening of the basicity of L leads to the transfer of a proton to L and to the emergence of the eventually symmetric LH+L cation. The loss of a proton by the vinyl cation converts it into a neutral reactive carbene molecule containing a C=C: moiety.

The formation of adducts with water and alcohol molecules by vinyl cations is unexpected, because according to quantum chemical calculations, they are energetically unfavorable and should not exist.

The very existence of these adducts means that the alleged high reactivity of vinyl carbocations is an overestimation.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www. mdpi.com/article/10.3390/molecules28031146/s1, Figure S1: Two locations of the C4H<sup>7</sup> + ·C4H7OH adduct in the crystal lattice of its salt with the {Cl<sup>11</sup> −} anion (not shown); Figure S2: The ATR IR spectrum of proton disolvate IVa; Table S1: Crystallographic data and details of the X-ray diffraction experiment.

**Author Contributions:** Conceptualization, E.S.S. and I.V.S.; methodology, E.S.S.; validation, I.Y.B.; formal analysis, I.Y.B. and I.V.S.; writing—review and editing, E.S.S.; supervision, E.S.S.; project administration, E.S.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported the Ministry of Science and Higher Education of the Russian Federation (state registration No. 1021052806375-6-1.4.3).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are available from the authors.

#### **References**


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