*2.1. Chemistry*

The hardly ionizable Chlorocomplexes [ReVII(O)Cl(Cat)2] were prepared using a procedure that was adapted from [26]. One equivalent (4.4 mg) of tetrabutylammonium tetrachlorooxorhenate(V) [(n-Bu4-N)(ReOCl4)] was dissolved in 1.5 mL of acetonitrile. Two equivalents of ligand 1,2-dihydroxybenzene and two equivalents of triethylamine (10% (*v*/*v*) solution in acetonitrile) were added and the reaction mixture was stirred for 3 days at laboratory temperature. Since catechol ligand lacks the free dissociable group, the yielding compound remains uncharged and its structural characterization by MS is impossible. Its presence in the reaction mixture was presumed entirely from a similarity between absorption spectra describing the formation of deprotonated pyrogallol analog. However, its ESI-MS structure identification is possible after the reaction with aniline (Figure 1), yielding an ESI ionizable reaction product [27].

Derivatives were prepared by mixing 10 µL of the reaction mixture described above with 50 µL of *p*- substituted aniline (5% (*v*/*v*) solution in acetonitrile). The reaction scheme of derivatization is shown in Figure 1. All prepared complexes are described in Table 1. For full chemical names see Table S6 in the Supplementary Materials.

**Figure 1.** Derivatization reaction of uncharged rhenium(VII) chlorocomplexes with *p*-substituted aniline. **Figure 1.** Derivatization reaction of uncharged rhenium(VII) chlorocomplexes with *p*-substituted aniline.of derivatization is shown in Figure 1. All prepared complexes are described in Table 1. For full chemical names see Table S6 in the Supplementary Materials.


Derivatives were prepared by mixing 10 μL of the reaction mixture described above **Table 1.** Labels and formulas of prepared rhenium complexes. **Table 1.** Labels and formulas of prepared rhenium complexes.

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1 Cl [ReVII(O)(Cat)2PClA]<sup>−</sup> a C18H12ClNO5Re <sup>a</sup> Deprotonated ion. <sup>a</sup> Deprotonated ion.

#### 3 H [ReVII(O)(Cat)2An]<sup>−</sup> a C18H13NO5Re *2.2. Molecular Modeling 2.2. Molecular Modeling*

4 CH3 [ReVII(O)(Cat)2PT]<sup>−</sup> a C19H15NO5Re 5 C(CH3)2 [ReVII(O)(Cat)2PIPA]<sup>−</sup> a C21H19NO5Re <sup>a</sup> Deprotonated ion. *2.2. Molecular Modeling*  Density functional theory could be a possible MS/MS prediction tool useful in structure elucidating. Molecular modeling proposed the significant change of molecular conformation of studied aniline derivatives before and after ionization. We illustrate the structures of neutral and deprotonated [Re(O)(Cat)2PBrA] in Figure 2. Density functional theory could be a possible MS/MS prediction tool useful in structure elucidating. Molecular modeling proposed the significant change of molecular conformation of studied aniline derivatives before and after ionization. We illustrate the structures of neutral and deprotonated [Re(O)(Cat)2PBrA] in Figure 2.

2 Br [ReVII(O)(Cat)2PBrA]<sup>−</sup> a C18H12BrNO5Re

**Figure 2.** The DFT calculated structures of neutral and deprotonated [Re(O)(Cat)2PBrA]. **Figure 2.** The DFT calculated structures of neutral and deprotonated [Re(O)(Cat)2PBrA].

**Figure 2.** The DFT calculated structures of neutral and deprotonated [Re(O)(Cat)2PBrA]. Evident is the shortening of the Re-N bond in the course of deprotonation. As it results from more detailed modeling, the shortening of the Re-N bond is accompanied by the prolongation of the Re-O(20) bond in a ligand, the increase in the Re-N-C(22) angle, Evident is the shortening of the Re-N bond in the course of deprotonation. As it results from more detailed modeling, the shortening of the Re-N bond is accompanied by the prolongation of the Re-O(20) bond in a ligand, the increase in the Re-N-C(22) angle, Evident is the shortening of the Re-N bond in the course of deprotonation. As it results from more detailed modeling, the shortening of the Re-N bond is accompanied by the prolongation of the Re-O(20) bond in a ligand, the increase in the Re-N-C(22) angle, and the decrease in the dihedral angle C(14)-O(22)-Re-O(19). For details, see the corresponding computation data in the Supplementary Materials. The extent of such structural changes depends on the aromatic substituent linked via derivatization. The decisive element here seems to be the basicity of the aniline derivative entering the SB reaction. The bar graph showing the correlation between Re-N bond shortening and the basicity of the used aniline derivative is shown in Figure 3. The pKa values of used aniline derivatives were obtained from [28].

were obtained from [28].

and the decrease in the dihedral angle C(14)-O(22)-Re-O(19). For details, see the corresponding computation data in the supplementary materials. The extent of such structural changes depends on the aromatic substituent linked via derivatization. The decisive element here seems to be the basicity of the aniline derivative entering the SB reaction. The bar graph showing the correlation between Re-N bond shortening and the basicity of the used aniline derivative is shown in Figure 3. The pKa values of used aniline derivatives

#### PClA-*p*-chloroaniline, PBrA-*p*-bromoaniline, An-aniline, PIPA-*p*-isopropylaniline, PT-*p*-toluidine. *2.3. High-Resolution Mass Spectrometry Characterization*

*2.3. High-Resolution Mass Spectrometry Characterization*  All molecular ions of prepared derivatives exhibit characteristic isotopic distributions. We compared the similarity between the calculated and experimental molecular ion isotopic patterns using the similarity index (SI) [29]. The [(1-SI) 100] values given in Table All molecular ions of prepared derivatives exhibit characteristic isotopic distributions. We compared the similarity between the calculated and experimental molecular ion isotopic patterns using the similarity index (SI) [29]. The [(1-SI) 100] values given in Table 2 indicate an apparent coincidence between the calculated and experimental spectra and the correct assignment of the elemental composition to the *m*/*z* values of the observed ions.

the correct assignment of the elemental composition to the *m*/*z* values of the observed ions. **Table 2.** Theoretical and experimental exact masses of molecular anions of studied derivatives. The error values express the difference between theoretical and experimental masses; the similarity in-**Table 2.** Theoretical and experimental exact masses of molecular anions of studied derivatives. The error values express the difference between theoretical and experimental masses; the similarity index expresses the resemblance between theoretical and experimental isotope patterns of molecular anions.

2 indicate an apparent coincidence between the calculated and experimental spectra and


[Re(O)(Cat)2An]<sup>−</sup> C18H13NO5Re 510.0357 510.0360 −0.3 −0.6 98.1 [Re(O)(Cat)2PT]<sup>−</sup> C19H15NO5Re 524.0514 524.0507 0.7 1.2 89.2 [Re(O)(Cat)2PIPA]<sup>−</sup> C21H19NO5Re 552.0827 552.0828 −0.2 −0.3 98.1 Excellent agreement between the isotope pattern calculated and that obtained by MS of the [Re(O)(Cat)2PBrA]- complex is documented in Figure 4. An analogous satisfactory agreement of exact mass and isotopic distribution for all other derivatives was observed. Excellent agreement between the isotope pattern calculated and that obtained by MS of the [Re(O)(Cat)2PBrA]- complex is documented in Figure 4. An analogous satisfactory agreement of exact mass and isotopic distribution for all other derivatives was observed.

[Re(O)(Cat)2PBrA]<sup>−</sup> C18H12BrNO5Re 587.9445 587.9440 0.4 0.8 94.2

with the aniline derivative. Therefore, UV-Vis absorption spectrophotometry can be applied for the evaluation of its reaction rate. The major absorption band at λmax = 630 nm and the minor one at λmax = 390 nm characterize the intensive blue-green coloration of the [ReVII(O)Cl(Cat)2] complex. The product of the reaction with the aniline derivative is pale yellow, showing an absorption maximum at 355 nm. The rate of the derivatization reaction with *p*-bromoaniline was followed by the UV/Vis absorption measurement. The corresponding spectra recorded at defined time intervals are shown in Figure 5. A decrease of more than 90% over the 15 min interval was observed for the absorption band at λmax = 630 nm. The proportional decrease in the height of the related absorption band at λmax = 390 nm was observed. After 50 min, both absorption maxima disappear in favor of a minor

**Figure 5.** UV/Vis kinetics of derivatization reaction of uncharged [ReVII(O)Cl(Cat)2] chlorocomplex

**Figure 4.** Experimental (**A**) and calculated (**B**) isotope pattern of [Re(O)(Cat)2PBrA]. **Figure 4.** Experimental (**A**) and calculated (**B**) isotope pattern of [Re(O)(Cat)2PBrA].

absorption band at λmax = 355 nm (see inset).

*2.4. Reaction Time-Course* 

with *p*-bromoaniline.

#### *2.4. Reaction Time-Course* A significant color change accompanies the reaction of the [ReVII(O)Cl(Cat)2] complex

*2.4. Reaction Time-Course* 

*Molecules* **2021**, *26*, x FOR PEER REVIEW 5 of 12

Excellent agreement between the isotope pattern calculated and that obtained by MS of the [Re(O)(Cat)2PBrA]- complex is documented in Figure 4. An analogous satisfactory agreement of exact mass and isotopic distribution for all other derivatives was observed.

A significant color change accompanies the reaction of the [ReVII(O)Cl(Cat)2] complex with the aniline derivative. Therefore, UV-Vis absorption spectrophotometry can be applied for the evaluation of its reaction rate. The major absorption band at λmax = 630 nm and the minor one at λmax = 390 nm characterize the intensive blue-green coloration of the [ReVII(O)Cl(Cat)2] complex. The product of the reaction with the aniline derivative is pale yellow, showing an absorption maximum at 355 nm. The rate of the derivatization reaction with *p*-bromoaniline was followed by the UV/Vis absorption measurement. The corresponding spectra recorded at defined time intervals are shown in Figure 5. A decrease of more than 90% over the 15 min interval was observed for the absorption band at λmax = 630 nm. The proportional decrease in the height of the related absorption band at λmax = 390 nm was observed. After 50 min, both absorption maxima disappear in favor of a minor absorption band at λmax = 355 nm (see inset). with the aniline derivative. Therefore, UV-Vis absorption spectrophotometry can be applied for the evaluation of its reaction rate. The major absorption band at λmax = 630 nm and the minor one at λmax = 390 nm characterize the intensive blue-green coloration of the [ReVII(O)Cl(Cat)2] complex. The product of the reaction with the aniline derivative is pale yellow, showing an absorption maximum at 355 nm. The rate of the derivatization reaction with *p*-bromoaniline was followed by the UV/Vis absorption measurement. The corresponding spectra recorded at defined time intervals are shown in Figure 5. A decrease of more than 90% over the 15 min interval was observed for the absorption band at λmax = 630 nm. The proportional decrease in the height of the related absorption band at λmax = 390 nm was observed. After 50 min, both absorption maxima disappear in favor of a minor absorption band at λmax = 355 nm (see inset).

**Figure 5.** UV/Vis kinetics of derivatization reaction of uncharged [ReVII(O)Cl(Cat)2] chlorocomplex with *p*-bromoaniline. **Figure 5.** UV/Vis kinetics of derivatization reaction of uncharged [ReVII(O)Cl(Cat)<sup>2</sup> ] chlorocomplex with *p*-bromoaniline.

An alternative insight into the mechanism of derivatization reaction has been provided by complementary ESI/MS kinetic measurement displayed in Figure 6. The intensity of ion *m*/*z* 588 (red line), as the desired derivatization product, achieves a maximum at approximately 15 min. This is consistent with the observed rate of the decrease in the [ReVII(O)Cl(Cat)2] absorption band (λmax = 630 nm). The appearance of ion *m*/*z* 454 (blue line) revealed another function of aniline derivative acting as a weakly basic accelerator of the ReVcomplex oxidation to a higher ReVI form. Therefore, we can observe a decrease in the intensity of peak *m*/*z* 419 (black line) due to the slow transformation of complex [Re<sup>V</sup> (O)(Cat)2] <sup>−</sup> present in the reaction mixture. As the ReVI complex species are prone to further oxidation, the intensity of ion *m*/*z* 454, reaching a maximum within 15 min, decreases at a moderate rate.

The time-course of ionic intensities helps to reveal the actual structure of ion *m*/*z* 454. Although the high-resolution MS data are available, there is still uncertainty about the exact structure of ion *m*/*z* 454, where both [ReVI (O)Cl(Cat)2] − and the adduct with chlorine [ReVII (O)(Cat)2]Cl<sup>−</sup> fit in the same mass. We believe that the shape of the time dependence of the ionic intensities presented in Figure 6 precludes the presence of a chlorine adduct.

An alternative insight into the mechanism of derivatization reaction has been provided by complementary ESI/MS kinetic measurement displayed in Figure 6. The intensity of ion *m*/*z* 588 (red line), as the desired derivatization product, achieves a maximum at approximately 15 min. This is consistent with the observed rate of the decrease in the [ReVII(O)Cl(Cat)2] absorption band (λmax = 630 nm). The appearance of ion *m*/*z* 454 (blue line) revealed another function of aniline derivative acting as a weakly basic accelerator of the ReVcomplex oxidation to a higher ReVI form. Therefore, we can observe a decrease in the intensity of peak *m*/*z* 419 (black line) due to the slow transformation of complex [ReV (O)(Cat)2]− present in the reaction mixture. As the ReVI complex species are prone to further oxidation, the intensity of ion *m*/*z* 454, reaching a maximum within 15 min, de-

**Figure 6.** ESI-MS kinetics of derivatization reaction of uncharged [ReVII(O)Cl(Cat)2] chlorocomplex **Figure 6.** ESI-MS kinetics of derivatization reaction of uncharged [ReVII(O)Cl(Cat)<sup>2</sup> ] chlorocomplex with *p*-bromoaniline.

### with *p*-bromoaniline. *2.5. Collision Induced Dissociation (CID)*

creases at a moderate rate.

The time-course of ionic intensities helps to reveal the actual structure of ion *m*/*z* 454. Although the high-resolution MS data are available, there is still uncertainty about the exact structure of ion *m*/*z* 454, where both [ReVI (O)Cl(Cat)2]− and the adduct with chlorine [ReVII (O)(Cat)2]Cl− fit in the same mass. We believe that the shape of the time dependence of the ionic intensities presented in Figure 6 precludes the presence of a chlorine adduct. *2.5. Collision Induced Dissociation (CID)*  The tandem mass spectrometry at 35eV collision energy in negative ionization mode was used to determine the structure of the prepared derivatives. The obtained MS/MS spectrum in Table 3 is very simple. The accurate mass measurement indicates that the main peak *m*/*z* 480 is formed by the loss of catechol moiety from the precursor marked by the blue diamond. It is evident that the isotopic profile retains the distribution confirming the presence of rhenium and bromine isotopes. It is even possible to observe the loss of the aromatic ring from the other catechol moiety to form ion M-2L *m*/*z* 404. Fragmentation The tandem mass spectrometry at 35eV collision energy in negative ionization mode was used to determine the structure of the prepared derivatives. The obtained MS/MS spectrum in Table 3 is very simple. The accurate mass measurement indicates that the main peak *m*/*z* 480 is formed by the loss of catechol moiety from the precursor marked by the blue diamond. It is evident that the isotopic profile retains the distribution confirming the presence of rhenium and bromine isotopes. It is even possible to observe the loss of the aromatic ring from the other catechol moiety to form ion M-2L *m*/*z* 404. Fragmentation behavior suggested the presence of a remarkably strong Re-N bond. On the other hand, the ion *m*/*z* 327 is formed by the loss of substituted aniline and the whole process ends in ReO<sup>4</sup> − and ReO<sup>3</sup> − ions resp. Since the same behavior was observed for almost all prepared complexes, the fragmentation pattern was demonstrated in this example only. The proposed fragmentation pathway is presented in Figure 7. High mass-accuracy measurements according to Table 3 and fragmentation patterns allowed us to identify the structure of all prepared derivatives.

behavior suggested the presence of a remarkably strong Re-N bond. On the other hand, the ion *m*/*z* 327 is formed by the loss of substituted aniline and the whole process ends in **Table 3.** Theoretical and experimental masses of [Re(O)(Cat)2PBrA] CID fragment ions measured at collision energy 35 eV. The error values express the difference between theoretical and experimental masses.


of all prepared derivatives.

**Ion Formula** 

masses. **Nom.**  *m***/***z*

**Figure 7.** Fragmentation scheme of [Re(O)(Cat)2PBrA]. **Figure 7.** Fragmentation scheme of [Re(O)(Cat)2PBrA].

According to the collision-induced dissociation results in Figure 8, it can be seen that the intensity of the product ion formed by the loss of one catechol ligand M-L (*m*/*z* 480) reaches a maximum at the same collision energy as the ion formed by simultaneous fragmentation of substituted aniline (*m*/*z* 327), but the intensity of this ion is only around 10% relative to the major peak. This is consistent with the observed shortening of the Re-N bond. According to the collision-induced dissociation results in Figure 8, it can be seen that the intensity of the product ion formed by the loss of one catechol ligand M-L (*m*/*z* 480) reaches a maximum at the same collision energy as the ion formed by simultaneous fragmentation of substituted aniline (*m*/*z* 327), but the intensity of this ion is only around 10% relative to the major peak. This is consistent with the observed shortening of the Re-N bond.

ReO4− and ReO3− ions resp. Since the same behavior was observed for almost all prepared complexes, the fragmentation pattern was demonstrated in this example only. The proposed fragmentation pathway is presented in Figure 7. High mass-accuracy measurements according to Table 3 and fragmentation patterns allowed us to identify the structure

**Table 3.** Theoretical and experimental masses of [Re(O)(Cat)2PBrA] CID fragment ions measured at collision energy 35 eV. The error values express the difference between theoretical and experimental

> **Measured**  *m***/***z*

588 C18H12BrNO5Re<sup>−</sup> 587.9462 587.9459 0.5 0.3 0.3 480 C12H8BrNO3Re<sup>−</sup> 479.9233 479.9271 −7.9 −3.8 −3.8 404 C6H4BrNO3Re<sup>−</sup> 403.892 403.8931 −2.8 −1.1 −1.1 327 C6H4O4Re<sup>−</sup> 326.9673 326.9681 −2.5 −0.8 −0.8 251 ReO4<sup>−</sup> 250.936 250.9354 2.3 0.6 0.6 235 ReO3<sup>−</sup> 234.9411 234.9383 11.7 2.8 2.8

**Error (mDa)** 

**Error (ppm)**  **Rel.Abundance (%)** 

**Theoretical**  *m***/***z*

The green curve on the CID diagram describing the formation of the ion *m*/*z* 251 does not exhibit a significant maximum, and it is evident that the formation of that ion corresponds to different processes. The spotting of this ion already at zero collision energy can be attributed to the decomposition of the complex; for example, by air humidity. Another mechanism of *m*/*z* 251 ion formation is the loss of the aromatic ring from *m*/*z* 327. Dissociation of the Re-N bond and cleavage of aniline from the *m*/*z* 404 ion occurs only at high CE.

Calculated significant deformation of the molecule, which should be linked both with the shortening of the Re-N bond and the prolongation of a Re-O(20) bond (approximately 0.2 Å) in a single ligand, has an experimental consequence in CID experiments. Such a bond is going to be the most easily fragmented, yielding ion *m*/*z* 480. The intensity of the simultaneously arising ion *m*/*z* 327 is then significantly lower due to the lower energetical convenience of such fragmentation pathway. As expected, the ion on the favorable fragmentation pathway has the highest negative energy. The difference in energies is about 177 kcal mol−<sup>1</sup> in favor of ion *m*/*z* 480. The energy differences were converted from Hartrees into kcal mol−<sup>1</sup> using a conversion factor of 627.5.

The fragmentation of all prepared complexes was analogous. The data are available in the Supplementary Materials (Figures S1–S12). We observed the only exception for [Re(O)(Cat)2PIPA]−. Here, the different behavior is not related to the change of bond length but the product stability of arising ions. As is evident from the corresponding CID diagram (Figure 8B), a significant shift to higher collision energies has been observed for the ion ReO<sup>3</sup> −. The collision energy where the ion *m*/*z* 235 reaches a maximum is almost 40 eV or 3.2 eV regarding ECM higher. Unlike the other complexes, the ReO<sup>3</sup> − ion (*m*/*z* 235) in a [Re(O)(Cat)2PIPA]− fragmentation pathway is not formed directly from ion M-2L but through the ion *m*/*z* 325 as an intermediate. Ion *m*/*z* 325 is formed by a loss of the methane molecule (Figure 9), where aryl-vinyl stabilization due to π electrons conjugation takes part. Such types of stabilization are unique for [Re(O)(Cat)2PIPA]<sup>−</sup> and not possible for other prepared complexes. We calculated that the difference in energies of the fragments with and without aryl-vinyl stabilization is around 48 kcal mol−<sup>1</sup> . The elemental composition of

ion *m*/*z* 325 was verified using HRMS. We have obtained the exact mass of 351.9993 Da by measuring, while the calculated value for C8H7NO3Re− is 351.9989 Da. It means the error is −1.0 ppm. *Molecules* **2021**, *26*, x FOR PEER REVIEW 8 of 12

**Figure 8.** CID diagram of the dependence of the relative intensity of fragmented ions on the collision energy: (**A**) [Re(O)(Cat)2PBrA]<sup>−</sup> (**B**) [Re(O)(Cat)2PIPA]<sup>−</sup>. **Figure 8.** CID diagram of the dependence of the relative intensity of fragmented ions on the collision energy: (**A**) [Re(O)(Cat)2PBrA]− (**B**) [Re(O)(Cat)2PIPA]−. tion of ion *m*/*z* 325 was verified using HRMS. We have obtained the exact mass of 351.9993 Da by measuring, while the calculated value for C8H7NO3Re− is 351.9989 Da. It means the error is −1.0 ppm.

simultaneously arising ion *m*/*z* 327 is then significantly lower due to the lower energetical convenience of such fragmentation pathway. As expected, the ion on the favorable frag-**Figure 9.** Proposed fragmentation mechanism of ion M-2L yielding from [Re(O)(Cat)2PIPA] com-**Figure 9.** Proposed fragmentation mechanism of ion M-2L yielding from [Re(O)(Cat)2PIPA] complex.

#### mentation pathway has the highest negative energy. The difference in energies is about **3. Materials and Methods**

plex.

#### 177 kcal mol−1 in favor of ion *m*/*z* 480. The energy differences were converted from Har-**3. Materials and Methods**  *3.1. Materials and Reagents*

*3.2. Instrumentation and Software* 

using a Stuard SA8 (Cole Parmer, UK) stirrer.

Parmer, USA) at a flow rate 3 μL min−1.

second syringe pump with acetonitrile.

trees into kcal mol−1 using a conversion factor of 627.5. The fragmentation of all prepared complexes was analogous. The data are available in the Supplementary Materials (Figures S1–S12). We observed the only exception for *3.1. Materials and Reagents*  Tetrabutylammonium tetrachlorooxorhenate(V), 4-Chloroaniline, 4-Bromoaniline, 4- Methylaniline, 4-isopropylaniline, aniline, 4-Methylcatechol, and 1,2-dihydroxybenzene Tetrabutylammonium tetrachlorooxorhenate(V), 4-Chloroaniline, 4-Bromoaniline, 4- Methylaniline, 4-isopropylaniline, aniline, 4-Methylcatechol, and 1,2-dihydroxybenzene were purchased from Sigma-Aldrich. Acetonitrile (HPLC grade) and triethylamine were

were purchased from Sigma-Aldrich. Acetonitrile (HPLC grade) and triethylamine were

ESI/MS experiments were conducted on a Bruker QqTOF compact instrument operated using Compass otofControl 4.0 (Bruker Daltonics, Bremen, Germany) software. Compass DataAnalysis 4.4 (Build 200.55.2969) (Bruker Daltonics, Bremen, Germany) software was used for data processing. Molecular structures and fragmentation schemes were drawn using ChemDraw (PerkinElmer Informatics, Waltham, MA, USA). Isotope patterns and exact masses of ions were calculated using IsotopePattern 3.0 (Build 201.9.27) (Bruker Daltonics, Bremen, Germany) utility. Analytical scales Kern ALJ 220-4 (Kern & Sohn, Balingen, Germany) were used to weigh solids. Stirring procedures were performed

ESI/MS data were collected in negative ion mode at scan range from *m*/*z* 50 to *m*/*z* 1000. The temperature of the drying gas was set to 220 °C at 3.0 L min−1 flow rate. Cone voltage was 2800 V. Samples were injected into the nebulizer by a syringe pump (Cole

Time-based ESI/MS measurement was performed by mixing the reactants in concentrated form and diluting the reaction mixture right before the ESI ion source using the

The isolation width of parent ions in CID experiments was set to 5 Da, the pressure of collision gas (nitrogen) in the collision cell was 2.5 × 10−3 mbar. Measurements were

[Re(O)(Cat)2PIPA]−. Here, the different behavior is not related to the change of bond length but the product stability of arising ions. As is evident from the corresponding CID diapurchased from Fisher Scientific. Nitrogen used as nebulizing and drying gas was generated by MS-NGM 11 (Bruker Daltonics, Bremen, Germany) nitrogen generator.
