Expect the unexpected. Isolation and characterisation of some unusual organometallic gold(I) complexes

Article abstract of DOI:10.1016/S0020-1693(03)00128-2

The reaction of equimolar amounts of 2-Li-3-MeC₆H₃PPh₂ and [AuBr(AsPh₃)] gives a mixture of the cyclometallated digold(I) complex [Au₂(μ-6-MeC₆H₃-2-PPh₂) ₂] and the 'half-cyclometallated' complex [(Ph₃As)Au(μ-6-MeC₆H₃-2-PPh ₂)AuBr] (1). This complex represents the first example of an intermediate of a cycloauration reaction that has been structurally characterised by X-ray diffraction. The reaction of [Au₂(μ-6-MeC₆H₃-2-PPh₂) ₂] with mercuric chloride gives the unusual heteronuclear Hg(II)/Au(I) complex [ClHg(μ-6-MeC₆H₃-2-PPh₂)AuCl] (2), arising from electrophilic cleavage of a gold-carbon σ-bond, and not the oxidative addition product.

Full text of DOI:10.1016/S0020-1693(03)00128-2

Inorganica Chimica Acta 352 (2003) 19ꢂ23
/
www.elsevier.com/locate/ica
Expect the unexpected. Isolation and characterisation of some
unusual organometallic gold(I) complexes
S.K. Bhargava a,*, F. Mohr a,1, A.C. Willis b
a Department of Applied Chemistry, RMIT University, GPO Box 2476V, Melbourne 3001, Vic., Australia
b Research School of Chemistry, Australian National University, Canberra 0200, ACT, Australia
Received 30 November 2001; accepted 20 February 2002
This paper is dedicated to our mentor and friend Professor Martin Bennett on the occasion of his retirement for a lifetime contribution to
organometallic chemistry
Abstract
The reaction of equimolar amounts of 2-Li-3-MeC6H3PPh2 and [AuBr(AsPh3)] gives a mixture of the cyclometallated digold(I)
complex [Au2(m-6-MeC6H3-2-PPh2)2] and the ‘half-cyclometallated’ complex [(Ph3As)Au(m-6-MeC6H3-2-PPh2)AuBr] (1). This
complex represents the first example of an intermediate of a cycloauration reaction that has been structurally characterised by X-ray
diffraction. The reaction of [Au2(m-6-MeC6H3-2-PPh2)2] with mercuric chloride gives the unusual heteronuclear Hg(II)/Au(I)
complex [ClHg(m-6-MeC6H3-2-PPh2)AuCl] (2), arising from electrophilic cleavage of a gold-carbon s-bond, and not the oxidative
addition product.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Binuclear; Gold; Cyclometallated; Heteronuclear
1. Introduction
elimination chemistry of dinuclear cyclometallated
gold(I) complexes of the type [Au2(m-C6H4ER2)2], where
The element gold and its varied, interesting and often
unusual chemistry have fascinated medieval alchemists
and modern-day researchers alike. In recent years, gold
chemistry has experienced a renaissance, evident in the
appearance of hundreds of research papers and com-
prehensive monographs. A large number of gold con-
taining structures have been reported over recent years,
out of which several examples stand out: hypercoordi-
Eꢁ
/
P, As and RꢁEt, Ph [7]; work that originated from
/
Martin Bennett’s group in Australia in the 1980s. The
digold(I) complexes [Au2(m-C6H4PR2)2] undergo oxida-
tive addition reactions with halogens to give metal-metal
bonded digold(II) complexes which, on warming, re-
arrange to give carbonꢂ
(Scheme 1), [7,8]. To further elucidate the mechanism
leading to CꢀC coupling we have studied the analogous
methyl substituted digold(I) complexes [Au2(m-n-
MeC6H3-2-PPh2)2], where nꢁ5, 6. In comparison to
/carbon coupled products
/
nated complexes [E(AuPPh3)n]mꢀ where Eꢁ
C [1,2], N
/
/
[3], O [4] and As [3], self-assembling macrocyclic singly-
and doubly-braided [2]catenanes [5,6] and a large variety
of homonuclear gold clusters. In our work we have
continued to study the oxidative addition and reductive
the unsubstituted parent complex, the 6-methyl substi-
tuted digold(II) analogue displays quite different reac-
tivity. Instead of rearranging to a C-C coupled product
these digold(II) complexes very rapidly isomerise to
heterovalent gold(I)/gold(III) complexes (Scheme 1),
[9,10]. During the course of this investigation we have
isolated and characterised two unusual and unexpected
compounds, the structures of these will be presented
here.
* Corresponding author. Tel./fax: ꢀ61-3-9925 3365.
/
E-mail address: suresh.bhargava@rmit.edu.au (S.K. Bhargava).
1
Present address: Department of Chemistry, The University of
Western Ontario, London, Ont., Canada N6A 5B7.
0020-1693/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0020-1693(03)00128-2

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S.K. Bhargava et al. / Inorganica Chimica Acta 352 (2003) 19ꢂ
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Scheme 1.
2. Experimental
ꢃ
/
788. The mixture was stirred at this temperature for
approximately 4 h then warmed to room temperature
overnight. The supernatant was removed by decantation
2.1. General procedures
and the colourless solid washed with ether (3ꢄ10 ml),
/
methanol (5 ml), hexane (15 ml) and dried in vacuo. The
resulting greyish solid was then dissolved in dichlor-
omethane (20 ml) and filtered through Celite. Concen-
tration of the filtrate under reduced pressure and
addition of hexane gave a colourless solid, which was
isolated by filtration and dried in air. 31P{1H} NMR
spectroscopy showed that the product consisted of
approximately 50:50 mixture of 1 and [Au2(m-6-
MeC6H3-2-PPh2)2]. Crystals suitable for X-ray diffrac-
tion were obtained by slow evaporation from CH2Cl2/
hexane. Data for 1: 31P{1H} NMR (CDCl3) d 36.4 (s);
The syntheses were performed under dry argon with
the use of standard Schlenk techniques, although the
solid gold complexes, once isolated, were air-stable.
Solvents were dried by standard methods, distilled, and
stored under nitrogen. The following instruments were
used for spectroscopic measurements: Varian Gemini
200 (1H at 200 MHz), Bruker Aspect 2000 (31P at 80.96
MHz), VG ZAB-2SEQ (EI mass spectra) and Perkin
Elmer FT 1800 (IR spectra as polyethylene disks in the
range 400ꢂ
150 cmꢃ1). The NMR chemical shifts (d) are
/
given in ppm relative to TMS (1H) and 85% H3PO4
(31P), referenced either to residual solvent signals (1H) or
externally (31P). Elemental analyses were performed by
the Microanalytical Laboratory of the Research School
of Chemistry at the Australian National University.
1H NMR (CDCl3) d 2.7 (s, 3H, CH3), 6.6ꢂ
7.5 (m, 28H,
/
Arom); IR (polyethylene disk, cmꢃ1) 235 (Au-Br).
2.2.2. [ClHg(m-6-MeC6H3-2-PPh2)AuCl] (2)
To a solution of [Au2(m-6-MeC6H3-2-PPh2)2] (200
mg, 0.21 mmol) in dichloromethane (20 ml) was added
HgCl2 (57 mg, 0.21 mmol) and stirred for 2 h. The
solution was concentrated under reduced pressure to
approximately 10 ml and hexane added to precipitate a
colourless solid. This was isolated by filtration, washed
with hexane and air-dried. The yield was 220 mg (71%).
2.2. Syntheses
The starting materials 2-Br-3-MeC6H3PPh2 [11], [Au-
Br(AsPh3)] [12] and [Au2(m-6-MeC6H3-2-PPh2)2] [9]
were prepared according to literature procedures.
2.2.1. [(Ph3As)Au(m-6-MeC6H3-2-PPh2)AuBr] (1)
n
A solution of BuLi (2.25 ml of a 1.6 M solution in
M.p.: 2708 (dec); 31P{1H} NMR (CDCl3) d 41.9 (3JPꢂHg
1
392 Hz); H NMR (CDCl3) d 2.6 (s, 3H, CH3), 6.8ꢂ
/
7.8
Cl]ꢀ, 507 [M-
HgCl2]ꢀ; IR (polyethylene disk,
cmꢃ1) 341.7 (Hg-Cl), 324.2 (Auꢂ
Cl); Anal. Found: C,
hexane, 3.6 mmol) was added dropwise to a solution of
2-Br-3-MeC6H3PPh2 (1.17 g, 3.3 mmol) in a 1:1 mixture
of ether and hexane at room temperature. The resulting
precipitate of 2-Li-3-MeC6H3PPh2 (0.649 g, 2.3 mmol)
was washed by decantation with hexane and suspended
in ether (25 ml). This was added to a stirred suspension
of [AuBr(AsPh3)] (1.34 g, 2.3 mmol) in ether (20 ml) at
(m, 13H, Arom); EIꢂ
/
MS m/z 709 [Mꢂ
/
HgCl]ꢀ, 471 [Mꢂ
/
/
30.47; H, 2.27; P, 3.89 Calc. for C19H16AuCl2HgP: C,
30.68; H, 2.17; P, 4.16. Crystals suitable for X-ray
diffraction were obtained by layering the mother liquor
with hexane.

S.K. Bhargava et al. / Inorganica Chimica Acta 352 (2003) 19ꢂ
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23
21
2.3. Structure determinations
phosphorus bonds. In addition, one gold atom is
bonded to bromide and the other coordinated to
˚
AsPh3. The Au-Br distance 2.394(2) A is similar to
˚
that in [AuBr(AsPh3)] [2.377(6) A] [20] and in
The structures were solved by direct methods (SIR 92)
[13] (1) or heavy-atom Patterson methods [14] (2) and
were expanded by the use of Fourier techniques [15].
Hydrogen atoms were included at geometrically deter-
mined positions, which were periodically recalculated
but not refined. Methyl hydrogen atoms were oriented
to best-fit peaks in difference electron density maps. The
neutral atom scattering factors were taken from Ref.
[16]; Df? and Dfƒ values and mass attenuation coeffi-
cients were taken from Ref. [17]. Anomalous dispersion
effects were included in Fcalc [18]. All calculations were
performed with the TEXSAN crystallographic software
package [19]. The crystallographic details for complexes
1 and 2 are listed in Table 1.
˚
[AuBr(PPh3)] [2.252(6) A] [21]. The Auꢀ
/
As distance
2.390(2) A is slightly greater than that of [AuBr(AsPh3)]
˚
˚
[2.342(5) A], consistent with the higher trans-influence
of the s-aryl group. The Au-P and Au-C distances
[2.245(3) and 2.07(1) A, respectively] are similar to those
˚
˚
P 2.296(2) A and Au-C 2.045(6)
in [AuPh(PPh3)] [Auꢀ
/
˚
A] [22].
Complex 1 represents the first example of an inter-
mediate in a cycloauration reaction involving P-donor
ligands that has been structurally characterised by X-ray
diffraction. It is evident that a further molecule of 2-Li-
3-MeC6H3PPh2 can displace the bromide to form a
second gold-carbon s-bond with loss of LiBr. In a
subsequent (or simultaneous) step, the phosphorus atom
displaces the triphenylarsine to form the second Au-P
bond and thus yield the fully cyclometallated product.
This suggests complex 1 is an intermediate in the
cycloauration reaction rather than merely a by-product.
Despite numerous attempts at developing a rational
synthesis for this complex, we have been unsuccessful so
far. It is noteworthy that analogous intermediates have
neither been observed nor isolated when phosphine
gold(I) precursors such as [AuBr(PPh3)] and
[AuBr(PEt3)] were used in the reaction. It may be that
these ‘half-cyclometallated’ complexes normally react
very quickly to form the fully cycloaurated products and
that, for some unknown reason, we have been lucky to
isolate this product before it could react any further.
Very few other reports of any intermediates involved
in cycloauration reactions have appeared in the litera-
ture. The C-coordinated 2-pyridyl complex [Au(2-
C5H4N)(AsPh3)] has been postulated as an intermediate
in the cyclometallation reaction of 2-LiC5H4N with
[AuCl(AsPh3)] to give the trinuclear complex [Au3(m-
C5H4N)3] [23]. However, the structure of this inter-
mediate has not been confirmed by X-ray crystallogra-
phy.
3. Results and discussion
Reaction of equimolar quantities of 2-Li-3-
MeC6H3PPh2 with [AuBr(AsPh3)] gives a colourless
solid, which shows two singlets of approximately equal
intensity at d 36.2 and 36.4 in the 31P {1H} NMR
spectrum; the former peak corresponding to the cyclo-
metallated gold(I) complex [Au2(m-6-MeC6H3-2-PPh2)2]
(Scheme 2). However, the identity of the second
component could not be established. A crystal of this
unknown compound was serendipitously selected and
analysed by X-ray diffraction. This showed that the
compound was the ‘half-cyclometallated’ complex
[(Ph3As)Au(m-6-MeC6H3-2-PPh2)AuBr] (1) shown in
Fig. 1; selected bond lengths and angles are listed in
Table 2.
The compound consists of two linearly coordinated
gold(I) atoms bridged by one molecule of the 6-
MeC6H3PPh2 ligand through gold-carbon and gold-
Table 1
Crystallographic data for compounds 1 and 2
Mercuric chloride reacts with low oxidation state
transition metal complexes resulting in products with a
variety of possible structures: both chlorides can be
1
2×1/2HgCl2
/
Formula
Formula weight
C37H31AsAuBrP
1055.38
P21/c
C19H16AuCl3Hg1.5
879.52
C2/c
P
displaced to give metalꢂ
/
mercuryꢂmetal bonds [24], only
/
Space group
˚
one chloride can be displaced to give M-HgCl com-
pounds [25,26] and in addition, HgCl2 can form simple
metal adducts [27,28] or form bridges between two
metals [29]. It was therefore expected that mercuric
chloride would oxidatively add to the digold(I) complex
resulting in the formation of a Cl-Hg-Au-Au-Cl unit.
However, the reaction of [Au2(m-6-MeC6H3-2-PPh2)2]
with HgCl2 gives a colourless solid (Scheme 3), which
shows a singlet at d 41.9 in the 31P{1H} NMR spectrum
a (A)
˚
8.958(2)
15.511(2)
25.032(1)
96.33(1)
3456.9(9)
4
27.277(3)
11.133(3)
17.882(4)
125.83(1)
4402(2)
8
b (A)
˚
c (A)
b (8)
3
V (A )
˚
Z
rcalc (g cmꢃ3
)
2.028
184.91
2.654
175.82
m (cmꢃ1
)
a
R
Rw
0.042
0.043
0.036
0.033
b
with 199Hg satellites (3JHgꢂP
with the expected oxidative addition product structure.
ꢁ392 Hz), inconsistent
/
a
Rꢁ
Rwꢁ
/
ajjFojꢃ
/
jFcjj/ajFoj.
/
b
/
[aw(jFojꢃ
jFcj)2/awFo2]1/2, where wꢁ
/
1/[s2(Fo)].

22
S.K. Bhargava et al. / Inorganica Chimica Acta 352 (2003) 19ꢂ
/
23
Scheme 2.
Fig. 1. Molecular structure of complex 1. Ellipsoids show 30%
probability levels. Hydrogen atoms are drawn as circles with small,
arbitrary radii.
Fig. 2. Molecular structure of complex 2. Ellipsoids show 30%
probability levels. Hydrogen atoms and the co-crystallised HgCl2 have
been omitted for clarity.
Table 2
Table 3
˚
˚
Selected bond distances (A) and angles (8) in complex 1
Selected bond distances (A) and angles (8) in complex 2
Au(1)
Au(2)ꢀ
Au(2)ꢀ
P(1)ꢀ
/
ꢀ ꢀ ꢀ
/
Au(2)
C(2)
As(1)
3.0927(8)
2.07(1)
Au(1)ꢀ
Au(1)ꢀ
As(1)ꢀ
Au(1)ꢀ
/
P(1)
Br(1)
2.245(3)
2.394(2)
173.0(3)
110.3(4)
Au(1)
Hg(1)ꢀ
Au(1)ꢀ
Cl(1)ꢀ
/
ꢀ ꢀ ꢀ
/
Hg(1)
3.2940(9)
2.270(6)
2.280(4)
176.1(2)
Hg(1)ꢀ
Au(1)ꢀ
Cl(2)ꢀ
Au(1)ꢀ
/
C(1)
2.07(1)
/
/
/
Cl(2)
/P(1)
2.233(4)
176.5(1)
109.1(5)
/
2.390(2)
176.5(1)
/
Au(2)ꢀ
/
C(2)
/Cl(1)
/
Hg(1)ꢀ
/
C(2)
/
Au(1)ꢀ
/
Br(1)
/
P(1)ꢀC(1)
/
/
Au(1)ꢀ
/
P(1)
/
P(1)ꢀC(1)
/
compound has been obtained from the reaction of
[Au2(m-C6H4PPh2)2] with HgCl2 [30]. The structure of
2 consists of linearly coordinated Hg(II) and Au(I)
atoms bonded to chloride atoms and bridged by a 3-
˚
MeC6H3PPh2 unit. The Au-Hg distance of 3.2940(9) A
is significantly longer than that in the non-methylated
˚
analogue [3.112(1) A] [30], indicative of very little or no
metalꢂmetal interaction. The Hg-C and Hg-Cl distances
[2.07(1) and 2.270(6) A, respectively] in 2 are very
similar to those observed in [HgClPh] [HgꢀC 2.044(9)
Cl 2.345(2) A] [31]. Furthermore, the Auꢀ
Scheme 3.
/
˚
The X-ray crystal structure (Fig. 2) shows that the
product is in fact the heteronuclear Au(I)/Hg(II) com-
plex [ClHg(m-6-MeC6H3-2-PPh2)AuCl] (2), which also
contains half a molecule of co-crystallised HgCl2. Table
3 lists important bond lengths and angles. A similar
/
˚
˚
A and Hgꢀ
/
/P
˚
and Au-Cl distances [2.233(4) and 2.280(4) A, respec-
tively] are identical (within experimental error) to those
in [AuCl(PPh3)] [32]. Complex 2 is formed by electro-

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23
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A.C. Willis, J. Am. Chem. Soc. 118 (1996) 10469.
philic cleavage of the s-aryl carbonꢂ
/
metal bond by
HgCl2, which is a well-established class of reaction.
[8] M.A. Bennett, S.K. Bhargava, K.D. Griffiths, G.B. Robertson,
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[9] S.K. Bhargava, F. Mohr, M.A. Bennett, L.L. Welling, A.C.
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4. Conclusions
[10] S.K. Bhargava, F. Mohr, M.A. Bennett, L.L. Welling, A.C.
Willis, Inorg. Chem. 40 (2001) 4271.
Serendipitously we have, for the first time, isolated
and structurally characterised an intermediate of a
cyclometallation reaction. Repeated attempts to ration-
ally prepare this complex have, so far, been unsuc-
cessful, with the proposed intermediate reacting
rapidly, giving the fully cycloaurated product. Mercuric
chloride is known to react with transition metal com-
plexes, giving a variety of possible structures. In this
case electrophilic cleavage of gold-carbon s-bonds
occurs and a heteronuclear Hg(II)/Au(I) complex is
formed.
[11] F. Mohr, Dissertation, RMIT University, Melbourne, 2001.
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Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 172010 and 172011. Copies of
this information may be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge, CB2
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