Straightforward synthesis of [Au(NHC)X] (NHC = N-heterocyclic carbene, X = Cl, Br, I) complexes

Article abstract of DOI:10.1039/c3cc43076f

An improved protocol for the synthesis of [Au(NHC)X] (X = Cl, Br, I) complexes is reported. This versatile one-step synthetic methodology proceeds under mild conditions, in air, using technical grade solvents, is scalable and is applicable to a wide range

Full text of DOI:10.1039/c3cc43076f

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Straightforward synthesis of [Au(NHC)X]
(NHC = N-heterocyclic carbene, X = Cl, Br, I)
complexes†
Cite this: Chem. Commun., 2013,
49, 5541
Received 25th April 2013,
Accepted 9th May 2013
´
´
´
Alba Collado, Adrian Gomez-Suarez, Anthony R. Martin, Alexandra M. Z. Slawin
and Steven P. Nolan*
DOI: 10.1039/c3cc43076f
www.rsc.org/chemcomm
An improved protocol for the synthesis of [Au(NHC)X] (X = Cl, Br, I)
complexes is reported. This versatile one-step synthetic methodology
proceeds under mild conditions, in air, using technical grade solvents,
is scalable and is applicable to a wide range of imidazolium and
imidazolidinium salts.
Homogeneous gold catalysis has experienced an amazingly
rapid growth since the beginning of the 21st century.1 Recently,
gold complexes bearing a NHC (NHC = N-heterocyclic carbene)
ligand have attracted increased attention. These complexes are
effective catalysts in a large number of valuable processes such
as enyne cycloisomerisation, hydroamination of alkenes and
alkynes, alkyne and nitrile hydration, polymerisation and many
others.2 Moreover, gold–NHC complexes have found applica-
tions in different fields,3 as they possess biomedical and
Scheme 1 Synthetic methods for [Au(NHC)Cl] complexes.
luminescent properties. Among them, [Au(NHC)Cl] complexes cannot be achieved by transmetallation routes.8 Therefore, the
are particularly interesting, as they have been used directly as development of improved, scalable, economical, and general
pre-catalysts or as synthons to prepare other Au–NHC species.4 procedures is highly desirable.
The most common procedures for the synthesis of [Au(NHC)Cl]
A close look at the literature reveals that some [Au(NHC)Cl]
complexes involve either: (a) the generation of a free carbene and complexes have been successfully prepared by direct treatment of
subsequent reaction with a gold-containing precursor,2b generally imidazolium salts with weak bases, such as NaOAc or K2CO3, in
[Au(DMS)Cl] or [Au(THT)Cl] (DMS = dimethylsulfide, THT = tetra- the presence of a gold complex.9 However, these procedures, which
hydrothiophene), or (b) carbene transfer from a Ag–5 or Cu–NHC6 have not been applied to the most common NHCs (Fig. 1), have
precursor (Scheme 1).7 Although both synthetic approaches are several drawbacks: they require not only high temperatures
efficient, they require multiple steps. In addition, inert conditions (80–120 1C) and long reaction times (9–48 h) but also environmen-
and excess of relatively expensive bases (NaH, KOtBu) are needed to tally unfriendly solvents such as xylenes, 3-chloropyridine or
generate the free carbene species and 1 equiv. of metal is wasted dimethylformamide. In this context, the challenge of developing
during the transmetallation reactions. Furthermore, neither of a more straightforward and general procedure for the synthesis of
these synthetic routes are general methodologies for common [Au(NHC)Cl] complexes using different NHCÁHCl salts (Fig. 1) and
Au–NHC complexes; e.g. [Au(IMes)Cl] or [Au(SIMes)Cl] cannot be [Au(DMS)Cl] in the presence of a weak base was tackled.
prepared effectively following the free carbene route,5a and the
synthesis of [Au(IPr*)Cl], containing the very large IPr* (1,3-bis(2,6- common and commercially available imidazolium salts. The pre-
bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) ligand, liminary experiment was carried out following the reaction condi-
tions reported by Wang,9c i.e. using 2 equiv. of NaOAc, 1 equiv. of
We first explored the reaction using IPrÁHCl (1), one of the most
EaStCHEM School of Chemistry, University of St. Andrews, St. Andrews, KY16 9ST,
[Au(DMS)Cl], and dry acetonitrile under a nitrogen atmosphere.
Gratifyingly, after 24 h at ambient temperature, we observed a
70% conversion to [Au(IPr)Cl] (2). Subsequent experiments per-
UK. E-mail: snolan@st-andrews.ac.uk; Fax: +44 (0)1334463808;
Tel: +44 (0)1334463763
† Electronic supplementary information (ESI) available: Optimisation details and
formed in air and with technical grade solvents revealed no detri-
mental impact of these protocol variations on the reaction outcome.
full characterisation data. CCDC 936081 and 936088. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3cc43076f
c
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Fig. 1 NHCÁHCl salts used in this study.
Therefore, subsequent experiments were carried out under these For [Au(IMes)Cl], the yield has been improved from 63% (obtained
user-friendly conditions. Optimisation studies revealed that complete by transmetallation) to 79% (Table 1, entry 2).5a Noteworthy is the
conversion could be obtained in acetone with only 1 equiv. of K2CO3 isolation of [Au(IPr*)Cl], which cannot be prepared by transmetalla-
after 1 h at 60 1C (see ESI† for details). Under these conditions, tion, in a 76% yield after 4 h (Table 1, entry 3).8 The preparation of
complex 2 was isolated as a white solid in 97% yield (Scheme 2).10 [Au(NHC)Cl] complexes containing N-alkyl unsaturated NHC ligands
Elemental analysis of complex 2 was carried out to confirm the high was also accomplished (Table 1, entries 6–9). Good yields were
purity of the material produced using the present methodology.
obtained (60–88%), although the reactions for ICyÁHCl, IDDÁHCl
and ItBuÁHCl did not reach completion (Table 1, entries 7–9). The
substitution of the backbone has a dramatic effect on the reaction
and only 53% yield of the desired gold complex was obtained when
using IPrMeÁHCl. However, this yield can be improved to 78% by
using 2 equiv. of base (Table 1, entry 10). This result represents a
significant improvement as the overall yield for the previously
reported two-step synthesis of [Au(IPrMe)Cl] is only 51%.11 Interest-
ingly, longer reaction times are required to reach full conversion for
saturated NHC ligands. The gold complexes were obtained in good
yields after 24 h (Table 1, entries 11–13). All complexes were
Scheme 2 Optimised conditions for the synthesis of [Au(IPr)Cl] (2).
To assess the generality of this procedure the preparation of
different [Au(NHC)Cl] compounds was investigated. These results
are summarized in Table 1.
1
characterised by H and 13C{1H} NMR spectroscopies and data for
the reported complexes matched literature spectra.8,11,12
This procedure proved to be efficient for the synthesis of both
saturated and unsaturated Au(I)–NHC complexes. The reaction reaches
completion in 1–4 h for the N-aryl unsaturated NHC ligands (Table 1,
entries 1–7). Good to excellent yields were obtained in all cases. A
comparison with previous procedures revealed that our optimised
synthetic route allows the preparation of [Au(NHC)Cl] in higher yields.
This is particularly true for [Au(IPr)Cl] and [Au(IMes)Cl] complexes.
[Au(IPr)Cl] was obtained in 97% yield (Table 1, entry 1), while 76%
yield is achieved following the free carbene route (ca. 60% overall
yield, taking into account that the free carbene must be isolated).5a
The utility of this methodology for the synthesis of some
[Au(NHC)Cl] complexes on a larger scale was also tested
(Table 2). Although the reactions can be successfully performed
using 1 equiv. of K2CO3, the addition of an excess of base
reduces the reaction time. Therefore, all larger scale reactions
were conducted with 3 equiv. of K2CO3.
Table 2 Large-scale reactionsa
Entry
NHCÁHCl (g)
[Au(DMS)Cl] (g)
Time (h)
Yield (g)
1
2
3
4
5
IPrÁHCl (21.7)
15.0
20.0
3.29
1.22
2.58
5
5
4
4
24
94% (29.8)
87% (31.6)
75% (3.91)
77% (2.11)
79% (3.74)
IMesÁHCl (23.1)
ICyÁHCl (3.00)
IPrMeÁHCl (1.87)
SIMesÁHCl (3.00)
Table 1 Scope of the [Au(NHC)Cl] synthesisa
Entry
NHCÁHCl
Time (h)
Yield (%)
a
1
2
3
4
5
6
7
8
IPrÁHCl
1
4
4
2
1
1
2
3
2
97
79
76
88
97
88
75
70
Reaction conditions: acetone, 60 1C, 1 equiv. of NHCÁHCl, 1 equiv. of
IMesÁHCl
IPr*ÁHCl
IPr*TolÁHCl
IXyÁHCl
[Au(DMS)Cl], 3 equiv. of K2CO3.
In order to gain further insight into the reaction mechanism
involved in this transformation, the reaction was performed in the
absence of base. The treatment of 1 equiv. of [Au(DMS)Cl] with
1 equiv. of IPrÁHCl under the above conditions gives rise to a new
species. This compound was isolated as a white solid in 95% yield
and unambiguously characterised by 1H, 13C{1H} NMR, X-ray
diffraction and elemental analyses as [IPrH][AuCl2] (3). In this
salt, the [AuCl2]À unit acts as a counterion and shows a nearly
linear geometry with a Cl1–Au–Cl2 angle of 175.0(4)1 (Fig. 2).12
IsBÁHCl
ICyÁHCl
IDDÁHCl
ItBuÁHCl
IPrMeÁHCl
SIPrÁHCl
SIMesÁHCl
SITbÁHCl
9
60
10
11
12
13
4
53 (78)b
78
24
24
24
82
68
a
Reaction conditions: 100 mg of NHCÁHCl, 1 equiv. of [Au(DMS)Cl],
b
1 equiv. of K2CO3, acetone, 60 1C. 2 equiv. of K2CO3.
c
5542 Chem. Commun., 2013, 49, 5541--5543
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In summary, a useful one-step procedure for the synthesis of
[Au(NHC)Cl] complexes has been developed. This methodology
is performed under mild conditions, does not require an inert
atmosphere and is applicable to a wide range of imidazolium
halide salts, including both saturated and unsaturated carbenes.
Further synthetic and theoretical studies dealing with the mecha-
nism of the reaction are currently ongoing.
The ERC (Advanced Investigator Award-FUNCAT), the EPSRC
and Syngenta are gratefully acknowledged for support of this
work. Umicore AG is acknowledged for their generous gift of
auric acid. We thank Mr Scott R. Patrick for helpful synthetic
contributions and Dr David J. Nelson for helpful discussions.
S.P.N. is a Royal Society Wolfson Research Merit Award holder.
Fig. 2 ORTEP representation of [IPrH][AuCl2] (3) showing 50% thermal ellipsoid
probability. Most H atoms were omitted for clarity.
The most salient feature of the 1H NMR spectrum of 3 in
CD2Cl2 is the signal corresponding to the NCHN proton that
appears at 8.89 ppm as a triplet (JH–H = 1.5 Hz). This signal is
shifted upfield 2.11 ppm relative to that of IPrÁHCl.13 Sub-
sequent treatment of acetone solutions of 3 with 1 equiv. of
K2CO3 afforded the final product 2. This result supports the
hypothesis that 3 is an intermediate species in this process. It
appears that the formation of 3 is the fastest step of the process
as this reaction occurs at room temperature within 10 min.
Due to the interesting nature of intermediate 3, i.e. a NHC salt
with a [AuCl2]À unit as counterion, we wondered if it was possible to
access similar intermediates using different NHCÁHX (X = Br, I) salts
and what the identity of final gold complex would be once these
species were treated with base. Therefore, reactions between
[Au(DMS)Cl] and IPrÁHBr (4) and IPrÁHI (5) were carried out,
affording [IPrH][AuClBr] (6) and [IPrH][AuClI] (7), respectively. Inter-
estingly, subsequent treatment of these species with one equivalent
of K2CO3 led to the formation of a single gold–NHC species in each
case, bromide (8) and iodide (9) respectively (Scheme 3).
Notes and references
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¨
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8 A. Gomez-Suarez, R. S. Ramon, O. Songis, A. M. Z. Slawin, C. S. J.
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10 [Au(IPr)Cl] was also successfully synthesized by using [Au(THT)Cl] (THT =
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11 S. Gaillard, X. Bantreil, A. M. Z. Slawin and S. P. Nolan, Dalton
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Scheme 3 Formation of [IPrH][XAuCl] and [Au(IPr)X] complexes.
It should be noted that the chloride derivative was never
observed during these reactions. This trend may be explained by
considering the halide trans effect series: I c Br > Cl.14 We
hypothesise that the halide exerting a higher trans effect would
labilise the bond trans to it and therefore would stay coordinated
´
12 P. de Fremont, N. M. Scott, E. D. Stevens, T. Ramnial, O. C.
Lightbody, C. L. B. Macdonald, J. A. C. Clyburne, C. D. Abernethy
and S. P. Nolan, Organometallics, 2005, 24, 6301–6309.
to the gold center in the final complex. Compounds 6–9 were 13 For the first reported synthesis of IPrÁHCl, see: J. Huang and
characterised by 1H and 13C{1H} NMR spectroscopies and by
elemental analysis. The general one-pot procedure can afford not
S. P. Nolan, J. Am. Chem. Soc., 1999, 121, 9889–9890 and for more
details see: L. Jafarpour, E. D. Stevens and S. P. Nolan, J. Organomet.
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only chlorides but also bromide and iodide complexes in one step. 14 F. R. Hartley, Chem. Soc. Rev., 1973, 2, 163–179.
c
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Chem. Commun., 2013, 49, 5541--5543 5543