106
I.M. Serafimova, P.E. Hoggard / Inorganica Chimica Acta 338 (2002) 105Á110
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2. Experimental
AuCl(PPh3), HAuCl4×
/
3H2O, CHCl3, CCl4, and PPh3
Aldrich, and AuCl from
were obtained from SigmaÁ
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Pfaltz and Bauer. AuCl3(PPh3) was prepared by the Cl2
oxidation of AuCl(PPh3) [1]. PPh3Cl2 was prepared
from PPh3 and Cl2 [10]. [Au(PPh3)2]Cl was made from
AuCl(PPh3) and PPh3 in acetone [11]. Chloroform was
HPLC grade, stabilized with EtOH.
For kinetic runs, 3.0 ml samples were pipetted into 1.0
cm quartz cuvettes and irradiated at 254 or 313 nm with
a 100-W mercury lamp in an Oriel Q housing, the light
from which was passed through an Oriel 125 mm
monochromator. At intervals, the sample would be
removed from the beam and the electronic spectrum
recorded on a Hewlett-Packard 8453 diode array
spectrometer.
Fig. 1. Sequential spectra from the 254 nm irradiation of a 3.6ꢃ
M solution of AuCl(PPh3) in CHCl3.
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10ꢀ5
ꢀ
any case much more closely associated with the AuCl4
moiety.
Starting concentrations of AuCl(PPh3) and
AuCl3(PPh3) in CHCl3 were determined from extinction
The photochemical behavior stands in apparent
contrast to the thermal reaction in the presence of
triphenylphosphine, in the course of which AuCl4 is
coefficients, determined from Beer’s Law plots: o276
2.05 (90.02)ꢃ
103 Mꢀ1 cmꢀ1 (a shoulder) for
AuCl(PPh3) and o334 1.279 (90.004)ꢃ
104 Mꢀ1
ꢂ
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ꢀ
ꢂ
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cmꢀ1 (lmax) for AuCl3(PPh3). The absorbance of
CHCl3 at 254 nm was determined to be 0.81, relative
to water, in a 10 cm cell, and thus AS in Eqs. (3) and (4)
was taken to be 0.081. To obtain concentrations of the
reactant during initial rate experiments, the extinction
coefficient of the (collective) product was estimated
from the maximum absorbance change at the analytical
wavelengths upon extended photolysis. The initial rate
was then determined from the slope of a concentration
versus time graph over approximately the first 3% of the
reaction. For more extended runs, concentrations of
AuCl(PPh3), AuCl3(PPh3), HAuCl4, PPh3, and PPh3Cl2
during the course of the reaction were determined from
a multicomponent analysis over the wavelength range
converted to AuCl3(PPh3), and then reduced to
AuCl(PPh3) [1]. It is quite likely that both sequences
proceed simultaneously under irradiation; however, they
are not precisely the reverse of each other, and the net
conversion of PPh3 to PPh3Cl2 would limit the practical
reversibility of the photochemical and thermal pro-
cesses.
When AuCl3(PPh3) was photolyzed at 254 nm in
chloroform, conversion to HAuCl4 was the dominant
reaction, and HAuCl4 was the sole gold photoproduct
when irradiated to exhaustion. Analysis of intermediate
spectra revealed, however, that some AuCl(PPh3) was
also formed. This is consonant with thermal reduction,
Eq. (2), of AuCl3(PPh3), once PPh3 is made available by
conversion of some AuCl3(PPh3) to HAuCl4.
250Á350 nm. Results were checked to ensure that the
/
Initial rates of reaction under 254 nm irradiation of
both AuCl(PPh3) and AuCl3(PPh3) were measured with
solutions of constant concentration as a function of
incident light intensity. The graph for AuCl3(PPh3) was
linear, passing through the origin. This is expected for a
metal-initiated reaction, but a number of solvent-
initiated reactions also exhibit a linear dependence of
the rate on the incident light intensity. When a radical
chain is part of the process, a square root dependence on
intensity usually results, arising from the bimolecular
termination step [3,9]. When radicals are consumed
primarily by the other reactants rather than themselves,
the rate is linearly related to intensity [9]. The slope of
mass balance of gold and of PPh3 was properly
maintained. Light intensities were measured in triplicate
by ferrioxalate actinometry [12,13].
3. Results
Qualitatively, the 254 nm photolysis of AuCl(PPh3) in
chloroform is characterized by the growing in of a
product peak at 334 nm, easily recognizable as
AuCl3(PPh3), and a subsequent fall in absorbance in
that region, together with a shift of the peak to 324 nm.
This is illustrated in Fig. 1. The final product spectrum
the graph for AuCl3(PPh3), 20.291.4 M per einstein,
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corresponds to a quantum yield of 0.09, based on total
light absorbed.
For AuCl(PPh3) the initial rate was not a linear
function of either I0 or I01/2, although the plot against
the square root of intensity was more nearly linear.
matches extremely well the spectrum of HAuCl4×
/
3H2O
dissolved in CHCl3. The constitution of that species is
not at all clear. The solid material doubtless contains
hydronium ion, and is a strong acid [14]. In chloroform
the proton may reside on one of the ligands, and is in