A Highly Asymmetric Gold(III) η3‐Allyl Complex

Abstract A highly asymmetric AuIII η3‐allyl complex has been generated by treating Au(η1‐allyl)Br(tpy) (tpy=2‐(p‐tolyl)pyridine) with AgNTf2. The resulting η3‐allyl complex has been characterized by NMR spectroscopy and X‐ray crystallography. DFT calculations and variable temperature 1H NMR suggest that the allyl ligand is highly fluxional.

Transition-metal allyl complexes have been thoroughly studied and are key intermediates in av ariety of metalcatalysed organic reactions,s uch as the widely used Pdcatalysed Tsuji-Trost reaction which in one step gives access to highly functional compounds via nucleophilic addition to the h 3 allyl in ar egio-and stereospecific manner. [1,2] Despite that the allyl ligand is one of the classical unsaturated, delocalized ligands in organometallic chemistry,A u III h 3 -allyl complexes have been rarely described in the literature.There are acouple of reports on DFT calculations of such complexes and one experimental study in the gas phase using mass spectrometry techniques. [3,4] Af ew Au III h 1 allyl complexes [5] have been reported together with ah andful of Au I h 1 -allyl complexes. [6] Herein, we report for the first time the generation and full characterization of an isolable Au III h 3allyl complex.
Tr eatment of Au(OAc F ) 2 (tpy) (1;O Ac F = OCOCF 3 )w ith allylmagnesium bromide according to our previously developed methodology [7] led to the formation of Au(h 1 -allyl)Br-(tpy) (2), with the allyl group trans to tpy-N (Scheme 1, left). Complex 2 was obtained in 52-69 %y ield and characterized by NMR, MS,e lemental analysis and X-ray diffraction analysis. [13] Thec haracteristic resonances of the protons on the allyl ligand are observed in the 1 HNMR spectrum of 2; the three vinylic signals are found at d = 6.28 (H b ,see labelling in Scheme 1), 5.48 (H c ), and 5.02 (H d ). Thet wo allylic hydrogens H a are chemically equivalent and give rise to one resonance at d 3.39. A 1 H-1 HNOESY experiment established that the h 1 allyl ligand is located trans to tpy-N;aNOE is observed between H 6' and H a ,H b ,and H c (Figure 1).
Addition of AgNTf 2 to aC D 2 Cl 2 solution of 2 led to the formation of Au(h 3 -allyl)(tpy) (3)a st he major product (Scheme 1, right), together with traces of what appeared to be adecomposition product. Complex 3 was characterized by NMR and X-ray diffraction analysis. [13] Ac omparison of the 1 HNMR spectra of 3 and 2 (Table 1a nd Supporting Information) shows that H b and H d are found at higher chemical shift in 3 compared to in 2 (Dd = 0.22 (H b )and 0.66 (H d )), whereas H c is observed at alower d (Dd = À0.18). The two H a are found at ahigher chemical shift (Dd = 0.41). Complex 3 can be described by the two Lewis (resonance) structures 3a and 3b (Scheme 1). The 1 HNMR data, however, suggest the prevalence of one structure over the other;t hree protons are observed in the vinylic region (H b , H c ,a nd H d ;s ee Table 1) and the two H a are found at as ignificantly lower ppm value.T his is rather unusual for h 3 allyl complexes;normally the anti protons (defined relative to the central proton which is usually found at around d = 6.5;H b in complex 3)a re found at d = 1-3, whereas the syn protons are found at larger ppm values,a round d = 2-5. [2] Furthermore,t here is only as mall increase (by 14 Hz) in 1 J(H a -C2) going from 2 to 3 (Table 1) indicating that the sp 3 hybridization of C2 remains essentially unchanged. This result also agrees with the thermodynamic preference of having the high trans influence C(sp 3 )end of the allyl ligand trans to the lower trans influence ligand tpy-N,instead of the higher trans influence tpy-C,and leads us to infer the structural preference of 3a over 3b.I ns ymmetric h 3 -allyl complexes the syn and anti H a protons usually give rise to two distinct signals. However,i fd ouble bond decoordination [8] followed by rotation around the MCH 2 ÀCHCH 2 bond and re-coordination occurs relatively fast on the NMR time scale,t he resonances for these two protons will coalesce into one averaged resonance.T he fact that ac oalesced signal is seen for the two H a ,b ut not for H c and H d ,s uggests that double bond decoordination/recoordination of the h 3 -allyl ligand occurs selectively trans to the tpy-C atom in 3a.Noevidence is seen in the NMR spectra for an analogous process starting from Lewis structure 3b which would lead to acoalescence of the resonances of H c and H d .T his supports the notion that resonance structure 3b is am inor contributor due to the unfavourable trans relationship between the C(sp 3 )end of the allyl group and the coordinating tpy-C atom.
Thestructure and dynamic behaviour of 3 were explored by DFT calculations at the PBE0 level, including solvation by dichloromethane (see Supporting Information for computa-tional details). Theo ptimized structure shows inequivalent CÀCbonds in the allyl ligand of 1.438 and 1.382 for C2-C3 and C3-C4, respectively,i na greement with 3a as the predominant Lewis structure (Scheme 1). Double bond decoordination to furnish an h 1 -allyl species occurred favourably only trans to the coordinating tpy-C atom and led to two structures with the empty coordination site trans to the tpy-C atom and the h 1 -allyl trans to the tpy-N atom (4,1 2.3 kcal mol À1 ;a nd 5,1 1.9 kcal mol À1 ,S cheme 2). Interestingly,t wo different TSs of similar energies (TS3-4 and TS5-3' ')w ere located connecting these two h 1 allylic intermediates with enantiomers 3 and 3' ',indicating the existence of two TSs for the double bond decoordination. Starting from ag iven enantiomer,t hese TSs correspond to clockwise and counterclockwise rotations of the AuÀCbond (see ESI). Thetwo h 1allyl intermediates 4 and 5 are also connected by aT S involving rotation of the s(C2-C3) bond (TS4-5). Theenergy associated with this TS (17.7 kcal mol À1 )i st he highest in the computed energy landscape that facilitates the exchange of H a and H a' ,with barriers that are consistent with aprocess that occurs at room temperature.
Theprocess described in Scheme 2(red pathway) involves the interconversion of one enantiomer of 3 to its enantiomeric counterpart (3' ')via achiral pathway.Ithas been argued [9] that such ap rocess is not in violation of the principle of microscopic reversibility provided that there exists ad egenerate alternative pathway,o fo pposite chirality but energetically degenerate to the first one (see Supporting Information). Burkey and co-workers [10] recently reported metallacycle ring inversions that were suggested to occur by chiral, degenerate Scheme 1. Top: Generation of Au III h 1 -a nd h 3 -allyl complexes 2 and 3.B ottom:C rystallographicstructure determination of 2 (left). [13] Owing to twinning and disorder limiting the high-resolution diffraction in the measured crystal, only Au and Br are refined as thermal ellipsoids (set at 50 % probability).O RTEP plot of the cationic part of complex 3 with thermal ellipsoids set at 50 %p robability (right).   pathways.I nterestingly,t he interconversion of the enantiomers 3 and 3' ' by this pathway does not involve a C s symmetric intermediate or transition state which might be considered to arise from double bond decoordination and Au À Ca nd C À C bond rotations.O ptimization of the h 1 -allyl geometry within C s symmetry constraints leads to a C s symmetric TS at 17.9 kcal mol À1 (blue pathway,S cheme 2). This transition state was found to directly connect 3 and 3' '.T he similar energies obtained for the symmetric pathway and the chiral one (Scheme 2) suggest the co-existence of the two pathways at the experimental conditions. Complex 3 slowly decomposes at ambient temperature and complete NMR characterization was therefore performed at 7 8 8C. Ther esonances of H a ,H d ,a nd H 6 ,a sw ell as several of the 13 CNMR resonances are broadened at this temperature (see Supporting Information). Thetemperaturedependent broadening phenomena in the 1 Ha nd 13 CNMR spectra further support the dynamic behaviour of the allyl ligand on the NMR time scale.S elected key 1 H-1 HN OE correlations in complex 3 are depicted in Figure 1. AN OE between H d and H 6 is observed, which is not observed in complex 2,i ndicating ac oordination of the double bond to Au, trans to the tpy-C atom. In contrast, H c (bonded to the same CasH d )shows aNOE with H 6' ,but upon increasing the intensity of the peaks in the NOESY spectrum, what appears to be aweak NOE between H c and H 6 becomes visible.These observations might indicate that 3,with the allyl ligand bound in an h 3 fashion, interconverts to the corresponding h 1 -allyl complex during the time scale of the NMR experiment, as depicted in Scheme 2.
Assuming the behaviour depicted in Scheme 2, af urther slowing of the process by lowering the temperature will cause the resonance of the two H a to split into two signals.T hus, decreasing the temperature to À42.3 8 8Cl ed to significant broadening of the resonances of H a ,H c ,a nd H d in the 1 HNMR spectrum of 3 (see Figure 2). At this point, the signals of H 6 (see Supporting Information) and H b are also broadened, but to alesser extent. Interestingly,upon lowering the temperature further, the resonance of H a undergoes decoalescence and eventually emerges as three resonances.A t À55.5 8 8Ct hese are significantly broadened and are barely discernible as three featureless,b roadened distortions of the baseline.A tÀ79.2 8 8Ct hese resonances,a td = 4.26, 3.82, and 3.09, are sharper and integrate for approximately 1H, 1H, and 2H, respectively (see the spectrum at the bottom of Figure 2 and Supporting Information). At this temperature,t wo resonances are also observed for H d (each integrating for ca. 1H), whereas the signals of H b and H c each appear as one broadened resonance (ca. 2H each). Furthermore,two sets of peaks for most of the resonances of the tpy ligand are observed (see Supporting Information). Theb roadening/ coalescence behaviour is reversible,a se videnced by the restoration of signals upon sample heating.B ased on these observations it is suggested that there is an interconversion between the h 3 -allyl complexes 3 and 3' ',a nd the h 1 -allyl complexes 4 and 5 (perhaps with NTf 2 or solvent coordinated trans to the tpy-C atom) in solution (Scheme 2). At À79.2 8 8C, this process is slow enough to enable the detection of coexisting h 3 (3/3' ')a nd h 1 (4/5,w ith an eventual coordinated Scheme 2. Double bond decoordination and subsequent rotation and recoordination in complex 3 as shown will cause an averagingoft he resonances of the two H a into one signal. Optimizedg eometries (PBE0-D3, SDD/6-311 + G**, SMD = dichloromethane) and DG energies (kcal mol À1 )for all intermediates and TSs involved in the equilibria between 3 and its enantiomer.Red = chiral pathway,blue = symmetric pathway.S ee text for details. counteranion or solvent molecule) forms by 1 HN MR spectroscopy.I na nh 1 -allyl complex, the two H a are chemically equivalent, and therefore it is suggested that the resonance at d = 3.09 arises from such ac omplex;t his chemical shift is slightly lower than that observed for the two chemically equivalent H a in h 1 -allyl complex 2 (d = 3.39) and nearly the same as that in [Au(h 1 -allyl)(CD 3 CN)(tpy)] + -[NTf 2 ] À (d = 3.12, see Supporting Information). Theresonances at d = 4.26 and 3.82 are thus assigned to h 3 -allyl complex 3. Based on the findings from low-temperature NMR spectroscopy,w hat is observed by 1 HNMR spectroscopy at room temperature is not strictly an h 3 -allyl complex, but rather the averaged signals arising from the h 3 -h 1 -h 3 interconversions whereby complex 3 interconverts to and equilibrates with an h 1 -allyl complex.
TheDFT free energies obtained for intermediates 4 and 5 do not account for the existence of h 1 allyl intermediates in solution. However,upon coordination of NTf 2 at Au (4-NTf 2 and 5-NTF 2 ,s ee Supporting Information), these species became almost isoenergetic to the h 3 -allyl complex 3 (DG = À3.6 kcal mol À1 ). [11] Therefore,t he equilibrium observed in solution may involve coordination and decoordination of NTf 2 (see Figure S33).
Crystallographic structure determination of complexes 2 and 3 were performed and selected parameters are given in Scheme 1. As can be seen from Scheme 1, complex 2 is an h 1 allyl complex with the allyl trans to the tpy-N atom and Br trans to the coordinating tpy-C atom, in full agreement with the NMR data. In complex 3,the double bond of the allyl has coordinated trans to tpy-C to form an h 3 -allyl complex as depicted in Scheme 1.
In 3,the C(sp 3 )end of the allyl ligand (C2) is more tightly bound to Au than the C(sp 2 )C =Ccarbon atoms (C3 and C4) with Au À Cb onds of 2.062(19), 2.21(2) and 2.35(2) , respectively,i ndicative of ah ighly asymmetric allyl complex which is best described by Lewis structure 3a and not 3b.
Theallyl ligand in 3 is more asymmetrically bonded than what is seen in related Pd II (N,C)c yclometalated complexes reported previously [12] (where the chelate N is ap yridine-N atom, and the chelate C is either an aryl-C or aNHC-C atom), with Pd-allyl bonds of 2.105(5)/2.095(4) (Pd-C2), 2.135(5)/ 2.152(4) (Pd-C3) and 2.257(5)/2.222(5) (Pd-C4). In addition, in complex 3,the DFT determined C2-C3 distance is significantly longer than C3-C4 (1.438 vs.1 .382 ), again indicating an asymmetric allyl complex. Thed istances are taken from DFT calculations because the experimental CÀC bond lengths of the allyl ligand have ahigh uncertainty due to the absence of high resolution diffraction signals,p robably originating from disorder and twinning in the crystals. However,t he differences in crystallographically determined bond lengths are still significant. DFT calculations were also used to determine the geometry expected for the isoelectronic,n eutral complex Pt(h 3 -allyl)(tpy) ( Figure S34). In this case,k ey bond lengths were found to be 1.433 for C2-C3, 1.402 for C3-C4, 2.091 for Pt-C2 and 2.221 for Pt-C4. While this system is also highly asymmetric,t he differences between the CÀCand MÀCbond lengths are larger for Au III (0.056 and 0.239 ,r espectively,f or M = Au ;0 .031 and 0.130 ,respectively,f or M = Pt).
In conclusion, we have generated and fully characterized the first Au III h 3 -allyl complex. [14] NMR spectroscopy and XRD analysis together with DFT calculations show that the allyl ligand bound to Au is highly asymmetric.T his asymmetric bonding appears to be dictated by the different trans influence of the coordinating atoms of the ancillary ligands (tpy-N vs.t py-C). We are currently investigating how this asymmetry will affect the reactivity of this class of complexes.