Synthesis, characterization and reactivity of N-heterocyclic carbene gold(III) complexes

Article abstract of DOI:10.1021/om060887t

A series of (NHC)Au^ICl (1, NHC = N,N′-bis(2,6- diisopropylphenyl)imidazol-2-ylidene (IPr); 2, NHC = N,N′-bis(2,4,6- trimethylphenyl)imidazol-2-ylidene (IMes); 3, NHC = N,N′-bis(2,6- diisopropylphenyl)-imidazolin-2-ylidene (SIPr); 4, NHC = N,N′-bis(2,4,6- trimethylphenyl)imidazolin-2-ylidene (SIMes); 5, NHC = N,N′- dicyclohexylimidazol-2-ylidene (ICy); 6, NHC = N,N′-diadamantylimidazol-2- ylidene (IAd); 7, NHC = N,N′-di-terf-butylimidazol-2-ylidene (I'Bu)) complexes were reacted with LiBr to generate [(IPr)AuBr] (8), [(IMes)AuBr] (9), [(SIPr)AuBr] (10), [(SIMes)AuBr] (11), [(ICy)AuBr] (12), [(IAd)-AuBr] (13), and [(I^tBu)AuBr] (14). These (NHC)Au^IBr complexes undergo oxidative addition of elemental bromine, leading to the new Au(III) complexes [(IPr)AuBr₃] (15), [(IMes)AuBr₃] (16), [(SIPr)AuBr ₃] (17), [(SIMes)AuBr₃] (18), [(ICy)AuBr₃] (19), [(IAd)AuBr₃] (20), and [(I^tBu)AuBr₃] (21). Complete characterization by NMR spectroscopy and single-crystal X-ray diffraction were performed in order to discern structural differences between organogold(I/III) congeners. A preliminary study examining the activity of (NHC)Au^III species on the addition of water to alkynes is also presented.

Full text of DOI:10.1021/om060887t

1376
Organometallics 2007, 26, 1376-1385
Synthesis, Characterization and Reactivity of N-Heterocyclic
Carbene Gold(III) Complexes
Pierre de Fre´mont,† Rohit Singh,† Edwin D. Stevens,† Jeffrey L. Petersen,‡ and
Steven P. Nolan*,†,§
Department of Chemistry, UniVersity of New Orleans, New Orleans, Louisiana 70148, Institut Catala`
d’InVestigacio´ Qu´ımica, AV. Pa¨ısos Catalans 16, 43007, Tarragona, Spain, and Department of Chemistry,
West Virginia UniVersity, Morgantown, West Virginia 26506
ReceiVed September 27, 2006
A series of (NHC)AuICl (1, NHC ) N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr); 2, NHC
) N,N′-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes); 3, NHC ) N,N′-bis(2,6-diisopropylphenyl)-
imidazolin-2-ylidene (SIPr); 4, NHC ) N,N′-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (SIMes); 5,
NHC ) N,N′-dicyclohexylimidazol-2-ylidene (ICy); 6, NHC ) N,N′-diadamantylimidazol-2-ylidene (IAd);
7, NHC ) N,N′-di-tert-butylimidazol-2-ylidene (ItBu)) complexes were reacted with LiBr to generate
[(IPr)AuBr] (8), [(IMes)AuBr] (9), [(SIPr)AuBr] (10), [(SIMes)AuBr] (11), [(ICy)AuBr] (12), [(IAd)-
AuBr] (13), and [(ItBu)AuBr] (14). These (NHC)AuIBr complexes undergo oxidative addition of elemental
bromine, leading to the new Au(III) complexes [(IPr)AuBr3] (15), [(IMes)AuBr3] (16), [(SIPr)AuBr3]
(17), [(SIMes)AuBr3] (18), [(ICy)AuBr3] (19), [(IAd)AuBr3] (20), and [(ItBu)AuBr3] (21). Complete
characterization by NMR spectroscopy and single-crystal X-ray diffraction were performed in order to
discern structural differences between organogold(I/III) congeners. A preliminary study examining the
activity of (NHC)AuIII species on the addition of water to alkynes is also presented.
tions have been achieved with low catalyst loading and high
turnover numbers. The gold(I) center must have two coordina-
tion sites occupied to ensure stability of the complexes and
thereby avoid reduction to gold(0).7 The most commonly
employed ligands so far have been phosphines (PR3)8 and, most
recently, N-heterocyclic carbenes (NHC).9 Both ligand families
exhibit strong σ-donation, and coordination of such ligands
results in good stability of the Au(I) complexes toward air,
moisture, and thermolysis. It is interesting to note that gold has
Introduction
Although, historically, organogold complexes have been
underutilized in organic synthesis, numerous publications have
recently emphasized the beneficial role of gold(I) in catalysis.1
Organic transformations such as skeletal rearrangements (cy-
cloisomerizations),2 carbene transfer reactions,3 indanization,4
oxidations,5 and hydrosilylations6 are examples of the diverse
chemistry mediated by organogold catalysts. Such transforma-
* To whom correspondence should be addressed at the Institut Catala`
d’Investigacio´ Qu´ımica (ICIQ). E-mail: snolan@iciq.es.
(6) Ito, H.; Yajima, T.; Tateiwa, J.; Hosomi, A. Chem. Commun. 2000,
981-982.
† University of New Orleans.
(7) (a) Baker, M. V.; Barnard. P. J.; Brayshaw, S. K.; Hickey, J. L.;
Skelton, B. W.; White, A. H. Dalton Trans. 2005, 1, 37-43. (b) Me´zailles,
N.; Ricard, L.; Gagosz, F. Org. Lett. 2005, 7, 4133-4136. (c) Ferrer, C.;
Echavarren, A. M. Angew. Chem., Int. Ed. 2006, 45, 1105-1109. (d) de
Fre´mont, P.; Stevens, E. D.; Fructos, M. R.; D´ıaz-Requejo, M. M.; Pe´rez,
P. J.; Nolan, S. P. Chem. Commun. 2006, 2045-2047. (e) Belting, V.;
Krause, N. Org. Lett. 2006, 8, 4489-4492. (f) Hashmi, A. S. K.; Blanco,
C.; Kurpejovic, E.; Frey, W.; Bats, J. W. AdV. Synth. Catal. 2006, 348,
709-713.
‡ West Virginia University.
§ Institut Catala` d’Investigacio´ Qu´ımica (ICIQ).
(1) (a) Teles, J. H.; Brode, S.; Chabanas, M. Angew. Chem., Int. Ed.
1998, 37, 1415-1418. (b) Nieto-Oberhuber, C.; Mun˜oz, M. P.; Bun˜uel,
E.; Nevado, C.; Ca´rdenas, D. J.; Echavarren, A. M. Angew. Chem., Int. Ed.
2004, 43, 2402-2406. (c) Kennedy-Smith, J. J.; Staben, S. T.; Toste, F. D.
J. Am. Chem. Soc. 2004, 126, 4526-4527. (d) Luzung, M. R.; Markham,
J. P.; Toste, F. D. J. Am. Chem. Soc. 2004, 126, 10858-10859. (e)
Johansson, M. J.; Gorin, D. J.; Staben, S. T.; Toste, F. D. J. Am. Chem.
Soc. 2005, 127, 18002-18003. (f) Gorin, D. J.; Davis, N. R.; Toste, F. D.
J. Am. Chem. Soc. 2005, 127, 11260-11261. (g) Fructos, M. R.; de Fre´mont,
P.; Nolan, S. P.; D´ıaz-Requejo, M. M.; Pe´rez, P. J. Organometallics 2006,
25, 2237-2241.
(8) (a) Muir, J. A.; Muir, M. M.; Pulgar, L. B. Acta Crystallogr. 1985,
C41, 1174-1176. (b) King, C.; Khan, M. N. L.; Staples, R. J.; Fackler, J.
P., Jr. Inorg. Chem. 1992, 31, 3236-3238. (c) Mu¨ller, T. E.; Green, J. C.;
Mingos, D. M. P.; McPartlin, C. M.; Whittingham, C.; Williams, D. J.;
Woodroffe, T. M. J. Organomet. Chem. 1998, 551, 313-330. (d) Rauben-
heimer, H. G.; Esterhuysen, M. W.; Timoshkin, A.; Chen, Y.; Frenking,
G. Organometallics 2002, 21, 3173-3181. (e) Stefanescu, D. M.; Yuen,
H. F.; Glueck, D. S.; Golen, J. A.; Zakharov, L. N.; Incarvito, C. D.;
Rheingold, A. L. Inorg. Chem. 2003, 42, 8891-8901. (f) Reiter, S. A.;
Nogai, S. D.; Schmidbaur, H. Dalton Trans. 2005, 247-255.
(9) (a) Cetinkaya, B.; Dixneuf, P.; Lappert, M. F. J. Chem. Soc., Dalton
Trans. 1974, 1827-1833. (b) Raubenheimer, H. G.; Lindeque, L.; Cronje,
S. J. Organomet. Chem. 1996, 511, 177-184. (c) Wang, H. M. J.; Chen,
C. Y. L.; Lin, I. J. B. Organometallics 1999, 18, 1216-1223. (d) Vicente,
J.; Chicote, M.-T.; Abrisqueta, M. D.; Alvarez-Falco´n, M. M.; Ram´ırez de
Arellano, M. C.; Jones, P. G. Organometallics 2003, 22, 4327-4333. (e)
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. (f) Barnard, P. J.;
Baker, M. V.; Berners-Price, S. J.; Skelton, B. W.; White, A. H. Dalton
Trans. 2004, 1038-1047. (g) Lin, I. J. B.; Vasam, C. S. Can. J. Chem.
2005, 83, 812-825. (h) Wang, J.-W.; Li, Q.-S.; Xu, F.-B.; Song, H.-B.;
Zhang, Z.-Z. Eur. J. Org. Chem. 2006, 1310-1316.
(2) (a) Zhang, L.; Kozmin, S. A. J. Am. Chem. Soc. 2004, 126, 11806-
11807. (b) Marion, N.; de Fre´mont, P.; Lemie`re, G.; Stevens, E. D.;
Fensterbank, L.; Malacria, M.; Nolan, S. P. Chem. Commun. 2006, 19,
2048-2050. (c) Nieto-Oberhuber, C.; Lo´pez, S.; Mun˜oz, M. P.; J´ımenez-
Nu´n˜ez, E.; Bun˜uel, E.; Ca´rdenas, D. J.; Echavarren, A. M. Chem. Eur. J.
2006, 12, 1694-1702. (d) J´ımenez-Nu´n˜ez, E.; Claverie, C. K.; Nieto-
Oberhuber, C.; Echavarren, A. M. Angew. Chem., Int. Ed. 2006, 45, 5452-
5455. (e) Sun, J.; Conley, M. P.; Zhang, L.; Kozmin, S. A. J. Am. Chem.
Soc. 2006, 128, 9705-9710. (f) Dube´, P.; Toste, F. D. J. Am. Chem. Soc.
2006, 128, 12062-12063.
(3) (a) Fructos, M. R.; Belderrain, T. R.; de Fre´mont, P.; Scott, N. M.;
Nolan, S. P.; D´ıaz-Requejo, M. M.; Pe´rez, P. J. Angew. Chem., Int. Ed.
2005, 44, 5284-5288.
(4) Marion, N.; D´ıez-Gonza´lez, S.; de Fre´mont, P.; Noble, A. R.; Nolan,
S. P. Angew. Chem., Int. Ed. 2006, 45, 3647-3650.
(5) Guan, B.; Xing, D.; Cai, G.; Wan, X.; Yu, N.; Fang, Z.; Yang, L.;
Shi, Z. J. Am. Chem. Soc. 2005, 127, 18004-18005.
10.1021/om060887t CCC: $37.00 © 2007 American Chemical Society
Publication on Web 02/07/2007

N-Heterocyclic Carbene Gold(III) Complexes
Organometallics, Vol. 26, No. 6, 2007 1377
Scheme 1. Formation of Gold(I) Complexes by Reduction
of HAuCl4
even a stronger affinity for N-heterocyclic carbene than for
phosphine and other Fisher acyclic carbenes.10 A broad range
of catalyzed transformations by inorganic gold(III) salts has been
reported in the literature; examples include hydroaminations,11
[4 + 2] benzannulations,12 functionalization of aromatic C-H
bonds,13 cycloisomerizations,14 and addition reactions to het-
erocycles.15 Most often AuX3 (X ) Cl, Br) salts are used
directly11-16 and only a limited number of examples of well-
defined organogold(III) complexes acting as catalysts are
known.17 No catalysis mediated by (PR3)- or (NHC)AuIII
complexes has been reported so far. This is quite surprising,
since the chemistry of the arsine,18 stilbine,19 phosphine,18b,c,20
and carbene21 gold(III) complexes was first examined in the
mid-1970s. Since these initial studies only a limited number of
publications have focused on this chemistry. Notable exceptions
are the extensive studies performed on gold(III) phosphine
complexes by Schmidbaur et al.22 Since then, C-tetrazolato,23
bis(thiazolinylidene), and bis(NHC) gold(III) complexes bearing
carbene moieties have been reported.24 Nevertheless, no example
of a mono(NHC) gold(III) complex has been reported, (4-
(10) Bonati, F.; Burini, A.; Pietroni, B. R.; Bovio, B. J. Organomet.
Chem. 1991, 408, 271-280.
methylthiazol-2-ylidene)AuCl3 being the closest related complex
reported so far.24a In order to expand the range of Au(III)
complexes known and hopefully to provide access to novel Au-
(III) architectures, we reasoned that our prior expertise in NHC
and Au(I) chemistry could be put to use in the synthesis of novel
Au(III) complexes bearing the electron-rich NHC ligands.
(11) Nishina, N.; Yamamoto, Y. Angew. Chem., Int. Ed. 2006, 45, 3314-
3317.
(12) Sato, K.; Asao, N.; Yamamoto, Y. J. Org. Chem. 2005, 70, 8977-
8981.
(13) Shi, Z.; He, C. J. Org. Chem. 2004, 69, 3669-3671.
(14) (a) Morita, N.; Krause, N. Org. Lett. 2004, 6, 4121-4123. (b) Kim,
N.; Kim, Y.; Park, W.; Sung, D.; Gupta, A. K.; Oh, C. H. Org. Lett. 2005,
7, 5289-5291. (c) Antoniotti, S.; Genin, E.; Michelet, V. Geneˆt, J.-P. J.
Am. Chem. Soc. 2005, 127, 9976-9977. (d) Barluenga, J.; Die´guez, A.;
Ferna´ndez, A.; Rodr´ıguez, F.; Fan˜ana´s, F. J. Angew. Chem., Int. Ed. 2006,
45, 2091-2093. (e) Liu, J.; Muth, E.; Flo¨rke, U.; Henkel, G.; Merz, K.;
Sauvageau, J.; Schwake, E.; Dyker, G. AdV. Synth. Catal. 2006, 348, 456-
462.
Results and Discussion
To eventually develop a general synthetic route leading to a
family of (NHC)AuX3 (X ) halide) complexes, we initially
examined possible approaches to a single target compound:
(IPr)AuX3. Previously, chlorine gas had been used to convert
(thiazolinylidene)AuCl to (thiazolinylidene)AuCl3.24 Because of
the very aggressive nature of chlorine gas, liquid bromine was
selected as a halogenation agent, as it does not require special
safety equipment and allows for a fairly straightforward and
general synthetic protocol.
We first attempted to generate a NHC-Au(III) complex from
a gold(III) salt, by direct reaction of the free carbene IPr (IPr
) 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with AuCl4-.
The reaction led to the formation of yellow metallic gold(0).
Study of the reaction mixture by 1H NMR spectroscopy showed
extensive signs of decomposition of the carbene and formation
of (IPr)AuCl, in a low yield (20%). This result is not surprising,
as the gold(III) cation possesses a very strong oxidant charac-
ter.25 Indeed, reduction of tetrachloroauric acid (HAuCl4) with
a 2-fold excess of stibine, arsine, or phosphine ligands is known
to generate the corresponding gold(I) complexes in good yield25
(Scheme 1).
(15) (a) Li, Z.; Shi, Z.; He, C. J. Organomet. Chem. 2005, 690, 5049-
5054. (b) Nair, V.; Vidya, N.; Abhilash, G. Tetrahedron Lett. 2006, 47,
2871-2873.
(16) (a) Hashmi, A. S. K.; Frost, T. M.; Bats, J. W. Org. Lett. 2001, 3,
3769-3771. (b) Diker, G.; Hildebrandt, D.; Liu, J.; Merz, K. Angew. Chem.,
Int. Ed. 2003, 42, 4399-4402. (c) Wei, C.; Li, C.-J. J. Am. Chem. Soc.
2003, 125, 9584-9585. (d) Yao, T.; Zhang, X.; Larock, R. C. J. Am. Chem.
Soc. 2004, 126, 11164-11165. (e) Yao, X.; Li, C.-J. J. Am. Chem. Soc.
2004, 126, 6884-6885. (f) Shi, Z.; He, C. J. Am. Chem. Soc. 2004, 126,
13596-13597. (g) Hashmi, A. S. K.; Schwarz, L.; Rubenbauer, P.; Blanco,
M. C. AdV. Synth. Catal. 2006, 348, 705-708. (h) Fu¨rstner, A.; Hannen,
P. Chem. Eur. J. 2006, 12, 3006-3019.
(17) (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, 45, 6545-
6547. (c) Gonza´lez-Arellano, C.; Corma, A.; Iglesias, M.; Sa´nchez, F. Chem.
Commun. 2005, 27, 3451-3453. (d) Lo, V. K.-Y.; Liu, Y.; Wong, M.-K.;
Che, C.-M. Org. Lett. 2006, 8, 1529-1532.
(18) (a) Duckworth, V. F.; Stephenson, N. C. Inorg. Chem. 1969, 8,
1661-1664. (b) Uson, R.; Laguna, A.; Buil, J. J. Organomet. Chem. 1975,
85, 403-408. (c) Godfrey, S. M.; Ho, N.; McAuliffe, C. A.; Pritchard, R.
G. Angew. Chem., Int. Ed. 1996, 35, 2344-2346.
(19) Vicente, J.; Arcas, A.; Mora, M.; Solans, X.; Font-Altaba, M. J.
Organomet. Chem. 1986, 309, 369-378.
(20) (a) Komiya, S.; Shibue, A. Organometallics 1985, 4, 684-687. (b)
Komiya, S.; Ishikawa, M.; Ozaki, S. Organometallics 1988, 7, 2238-2239.
(c) Blanco, M. C.; Ferna´ndez, E. J.; Olmos, M. E.; Pe´rez, J. Organometallics
2004, 23, 4373-4381.
(21) (a) Minghetti, G.; Bonati, F. J. Organomet. Chem. 1973, 54, C62-
C63. (b) Minghetti, G.; Bonati, F. J. Organomet. Chem. 1974, 73, C43-
C44. (c) Manojlovic´-Muir, L. J. Organomet. Chem. 1974, 73, C45-C46.
(d) Uso´n, R.; Laguna, A.; Brun, P.; Laguna, M.; Abad, M. J. Organomet.
Chem. 1981, 218, 265-273.
(22) (a) Schneider, D.; Schier, A.; Schmidbaur, H. Dalton Trans. 2004,
1995-2005. (b) Schneider, D.; Schuster, O.; Schmidbaur, H. Dalton Trans.
2005, 1940-1947.
(23) Welhan, M.; Thiel, R.; Fuchs, J.; Beck, W.; Fehlhammer, W. P. J.
Organomet. Chem. 2000, 613, 159-169.
(24) (a) Raubenheimer, H. G.; Olivier, P. J.; Lindeque, L.; Desmet, M.;
Hrusak, J.; Kruger, G. J. J. Organomet. Chem. 1997, 544, 91-100. (b)
Ku¨hlkamp, P.; Raubenheimer, H. G.; Field, J. S.; Desmet, M. J. Organomet.
Chem. 1998, 552, 69-74.
Oxidative addition of bromine by (IPr)AuCl (1) gave an
1
orange powder in high yield. H NMR analysis provided a
spectrum with the same pattern as that found for (IPr)AuCl,
but with significant changes in the chemical shifts for all protons.
We also noticed that the septuplet assigned to the protons from
the diisopropyl group was split into two distinct multiplets at
2.99 and 2.96 ppm. We attribute this small splitting to the
existence of two different complexes: (IPr)AuBr3 and (IPr)AuBr2-
Cl. 13C NMR spectra also support this hypothesis, as two
complexes with similar carbon skeletons and very similar signals
are observed.
(25) Schmidbaur, H. Gold, Progress in Chemistry, Biochemistry and
Technology; Wiley: West Sussex, England, 1999; p 358.

1378 Organometallics, Vol. 26, No. 6, 2007
de Fre´mont et al.
Figure 1. (NHC)AuICl complexes used as starting materials in this study.
Table 1. Chemical Shifts of the Carbenic Carbon in NMR
for the Gold(I) Halide Complexes
To exclude the formation of a mixture of (IPr)AuBr3-xClx,
we proceeded to convert the reported7a,26 (NHC)AuI chloride
complexes (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)-
gold(I) chloride (1; (IPr)AuCl26), (1,3-bis(2,4,6-trimethylphen-
δC
(ppm)
δC
(ppm)
∆δC
(ppm)
(NHC)AuCl
(NHC)AuBr
(IPr)AuCla (1)
(IMes)AuClb (2)
(SIPr)AuClb (3)
(SIMes)AuClb (4)
(ICy)AuCla (5)
(IAd)AuCla (6)
(ItBu)AuCla (7)
175.1
173.4
196.1
195.0
168.0
166.3
168.2
(IPr)AuBrb (8)
179.0
176.7
199.0
198.1
172.1
170.2
172.4
+3.9
+3.3
+2.9
+3.1
+4.1
+3.9
+4.2
yl)imidazol-2-ylidene)gold(I) chloride (2; (IMes)AuCl26), (1,3-
bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene)gold(I) chloride
(3; (SIPr)AuCl26), (1,3-bis(2,4,6-trimethylphenyl)imidazolidin-
2-ylidene)gold(I) chloride (4; (SIMes)AuCl26), (1,3-bis(cyclo-
hexyl)imidazol-2-ylidene)gold(I) chloride (5; (ICy)AuCl26), (1,3-
bis(adamantyl)imidazol-2-ylidene)gold(I) chloride (6; (IAd)-
AuCl26), and (1,3-bis(tert-butyl)imidazol-2-ylidene)gold(I) chlo-
ride (7; (ItBu)AuCl7a) (Figure 1) into their (NHC)AuI bromide
relatives 8-14 by use of a metathetical reaction with LiBr. As
the gold(I) cation is one of the softest available acids, we
suspected that the bromide anion would easily replace the
chloride anion.
(IMes)AuBrb (9)
(SIPr)AuBrb (10)
(SIMes)AuBrb (11)
(ICy)AuBrb (12)
(IAd)AuBrb (13)
(ItBu)AuBrb (14)
a NMR recorded in CD2Cl2. b NMR recorded in CDCl3.
congeners of the (NHC)AuCl and (NHC)AuBr series. The
substitution of a chloride by a bromide has a very small effect
on the environment seen by the protons of the different
complexes. 13C NMR spectra display resonances for the different
carbenic carbons between 166.3 and 175.1 ppm for the
unsaturated imidazole moieties and around 195 ppm for the
saturated imidazole moieties (Table 1). The intensity of this
resonance is weak, since the carbenic carbon is a quaternary
center and is affected by the quadrupolar moment of the gold
The complexes (NHC)AuCl (1-7) were stirred with a large
excess of lithium bromide, at room temperature, with acetone
or THF as solvent. The reactions are not sensitive to air and
provide the desired bromide complexes (IPr)AuBr (8), (IMes)-
AuBr (9), (SIPr)AuBr (10), (SIMes)AuBr (11), (ICy)AuBr (12),
(IAd)AuBr (13), and (ItBu)AuBr (14) as white powders (eq 1).
The protocol furnishes the products in good yields after stirring
for 24 h.
3
atom (I ) /2).
Herrmann et al.27 have postulated that the chemical shift of
the carbenic carbon can be correlated to the acidity of the metal
to which the NHC is bound. Indeed, a free NHC ligand, with
no electronic donation toward a Lewis acid, would have a very
low-field signal, usually above 200 ppm, reflecting the avail-
ability of an excess of electron density on the carbene carbon.
In contrast, a bond with a metal will displace the chemical shift
to a higher field value when the electronic density from the
carbene is partially transferred to the metal by σ donation. By
comparison of the chemical shifts of the carbenic carbon,
between the chloride and the bromide series of the gold(I)
complexes, a consistent shift of 3-4 ppm to lower field due to
the halide exchange was observed that we attribute to a small
variation of the acidity of the gold center. We reasoned that the
metal acidity is less, due to the lower electronegativity of
bromine versus that of chlorine. This result is in good agreement
with the study published by Baker et al.7a Crystals of (IPr)-
AuBr (8) were grown by slow diffusion in a mixture of DCM
and hexane and allowed us to perform a single-crystal X-ray
diffraction study (Figure 2).
(NHC)AuCl + LiBr acetone8(NHC)AuBr + LiCl (1)
It is interesting to note that while these complexes are
air-stable, the presence of water leads to rapid decomposition
with appearance of purple colloidal gold(0) and formation of
imidazolium salts. This trend is strongly accentuated for
complexes bearing saturated carbene moieties (10 and 11).
While Baker et al.7a reported a reaction time of 16 h to convert
(ItBu)AuCl (7) into (ItBu)AuBr (14), we selected the longer
reaction time of 24 h as a general reaction time, as we noticed
that the metathesis reaction could require longer time to reach
completion as a function of the NHC. This reaction time is then
general and has not been optimized for each NHC employed.
1
The H NMR spectra of complexes 8, 9, and 12-14 display a
low-field singlet between 6.95 and 7.17 ppm, assigned to the
two protons located on the unsaturated imidazole backbone.
Spectra of complexes 10 and 11 display a more upfield singlet
at 4.04 and 3.97 ppm, respectively, assigned to the four protons
located on the saturated imidazole backbone. For all complexes,
all signals expected for the N-aryl and N-alkyl chains are
present. As expected, no significant change in the chemical shift
(less than 0.1 ppm) is visible for the signals attributed to
The gold atom is two-coordinate, as is usual for gold(I)
complexes, and exhibits a linear geometry with a C(1)-Au-
Br bond angle value of 180.0°. The C(1)-Au bond length (1.975
Å) is in good agreement with those for reported NHC-gold(I)
complexes.7a,26,28 The Au-Br bond length (2.381 Å) is in the
(26) de Fre´mont, P.; Scott, N. M.; Stevens, E. D.; Nolan, S. P.
Organometallics 2005, 24, 2411-2418.
(27) Herrmann, W. A.; Runte, O.; Artus, G. J. Organomet. Chem. 1995,
501, C1-C4.

N-Heterocyclic Carbene Gold(III) Complexes
Organometallics, Vol. 26, No. 6, 2007 1379
1
The H NMR spectra of the complexes 15, 16, and 19-21
display a low-field singlet between 7.21 and 7.53 ppm, assigned
to the two protons located on the unsaturated imidazole
backbone. The chemical shifts are slightly shifted downfield,
in comparison to the shifts for the (NHC)AuBr series, likely
due to the double bond being less rich in electron density. It is
reasonable to assume that gold(III), being more acidic than gold-
(I), induces a greater delocalization of the electronic density
from the carbon-carbon double bond to the carbenic carbene,
through the entire aromatic system. There is no sign of attack
by the bromine on the double bond. 1H NMR spectra of 17 and
18 display more upfield singlets at 4.29 and 4.23 ppm,
respectively, assigned to the four protons located on the saturated
imidazole backbone. For all complexes, signals expected for
the N-aryl and N-alkyl chains are present. 13C NMR resonances
of the different carbenic carbons are characterized by a weak
upfield signal between 132.9 and 146.2 ppm for the unsaturated
imidazole moieties and around 173 ppm for the saturated
imidazole moieties (Table 2).
Differences in the carbenic carbon shifts between the two
series of gold bromide complexes are found to be between 24.9
and 38.3 ppm. However, the shifts are within the range for the
oxidation of gold(I) thiazolinylidene chloride reported by
Raubenheimer et al.24 Expectedly, the differences indicate an
increase of acidity of the gold atom associated with an increase
in oxidation state (Table 2). It is reasonable to assume that a
smaller upfield shift indicates an attenuated acidity of the gold
atom, likely due to a better donation of the carbene moieties. If
this is correct, the saturated SIPr and SIMes carbenes provide
the greatest electronic density to the gold(III) cation. A
comparison between unsaturated carbenes bearing aromatic and
alkyl R groups is also possible. Interestingly, IPr and IMes
appear to be better σ donors than IAd, ICy, and ItBu. All NHC
ligands display the same donor property trends as seen for the
gold(I) complexes.26 It is also interesting to note that the
chemical shifts of the carbenic carbon in these gold(III)
complexes, especially the complexes bearing the alkyl R group,
are extremely close to the reported value for the imidazolium
salts, with a difference of 0.9-2.3 ppm. Unfortunately, we
cannot unequivocally quantify in an absolute sense the electronic
effect associated with electronic density residing on the carbenic
carbene, as the -AuIIIBr3 moiety is not isolobal with the acidic
proton borne by imidazolium salts.7a
Figure 2. Ball-and-stick representation of (IPr)AuBr. Hydrogen
atoms have been omitted for clarity.
range of those for known bromide gold(I) salts and complexes.7a,29
The minimal Au‚‚‚Au distance is 8.431 Å, excluding any
aurophilic interactions, which require a distance shorter than
3.60 Å between gold(I) cations.30 There is no major structural
difference between (IPr)AuCl (1) and (IPr)AuBr (8). Crystals
of other (NHC)AuBr complexes, described in this paper, can
be grown in a mixture of DCM and heptane.
Direct addition of a slight excess of elemental bromine (Br2)
to solutions of the complexes (IPr)AuBr (8), (IMes)AuBr (9),
(SIPr)AuBr (10), (SIMes)AuBr (11), (ICy)AuBr (12), (IAd)-
AuBr (13), and (ItBu)AuBr (14) gives the desired complexes
(IPr)AuBr3 (15), (IMes)AuBr3 (16), (SIPr)AuBr3 (17), (SIMes)-
AuBr3 (18), (ICy)AuBr3 (19), (IAd)AuBr3 (20), and (ItBu)AuBr3
(21) in good yields, as yellow or orange powders stable in air
(eq 2).
CH2Cl2
(NHC)AuBr + Br2 oxidative addition8 (NHC)AuBr3
(2)
Initially, the reactions were allowed to proceed overnight,
but we noticed that the reactions were very fast at room
temperature (even at -78 °C) and were complete in less than
1/2 h. This is not surprising, since redox reactions involving
metals are known to proceed with rapid kinetics. We did not
observe any NHC-Au bond cleavage or rearrangement by using
bulky carbenes, such as IAd and ItBu, as reported for the
oxidation of sterically demanding gold(I) phosphines.22b At-
tempts to synthesize (IMes)AuBr3 (16) and (SIMes)AuBr3 (18)
at room temperature failed and gave decomposition products
with no trace of the desired complexes, even when a substo-
ichiometric amount of bromine was used. At -78 °C, the
reaction proceeded smoothly without any trace of decomposition
product. These particular synthetic conditions for the complexes
bearing the IMes and SIMes moieties again illustrate the
difference in reactivity encountered with these two carbenes on
the chemistry of metals from group 1131 (Scheme 2).
To unambiguously characterize all these new gold(III)
complexes, X-ray-quality crystals were grown in a mixture of
DCM and heptane. Ball-and-stick representations are provided
in Figures 3-5, and crystallographic data are given in Table 5.
All (NHC)AuBr3 complexes have a four-coordinate gold
atom, in a square-planar environment, as expected for d8 metals.
The C(1)-Au-Br(2) and Br(1)-Au-Br(3) bonds are nearly
linear, with angles between 173.73 and 178.66° (Table 3).
All C(1)-Au distances lie in the range of 2.01-2.05 Å,
regardless of whether the gold center bears a saturated or
unsaturated imidazole motif (Table 4). These metrical param-
eters are in close agreement with those of reported organogold-
(III) complexes32 possessing carbon-gold bond lengths between
2.01 and 2.07 Å. There is no discernible correlation between
the gold-carbon bond length and the electronic or steric
parameters associated with the NHCs employed.
(28) (a) Wang, H. M. J.; Chen, C. Y. L.; Lin, I. J. B. Organometallics
1999, 18, 1216-1223. (b) Vicente, J.; Chicote, M.-T.; Abrisqueta, M. D.;
Alvarez-Falco´n, M. M.; Ramirez de Arellano, M. C.; Jones, P. G.
Organometallics 2003, 22, 4327-4333. (c) de Fre´mont, P.; Stevens, E. D.;
Eelman, M. D.; Fogg, D. E.; Nolan, S. P. Organometallics 2006, 25, 5824-
5828.
All Br-Au distances were found to be between 2.38 and 2.47
(29) Beurskens, P. T.; Blaauw, H. J. A.; Cras, J. A.; Steggerda, J. J.
Inorg. Chem. 1968, 7, 805-810.
Å (Table 4). They are similar to those of reported Br-AuIII
(30) White-Morris, R. L.; Olmstead, M. M.; Jiang, F.; Tinti, D. S.; Balch,
A. L. J. Am. Chem. Soc. 2002, 124, 2327-2336.
(32) (a) Cinellu, M. A.; Minghetti, G.; Pinna, M. V.; Stoccoro, S.; Zucca,
A.; Manassero, M. J. Chem. Soc., Dalton Trans. 1999, 2823-2831. (b)
Wile, B. M.; Burford, R. J.; McDonald, R.; Ferguson, M. J.; Stradiotto, M.
Organometallics 2006, 25, 1028-1035.
(31) de Fre´mont, 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.

1380 Organometallics, Vol. 26, No. 6, 2007
Scheme 2. Reactivity of Complexes Bearing Different NHC Moieties
de Fre´mont et al.
Table 2. NMR Chemical Shifts of the Carbenic Carbon for
(III) complexes synthesized, (IPr)AuBr3 displays the best
catalytic activity (Table 6).
the Au-Br Complexes
The solvent screening indicates that an alcohol is required
for the catalysis to proceed efficiently (Table 7). There is no
trace of enol ethers or acetals resulting from the addition of
alcohol to the alkynes as a competing reaction.
δC
δC
∆δC
(NHC)AuBr
(ppm)
(NHC)AuBr3
(ppm)
(ppm)
(IPr)AuBra (8)
179.0
176.7
199.0
198.1
172.1
170.2
172.4
(IPr)AuBr3a (15)
(IMes)AuBr3a (16)
(SIPr)AuBr3a (17)
(SIMes)AuBr3a (18)
(ICy)AuBr3a (19)
(IAd)AuBr3a (20)
(ItBu)AuBr3a (21)
146.2
144.4
174.1
172.3
136.8
132.9
134.2
-32.8
-32.3
-24.9
-25.8
-35.3
-37.3
-38.3
(IMes)AuBra (9)
(SIPr)AuBra (10)
(SIMes)AuBra (11)
(ICy)AuBra (12)
(IAd)AuBra (13)
(ItBu)AuBra (14)
The present complexes were inefficient with internal alkynes,
confirming the likely formation of a gold(III) vinyl secondary
carbocation as an early reaction intermediate14d,35 (Table 8).
However, the most dramatic result is the acceleration obtained
when 1 equiv of a silver(I) salt is added as cocatalyst. This
permits the rapid and quantitative formation of products while
reducing catalyst loading from 10 to 2 mol % (Table 9).
These initial catalytic observations raise many questions in
term of mechanism of activation and nature of the true catalytic
species. Ongoing studies in our laboratories are aimed at
answering these questions.
(NHC)‚HCl
δ
C (ppm)
∆δC (ppm)
(IPr)‚HClb
132.2
134.8
160.0
160.2
134.5
132.1
132.7
+14.0
+9.6
(IMes)‚HClb
(SIPr)‚HClb
(SIMes)‚HClb
(ICy)‚HClb
(IAd)‚HClb
(ItBu)‚HClb
+14.1
+12.1
+2.3
+0.8
+1.5
a
b
NMR recorded in CDCl3. NMR recorded in DMSO-d6.
Conclusion
We report the synthesis of the first series of well-defined
(NHC)AuIII complexes. Their straightforward synthesis can be
carried out under aerobic conditions by oxidative addition of
elemental bromine to the corresponding (NHC)AuI precursor.
NMR and crystallographic data provide detailed information
concerning the steric constraints and electronic effects produced
by the different carbene environments around the Au(III) center.
We also report the first use of a (NHC)AuIII complex to catalyze
an organic transformation. While the initial catalytic tests
provide modest results, addition of a silver salt as a cocatalyst
allowed the formation of a very efficient catalytic system. We
are currently investigating the possible mechanism at play in
this and related reactions.
complexes, where the bromide-gold(III) bond lengths are
between 2.38 and 2.65 Å.22b,33 The carbene ligands (except IPr
and IAd) induce a trans influence with a lengthening of the
Au-Br(2) bond. This effect is less pronounced than that
observed for the phosphine-AuBr3 complexes described by
Schmidbaur et al.22b It is surprising to observe that, while there
is no visible trans effect for the complex (IAd)AuBr3 (20), the
three different Au-Br bonds are slightly longer than expected
when compared to our other gold(III) complexes. There is no
close contact between gold atoms. This is not surprising, since
aurophilic interactions only apply for d10 gold(I) cations.34 There
is no insertion of bromine in the crystal lattices, as reported for
some gold(III) phosphine tribromide complexes.22b
We were interested in testing the catalytic activity of our new
well-defined gold(III) complexes to mediate the addition of
water to alkynes. As gold(I) and gold(III) salts are known to
catalyze this reaction, we used a reported work35 as reference
to gauge the catalytic activity of our system. Of the many Au-
Experimental Section
General Considerations. All reactions using (NHC)AuCl or
(NHC)AuBr as starting material were carried out in air. All alkynes
were used as received (Aldrich, Acros). All reactions were carried
out open to air unless indicated otherwise. Solvents for NMR
spectroscopy were dried over molecular sieves. NMR spectra were
collected on a 500 or 400 MHz Varian Gemini spectrometer. Flash
(33) (a) Burawoy, A.; Gibson, C. S.; Hampson, G. C.; Powell, H. M. J.
Chem. Soc. 1937, 2, 1690-1695. (b) Perutz, M. F.; Weisz, O. J. Chem.
Soc. 1946, 438-442. (c) Lo¨rcher, K. P.; Stra¨hle, J. Z. Naturforsch., B:
Anorg. Chem., Org. Chem. 1975, 30, 662-664. (d) Fabretti, A. C.; Giusti,
A.; Malavasi, W. J. Chem. Soc., Dalton Trans. 1990, 3091-3093.
(34) Pyykko¨, P.; Tamm, T. Organometallics 1998, 17, 4842-4852.
(35) Schneider, S. K.; Herrmann, W. A; Herdtweck, E. Z. Anorg. Allg.
Chem. 2003, 629, 2363-2370.

N-Heterocyclic Carbene Gold(III) Complexes
Organometallics, Vol. 26, No. 6, 2007 1381
Figure 3. Ball-and-stick representations of (IPr)AuBr3 (15) and (IMes)AuBr3 (16). Hydrogen atoms have been omitted for clarity.
Figure 4. Ball-and-stick representations of (SIPr)AuBr3 (17) and (SIMes)AuBr3 (18). Most hydrogen atoms have been omitted for clarity.
chromatography was performed on silica gel (230-400 mesh;
Natland International Corp.). Elemental analyses were performed
by Robertson Microlit Laboratories. (NHC)AuCl complexes were
synthesized according to literature procedures.26
179.0 (s, C carbene), 145.8 (s, CH aromatic), 134.2 (s, CH
aromatic), 131.0 (s, CH aromatic), 124.5 (s, CH imidazole), 123.3
(s, CH aromatic), 29.0 (s, CH (CH3)2), 24.7 (s, CH (CH3)2), 24.2
(s, CH (CH3)2). Anal. Calcd for C27H36N2AuBr (665.21): C, 48.74;
H, 5.41; N, 4.21. Found: C, 48.68; H, 5.17; N, 3.94.
Synthesis of [(IMes)AuBr] (9). A protocol similar to that used
for 8 gave 9 (from 0.90 g, 1.68 mmol, of 2) as a white solid.
Yield: 0.780 g (80%). 1H NMR (CDCl3): δ 7.09 (s, 2H, CH
imidazole), 6.98 (s, 4H, CH aromatic), 2.33 (s, 6H, CH3), 2.10 (s,
12H, CH3). 13C NMR (CDCl3): δ (ppm) 176.7 (s, C carbene), 139.7
(s, CH aromatic), 134.6 (s, CH aromatic), 134.5 (s, CH aromatic),
129.4 (s, CH aromatic), 122.0 (s, CH imidazole), 21.1 (s, CH3),
17.7 (s, CH3). Anal. Calcd for C21H24N2AuBr (581.02): C, 43.39;
H, 4.16; N, 4.82. Found: C, 43.51 H, 3.88; N, 4.66.
Synthesis of [(IPr)AuBr] (8). In a flask, (IPr)AuCl (1; 1.00 g,
1 equiv, 1.61 mmol) was dissolved in 5 mL of acetone with LiBr
(1.19 g, 8.5 equiv, 13.70 mmol) and the solution was stirred at
room temperature for 24 h. The acetone was removed by vacuum
and 2 mL of DCM added to the residue. The organic phase was
dried over MgSO4, since LiBr is extremely hygroscopic. The
solution was filtered over a plug of silica gel (3 g). After reduction
of the volume of DCM to 0.5 mL, 5 mL of pentane was added,
which led to the appearance of a white precipitate. This precipitate
was filtered, washed with 5 mL of cold pentane, and dried to afford
1
the desired complex. Yield: 0.94 g (87%). H NMR (CDCl3): δ
Synthesis of [(SIPr)AuBr] (10). In a flask (SIPr)AuCl (3; 1.00
g, 1 equiv, 1.61 mmol) was dissolved in 5 mL of acetone, and
LiBr (1.19 g, 8.5 equiv, 13.70 mmol) was added. This solution
was stirred at room temperature for 48 h. The acetone was removed
by vacuum and replaced by DCM. This solution was filtered over
7.50 (t, J ) 8.0 Hz, 2H, CH aromatic), 7.28 (d, J ) 8.0 Hz, 4H,
CH aromatic), 7.17 (s, 2H, CH imidazole), 2.56 (septet, J ) 7.0
Hz, 4H, CH(CH3)2), 1.34 (d, J ) 7.0 Hz, 12H, CH (CH3)2), 1.22
(d, J ) 7.0 Hz, 12H, CH (CH3)2). 13C NMR (CDCl3): δ (ppm)

1382 Organometallics, Vol. 26, No. 6, 2007
de Fre´mont et al.
Figure 5. Ball-and-stick representations of (ICy)AuBr3 (19), (IAd)AuBr3 (20), and (ItBu)AuBr3 (21). Hydrogen atoms have been omitted
for clarity.
Table 3. Selected Bond Angle Values (deg) for (NHC)AuBr3 Complexes
(NHC)AuBr3
C(1)-Au-Br(1)
C(1)-Au-Br(2)
C(1)-Au-Br(3)
Br(1)-Au-Br(2)
Br(1)-Au-Br(3)
Br(2)-Au-Br(3)
(IPr)AuBr3 (15)
(IMes)AuBr3 (16)
(SIPr)AuBr3 (17)
(SIMes)AuBr3 (18)
(ICy)AuBr3 (19)
(IAd)AuBr3 (20)
(ItBu)AuBr3 (21)
92.3(5)
91.4(2)
88.0(4)
91.0(4)
89.2(6)
87.80(15)
90.69(14)
178.2(5)
178.0(2)
174.9(4)
175.9(4)
178.3(6)
178.66(16)
177.97(14)
88.8(5)
89.1(2)
93.0(4)
90.9(4)
86.4(6)
85.93(15)
87.16(14)
87.66(15)
89.69(7)
89.89(7)
88.71(7)
91.92(11)
93.53(2)
91.32(2)
178.79(12)
177.78(5)
177.76(4)
176.60(7)
175.45(11)
173.73(3)
177.82(2)
91.21(16)
89.83(6)
89.28(7)
89.66(7)
92.51(10)
92.73(2)
90.83(3)
Yield: 0.980 g (78%). 1H NMR (CDCl3): δ 6.95 (s, 2H, CH
imidazole), 4.57 (m, 2H, NCH cyclohexyl), 2.07 (m, 4H, CH2),
1.86 (m, 4H, CH2), 1.73 (m, 2H, CH2), 1.56 (m, 4H, CH2), 1.43
(m, 4H, CH2), 1.22 (m, 2H, CH). 13C NMR (CDCl3): δ (ppm)
172.1 (s, C carbene), 117.3 (s, CH imidazole), 61.0 (s, NCH
cyclohexyl), 34.3 (s, CH2), 25.5 (s, CH2), 25.3 (s, CH2). Anal. Calcd
for C15H24N2AuBr (509.20): C, 35.38; H, 4.75; N, 5.50. Found:
C, 35.50; H, 4.73; N, 5.30.
Table 4. Selected Au-X Bond Distances (Å) in (NHC)AuBr3
Complexes
(NHC)AuBr3
Au-C(1) Au-Br(1) Au-Br(2) Au-Br(3)
2.048(19) 2.384(3) 2.386(4) 2.397(3)
2.009(8) 2.4156(15) 2.4224(12) 2.4123(14)
2.042(13) 2.405(2) 2.4452(18) 2.408(2)
(IPr)AuBr3 (15)
(IMes)AuBr3 (16)
(SIPr)AuBr3 (17)
(SIMes)AuBr3 (18) 2.052(13) 2.4108(15) 2.4468(17) 2.4169(17)
(ICy)AuBr3 (19)
(IAd)AuBr3 (20)
(ItBu)AuBr3 (21)
2.04(2)
2.052(6)
2.015(5)
2.405(3)
2.4465(8)
2.4209(6)
2.444(3)
2.4496(7)
2.4403(6)
2.410(3)
2.4426(7)
2.4638(6)
Synthesis of [(IAd)AuBr] (13). A protocol similar to that used
for 8 gave 13 (from 1.00 g, 1.76 mmol, of 6) as a white solid.
Yield: 0.590 g (55%). 1H NMR (CDCl3): δ 7.08 (s, 2H, CH
imidazole), 2.56 (m, 14H, CH2 adamantyl), 2.26 (m, 6H, CH2
adamantyl), 1.75 (m, 10H, CH2 adamantyl). 13C NMR (CDCl3): δ
(ppm) 170.2 (C carbene), 115.2 (s, CH imidazole), 59.2 (s, NCH
adamantyl), 44.0 (s, CH2), 35.7 (s, CH2), 29.8 (s, CH2). Anal. Calcd
for C23H32N2AuBr (613.10): C, 45.04; H, 5.26; N, 4.57. Found:
C, 44.97; H, 5.01; N, 4.44.
Synthesis of [(ItBu)AuBr]7a (14). A protocol similar to that used
for 8 gave 14 (from 1.00 g, 2.42 mmol, of 7) as a white solid.
Yield: 0.810 g (73%). 1H NMR (CDCl3): δ 7.09 (s, 2H, CH
imidazole), 1.87 (s, 18H, C(CH3)3). 13C NMR (CDCl3): δ (ppm)
172.4 (s, C carbene), 116.4 (s, CH imidazole), 59.2 (s, C(CH3)3),
31.9 (s, C(CH3)3).
a plug of silica gel and dried over MgSO4. After filtration and
reduction of the volume of DCM to 0.5 mL, 5 mL of pentane was
added until appearance of a white precipitate. This precipitate was
filtered, washed with pentane, and dried to afford the desired
complex. It is worthy of note that washing the complex with water
leads to its decomposition to the corresponding imidazolium salt.
1
Yield: 0.610 g (57%). H NMR (CDCl3): δ 7.41 (t, J ) 7.5 Hz,
2H, CH aromatic), 7.23 (d, J ) 7.5 Hz, 4H, CH aromatic), 4.04 (s,
4H, CH2 imidazole), 3.05 (septet, J ) 6.5 Hz, 4H, CH(CH3)2), 1.41
(d, J ) 6.5 Hz, 12H, CH(CH3)2), 1.33 (d, J ) 6.5 Hz, 12H, CH-
(CH3)2). 13C NMR (CDCl3): δ (ppm) 199.0 (s, C carbene), 146.7
(s, CH aromatic), 134.1 (s, CH aromatic), 130.2 (s, CH aromatic),
124.8 (s, CH aromatic), 53.7 (s, CH2 imidazole), 29.2 (s, CH(CH3)2),
25.3 (s, CH(CH3)2), 24.3 (s, CH(CH3)2). Anal. Calcd for C27H38N2-
AuBr (667.14): C, 48.58; H, 5.74; N, 4.20. Found: C, 48.60; H,
5.60; N, 4.05.
Synthesis of [(IPr)AuBr3] (15). In a flask (IPr)AuBr (8; 0.840
g, 1 equiv, 1.262 mmol) was dissolved in 5 mL of DCM with
bromine (0.240 g, 1.2 equiv, 1.514 mmol). The solution was stirred
at room temperature for 1 h. The volume of DCM was reduced to
0.5 mL by vacuum, at the same time removing the excess bromine.
Then 5 mL of pentane was added to produce an orange precipitate.
This solid was collected on a filter, washed with 5 mL of pentane,
and dried to afford the desired complex. Yield: 0.870 g (84%). 1H
NMR (CDCl3): δ 7.54 (t, J ) 7.5 Hz, 2H, CH aromatic), 7.35 (d,
J ) 7.5 Hz, 4H, CH aromatic), 7.35 (s, 2H, CH imidazole), 2.99
(septet, J ) 6.5 Hz, 4H, CH(CH3)2), 1.41 (d, J ) 6.5 Hz, 12H,
CH(CH3)2), 1.13 (d, J ) 6.5 Hz, 12H, CH(CH3)2). 13C NMR
(CDCl3): δ (ppm) 146.2 (s, C carbene), 145.8 (s, CH aromatic),
132.6 (s, CH aromatic), 131.6 (s, CH aromatic), 126.1 (s, CH
Synthesis of [(SIMes)AuBr] (11). A protocol similar to that
used for 10 provided 11 (from 1.00 g, 1.86 mmol, of 4) as a white
1
solid. Yield: 0.780 g (72%). H NMR (CDCl3): δ 6.93 (s, 4H,
CH aromatic), 3.97 (s, 4H, CH2 imidazole), 2.31 (s, 12H, CH3),
2.29 (s, 6H, CH3). 13C NMR (CDCl3): δ (ppm) 198.1 (s, C carbene),
139.1 (s, CH aromatic), 135.7 (s, CH aromatic), 134.7 (s, CH
aromatic), 130.0 (s, CH aromatic), 50.9 (s, CH2 imidazole), 21.3
(s, CH3), 18.2 (s, CH3). Anal. Calcd for C21H26N2AuBr (583.08):
C, 43.24; H, 4.49; N, 4.80. Found: C, 43.14 H, 4.22; N, 4.69.
Synthesis of [(ICy)AuBr] (12). A protocol similar to that used
for 8 gave 12 (from 1.15 g, 2.47 mmol, of 5) as a white solid.

N-Heterocyclic Carbene Gold(III) Complexes
Organometallics, Vol. 26, No. 6, 2007 1383
Table 5. Crystallographic Data for Complexes 8 and 15-21
8
15
16
17
formula
Mr
C27H36N2AuBr
665.45
C27.50H37N2AuBr3
867.74
C21H24N2AuBr3
741.12
C28H40N2AuBr3Cl2
912.22
cryst syst
space group
cell constants
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
orthorhombic
Pccn
monoclinic
P21/n
orthorhombic
P212121
monoclinic
P21/c
10.9117(6)
12.6771(7)
19.9643(10)
90.00
90.00
90.00
16.950(2)
19.403(3)
20.417(3)
90.00
110.421(3)
90.00
10.6845(6)
14.2833(8)
15.7115(9)
90.00
90.00
90.00
10.677(2)
16.593(4)
19.306(4)
90.00
99.664(4)
90.00
2761.6 (3)
4
6293.0(15)
8
2397.7(2)
4
3371.5(12)
4
Z
λ (Å)
0.710 73
1.601
6.789
1304
295(2)
0.710 73
1.832
8.588
3336
295(2)
0.710 73
2.053
11.143
1392
295(2)
0.710 73
1.797
8.096
1760
295(2)
F(calcd) (g/cm3)
µ (mm-1
F(000)
T (K)
)
2θmax (deg)
55.04
17 717
3140
0.0505
52.774
39 800
8206
0.0574
46.718
25 184
3137
0.0410
53.004
16 323
4009
0.0820
no. of rflns measd
no. of indep rflns
Rint
no. of data/restraints/params
R
2026/0/146
0.0758
5509/563/638
0.0967
2891/705/330
0.0701
2934/335/377
0.0992
w (F2 all rflns)
R (F, >4σ(F))
0.0321
+0.928, -0.986
0.0495
+0.925, -1.413
0.0300
+0.744, -0.530
0.0589
+1.130, -0.775
max, min ∆F (e Å-3
)
18
19
20
21
formula
Mr
cryst syst
space group
cell constants
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C
21.50H27.01N2AuBr3Cl1.01
C
15.50H25N2AuBr3Cl
C23H32N2AuBr3
773.20
monoclinic
P21/c
C11H20N2AuBr3
616.99
monoclinic
P21/c
785.90
monoclinic
P21/c
711.52
orthorhombic
Pccn
8.7226(8)
16.0470(15)
19.7576(18)
90.00
95.611(2)
90.00
15.0878(9)
19.9495(13)
14.6881(9)
90.00
90.00
90.00
16.4445(16)
11.1131(10)
12.9782(12)
90.00
90.044(2)
90.00
9.2947(5)
15.3944(8)
12.1528(7)
90.00
96.7460(10)
90.00
2752.3(4)
4
4421.0(5)
8
2371.8(4)
4
1726.86(16)
4
Z
λ (Å)
0.710 73
1.897
9.808
1485
295(2)
0.710 73
2.138
12.198
2664
295(2)
0.710 73
2.165
11.270
1472
295(2)
0.710 73
2.373
15.445
1136
295(2)
F(calcd) (g/cm3)
µ (mm-1
F(000)
T (K)
)
2θmax (deg)
no. of rflns measd
no. of indep rflns
Rint
61.21
39 906
3600
0.0603
58.484
36 155
2892
0.0586
46.718
21 229
3429
0.0494
52.846
21 428
3569
0.0418
no. of data/restraints/params
R
3197/240/278
0.1291
2609/186/217
0.1339
2888/280/390
0.0725
3107/491/235
0.0548
w (F2 all rflns)
R (F, >4σ(F))
0.0490
+1.610, -0.797
0.0438
+0.778, -1.198
0.0295
+1.244, -1.443
0.0259
+1.127, -0.907
max, min ∆F (e Å-3
)
Table 6. Screening of (NHC)AuBr3 Complexes in Addition
of Water to Alkynesa
aromatic), 124.7 (s, CH imidazole), 29.1 (s, CH(CH3)2), 26.5 (s,
CH(CH3)2), 23.0 (s, CH(CH3)2). Anal. Calcd for C27H36N2AuBr3
(825.27): C, 39.30; H, 4.40; N, 3.39. Found: C, 39.26; H, 4.12;
N, 3.28.
Synthesis of [(IMes)AuBr3] (16). In a flask (IMes)AuBr (9;
0.100 g, 1 equiv, 0.179 mmol) was dissolved in 1 mL of DCM
and cooled to -78 °C, and then bromine (0.040 g, 1.2 equiv, 0.214
mmol) was added and the solution was stirred for 20 min. The
excess bromine was removed by vacuum while the temperature
was slowly raised to room temperature. Then 0.5 mL of DCM was
added, followed by 5 mL of pentane, until the appearance of an
orange precipitate. This suspension was filtered and the solid washed
with pentane and dried to afford the desired complex. Yield: 0.120
entry
catalyst
time (h)
yield (%)b
1
2
3
4
5
6
7
-
-
-
24
24
24
48
48
NR
NR
94
95
91
IPr
AuCl3
(IPr)AuBr3
(IMes)AuBr3
(IAd)AuBr3
(ItBu)AuBr3
35
92
1
a Reaction conditions: 10 mol % of catalyst, 0.5 mL of H2O, 0.5 mL of
methanol, 1 mmol of phenylacetylene. b GC yields, an average of two runs.
NR ) no reaction.
g (94%). H NMR (CDCl3): δ 7.32 (s, 2H, CH imidazole), 7.06
(s, 4H, CH aromatic), 2.37 (s, 6H, CH3), 2.29 (s, 12H, CH3). 13C
NMR (CDCl3): δ (ppm) 144.4 (s, C carbene), 140.9 (s, CH
aromatic), 135.2 (s, CH aromatic), 132.8 (s, CH aromatic), 130.1
(s, CH aromatic), 125.9 (s, CH2 imidazole), 21.3 (s, CH3), 19.7 (s,
CH3). Anal. Calcd for C21H24N2AuBr3 (740.82): C, 34.03; H, 3.26;
N, 3.78. Found: C, 34.13; H, 3.49; N, 3.52.

1384 Organometallics, Vol. 26, No. 6, 2007
de Fre´mont et al.
Table 7. Solvent Screening in Addition of Water to Alkynesa
Table 9. Effect of a Silver Salt on Catalytic Addition of
Water to Alkynesa
entry
solvent
time (h)
yield (%)b
1
2
3
4
5
MeOH
IPA
H2O
MeCN
THF
24
36
48
95
47
62c
NR
NR
a Reaction conditions: 10 mol % of catalyst, 0.5 mL of H2O, 0.5 mL of
methanol, 1 mmol of phenylacetylene. b GC yields, an average of two runs.
NR ) no reaction. c Small amount of acetone added to solubilize the
catalyst.
loading (mol %)
catalyst
(IPr)AuBr3
(IPr)AuBr3 + AgPF6
AgPF6
time (h)
yield (%)b
10
2
10
24
1
24
95
99
NR
Table 8. Various Alkynes Screened in Addition of Water to
Alkynesa
a Reaction conditions: 0.5 mL of H2O, 0.5 mL of methanol, 1 mmol of
phenylacetylene. b GC yields, an average of two runs. NR ) no reaction.
(s, CH3), 20.3 (s, CH3). Anal. Calcd for C21H26N2AuBr3 (742.88):
C, 33.94; H, 3.53; N, 3.77. Found: C, 34.19; H, 3.57; N, 3.66.
Synthesis of [(ICy)AuBr3] (19). A protocol similar to that used
for 15 gave 19 (from 0.365 g, 0.789 mmol of 12) as a yellow solid.
Yield: 0.470 g (89%). 1H NMR (CDCl3): δ 7.21 (s, 2H, CH
imidazole), 4.50 (m, 2H, NCH cyclohexyl), 2.24 (m, 4H, CH2),
1.90 (m, 4H, CH2), 1.77 (m, 2H, CH2), 1.51 (m, 4H, CH2), 1.46
(m, 4H, CH2), 1.21 (m, 2H, CH). 13C NMR (CDCl3): δ (ppm)
136.8 (s, C carbene), 120.5 (s, CH imidazole), 61.1 (s, NCH
cyclohexyl), 33.4 (s, CH2), 25.3 (s, CH2), 25.1 (s, CH2). Anal. Calcd
for C15H24N2AuBr3 (668.82): C, 26.93; H, 3.63; N, 4.19. Found:
C, 26.89; H, 3.56; N, 3.99.
Synthesis of [(IAd)AuBr3] (20). A procedure similar to that used
for 15 gave 20 (from 0.185 g, 0.325 mmol, of 13) as a yellow
1
solid. Yield: 0.240 g (95%). H NMR (CDCl3): δ 7.53 (s, 2H,
CH imidazole), 2.57 (m, 14H, CH2 adamantyl), 2.32 (m, 6H, CH2
adamantyl), 1.75 (m, 10H, CH2 adamantyl). 13C NMR (CDCl3): δ
(ppm) ) 132.9 (C carbene), 121.4 (s, CH imidazole), 63.8 (s, NCH
adamantyl), 44.1 (s, CH2), 35.6 (s, CH2), 30.2 (s, CH2). Anal. Calcd
for C23H32N2AuBr3 (778.93): C, 35.73; H, 4.17; N, 3.62. Found:
C, 35.43; H, 4.05; N, 3.54.
Synthesis of [(ItBu)AuBr3] (21). A protocol similar to that used
for 15 gave 21 (from 0.360 g, 871 mmol, of 14) as a yellow solid.
Yield: 0.500 g (93%). 1H NMR (CDCl3): δ 7.49 (s, 2H, CH
imidazole), 1.92 (s, 18H, C(CH3)3). 13C NMR (CDCl3): δ (ppm)
134.2 (s, C carbene), 122.7 (s, CH imidazole), 62.6 (s, C(CH3)3),
32.2 (s, C(CH3)3). Anal. Calcd for C11H20N2AuBr3 (616.78): C,
21.41; H, 3.27; N, 4.54. Found: C, 21.59; H, 3.28; N, 4.47.
a Reaction conditions: 0.5 mL of H2O, 0.5 mL of methanol, 1 mmol of
alkyne. b GC yields, isolated yields in parentheses, an average of two runs.
NR ) no reaction. c Temperature 25 °C.
Synthesis of [(SIPr)AuBr3] (17). A protocol similar to that used
for 15 gave 17 (from 0.280 g, 0.448 mmol, of 10) as a orange
solid. Yield: 0.360 g (97%). 1H NMR (CDCl3): δ 7.43 (t, J ) 7.0
Hz, 2H, CH aromatic), 7.26 (d, J ) 7.0 Hz, 4H, CH aromatic),
4.29 (s, 4H, CH2 imidazole), 3.41 (septet, J ) 6.5 Hz, 4H,
CH(CH3)2), 1.46 (d, J ) 6.5 Hz, 12H, CH(CH3)2), 1.25 (d, J ) 6.5
Hz, 12H, CH(CH3)2). 13C NMR (CDCl3): δ (ppm) 174.1 (s, C
carbene), 147.0 (s, CH aromatic), 132.9 (s, CH aromatic), 131.1
(s, CH aromatic), 125.4 (s, CH aromatic), 55.0 (s, CH2 imidazole),
29.3 (s, CH(CH3)2), 27.4 (s, CH(CH3)2), 24.2 (s, CH(CH3)2). Anal.
Calcd for C27H38N2AuBr3 (827.94): C, 39.20; H, 4.63; N, 3.39.
Found: C, 39.52; H, 4.66; N, 3.32.
Screening of Substrates in Catalytic Addition of Water to
Terminal Alkynes. Into a reaction vessel equipped with a magnetic
stirring bar and a reflux condenser were placed the catalyst ((IPr)-
AuBr3; 10 mol %, 83 mg), distilled water (0.5 mL), and methanol
(5 mL). A 1 mmol portion of the indicated alkyne was then added.
The resulting mixture was refluxed and stirred using a magnetic
plate in an oil bath for the indicated time. The reactions were
monitored by gas chromatography. After reaching maximum
conversion, the reaction mixture was cooled to room temperature.
Prior to workup, the reaction mixture was passed through a short
silica column. The resulting filtrate was concentrated under reduced
pressure and the residue diluted with methyl tert-butyl ether or
diethyl ether and washed with brine. The ethereal solution was dried
over magnesium sulfate. The solvent was then evaporated in vacuo.
When necessary, the product was purified by flash chromatography
on silica gel with hexanes or a 2-10% mixture of ethyl acetate in
hexanes.
Synthesis of [(SIMes)AuBr3] (18). A preparation similar to that
used for 9 gave 18 (from 0.100 g, 0.185 mmol, of 11) as a orange
1
solid. Yield: 0.128 g (94%). H NMR (CDCl3): δ 6.96 (s, 4H,
CH aromatic), 4.23 (s, 4H, CH2 imidazole), 2.54 (s, 12H, CH3),
2.31 (s, 6H, CH3). 13C NMR (CDCl3): δ (ppm) 172.3 (s, C carbene),
140.1 (s, CH aromatic), 135.8 (s, CH aromatic), 132.1 (s, CH
aromatic), 130.3 (s, CH aromatic), 53.3 (s, CH2 imidazole), 21.2

N-Heterocyclic Carbene Gold(III) Complexes
Organometallics, Vol. 26, No. 6, 2007 1385
Isolated Products. 1-(4-(Dimethylamino)phenyl)ethanone36
(Table 7, Entry 1). The procedure afforded 133 mg (92%) of the
product.
Acknowledgment. The National Science Foundation (NSF)
is gratefully acknowledged for financial support of this work,
as are Umicore AG, Eli Lilly, and Boehringer Ingelheim
Pharmaceuticals for materials support and unrestricted grants.
We wish to thank the University of Ottawa and its Chemistry
Department and Pfizer for hosting our group and group members
during our time away from the University of New Orleans.
1-(4-Methoxyphenyl)ethanone37 (Table 7, Entry 2), The
procedure afforded 118 mg (90%) of the product.
1-Phenylethanone38 (Table 7, Entry 3), The procedure afforded
105 mg (88%) of the product.
(4-Acetylphenyl)acetonitrile39 (Table 7, Entry 4). The proce-
dure afforded 121 mg (86%) of the product.
1-Hydroxy-1,1-diphenylpropan-2-one40 (Table 7, Entry 5).
The procedure afforded 74 mg (36%) of the product.
1-(4-Chlorophenyl)ethanone41 (Table 7, Entry 6). The proce-
dure afforded 118 mg (77%) of the product.
Supporting Information Available: Crystallographic informa-
tion files (CIF) of the complexes 8 and 15-21. This material is
available free of charge via the Internet at http://pubs.acs.org. These
files have also been deposited with the CCDC, 12 Union Road,
Cambridge CB2 1EZ, U.K., and can be obtained on request free
of charge, by quoting the publication citation and deposition
numbers 621434-621442.
(36) Klingsberg, E.; Schreiber, A. M. J. Am. Chem. Soc. 1962, 84, 2941-
2944.
(37) Cacchi, S.; Fabrizi, G.; Gavazza, F.; Goggiamani, A. Org. Lett. 2003,
5, 289-291.
(38) The product is commercially available, and the spectra of the isolated
product were compared with spectra from a sample obtained from Aldrich.
(39) Wu, L.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 15824-15832.
(40) Hou, Z.; Takamine, K.; Aoki, O.; Shiraishi, H.; Fujiwara, Y.;Tan-
iguchi, H. J. Org. Chem. 1988, 53, 6077-6084.
OM060887T
(41) Murphy, J. A.; Commeureuc, A. G. J.; Snaddon, T. N.; McGuire,
T. M.; Khan, T. A.; Hisler, K.; Dewis, M. L.; Carling, R. Org. Lett. 2005,
7, 1427-1429.