Organometallic gold complexes of carborane. Theoretical comparative analysis of ortho, meta, and para derivatives and luminescence studies

Article abstract of DOI:10.1039/b820803d

The synthesis and X-ray analysis of complexes [(μ-1,12-C ₂B₁₀H₁₀){Au(PPh₃)}₂] and [(μ-1,2-C₂B₁₀H₁₀){Au(PMe₃)} ₂] have provided the experimental d

Full text of DOI:10.1039/b820803d

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www.rsc.org/dalton | Dalton Transactions
Organometallic gold complexes of carborane. Theoretical comparative
analysis of ortho, meta, and para derivatives and luminescence studies†
Olga Crespo,a M. Concepcio´n Gimeno,a Antonio Laguna,*a Isaura Ospino,a Gabriel Aullo´nb and
Josep M. Olivac
Received 21st November 2008, Accepted 18th February 2009
First published as an Advance Article on the web 26th March 2009
DOI: 10.1039/b820803d
The synthesis and X-ray analysis of complexes [(m-1,12-C2B10H10){Au(PPh3)}2] and [(m-1,2-C2B10H10)-
{Au(PMe3)}2] have provided the experimental data needed to analyse two points. The first point is the
use of these data to carry out a computational study with the aim of comparing the electronic structures
and relative stabilities of the organometallic isomers [(m-1,n-C2B10H10){Au(PR3)}2] (n = 2, 7, 12; R =
Ph, Me) with those of the parent carborane clusters ortho-, meta- and para-carborane and the influence
of the monophosphine substituents. The second point is focused in the influence of the steric demand of
the monophosphine in the presence or not of aurophilic interactions in the ortho derivatives
[(m-1,2-C2B10H10){Au(PR3)}2] (R = Me, Ph). The photoluminescent behaviour of both the carboranes
and the organometallic complexes is presented.
electronic transmission via the para-carborane cage has been
described. Thus it has been used as a building block in calamitic
molecules in which the effect of the carborane rings on the
Introduction
Closo-carboranes are rigid three-dimensional aromatic electron
withdrawing species in the sequence ortho > meta > para.1–4
They exhibit great chemical, electrochemical and thermal sta-
bilities which make them attractive as building blocks for the
synthesis of a great variety of species. Upon deprotonation of the
carbon atoms highly nucleophilic carbanions are afforded. Thus,
organogold complexes with s Au–C bonds have been reported5
for ortho- and meta-carborane with stoichiometries [(m-1,n-
C2B10H10){Au(PR3)}2] [n = 2 or 7], [(1-R-2-C2B10H10)(AuL)] [R =
C5NH4; L = PPh3. R = MeOCH2, Ph, SitBuMe2; L = PR3, AsPh3.
R = Me; L = AsPh3], PPN[Au(1-SitBuMe2-2-C2B10H10)2] and
[(1,2-C2B10H11){Au(PPh3)}], the crystal structures of some of them
have been characterized, as well as that of the bis-ortho-carborane
compound [{m-(H10B10C2-C2B10H10)}{Au(PPh3)}2]. These studies
reveal that the latter displays an aurophillic interaction be-
tween the two gold centres, whereas in the ortho-derivative [{m-
(C2B10H10){Au(PPh3)}2] the gold ◊ ◊ ◊ gold distance is too long to be
considered as an aurophilic interaction.
mesogenic properties has been analysed.6,7 As a consequence
the synthesis and study of para-carborane complexes, which
are almost unexplored compared with the widely studied ortho-
carborane coordination compounds, has increased. The electronic
transmission via the para-carborane cage has been described in the
cobalt systems8 [{Co2C2(SiMe3)(CO)4(dppm)}2(1,12-C2B10H10)]
5
and in luminescent iron complexes which contain the unit [1-{h -
CpFe(CO)2}-1,12-C2B10H10}]-. Few organometallic complexes of
para-carborane have been structurally characterized which in-
5
clude the dinuclear [1,12-{Fe(h -Cp)(CO)2}2(1,12-C2B10H10)] and
5
mononuclear [1-{Fe(h -Cp)(CO)(PPh3)}-(1,12-C2B10H11)] iron
complexes,9,10 the mercury derivatives [Hg(1-Ph-12-C2B10H10)2],
[HgCl(1-Ph-12-C2B10H10)(2,2¢-bipyridyl)] and the result of the
reaction of the former of these mercury complexes with 2,2¢-
bypiridyl and 2,2¢-bypirimidyl.11 No organometallic gold deriva-
tives of the para isomer have been described as far as we know
and complexes with C-functionalized carborane ligands from
this isomer are scarcely represented. These include gold deriva-
It is noticeable the lack of similar organogold complexes derived
from para-carborane. Nevertheless, the orientation of the carbon
atoms in para-carborane has attracted the attention of chemists.
Para-carborane allows the synthesis of rod-like molecules and
∫
tives with the 1,12-(HC C)2-1,12-C2B10H10, and 1,12-(PPh2)2-1,12-
C2B10H10 ligands.12,13 Among them, only the crystal structures
∫
of [{m-1,12-(C C)2-1,12-C2B10H10}{Au{P(4-OMeC6H4)3}}2] and
∫
PPN[Au{1,12-(C CH)2-1,12-C2B10H10}2] have been reported.
Our aim in this work is to carry out a theoretical com-
parative analysis of organogold complexes of the ortho-, meta,
and para isomers. With this purpose we have synthesised and
structurally characterized the first organometallic gold derivative
of para-carborane whose data also allows comparison between
the organogold [(m-1,n-C2B10H10){Au(PPh3)}2] (n = 2, 7, 12)
and the corresponding carborane isomers. The dinuclear [(m-1,2-
C2B10H10){Au(PMe3)}2] has also been synthesised and its crystal
structure elucidated in order to compare the modification of the
stability of the carborane gold complexes with the monophosphine
substituents and the influence of the steric demands of the
aDepartamento de Qu´ımica Inorga´nica, Instituto de Ciencia de materiales de
Arago´n, Universidad de Zaragoza-CSIC, 50009, Zaragoza, Spain
bDepartament de Qu´ımica Inorga`nica and Institut de Qu´ımica Teo`rica i
Computacional (IQTCUB), Universitat de Barcelona, Diagonal 647, 08028,
Barcelona, Spain
cInstituto de Qu´ımica-F´ısica Rocasolano, Consejo Superior de Investiga-
ciones Cient`ıficas, Serrano, 119, E-28006, Madrid, Spain
† Electronic supplementary information (ESI) available: optimized geome-
tries of the gold complexes [(m-1, n-C2B10H10){Au(PPh3)}2], with n = {2, 7,
12} and the frontier orbitals in complexes [(m-1, n-C2B10H10){Au(PPh3)}2],
with n = {2, 7}. CCDC reference numbers 710441 & 710442. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b820803d
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Table 1 Details of data collection and structure refinement for 1 and 2
monophosphine in the presence or not of aurophilic interactions
in the ortho-derivatives. Photoluminescent steady-state studies
have been carried out both for the carborane isomers and the
organometallic complexes.
Compound
1
2
Formula
Mr
Habit
C38H40Au2B10P2
1060.67
colourless plate
0.33 ¥ 0.10 ¥ 0.06
monoclinic
P21/c
C8H28Au2B10P2
688.28
colourless prism
0.16 ¥ 0.15 ¥ 0.09
monoclinic
C2/c
Experimental and computational methodologies
Crystal size (mm)
Crystal system
Space group
Cell constants:
Instrumentation. C, H, N and S analyses were carried out
with a Perkin-Elmer 2400 microanalyzer. NMR spectra were
recorded on Bruker ARX 400 spectrometers in CDCl3. Chemical
shifts are cited relative to SiMe4 (1H, external), and 85% H3PO4
(31P, external). Steady-state photoluminescence spectra were
recorded with a Jobin-Yvon Horiba fluorolog FL-3-11 spectrome-
ter using band pathways of 3 nm for both excitation and emission.
The solid samples were prepared by mixing the compounds with
silica gel.
˚
a (A)
8.1035(16)
11.757(2)
20.581(4)
97.18(3)
1945.4(7)
2
1.811
7.644
1012
16.807(3)
13.648(3)
18.454(4))
102.73(3)
4128.7(14)
8
2.215
14.338
2512
˚
b (A)
˚
c (A)
b (◦)
3
˚
V (A )
Z
Dx (Mg m-3)
m (mm-1)
F(000)
T (◦C)
-100(2)
52.0
-100(2)
52.0
Starting materials. The starting materials [AuCl(PPh3)]14 and
[AuCl(PMe3)]15 were prepared according to published procedures.
All other reagents are commercially available. Reaction solvents
were reagent grade and were distilled from the appropriate drying
agents under argon prior to use.
2qmax
No. of refl.:
measured
independent
Transmissions
Rint
28940
3803
0.66-0.19
0.031
235
61066
4049
0.36-0.21
0.028
205
Parameters
Restraints
wR(F2, all Refl.)
R(F, >4s(F))
S
Synthesis of [(l-1,12-C2B10H10){Au(PPh3)}2] (1)
9
0
0.058
0.023
1.16
0.042
0.020
1.14
To a solution of para-carborane (28.8 mg, 0.2 mmol) in diethyl
ether at 0 ◦C, under an argon atmosphere, nBuLi (hexane solution
1,6 M, 0.38 mL, 0.6 mmol) was added and the mixture was stirred
for one hour. The solution was warmed to room temperature and
[AuCl(PPh3)] (172 mg, 0.35 mmol) was added. The suspension
was stirred for 90 min and the remaining solid filtered over Celite.
The filtrate was evaporated (ca. 5 mL). Addition of n-hexane
(ca. 10 mL) afforded a white solid. Yield 137.4 mg (74%). Analyti-
cal data, Found: C 43.15, H 3.75; Calculated for C38H40Au2B10P2:
-3
˚
max. Dr (e A )
1.35
1.02
fibers and transferred to the cold gas stream of the diffractometer.
Data were collected using monochromated Mo Ka radiation (l =
0.71073) in w scans. An absorption correction with the program
SADABS16 based on multiple scans was used. The structures were
solved by direct methods (1) or the heavy atom method (2) and
refined on F2 using the program SHELXL-97.17 All of the non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were included using a riding model. Details of data collection
and refinement are shown in Table 1. The residual electron density
peaks are near one of the gold atoms in both cases.
1
C 43.05, H 3.70. 31P { H} NMR: 38.0 ppm (s); 1H NMR: 1–3 ppm
(m, br, 10H, BH); 7.3-7.6 (m, br, 30H, Ph).
Synthesis of [(l-1,2-C2B10H10){Au(PMe3)}2] (2)
In a similar procedure to that described for 1 nBuLi (hexane
solution 1,6 M, 0.38 mL, 0.6 mmol) was added to a solution
of ortho-carborane (28.8 mg, 0.2 mmol) in tetrahydrofuran at
Computational method
0
◦C under an argon atmosphere and the mixture was stirred
All density functional calculations were performed with the suite
of programs Gaussian0318 with the Hartree–Fock/DFT hybrid
functional B3LYP method.19 Effective core pseudopotentials and
their associated double-z basis sets, known as LANL2DZ, were
used for Au.20 The basis set 6–31G* was used for all other atoms.21
As regards to geometry optimizations, we started off with the
geometries from the crystal structures22,23 and reached energy
minima for the ortho-, meta- and para-carborane complexes [(m-
1,n-C2B10H10){Au(PPh3)}2], with n = 2, 7, and 12 respectively.
All geometries correspond to energy minima characterized by
vibrational analysis.
for 2.5 h. The solution was warmed to room temperature and
[AuCl(PMe3)] (107.8 mg, 0.35 mmol) was added. The suspension
was stirred for two days and the remaining solid filtered over Celite.
The filtrate was evaporated (ca. 5 mL). Addition of n-hexane (ca.
10 mL) afforded 54 mg of a yellow–orange solid which contains a
mixture of three different compounds from which 2 represents ª
64% (based on integration of 1H NMR resonances). Spectroscopic
1
1
data for 2: 31P { H} NMR: 0.0 ppm (s); H NMR: 1–3 ppm (m,
br, BH), 1.48 ppm [J(PH) = 10 Hz, Me]. Other NMR resonances:
31P{ H}–1.4 (s); 1H 1.73 [t, J(PH) = 4 Hz)], (ª 18%) and 31P{ H}
1
1
4.3 ppm (s); 1H 1.58 [d, J(PH = 10 Hz)] (ª 18%).
Results and discussion
X-Ray studies
Preparation and crystal sctructure of [(l-1,12-C2B10H10)-
{Au(PPh3)}2] (1) and [(l-1,2-C2B10H10){Au(PMe3)}2] (2)
Crystals of complexes 1 and 2 were obtained by addition of
n-hexane to a solution of the compounds in dichloromethane (1) or
CDCl3 (2). Data were recorded on a Xcalibur Oxford Diffraction
diffractometer. The crystals were mounted in an inert oil on glass
Reaction of Li2(1,12-C2B10H10) with [AuCl(PPh3)] in Et2O in molar
ratio 1:2 affords the dinuclear compound [(m-1,12-C2B10H10)-
3808 | Dalton Trans., 2009, 3807–3813
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{Au(PPh3)}2] (1). Complex [(m-1,2-C2B10H10){Au(PMe3)}2] (2)
may be prepared from [Li2(1,2-C2B10H10)] and [AuCl(PMe3)] in
Et2O or THF as solvents. Reaction times are longer than for the
para-carborane derivative (1). Compound 2 is not isolated as a
pure compound, it represents the majority product of a mixture of
three species. 1H NMR studies of the crystals of 2 (see below) has
lead to spectral identification of the complex (see Experimental).
1
In the 31P{ H} NMR spectrum of 1 the two equivalent phosphorus
appear as a broad signal at d = 38 ppm, at a very similar field to
that found for [(m-1,n-C2B10H10){Au(PPh3)}2] [d = 38.5 (n = 2)
or 40.5 (n = 7)]. The resonance corresponding to compound
2 is observed at much higher field (0.0 ppm). The 1H NMR
spectra of 1 and 2 show a very broad resonance corresponding
to the BH hydrogen atoms between 1 and 3 ppm, one multiplet
corresponding to the phenyl hydrogen atoms between 7 and 8 ppm
for 1 and that corresponding to the presence of the methyl groups
at 1.48 ppm in 2. No signal corresponding to the presence of
unsubstituted carborane is found in these spectra.
Fig. 2 Molecular structure of [(m-1,2-C2B10H10){Au(PMe3)}2] (2). Hydro-
gen atoms have been omitted for clarity. Ellipsoids are drawn at the 50%
probability level.
C–Au–P angles of 175.75(10)◦ (1) which are in between those
found in the ortho [178.9(4)◦, 174.2(4)◦] and meta [179.3(2)◦,
174.1(2)◦] complexes with PPh3 as monophosphine, and are similar
to that found in 2 [175.87(11)◦, 175.76(10)◦]. Au–P and Au–C
distances found in 1 and 2 compare well with those found in
the analogous complexes with the ortho-, or meta-carborane
isomers [(m-1,n-C2B10H10){Au(PPh3)}2] (n = 2, 7; see Table 2).
Interestingly, the steric effects do not govern the presence of
aurophilic interactions which are absent in both 2 and in [(m-
1,2-C2B10H10){Au(PPh3)}2] despite the difference of the phosphine
substituents.
The structures of 1 and 2 (Figs. 1 and 2) show the expected
icosahedral cage for the C2B10 unit having a linear coordination
for gold atoms that display a slightly distorted geometry with
Luminescence studies
Steady-state photoluminescence studies of the three isomers of
carborane as well as the ortho and para derivatives have been
carried out. The three carborane isomers are weakly luminescent
in the solid state, both at room temperature and 77 K (table 3).
Fig. 1 Molecular structure of [(m-1,12-C2B10H10){Au(PPh3)}2] (1). Hy-
drogen atoms have been omitted for clarity. Ellipsoids are drawn at the
50% probability level.
Table 2 Bond distances in the different isomers [(m-1,n-C2B10H10){Au(PPh3)}2]: ortho-, meta- and para- [n = 2, 7, and 12 (1)] and [(m-1,2-
C2B10H10){Au(PMe3)}2] (2)
˚
Experimental values (A)
Distance
Orthoa with PPh3
Ortho with PMe3
Metab with PPh3
Para with PPh3
C–Cc
C–Au
1.71(2)
1.693(5)
2.049(4)
2.068(4)
2.2623(12)
2.2731(10)
3.5223(7)
—
2.058(4)
2.055(14)
2.033(15)
2.270(4)
2.273(5)
3.567(1)
2.054(7)
2.047(7)
2.265(2)
2.271(2)
6.14
Au–P
2.2738(10)
—
Au ◊ ◊ ◊ Au
˚
Computed values (A)
PPh3
PMe3
Distance
ortho
meta
para
ortho
meta
para
C ◊ ◊ ◊ C
C–Au
Au–P
1.687
2.077
2.378
3.762
—
—
1.686
2.076
2.366
3.799
—
—
2.078
2.380
—
2.078
2.384
—
2.078
2.368
—
2.079
2.368
—
Au ◊ ◊ ◊ Au
a Data from reference 22. b Data from reference 23. c For carborane carbon atoms.
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Table 3 Emission maxima (nm) for the three carborane isomers and their
organogold complexes in the solid state
Table 4 Computed relative stabilities (in kcal/mol) for compounds [(1,n-
C2B10H10)Z2] and (n = 2, 7, 12), with Z = H, AuPPh3 or AuPMe3. Zero
energies are taken arbitrarily to each para-carborane isomer
Compound
Room T
77 K
Compound
ortho
meta
para
Ortho-carborane
356
380
345
368, 500
391, 500
394, 527
367, 506
354
390
320
399, 500
403, 516
396, 513
389, 555
Meta-carborane
[(1,n-C2B10H12)]
+19.7
+11.7
+13.6
+3.4
+0.4
+0.5
0.0
0.0
0.0
Para-carborane
[(1,n-C2B10H10){Au(PPh3)}2]
[(m-1,2-C2B10H10){Au(PPh3)}2]
[(m-1,2-C2B10H10){Au(PMe3)} ]
[(m-1,7-C2B10H10){Au(PPh3)}22]
[(m-1,12-C2B10H10){Au(PPh3)}2]
[(1,n-C2B10H10){Au(PMe3)}2]
They display emissions between 345 and 390 nm (table 3). The
corresponding excitation maxima appear between 285 and 310 nm.
Luminescence studies of ortho-carborane have been reported24
which for this isomer describe a weak emission at 395 nm at room
temperature as well as a structured emission at 410 nm both in
the solid state at 77 K and in ethanol frozen solutions. We have
observed emission near 390 nm for the three carborane isomers.
The band at lower energies has not been observed by us, although
we have seen a very weak shoulder (whose tail goes on almost until
500 nm) in the band described above upon excitation near 260 nm.
Upon excitation at lower energies the intensity of the shoulder
decreases and even or almost disappears and the emission near
390 nm is always present.
Fig. 4 Main parameters for the optimized geometry of complex
[(m-1,12-C2B10H10){Au(PPh3)}2]. Ci symmetry point-group.
The organometallic derivatives display two emission bands
(Table 3) whose maxima appear at about 380 and 520 nm. Emission
at higher energies resembles that of the unsubstituted carboranes
and we propose for this band an intraligand (IL-carborane)
origin. The relative intensity of both emissions depends on the
excitation wavelength. The high energy band always displays very
low intensity and even disappears. The spectra shown in Fig. 3 have
been recorded with an excitation wavelength of 300 nm. Metal to
ligand charge transitions (MLCT) could be responsible for these
emissions near 520 nm.
imental and computed distances show good agreement. Similar
results have been obtained for the ortho- and meta-complexes
[(m-1,n-C2B10H10){Au(PPh3)}2] (n = 2, 7). The ESI† contains the
atomic coordinates for the optimized geometries.
The relative stability of the three isomers for three families
of complexes: (i) the unsubstituted carboranes, (ii) disubstituted
with two “AuPPh3” fragments and (iii) disubstituted with two
“AuPMe3” fragments, have been analysed and are presented in
Table 4. Among each family of complexes the relative stability
shows the same order: para ≥ meta > ortho. These studies are
in agreement with the previously reported theoretical studies for
the carborane isomers [1b,c]. The organometallic gold complexes
show in all cases higher stabilities than the corresponding parent
carboranes. This stabilization may arise from the fact that the
gold–phosphine fragment can provide more electron density to
the carborane carbon atoms, compared with hydrogen. A different
analysis can be carried out in order to compare those species
which contain the same carborane isomer. These data suggest
that coordination of the “Au(PPh3)” fragment leads to more
stable complexes than the coordination of “Au(PMe3)” with higher
differences found for the ortho isomers. The electronic properties of
PPh3, compared with PMe3 (balance between s-donor/p-acceptor
character) could be responsible for these differences. Furthermore,
the crystallographic data for the ortho-carborane gold complexes
show the absence of aurophilic interactions that could contribute
to the stability of these compounds. Consequently, steric factors do
not play a great influence, either on the stability of the compounds
or in the presence of gold ◊ ◊ ◊ gold contacts.
Fig. 3 Emission spectra of the organogold complexes [(m-1,n-C2B10H10)-
{Au(PR3)}2] (n = 2 (R = Ph, Me), 7 (R = Ph), 12 (R = Ph). Excitation
wavelength 310 nm.
Fig. 5 displays some frontier orbitals for the para complex
[(m-1,12-C2B10H10){Au(PPh3)}2], whereas the ESI† contains the
plots for the respective frontier orbitals of the ortho- and meta-
complexes. The four higher occupied molecular orbitals are
responsible for Au–P and Au–C bonds: two of the latter are
displayed in Fig. 5a and Fig. 5b, corresponding to symmetric and
antisymmetric combinations of their localized bonds. We should
Computational studies of [(l-1,n-C2B10H10){Au(PPh3)}2], n = 2, 7,
and 12
In Fig. 4 we display the optimized geometry of the para-
complex [(m-1,12-C2B10H10){Au(PPh3)}2]. Comparison of exper-
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Table 5 NPA charges (in electrons) in ortho-, meta-, and para-carborane,
and the corresponding complexes [(m-1,n-C2B10H10)Z2] (n = 2, 7, 12), with
Z = H or AuPPh3 fragment
ortho
H
AuPPh3
AuPMe3
C
-0.548
0.147
-0.012
-0.181
-0.154
—
-0.719
0.140
-0.016
-0.178
-0.153
0.355
-0.714
0.145
-0.011
-0.176
-0.151
0.292
BA
BB
BC
BD
Au
P
—
1.006
0.970
meta
H
AuPPh3
AuPMe3
C
-0.708
0.147
-0.014
0.010
-0.181
—
-0.871
0.142
-0.015
0.007
-0.178
0.351
1.003
-0.869
0.145
-0.011
0.010
-0.176
0.298
0.969
BA
BB
BC
BD
Au
P
—
para
H
AuPPh3
AuPMe3
C
B
Au
P
-0.696
-0.013
-0.855
0.015
0.348
1.003
-0.859
-0.010
0.295
—
0.972
together with the same charges of the parent carboranes for
comparative purposes.
Fig. 5 Selected molecular orbitals for optimized complex [(m-1,12-
C2B10H10){Au(PPh3)}2]: (a) HOMO-3; (b) HOMO; and (c) LUMO. Orbital
function isovalue = 0.02.
highlight the contribution from the cage boron atoms on the
symmetric orbital (HOMO), which delocalizes electron density
onto the carborane cage. A similar situation applies for the meta-
and ortho-complexes.
The LUMO of the para-complex is shown in Fig. 5c, whose wave
function amplitude leads to the conclusion that a nucleophilic
attack should be directed mainly towards the carborane cage.
Thus, the nucleophilic/electrophilic character of these complexes
lie mainly onto the {P–Au–carborane–Au–P} central part and the
exo phenyl groups respectively, with the exception of the para-
complex, the latter having the LUMO or more accessible orbital
for surplus electrons along the whole skeleton of the complex (see
ESI†).
Electronic charges are purely formal and can be obtained
through different population analyses. Since natural population
analysis (NPA) shows a more “realistic” scheme as compared
to the Mulliken charges, we have only included here the NPA
analysis, although the same trends can be observed in the Mulliken
population analysis (see ESI†). Table 5 gathers the natural bond
order (NBO) charges for the ortho-, meta- and para-complexes
For ortho- and meta-C2B10H12 isomers, the charges on the boron
atoms depend on their positions as compared to the charges on
the carbon atoms, with variations ranging up to 0.33, whereas the
charges on boron atoms for para-C2B10H12 are equal due to the
symmetry of the cluster. However, the para- and meta-C2B10H12
isomers have similar charges on carbon atoms: ~ -0.71, with a
lower change on the carbon (-0.55) for the ortho-C2B10H12 isomer;
the latter can be attributed to the relative stability found in these
molecules (Table 2). These variations are also transferred to the
related complexes when H is substituted by Au(PR3), an indication
that the electronic structure of the isomers for each molecular
species can be transferred between each other: [H ↔ Au(PR3)].
Finally, an important increase of negative charge of ~ -0.2 is
observed for carbon atoms in organogold complexes, whereas
charges on boron atoms remain almost unchanged.
The NPA charges on Au atoms are 0.35 and 0.29 for all isomers
having R = Ph and Me, respectively, whereas the charge on P has
about 1.0. These data support the changes in the donor nature of
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the phosphine ligands. These values for triphenylphospine can be
compared with those computed on the experimental geometries
of [AuCl(PPh3)]25 and [AuPh(PPh3)]26 with the same theoretical
model: The NPA charges on P are 1.05 and 1.00, respectively,
as is expected for a gold(I)–phosphine fragment. However, the
NPA charges on Au, 0.19 and 0.17 respectively, have a decrease
of ca. 0.2e as compared to the carborane complexes. Hence,
the increase of the polarity in the X–Au(PPh3) bonds can be
related to the electron-deficiency nature of carborane boxes (as
reflected in the HOMO orbital plot, see Fig. 5b and ESI†) together
with the electron withdrawing role of the cages in the [(m-1,n-
C2B10H10){Au(PPh3)}2] (n = 2, 7, 12) complexes, as compared to
well known compounds, such as [AuX(PPh3)] (X = Cl, Ph).
geometry (Fig. 6a) is compared with that of the analogous
mercuracarborane derivative described by Hawthorne et al.27
Acknowledgements
We thank the Ministerio de Educacion y Ciencia No. CTQ2007-
67273-C02-01 and MAT2006-13646-C03-02 for financial support.
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