5536 Inorganic Chemistry, Vol. 49, No. 12, 2010
Bojan et al.
with a plausible bonding explanation becomes an interesting
challenge. Therefore, taking all the experimental structural
information into account we have carried out ONIOM43
DFT/UFF44 calculations as implemented in Gaussian0345 to
gain insight into the bonding nature of SbR3 ligands in
homoleptic gold-stibine adducts. These were also compared
with phosphine-gold(I) compounds. We have carried out
theoreticalcalculations on modelsystems of the type ER3 and
[Au(ER3)n]þ (E = P or Sb; R = Ph or Mes; n = 2, 3, or 4)
(Figure 6). By making use of the ONIOM approach we have
fully optimized all molecular model systems at the DFT/UFF
level of theory which accounts for complete molecules,
including all the ligand atoms. After optimization, all model
systems were recalculated at the DFT/B3LYP level of theory
to have a single point electronic structure at the optimized
geometry. A summary of the already experimentally reported
X-ray diffraction results, structural parameters, and the
theoretical results obtained are given in Tables 5 and 6,
respectively. From these results we can infer several interest-
ing conclusions.
We first analyzed the influence of the stibine aryl groups
(phenyl vs mesityl) on the coordination with Au(I) centers
by NBO53 analysis and BSSE-corrected (using the counter-
poise correction)54 Au-Sb dissociation energy calculations
of two-, three-, and four-coordinated [Au(SbR3)n]þ (R = Ph
or Mes; n = 2, 3, and 4) cationic units. If we focus on the
bonding abilities of SbPh3 and SbMes3 ligands toward gold
we observe several interesting results that are comparable to
the obtained experimental results. First, the fully optimized
free ligands show similar structural parameters to those
observed in the X-ray diffraction studies of SbPh3 and
SbMes3, respectively. It is worth mentioning that the calcu-
lated C-Sb-C angles are very close to the experimental
Table 5. Selected Experimental X-ray Diffraction Structural Parameters for Free
Ligands and for Homoleptic Stibine- and Phosphine-Gold(I) Complexes
[Au(ER3)n]þ (E = P or Sb; R = Ph or Mes; n = 2, 3, or 4)
Au-E
E-C
C-E-C
ref
SbPh3
2.14-2.15 95.6-98.0
2.15-2.16 97.8-100.9
2.15-2.18 97.0-106.8
2.18-2.18 103.6-105.9 35
1.83-1.83 101.9-102.5 46
1.83-1.84 103.6-105.9 47
36
this work
this work
SbPh2Mes
SbPhMes2
SbMes3
PPh3
PMes3
[Au(SbPh3)4]þ
2.65-2.66 2.10-2.14 99.1-102.1
2.12-2.14 98.7-110.6
32
this work
[Au(SbPh2Mes)3]þ 2.62
[Au(SbMes3)2]þ
[Au(PPh3)4]þ
[Au(PPh3)3]þ
[Au(PPh3)2]þ
[Au(PMes3)2]þ
2.58
2.35-2.53 1.69-1.76 88.3-114.6
2.14-2.15 103.8-115.0 this work
48, 49
2.37-2.40 1.80-1.83 102.9-106.5 50
2.31-2.31 1.78-1.81 104.6-107.5 51
2.35
1.82-1.83 111.5-113.3 52
values, with the expected larger angle for the mesityl ligand
(105.1° theoretical; 105.0° experimental) than for the phenyl
ligand (97.2° theoretical; 96.0° experimental).
Second, we analyzed the different coordination abilities of
SbPh3 and SbMes3 toward gold in homoleptic complexes
by using theoretical models of the type [Au(SbR3)n]þ (R =
PhorMes; n = 2, 3, and4) (Figure 6). Inthe case of theSbPh3
ligand, the three possible coordination environments, linear,
trigonal planar, and tetrahedral, each reach a local minimum
with Au-Sb dissociation energies of 53.1, 31.81, and 14.16
kcal mol-1, respectively, and with Au-Sb distances within
3
the range of coordination bonds (see Tables 5 and 6). In
contrast, when the aryl substituent of the stibine ligand is
mesityl, only the homoleptic model with two stibine ligands
in a linear coordination geometry, [Au(SbMes3)2]þ, converges
to a local minimum showing an Au-Sb dissociation energy
of 48.3 kcal mol-1, close to that observed with triphenyl-
3
stibine. WhenthreeorfourSbMes3 ligandsare placed around
gold in the model system, the optimization run does not
converge and tends to dissociate one or two stibine ligands,
respectively, leading to [Au(SbMes3)2]þ fragments. These re-
sults agree with the previously reported experimental data
on SbPh3 ligands and with those reported in this work for
bulkier SbMesPh2, SbMes2Ph, and SbMes3 ligands. Thus,
while in the case of SbPh3 it is possible to achieve tetrahedral
coordination, the coordination number is reduced when
mesityl substituents are included in the stibine. In the case
of the complex [Au(SbMesPh2)3]ClO4 (11), three stibine
ligands in a trigonal planar coordination environment are
observed, whereas for complexes [Au(SbMes2Ph)2]ClO4 (10)
or [Au(SbMes3)2]ClO4 (7) two stibines in a linear coordina-
tion around the gold atom are found. If the electronic
properties for the model systems [Au(SbR3)2]þ (R = Ph or
Mes) are compared, both the NBO charges on gold and anti-
mony, and the natural electron configurations (see Table 6
and Supporting Information) are quite similar for each
molecule. Therefore, it seems that the coordination environ-
ment is governed by steric effects rather than electronic ones.
Another interesting feature isthe different bonding of PPh3
and SbPh3 ligands to Au(I) in homoleptic complexes. We
have analyzed this theoretically by using model systems of the
type [Au(EPh3)n]þ (E = P or Sb; n = 2, 3, and 4). With these
calculations we try to reproduce the observed experimental
tendency inwhichthenumberof PPh3 ligands bondedto gold
is usually 2 (linear) or 3 (trigonal planar) and, to a lesser
extent, 4, whereas in the case of SbPh3 only one type of
coordination environment consisting of 4 SbPh3 ligands in a
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