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Article

Synthesis of Tris(trifluoromethyl)nickelates(II)—Coping with “The C2F5 Problem”

by
Sascha A. Schäfer
,
Rose Jordan
*,
Katharina M. Klupsch
,
Felix Carl-Heinz Herwede
and
Axel Klein
*
Faculty of Mathematics and Natural Sciences, Department of Chemistry and Biochemistry, Institute for Inorganic and Materials Chemistry, University of Cologne, Greinstrasse 6, 50939 Koeln, Germany
*
Authors to whom correspondence should be addressed.
Inorganics 2024, 12(7), 187; https://doi.org/10.3390/inorganics12070187
Submission received: 12 June 2024 / Revised: 2 July 2024 / Accepted: 3 July 2024 / Published: 5 July 2024
(This article belongs to the Special Issue State-of-the-Art Inorganic Chemistry in Germany)
Browse Figures
Versions Notes

Abstract

:
When synthesizing the versatile precursors (NMe4)[Ni(CF3)3(MeCN)] we recently encountered the problem that marked amounts of C2F5 were incorporated instead of CF3 under the chosen reaction conditions forming mixed-ligand nickelates [Ni(CF3)x(C2F5)y(MeCN)]− (x + y = 3). We studied the three products with y = 0, 1, or 2, using 19F nuclear magnetic resonance (NMR) spectroscopy and single-crystal X-ray diffraction. We were able to trace the reaction mechanism and solve the problem by modifying the experimental conditions.

Graphical Abstract

1. Introduction

Besides copper, nickel is a promising platform for the development of new synthetic methodologies for stoichiometric or catalytic trifluoromethylation reactions [1,2,3,4,5,6,7,8,9,10,11] and recently various CF3 nickel complexes [Ni(CF3)4]2−, Ni(CF3)3(L)]−, [Ni(CF3)2(L)2], and [Ni(CF3)(L)3]+ (L = neutral ligands such as nitriles, phosphines or carbenes) have come to the focus of interest [11,12,13,14,15,16].
[Ni(CF3)4]2− can catalyze C–H bond trifluoromethylations of electron-rich (hetero)arenes in up to 99% yield (Scheme 1A) using Umemoto Reagent II (2,8-difluoro-S-(trifluoromethyl)dibenzothiophenium triflate) in DMSO [15].
Using 5 mol% of (NMe4)[Ni(CF3)(C4F8)(MeCN)], which contains a chelating perfluorobutane-diide ligand, as catalyst for the reaction of 1,3,5-trimethoxybenzene with Umemoto Reagent II in DMSO as solvent gave the trifluoromethylated product in 83% yield (Scheme 1B) [14]. For comparison, using [Ni(CF3)3(MeCN)]− as catalyst gave only a 78% yield in the same reaction, while [Ni(CF3)4]2− gave 96% yield [15]. Thus, the specific catalyst design is important.
We recently contributed to this topic by exploring the synthesis, structures, and electrochemical properties of tris(trifluoromethyl)nickelates(II) of the type (NMe4)[Ni(CF3)3(F-NHC)] containing fluorinated N-heterocyclic carbene ligands (F-NHC) of the N,N-di(perfluorophenyl)imidazolylidene type [14]. These carbene complexes were synthesized from the previously reported (NMe4)[Ni(CF3)3(MeCN)] (1) and the corresponding carbene ligands. The precursor 1, as well as its bis- and tetrakis-CF3 congeners, are prepared through transmetalation of the CF3 ligands from silver. In the course of our syntheses of 1 and its use as precursor for [Ni(CF3)3(L)]− complexes, we observed in 19F nuclear magnetic resonance (NMR) spectra and crystal structures of the products that CF3 groups were partially replaced by C2F5 groups [14] as a result of unintended chain elongation—which we called “the C2F5 problem”. However, there is increasing interest in the chemistry of the C2F5 group [2,4,12,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33], including the use of the Ruppert–Prakash reagent Si(CH3)3(C2F5) (TMS–C2F5) [17] for perfluoroethylation reactions with applications in bioactive molecules [27]. Compared with the CF3 group, the far less explored C2F5 provides increased steric demand, higher electronegativity, and lipophilicity [34,35,36,37] that render it an attractive alternative to CF3 [21,25,26,27,33]. The formation of C2F5 during our syntheses of Ni(CF3)3 derivatives is, therefore, interesting regarding potential targeted use of the method for the synthesis of C2F5 complexes. On the other hand, understanding the phenomenon of unintended C2F5 formation during the synthesis of tris(trifluoromethyl)nickelates(II) is also relevant to any future work on Ni(CF3)n complexes in order to avoid “the C2F5 problem”. While this chemistry is extremely interesting, the synthetic work is not trivial and requires careful consideration and optimization regarding setups, reagents, and reaction conditions, especially when using Schlenk techniques as opposed to a glovebox.
Herein, we report on the characterization of mixed CF3/C2F5-containing Ni(II) complexes formed through perfluoroalkyl chain elongation, and present an assessment of the underlying mechanism, and how we were able to prevent this unwanted side reaction through modification of the reaction conditions.

2. Results and Discussion

2.1. Synthesis

In the initial reaction protocol for the synthesis of the Ni(II) tris-CF3 precursor (NMe4)[Ni(CF3)3(MeCN)] (1) (Scheme 2) adapted from Vicic et al. [15], we reacted AgF with the Ruppert–Prakash reagent Si(CH3)3CF3 (TMS–CF3) [17] at ambient temperature in MeCN to form AgCF3.
AgCF3 is reported to exist in solution in a disproportionation equilibrium with AgI[AgI(CF3)2], or to decay to AgI[AgIII(CF3)4] under elimination of 2 equivalents Ag0 (elemental silver), depending on the temperature and the presence of less noble metal ions, which catalyze the elimination reaction [38,39]. As we usually observed a silver mirror in the flask, we assume the formation of AgI[AgIII(CF3)4] in significant amounts.
The solution was decanted from the produced solids after 2 h and added to a suspension of [NiBr2(dme)] (dme = 1,2-dimethoxyethane) and NMe4I in MeCN. After 2 days reaction time, workup was performed and the product was analyzed primarily via 19F NMR (full analytics in the Section 3).

2.2. NMR Spectroscopy and Stereoselectivity

The 19F NMR spectra (Figure 1, complete spectra in the Supplementary Materials) showed the coexistence of (NMe4)[Ni(CF3)3(MeCN)] (1), cis-(NMe4)[Ni(CF3)2(C2F5)(MeCN)] (2), and trans-(NMe4)[Ni(CF3)(C2F5)2(MeCN)] (3) in an approximate ratio of 50/40/10% and confirmed the stereochemistry shown in Scheme 2. The fourth possible compound (NMe4)[Ni(C2F5)3(MeCN)] was not observed.
The signal of the CF3 ligand trans to the MeCN ligand, which is observed at −25.1 ppm for 1, is shifted slightly and progressively downfield upon replacement of one or both CF3 ligands cis to MeCN with C2F5. Interestingly, only the cis CF3 ligands are prone to replacement by C2F5, while the formation of trans-[Ni(CF3)2(C2F5)(MeCN)]− with C2F5 located trans to the coordinated MeCN has not yet been observed. This is fully in line with the tris-C2F5 derivative [Ni(C2F5)3(MeCN)]− not being observed at all, not even in traces by the very sensitive 19F NMR method. The CF3 ligand trans to MeCN seems to be far less prone to be replaced by C2F5. Both the selective trans stereochemistry of [Ni(CF3)(C2F5)2(MeCN)]− (3) and the non-observation of [Ni(C2F5)3(MeCN)]− might be due to the increased steric bulk of C2F5 compared with CF3. An alternative explanation would be the special CF3 position trans to the weaker MeCN ligand being electronically disfavored for C2F5 replacement.
The signals of the cis CF3 groups in 1 and 2 are superimposed at −31.0 ppm. The C2F5 ligands in 2 and 3 show signals in the regions around −82 and –109 ppm, corresponding to the –CF3 and –CF2– sub-moieties, respectively. The C2F5 ligands in 3 are slightly de-shielded compared to the one in 2, but overall the shifts of the C2F5 ligands in 2 and 3 are very similar to those of cis-(NMe4)[Ni(C2F5)2(OAc)] [18].

2.3. Single-Crystal X-ray Diffraction (sc-XRD)

We were unable to separate the precursor complexes 1 to 3 by crystallization but found that extraction of the precursor mixture with THF yields fractions that contain only minor amounts of 1, suggesting that C2F5 substitution does have a significant effect on solubility [26,27]. Recrystallization of such a sample and sc-XRD study showed that 2 is contaminated with approximately 20% 1 in the solid state. Remarkably, CF3 and C2F5 fractionally occupy the same position in the otherwise identical structures (monoclinic, P21/n). The structure of the complex species of 1 was previously reported in the structure of (PPh4)[Ni(CF3)3(MeCN)] (P1) [16] and we refined the structure of 2 with an 80:20 occupancy of 2:1. This explains while recrystallization does not allow separation of 1 and 2. Further, the structure of 2 confirmed our interpretation from 19F NMR of the C2F5 group as located cis to MeCN (Figure 2).
The Ni(II) center in 2 adopts a distorted square planar coordination with the “trans” angles N1–Ni–C3 and C4–Ni–C1 of around 171° deviating markedly from the ideal 180° (Table 1). This represents a slight distortion towards tetrahedral with a τ4 value of 0.13 (ideal tetrahedral = 1, ideal square planar = 0) [38]. In the previously reported structure (PPh4)[Ni(CF3)3(MeCN)] (P1) the corresponding angles of about 178° are much closer to the ideal 180° and the τ4 value of 0.03 is much smaller. At the same time, the bonding distances around Ni(II) are very similar for 2 and P1 with Ni–C3 being the shortest in agreement with the weaker MeCN ligand in trans position to this CF3 group. The corresponding Ni–CF3 bond in P1 and the Ni–CF2CF3 bond in 2 have virtually the same length.
Further, the C2F5 C–C bond with 1.513(3) Å and the Ni–C–C angle of 119.6(1)° in 2 are very similar to those found in the related complexes (NMe4)[Ni(CF3)2(C2F5)(F-NHC)] (F-NHC = N,N-bis(perfluorophenyl)imidazolylidene) [14], [Ni(C4F8)(C2F5)(MeCN)] [14], (NMe4)[Ni(C2F5)2(OAc)] [18], or [Ni(t-Bu2terpy)(C2F5)2] (t-Bu2-terpy = 4,4″-di-tert-butyl-2,2′;6′,6″-terpyridine) [39] showing a C–C bond length of around 1.51 Å and Ni–C–C angles ranging from 115 to 120°.
From this comparison, we conclude that, while the coordination around Ni(II) is flexible concerning the trans angles, the length of the Ni–C bonds are quite invariable, which points to a strong σ-donating power of the CF3 and C2F5 ligands. The C–C bond length of the C2F5 ligand is also rather invariant and the space requirement for the CF3 to C2F5 replacement in 1 to 2 is provided through the flexible geometry around Ni(II) in addition to some flexibility of the Ni–C–C angle.

2.4. Reaction with N-Heterocyclic Carbene (NHC) Ligands

When the mixed precursor material (NMe4)[Ni(CF3)x(C2F5)y(MeCN)] (x + y = 3; 1 to 3) was reacted with fluorinated N-heterocyclic carbene (NHC) ligands as reported elsewhere [14], we observed significant amounts of cis-(NMe4)[Ni(CF3)2(C2F5)(L)] via 19F NMR, meaning that the C2F5-containing precursor readily reacts with carbene ligands. For example, in the case of the fluorinated NHC ligand N,N-bis(2,4-difluorophenyl)imidazolylidene (F-NHC), the spectra (Figure 3) show that even a crystalline sample of the material contains only approximately 75 to 80% (NMe4)[Ni(CF3)3(F-NHC)] (4) and 20 to 25% cis-(NMe4)[Ni(CF3)2(C2F5)(F-NHC)] (5).
sc-XRD of the sample, as well as of other fluorinated NHC complexes from the same precursor as reported previously [14], confirmed the coexistence of 4 and 5 in the form of CF3/C2F5 replacement in the crystal structure as seen for the precursor. As for the pair 1 and 2, the CF3 and C2F5 ligands occupy the same positions in the otherwise identical crystal structure. Refinement of the “disordered” crystal structure of 4/5 yielded an occupancy of approximately 85% CF3 and 15% C2F5 [14]. Therefore, 4 and 5 cannot be separated by crystallization to obtain pure 4 either. This is a somewhat lower content of 5 than we concluded from the NMR spectra for the bulk crystalline sample, though in a similar range. Judging from the varying C2F5 contents in the structures of different carbene complexes from our previously published study [14], it is reasonable to assume that C2F5 incorporation may vary between crystals both statistically and throughout the process of crystal formation from a given solution. Pure 4, including a crystal structure, was previously obtained from pure 1 precursor [14].

2.5. Mechanistic Considerations on the CF3 to C2F5 Conversion and Refined Reaction Procedure

It has long been known that conditions for the preparation of metal-CF3 complexes can also generate metal-C2F5 byproducts, and M=CF2 intermediates have been proposed [29,40,41,42,43,44]. The formation of difluorocarbene requires the elimination of fluoride from CF3, which is promoted by Lewis acids such as (hard) metal cations [45] or protons. We assume that C2F5 via CF2 insertion is formed at Ag during the reaction of TMS–CF3 with AgF since chain elongation during a later step at Ni would probably lead to the formation of Ni–F complexes as significant byproducts. For example, a common side product of moisture-induced decomposition of Ni–CF3 complexes, where protons promote the elimination of fluoride from CF3, is the fluoride-bridged dimer {[Ni(CF3)2F]}2 [14], which can also result from thermal decomposition [39].
Nebra et al. reported that an AgIII=CF2 complex can be generated from [AgIII(CF3)4]− in the presence of K∙∙∙F interactions, though the reaction is highly endothermic and the resulting [AgIII(CF3)3(CF2)] species is very reactive due to negligible π-backdonation from AgIII to difluorocarbene according to their DFT analysis [45]. However, they do not mention C2F5 formation during their trifluoromethylation experiments with [AgIII(CF3)4]− in the presence of K+ [45]. For the formation of Cu–C2F5 (and even Cu–C3F7) from Cu–CF3, Hu et al. concluded that the plausible intermediate is a difluorocarbene complex CuI=CF2 [29]. Combining these two pieces of information, we believe that the chain elongation is occurring at AgI, rather than AgIII, in our case. Due to the higher π-backdonation ability of AgI, AgI=CF2 formation promoted by M∙∙∙F interactions is probably more facile from AgI–CF3. Thus, considering that both the disproportionation of AgI–CF3 into AgI[AgIII(CF3)4] and Ag0 and α-fluoride elimination are promoted by Lewis acids [40,41,45], we are looking at two competing, opposing effects of the presence of trace metals, or moisture, on C2F5 formation during the reaction of TMS–CF3 with AgF. While the formation of AgI[AgIII(CF3)4] can occur at any temperature above −30 °C, α-fluoride elimination is additionally promoted thermally.
We thus concluded that the C2F5-containing by-products are generated when the reaction of TMS–CF3 with AgF is performed in concentrated MeCN solution at elevated temperatures. The formation of C2F5 was facilitated by the exothermic reaction during upscaling experiments with high reagent concentrations. Based on this, we modified the reaction protocol for the synthesis of (NMe4)[Ni(CF3)3(MeCN)] (1) (Scheme 3). Reducing the overall concentration of reagents and employing better heat management during the reaction of AgF with TMS–CF3, while maintaining the original conditions for the reaction of the resulting Ag–CF3 with Ni(II), allowed us to eliminate the formation of the C2F5-containing 2 and 3. This further supports our assumption that the chain elongation occurs at Ag as opposed to Ni as stated earlier. Furthermore, we note that the quality of the AgF used for the reaction is extremely vital to the success of the synthesis with respect to the “C2F5 problem” due to the complex effects of the presence of free Lewis acid on AgI[AgIII(CF3)4] vs. AgI=CF2 formation.
If Ag–C2F5 species are, thus, the reason for the formation of the observed mixed-ligand nickelates [Ni(CF3)x(C2F5)y(MeCN)] (x + y = 3), the observed stereoselectivity of cis-(NMe4)[Ni(CF3)2(C2F5)(MeCN)] (2) and trans-(NMe4)[Ni(CF3)(C2F5)2(MeCN)] (3) would then simply be a matter of the increased bulkiness of C2F5 compared with CF3. To further support such Ag–C2F5 species, we tried to study them by 19F NMR in solutions containing TMS–CF3 and AgF, but failed due to the formation of elemental Ag impeding NMR.

3. Materials and Methods

3.1. NMR Spectroscopy

19F NMR spectra were recorded on a Bruker Avance II NMR spectrometer (Bruker, Rheinhausen, Germany) operating at 282 MHz and referenced to α,α,α-trifluorotoluene as an internal standard (δ = −63.7 ppm).

3.2. Single-Crystal X-ray Diffraction

X-ray crystal structure determination of cis-(NMe4)[Ni(CF3)2(C2F5)(MeCN)] (2) was carried out on a Bruker D8 Venture diffractometer including a Bruker Photon 100 CMOS detector (both Bruker, Rheinhausen, Germany) at 100(2) K using Mo Kα (λ = 0.71073 Å) radiation. The crystal data were collected using APEX4 v2021.10-0 [46]. The structures were solved by dual-space methods using SHELXT, and the refinement was carried out with SHELXL employing the full-matrix least-squares methods on FO2 < 2σ(FO2) as implemented in ShelXle [47,48,49]. The non-hydrogen atoms were refined with anisotropic displacement parameters, and hydrogen atoms were included by using appropriate riding models.

3.3. General Synthesis Conditions

All manipulations were performed using standard Schlenk techniques. Solvents were freed of oxygen by purging with Ar and dried using freshly activated 3 Å molecular sieves.

3.4. Materials

AgF was obtained from Acros Organics (Geel, Belgium), Fluorochem (Glossop, UK), Carbolution (St. Ingbert, Germany), or BLDpharm (Reinbek, Germany) and used without further purification. (CH3)3SiCF3 was received from ABCR (Karlsruhe, Germany). [NiBr2(dme)] was prepared according to a reported procedure [50].

3.5. Synthetic Procedures

3.5.1. Preparation of (NMe4)[Ni(CF3)3(MeCN)] (1, Initial Procedure)

The reagents, 3.0 g (9.72 mmol; 1.0 eq.) [NiBr2(DME)] and 1.96 g (9.72 mmol; 1.0 eq.) NMe4I, were weighed under ambient conditions and quickly transferred into a flame-dried Schlenk-flask. The solids were dried under reduced pressure at 60 °C for 1 h. AgF (3.7 g; 29.16 mmol; 3.0 eq.) was weighed under ambient conditions, quickly transferred into a separate, flame-dried Schlenk-flask, and dried under reduced pressure at 60 °C for 1 h. After cooling to RT, both flasks were backfilled with Ar. MeCN (25 mL) was added to each flask. (CH3)3SiCF3 (5.02 mL; 4.84 g; 34.02 mmol; 3.5 eq.) was quickly added to the suspended AgF. The resulting solution was stirred for 2 h under exclusion of light. Then, the AgCF3-containing solution was transferred via a cannula into the [NiBr2(DME)]/NMe4I suspension. The resulting bright yellow suspension was stirred vigorously for 2 d at RT under exclusion of light. The suspension was filtered over Celite® and evaporated. A total of 2.22 g of a bright yellow solid was isolated. The solid contained a 5:4:1 mixture of (NMe4)[Ni(CF3)3(MeCN)] (1): (NMe4)[Ni(CF3)2(C2F5)(MeCN)] (2): (NMe4)[Ni(CF3)(C2F5)2(MeCN)] (3). (1) 19F NMR (282 MHz, MeCN-d3): δ [ppm] = −25.11 (septet, J = 4.7 Hz, 1, trans-CF3), −25.22 (multiplet, 2, trans-CF3), −25.35 (multiplet, 3, 3F, trans-CF3), −30.99 (multiplet, 1 and 2, cis-CF3), −81.69 (quartet, J = 4.1 Hz, 3, C2F5), −81.97 (quartet, J = 3.8 Hz, 2, C2F5), −108.46 (quartet, J = 8.3 Hz, 3, C2F5), −109.32 (quartet, J = 8.1 Hz, 2, C2F5).

3.5.2. Preparation of (NMe4)[Ni(CF3)3(MeCN)] (1, Refined Procedure)

The reagents, 2.5 g (8.10 mmol; 1.0 eq.) [NiBr2(DME)] and 0.89 g (8.10 mmol; 1.0 eq.) NMe4Cl, were weighed under ambient conditions and quickly transferred into a flame-dried Schlenk-flask. The solids were dried under reduced pressure at 80 °C for 1 h. AgF (3.19 g; 25.11 mmol; 3.1 eq.) was weighed under ambient conditions, quickly transferred into a separate, flame-dried Schlenk-flask, and dried under reduced pressure at RT for 1 h. After cooling to RT both flasks were backfilled with Ar. MeCN (40 mL) was added to each flask. (CH3)3SiCF3 (3.55 mL; 3.69 g; 25.92 mmol; 3.2 eq.) was added to a third flame-dried Schlenk-flask and diluted with 15 mL MeCN. All three flasks were cooled to 0 °C. The diluted (CH3)3SiCF3 was added dropwise to the suspended AgF. The resulting solution was stirred for 15 min under exclusion of light. Afterwards, the AgCF3-containing solution was transferred via a cannula into the [NiBr2(DME)]/NMe4Cl suspension. The resulting bright yellow suspension was stirred vigorously for 2 d at RT under exclusion of light. The suspension was filtered over Celite® and partially evaporated. A total of 66 mL of a bright yellow MeCN solution was recovered. The concentration of (NMe4)[Ni(CF3)3(MeCN)] (1) in the solution was determined using quantitative NMR. The solution contained 0.082 mmol/mL, corresponding to 5.41 mmol (67%) (NMe4)[Ni(CF3)3(MeCN)]. 19F NMR (282 MHz, CD3CN): δ [ppm] = −25.11 (septet, J = 4.6 Hz, 3F), −30.98 (quartet, J = 4.6 Hz, 6F).

4. Conclusions

In our recent attempts to synthesize the versatile precursor (NMe4)[Ni(CF3)3(MeCN)] (1) we had observed that marked amounts of C2F5 were incorporated instead of CF3 under the chosen reaction conditions with the formation of mixed-ligand nickelates [Ni(CF3)x(C2F5)y(MeCN)]− (x + y = 3). The C2F5-containing precursor derivatives readily form NHC complexes like the tris-CF3 precursor 1. We studied the precursors with x = 3 (1), x = 2 (cis-2), and x = 1 (trans-3) and their stereochemistry, as well as the corresponding F-NHC complexes (F-NHC = N,N-bis(2,4-difluorophenyl)imidazolylidene) using 19F nuclear magnetic resonance (NMR) spectroscopy and single-crystal X-ray diffraction, and surprisingly found significant CF3/C2F5 replacement in the solid-state structures, explaining our failure to purify the materials through crystallization. We were able to trace the reaction mechanism and assume transmetalating Ag-C2F5 species to be responsible for the C2F5 ligands in the nickelates, and we finally solved the problem by modifying the experimental conditions. We found that the synthesis of (NMe4)[Ni(CF3)3(MeCN)] (1) must be carried out avoiding high concentrations of TMS–CF3 and excessive formation of heat. For larger reaction batches, efficient cooling of the reaction vessel must be provided to suppress formation of C2F5 species (NMe4)[Ni(CF3)x(C2F5)y(MeCN)] (x + y = 3). Furthermore, due to the complex role of Lewis acid impurities in the reactions of Ag–CF3 complexes, it should be kept in mind that trace impurities in AgF may also have a notable influence on C2F5 formation during the synthesis of 1.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics12070187/s1. Figure S1: 19F NMR spectrum (282 MHz, MeCN-d3) of the mixture of species (NMe4)[Ni(CF3)x(C2F5)y(MeCN)] (x + y = 3) with partial CF3 to C2F5 replacement; Figure S2: 1H NMR spectrum (300 MHz, MeCN-d3) of the mixture of species (NMe4)[Ni(CF3)x(C2F5)y(MeCN)] (x + y = 3) with partial CF3 to C2F5 replacement; Figure S3: 19F NMR spectrum (282 MHz, MeCN-d3) of the mixture of species (NMe4)[Ni(CF3)x(C2F5)y(F-NHC)] (x + y = 3, F-NHC = N,N-bis(2,4-difluorophenyl)imidazolylidene) with partial CF3 to C2F5 replacement; Figure S4: 1H NMR spectrum (300 MHz, MeCN-d3) of the mixture of species (NMe4)[Ni(CF3)x(C2F5)y(F-NHC)] (x + y = 3, F-NHC = N,N-bis(2,4-difluorophenyl)imidazolylidene) with partial CF3 to C2F5 replacement; Figure S5: 19F NMR spectrum (282 MHz, MeCN-d3) of (NMe4)[Ni(CF3)3(MeCN)] (1) prepared using the refined reaction procedure; Figure S6: Crystal structure of (NMe4)[Ni(CF3)x(C2F5)y(MeCN)] (x = 2.2, y = 0.8) viewed along the crystallographic a and b axes. The split occupancy of 20% (NMe4)[Ni(CF3)3(MeCN)] was omitted for clarity; Figure S7: Molecular structures of cis-(NMe4)[Ni(CF3)2(C2F5)(MeCN)] (left) and (NMe4)[Ni(CF3)3(MeCN)] (right) from sc-XRD of (NMe4)[Ni(CF3)x(C2F5)y(MeCN)] (x = 2.2, y = 0.8); Figure S8: Molecular structures of cis-[Ni(CF3)2(C2F5)(F-NHC)]− (F-NHC = N,N-bis(2,4-difluorophenyl)imidazolylidene, left) and [Ni(CF3)3(F-NHC)]− (right) from sc-XRD of (NMe4)[Ni(CF3)x(C2F5)y(MeCN)] (x = 2.85, y = 0.15); Table S1: Crystallographic and structure refinement data for (NMe4)[Ni(CF3)x(C2F5)y(MeCN)] (x = 2.2, y = 0.8); Table S2: Selected bond lengths (Å) and angles (°) of cis-(NMe4)[Ni(CF3)2(C2F5)(MeCN)] from sc-XRD.

Author Contributions

Conceptualization, A.K.; methodology, S.A.S. and A.K.; investigation, S.A.S., K.M.K. and F.C.-H.H.; resources, A.K.; data curation, S.A.S. and R.J.; visualization, S.A.S. and R.J.; supervision and project administration, A.K.; manuscript original draft, R.J. and A.K.; manuscript editing, S.A.S., R.J. and A.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the University of Cologne (S.S.), Studienstiftung des Deutschen Volkes (R.J.), Deutsche Forschungsgemeinschaft (DFG Priority Program 2102 “Light-controlled Reactivity of Metal Complexes”), KL1194/16-1 and 16-2 (A.K.).

Data Availability Statement

The crystal structure of cis-(NMe4)[Ni(CF3)2(C2F5)(MeCN)] (2) is deposited as CCDC 2218331; data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by e-mailing to [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK. Other original contributions presented in the study are included in the article/Supplementary Materials, further inquiries can be directed to the corresponding authors.

Acknowledgments

We thank Scott T. Shreiber and David A. Vicic, LeHigh University, Bethlehem, PA, USA for helpful comments.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 12 00187 sch001
Scheme 1. Trifluoromethylation with Umemoto Reagent II, (A) using (NMe4)2[Ni(CF3)4] as catalyst, R = (hetero)aromatics, or (B) using (NMe4)[Ni(CF3)(C4F8)(MeCN)] as catalyst; adapted from refs. [14,15].
Scheme 1. Trifluoromethylation with Umemoto Reagent II, (A) using (NMe4)2[Ni(CF3)4] as catalyst, R = (hetero)aromatics, or (B) using (NMe4)[Ni(CF3)(C4F8)(MeCN)] as catalyst; adapted from refs. [14,15].
Inorganics 12 00187 sch001
Inorganics 12 00187 sch002
Scheme 2. Initial reaction protocol for the synthesis of (NMe4)[Ni(CF3)3(MeCN)] (1), producing mixtures of 1 and the perfluoroethyl derivatives (NMe4)[Ni(CF3)2(C2F5)(MeCN)] (2) and (NMe4)[Ni(CF3)(C2F5)2(MeCN)] (3).
Scheme 2. Initial reaction protocol for the synthesis of (NMe4)[Ni(CF3)3(MeCN)] (1), producing mixtures of 1 and the perfluoroethyl derivatives (NMe4)[Ni(CF3)2(C2F5)(MeCN)] (2) and (NMe4)[Ni(CF3)(C2F5)2(MeCN)] (3).
Inorganics 12 00187 sch002
Inorganics 12 00187 g001
Figure 1. 19F NMR spectrum (282 MHz, MeCN-d3) of (NMe4)[Ni(CF3)x(C2F5)y(MeCN)] (x + y = 3), 1, 2, and 3, with partial CF3 to C2F5 replacement.
Figure 1. 19F NMR spectrum (282 MHz, MeCN-d3) of (NMe4)[Ni(CF3)x(C2F5)y(MeCN)] (x + y = 3), 1, 2, and 3, with partial CF3 to C2F5 replacement.
Inorganics 12 00187 g001
Inorganics 12 00187 g002
Figure 2. Crystal structure of cis-(NMe4)[Ni(CF3)2(C2F5)(MeCN)] (2) viewed along the crystallographic a axis (left) and molecular structure (right), shown as an ORTEP plot at 40% probability (H atoms omitted). The split occupancy of 20% (NMe4)[Ni(CF3)3(MeCN)] (1) was omitted for clarity.
Figure 2. Crystal structure of cis-(NMe4)[Ni(CF3)2(C2F5)(MeCN)] (2) viewed along the crystallographic a axis (left) and molecular structure (right), shown as an ORTEP plot at 40% probability (H atoms omitted). The split occupancy of 20% (NMe4)[Ni(CF3)3(MeCN)] (1) was omitted for clarity.
Inorganics 12 00187 g002
Inorganics 12 00187 g003
Figure 3. 19F NMR spectrum (282 MHz, MeCN-d3) of (NMe4)[Ni(CF3)x(C2F5)y(F-NHC)] (x + y = 3), 4 and 5 with partial CF3 to C2F5 replacement.
Figure 3. 19F NMR spectrum (282 MHz, MeCN-d3) of (NMe4)[Ni(CF3)x(C2F5)y(F-NHC)] (x + y = 3), 4 and 5 with partial CF3 to C2F5 replacement.
Inorganics 12 00187 g003
Inorganics 12 00187 sch003
Scheme 3. Refined reaction protocol for the synthesis of (NMe4)[Ni(CF3)3(MeCN)] (1).
Scheme 3. Refined reaction protocol for the synthesis of (NMe4)[Ni(CF3)3(MeCN)] (1).
Inorganics 12 00187 sch003
Table 1. Selected molecular metrics of cis-(NMe4)[Ni(CF3)2(C2F5)(MeCN)] (2) and (PPh4)[Ni(CF3)3(MeCN)] (P1) a.
Table 1. Selected molecular metrics of cis-(NMe4)[Ni(CF3)2(C2F5)(MeCN)] (2) and (PPh4)[Ni(CF3)3(MeCN)] (P1) a.
2P1 2P1
Distances (Å) Angles (°)
Ni–C11.959(2)1.953(2)N1–Ni–C188.86(6)89.43(8)
Ni–C41.948(2)1.938(2)C1–Ni–C392.71(6)90.15(9)
Ni–C31.897(2)1.885(2)C4–Ni–N189.48(6)90.28(8)
C3–Ni–C490.41(6)90.06(9)
Ni–N11.895(1)1.882(1)N1−Ni1−C3170.75(6)177.6(1)
C4−Ni1−C1170.69(6)177.9(1)
361.46(6)359.92(9)
a From sc-XRD. The structure of P1 is reported in ref. [16].
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Schäfer, S.A.; Jordan, R.; Klupsch, K.M.; Herwede, F.C.-H.; Klein, A. Synthesis of Tris(trifluoromethyl)nickelates(II)—Coping with “The C2F5 Problem”. Inorganics 2024, 12, 187. https://doi.org/10.3390/inorganics12070187

AMA Style

Schäfer SA, Jordan R, Klupsch KM, Herwede FC-H, Klein A. Synthesis of Tris(trifluoromethyl)nickelates(II)—Coping with “The C2F5 Problem”. Inorganics. 2024; 12(7):187. https://doi.org/10.3390/inorganics12070187

Chicago/Turabian Style

Schäfer, Sascha A., Rose Jordan, Katharina M. Klupsch, Felix Carl-Heinz Herwede, and Axel Klein. 2024. "Synthesis of Tris(trifluoromethyl)nickelates(II)—Coping with “The C2F5 Problem”" Inorganics 12, no. 7: 187. https://doi.org/10.3390/inorganics12070187

APA Style

Schäfer, S. A., Jordan, R., Klupsch, K. M., Herwede, F. C. -H., & Klein, A. (2024). Synthesis of Tris(trifluoromethyl)nickelates(II)—Coping with “The C2F5 Problem”. Inorganics, 12(7), 187. https://doi.org/10.3390/inorganics12070187

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