Trifluoromethylation of [AuF3(SIMes)]: Preparation and Characterization of [Au(CF3)xF3−x(SIMes)] (x=1–3) Complexes

Abstract Trifluoromethylation of [AuF3(SIMes)] with the Ruppert–Prakash reagent TMSCF3 in the presence of CsF yields the product series [Au(CF3)xF3−x(SIMes)] (x=1–3). The degree of trifluoromethylation is solvent dependent and the ratio of the species can be controlled by varying the stoichiometry of the reaction, as evidenced from the 19F NMR spectra of the corresponding reaction mixtures. The molecular structures in the solid state of trans‐[Au(CF3)F2(SIMes)] and [Au(CF3)3(SIMes)] are presented, together with a selective route for the synthesis of the latter complex. Correlation of the calculated SIMes affinity with the carbene carbon chemical shift in the 13C NMR spectrum reveals that trans‐[Au(CF3)F2(SIMes)] and [Au(CF3)3(SIMes)] nicely follow the trend in Lewis acidities of related organo gold(III) complexes. Furthermore, a new correlation between the Au−Ccarbene bond length of the molecular structure in the solid state and the chemical shift of the carbene carbon in the 13C NMR spectrum is presented.

Herein,w er eport on the synthesis and characterization of the hitherto unknown series [Au(CF 3

Results and Discussion
The molecular structureo f[ AuF 3 (SIMes)] in the solid state reveals that the AuÀFb ond with the fluorido ligand in trans position to the SIMes ligand is about 5pml onger than those to the cis-fluorido ligands. [14] In accordance with this finding, as electives ubstitution of the trans-fluorido ligand by ac hlorido or ap entafluoridoorthotellurato (OTeF 5 )l igand was recently reportedb yu s. [43] Hence, the trifluoromethylation of [AuF 3 (SIMes)] is expectedt oy ield trans-[Au(CF 3 )F 2 (SIMes)] (1). Indeed, when TMSCF 3 is condensed into aD CM solution of [AuF 3 (SIMes)] at À80 8Ci nt he presence of the nucleophilic fluoride source CsF,c ompound 1 is formed. 19 FNMR spectroscopy investigationss how that first only compound 1 is formed, but during the consumption of [AuF 3 (SIMes)],a second substitution reactiono faf luorido ligand by at rifluoromethyl group is observed, forming cis-[Au(CF 3 ) 2 F(SIMes)] (2)i n solution (cf. Scheme 3). After af ew hours, no [AuF 3 (SIMes)] is left andt he ratio of the products stays constantf or several days, even though unreacted TMSCF 3 is left in the reaction mixture (see Supporting Information, Figures S9-S15). Therefore, the reactionp roceeds rather fast and the reactiont ime does not have ar elevant influence on the product ratio. Instead, the stoichiometry of the reactants has ad ecisive impact on the ratio between 1 and 2.A se xpected, the formationo f trans-[Au(CF 3 )F 2 (SIMes)] (1)i sf avored by % 0.5 equivalents of TMSCF 3 ,w hilea ne xcesso fT MSCF 3 increases the amount of cis-[Au(CF 3 ) 2 F(SIMes)] (2), as shown in Ta ble 1. HCF 3 and trans-[AuClF 2 (SIMes)] are formed as by-products,t he presence of the former being probably due to side reactions of the highly basic transient CF 3 À anion with any proton sources, for example, from the solvent, [44,45,46] while the latter is formed by a chlorine/fluorine exchange reactionb etween [AuF 3 (SIMes)] and DCM, possibly promoted by the fluoridea nions of CsF. [14] The amount of by-products depends on the stoichiometry.T he more TMSCF 3 is used, the more HCF 3 is formed, while less [AuClF 2 (SIMes)] is present( see Supporting Information, Figures S10, S13 and S15). Compounds 1 and 2 partially decompose in solution at room temperature within af ew days, leadingt o the formation of elemental gold. However,t he 19 FNMR spectra still show signals of the products after several weeks.
If the trifluoromethylation of [AuF 3 (SIMes)] is performed in THF,c ompound 1 is not observed in the 19 FNMR spectrumo f the reaction mixture. Instead, compound 2 and the three times substituted complex [Au(CF 3 ) 3 (SIMes)] (3)a re formed (cf. Scheme 3). The reason for the formation of highers ubstituted products in THF is most likely the better solubility of CsF in THF compared to DCM. Thisl eads to an enhanced activation of the TMSCF 3 formingapentacoordinated silicon(IV) anion, which acts as ah ighly potent CF 3 transfer reagent, as shown by Naumann et al. [46] In contrast to the reactioni nD CM, no TMSCF 3 is left in the reactionm ixture after af ew hours (see Supporting Information, Figures S16a nd S17). The stoichiometry of the reactants hasasignificant influence on the product ratio. When [AuF 3 (SIMes)] and TMSCF 3 are used in roughlya 1:1r atio, compounds 2 and 3 are formed in almost equal amounts. If only about half an equivalent of TMSCF 3 is used, five times more 2 than 3 is formed (cf. Ta ble 2). The transfer of two or three CF 3 groups, even though not more than one equivalent of TMSCF 3 is used, can most likely be explained by the lower solubility of [AuF 3 (SIMes)] in THF.F urthermore, the formation of HCF 3 and the [Au(CF 3 ) 4 ] À anion as by-products is observed, which increases with the amounts of TMSCF 3 (see Supporting Information, Figures S16 and S17). The existence of the former is probablyd ue to ar eaction of the highly basic CF 3 À anion, which is abstracted from the pentacoordinated silicon(IV) anion, with proton sources in the reaction mixture, [44,46] while the latter is possibly formed by the trifluoromethylation of traces of the [AuF 4 ] À anion.
[Au(CF 3 ) 3 (SIMes)] can be isolated via ad ifferent route, starting from the literature-knownc omplex [Au(CF 3 ) 3 (NCCH 3 )] [42] by substitution of the acetonitrile ligand with SIMes. Ar oomt emperatures olution of 3 in DCM or THF is stable for several weeks. In pure form, compound 3 can be stored under an argon atmosphere at room temperature for months and it is stable under air for severald ays withoutd ecomposition. The synthetic routes for the preparation of the target compounds (3)a re summarized in Scheme 3a nd an overview on the chemical shifts in the 19 FNMR spectra,w hichw ill be discussed below,i sg iven in Table 3.

Scheme3.Reactionscheme for the preparation of the target compounds trans-[Au(CF 3 )F 2 (SIMes)] (1), cis-[Au(CF 3 ) 2 F(SIMes)] (2)a nd [Au(CF 3 ) 3 (SIMes)] (3). They can be prepared by trifluoromethylation of [AuF 3 (SIMes)]i nDCM (top) and THF (middle), andcompound 3 can also be prepared by ligand substitution of [Au(CF 3 ) 3 (NCCH 3 )] (bottom).
In case of the trifluoromethylation, the outcome of the reactiond epends on the solvent. In eitherc ase, the product ratio can be controlled by the stoichiometry of the reactants, see Ta ble 1a nd Table 2f or DCM and THF,respectively. Table 1. Product ratio dependence of the stoichiometric factors in the reaction between [AuF 3 (SIMes)] and TMSCF 3 in DCM determined by the integral ratios of the signals in the 19 FNMR spectra (seeS upportingI nformation, Figures S9-S15). Note that the amount of TMSCF 3 can slightlyd eviate from the values listed in the table, due to the inherent uncertainty of the used manometer for determining the pressure of TMSCF 3 .  Table 2. Product ratio dependence of the stoichiometric factors in the reaction between [AuF 3 (SIMes)]a nd TMSCF 3 in THF determined by the integral ratioso ft he signals in the 19 FNMR spectra (see Supporting Information, Figures S16 and S17). Note that the amounto fT MSCF 3 can slightly deviate from the values listed in the table, due to the inherentu ncertainty of the used manometer for determining the pressure of TMSCF 3 .  Table 3. 19 FNMR spectroscopic data of the target compounds trans- The subscripts ca nd ts tand for CF 3 groups in cis or trans position to the SIMes ligand, respectively. [a] Compound The 19 FNMR spectrum of trans-[Au(CF 3 )F 2 (SIMes)] (1)s hows at riplet at À46.5 ppm and aq uartet at À329.7 ppm with a 3 J( 19 F, 19 F) coupling constant of 18 Hz, which correspondt ot he trifluoromethyla nd the two fluorido ligands, respectively (see Figure 1). The former resonance is in the upfield range of chemicals hifts of the corresponding trifluoromethyl groups in literature-known fluorido trifluoromethylg old(III) complexes [9,10,12,37] and almost identical to the one in the [trans-Au(CF 3 ) 2 F 2 ] À anion (d = À46.2 ppm). [12] The latter is 4 ppm À19 ppm upfield shifted compared to the cis-fluorido ligands in the literature-known trans-[AuF 2 X(SIMes)] (X = Cl, F, OTeF 5 ) complexes. [14,43] An interesting feature of metal complexes with NHC ligands is the chemical shift of the carbene carbona tom in the 13 CNMR spectra,w hich was provent ob eam easure of the Lewisa cidity of the metal center. [14,43,47] In the 1 H, 13 C HMBC NMRs pectrum of 1,t he resonance of the carbene carbon atom was detected at 192.3 ppm, which is 26 ppm À45 ppm downfield shifted compared to the literature-known trans-[AuF 2 X(SIMes)] (X = Cl, F, OTeF 5 ) [14,43] complexes (cf. Ta ble 4) due to the weaker Lewis acidity of the gold center in compound 1.T his is in good agreement with the corresponding AuÀC carbene bond lengthsi nt he solid state andt he calculated dissociation energy of the AuÀC carbene bond, as discussed in detail below.
The 19 FNMR spectrum of cis-[Au(CF 3 ) 2 F(SIMes)] (2)( see Figure3)c onsistso ft wo doublets of quartets at À23.9 ppm and À41.2 ppm, where the latter appearst ob easextet due to the coupling constants of 7Hza nd 14 Hz, and aq uartet of quartets at À254.0 ppm in an integral ratio of 3:3:1. The latter resonance belongs to the remaining fluoridol igand, while the two other signals are due to the two chemically inequivalent . CF 3 groups.T he chemical shifts are all similar to the literatureknown [Au(CF 3 ) 3 F] À anion. [10] The 3 J( 19 F, 19 F) coupling constants between the fluorine nuclei of the trifluoromethyl groups and the fluorido ligand are 57 Hz and 14 Hz for the signals at À23.9 ppm and À41.2 ppm, respectively.I ti sk nown that trans coupling constants are usually larger than cis couplingc onstants, [37] for example, in the [Au(CF 3 ) 3 F] À anion, the coupling constantsa re 55.8 Hz (trans)a nd 12.3 Hz (cis). [10] Furthermore, the coupling constant of 14 Hz is in good agreement with the 3 J( 19 F, 19 F) cis couplingi n1 (18 Hz) and the literature-known trans-[Au(CF 3 ) 2 F 2 ] À anion (16.5 Hz). [12] Therefore, the signals at À23.9 ppm and À41.2 ppm can be assigned to the CF 3 groups cis and trans to the carbene ligand, respectively (cf. Figure 3 and Ta ble3). Figure 4s hows the 19 FNMR spectrum of [Au(CF 3 ) 3 (SIMes)] (3), which consists of aq uartet at À31.5 ppm and as epteta t À34.3 ppm with an integral ratio of 2:1. The quartet belongs to the two trans-positioned CF 3 groups andt he septet to the CF 3 group in trans-position to the SIMes ligand.T he coupling constant of 7Hzi si dentical to the 4 J( 19 F, 19 F) couplingc onstant of the two CF 3 groups in compound 2 and fits nicely within the range of neutral complexes containing the Au(CF 3 ) 3 fragment prepared by Menjóne tal. (6.0-7.5 Hz). [42] Compared to the acetonitrile complex [Au(CF 3 ) 3 (NCCH 3 )],w hich was used as as tartingm aterial fort he selective synthesis of compound 3, the relative positionofthe two resonancesa re interchanged. [42] In the 1 H, 13 CH MBC NMR spectrum, the resonanceo ft he carbene carbon atom in compound 3 is observeda t1 91.4 ppm, which is only 1ppm upfield shiftedc ompared to compound 1, and thus points towards as imilar Lewis acidity (cf. discussion below).
In order to obtain single crystals of compound 3 suitablef or X-ray diffraction, ap ure sample of 3 prepared by the reaction between [Au(CF 3 ) 3 (NCCH 3 )] and SIMes was dissolved in dichloromethane, chloroform, acetone or tetrahydrofuran, and each of the solutions was layered by n-hexanea t58C. From all four solvents, compound 3 crystallizes in the monoclinic space group P2 1 /c with as quare planar coordination aroundt he gold center and contains half ac o-crystallized, disordered solvent molecule. Since the four structures are homeotypic (see Supporting Information, Figures S3-S6), the followingd iscussion is based on the structural data of [Au(CF 3 ) 3 (SIMes)]·0.5 CH 2 Cl 2 ( Figure 5). The AuÀCF 3 bond lengths to the two CF 3 groups in cis position to the SIMes ligand (208.3(3) pm, 208.6(3) pm) are in the typical range for neutral [Au(CF 3 ) 3 L] complexes (207.4(9)-209.8(2) pm) [42] and similar to those in [NBu 4 ]-[Au(CF 3 ) 4 ]( 207.5(6) pm, 208.5(7) pm). [34] The AuÀCF 3 bond length of the CF 3 group trans to the SIMes ligand (207.8(3)pm) is comparable to the other two AuÀCF 3 bonds and in the upperr ange of neutral[ Au(CF 3 ) 3 L] complexes (200.1(3)-209.0(3) pm). [42] The AuÀCb ond to the SIMes ligand (208.1(2) pm) is slightly longert han in compound 1 (203.5(9) pm;cf. Figure 2) and 3-11pml onger than in other literature-known [AuX 3 (SIMes)] complexes (X = F, Cl, Br). [14,48] The elongation of this AuÀCb ond is due to the strong trans-influence of the CF 3 group compared to the halides, [49][50][51] which also explains why the AuÀCF 3 bond lengths of the two trans-standingC F 3 groups are usuallyl onger than that of the CF 3 group trans to the donor ligand in similarc omplexes. [42] Recently,o ur group reported on the calculation and experimental access to the "SIMes affinity" of gold(III) moieties. Therein, the Gibbs free energy D r G diss of the dissociation of the SIMes ligand from [AuF 2 X(SIMes)] and [AuX 3 (SIMes)] complexes (X = Cl, F, OTeF 5 )f orming SIMes and the corresponding [AuF 2 X] or [AuX 3 ]f ragment on the RI-B3LYP-D3/def2-TZVPPl evel of theorya t08Cw ere calculated. [43] ThisS IMes affinity can be correlatedw ith the chemical shift of the carbene carbon atom in the 13 CNMR spectrum. We found an early linear relationship between an increaseo ft he SIMes affinity and an upfield shift in the 13 CNMR spectrum. [43] We have now determined the SIMes affinitieso ftrans-[Au(CF 3 )F 2 (SIMes)] (1)a nd [Au(CF 3 ) 3 (SIMes)] (3)t ob e2 51 kJ mol À1 and 240 kJ mol À1 ,r espectively.T he corresponding complexes with Cl, Fa nd/or OTeF 5 ligandse xhibit significantly higher SIMes affinities, ranging from 300 kJ mol À1 to 430 kJ mol À1 .T hist rend is in accordance with the chemical shifts of the carbene carbon atom in the 13 CNMR spectra, as depicted in Figure 6. Theoretical studies show that ad ownfield shift of the carbene carbon atom in gold complexes resultsf rom ad eshielding of the carbene  carbon atom by ligandsw ith as trong trans-influence. [52] Our results are in good agreementw ith the literature, where the trifluoromethyl group was determined to have a trans-influence in the order of the methyl group, which is one of the strongest trans-influencing ligands, and am uch stronger transinfluence than the halides. [49][50][51]53] Ar ecent study on hydrido gold(III) complexes showed that also the cis-influencec an change the chemical shift. [54] However,t his is not the case in our trifluoromethyl gold complexes, since compounds 1 and 3 have similar SIMes affinities and 13 C carbene chemical shifts.
Regarding the series of [AuF 2 X(SIMes)] complexes (X = CF 3 , Cl, F, OTeF 5 ), the theoretically determined SIMes affinity is a good measure for the strength of the AuÀC carbene bond, which is inverselyp roportionalt ot he experimentally determined AuÀ C carbene bond length in them olecular structures in the solid state. Ta ble 4l ists the SIMesa ffinities, gold carbon distances and 13 Cc hemical shifts of the carbene carbon atomi nt hese complexes.F igure 7s hows the nearly linear relationship between the AuÀC carbene bond length and the chemical shift of the carbene carbon atom, which underlines the use of the calculated Gibbs free energies as am easureo ft he strength of the AuÀC carbene bond. The trans-influence rises in the order OTeF 5 < F < Cl ! CF 3 .I ns ummary,t he higher trans-influence of the CF 3 ligand in 1 leads to alower Lewis acidity of the gold(III) center,r esulting in al ongerA u ÀC carbene bond length and ad eshieldingo ft he carbene carbon atom. . In both cases, the product ratio does not change significantly with longerr eactiont imes, but it can be controlled by the stoichiometry of the reaction, as evidenced by the 19 FNMR spectra.M ore TMSCF 3 leads to al arger amount of the higher-substituted product. Furthermore, for the selective preparation of compound 3,a na lternative synthetic route startingf rom [Au(CF 3 ) 3 (NCCH 3 )] is described. The 13 CNMR shifts of the carbene carbon atoms in compounds 1 and 3 can be correlated with the calculated dissociation energies of the AuÀ C carbene bond, revealing as ignificantly lower Lewis acidity compared to literature-known [AuF 2 X(SIMes)] or [AuX 3 (SIMes)] complexes (X = Cl, F, OTeF 5 ). These dissociation energies are in accordancew ith the trend in the AuÀC carbene bond lengthsi nt he [AuF 2 X(SIMes)] series. This article offers an ew synthetic route to the rare group of fluorido trifluoromethyl gold(III) complexes which could find interesting properties for gold-mediated coupling reactions.

Experimental Section
CAUTION!S trong Oxidizers!A ll reactions should be performed under strictly anhydrous conditions. The combination of AuF 3 with organic materials can lead to violent reactions. On contact with only small amounts of moisture, all used fluoride-containing compounds decompose under the formation of HF.T herefore, appropriate treatment procedures should be available in case of ac ontamination with HF-containing solutions. Materials, chemicals and procedures:A ll experiments were performed under rigorous exclusion of air and moisture using standard Schlenk techniques. All solids were handled in an MBRAUN UNIlab plus glovebox with an argon atmosphere (O 2 < 0.5 ppm, Figure 6. Correlationb etween the calculated SIMes affinity (ÀD r G diss )w ith the chemical shifts of the carbenec arbon atoms in the 13 CNMR spectra (d( 13 C carbene )) of trans-[Au(CF 3 )F 2 (SIMes)] (1), [Au(CF 3 ) 3 (SIMes)] (3)( bothh ighlightedi nb old)a nd similar, literature-known compounds. [43] The 13 Cc hemical shift of uncoordinatedS IMes, whichh as by definition aS IMes affinity of 0kJmol À1 ,isa lso included. [55]  H 2 O < 0.5 ppm). Solvents were dried using freshly ground CaH 2 in case of CH 2 Cl 2 ,C HCl 3 ,C D 2 Cl 2 and ortho-difluorobenzene, SICA-PENT in case of CH 3 CN, potassium in case of Et 2 Oa nd sodium in case of THF, n-pentane and n-hexane. Acetone was distilled prior to use. All solvents were stored over 4 molecular sieves, except for CH 3 CN, which was stored over 3 molecular sieves. AuF 3 , [56] 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene (SIMes) [57] and [Au(CF 3 ) 3 (NCCH 3 )] [42] were prepared using literatureknown methods. Raman spectra were recorded at room temperature using aB ruker MultiRAM FT-Raman spectrometer with a 1064 nm wavelength ND:YAG laser.T he spectra were measured directly inside the reaction flask with al aser power of 30 mW and 64 scans with ar esolution of 2cm À1 .I Rs pectra were measured at room temperature inside ag lovebox under argon atmosphere using aB ruker ALPHA FTIR spectrometer with ad iamond ATRa ttachment with 32 scans and ar esolution of 4cm À1 .R aman and IR spectra were processed using OPUS 7.5 and Origin 9.1 [58] was used for their graphical representation. NMR spectra were recorded using aJ EOL 400 MHz ECZ or ECS spectrometer and all chemical shifts are referenced as defined in the IUPAC recommendations of 2001. [59] MestReNova 14.0 was used to process the spectra and for their graphical representation. X-ray diffraction measurements were performed on aB ruker D8 Venture with MoK a (l = 0.71073 nm) radiation at 100 K. Single crystals were picked in perfluoroether oil at 0 8Cu nder nitrogen atmosphere and mounted on a0 .15 mm Mitegen micromount. They were solved using the ShelXT [60] structure solution program with intrinsic phasing and were refined with the refinement package ShelXL [61] using least squares minimizations by using the program OLEX2. [62] Diamond 3 and POV-Ray 3.7 were used for their graphical representation. Quantum chemical calculations were performed using the functional B3LYP [63] with RI [64] and Grimme-D3 [65] and the basis set def2-TZVPP [66] as incorporated in TURBOMOLE. [67] Preparation of [AuF 3 (SIMes)]:T he synthesis of [AuF 3 (SIMes)] was based on al iterature-known procedure [43] with slight deviations and upscaling of the synthesis. In at ypical experiment, AuF 3 (400 mg, 1.58 mmol, 1equiv.) and SIMes (483 mg, 1.58 mmol, 1equiv.) were dissolved in dichloromethane (20 mL) and stirred for 30 minutes at À80 8C. Ortho-difluorobenzene (20 mL) was added and the mixture was stirred for 30 minutes at À80 8C. Volatiles were removed under reduced pressure at À40 8Cu ntil ad ark grey precipitate was formed. The resulting yellowish solution was filtered off. ortho-Difluorobenzene (20 mL) was added to the solid residue, the mixture was stirred for 30 minutes at À40 8Ca nd the solution was filtered off. This washing process was repeated until the filtrated solution was colorless (usually,t hree times were sufficient). Dichloromethane (20 mL) was added and the mixture was stirred for 30 minutes at À80 8C, yielding ad ark solution. The mixture was filtered through ah ydrophobic PTFE filter (0.2 mm). ortho-Difluorobenzene (20 mL) was added to the colorless solution, the mixture was stirred for 30 minutes at À80 8Ca nd volatiles were removed at À40 8Cu ntil ac olorless precipitate was formed. The colorless solution was filtered off and residual solvent was removed under reduced pressure. The product (206 mg, 0.368 mmol, 23 %) was obtained as ac olorless powder. 1 17.8 mmol, 1equiv.) were dissolved in dichloromethane (1 mL) and at À196 8C, trimethyl(trifluoromethyl)silane (2.5 mg, 17.5 mmol, 1equiv.) was condensed onto this mixture. The mixture was allowed to warm up to À80 8C, the resulting solution was stirred for 1hour,w armed to room temperature and left stirring overnight. Main reaction products identified by 19 FNMR spectroscopy are trans-[Au(CF 3 )F 2 (SIMes)] (1)a nd cis-[Au(CF 3 ) 2 F(SIMes)] (2), HCF 3 and trans-[AuClF 2 (SIMes)] are formed as by-products (see Figures S9-S15). Single crystals of 1 suitable for X-ray diffraction were obtained by slow vapor diffusion of n-pentane into ad ichloromethane solution at 5 8C. Trifluoromethylation of [AuF 3 (SIMes)] in tetrahydrofuran:I na typical experiment, CsF (2.7 mg, 17.8 mmol, 1equiv.) was dissolved in tetrahydrofuran (1 mL) and at À196 8C, trimethyl(trifluoromethyl)silane (2.5 mg, 17.5 mmol, 1equiv.) was added. The mixture was allowed to warm up to À80 8Ca nd the resulting solution was stirred for 30 minutes. Thereafter,asolution of [AuF 3 (SIMes)] (10 mg, 17.8 mmol, 1equiv.) in tetrahydrofuran (1 mL) was added and the resulting mixture was stirred for 1h our at À80 8C, warmed to room temperature and left stirring overnight. Main reaction products identified by 19 FNMR spectroscopy are cis-  Crystallographic data:D eposition numbers 2001090, 2000997, 2000994, 2000995, and 2000996 (1, 3a, 3b, 3c,a nd 3d)c ontain the supplementary crystallographic data for this paper.T hese data are provided free of charge by the joint Cambridge Crystallographic Data Centre and FachinformationszentrumK arlsruhe Access Structures service.