Synthesis, reactivity, and electrochemical studies of gold(I) and gold(III) complexes supported by N-heterocyclic carbenes and their application in catalysis

Article abstract of DOI:10.1021/om1005484

In the present study we describe the efficient synthesis of various Au(I) complexes supported by NHC ligands. Some of these ligands have a pendant pyridine arm that is linked with various tethers (CH₂)_n to the NHC backbone (n = 0-2). The chloride in the Au(I) complexes is easily and cleanly replaced by an aryl group upon reaction with an aryl-Grignard reagent. The thus obtained aryl Au(I) complexes are cleanly oxidized to the corresponding Au(III) complexes with phenyliodoso dichloride, as are the corresponding halide Au(I) complexes. The attempted salt metathesis with the parent Au(III) complex led to the oxidative coupling of the aryl residues with formation of the Au(I) complex. Some of the complexes are promising catalysts in the cycloisomerization of an ω-alkynylfuran to isobenzofuranol in the presence of a silver salt. For those precursors with pendant pyridine arms, a cationic dimeric Au complex was isolated and characterized, which represents a catalyst resting state and forms under reaction conditions.

Full text of DOI:10.1021/om1005484

4448 Organometallics 2010, 29, 4448–4458
DOI: 10.1021/om1005484
Synthesis, Reactivity, and Electrochemical Studies of Gold(I) and
Gold(III) Complexes Supported by N-Heterocyclic Carbenes and Their
Application in Catalysis
‡
Marek Pazicky, Annette Loos, Maria Joao Ferreira, Daniel Serra, Nikolai Vinokurov,
†
†
†
ꢀ
ꢁ †
Frank Rominger, Christoph Jakel, A. Stephen K. Hashmi, and Michael Limbach*
~
‡
†
‡
,†,§
€
†CaRLa, Catalysis Research Laboratory, Im Neuenheimer Feld 584, 69120 Heidelberg, Germany,
‡
€
Organisch-Chemisches Institut, Ruprecht-Karls-Universitat Heidelberg, Im Neuenheimer Feld 270,
69120 Heidelberg, Germany, and §BASF SE, Basic Chemicals Research, GCB/C-M313,
67056 Ludwigshafen, Germany
Received June 3, 2010
In the present study we describe the efficient synthesis of various Au(I) complexes supported by
NHC ligands. Some of these ligands have a pendant pyridine arm that is linked with various tethers
(CH2)n to the NHC backbone (n = 0-2). The chloride in the Au(I) complexes is easily and cleanly
replaced by an aryl group upon reaction with an aryl-Grignard reagent. The thus obtained aryl Au(I)
complexes are cleanly oxidized to the corresponding Au(III) complexes with phenyliodoso dichlor-
ide, as are the corresponding halide Au(I) complexes. The attempted salt metathesis with the parent
Au(III) complex led to the oxidative coupling of the aryl residues with formation of the Au(I) com-
plex. Some of the complexes are promising catalysts in the cycloisomerization of an ω-alkynylfuran
to isobenzofuranol in the presence of a silver salt. For those precursors with pendant pyridine arms, a
cationic dimeric Au complex was isolated and characterized, which represents a catalyst resting state
and forms under reaction conditions.
Introduction
from homogeneous and heterogeneous catalysis5-16 to
medicinal chemistry.17,18
Although the first efforts to prepare organogold com-
pounds were made ca. 160 years ago,1 it took nearly 50
years until their existence was proven by Pope and Gibson
in 1907.2 It took a further ca. 40 years for Gilman to isolate
the first trialkyl gold complex3 and another 20 for Coates
to introduce phosphines as ligands in gold complexes.4
From that point on the field of organogold chemistry
“exploded”, and the application of the meanwhile uncoun-
table number of various Au(I) and Au(III) complexes spans
In homogeneous catalysis both simple salts and well-
defined Au(I) and Au(III) complexes have been used, with
the latter usually supported by good σ donors. Among these,
N-heterocyclic carbenes (NHCs) have recently excelled in
supporting Au(I) as well as Au(III) complexes.13,19-21 Here,
we report the synthesis of some new Au(I) and Au(III)
complexes containing both simple NHCs and NHCs with
pendant arms containing pyridine moieties. The redox chem-
istry of these complexes, their molecular structure, and their
application in the synthesis of phenols are reported. The
importance of the pyridine arms in supporting cationic Au(I)
catalysts is also discussed.
*To whom correspondence should be addressed. E-mail:
michael.limbach@basf.com.
(1) Frankland, E. J. Chem. Soc. 1850, 4, 297–299.
(2) Pope, W. J.; Gibson, C. S. J. J. Chem. Soc. 1907, 91, 2061–2066.
(3) Gilman, H.; Woods, L. A. J. Am. Chem. Soc. 1948, 70, 550–552.
(4) Coates, G. E.; Parkin, C. J. J. Chem. Soc. 1963, 421–429.
(5) Brenzovich, W. E.; Benitez, D.; Lackner, A. D.; Shunatona, H. P.;
Tkatchouk, E.; Goddard, W. A., III; Toste, F. D. Angew. Chem., Int. Ed.
2010, 49, 5519–5522.
Results and Discussion
Synthesis and Characterization. The N-heterocyclic car-
bene ligands used in this work are depicted in Figure 1. While
the imidazolium salt L1a HCl is commercially available, in
3
(6) Melhado, A. D.; Brenzovich, W. E.; Lackner, A. D.; Toste, F. D.
J. Am. Chem. Soc. 2010, 132, 8885–8887.
analogy to a procedure reported by Echavarren et al. the
(7) Shapiro, N. D.; Toste, F. D. Synlett 2010, 675–691.
(8) Arcadi, A. Chem. Rev. 2008, 108, 3266–3325.
(9) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006,
45, 7896–7936.
(15) Jansen, M. Chem. Soc. Rev. 2008, 37, 1826–1835.
(16) Corma, A.; Garcia, H. Chem. Soc. Rev. 2008, 37, 2096–2126.
(17) Wang, X. Y.; Guo, Z. J. Prog. Chem. 2009, 21, 845–855.
(18) Navarro, M. Coord. Chem. Rev. 2009, 253, 1619–1626.
(19) Glorius, F., Ed. N-Heterocyclic Carbenes in Transtion Metal
Catalysis; Springer-Verlag: Berlin, 2007.
(10) Skouta, R.; Li, C.-J. Tetrahedron 2008, 64, 4917–4938.
€
(11) Furstner, A.; Davies, P. W. Angew. Chem., Int. Ed. 2007, 46,
3410–3449.
€
(12) Furstner, A. Chem. Soc. Rev. 2009, 38, 3208–3221.
(20) Nolan, S. P. N-Heterocyclic Carbenes in Synthesis; Wiley-VCH:
Weinheim, 2006.
(21) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47,
3122–3172.
(13) Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776–1782.
(14) Yam, V. W. W.; Cheng, E. C. C. Chem. Soc. Rev. 2008, 37, 1806–
1813.
r
pubs.acs.org/Organometallics
Published on Web 09/21/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 20, 2010 4449
Figure 1. Various NHC ligands L1a-c and L2a-c used to prepare Au(I) complexes 1a-f.
Complexes 1a-f were successfully oxidized with PhICl2
26-29
to give the corresponding Au(III) complexes [AuCl3(L)]
2a-f in very good yields (92-97%), as recently reported
by Nolan et al. for 2a.27 PhICl2 acts both as oxidant and as
chlorinating agent. The presence of chlorine proved crucial
in the stabilization of the Au(III) centers, as observed both in
preparative reactions and in the electrochemical studies (cf.
below). The attempted oxidation of 1e with oxidants other
than PhICl2, e.g., Ph2IBr, PhI(O2CCH3)2, or PhI(O2CCF3)2
failed or gave a complex product mixture.
The solid-state molecular structures of complexes 2b-f are
depicted in Figure 4. A comparison between corresponding
complexes 1a-f and 2a-f revealed that a higher oxidation
state does not translate into the shortening of the Au-
carbene or the Au-Cltrans distances (cf. Table 2; Au-Ccarbene
for 1a-f ranges from 1.972(4) to 1.999(5) A compared to
1.977(6)-2.010(4) A for 2a-f and Au-Cltrans ranges from
2.2758(12) to 2.3183(15) A for 1a-f compared to 2.306-
(2)-2.3201(13) A for 2a-f), which correlates well to avail-
able structural data for this type of compounds.24
The δ(Ccarbene) is shifted between 20 and 30 ppm upfield
for complexes 2a-f when compared to the chemical shifts
observed for the Au(I) complexes 1a-f (cf. Table 1). This
correlates well with the increasing number of good σ donors
in the coordination sphere30,31 of the metal for such
complexes.24,27 The geometry around the Au center is essen-
tially square planar (Ccarbene-Au-Cltrans angle ranges from
176° to 180° for 1a-f compared to 178-179° for 2a-f, cf.
Table 3). There is no interaction between the pyridine and the
Figure 2. ORTEP diagram of L2c HI H2O with 50% prob-
3
3
ability ellipsoids. Most hydrogen atoms are omitted for clarity.
Selected bond lengths (A) and angles (deg): C1-N1 1.333(3),
C1-N2 1.330(3), C4-N2 1.373(3), C3-N1 1.384(3), C3-C4
1.354(4), N3-O1 2.848, I1-O1 3.640, N3-H1b 2.061, I1-H1a
2.864, C1-N2-C21-N3 0.43.
ligand precursors L1b,22 L2a,23 and L2b,23 i.e., L1b HI,
L2a HBr, and L2b HBr, were prepared in 78%, 95%, and
3
3
3
82% yield, respectively. The new imidazolium salts L1c HI
3
and L2c HI were synthesized by alkylation of the corre-
3
spondingly substituted alkyl and aryl imidazoles with sub-
sequent crystallization in 86% and 91% yield, respectively
(cf. Experimental Section). The hygroscopic imidazolium
salt L2c HI crystallized as a water adduct L2c HI H2O,
giving suitable crystals for single-crystal X-ray diffraction.
The solid-state molecular structure is depicted in Figure 2
and shows that the imidazolium and the pyridine rings are
almost parallel (0.43°). The water molecule forms hydrogen
bridges to both the nitrogen of the pyridine (2.061 A) and the
iodide (2.864 A).
3
3
3
Au center for 2d-f, due to the large N Au distances (cf.
3 3 3
Table 3), nor is there an interaction between the pyridine’s π
system and the metal center for complexes 2e and 2f. The
closest distances of the pyridine carbon atoms to the Au
center are around 3.3 and 4.0 A, respectively.
The Au(I) complexes 1a-f were obtained from the corre-
sponding imidazolium salts by transmetalation of the in situ
generated silver carbenes with (Me2S)AuCl24 and were iso-
lated in moderate to almost quantitative yields (57-99%)
after column chromatography on basic alumina (cf. Scheme 1).
Recrystallization from hexanes yielded single crystals suita-
ble for X-ray analysis of the known Au(I) complex 1b22,25 as
well as of the new Au(I) complexes 1c-f. They show spectro-
scopic and geometric features expected for Au(I) NHC com-
plexes, namely, a 13C NMR carbene signal at around 170
ppm (cf. Table 1) and a quasi linear geometry around the Au
atom (cf. Table 3 and Figure 3). Interestingly, the pendant
pyridine arm in complexes 1d-f does not interact with the
Au center, as deduced from the rather long Au-N distance
(cf. Table 3).
The reaction of 1a with the Grignard reagents BrMgC6H6
and BrMgC6F5 gave, according to Hashmi’s protocol,32 after
flash column chromatography on alumina and subsequent
crystallization from THF/hexane mixtures, [Au(C6H5)(L1a)]
(3a) (in 96% yield) and [Au(C6F5)(L1a)] (3b) (in 77% yield),
respectively (Scheme 1). Both 3a and 3b decompose upon
long contact times on alumina, as indicated by the formation
of purple Au-nanoparticles. Nevertheless, complexes 3a and
3b were fully characterized (for X-ray structures, cf. Figure 5).
Compared to the parent complex 1a, the chemical shifts of
(27) Gaillard, S.; Slawin, A. M. Z.; Bonura, A. T.; Stevens, E. D.;
Nolan, S. P. Organometallics 2010, 29, 394–402.
(28) Bhargava, S. K.; Mohr, F.; Bennett, M. A.; Welling, L. L.; Willis,
A. C. Organometallics 2000, 19, 5628–5635.
~
ꢁ
ꢁ
ꢁ~
(22) Nieto-Oberhuber, C.; Munoz, M. P.; Lopez, S.; Jimenez-Nunez,
E.; Nevado, C.; Herrero-Gomez, E.; Raducan, M.; Echavarren, A. M.
ꢁ
(29) Teets, T. S.; Nocera, D. G. J. Am. Chem. Soc. 2009, 131, 7411–
7420.
Chem. Eur. J. 2006, 12, 1677–1693.
ꢁ
(23) Tulloch, A. A.; Danopoulos, A. A.; Winston, S.; Kleinhenz, S.;
Eastham, G. J. Chem. Soc., Dalton Trans. 2000, 4499–4506.
(24) 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.
(25) Nieto-Oberhuber, C.; Lopez, S.; Echavarren, A. M. J. Am.
Chem. Soc. 2005, 127, 6178–6179.
(30) de Fremont, P.; Singh, R.; Stevens, E. D.; Petersen, J. L.; Nolan,
S. P. Organometallics 2007, 26, 1376–1385.
(31) Jothibasua, R.; Huynh, H. V.; Koh, L. L. J. Organomet. Chem.
2008, 693, 374–380.
€
€
(32) Hashmi, A. S. K.; Lothschutz, C.; Dopp, R.; Rudolph, M.;
Ramamurthi, T.; Rominger, F. Angew. Chem., Int. Ed. 2009, 48, 8243–
8246.
(26) Zhao, X.-F.; Zhang, C. Synthesis 2007, 4, 551–557.

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Pazꢀicky et al.
4450 Organometallics, Vol. 29, No. 20, 2010
Scheme 1
Table 1. Chemical Shifts (ppm) of the Carbenic Carbon (δC) of the NHC Salts L1a HCl, L1b, c HI, L2a,b HBr, L2c HI, [AuCl(L)]
3
3
3
3
(1a-f), [AuCl3(L)] (2a-f), [AuAr(L)] (3a,b), [AuArCl2(L)] (4a,b), and [Au(L)NTf2]2 (5a,b)
ΔδC (ppm) =
L
δ
C (ppm) of L HXa
δ
C (ppm) of 1a-fa
δC (ppm) of 2a-f
δ
C (ppm) of 3a,ba
δ
C (ppm) of 4a,ba
δ
C (ppm) of 5a,bb δC(1) - δC(2)
3
L1a
L1b
L1c
L2a
L2b
L2c
141.11
137.46
135.12
135.61
137.50
137.03
173.44
172.82
169.36
170.90
172.33
171.57
144.72a
143.01a
138.50a
149.40c
152.51a
146.56a
195.81, 190.58
180.35, 167.13
28.72
29.81
30.86
21.50
19.82
25.01
165.03
164.67
a NMR in CDCl3. b NMR in CD2Cl2. c NMR in THF-d8.
Figure 3. ORTEP diagram of 1b-f with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths and
angles are depicted in Tables 2 and 3, respectively.
the carbene carbons are deshielded (173.44 ppm for 1a vs
195.81and190.58ppm for 3aand 3b, respectively, cf. Table1).
This implies a poorer π-donor ability of the aromatic rings in
3a and 3b in comparison to chloride, a fact that is explainable
by the torsion angle between the NHC and the aromatic
rings, which allows only a poor overlap between the orbitals
involved in NHC coordination and the π system of the
aromatic rings (see Table 3).
The oxidation of complexes 3a and 3b in the presence of
PhICl2 proceeds smoothly to give [Au(C6H5)Cl2(L1a)] (4a)
and [Au(C6F5)Cl2(L1a)] (4b) in high yields (78% and 93%,
respectively, cf. Scheme 1). Both compounds were precipi-
tated from THF/water mixtures and purified by flash col-
umn chromatography on basic alumina. Again, after long
contact with alumina, the complexes decomposed, as indi-
cated by the formation of purple Au-nanoparticles. Besides
4a, ca. 17% of 2a was isolated from the reaction mixture.
This is possibly originated from the reductive elimination of
chlorobenzene, a common decomposition product,33 from
4a, with subsequent oxidation of the Au(I) species 1a to 2a
under reaction conditions. A transmetalation route cannot
ꢁ
(33) Uson, R.; Laguna, A.; Pardo, J. Syn. React. Inorg. Metal-Org.
Chem. 1974, 4, 499–513.

Article
Organometallics, Vol. 29, No. 20, 2010 4451
Figure 4. ORTEP diagram of 2b-f with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths and
angles are depicted in Tables 2 and 3, respectively.
˚
Table 2. Selected Au-Ccarbene, Au-Xtrans (X = Cl, C, N), Au-Clcis, Au Au, and Au-N(py) Distances (A) for [AuCl(L)] (1a-f),
3 3 3
[AuCl3(L)] (2a-f), [AuAr(L)] (3a,b), [AuArCl2(L)] (4a,b), and [Au(L)NTf2]2 (5a,b)
complex
Au-Ccarbene
Au-Xtrans
Au-Clcis
Au-Clcis
Au Au
Au-N(py)
3 3 3
1a62-64
1b
1c
1.999(5)
1.975(4)
1.995(4)
1.992(7)
1.974(4)
1.972(4)
1.977(6)
1.994(4)
2.010(4)
2.003(5)
1.999(5)
1.997(5)
2.002(11)
2.001(4)
2.077(6)
2.051(10)
1.984(8)
1.960(9)
1.992(4)
2.2758(12)a
2.2892(10)a
2.3143(11)a
2.3183(15)a
2.3183(9)a
2.290(4)a
8.446
3.764
6.249
3.3491(5)
4.020
3.324-6.708
9.398
6.415
10.305
6.943
7.598
8.888
9.704
10.867
9.904
8.773
1d
1e
4.817
4.969
3.606-4.822
1fb
2a27
2b
2.306(2)a
2.276(2)
2.268(2)
2.3160(13)a
2.3154(12)a
2.3112(14)a
2.3201(13)a
2.3162(11)a
2.072(13)c
2.034(5)c
2.2614(14)
2.2859(12)
2.3036(14)
2.2906(12)
2.2787(11)
2.2886(13)
2.2812(12)
2.2970(14)
2.2905(11)
2.2802(11)
2c
2d
2e
2f
3a
3b
4a
2.737
3.836
4.393
2.085(6)c
2.2805(17)
2.276(3)
2.2872(16)
2.276(3)
4b
5a
2.063(12)c
2.066(7)d
e
e
3.407
2.068(6)d
5b
2.077(3)d
3.2269(3)
a Xtrans = Cl. b Four independent molecules. c Xtrans = C. d Xtrans = N(py). e Not relevant.
be excluded, however.5 Compounds 4a and 4b are, to the best
of our knowledge, the first NHC complexes of Au(III) of the
type [AuArCl2(L)], although complexes of this type with
other σ-donor ligands34-40 or with alkyl instead of aryl
residues41,42 have been reported. A comparison of the che-
mical shifts for the δ(Ccarbene) for 3a,b and 4a,b shows a clear
upfield shift of 15-23 ppm, resulting from the presence
of more σ donors in the metal’s coordination sphere
(Table 1).30,31 The structural parameters of complexes 3a,b
and 4a,b are comparable to 1a-f and 2a-f (cf. Figure 6;
Tables 1-3).
Curiously, the reaction of complex 2e and p-methoxyphe-
nylmagnesium bromide gave 1e in 87% yield instead of the
expected aryl-Au(III) complex of type 4. This is accompa-
nied by the formation of 4,40-bismethoxybiphenyl in 81%
isolated yield (Scheme 1). C-C coupling reactions involving
(34) Liddle, K. S.; Parkin, C. J. Chem. Soc., Chem. Commun. 1972, 26.
ꢁ
(35) Uson, I.; Laguna, A.; Vicente, J. Syn. React. Inorg. Metal-Org.
Chem. 1976, 6, 293–305.
(36) Vicente, J.; Bermudez, M. D.; Chicote, M. T.; Sanchez-Santano,
ꢁ
M. J. J. Organomet. Chem. 1989, 371, 129–135.
(37) Vicente, J.; Arcas, A.; Mora, M.; Solans, X.; Font-Altaba, M.
J. Organomet. Chem. 1986, 309, 369–378.
(38) Uson, R.; Laguna, A.; Bergareche, B. J. Organomet. Chem. 1980,
184, 411–416.
(39) Uson, R.; Laguna, A.; Vicente, J. J. Organomet. Chem. 1975, 86,
415–421.
(40) Vaughan, L. G.; Sheppard, W. A. J. Am. Chem. Soc. 1969, 91,
6151–6156.
(41) Mankad, N. P.; Toste, F. D. J. Am. Chem. Soc. 2010, doi:
10.1021/ja106257n.
(42) Scott, V. J.; Labinger, J. A.; Bercaw, J. E. Organometallics 2010,
doi: 10.1021/om1006566.

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Pazꢀicky et al.
4452 Organometallics, Vol. 29, No. 20, 2010
Table 3. Selected Angles and Torsion Angles (deg) for [AuCl(L)] (1a-f), [AuCl3(L)] (2a-f), [AuAr(L)] (3a,b), [AuArCl2(L)] (4a,b), and
[Au(L)NTf2]2 (5a,b)
complex
Ccarbene-Au-Xtrans
Ccarbene-Au-Clcis
Xtrans-Au-Clcis
Clcis-Au-Clcis
torsion angle
1a62-64
1b
1c
180.0a
178.25(11)a
177.74(13)a
175.7(2)a
1d
1e
176.89(12)a
175.5-179.6a
178.10(18)
179.19(14)a
1fb
2a27
2b
88.50(18)
88.52(13)
88.42(13)
88.70(14)
88.86(14)
91.94(14)
88.17(14)
90.15(13)
88.01(13)
89.09(12)
90.20(12)
90.01(10)
91.43(5)
91.61(5)
90.54(6)a
91.88(6)
90.25(6)
89.65(5)
91.56(4)
90.28(5)
90.82(4)
89.92(4)
178.53(7)
176.53(5)
73.9c
71.09c
2c
2d
2e
2f
179.21(14)a
177.70(14)a
178.16(14)a
177.68(13)a
174.38(6)
179.66(6)
178.12(4)
178.95(4)
89.04c
86.44c
66.97c
62.60c
3a
3b
4a
178.5(5)d
176.52(17)d
178.6(2)d
44.81e
16.71e
14.11e
90.37(17)
91.04(17)
91.19(6)
88.54(18)
90.06(18)
88.81(6)
178.53(7)
4b
5a
180.00(2)d
174.6(3) f
170.9(3) f
175.24(14) f
21.41e
34.53g
45.68g
67.87g
5b
a Xtrans = Cl. b Four independent molecules. c Angle N-Ccarbene-Au-Clcis
. =
d Xtrans = C. e Angle between the aromatic and the carbene rings. f Xtrans
N(py). g Angle between the pyridine and the carbene ring.
Figure 5. ORTEP diagrams of 3a and 3b with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond
lengths and angles are depicted in Tables 2 and 3, respectively.
Grignard reagents in the presence of Au(III) centers43-47
usually occur by radical coupling, in which case the Au(III)
complex 2e would act as an oxidant. In that case reduction
would be faster than chloride substitution. This has been
shown previously for several C-C coupling reactions
mediated by Au(I) and Au(III) complexes, although the fate
of the Au species often remained unclear.46,48,49 An inner-
sphere reductive elimination pathway cannot be excluded,
however.5,42
The abstraction of one chloride ligand in complexes 1e and
1f by means of a silver salt (Scheme 1) gave in very good
yields (93% and 96%, respectively) the cationic Au(I) com-
plexes [Au(L2b)]2(NTf2)2 (5a) and [Au(L2c)]2(NTf2)2, (5b),
respectively. Now, the pyridine pendant arm stabilizes the
cationic Au(I) center as an L-type donor, similarly to what
has been reported for other cationic Au(I) complexes.24,50
Due to the preference for a linear coordination geometry of
Au(I) centers, the pyridine arm coordinates to a second Au(I)
center, thus giving a dimeric bicationic structure with two
triflimide units as counterions (cf. Figure 7). The 13C{1H}
NMR chemical shifts for the carbene carbon observed for
these two complexes (165.03 ppm for 5a and 164.67 ppm for
5b) agree with the chemical shifts reported for cationic
complexes of the type [Au(NHC)(L)]þ (from 165.7 to 190.4
(43) Hashmi, A. S. K.; Blanco, M. C.; Fischer, D.; Bats, J. W. Eur. J.
Org. Chem. 2006, 1387.
ꢁ
ꢁ
(44) Uson, R.; Laguna, A.; Fernandez, E. J.; Mendia, A.; Jones, P. G.
J. Organomet. Chem. 1988, 350, 129–138.
(45) Vaughan, L. G.; Sheppard, W. A. J. Organomet. Chem. 1970, 22,
739–742.
(46) Zhang, G.; Peng, Y.; Cui, L.; Zhang, L. Angew. Chem., Int. Ed.
ppm), cf. Table 3.24 Although the Au Au distance for both
3 3 3
complexes is short (3.407 and 3.2269(3) A for 5a and 5b,
respectively), there is no Au-Au bond. This is believed to be
partly due to the aurophilic attraction between two Au(I)
2009, 48, 3112–3115.
ꢁ
(47) Gonzalez-Arellano, C.; Abad, A.; Corma, A.; Garcıa, H.;
´
ꢁ
Iglesias, M.; Sanchez, F. Angew. Chem., Int. Ed. 2007, 46, 1536–1538.
(48) Hashmi, A. S. K.; Ramamurthi, T. D.; Rominger, F. J. Orga-
nomet. Chem. 2009, 694, 592–597.
(49) Peng, Y.; Cui, L.; Zhang, G.; Zhang, L. J. Am. Chem. Soc. 2009,
131, 5062–5063.
(50) Khin, C.; Hashmi, A. S. K.; Rominger, F. Eur. J. Inorg. Chem.
2010, 1063–1069.

Article
Organometallics, Vol. 29, No. 20, 2010 4453
Figure 6. ORTEP diagram of 4a and 4b with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths
and angles are depicted in Tables 2 and 3, respectively.
Figure 7. ORTEP diagram of 5a and 5b with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths
and angles are depicted in Tables 2 and 3, respectively.
ions (typical values for these interaction are reported be-
tween 2.75 and 3.4 A)11 and partly due to the geometry of the
complex. Here, packing obviously plays a key role, since
substitution of the mesityl group in L2b for a methyl group
reduced to Au(0) at -1130 mV and was not reoxidized going
back to anodic potential. When the same experiment was run
with reversion of the scanning direction, we observed the
oxidation of Cl- to Cl2 at -1360 mV,52 but no further oxidation
wave from Au(I) to Au(III) was detected. On the basis of
literature,53 we believe that once chloride is oxidized to chlorine,
the Au(I) centers present in the vicinity of the electrode is then
rapidly oxidized to Au(III) by Cl2 (the reaction is analogous to
the one described above for PhICl2). The oxidation to the
chemically formed Au(III) is then evident once the scanning
direction is again reversed, since now a reduction wave from
Au(III) to Au(I) at -400 mV is indeed observed (Figure 8c).
The potential values measured do not seem to correlate
with the presence of the tethered pyridine or with the length
of the tether.
Catalytic Studies. Complexes 1b-f and 2e were tested as
catalysts in the cycloisomerization of the ω-alkynylfuran 6 to
give 4-isobenzofuranol (7, Scheme 2). This reaction has been
reported by Hashmi et al.54 as a standard test reaction for
new Au complexes and also by others using platinum.55-57
More recently cationic Au(I) complexes supported by phos-
phines58 and 1,2,3-triazoles were also used in this reaction.59
causes a significant shortening of the Au Au distance
3 3 3
(3.407 A for 5a vs 3.1730(5) A for [Au2(CH3im(CH2py))2]-
(BF4)2).51 Furthermore, Au(I) complexes 1d and 1f also
show similar Au Au distances in the solid state (see
3 3 3
Table 2), but the electrochemical studies on these complexes
rule out the presence of Au(II) species in the oxidation of
these complexes (see below).
Electrochemical Studies. The electrochemistry of the Au(I)
complexes 1b and 1d-f and the Au(III) analogues 2b and
2d-f was studied by cyclic voltammetry (CV) in acetonitrile
using nBu4NPF6 (0.1 M) as electrolyte (cf. Figure 8, Table 4).
The cyclic voltammograms of the Au(I) complexes 1b and
1d-f show a reduction wave at -914, -1160, -980, and
-1130 mV, respectively, versus the redox couple ferrocene/
ferrocenium, indicating the reduction of the complex to
metallic Au(0). Upon reversal of the scanning direction, no
oxidation wave was detected in the absence of nBu4NCl
(Figure 8a). For the Au(III) complexes 2b and 2d-f a
reduction wave was obtained at -247, -120, -300, and
-400 mV, respectively, to give the corresponding Au(I)
complexes (Figure 8b). Complexes 2d and 2e were in turn
further reduced to Au(0) at -1030 and -1140 mV. In case of
2b and 2f no reduction wave from Au(I) to Au(0) was observed.
Reoxidation from Au(I) to Au(III) was observed for 2d and 2e
at -950 and -1000 mV, respectively, but not for 2b and 2f.
When the CV experiment for 1f was repeated in the pre-
sence of nBu4NCl (3.6 mM) the Au(I) complex (5.7 mM) was
(52) Bratsch, S. G. J. Chem. Phys. Ref. Data 1989, 18, 21.
€
(53) Kuhlkamp, P.; Raubenheimer, H. G.; Field, J. S.; Desmet, M.
J. Organomet. Chem. 1998, 552, 69–74.
€
ꢁ
(54) Hashmi, A. S. K.; Wolfle, M.; Ata, F.; Hamzic, M.; Salathe, R.;
Frey, W. Adv. Synth. Catal. 2006, 348, 2501–2508.
(55) Martın-Matute, B.; Cardenas, D. J.; Echavarren, A. M. Angew.
´
Chem., Int. Ed. 2001, 113, 4890–4893.
(56) Martin-Matute, B.; Nevado, C.; Cardenas, D. J.; Echavarren,
A. M. J. Am. Chem. Soc. 2003, 125, 5757–5766.
ꢁ
(57) Echavarren, A. M.; Mendez, B.; Munoz, M. P.; Nevado, C.;
ꢁ
´
n-Matute, B.; Nieto-Oberhuber, C.; Cardenas, D. J. Pure Appl.
Martı
Chem. 2004, 76, 453–463.
(51) Catalano, V. J.; Moore, A. L. Inorg. Chem. 2005, 44, 6558–6566.

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Pazꢀicky et al.
4454 Organometallics, Vol. 29, No. 20, 2010
Figure 8. Electrochemical study. (a) and (b) GC, Pt, SCE in acetonitrile 0.1 M solution of nNBu4PF6 (initial potential 0 V vs ferrocene,
vertex potential þ1.5 V vs ferrocene, vertex potential -1.5 V vs ferrocene, final potential 0 V vs ferrocene, scan rate 25 mV/s). (c) For
compound 1f, GC, Pt, SCE in acetonitrile 0.1 M solution of nNBu4PF6 (all potentials vs ferrocene, scan rate 25 mV/s, 10 mg of nNBu4Cl
(3.6 mM) added as source of chloride). The concentration of the gold complexes ranges from 5.0 to 6.9 mM.
Table 4. Electrochemical Study of Gold(I) [AuCl(L2a)] (1d),
[AuCl(L2b)] (1e), and [AuCl(L2c)] (1f) and Gold(III) Complexes
[AuCl3(L2a)] (2d), [AuCl3(L2b)] (2e), and [AuCl3(L2c)] (2f)
Scheme 2
reduction (III-I)
[mV]
reduction (I-0)
[mV]
oxidation (I-III)
[mV]
complex
1b
1d
1e
1f
-914
-1160
-980
-1130
2b
2d
2e
2f
-247
-120
-300
-400
further conversion of the starting material, pointing to a fast
catalyst deactivation.
-1030
-1140
-950
-1000
The Au(I) complexes 1b-f (Table 5, entries 1-5) gave a
low overall conversion to 7, with a maximum yield of 57%
(with 1f) but decent turnover numbers of 40-228 [mol]/[mol]
at a catalyst loading of 0.25 mol % in chloroform. The length
of the tether (CH2)n between the pyridine ring and the NHC
backbone has a major influence on the reaction outcome;
that is, in the series of complexes 1d-f with increasing
n, yields increase from 10% (n = 0) to over 37% (n = 1) to
57% (n = 2), respectively (entries 3-5). Au(III) complex 2e
gives slightly lower yields when compared to the corresponding
Au(I) analogue 1e under the same conditions (28% vs 37%,
entry 6 vs entry 4).
All reactions were done at room temperature in NMR tubes,
with the exception of one preparative experiment (Table 5,
entry 16). Conversion and yield were monitored by 1H
NMR. The reactions turned out to be complete within one
hour. Control NMR experiments after 24 h showed no
(58) Hashmi, A. S. K.; Loos, A.; Littmann, A.; Braun, I.; Knight, J.;
Doherty, S.; Rominger, F. Adv. Synth. Catal. 2009, 351, 576–582.
(59) Chen, Y.; Yan, W.; Akhmedov, N. G.; Shi, X. Org. Lett. 2010,
12, 344–347.

Article
Organometallics, Vol. 29, No. 20, 2010 4455
Table 5. Gold-Catalyzed Synthesis of Dihydroisobenzofuran-4-ol
(7), cf. Scheme 2,a
TON, entry 9 vs 18, respectively). As 5a and 5b are both
coordinatively saturated due to the proximity of the two-
coordinate Au(I) centers, they have no open coordination
sites to act as catalytically active species. However, they
could break up under reaction conditions into monomeric
species or decoordinate one or two pyridine arms to allow for
substrate coordination.
entry
catalyst
catalyst (mol %)
yield (%)
TONb
1
2
3
4
5
6
7
8
1b
1c
1d
1e
1f
2e
1f
1f
1f
1f
1f
1d
1e
1f
1f
1f
5a
5b
0.25
0.25
0.25
0.25
0.25
0.25
2.00
0.50
0.25
0.10
0.05
0.25
0.25
0.25
0.25
0.25
0.13
0.13
30c
42c
10c
37c
57c
28c
70c
67c
82d
38c
30c
30d
24d
25d,e
4d,f
87g
74d,h
72d,h
120
168
40
148
228
112
35
134
328
380
600
120
96
Conclusions
We reported new NHC gold complexes, supported by both
tethered and untethered ligands. In the case of ligands
bearing a NHC with a pyridine side arm, it was found that
pyridine is only coordinated to the gold center in the case of
dimeric cationic complexes, which formed under reaction
conditions and turned out to be at least a catalyst resting
state in a cycloisomerization reaction.
(NHC)AuCl complexes were successfully oxidized to their
Au(III) counterparts by use of PhICl2 in good yields. It was
found that the presence of chlorine in the media is crucial for
the formation of the Au(III) complexes. By cyclovoltamme-
try we have shown that chlorine is first generated in situ by
oxidation of chloride, which in turn oxidizes complexes of
the type (NHC)AuCl.
9
10
11
12
13
14
15
16
17
18
100
16
348
569
554
a All reactions were done in an NMR tube with the exception of entry
16. All reactions were done at room temperature and monitored by
NMR after one hour. b TON = n(7)/n(catalyst). c NMR experiment in
CDCl3. d NMR experiment in CD2Cl2. e AgSbF6 was used instead of
AgNTf2. f AgOTf was used instead of AgNTf2. g Isolated yield of double
scaled reaction. h No silver salt added.
The same oxidation methodology was used to synthesize
(NHC)AuArCl2 from (NHC)AuAr, a reaction that have not
yet been reported in Au-NHC chemistry. The attempted
synthesis of (NHC)AuArCl2 via arylation of (NHC)AuCl3
failed, with bisaryl and (NHC)AuCl as byproduct.
In the cycloisomerization of ω-alkynylfurans these new
organogold complexes are active even with a problematic
substrate. Here, the pendant pyridine arm with the NHC
backbone possibly stabilizes intermediates such as 5a and 5b.
These turned out to be exceptionally effective precatalysts in
this reaction.
For catalyst 1f a systematic reduction of the catalyst loading
from 2 to 0.05 mol % led to a decrease in yield (from 57% to
30%, entries 5, 9, 10, 11, 16) but to an increase in TON (from
35 to 600 [mol]/[mol]). A strong dependency of yield and
TON was observed by solvent change from chloroform to
dichloromethane for 1f (57% vs 82% yield and 228 vs 328
TON, entry 5 vs 9) and 1d (10% vs 30% yield and 40 vs 120
TON, entry 3 vs 12), but not for 1e (37% vs 24% yield and
148 vs 96 TON, entry 4 vs 13, respectively).
The counterions had a crucial impact on the yield and
TON: replacement of Gagosz’s60 NTf2- by SbF6- or TfO-
in dichloromethane decreased the yields and TONs drama-
tically (82% vs 25% and 4% yield, 328 vs 100 and 16 TON,
entry 9 vs 14 and 15, respectively). This is in agreement with
the stabilization of Au(I) cations by coordinating ligands,
which has been associated with higher conversions in this
reaction by slowing catalyst decomposition pathways.50,59
The most active catalyst under the screening conditions
(NMR scale) is 1f, which gave 7 in 82% yield (328 TON,
entry 9). The same reaction on a preparative scale (entry 16)
gave 7 in an isolated yield of 87%, indicated by an even
higher TON of 348. This by far exceeds the yield of 68%
obtained with state-of-the-art Au(I) phosphine complexes.58
Interestingly, both complexes 1e and 1f, which both bear a
pyridine pendant arm linked via a methylene and ethylene
tether to the NHC’s backbone, dimerize in the presence of
AgNTf2 to give 5a and 5b (Figure 7). As the conditions leading
to the formation of the dimer are similar to the conditions for
the cycloisomerization, we presume that 5a and 5b form in
situ under reaction conditions and represent at least a
catalyst resting state. We therefore tested 5a and 5b, without
any added base and with the same concentration of Au (i.e.,
with half of the catalyst loadings of 5a and 5b) and obtained
7 in dramatically higher yield and TON (1e vs 5a: 24% vs
74% yield, 96 vs 569 TON, entry 13 vs 17, respectively) or
similar TON and yield (1f vs 5b: 82% vs 72% yield, 328 vs 554
Experimental Section
All reactions and manipulations were performed under an
argon atmosphere using standard Schlenk techniques. Hexane,
tetrahydrofuran, and dichloromethane were dried with an
MBraun solvent purification system. Pentane, heptane, aceto-
nitrile, diethyl ether, and chloroform were purchased as high-
purity anhydrous solvents and used as received. All solvents
were saturated with argon prior to use. All deuterated solvents
were degassed via freeze-pump-thaw cycles and stored over
molecular sieves (4 A). 1H and 13C{1H} spectra were recorded at
room temperature on a Bruker 250 spectrometer operating at
200 and 50 MHz. Additionally 1H, 13C{1H}, and 19F{1H} NMR
were recorded on a Bruker Avance II 400 operating at 400, 100,
and 376 MHz, respectively, or on a Bruker 300, operating at 300
and 75 MHz with chemical shifts (δ, ppm) reported relative to
the solvent peaks (1H NMR, 13C NMR). Peak assignment of the
carbene of 3b and 4b was accomplished by gHMBC. For X-ray
crystal structure analyses suitable crystals were mounted with a
perfluorinated polyether oil on nylon loops and cooled imme-
diately on the goniometer head. Data collection was performed
with Mo KR radiation (graphite monochromator) on a Bruker
Smart (1d, 2c-f, 3a, 4a, 4b, 5a, 5b) or a Bruker APEX (1b, 1c, 1e,
1f, 2b, 3b) at 200 K. Structures were solved by direct methods
and refined by full-matrix least-squares against F2. All non-
hydrogen atoms were given anisotropic displacement para-
meters. Hydrogen atoms were included in calculated positions.
Calculations were performed using the SHELXTL software
package.61 Melting points and IR were carried out on a Stuart
SMP 30 instrument with a gradient of 1 °C/min and on a Varian
ꢁ
(60) Mezailles, N.; Ricard, L.; Gagosz, F. Org. Lett. 2005, 7, 4133–
4136.

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Pazꢀicky et al.
4456 Organometallics, Vol. 29, No. 20, 2010
2000 FT-IR Scimitar Series, respectively. TLC analyses were
performed on commercially available Macherey-Nagel Poly-
gram SILG/UV254 and ALOX/UV254 precoated plastic sheets.
Elemental analyses were determined on an Elementar Vario EL
CHN analyzer, and mass spectra (FABþ, HR FABþ) were
obtained on a JEOL MStation JMS 700 instrument. All electro-
chemical experiments were conducted with a Princeton Applied
Research potentiostat/galvanostat model 263A and the corre-
sponding software PowerSuite 2.11. The three-electrode system
was used. A glassy carbon (GC) working electrode (2 mm
diameter) was employed. The pseudo-reference electrode was
a platinum wire immersed in a solution of 0.1 M tetrabutylam-
monium hexafluorophosphate (nBu4NPF6) in acetonitrile. Pla-
tinum wire was used as the auxiliary electrode. In each case, IR
compensation was applied in order to reduce the acetonitrile
resistance. The scan rate for all CV measurements was 25 mV/s.
Ferrocene was added for each measurement as an internal
standard. Experiments were conducted under strict inert condi-
tions to exclude interactions with oxygen and moisture. Mea-
surements were taken at room temperature (25 °C) in acetoni-
trile as solvent. nBu4NPF6 and/or nBu4NCl were used as
supporting electrolyte. All other starting materials were pur-
chased in reagent grade purity from Acros, Aldrich, Fluka, or
Strem and used without further purification. Ligand precursor
the dark at room temperature. The mixture was filtered, and
[AuCl(SMe2)] (2.70 mmol) was added. The resulting mixture
was stirred for 24 h at room temperature in the dark before it was
filtered through Celite. The solvent was removed under vacuum
and the residue redissolved in a minimum amount of dichlor-
omethane. The product was precipitated as a white solid upon
addition of pentane and purified by column chromatography on
basic alumina (hexane/THF, 10:1-1:100).
Chloro[1,3-dihydro-1-methyl-3-adamantyl-2H-imidazol-2-yli-
dene]gold [AuCl(L1c)] (1c): 700 mg, 57% yield; mp 202-204 °C
(hexane); Rf = 0.43 (THF/hexane, 1:1); H NMR (200 MHz,
1
CDCl3) 1.76 (s, 6H), 2.25 (s, 3H), 2.49 (s, 6H), 3.90 (s, 3H), 6.91
(br s, 1H), 7.10 (br s, 1H); 13C{1H} NMR (50 MHz, CDCl3)
30.04, 35.95, 39.94, 44.52, 59.57, 117.78, 119.96, 169.36; HRMS
(FAB) calcd for C14H20N2Au m/z 413.1292 (M)þ, found
413.1304; for C14H20N235ClAu m/z 448.0981 (M)þ, found
448.0999. Anal. Calcd for C14H20AuN2Cl: C 37.47, H 4.49, N
6.24. Found: C 37.37, H 4.50, N 6.10.
Chloro[1,3-dihydro-1-(2-pyridinyl)-3-(2,4,6-trimethylphenyl)-
2H-imidazol-2-ylidene]gold [AuCl(L2a)] (1d): 570 mg, 84%
yield; mp 209-212 °C (hexane); Rf = 0.57 (THF/hexane, 1:1);
1H NMR (CDCl3, 200 MHz) 2.02 (s, 6H), 2.27 (s, 3H), 6.92 (s,
2H), 6.98 (d, 1H, J = 2.0 Hz), 7.38 (ddd, 1H, J = 7.5 Hz, J = 2.0
Hz, J = 1.0 Hz), 7.87 (ddd, 1H, J = 7.5 Hz, J = 2.0 Hz, J = 2.0
Hz), 8.01 (d, 1H, J = 2.0 Hz), 8.49 (ddd, 1H, J = 8.0 Hz, J = 2.0
Hz, J = 1.0 Hz), 8.75 (dd, 1H, J = 8.0 Hz, J = 1.0 Hz); 13C{1H}
(CDCl3, 50 MHz) 17.94, 21.16, 117.34, 120.36, 122.47, 124.11,
129.56, 134.60, 135.05, 139.10, 139.93, 148.80, 150.26, 170.90;
HRMS (FAB) calcd for C17H1835ClN3Au m/z 496.0855 (M -
H)þ, found 496.0894. Anal. Calcd for C17H17ClN3Au: C 41.19,
H 3.46, N 8.48. Found: C 41.75, H 3.63, N 8.09.
Chloro[1,3-dihydro-1-(2-pyridinylmethyl)-3-(2,4,6-trimethyl-
phenyl)-2H-imidazol-2-ylidene]gold [AuCl(L2b)] (1e): 640 mg,
92% yield; mp 181-184 °C (hexane); Rf = 0.40 (THF/hexane,
1:1); 1H NMR (CDCl3, 200 MHz) 2.01 (s, 6H), 2.33 (s, 3H), 5.59
(s, 2H), 6.89 (d, 1H, J = 2.0 Hz), 6.95 (s, 2H), 7.27 (m, 1H), 7.36
(d, 1H, J = 2.0 Hz), 7.46 (d, 1H, J = 8.0 Hz), 7.74 (ddd, 1H, J =
8.0 Hz, J = 8.0 Hz, J = 2.0 Hz), 8.62 (d, 1H, J = 4.0 Hz);
13C{1H} NMR (CDCl3, 50 MHz) 17.83, 21.11, 56.50, 121.39,
122.52, 123.50, 129.10, 129.39, 134.64, 134.73, 137.45, 139.65,
149.75, 154.84, 172.33; HRMS (FAB) calcd for
C18H2035ClN3Au m/z 510.1011 (M - H)þ, found 510.1004.
Anal. Calcd for C18H19ClN3Au: C 42.41, H 3.76, N 8.24.
Found: C 42.46, H 3.89, N 8.04.
L1a HCl was purchased from Aldrich and used as received,
while 6 was prepared according to a literature procedure.54
3-Adamantylmethylimidazolium Iodide (L1c HI). Methyl io-
3
3
dide (3.80 mL, 8.80 g, 61.8 mmol) was added dropwise to a
solution of N-adamantylimidazole (2.50 g, 12.4 mmol) in THF
(8 mL). The mixture was heated to reflux for 48 h. The product
was isolated as a white solid by filtration (3.64 g, 86% yield). The
solid was used without any further purification: mp 227-230 °C
(THF); 1H NMR (200 MHz, CDCl3) 1.78 (s, 6H), 2.23 (s, 6H),
2.31 (s, 3H), 4.19 (s, 3H), 7.44 (br s, 2H), 10.17 (s, 1H); 13C{1H}
NMR (50 MHz, CDCl3) 29.54, 35.41, 37.36, 43.03, 60.88,
118.95, 124.01, 135.12; IR (CH2Cl2) 3456, 3062, 2909, 2849,
1555, 1448, 1166, 918, 812. Anal. Calcd for C14H21IN2: C 48.85,
H 6.15, N 8.14. Found: C 48.96, H 6.02, N 8.28.
1-(2-Pyridinylethyl)-3-mesitylimidazolium Iodide (L2c HI).
3
2-(2-Chloroethyl)pyridine (1.42 g, 10.0 mmol), 1-mesitylimida-
zole (2.05 g, 11.0 mmol), and sodium iodide (1.50 g, 10.0 mmol)
were dissolved in acetone (10 mL). This solution was sealed in a
high-pressure tube (50 mL) and heated to 60 °C for 24 h. The
contents of the reaction vessel were poured into dichloro-
methane (20 mL) and filtered through Celite. Diethyl ether
(10 mL) was then added, which caused an oil to separate. This
was recrystallized from dichloromethane/diethyl ether to yield a
highly hygroscopic solid, which was dried under vacuum and
kept under argon (4.20 g, 91% yield): mp 112-115 °C (hexane);
1H NMR (CDCl3, 200 MHz) 1.94 (s, 6H), 2.31 (s, 3H), 3.60
(t, 2H, J = 6.0 Hz), 5.17 (t, 2H, J = 6.0 Hz), 6.96 (s, 2H), 7.06
(m, 1H), 7.15 (m, 1H), 7.44 (m, 1H), 7.63 (m, 1H), 7.87 (m, 1H),
8.46 (m, 1H), 9.75 (br s, 1H); 13C{1H} NMR (CDCl3, 50 MHz)
17.51, 21.01, 37.60, 48.99, 122.23, 123.10, 124.06, 124.25, 129.62,
130.37, 134.06, 136.94, 137.03, 141.04, 149.17, 155.87; IR
(CH2Cl2): 3466, 3063, 2193, 1593, 1568, 1477, 1438, 1202, 1158,
919, 731. Anal. Calcd for C19H22IN3: C 54.42, H 5.29, N 10.02.
Found: C 55.55, H 5.42, N 10.25; HRMS (FAB) calcd for
C19H22IN3 m/z 292.18136 (M)þ, found 292.18079.
Chloro[1,3-dihydro-1-(2-pyridinylethyl)-3-(2,4,6-trimethylphenyl)-
2H-imidazol-2-ylidene]gold [AuCl(L2c)] (1f): 620 mg, 87% yield;
1
mp 127-130 °C (hexane); Rf = 0.26 (THF/hexane, 1:1); H
NMR (CDCl3, 200 MHz) 1.94 (s, 6H), 2.32 (s, 3H), 3.45 (t, 2H,
J = 8.0 Hz), 4.74 (t, 2H, J = 8.0 Hz), 6.72 (d, 1H, J = 2.0 Hz),
6.93 (s, 2H), 6.97 (m, 1H), 7.19 (d, 2H, J = 8.0 Hz), 7.61 (ddd,
1H, J = 8.0 Hz, J = 8.0 Hz, J = 2.0 Hz), 8.58 (d, 1H, J = 4.0
Hz); 13C{1H} NMR (CDCl3, 50 MHz) 17.74, 21.10, 30.33,
34.22, 39.53, 50.72, 121.32, 121.55, 122.15, 124.08, 125.51,
129.33, 134.65, 134.76, 135.77, 136.86, 139.54, 149.60, 156.82,
171.57; HRMS (FAB) calcd for C19H2235ClN3Au m/z 524.1168
(M - H)þ, found 524.1163. Anal. Calcd for C19H21ClN3Au: C
43.57, H 4.04, N 8.02. Found: C 46.06, H 4.51, N 7.47.
General Procedure for the Oxidation of 1b-f with PhICl2 to
Give Au(III) Complexes 2b-f. At 0 °C to a solution of complex
1b-f (700 μmol) in dichloromethane (20 mL) was added drop-
wise a solution of PhICl2 (800 μmol) in dichloromethane
(10 mL). The reaction mixture was stirred for 24 h in the dark at
room temperature. The solvent was then evaporated, and the
yellow solid was washed with hexane. The pure product was
dried under high vacuum.
Trichloro[1,3-dihydro-1-methyl-3-(2,4,6-trimethylphenyl)-2H-
imidazol-2-ylidene]gold [AuCl3(L1b)] (2b): 320 mg, 92% yield;
mp 215-218 °C (hexane); 1H NMR (200 MHz, CDCl3) 2.15 (s,
6H), 2.36 (s, 3H), 4.12 (s, 3H), 7.00 (s, 2H), 7.12 (d, 1H, J = 2.0
Hz), 7.38 (d, 1H, J = 2.0 Hz); 13C{1H} NMR (50 MHz, CDCl3)
General Procedure for the Synthesis of Au(I) Complexes 1c-f.
A mixture of silver(I) oxide (1.80 mmol) and ligand precursor
(2.90 mmol) in dichloromethane (50 mL) was stirred for 24 h in
(61) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
(62) Baker, M. V.; Barnard, P. J.; Berners-Price, S. J.; Bayshaw, S. K.;
Hickey, J. L.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 2005,
690, 5625–5635.
(63) Singh, S.; Kumar, S. S.; Janik, V.; Roesky, H. W.; Schmidt,
H.-G.; Noltemeyer, M. Eur. J. Inorg. Chem. 2005, 3057–3062.
ꢁ
(64) de Fremont, P.; Scott, N. M.; Stevens, E. D.; Nolan, S. P.
Organometallics 2005, 24, 2411–2418.

Article
Organometallics, Vol. 29, No. 20, 2010 4457
18.67, 21.36, 38.72, 125.31, 125.66, 130.09, 132.38, 135.62,
141.21, 143.01; HRMS (FAB) calcd for C13H16N2Au m/z
397.0979 (M)þ, found 397.0984; for C13H16N235ClAu m/z
432.0668 (M)þ, found 432.0653. Anal. Calcd for C13H16AuN2Cl3:
C 31.00, H 3.20, N 5.56. Found: C 30.70, H 3.12, N 5.41.
Trichloro[1,3-dihydro-1-methyl-3-adamantyl-2H-imidazol-2-
ylidene]gold [AuCl3(L1c)] (2c): 330 mg, 94% yield; mp 265-
268 °C (hexane); 1H NMR (200 MHz, CDCl3) 1.79 (br s, 6H),
2.35 (br s, 3H), 2.48 (br s, 6H), 4.04 (s, 3H), 7.22 (br s, 1H), 7.43
(br s, 1H); 13C{1H} NMR (50 MHz, CDCl3) 30.04, 35.55, 39.23,
44.12, 62.84, 121.36, 124.54, 138.50; HRMS (FAB) calcd for
C14H20N2Au m/z 413.1292 (M)þ, found 413.1327. Anal. Calcd
for C14H20AuN2Cl3: C 32.36, H 3.88, N 5.39. Found: C 32.34, H
3.85, N 5.33.
Trichloro[1,3-dihydro-1-(2-pyridinyl)-3-(2,4,6-trimethylphenyl)-
2H-imidazol-2-ylidene]gold [AuCl3(L2a)] (2d): 55 mg, 96% yield;
mp 213-216 °C (hexane); 1H NMR (THF-d8, 200 MHz) 2.13 (s,
6H), 2.25 (s, 3H), 6.97 (s, 2H), 7.47 (dd, 1H, J = 8.0 Hz, J = 4.0
Hz), 7.63 (d, 1H, J = 2.0 Hz),7.98 (m, 2H), 8.25 (d, 2H, J = 2.0
Hz), 8.60 (dd, 1H, J = 4.0 Hz, J = 2.0 Hz); 13C{1H} NMR
(THF-d8, 50 MHz) 17.96, 20.30, 116.98, 122.99, 124.88, 129.64,
135.64, 139.62, 140.73, 149.40; HRMS (FAB) calcd for
C17H17N335Cl2Au m/z 530.0465 (M)þ, found 530.0459. Anal.
Calcd for C17H17AuCl3N3: C 36.03, H 3.02, N 7.42. Found: C
35.57, H 3.28, N 7.90.
Pentafluorophenyl[1,3-dihydro-1,3-bis(2,4,6-trimethylphenyl)-
2H-imidazol-2-ylidene]gold [Au(C6F5)(L1a)] (3b): 1.16 g, 77%
yield; mp 180-183 °C (heptane); Rf = 0.52 (THF/hexane, 3:10);
1H NMR (CHCl3, 200 MHz) 2.18 (s, 12H), 2.36 (s, 6H), 7.03 (br
s, 4H), 7.13 (s, 2H); 13C{1H} NMR (CDCl3, 50 MHz) 17.86,
21.17, 122.17, 129.21, 129.30, 134.73, 134.82, 139.48, 190.58; 19
F
NMR (CDCl3, 376 MHz) -163.67, -160.80, -116.02; HRMS
(FAB) calcd for C27H24N2F5Au m/z 668.1525 (M)þ, found
668.1535. Anal. Calcd for C27H24N2F5Au: C 48.51, H 3.62, N
4.19. Found: C 47.10, H 3.61, N 3.76.
Phenyl[1,3-dihydro-1,3-bis(2,4,6-trimethylphenyl)-2H-imida-
zol-2-ylidene]gold Dichloride [Au(C6H5)Cl2(L1a)] (4a). To a
solution of 3a (1.20 g; 2.10 mmol) in dichloromethane (30 mL)
was added PhICl2 (710 mg; 2.60 mmol) at 0 °C. The reaction
mixture was stirred for 3 h in the dark at room temperature, and
1
the conversion was monitored by H NMR until completion.
Evaporation of the solvent, purification of the residue by flash
chromatography on neutral alumina (THF/hexane, 1:9-1:1),
and drying under high vacuum yielded 4a as a colorless solid
(1.05 g, 78% yield). Additional washing of the column with THF
gave a second fraction, which was crystallized from dichloro-
methane/hexane to give 2a (220 mg, 17% yield) as a yellow solid.
4a: mp 203-206 °C (heptane); Rf = 0.46 (THF/hexane, 3:10);
1H NMR (CDCl3, 200 MHz) 2.30 (s, 12H), 2.40 (s, 6H),
6.85-7.00 (m, 5H), 7.05 (s, 4H), 7.18 (s, 2H); 13C{1H} NMR
(CHCl3, 50 MHz) 18.41, 21.21, 123.76, 125.62, 128.21, 129.46,
132.72, 133.88, 135.60, 139.84, 146.12, 180.35; HRMS (FAB)
calcd for C27H30N235Cl2Au m/z 649.1452 (M þ H)þ, found
649.1474. Anal. Calcd for C27H29N2Cl2Au: C 49.94, H 4.50, N
4.31. Found: C 49.98, H 4.51, N 4.25. 2a: 1H NMR (CDCl3, 200
MHz) 2.23 (s, 12H), 2.34 (s, 6H), 7.00 (s, 4H), 7.23 (s, 2H);
13C{1H} NMR (CHCl3, 50 MHz) 18.61, 21.19, 125.61, 129.97,
132.36, 135.32, 141.03, 144.72.
Pentafluorophenyl[1,3-dihydro-1,3-bis(2,4,6-trimethylphenyl)-
2H-imidazol-2-ylidene]gold Dichloride [Au(C6F5)Cl2(L1a)] (4b).
To a solution of 3b (800 mg; 1.2 mmol) in dichloromethane (20
mL) was added PhICl2 (460 mg; 1.7 mmol) at 0 °C. The reaction
mixture was stirred for 18 h in the dark at room temperature.
The solvent was evaporated and the solid was washed with
hexane. Drying under high vacuum yielded 4b as a colorless
solid (820 mg, 93% yield): mp 232-235 °C (heptane); Rf = 0.45
(THF/hexane, 3:10); 1H NMR (CDCl3, 200 MHz) 2.29 (s, 12H),
2.40 (s, 6H), 7.08 (s, 4H), 7.27 (s, 2H); 13C NMR (CDCl3, 50 MHz)
18.40, 21.21, 124.38, 129.55, 133.04, 135.25, 140.43, 167.13; 19F
NMR (CDCl3, 376 MHz) -162.31, -158.47, -125.58; HRMS
(FAB) calcd for C27H24N235ClF5Au m/z 703.1214 (M)þ, found
703.1185; C27H24N237ClF5Au m/z 705.1184 (M)þ, found
705.1193. Anal. Calcd for C27H24N2Cl2F5Au: C 43.86, H 3.27,
N 3.79. Found: C 43.72, H 3.48, N 3.59.
Trichloro[1,3-dihydro-1-(2-pyridinylmethyl)-3-(2,4,6-trimethyl-
phenyl)-2H-imidazol-2-ylidene]gold [AuCl3(L2b)] (2e): 540 mg,
1
95% yield; mp 187-190 °C (hexane); H NMR (CDCl3, 200
MHz) 2.06 (s, 6H), 2.26 (s, 3H), 5.59 (s, 2H), 6.90 (s, 2H), 7.04 (d,
1H, J = 2.0 Hz), 7.27 (dd, 1H, J = 8.0 Hz, J = 6.0 Hz), 7.52 (d,
1H, J = 8.0 Hz), 7.58 (d, 1H, J = 2.0 Hz), 7.72 (m, 1H), 8.57 (d,
1H, J = 4.0 Hz, J = 1.0 Hz); 13C{1H} NMR (CDCl3, 50 MHz)
18.50, 21.13, 56.21, 124.07, 124.23, 124.51, 125.24, 129.86,
132.32, 135.37, 137.77, 140.94, 143.24, 150.23, 152.51; HRMS
(FAB) calcd for C18H19N335Cl2Au m/z 544.0622 (M)þ, found
544.0639. Anal. Calcd for C18H19Cl3N3Au: C 37.23, H 3.30, N
7.24. Found: C 37.69, H 3.40, N 6.94.
Trichloro[1,3-dihydro-1-(2-pyridinylethyl)-3-(2,4,6-trimethyl-
phenyl)-2H-imidazol-2-ylidene]gold [AuCl3(L2c)] (2f): 220 mg,
1
97% yield; mp 203-206 °C (hexane); H NMR (CDCl3, 200
MHz) 2.02 (s, 6H), 2.27 (s, 3H), 3.51 (t, 2H, J = 2.0 Hz), 4.88 (t,
2H, J = 2.0 Hz), 6.83 (d, 1H, J = 1.0 Hz), 6.91 (s, 2H), 7.04 (d,
1H, J = 1.0 Hz), 7.14 (m, 2H), 7.57 (dd, 1H, J = 6.0 Hz, J = 1.0
Hz), 8.55 (br s,1H); 13C{1H} NMR (CDCl3, 50 MHz) 18.38,
21.14, 124.73, 129.87, 132.20, 135.34, 137.10, 140.95, 142.64,
146.56; HRMS (FAB) calcd for C19H21N335Cl2Au m/z 558.0778
(M)þ, found 558.0810; for C19H22N335Cl3Au m/z 594.0545
(M þ H)þ, found 594.0491. Anal. Calcd for C19H21Cl3N3Au:
C 38.37, H 3.56, N 7.07. Found: C 37.27, H 3.69, N 6.62.
General Procedure for Arylation of 1a to Give Au(I) Complexes
3a,b. At 0 °C to a solution of complex 1a (3.80 mmol) in THF
(80 mL) was added a solution of Grignard reagent (C6H5MgBr
for 3a and C6F5MgBr for 3b, each 15.0 mmol). The mixture
was stirred at room temperature for 3-5 h. Water (drops) was
added, and a precipitate crashed out of solution. This solid
was filtered off, the solution was concentrated, and the residue
was purified on neutral alumina eluted with THF/hexane
(1:9-1:1) (this has to be done quickly since the TLC showed
complex decomposition to purple Au-nanoparticles). The
products were crystallized from CHCl3/heptane as colorless
solids.
Reaction between 2e and p-Methoxyphenylmagnesium Bro-
mide. At -20 °C to a solution of 2e (50 mg, 86.1 μmol) in dry
THF (2 mL) was added dropwise p-methoxyphenylmagnesium
bromide (100 μL; 103 μmol; 1 M in THF). The reaction mixture
was stirred for 1 h at this temperature, and then slowly the
temperature was raised to 0 °C. After slow addition of water the
product was filtered through a plug of Celite and concentrated
to dryness. The crude reaction mixture was purified on basic
alumina (dichloromethane/ethanol gradient mixtures). 4,40-Bis-
methoxy-1,10-biphenyl (first fraction) was isolated as a colorless
oil (15 mg, 81% yield) followed by 1e (38 mg, 87% yield).
1
4,40-Bismethoxy-1,10-biphenyl: H NMR (CDCl3, 200 MHz)
Phenyl[1,3-dihydro-1,3-bis(2,4,6-trimethylphenyl)-2H-imidazol-
2-ylidene]gold [Au(C6H5)(L1a)] (3a): 2.11 g, 96% yield; mp 242-
245 °C (heptane); Rf = 0.46 (THF/hexane, 3:10); 1H NMR
(CDCl3, 200 MHz) 2.19 (s, 12H), 2.37 (s, 6H), 6.84-7.18 (m,
11H); 13C{1H} NMR (CDCl3, 50 MHz) 18.21, 21.38, 122.01,
124.70, 126.97, 129.39, 135.14, 135.54, 139.27, 140.67, 169.34,
195.81; HRMS (FAB) calcd C27H30N2Au m/z 579.2075 (M þ
H)þ, found 579.2065. Anal. Calcd for C27H29N2Au: C 56.06, H
5.05, N 4.84. Found: C 55.46, H 4.89, N 5.06.
2.01 (s, 6H), 2.33 (s, 3H), 5.59 (s, 2H), 6.89 (d, 1H, J = 2.0 Hz),
6.95 (s, 2H), 7.27 (m, 1H), 7.36 (d, 1H, J = 2.0 Hz), 7.46 (d, 1H,
J = 8.0 Hz), 7.74 (ddd, 1H, J = 8.0 Hz, J = 8.0 Hz, J = 2.0 Hz),
8.62 (d, 1H, J = 4.0 Hz).
General Procedure for the Synthesis of the Cationic Au(I) Com-
plexes 5a,b. To a solution of 1e or 1f (200μmol) in dichloromethane
(3 mL) was added AgNTf2 (200 μmol). The solution was stirred for
5 min and filtered over Celite. The pad of Celite was further washed
twice with dichloromethane (2 mL), and the resulting solution was

ꢁ
Pazꢀicky et al.
4458 Organometallics, Vol. 29, No. 20, 2010
reduced under vacuum to 1 mL. Pentane (10 mL) was added,
resulting in the immediate precipitation of a white solid. The
product was filtered, further washed with pentane (3 ꢀ 5 mL), and
dried under high vacuum.
methane (1 mL) were added gold complex 1f (529 μg, 1.00 μmol)
and AgNTf2 (392 μg, 1.00 μmol). The solution was stirred for 3 h
and purified by flash column chromatography (petroleum ether/
ethyl acetate, 5:1, Rf = 0.24). The product was isolated as
colorless solid (53 mg, 87% yield): 1H NMR (CDCl3, 300 MHz)
2.26 (s, 3H), 4.96 (s, 1H), 5.08-5.15 (m, 4H), 6.72 (d, 1H, J = 7.5
Hz), 7.05 (d, 1H, J = 7.5 Hz); 13C{1H} NMR (CDCl3, 75 MHz)
15.22, 71.77, 74.14, 113.02, 121.83, 125.27, 130.63, 139.28,
148.40.
[Au(L2b)]2(NTf2)2 (5a): 140 mg, 93% yield; mp 85-88 °C
(heptane); Rf = 0.40 (THF/hexane, 1:1); 1H NMR (CD2Cl2, 400
MHz) 1.98 (br s, 6H), 2.33 (s, 3H), 5.97 (br s, 1 H), 7.04 (s, 2H),
7.14 (s, 1H), 7.62 (s, 1H), 7.68 (dd, 1H, J = 4.0 Hz, J = 8.0 Hz),
8.07 (d, 1H, J = 8.0 Hz), 8.12 (d, 1H, J = 4.0 Hz), 8.24 (dd, 1H,
J = 8.0 Hz, J = 8.0 Hz); 13C{1H} NMR (CD2Cl2, 100 MHz)
18.19, 21.29, 57.40, 118.69, 121.88, 123.10, 125.04, 127.33,
128.77, 129.95, 134.67, 135.09, 141.08, 143.16, 152.57, 155.03,
165.03; 19F NMR (CD2Cl2, 376 MHz) -79.40; MS (FAB) for
C36H38N6Au m/z 751.28 (M)þ, (L2bAuL2b)þ. Anal. Calcd for
C20H19AuF6N4O4S2: C 31.84, H 2.54, N 7.43. Found: C 31.93,
H 2.72, N 7.36.
[Au(L2c)]2(NTf2)2 (5b): 150 mg, 96% yield; mp 180 °C (dec,
heptane); Rf = 0.29 (THF/hexane, 1:1); 1H NMR (CD2Cl2, 400
MHz) 1.78 (s, 6H), 2.32 (s, 3H), 3.66 (t, 2H, J = 8.0 Hz), 5.11 (t,
2H, J = 8.0 Hz), 7.00 (br s, 3H), 7.56 (m, 2H), 7.67 (d, 1H, J =
8.0 Hz), 7.98 (dd, 1H, J = 8.0 Hz, J = 8.0 Hz), 8.15 (d, 1H, J =
4.0 Hz); 13C{1H} NMR (CD2Cl2, 100 MHz) 17.92, 21.24, 39.69,
50.20, 121.98, 122.99, 124.62, 125.75, 129.23, 129.88, 134.50,
134.96, 140.98, 142.01, 151.90, 157.58, 164.67; 19F NMR
(CD2Cl2, 376 MHz) -79.35; MS (FAB) for C38H42N6Au m/z
779.31 (M)þ, (L2cAuL2c)þ. Anal. Calcd for C21H21AuF6-
N4O4S2: C 32.82, H 2.75, N 7.29. Found: C 32.20, H 2.86, N
7.17.
Acknowledgment. M.P., M.J.F., D.S., N.V., C.J., and
M.L. work at CaRLa of Heidelberg University, being co-
financed by University of Heidelberg, the state of Baden-
€
Wurttemberg, and BASF SE. Support of these institu-
tions is greatly acknowledged. The authors thank D.
Gutruf (AK Gade, Anorganisch-Chemisches Institut,
€
Ruprecht-Karls-Universitat Heidelberg) for collecting
electrochemical data and Dr. F. Stecker (BASF, GCI/
E-M311) for their interpretation. The authors are in-
debted to Kenneth G. Caulton, Indiana University,
Bloomington, for careful proofreading of the final manu-
script.
Supporting Information Available: Crystallographic informa-
tion files (CIF) of compounds L2c HI, 1b-f, 2b-f, and 3-5;
3
cyclic voltammetry data and NMR spectra of all compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org. Files CCDC 771118 (ligand L2c), 771119 (1b),
771120 (1c), 771121 (1d), 771122 (1e), 771123 (1f), 771124 (2b),
771125 (2c), 771126 (2d), 771127 (2e), 771128 (2f), 771129 (3a),
771130 (3b), 771131 (4a), 771132 (4b), 771133 (5a), and 771134
(5b) 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.
General Procedure for the Catalytic Studies. To a solution of 6
(30.0 mg, 200 μmol) in deuterated solvent (CDCl3 or CD2Cl2,
0.5 mL) containing internal standard (1,3,5-tri-tert-butylben-
zene, 2 mg) was added a 1:1 mixture of gold complex and silver
salt (0.05-4 mol %). The NMR tube was shaken for 1 h, and the
1
yield was determined by integration of the corresponding H
NMR signals in comparison to the internal standard.
4-Isobenzofuranol (7). To a solution of 2-methyl-5-[(2-pro-
pyn-1-yloxy)methyl]furan (6, 60.8 mg, 405 μmol) in dichloro-