Synthetic and Structural Studies of [AuCl3(NHC)] Complexes

394

Organometallics 2010, 29, 394–402 DOI: 10.1021/om900814e

Synthetic and Structural Studies of [AuCl3(NHC)] Complexes Sylvain Gaillard,† Alexandra M. Z. Slawin,† Allen T. Bonura,‡ Edwin D. Stevens,‡ and Steven P. Nolan*,† †

School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, United Kingdom and ‡Department of Chemistry, University of New Orleans, 2000 Lakeshore Drive, New Orleans, Louisiana 70148 Received September 17, 2009

A series of N-heterocyclic carbene complexes [AuCl(NHC)] 1 [NHC: IPr, a; SIPr, b; IPrMe, c; IPrCl, d; IMes, e; SIMes, f; ItBu, g; IAd, h; and ICy, i] were reacted with chlorine gas or PhICl2 to afford [AuCl3(IPr)], 2a; [AuCl3(SIPr)], 2b; [AuCl3(IPrMe)], 2c; [AuCl3(IPrCl)], 2d; [AuCl3(IMes)], 2e; [AuCl3(SIMes)], 2f; [AuCl3(ItBu)], 2g; [AuCl3(IAd)], 2h; and [AuCl3(ICy)], 2i, respectively. Complete characterization by 1H and 13C NMR spectroscopies as well as by single-crystal X-ray diffraction was performed in order to discern electronic and structural differences between organogold(I/III) congeners. Calculations of the buried volume of the NHC ligand in complexes [AuCl3(NHC)] 2 were also performed in order to evaluate the steric hindrance of the NHC. These data permit comparisons with the related complexes [AuCl(NHC)] 1 and [AuBr3(NHC)]. Introduction During the past decade, organogold chemistry and catalysis have undergone a veritable rebirth. The most commonly used species in gold catalysis are gold(I/III) salts or gold(I) complexes with ligands such as phosphines and NHCs (N-heterocyclic carbenes).1 Among the wide range of applications of gold, gold(I) complexes have proven to be particularly efficient in natural product synthesis.2 In comparison, organogold(III) complexes are rarely described as *Corresponding author. Fax: (þ) 44(0)1334463808. E-mail: snolan@ st-andrews.ac.uk. (1) (a) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180–3211. (b) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239–3265. (c) Arcadi, A. Chem. Rev. 2008, 108, 3266–3325. (d) Jim
enez-N
u~nez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326–3350. (e) Skouta, R.; Li, C.-J. Tetrahedron 2008, 64, 4917–4938. (f) Muzart, J. Tetrahedron 2008, 64, 5815– 5849. (g) Gorin, D. J.; Sherry, B. D.; Toste, F. D. Chem. Rev. 2008, 108, 3351–3378. (h) Díez-Gonz
alez, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612–3676. For reviews on (NHC) gold complexes, see: (i) Glorius, F., Ed. N-Heterocyclic Carbenes in Transition Metal Catalysis; Springer-Verlag: Berlin, Germany, 2007. (j) Nolan, S. P., Ed. N-Heterocyclic Carbenes in Synthesis; Wiley-VCH: Weinheim, Germany, 2006. (k) Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776–1782. (l) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122–3172. (2) Hashmi, A. S. K.; Rudolph, M. Chem. Soc. Rev. 2008, 37, 1766–1775. (3) (a) Casado, R.; Contel, M.; Laguna, M.; Romero, P.; Sanz, S. J. Am. Chem. Soc. 2003, 125, 11925–11935. (b) Hashmi, A. S. K.; Weyrauch, J. P.; Rudolph, M.; Kurpejovic, E. Angew. Chem., Int. Ed. 2004, 43, 6545–6547. (c) Lo, V. K.-Y.; Liu, Y.; Wong, M.-K.; Che, C.-M. Org. Lett. 2006, 8, 1529–1532. (4) (a) Marcon, G.; Carotti, S.; Coronnello, M.; Messori, L.; Mini, E.; Orioli, P.; Mazzei, T.; Cinellu, M. A.; Minghetti, G. J. Med. Chem. 2002, 45, 1672–1677. (b) Messori, L.; Marcon, G.; Orioli, P. Bio. Inorg. Chem. Appl. 2003, 177–187. (c) Messori, L.; Marcon, G.; Cinellu, M. A.; Coronnello, M.; Mini, E.; Gabbiani, C.; Orioli, P. Bioorg. Med. Chem. 2004, 12, 6039–6043. (d) Barnard, P. J.; Berners-Price, S. J. Coord. Chem. Rev. 2007, 251, 1889–1902. (e) Raubenheimer, H. G.; Cronje, S. Chem. Soc. Rev. 2008, 37, 1998–2011. (f) Milacic, V.; Dou, Q. P. Coord. Chem. Rev. 2009, 253, 1649–1660. (g) Abdou, H. E.; Mohamed, A. A.; Fackler, J. P., Jr.; Burini, A.; Galassi, R.; L
opez-de-Luzuriaga, J. M.; Olmos, M. E. Coord. Chem. Rev. 2009, 253, 1661–1669. (h) Ott, I. Coord. Chem. Rev. 2009, 253, 1670–1681. (i) Hindi, K. M.; Panzer, M. J.; Cannon, C. L.; Youngs, W. J. Chem. Rev. 2009, 109, 3859–3884. (j) Teyssot, M. L.; Jarousse, A. S.; Manin, M.; Chevry, A.; Roche, S.; Norre, F.; Beaudoin, C.; Morel, L.; Boyer, D.; Mahiou, R.; Gautier, A. Dalton Trans 2009, 6894–6902. pubs.acs.org/Organometallics

Published on Web 12/18/2009

catalytic mediators.3 Moreover, in the past few years, gold(I/ III) complexes have found renewed interest as anticancer drugs4 and have also been investigated for their photoluminescence properties.5 In spite of this recent attention, a limited number of reports have highlighted the synthesis and characterization of well-defined neutral gold(III) trihalide complexes. To the best of our knowledge, [AuCl3(Py)] and [AuBr3(Py)] (Py = pyridine) were the first gold(III) complexes, reported by Gibson and Colles in 1931.6 Since then, the synthesis of [AuI3(arsine)],7 [AuX3(phosphine)] (X=Cl and I),7,8 [AuX3(Rphen)] (Rphen=2,9-dialkyl1,10-phenanthroline) (X= Cl and Br),9 and [AuX3(NHC)]10 (5) (a) Yam, V. W.-W.; Lo, K. K.-W. Chem. Soc. Rev. 1999, 28, 323– 334. (b) Brandys, M. C.; Puddephatt, R. J. J. Am. Chem. Soc. 2001, 123, 4839–4840. (c) Yam, V. W.-W.; Chan, C.-L.; Li, C.-K.; Wong, K. M.-C. Coord. Chem. Rev. 2001, 216-217, 173–194. (d) White-Morris, R. L.; Olmstead, M. M.; Balch, A. L. J. Am. Chem. Soc. 2003, 125, 1033–1040. (e) Fern
andez, E. J.; Laguna, A.; L
opez-de-Luzuriaga, J. Dalton Trans. 2007, 1969–1981. (f) Jothibasu, R.; Huynh, H. V.; Koh, L. L. J. Organomet. Chem. 2008, 693, 374–380. (g) Jean-Baptiste dit Dominique, F.; Gornitzka, H.; Sournia-Saquet, A.; Hemmert, C. Dalton Trans. 2009, 340–352. (6) Gibson, C. S.; Colles, W. M. J. Chem. Soc. 1931, 2407–2416. (7) Godfrey, S. M.; Ho, N.; McAuliffe, C. A.; Pritchard, R. G. Angew. Chem., Int, Ed. Engl. 1996, 35, 2343–2346. (8) (a) Bhargava, S. K.; Mohr, F.; Bennett, M. A.; Welling, L. L.; Willis, A. C. Organometallics 2000, 19, 5628–5635. (b) Schneider, D.; Schier, A.; Schmidbaur, H. Dalton Trans. 2004, 1995–2005. (c) Schneider, D.; Schuster, O.; Schmidbaur, H. Dalton Trans. 2005, 1940–1947. (d) Teets, T. S.; Nocera, D. G. J. Am. Chem. Soc. 2009, 131, 7411–7420. (9) (a) Robinson, W. T.; Sinn, E. J. Chem. Soc., Dalton Trans. 1975, 726–731. (b) Hudson, Z. D.; Sanghvi, C. D.; Rhine, M. A.; Ng, J. J.; Bunge, S. D.; Hardcastle, K. I.; Saadein, M. R.; MacBeth, C. E.; Eichler, J. F. Dalton Trans. 2009, 7473–7480. (10) (a) Raubenheimer, H. G.; Olivier, P. J.; Lindeque, L.; Desmet, M.; Hrusak, J.; Kruger, G. J. J. Organomet. Chem. 1997, 544, 91–100. (b) de Fr
emont, P.; Singh, R.; Stevens, E. D.; Petersen, J. L.; Nolan, S. P. Organometallics 2007, 26, 1376–1385. (c) Kessler, F.; Szesni, N.; Maass, C.; Hohberger, C.; Weibert, B.; Fischer, H. J. Organomet. Chem. 2007, 692, 3005–3018. (d) Samantaray, M. K.; Pang, K.; Shaikh, M. M.; Ghosh, P. Dalton Trans. 2008, 4893–4902. (e) Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B. Chem. Rev. 2009, 109, 3561– 3598. (f) Gaillard, S.; Bantreil, X.; Slawin, A. M. Z.; Nolan, S. P. Dalton Trans. 2009, 6967–6971. (g) Han, X.; Koh, L. L.; Weng, Z.; Hor, T. S. A. Dalton Trans. 2009, 7248–7252. r 2009 American Chemical Society

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Figure 1. NHC ligands a-i used in this study. Scheme 1. Oxidative Addition of Chlorine on [AuCl(IPrMe)], 1c

(X=Cl, Br, and I) have been achieved. Among [AuX3(NHC)] complexes, only one study has appeared on the synthesis of complexes bearing imidazolyl as NHC and chloride as halide atoms.10a In this context, the development of new [AuCl3(NHC)] complexes and novel, straightforward strategies to synthesize them continue to be an area of great activity and interest. Recently, we have reported the synthesis and characterization of a series of [AuBr3(NHC)] complexes.10b Catalytic activity of such complexes was also investigated and compared to gold(III) salts in alkyne hydration and in the polymerization of styrene.11 It is suspected that the active gold species in this chemistry is in fact a (NHC)Au(I) species generated by reductive elimination of the halogen. As the Au-Cl bond should be stronger than the analogous Au-Br bond, the synthesis of [AuCl3(NHC)] 2 complexes was next targeted. In a previous study, we reported preliminary results on (NHC)gold(III) chloride synthesis bearing 4,5-dimethyl-N,N0 -bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPrMe) c (Figure 1) by oxidative addition of chlorine gas on the corresponding gold(I) complexes.10f In this instance, an interesting mixture of two products with one resulting from C-H bond activation of a backbone methyl group was obtained (Scheme 1). Here we report on the oxidation of [AuCl(NHC)] 1 complexes with chlorine gas or PhICl2 as chlorinating agent leading to [AuCl3(NHC)] 2 complexes (Figure 1). We also present an analysis of the steric hindrance of different NHCs coordinated around the gold center (Figure 1), by calculating the buried volume (%VBur) value for these ligands in the present relatively unencumbered gold systems.

Results Initially, the synthesis of [AuCl3(IPr)], 2a, was attempted via a substitution reaction involving the trichloropyri(11) Urbano, J.; Hornigo, J.; de Fr
emont, P.; Nolan, S. P.; D
azRequejo, M. M.; P
erez, P. J. Chem. Commun. 2008, 759–761.

Scheme 2. Oxidative Addition of Chlorine Gas on [AuCl(IPr)], 1a

dinegold(III)12 complex with the free carbene IPr a (N,N0 bis(2,6-diisopropylphenyl)imidazol-2-ylidene). This procedure led to the formation of the expected complex 2a, with concomitant formation of [AuCl(IPr)], 1a, as the major product resulting from gold reduction.10b An alternative route, based on earlier work of Raubenheimer et al.,10a,13 and by analogy with our previous study on the [AuBr3(NHC)] synthesis,10b was envisaged where chlorine gas14 is used as an oxidative reagent enabling the synthesis of [AuCl3(NHC)] 2. Bubbling chlorine gas in a solution of commercially available [AuCl(IPr)],15 1a, in dichloromethane at room temperature resulted in significant decomposition of 1a. Nevertheless, when the reaction was carried out at -78 °C and slowly warmed to room temperature over 5 h, the expected [AuCl3(IPr)], 2a, was obtained as a pale yellow powder in 95% yield (Scheme 2). The use of low temperature at the beginning of the reaction caused the chlorine gas condensation (bp = -34 °C) and ultimately diminishes its oxidative activity. 1 H NMR spectroscopic analysis provided similar chemical shifts to those found for [AuBr3(IPr)]. The most significant (12) Gibson, C. S.; Colles, W. M. J. Chem. Soc. 1931, 2407–2416. (13) K€ uhlkamp, P.; Raubenheimer, H. G.; Field, J. S.; Desmet, M. J. Organomet. Chem. 1998, 552, 69–74. (14) CAUTION: the use of chlorine gas should be conducted in a wellventilated hood. (15) Fructos, M. R.; Belderrain, T. R.; de Fr
emont, P.; Scott, N. M.; Nolan, S. P.; D
az-Requejo, M. M.; P
erez, P. J. Angew. Chem., Int. Ed. 2005, 44, 5284–5288.

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Gaillard et al. Scheme 3. Synthesis of [AuCl(IPrCl)], 1d

Figure 2. Ball-and-stick representation of [AuCl3(IPr)], 2a. H atoms are omitted for clarity.

difference is an upfield shift in the septet assigned to the isopropyl proton (2a: 2.85 ppm; [AuBr3(IPr)]: 2.99 ppm) due to the difference in electronegativity of the halogen atoms. In the 13C NMR spectrum, the chemical shift of the carbenic carbon of the IPr in 2a is similar to the analogous bromide complexes (2a: 145.7 ppm; [AuBr3(IPr)]: 146.2 ppm). X-ray-quality crystals of complex 2a were grown by slow evaporation of a saturated solution of the crude product in dichloromethane/hexane (Figure 2).16 The gold atom is fourcoordinate, as usual for gold(III) complexes, and exhibits a square-planar geometry. Selected bond distances and angles are summarized in Tables 3 and 4. In an effort to extend our methodology to other NHCs, we synthesized a series of [AuCl(NHC)] [NHC = N,N0 -bis(2,6diisopropylphenyl)imidazolin-2-ylidene (SIPr), 1b; 4,5-dimethyl-N,N0 -bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPrMe), 1c; 4,5-dichloro-N,N0 -bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPrCl), 1d; N,N0 -bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), 1e; N,N0 -bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (SIMes), 1f; N,N0 -di-tert-butylimidazol2-ylidene (ItBu), 1g; N,N0 -diadamantylimidazol-2-ylidene (IAd), 1h; N,N0 -dicyclohexylimidazol-2-ylidene (ICy), 1i] (Figure 1).17 For the new complex 1d, 5 was first synthesized from 4 by modification of the Arduengo procedure.18 The silver NHC complexes are often employed as NHC transfer agents, and the related [AgCl(IPrCl)], 6, can be synthesized in low yield (28%) using the procedure involving silver(I) oxide and 5.19 Finally, complex 1d was prepared in 31% yield by a transmetalation reaction of IPrCl d from [AgCl(IPrCl)] (6) to [(Me2S)AuCl]20 (Scheme 3).10b,19,21 Due to the low overall yield (7.3%) of 6 from 3 and the long time reaction (1 week) of this route, a onepot multistep reaction was attempted. Initially following the Arduengo procedure18 for d, IPr 3 HBF4 (4) was deprotonated (16) CCDC 743930-743940 for compounds 1d, 2a-i, and 6 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. (17) de Fr
emont, P.; Scott, N. M.; Stevens, E. D.; Nolan, S. P. Organometallics 2005, 24, 2411–2418. (18) Arduengo, A. J.III; Krafczyk, R.; Schumutzler, R.; Craig, H. A.; Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999, 55, 14523–14534. (19) (a) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972– 975. (b) Schneider, S. K.; Hermann, W. A.; Herdtweck, E. Z. Anorg. Allg. Chem. 2003, 629, 2363–2370. (c) Hu, X.; Castro-Rodriguez, I.; Olsen, K.; Meyer, K. Organometallics 2004, 23, 755–764. (20) Dash, K. C.; Schmidbaur, H. Chem. Ber. 1973, 106, 1221–1225. (21) Wang, H. M. J.; Vasam, C. S.; Tsai, T. Y. R.; Chen, S.-H.; Chang, A. H. H.; Lin, I. J. B. Organometallics 2005, 24, 486–493.

by NaH in the presence of a catalytic amount of KOtBu to generate the free carbene IPr, a. Then, CCl4 was added to afford the free carbene IPrCl d. Finally, following the strategy of Sadighi,22 [(Me2S)AuCl] was added to the solution of d to furnish directly [AuCl(IPrCl)], 1d, after 2 days with a 56% yield (Scheme 3). The slow transmetalation of IPrCl d from 6 to [(Me2S)AuCl] may very well be caused by the electronic and structural properties of 6. In the 13C NMR, the coupling constants J109/107Ag-13C for 6, which depend on the magnetogyric ratio of these nuclei,23 were resolved at 273.8 and 236.9 Hz, respectively. These important values of J109/107Ag-13C indicate a relatively strong Ag-Ccarbene bond.24 To unambiguously characterize 6 and 1d, X-ray-quality crystals of both complexes were grown by slow evaporation of mixtures of chloroform/hexane.16 Ball-and-stick representations are provided in Figure 3. From these data, the Ag-Ccarbene bond length of 6 was calculated as 2.064(12) A˚ and revealed a very short AgCcarbene bond compared to previously reported bond distances (between 2.056 and 2.095 A˚).24,25 It thus confirmed the strong interaction between the ligand and the silver atom, which was suspected as the cause for the extremely slow transmetalation of d. The particularity of ligand d is also evident in [AuCl(IPrCl)], 1d. The Au-Ccarbene bond is one of the shortest observed in the [AuCl(NHC)] 1 series, with a bond length of 1.91(2) A˚, which typically range from 1.942 to 2.018 A˚ for Au-Ccarbene in 1. Nevertheless, the chemical shift of the carbene in 13C NMR was found at 175.1 ppm, which is very close to that found for 1a (175.5 ppm) (Table 2). All structural and spectroscopic data indicate that the substitution at the backbone may very well influence the metalligand interaction and possibly also catalytic activity. Once all [AuCl(NHC)] complexes 1a-i were on hand, we explored the compatibility of the reaction shown in Scheme 3 with other NHC ligands presented in Figure 1. The results of the oxidative addition of chlorine [using either Cl2 gas (method A) or PhICl2 (method B)] onto gold(I) precursors to furnish (NHC)gold(III) are summarized in Table 1. Method A. Carefully bubbling chlorine gas in a solution of [AuCl(NHC)] 1 furnished the expected [AuCl3(NHC)] 2 as pale yellow air-stable powders with yields ranging from 79% (22) Akana, J. A.; Bhattacharyya, K. X.; M€ uller, P.; Sadighi, J. P. J. Am. Chem. Soc. 2007, 129, 7736–7738. (23) Arduengo, A. J.III; Dias, H. V. R.; Calabrese, J. C.; Davidson, F. Organometallics 1993, 12, 3405–3409. (24) (a) Ramnial, T.; Albernethy, C. D.; Spicer, M. D.; McKenzie, I. D.; Gay, I. D.; Clyburne, J. A. C. Inorg. Chem. 2003, 42, 1391–1393. (b) Garrison, J. C.; Youngs, W. J. Chem. Rev. 2005, 105, 3978–4008. (c) de Fr
emont, P.; Scott, N. M.; Stevens, E. D.; Ramnial, T.; Lightbody, O. C.; Macdonald, C. L. B.; Clyburne, J. A. C.; Abernethy, C. D.; Nolan, S. P. Organometallics 2005, 24, 6301–6309. (25) Tapu, D.; Dixon, D. A.; Roe, C. Chem. Rev. 2009, 109, 3385– 3407.

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Figure 3. Ball-and-stick representation of [AgCl(IPrCl)] (6) (left) and [AuCl(IPrCl)] (1d) (right). H atoms are omitted for clarity. Selected bond distances (A˚) and angles (deg), for 6: Ag1-C1 2.064(12), Ag1-Cl1 2.278(4), C1-Ag1-Cl1 180.000(1). For 1d: Au1-Ccarbene 1.91(2), Au1-Cl1 2.280(6), Ccarbene-Au1-Cl1 178.1(7). Table 1. Synthesis of [AuCl3(NHC)] Complexes

AuClðNHCÞ method A or B AuCl3 ðNHCÞ s 1 2 CH2 Cl2 , T , t yield of 2 (%)a entry

AuCl(NHC)

method A

method B

1 2 3 4 5 6 7 8 9

IPr 1a SIPr 1b IPrMe 1c IPrCl 1d IMes 1e SIMes 1f ItBu 1g IAd 1h ICy 1i

95 90 88b 80 79c 0d 96 91 0d

95 92 90 92 94 95 96 92 91

a Isolated yield. Method A: excess of chlorine was bubbled at -78 °C in a solution of (NHC)gold(I) 1 in dichloromethane, and the reaction mixture was slowly warmed to room temperature in 5 h under stirring (except for 1e, 1f, and 1i, which ran at -78 °C during 7 h). Method B: In a vial, (NHC)gold(I) 1 (1 equiv) and PhICl2 (1.1 equiv) were dissolved in dichloromethane and stirred at room temperature overnight. b Mixture of 2c and 3 was obtained in the ratio 1.5:1. c Reaction carried out at -78 °C during 7 h. d Product detected among decomposition.

to 96% (Table 1, entries 1-5 and 7, 8). In the case of 1c (Table 1, entry 3), we previously reported a concomitant formation of 3 with [AuCl3(IPrMe)] (2c), resulting from a CH insertion on the methyl framework of the backbone (Scheme 1).10f Another precedent has been reported by Ghosh et al. with bromine, which reacted with a framework of the ligand after the oxidation of the metal center.10d It is noteworthy that no addition of chlorine to the double bond of the unsaturated carbenes was detected. Under these conditions, attempts to synthesize [AuCl3(IMes)] (1e) and [AuCl3(SIMes)] (1f) failed and led only to decomposition products. Nevertheless, when the reactions were carried out at -78 °C during 7 h, 1e was obtained with 79% yield and 1f was also observed but was surprisingly unstable under these conditions (Table 1, entries 5 and 6). In the case of [AuCl3(ICy)] (1i), only decomposition was obtained even if the reaction mixture was kept at -78 °C (Table 1, entry 9). Method B. To improve and generalize the syntheses of [AuCl3(NHC)] 2, we turned our attention to an alternative

chlorinating agent. The use of the chlorine-based oxidant iodobenzene dichloride (PhICl2),26 already reported for the oxidation of Pt(II),27 Pd(II),28 and Mo(II),29 metals of group V,30 group VI,31 and Au(I)8a,d was explored. Fortunately, in the presence of 1.1 equiv of PhICl2 in dichloromethane at room temperature, all (NHC)gold(I) precursors 1a-i were quantitatively and cleanly oxidized into the expected [AuCl3(NHC)] complexes 2a-i with yields ranging from 90% to 96% (Table 1). By this method, all yields were generally improved. Due to these softer conditions, the selectivity of the reaction was also improved, especially in the case of [AuCl(IPrMe)] 1c (Table 1, entry 3), where no byproduct 3 was observed (Scheme 1). Finally, among the two methods, the second with the use of PhICl2 presents several advantages. This chlorinating reagent is easily handled and can be quickly prepared on a large scale.26b The use of PhICl2 also permits conducting the reactions at room temperature, which is more economical and practical than the protocol of method A.

Discussion H and 13C NMR Data. The 1H NMR spectra of complexes bearing unsaturated NHCs without substitution on the backbone, 2a, 2e, 2g, 2h, and 2i, displayed a low-field singlet between 7.30 and 7.50 ppm assigned to the two protons of the imidazole backbone, versus the range of 1

(26) (a) Zielinska, A.; Skulski, L. Tetrahedron Lett. 2004, 45, 1087– 1089. (b) Zhao, X.-F.; Zhang, C. Synthesis 2007, 4, 551–557. (27) (a) Baxter, L. A. M.; Heath, G. A.; Raptis, R. G.; Willis, A. C. J. Am. Chem. Soc. 1992, 114, 6944–6946. (b) Bennett, M. A.; Bhargava, S. K.; Bond, A. M.; Edwards, A. J.; Guo, S.-X.; Priv
er, S. H.; Rae, A. D.; Willis, A. C. Inorg. Chem. 2004, 43, 7752–7763. (c) Bennett, M. A.; Bhargava, S. K.; Messelh€ausser, J.; Priv
er, S. H.; Welling, L. L.; Willis, A. C. Dalton Trans. 2007, 3158–3169. (d) Whitfield, S. R.; Sanford, M. S. Organometallics 2008, 27, 1683–1689. (e) Mamtora, J.; Crosby, S. H.; Newman, C. P.; Clarkson, G. J.; Rourke, J. P. Organometallics 2008, 27, 5559–5565. (28) (a) Cotton, F. A.; Koshevoy, I. O.; Lahuerta, P.; Murillo, C. A.; Sana
u, M.; Ubeda, M. A.; Zhao, Q. J. Am. Chem. Soc. 2006, 128, 13674– 13675. (b) Whitfield, S. R.; Sandford, M. S. J. Am. Chem. Soc. 2007, 129, 15142–15143. (29) Baya, M.; Houghton, J.; Daran, J.-C.; Poli, R.; Male, M.; Albinati, A.; Gutman, M. Chem.-Eur. J. 2007, 13, 5347–5359. (30) (a) Witte, P. T.; Meetsma, A.; Hessen, B. Organometallics 1999, 18, 2944–2946. (b) Hayton, T. W.; Legzdins, P.; Patrick, B. O. Inorg. Chem. 2002, 41, 5388–5396. (31) Filippou, A. C.; Winter, J. G.; Kociok-K€ ohn, G.; Troll, C.; Hinz, I. Organometallics 1999, 18, 2649–2659.

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Table 2. Chemical Shift (ppm) of the Carbenic Carbon (δC) of Free NHCs, NHC Salts, and [AuCl(NHC)] 1, [AuCl3(NHC)] 2, and [AuBr3(NHC)] Complexes10b,f,17,18,25,33

NHC IPr a SIPr b IPrMe c IPrCl d IMes e SIMes f ItBu g IAd h ICy i

δC of free NHC

δC of NHC 3 HCl salt

δC of 1

δC of 2

220.6c 244.0c

132.2e 160.0e 136.0f 140.2g 134.8e 160.2e 132.7e 132.1e 134.9a

175.5 196.1 171.4 175.1 173.4 195.0 168.2 166.3b 168.0

145.7 172.8 141.4b 148.3 144.7 171.2b 135.7 134.2 138.7

220.6c 219.7d 243.8c 213.2d 211.4d 210.1d

a

a

δC of [AuBr3(NHC)]a 146.2 174.1 144.4 172.3 134.2 132.9 136.8

ΔδC (ppm) between NHC salt and 2

ΔδC (ppm) between 2 and 1

ΔδC (ppm) between [AuBr3(NHC)] and 2

13.5 12.8 5.4 8.1 9.9 11.0 3.0 2.1 3.8

29.4 23.3 24.6 25.7 28.7 23.8 32.5 32.1 29.3

-0.5 -1.3 0.3 -1.1 1.5 1.3 1.9

a NMR recorded in CDCl3. b NMR recorded in CD2Cl2. c NMR recorded in d6-benzene. d NMR recorded in d8-tetrahydrofuran (d8-THF). e NMR recorded in d6-dimethylsulfoxide (d6-DMSO). f NMR recorded in d2-water (D2O). g NMR recorded in d4-methanol (d4-MeOD).

Table 3. Selected Au-Ccarbene and Au-Cl Bond Distances (A˚) for Complexes 2a-i complex [AuCl3(IPr)], 2a [AuCl3(SIPr)], 2b [AuCl3(IPrMe)], 2c [AuCl3(IPrCl)], 2d [AuCl3(IMes)], 2e [AuCl3(SIMes)], 2f [AuCl3(ItBu)], 2g [AuCl3(IAd)], 2h [AuCl3(ICy)], 2i

AuCcarbene

AuCl(trans)

AuCl(cis)

AuCl(cis)

2.013(9) 2.002(9) 2.010(8) 1.975(13) 1.998(10) 1.977(6) 2.013(11) 2.006(4) 2.009(10) 1.986(18)

2.306(3) 2.303(3) 2.311(2) 2.315(3) 2.298(3) 2.306(2) 2.315(3) 2.320(13) 2.325(3) 2.307(5)

2.270(3) 2.272(3) 2.272(2) 2.284(4) 2.258(3) 2.276(2) 2.278(3) 2.2872(9) 2.284(3) 2.281(5)

2.259(3) 2.255(3) 2.270(2) 2.250(4) 2.274(4) 2.268(2) 2.258(3) 2.2864(9) 2.295(3) 2.274(1)

7.04 and 7.24 ppm for 1a, 1e, 1g, 1h, and 1i. This significant variation already observed for the [Au(NHC)Br3] complexes family indicated an important decrease of the electron density of the double bond. The consequence could be an increase of the acidic character of the gold center due to the presence of two more halides as ligands. For the same reason, saturated NHCs possessing a singlet correspond to the four protons of the backbone with a signal at 4.28 ppm for 2b and 4.24 ppm for 2f versus 4.06 and 3.98 ppm in [AuCl(SIPr)] (1b) and [AuCl(SIMes)] (1f), respectively. 13 C NMR is commonly accepted as one of the most powerful analytical tools for the study of NHCs.25 The 13C NMR spectra of complexes 2a-i displayed signals for the carbenic carbon ranging between 134.2 and 148.3 ppm for unsaturated NHC and around 172 ppm for saturated NHC (Table 2). The carbenic carbon chemical shift (δC) of [AuCl3(NHC)] 2a-i were also compared to their analogous [AuBr3(NHC)] and [AuCl(NHC)] complexes 1a-i (Figure 4 and Table 2). Herrmann et al. have postulated a correlation between the δC of the NHC and the acidity of the coordinated metal center.32 The carbenic carbon of [AuCl3(NHC)] complexes 2g-i with an N-alkyl-substituent displayed signals below 140 ppm and indicated an increase of the Lewis acidity of the gold center. With regard to complexes 2a, 2c, and 2d, a chloride substituent (148.3 ppm) decreased the Lewis acidity of the metal center compared to unsubstituted IPr (145.7 ppm), whereas methyl groups (141.4 ppm) increased the latter. Free NHCs have a δC usually above 200 ppm and NHC salts around 130 ppm for unsaturated NHCs and 160 ppm for saturated NHCs (Figure 4). The differences in δC between [AuCl3(NHC)] 2a-i and the (32) Herrmann, W. A.; Runte, O.; Artus, G. J. Organomet. Chem. 1995, 501, C1–C4.

imidazolium salt (ΔδC) were calculated to be between 2.1 and 13.5 ppm. Among organometallic complexes bearing NHC, the δC values of complexes 2 present the most upfield resonances, close to that of the NHC 3 HCl salt. This is also in good agreement with our previous study on [AuBr3(NHC)] complexes (Table 2).10b These data highlight the strong Lewis acidity of the AuCl3 moiety in complexes 2a-i. Differences of the carbenic carbon signal (ΔδC) between [AuCl(NHC)] 1a-i and [AuCl3(NHC)] 2a-i were found to range between 23.3 and 32.5 ppm (ΔδC were included between 24.9 and 38.2 ppm for [AuBr(NHC)] and their analogous [AuBr3(NHC)]). This trend results from the change in oxidation state of the gold center as observed by Raubenheimer and co-workers.10a When the δC values of [AuCl3(NHC)] 2a-i and [AuBr3(NHC)] complexes were compared, no significant differences appeared (-1.5 to 1.3 ppm), implying that the chemical property was independent of the nature of the halide.34 Crystallographic Data. To unambiguously characterize all new complexes 2a-i, X-ray-quality crystals were grown by slow evaporation of a solution of complexes 2a-i in chlorinated solvent (dichloromethane or chloroform with hexane) or by slow diffusion of pentane in dichloromethane.16 Balland-stick representations are provided in Figure 5. The Au-Cl(trans) bond lengths were found to range between 2.298 and 2.325 A˚ and are longer than the Au-Cl(cis), measured as 2.258 to 2.295 A˚. All Au-Ccarbene bond distances range from 1.975 to 2.013 A˚ (Table 3). Usually, Au-Ccarbene bond lengths in (NHC)gold(III) complexes are between 2.00 and 2.13 A˚,10a,d,36 whereas in the N,Sheterocyclic carbene gold(III) complexes they are 1.97 and 2.02 A˚.10c Noteworthy, the 2c and 2d have shorter AuCcarbene bond lengths than 2a. This is in perfect agreement with the bond length trends observed for these NHCs in the silver(I) and gold(I) series. In all cases, the Ccarbene-Au-Cl(trans) and Cl(cis)-Au-Cl(cis) bonds are nearly linear, with (33) Baker, M. V.; Barnard, P. J.; Brayshaw, S. K.; Hickey, J. L.; Skelton, B. W.; White, A. H. Dalton Trans. 2005, 37–43. (34) (a) Arduengo, A. J.III; Harlow, R. L.; Kline, M. J. J. Am. Chem. Soc. 1991, 113, 361–363. (b) Arduengo, A. J.III; Dias, H. V. R.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1992, 114, 5530–5534. (c) Arduengo, A. J., III; Bock, H.; Chen, H.; Denk, M.; Dixon, A. D.; Green, J. C.; Herrmann, W. A.; Jones, N. L.; Wagner, M.; West, R. J. Am. Chem. Soc. 1994, 116, 6641–6649. (d) Arduengo, A. J.III; Goerlich, J. R.; Marshall, W. J. J. Am. Chem. Soc. 1995, 117, 11027–11028. (e) Herrmann, W. A.; K€ocher, C.; Goossen, L. J.; Artus, G. R. Chem.;Eur. J. 1996, 2, 1627–1636. (35) Manojlovi
c-Muir, L. J. Organomet. Chem. 1974, 73, C45–C46. (36) The Web-based free-access SambVca software is available online at http://www.molnac.unisa.it/OMtools/sambvca.php.

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Table 4. Selected Angle Values and Torsion Angles of N-Ccarbene-Au-Cl(cis) (deg) for Complexes 2a-i complex [AuCl3(IPr)], 2a [AuCl3(SIPr)], 2b [AuCl3(IPrMe)], 2c [AuCl3(IPrCl)], 2d [AuCl3(IMes)], 2e [AuCl3(SIMes)], 2f [AuCl3(ItBu)], 2g [AuCl3(IAd)], 2h [AuCl3(ICy)], 2i

Ccarbene-Au-Cl(trans)

Ccarbene-Au-Cl(cis)

Cl(trans)-Au-Cl(cis)

Cl(cis)-Au-Cl(cis)

N1-Ccarbene-Au-Cl(cis) torsion angle

178.2(3) 178.1(3) 175.2(2) 177.3(3) 178.6(3) 178.10(18) 176.6(3) 177.53(13) 178.6(3) 179.1(5)

90.3(3) 88.9(3) 88.2(2) 87.6(3) 91.1(3) 88.50(18) 90.2(3) 87.00(13) 87.5(3) 88.5(5)

89.43(12) 89.06(12) 90.25(9) 89.82(14) 90.44(13) 90.01(10) 89.86(12) 91.20(5) 93.06(11) 91.01(18)

179.70(11) 179.46(13) 178.58(9) 179.48(16) 179.18(10) 178.53(7) 177.26(12) 177.94(5) 174.06(11) 176.87(18)

74.6 74.6 69.7 82.8 86.4 73.9 70.8 85.0 86.5 77.0

Figure 4. Range of NHC species function of their carbenic carbon chemical shift (δ in ppm) in 13C NMR.

angles between 175.2° and 179.7° (Table 4). These data confirm the expected square-planar geometry for all complexes. Interestingly, the plane defined by the AuCl3 moiety and the plane defined by the imidazole ring are not perfectly perpendicular. The Cl(cis)-Au-Ccarbene-N torsion angles measured for complexes 2a-i are between 69.7° and 86.5° (Table 4). The most twisted complexes were 2b and 2f, with a torsion angle around 70° due to the flexibility of the unsaturated imidazole ring. With bulky N-substituents on the NHC such as IAd h (86.5°) and ItBu g (84.9°) the torsion angles in the complexes were closer to 90° due to the steric hindrance of the substituents that surround the metal center. When the backbone of the imidazole ring was substituted such as in IPrMe c (86.4°) and IPrCl d (82.8°), the 2,6diisopropylphenyl substituents of the NHC were “pushed” toward the metal center and the torsion angles found were nearly 90°. Percent Buried Volume (%VBur). For a more in-depth understanding of the structural properties of NHC ligands a-i, we examined the steric hindrance brought about by this ligand family around the metal center. To quantify this steric influence, the percent buried volume (%VBur) of each of these ligands was calculated using the SambVca method developed by Cavallo and co-workers.36 The calculation procedure has already been reported.37 In this model, the %VBur value represents the portion of a sphere, centered around the metal atom, occupied by the ligand (Figure 6). All calculated buried volumes range from 27.7% (for ICy i) to 38.8% (for SIPr b) (Table 5, entries 2 and 9). Surprisingly, IAd h and ItBu g, which are commonly accepted as the bulkiest ligands in the studied series, revealed a large but not largest value of 36% (Table 5, entries 7 and 8). Noteworthy is an important difference between values for 2f and 2c [SIMes f (34.7%) and SIPr b (38.8%)], which is probably due to the difference in torsion angle of the N(1)-C-C-N(2), 10.5° (37) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo, L. Eur. J. Inorg. Chem. 2009, 1759–1766. (38) For the values obtained for the real bond length for Au-C of each complex see Supporting Information.

and 19.6°, respectively (Table 5, entries 2 and 6). This torsion angle in the imidazole ring affects the proximity of N-substituents of the aromatic rings to the metal. The NHC buried volume for ligands a-i in the [AuBr3(NHC)] and [AuCl(NHC)] 1a-i complexes were also calculated to evaluate the influence of the halide and complex geometry on the %VBur (Table 5). In the series of [AuBr3(NHC)], the buried volume ranges between 26.8% (for ICy i) and 37.6% (for SIPr b) (Table 1, entries 2 and 9). These values are similar to those found for the [AuBr3(NHC)] complexes. These results permit us to conclude that the nature of the halide does not influence the conformation of the NHC ligand a-i in the present complexes. In the case of [AuCl(NHC)], %VBur values fall between 27.6% (for ICy i) and 47.4% (for SIPr b) (Table 1, entries 2 and 9). The buried volumes of NHC a-i between gold(I) chloride and gold(III) chloride complexes were compared, and an increase of the values was observed in gold(I) complexes (Table 5). This observation is related to the geometry of the complexes, linear for gold(I) and square planar for gold(III). In the case of linear gold(I) complexes, more space is available to the NHC in the absence of the two halides and generally the % VBur values are larger. Interestingly, for small NHCs such as ICy i, no interaction between the ligand and the halides appeared in gold(III) complexes 2i, where the buried volume was found to be the same as in 1i. Noteworthy, IPr a, SIPr b, IPrMe c, and IPrCl d in gold(I) chloride complexes exhibited a buried volume increased by around 10 units compared to their analogous gold(III) complexes 2a, 2b, 2c, and 2d. To understand this particularity of the 2,6-diisopropylphenyl substituent in the NHC compared to mesityl framework, the distances between the four carbon atoms in ortho position on the aromatic group to the gold center (d) were measured for 1a, 1e, 2a, and 2e (Figure 7). For gold(III) chlorides 2a and 2e, all distances d measured were similar in IPr a and IMes e (between 3.993 and 4.637 A˚), as well as in the gold(I) chloride series (between 3.984 and 4.212 A˚). This observation is in agreement with the similar buried volumes between IPr a and IMes e in the gold(III) chloride series. Then, distances between the four closest methyl groups of the isopropyl group to the gold center (d0 ) were measured for 1a and 2a (Figure 7). In the case of 1a, d0 were found to measure 3.886 to 4.235 A˚, whereas they are significantly longer, at 4.228 to 5.123 A˚, in [AuCl3(IPr)], 2a. This difference parallels the %VBur values and may very well result from the electronic repulsion of the methyl groups by the chlorine atoms in [AuCl3(IPr)], 2a. The shorter AuCcarbene bond length in linear gold(I) (1.974 A˚ on average) compared to gold(III) (1.999 A˚ on average) could also be another factor influencing this interaction.

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Figure 5. Ball-and-stick representations of complexes [AuCl3(SIPr)], 2b; [AuCl3(IPrMe)], 2c; [AuCl3(IPrCl)], 2d; [AuCl3(IMes)], 2e; [AuCl3(SIMes)], 2f; [AuCl3(ItBu)], 2g; [AuCl3(IAd)], 2h; and [AuCl3(ICy)], 2i. Most hydrogen atoms are omitted for clarity. Selected bond distances (A˚) and angles (deg) are summarized in Tables 3 and 4.

Figure 7. Representation of distances d and d0 in complexes 2a and 2e. Figure 6. Graphical Representation of the Sphere Used for the %VBur Calculations. Table 5. Steric Parameter (%VBur) Calculated for NHCs a-i in [AuCl3(NHC)] 2a-i, [AuBr3(NHC)], and [AuCl(NHC)] Complexes 1a-i entry 1 2 3 4 5 6 7 8 9

ligand

%VBur for 2a

IPr a SIPr b IPrMe c IPrCl d IMes e SIMes f ItBu g IAd h ICy i

34.9 38.8 35.7 35.5 34.2 34.7 36.0 36.5 27.7

%VBur for [AuBr3(NHC)]a 33.3 37.6 34.1 34.0 35.8 35.8 26.8

%VBur for 1a 45.6 47.4 44.5 43.8 36.5 37.1 n.a. 40.0 27.6

a Calculation were achieved using a crystallographically determined bond length for the Au-Ccarbene bond and the radius of sphere was fixed at R = 3.5 A˚.38

Conclusion In summary, we have described the syntheses of a series of [AuCl3(NHC)] 2 complexes in excellent yields by oxidative addition of chlorine onto AuCl(NHC) 1 with chlorine gas or PhICl2. PhICl2 was revealed to be more selective than chlorine gas especially in the case of IPrMe and precluded decomposition in the case of ICy and SIMes. This reagent presents several advantages compared to chlorine gas. Indeed, PhICl2 is easily handled, resulting in a good control of

the reaction stoichiometry. Moreover, reactions were performed at room temperature instead of at low temperature when chlorine gas was used. All new complexes were fully characterized by NMR and X-ray analysis. The X-ray data permitted the examination of the steric effects of the most common NHCs by calculation of the buried volume (%VBur) values. This new computational steric model proves efficient in rapidly affording measurable descriptors for this highly important ligand parameter. We have also extended this study to IPrMe and IPrCl in order to evaluate the influence of substitution on the NHC backbone. Studies aimed at exploring the effects of this substitution on metal-catalyzed reactions are ongoing in our laboratories.

Experimental Section General Considerations. All reactions were carried out using standard Schlenk techniques under an atmosphere of dry argon. Anhydrous solvents were either distilled from appropriate drying agents or purchased from Aldrich and degassed prior to use by purging with dry argon and kept over molecular sieves. Solvents for NMR spectroscopy were degassed with argon and dried over molecular sieves. NMR spectra were recorded on a 400 MHz Varian Gemini spectrometer. Elemental analyses were performed by St Andrews analytical services. AuCl and SMe2 were purchased from Strem and Acros, respectively, and [Au(SMe2)Cl] was prepared according to the reported procedure.20 Synthesis of [AuCl(IPrCl)] (1d). In a Schlenk tube, IPr 3 HBF4 (953 mg, 2 mmol) was added to a suspension of NaH (96 mg, 4 mmol) in THF (10 mL). A spatula of KOtBu was added, and

Article the reaction mixture was stirred at room temperature overnight. The solution was filtered and CCl4 (386 μL, 4 mmol) was added to the filtrate. The reaction mixture was stirred an additional 3 h, and AuCl(SMe2) (589.1 mg, 2 mmol) was added. The reaction mixture was stirred in darkness at room temperature during 14 h. The reaction mixture was filtered on a pad of silica, and solvent was removed under vacuum. The crude mixture was dissolved in dichloromethane, and charcoal was added. The resulting mixture was stirred at room temperature overnight. The solution was filtered on a pad of silica. Solvent was reduced to 2 mL under vacuum, and hexane (10 mL) was added. The resulting pale yellow precipitate was filtered, washed with hexane (3  5 mL), and dried under vacuum. Yield: 771 mg (56%). 1H NMR (400 MHz, CDCl3): δ 7.56 (t, J = 7.8 Hz, 2H, CH aromatic), 7.32 (d, J=7.8 Hz, 4H, CH aromatic), 2.45 (sept, J = 6.8 Hz, 4H, CH(CH3)2), 1.35 (d, J = 6.8 Hz, 12H, CH(CH3)2), 1.26 (d, J = 6.8 Hz, 12H, CH(CH3)2) ppm. 13C NMR (100 MHz, CDCl3): δ 175.1 (s, C carbene), 146.0 (s, C aromatic), 131.6 (s, CH aromatic), 130.0 (s, C aromatic), 124.6 (s, CH aromatic), 118.9 (s, CCl imidazole), 29.1 (s, CH(CH3)2), 24.6 (s, CH(CH3)2), 23.4 (s, CH(CH3)2) ppm. Anal. Calcd for C29H40N2AuCl (689.90): C, 47.01; H, 4.97; N, 4.06. Found: C, 47.05; H, 4.72; N, 3.90. Synthesis of [AuCl3(IPr)] (2a). Method A: Excess chlorine was bubbled into a solution of 1a (200 mg, 0.32 mmol) in dichloromethane (4 mL) at -78 °C, and the reaction mixture was slowly warmed to room temperature over 5 h with stirring. The solvent volume was reduced by half under vacuum, and hexane (10 mL) was added. The resulting precipitate was collected and washed with hexane (3  5 mL). The solid was dried under vacuum to afford 2a as a pale yellow powder. Yield: 209.7 mg (95%). Method B: In a vial, 1a (1 equiv) and PhICl2 (1.1 equiv) were dissolved in dichloromethane and stirred at room temperature overnight. Solvent volume was reduced by half under vacuum, and hexane was added. The resulting precipitate was collected and washed with hexane. The solid was dried under vacuum to afford 2a as a pale yellow powder. Yield: 209.7 mg (95%). 1H NMR (400 MHz, CDCl3): δ 7.56 (t, J = 7.8 Hz, 2H, CH aromatic), 7.36 (d, J = 7.8 Hz, 4H, CH aromatic), 7.34 (s, 2H, CH imidazole); 2.85 (sept, J = 6.7 Hz, 4H, CH(CH3)2), 1.40 (d, J = 6.7 Hz, 12H, CH(CH3)2), 1.13 (d, J = 6.7 Hz, 12H, CH(CH3)2) ppm. 13C NMR (100 MHz, CDCl3): δ 146.0 (s, C aromatic), 145.7 (s, C carbene), 132.2 (s, C aromatic), 131.8 (s, CH imidazole), 126.4 (s, CH aromatic), 124.7 (s, CH aromatic), 29.0 (s, CH(CH3)2), 26.5 (s, CH(CH3)2), 22.7 (s, CH(CH3)2) ppm. Anal. Calcd for C27H36N2AuCl3 (691.91): C, 46.87; H, 5.24; N, 4.05. Found: C, 46.96; H, 4.82; N, 3.94. Synthesis of [AuCl3(SIPr)] (2b). Method A afforded 2b as a pale yellow solid. Yield: 300 mg (90%). Method B provided 2b as a pale yellow solid. Yield: 102.5 mg (92%). 1H NMR (400 MHz, CDCl3): δ 7.46 (t, J = 7.8 Hz, 2H, CH aromatic), 7.29 (d, J=7.8 Hz, 4H, CH aromatic), 4.28 (s, 4H, CH2 imidazole), 3.33 (sept, J = 6.7 Hz, 4H, CH(CH3)2), 1.46 (d, J = 6.7 Hz, 12H, CH(CH3)2), 1.27 (d, J = 6.7 Hz, 12H, CH(CH3)2) ppm. 13C NMR (100 MHz, CDCl3): δ 172.8 (s, C carbene), 146.9 (s, C aromatic), 132.0 (s, C aromatic), 131.1 (s, CH aromatic), 125.2 (s, CH aromatic), 54.7 (s, CH2 imidazole), 29.0 (s, CH(CH3)2), 27.1 (s, CH(CH3)2), 23.6 (s, CH(CH3)2) ppm. Anal. Calcd for C27H38N2AuCl3 (693.93): C, 46.73; H, 5.52; N, 3.64. Found: C, 46.49; H, 5.16; N, 3.64. Synthesis of [AuCl3(IPrMe)] (2c). Method A: A preparation method similar to that used for compound 2a gave an inseparable mixture of 2c and 3 in a ratio 1.5:1 in favor of 2c as a pale yellow solid. Yield: 67 mg (88%). Method B: A preparation method similar to that used for compound 2a gave 2c as a pale yellow solid. Yield: 99.7 mg (90%). 1H NMR (400 MHz, CD2Cl2): δ 7.61 (t, J = 7.8 Hz, 2H, CH aromatic), 7.42 (d, J=7.8 Hz, 4H, CH aromatic), 2.82 (sept, J = 6.7 Hz, 4H, CH(CH3)2), 2.08 (s, 6H, CH3 imidazole), 1.39 (d, J = 6.6 Hz, 12H, CH(CH3)2), 1.12 (d, J = 6.8 Hz, 12H,

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CH(CH3)2) ppm. 13C NMR (100 MHz, CD2Cl2): δ 147.2 (s, C aromatic), 141.4 (s, C carbene), 132.0 (s, C aromatic), 131.3 (s, C imidazole), 131.0 (s, C aromatic), 125.7 (s, CH aromatic), 29.1 (s, CH(CH3)2), 25.5 (s, CH(CH3)2), 24.7 (s, CH(CH3)2), 11.3 (s, CH3 imidazole) ppm. Anal. Calcd for C29H40N2AuCl3 (718.19): C, 48.38; H, 5.60; N, 3.89. Found: C, 48.28; H, 5.50; N, 3.79. Synthesis of [AuCl3(IPrCl)] (2d). Method A afforded 2d as a pale yellow solid. Yield: 131.7 mg (80%). Method B gave 2d as a pale yellow solid. Yield: 84 mg (92%).1H NMR (400 MHz, CDCl3): δ 7.61 (t, J = 7.8 Hz, 2H, CH aromatic), 7.40 (d, J = 7.8 Hz, 4H, CH aromatic), 2.79 (sept, J=6.6 Hz, 4H, CH(CH3)2), 1.41 (d, J = 6.5 Hz, 12H, CH(CH3)2), 1.20 (d, J = 6.8 Hz, 12H, CH(CH3)2) ppm. 13C NMR (100 MHz, CDCl3): δ 148.4 (s, C carbene), 147.0 (s, C aromatic), 132.7 (s, CH aromatic), 129.6 (s, C aromatic), 125.6 (CH, aromatic), 123.0 (s, CCl imidazole), 29.2 (s, CH(CH3)2), 25.5 (s, CH(CH3)2), 24.3 (s, CH(CH3)2) ppm. Anal. Calcd for C27H34N2AuCl5 (758.08): C, 42.62; H, 4.50; N, 3.68. Found: C, 42.44; H, 4.50; N, 3.67. Synthesis of [AuCl3(IMes)] (2e). Method A: Excess chlorine was bubbled into a solution of 1e (100 mg, 0.186 mmol) in dichloromethane (2 mL) at -78 °C, and the reaction mixture was stirred at -78 °C during 7 h. The solvent volume was reduced by half under vacuum, and hexane (5 mL) was added. The resulting precipitate was collected and washed with hexane (3  5 mL). The solid was dried under vacuum to afford 2e as a pale yellow powder. Yield: 89.7 mg (79%). Method B afforded 2e as a pale yellow solid. Yield: 105.7 mg (94%). 1H NMR (400 MHz, CDCl3): δ 7.30 (s, 2H, CH imidazole), 7.03 (s, 4H, CH aromatic), 2.36 (s, 6H, CH3 mesityl), 2.25 (s, 12H, CH3 mesityl) ppm. 13C NMR (100 MHz, CDCl3): δ 144.6 (s, C carbene), 141.0 (s, C aromatic), 135.3 (s, C aromatic), 132.3 (s, C aromatic), 129.9 (s, CH aromatic), 125.6 (s, CH imidazole), 21.2 (s, CH3 mesityl), 18.5 (s, CH3 mesityl) ppm. Anal. Calcd for C27H36N2AuCl3 with 10% of dichloromethane (606.06): C, 41.08; H, 3.96; N, 4.54. Found: C, 41.08; H, 3.56; N, 4.35. Synthesis of [AuCl3(SIMes)] (2f). Method B: A preparation method similar to that used for compound 2a gave 2f as a pale yellow solid. Yield: 108 mg (95%). 1H NMR (400 MHz, CD2Cl2): δ 7.03 (s, 4H, CH aromatic), 4.24 (s, 4H, CH imidazole), 2.48 (s, 12H, CH3 mesityl), 2.34 (s, 6H, CH3 mesityl) ppm. 13C NMR (100 MHz, CD2Cl2): δ 171.5 (s, C carbene), 140.7 (s, C aromatic), 136.6 (s, C aromatic), 132.2 (s, C aromatic), 130.3 (s, CH aromatic), 52.7 (s, CH2 imidazole), 21.2 (s, CH3 mesityl), 19.0 (s, CH3 mesityl) ppm. Anal. Calcd for C21H26N2AuCl3 (608.08): C, 41.36; H, 4.30; N, 4.59. Found: C, 41.71; H, 4.12; N, 4.23. Synthesis of [AuCl3(ItBu)] (2g). Method A gave 2g as a pale yellow solid. Yield: 226 mg (96%). Method B provided 2g as a pale yellow solid. Yield: 111.9 mg (96%). 1H NMR (400 MHz, CDCl3): δ 7.45 (s, CH imidazole), 1.96 (s, 18H, C(CH3)3) ppm. 13 C NMR (100 MHz, CDCl3): δ 135.7 (s, C carbene), 122.1 (s, CH imidazole), 62.5 (s, C(CH3)3), 31.9 (s, C(CH3)3) ppm. Anal. Calcd for C11H20N2AuCl3 (483.62): C, 27.32; H, 4.17; N, 5.79. Found: C, 27.43; H, 3.97; N, 5.66. Synthesis of [AuCl3(IAd)] (2h). Method A afforded 2h as a pale yellow solid. Yield: 204.2 mg (91%). Method B gave 2h as a pale yellow solid. Yield: 102.4 mg (92%). 1H NMR (300 MHz, CDCl3): δ 7.51 (s, CH imidazole), 2.58 (d, J = 2.8 Hz, 12H, adamantyl), 2.34 (s, 4H, adamantyl), 1.77 (t, J = 2.8 Hz, 12H, adamantyl) ppm. 13C NMR (100 MHz, CDCl3): δ 134.2 (s, C carbene), 121.0 (s, CH imidazole), 63.8 (s, N-C adamantyl), 43.9 (s, CH2 adamantyl), 35.3 (s, CH2 adamantyl), 30.0 (s, CH adamantyl) ppm. Anal. Calcd for C23H32N2AuCl3 (639.83): C, 43.17; H, 5.04; N, 4.38. Found: C, 43.46; H, 5.02; N, 4.07. Synthesis of [AuCl3(ICy)] (2i). Method B: A preparation method similar to that used for compound 2a gave 2i as a pale yellow solid. Yield: 104.6 mg (91%). 1H NMR (400 MHz, CDCl3): δ 7.21 (s, 2H, CH imidazole), 4.57-4.68 (m, 2H, CH cyclohexyl), 2.22-2.29 (m, 4H, CH2 cyclohexyl), 1.88-1.95 (m, 4H, CH2

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cyclohexyl), 1.76-1.84 (m, 2H, CH2 cyclohexyl), 1.45-1.63 (m, 8H, CH2 cyclohexyl), 1.15-1.31 (m, 2H, CH2 cyclohexyl) ppm. 13 C NMR (100 MHz, CDCl3): δ 138.7 (s, C carbene), 120.2 (s, CH imidazole), 61.1 (s, CH cyclohexyl), 33.4 (s, CH2 cyclohexyl), 25.1 (s, CH2 cyclohexyl), 24.8 (s, CH2 cyclohexyl) ppm. Anal. Calcd for C15H24N2AuCl3 (534.07): C, 33.63; H, 4.52; N, 5.23. Found: C, 33.88; H, 4.51; N, 5.05. Synthesis of IPrCl 3 HCl (5). In a Schlenk under argon, a spatula of KOtBu was added to a solution of IPr 3 HBF4 (1.0 g, 2.1 mmol) and NaH (100.8 mg, 4.2 mmol) in THF (15 mL). The reaction was stirred at room temperature overnight. The reaction mixture was filtered under argon, and CCl4 (405 μL, 4.2 mmol) was added. The resulting solution was stirred an additional 3 h at room temperature. A solution of HCl in dioxane (4 M) was added, and the resulting precipitate was collected to furnish 5 as a white powder. Yield: 873.4 mg (84%). (The H of the imidazolium salt 5 was exchanged by D in CD3OD.) 1H NMR of d1-5 (400 MHz, CD3OD): δ 7.75 (t, J = 7.8 Hz, 2H, CH aromatic), 7.58 (d, J = 7.8 Hz, 4H, CH aromatic), 2.46 (sept, J = 6.8 Hz, 4H, CH(CH3)2), 1.36 (d, J=6.8 Hz, 12H, CH(CH3)2), 1.25 (d, J = 6.8 Hz, 12H, CH(CH3)2). 13C NMR of d1-5 (100 MHz, CD3OD): δ 147.3 (s, C aromatic), 140.0 (t, CD imidazole), 134.6 (s, CH aromatic), 128.2 (s, C aromatic), 126.6 (s, CH aromatic), 124.1 (s, C imidazole), 30.7 (s, CH(CH3)2), 25.1 (s, CH(CH3)2), 23.4 (s, CH(CH3)2). Anal. Calcd for C27H35N2Cl3 (492.19): C, 65.65; H, 7.14; N, 5.67. Found: C, 65.79; H, 7.05; N, 5. 48.

Gaillard et al. Synthesis of [AgCl(IPrCl)] (6). In a flask, Ag2O (91.5 mg, 0.39 mmol) was added to a solution of IPrCl 3 HCl, 5 (300 mg, 0.60 mmol), in dichloromethane (3 mL). The reaction mixture was stirred in the dark at room temperature during 3 h and was then filtered on a pad of Celite. Solvent was removed under vacuum, and hexane was added (10 mL). The resulting precipitate was collected by filtration, washed with hexane (3  5 mL), and dried under vacuum to afford 6 as a white powder. Yield: 112.9 mg (31%). 1H NMR (400 MHz, CDCl3): δ 7.56 (t, J = 7.8 Hz, 2H, CH aromatic), 7.33 (d, J=7.8 Hz, 4H, CH aromatic), 2.45 (sept, J = 6.8 Hz, 4H, CH(CH3)2), 1.28 (d, J = 6.8 Hz, 12H, CH(CH3)2), 1.26 (d, J = 6.9 Hz, 12H, CH(CH3)2) ppm. 13C NMR (100 MHz, CDCl3): δ 183.6 (dd, JC-109Ag = 273.8 Hz, JC-107Ag = 236.9 Hz, C carbene), 145.9 (s, C aromatic), 131.6 (s, CH aromatic), 131.6 (s, C aromatic), 124.6 (s, CH aromatic), 119.5 (d, JAg-C = 9.3 Hz, C imidazole), 29.0 (s, CH(CH3)2), 24.9 (s, CH(CH3)2), 23.3 (s, CH(CH3)2) ppm. Anal. Calcd for C27H34N2AgCl3 (600.80): C, 53.98; H, 5.70; N, 4.66. Found: C, 54.14; H, 5.64; N, 4.68.

Acknowledgment. Support of this work by the ERC (Advanced Researcher Grant to S.P.N.) and the EPSRC is gratefully acknowledged. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.