Oxidative addition reactions of methyl substituted binuclear gold complexes studied by X-ray photoelectron spectroscopy

Article abstract of DOI:10.1016/S0022-328X(00)00228-X

X-ray photoelectron spectra of homovalent binuclear gold complexes with oxidation states of +I, +II, +III as well as heterovalent gold(I)-gold(III) species were obtained. The binding energies for the gold 4f peaks increase as the gold oxidation state incr

Full text of DOI:10.1016/S0022-328X(00)00228-X

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Journal of Organometallic Chemistry 607 (2000) 93–96
Oxidative addition reactions of methyl substituted binuclear gold
complexes studied by X-ray photoelectron spectroscopy
a,
a
b
S.K. Bhargava *, F. Mohr , J.D. Gorman
a
Department of Applied Chemistry, RMIT Uni6ersity, GPO Box 2476V, Melbourne, 3001 Vic., Australia
b
Department of Applied Physics, RMIT Uni6ersity, Melbourne, 3001 Vic., Australia
Received 3 March 2000; received in revised form 16 April 2000
This paper is dedicated to Professor Martin Bennett on the occasion of his retirement for a lifetime contribution to organometallic chemistry.
Abstract
X-ray photoelectron spectra of homovalent binuclear gold complexes with oxidation states of +I, +II, +III as well as
heterovalent gold(I)–gold(III) species were obtained. The binding energies for the gold 4f peaks increase as the gold oxidation
state increases. Residual gas analysis showed that the gold(I)–gold(III) heterovalent compound is photoreduced to a homovalent
gold(I) compound. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: XPS; Binuclear; Gold; Cyclometallated; Heterovalent; Photoreduction
1. Introduction
oxidative addition (OXAD) reactions forming a variety
of complexes with different oxidation states (Scheme 1).
The technique of X-ray photoelectron spectroscopy
(
XPS) has been widely used to determine the oxidation
state of metals in organometallic and inorganic com-
pounds [1]. The main advantage of XPS over other
spectroscopy techniques like NMR and MS, is that
solid samples are used, making it ideal for analysis of
organometallic compounds that isomerise or decom-
pose in solution, or are too insoluble for solution
techniques. Examples include chromium, tungsten and
molybdenum nitrosyl complexes [2], Group IV and V
phenyl compounds [3] and a variety of gold and plat-
inum complexes [4]. The most common oxidation states
of gold are +I and +III, however in recent times,
gold oxidation states of −I, +II, +IV and +V have
been reported [5]. Due to poor solubility and the non-
crystalline nature of many gold compounds, XPS has
been the easiest and often only means to determine the
oxidation state of this metal.
2
. Results and discussion
Oxidative addition of Cl to colourless 1 at −70°
2
affords the bright yellow gold(II) compound 2. This
compound spontaneously isomerises to the colourless
heterovalent compound 3 above about −20°. Addition
of a further equivalent of Cl to 3 affords a yellow
2
solution of which yellow 4 precipitates out on standing.
All compounds have been fully characterised [6]. The
binding energies of the Au 4f7/2 P 2p and Cl 2p core
levels and full width at half maximum values (FWHM)
for the four complexes are shown in Table 1.
It can clearly be seen that the Au BE values increase
in magnitude as the formal oxidation state of gold
increases from +I to +III. The Au 4f BE values are
slightly higher than those reported for binuclear ylide
complexes of gold(I), gold(II) and gold(III) [7]. More-
over, the BE values are ca. 1 eV lower than those of the
analogous binuclear gold compounds without the
methyl substituent [8]. The gold 4f7/2 BE value increases
by ca. 1 eV as the oxidation state increases from 0 to
This work illustrates the use of XPS in determining
the oxidation states of gold in a sequence of chlorine
*
Corresponding author. Tel.: +61-3-9925-3365; fax: +61-3-9639-
321.
E-mail address: suresh.bhargava@rmit.edu.au (S.K. Bhargava).
1
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00228-X

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S.K. Bharga6a et al. / Journal of Organometallic Chemistry 607 (2000) 93–96
Scheme 1.
+
I and +II. The increase in BE value, as the oxida-
tion state is increased from +II to +III is only about
.5 eV. The change in BE values is however sufficient to
one gold oxidation state in 2, thus confirm the presence
of a gold(II)–gold(II) centre. The BE values are in
good agreement with those reported for non-methy-
lated analogues [8].
0
allow identification of the oxidation state of gold in the
system with some confidence. The BE values for P 2p
and Cl 2p do not change significantly as the gold
oxidation state is increased, illustrating that the gold
centre undergoes greater changes both in terms of
co-ordination number and co-ordination geometry,
which changes from linear gold(I) to square planar
gold(III) as the oxidation state is increased. The only
exception is the P 2p BE value for 4, which is 0.4 eV
higher that in the other three complexes. This suggests
that the four chlorine atoms remove some electron
density from phosphorus through the gold atoms.
Gold(II) compounds used to be considered rare but,
with recent work on binuclear compounds, gold(II) is
now considered a fairly common oxidation state of the
metal [9]. Early work suggesting the presence of
gold(II) often proved to be heterovalent +I and +III
states which gave an average oxidation state of +II.
The study of non-methylated cyclometallated gold(II)
Heterovalent binuclear gold compounds are relatively
rare, especially those containing AuꢀPꢀC rings like
compound 3. A similar compound to 3 is formed
during the OXAD reaction with nitromethane, to give a
binuclear gold(II) ylide compound [11], however no
XPS measurements have been reported on this com-
pound. The Au 4f core level spectrum of 3 (Fig. 1)
shows considerable broadening of the Au 4f doublet.
Curve fitting of the data results in two distinct peaks,
whose BE values indicate the presence of both gold(I)
197
and gold(III). Au M o¨ ssbauer spectroscopy of 3 has
confirmed the presence of both gold(I) and gold(II)
[12]. During our XPS measurements it was observed
that the gold 4f peak slowly changed over time from the
mixed gold(I), gold(III) state to a predominantly
gold(I) state. Fig. 2 shows the change occurring in the
Au 4f doublet over a period of 40 min. At the same
time an increase in the analysis chamber pressure was
observed. This suggested photo stimulated reduction of
the compound. Such photoreduction occurring during
XPS measurements have been observed in the study of
1
97
analogues of 2 by
shown that each gold atom is present in an identical d
state [10]. Our XPS results confirm the presence of only
Au M o¨ ssbauer spectroscopy has
9
Table 1
BE and FWHM values (eV) for complexes 1–4 (all data 90.05)
Compound
Au oxidation state
Au 4f7/2
FWHM
P 2p
Cl 2p
Au metal
0
84.0
1.1
1
2
3
4
+I
+II
+I, +III
+III
85.1
86.5
85.2, 86.7
86.9
1.7
1.9
1.7, 1.9
1.9
132.1
132.1
132.1
132.5
198.7
198.9
198.7

S.K. Bharga6a et al. / Journal of Organometallic Chemistry 607 (2000) 93–96
95
in the solid state are consistent with reduction experi-
ments carried out with 3 in solution. Observation of
photoreduction and identifying chamber gas compo-
nents can provide additional structural information,
akin to observing both molecular ion and fragment
peaks in mass spectroscopy. Refluxing 3 in a high
boiling solvent for a prolonged period of time causes
reduction to 1 as well as the formation of several other
products. However treating 3 with reducing agents such
as Cu or Zn powder only leads to the precipitation of
elemental gold. Photoreduction is a common occur-
rence in gold compounds, especially gold(I) compounds
such as Au(CO)Cl which are easily photoreduced to
metallic gold in light at room temperature [14].
Fig. 1. Au 4f7/2 core level spectra of complex 3 showing photoreduc-
tion over time.
3
. Conclusions
In this work we have shown that XPS is an impor-
tant additional technique in structure determination of
gold compounds, particularly for compounds that are
insoluble or unstable in solution and hence cannot be
fully characterised by other spectroscopic techniques.
The change in BE values of the Au 4f doublets can be
used to identify gold oxidation states of +I, +II,
+
III in both homo- and heterovalent systems. The BE
values of co-ordinated ligands are not significantly af-
fected by changes in the gold oxidation state. Photore-
duction by the X-ray beam combined with residual gas
analysis by mass spectroscopy can be utilised as a
further tool in the structural elucidation of gold species.
Fig. 2. Mass spectrum of residual chamber gas analysed during
irradiation of 3.
some platinum compounds [13]. This photoreduction of
4. Experimental
3
could be accelerated by irradiating the sample with a
1
0 keV electron beam for a short period of time.
The gold compounds 1–4 were prepared according
to a modified literature procedure [15]. XPS was per-
formed on a VG Microlab 310F with an Al anode
unmonochromated X-ray source operated at a power of
300 W and 15 kV excitation voltage.
Examination of the gold 4f doublet during irradiation
showed that after only 60 s of exposure the peak curve
fitted to give 54% gold(I) and after ca. 10 min of
irradiation this had further reduced with ca. 66% being
present as gold(I). Mass spectra of the residual chamber
gas were recorded for compound 3, both before and
during X-ray irradiation. Fig. 2 shows the mass spec-
trum of the residual chamber gas obtained after
irradiation.
The majority of the peaks can be assigned to decom-
position products of the organic moieties of the com-
pound, including various alkyl and aryl fragments. Due
to the limited mass range of the instruments no phos-
phine fragments could be observed. The two major
peaks at 35 and 37 Da with the isotopic ratio of 3:1 can
be assigned to loss of chlorine. This seems to indicate
that the radiation causes reduction to a gold(I) species
accompanied by the loss of chlorine and decomposition
of the compound. These observations of reduction of 3
The energy of the Al–K line was taken to be 1486.6
a
eV. The sample was tilted such that escaping electrons
were collected by the electron analyser normal to the
sample surface unless otherwise stated. The circular
area of the sample from which escaping electrons were
detected was ca. 2 mm in diameter. All survey spectra
were performed in Constant Analyser Energy (CAE)
mode at a pass energy of 100 eV at 1 eV steps between
data points. All single element scans were performed in
CAE mode at a pass energy of 20 and 0.1 eV steps. The
spectrometer was calibrated with a sputtered copper
(99.999%) sample and deposited gold on silicon sample
giving Cu 2p , Cu (KLL), Au 4f binding energies of
3/2
7/2
932.60, 334.8 eV (KE) and 84.00 eV, respectively. All
samples for analysis in the Microlab were mounted on

96
S.K. Bharga6a et al. / Journal of Organometallic Chemistry 607 (2000) 93–96
Battistoni, G. Mattogno, D.M.P. Mingos, J. Electron Spectrosc.
Relat. Phenom. 49 (1989) 273. (c) P.M.Th.M. Van Attekum,
J.W.A Van Der Velden, J.M. Trooster, Inorg. Chem. 19 (1990)
aluminium stubs using double sided adhesive conduct-
ing carbon tape. The base pressure of the analysis
−
11
chamber was 6×10
torr before samples were intro-
7
01. (d) H. Kitagawa, N. Kojima, T Nakajima, J. Chem. Soc.
duced. Charge correction was with respect to the low
binding energy C 1s component taken to be 285.0 eV.
The Au metal standard was prepared by vacuum de-
positing Au onto an aluminium stub. The four Au
complexes (1–4) were analysed as powders on double
sided carbon tape. Residual Gas Analysis was per-
formed using a Dycor Quadrupole Gas Analyser Model
M100 with a resolution of 0.5 amu at 10% height over
a range from 1 to 100 Da.
Dalton Trans. (1991) 3121. (f) Y.M. Shul’ga, N.Y. Shul’ga, A.
Graja, Solid State Commun. 8 (1998) 491.
5] H. Schmidbaur, Gold, Progress in Chemistry, Biochemistry and
Technology, Wiley, New York, 1999.
6] The synthesis and full characterisation of compounds 1–4 will be
published separately.
[
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Vondel, G.P. Van der Kelen, J. Chem. Soc. Chem. Commun.
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7
[
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9] A. Laguna, M. Laguna, Coord. Chem. Rev. (1999) 837.
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