Synthesis, structure and bonding of a digold complex with bridging triphenylstannyl ligands

Article abstract of DOI:10.1016/j.jorganchem.2015.01.021

The reaction of Au(PPh₃)Ph with HSnPh₃ yielded the new digold-ditin complex, [Au(PPh₃)(μ-SnPh₃)]₂, 6 in 52% yield. Benzene was also formed. Sn₂Ph₆ is a major coproduct that was formed by the degradation of 6. Compound 6 was characterized structurally by a single-crystal X-ray diffraction analysis. The molecule contains two Au(PPh₃) groups that are joined by a strong Au-Au bond (2.5590(5) ? in length) that is bridged by two SnPh₃ groups. The metal-metal bonding was analyzed by DFT Mo calculations. The Au-Sn bonding is represented by the HOMO which is a four center: two electron bond.

Full text of DOI:10.1016/j.jorganchem.2015.01.021

Journal of Organometallic Chemistry 795 (2015) 40e44
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, structure and bonding of a digold complex with bridging
triphenylstannyl ligands
Richard D. Adams*, Yuen Onn Wong, Qiang Zhang
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of Au(PPh3)Ph with HSnPh3 yielded the new digoldeditin complex, [Au(PPh3)(meSnPh3)]2, 6
Received 30 November 2014
Received in revised form
15 January 2015
Accepted 25 January 2015
Available online 13 February 2015
in 52% yield. Benzene was also formed. Sn2Ph6 is a major coproduct that was formed by the degradation
of 6. Compound 6 was characterized structurally by a single-crystal X-ray diffraction analysis. The
molecule contains two Au(PPh3) groups that are joined by a strong AueAu bond (2.5590(5) Å in length)
that is bridged by two SnPh3 groups. The metalemetal bonding was analyzed by DFT Mo calculations.
The AueSn bonding is represented by the HOMO which is a four center: two electron bond.
© 2015 Elsevier B.V. All rights reserved.
Dedicated to Professor Hubert Schmidbaur
on the occasion of his 80th birthday.
Keywords:
Gold
Tin
Molecular orbitals
Crystal structure
Introduction
We have recently found that aryl containing gold phosphine
complexes, Au(PPh3)Ar, Ar ¼ Ph, Np, Py, react with certain dirhe-
Interest in the organometallic chemistry of gold has grown
rapidly in recent years following the discoveries that gold nano-
particles supported on metal oxides exhibit surprisingly high ac-
tivity for the catalytic oxidation of CO and selected hydrocarbons
[1]. Studies have also shown bimetallic catalysts containing gold
exhibit even better activity for oxidation catalysis [2]. Tin is well
known for its ability to serve as a modifier of heterogeneous metal
catalysts [3]. A recent study has shown that Au/SnO2 catalyst ex-
hibits oxidation activity comparable to Au/TiO2, one of the most
active catalytic gold oxidation systems [4]. Gold complexes can also
perform novel forms of dual catalysis homogeneously when com-
bined with other metals, most notably with palladium [5].
nium [11] and triosmium [12] carbonyl complexes to yield elec-
tronically unsaturated complexes containing bridging aryl ligands.
In the present work, we have investigated the reaction of Au(PPh3)
Ph with HSnPh3. The principal product is a new digoldeditin
complex [Au(PPh3) (m-SnPh3)]2, 6 that contains two bridging SnPh3
ligands. The results of our studies of the synthesis, structural
characterization and bonding in this compound are described in
this report.
Experimental details
General data
There are very few examples of organometallic goldetin com-
plexes in the literature. Examples with terminally-coordinated tin
ligands include Au(PMe2Ph)2(SnCl3), 1, AueSn ¼ 2.881(1) Å [6] and
Au(PPh3)[Sn{N(p-tol)SiMe2}3SiMe], 2 [7], see Scheme 1.
Reagent grade solvents were dried by the standard procedures
and were freshly distilled prior to use. Infrared spectra were
recorded on a Thermo Nicolet Avatar 360 FT-IR spectrophotometer.
1H NMR and 31P{1H} NMR were recorded on a Varian Mercury 400
spectrometer operating at 400.1 and 161.9 MHz respectively. 31P
{1H} NMR spectra were referenced externally by using 85% ortho-
H3PO4. Solid state 119Sn cross-polarization magic angle spinning
(CP-MAS) spectra were collected on a Bruker Avance III-HD
500 MHz spectrometer fitted with a 1.9 mm MAS probe and were
Compounds [Au4(PPh3)4(
SnCB10H11]2, 4 [9] and the dianion compound [Bu3NH]2[(PPh3)
Au( -SnB11H11)]2, 5 [10] have bridging Sn ligands, Scheme 2.
m-SnCl3)2], 3 [8] and Au4(PPh3)4[m-
m
* Corresponding author.
E-mail address: Adamsrd@mailbox.sc.edu (R.D. Adams).
http://dx.doi.org/10.1016/j.jorganchem.2015.01.021
0022-328X/© 2015 Elsevier B.V. All rights reserved.

R.D. Adams et al. / Journal of Organometallic Chemistry 795 (2015) 40e44
41
Table 1
Crystallographic data for 6.
Empirical formula
Formula weight
Crystal system
Au2Sn2P2C72
1618.54
Orthorhombic
H60$CH2Cl2
Lattice parameters
a (Å)
b (Å)
c (Å)
14.2638(6)
22.9477(9)
9.8436(4)
90.00
90.00
90.00
3222.0(2)
P21212
2
1.71
5.437
a
b
g
(deg)
(deg)
(deg)
V (Å3)
Scheme 1. Line structures for compounds 1 and 2.
Space group
Z value
rcalc (g/cm3)
m
(Mo K
Temperature (K)
qmax(ꢀ)
No. obs. (I > 2
a
) (mmꢁ1
)
referenced against SnPh4. The spectra was collected at ambient
temperature with a sample rotation rate of 20 kHz. HSnPh3 was
obtained from SIGMA-ALDRICH and was used without further pu-
rification. Au(PPh3)Ph was prepared according to the previously
reported procedure [13]. Product separations were performed by
TLC in air on Analtech 0.25 mm alumina 60 Å F254 glass plates.
294(2)
47.50
7132
2
s(I))
No. parameters
364
1.119
0.001
0.0424; 0.1092
Multi-Scan
1.000/0.699
2.019
Goodness of fit (GOF)a
Max. shift on final cycle of refinement
Residualsa: R; Rw
Absorption correction, max/min
Reaction of HSnPh3 with Au(PPh3)Ph
Largest peak in Final Diff. Map (eꢁ/Å3)
a
1/2
R ¼ Shkl(rrFobsr ꢁ rFcalcrr)/ShklrFobsr; Rw ¼ [Shklw(rFobsr ꢁ rFcalcr)2/ShklwFo2bs
]
;
30.0 mg (0.026 mmol) of Au(PPh3)Ph was added to 18.6 mg
(0.052 mmol) of HSnPh3 dissolved in 10 mL of benzene. The so-
lution was allowed to stir for 7 h at room temperature. A red-
orange solution formed and the solvent was then removed in
vacuo. The residue was extracted in methylene chloride and
separated by TLC by using hexane solvent to elute a yellow band of
w ¼ 1/
s
2(Fobs); GOF ¼ [Shklw(rFobsr ꢁ rFcalcr)2/(ndata e nvari)]1/2
.
program SADABS. The structure was solved by a combination of
direct methods and difference Fourier syntheses, and refined by
full-matrix least-squares on F2, using the SHELXTL software pack-
age [15]. All non-hydrogen atoms were refined with anisotropic
displacement parameters. The hydrogen atoms were placed in
geometrically idealized positions and included as standard riding
atoms during the final cycles of least-squares refinements. Com-
pound 6 crystallized in the orthorhombic crystal system with one
equivalent of CH2Cl2 cocrystallized from the crystallization solvent.
The space group P21212 was uniquely identified by the pattern of
systematic absences observed in the data. Crystal data, data
collection parameters, and results of the analyses are listed in
Table 1.
[Au(PPh3)(m-SnPh3)]2, 6 18.8 mg (52%) and Sn2Ph6, 8.6 mg (46%).
The orange crystals of pure 6 can be physically separated from the
colorless crystals of the Sn2Ph6. Spectral data for 6: 1H NMR
(CD2Cl2, in ppm)
d
¼ 7.50 (m, 15H), 7.35 (m, 15H). 119Sn CP-MAS (in
ppm)
d
¼ 121.01 (s). 31P NMR (CD2Cl2, in ppm)
d
¼ 43.33(s). Anal.
Calcd for 1: C, 53.42; H, 3.73. Found: C, 56.65; H, 4.10. Samples of 6
inevitably contain small amounts of Sn2(C6H5)6 formed by its
decomposition. This could explain the high measurements for C
and H.
Crystallographic analysis
Orange single crystals of 6 crystallize together with colorless
crystals of Sn2Ph6 (a decomposition product) upon slow evapora-
tion of solvent from a solution in methylene chloride at 15 ꢀC. An
orange crystal of 6 was glued onto the end of a thin glass fiber. X-ray
intensity data were measured by using a Bruker SMART APEX CCD-
Computational details
Density functional theory (DFT) calculations were performed
with the Amsterdam Density Functional (ADF) suite of programs
[16] by using the PBEsol functional [17] with valence quadruple-
based diffractometer by using Mo K
a
radiation (
l
¼ 0.71073 Å). The
z
þ 4 polarization function, relativistically-optimized (QZ4P) basis
raw data frames were integrated with the SAINT þ program by
using a narrow-frame integration algorithm [14]. Correction for
Lorentz and polarization effects were also applied by using SAINTþ.
An empirical absorption correction based on the multiple mea-
surement of equivalent reflections was applied by using the
sets for the gold, tin, phosphorus, carbon and hydrogen atoms with
frozen cores. The molecular orbitals for compound 6 and their
energies were determined by geometry-optimized calculations
with scalar relativistic corrections that were initiated by using the
atom positional parameters for the molecule as determined from
Scheme 2. Structures of 3, 4 and the anion of 5.

42
R.D. Adams et al. / Journal of Organometallic Chemistry 795 (2015) 40e44
the crystal structure analysis. The geometry-optimized coordinates
of 6 are given in Table S1.
the terminally coordinated tin ligand in compound 2,
AueSn ¼ 2.5651(13) Å [7], but are similar to the bridging AueSn
distances reported for the compounds 3, 2.8150(7) Å and 2.9725(8)
Å [8]; 4, 2.891(1) Å, 2.727(1) Å, 2.936(1) Å, and 2.757(1) [9] and the
dianion 5, 2.737(1) Å and 2.761(1) Å [10] (see Scheme 2). The
Results and discussion
AueSneAu angle in
6
(53.08ꢀ) is slightly smaller than the
The reaction of Au(PPh3)Ph with HSnPh3 yielded the new
digoldeditin complex, [Au(PPh3)(m-SnPh3)]2, 6 in 52% yield. Sn2Ph6
AueSneAu angle (57.03ꢀ) in 5 as a result of the shorter AueAu
distance in 6. The Au2Sn2 ring is almost planar; the dihedral angle
between the two Au2Sn triangles is 163.57(2)ꢀ. It is unlikely that
there is any significant direct SneSn bonding interaction, because
the Sn … Sn distance is very long at 5.069(1) Å. Compound 6 does
not exhibit a 119Sn NMR signal in solutions; however, a 119Sn CP-
was a major coproduct that was obtained in 46% yield. Sn2Ph6 was
subsequently found to be formed by the degradation of 6. The
formation of benzene was observed when the reaction was per-
formed in an NMR tube in CD2Cl2 solvent. Compound 6 was char-
acterized structurally by a single-crystal X-ray diffraction analysis.
An ORTEP diagram of its molecular structure is shown in Fig. 1. The
molecule contains two Au(PPh3) groups that are joined by a strong
AueAu bond that is bridged by two SnPh3 groups. In the solid state
the molecule lies on a crystallographic two-fold rotation axis that
lies perpendicular to the AueAu bond. The AueAu distance in 6 is
short, 2.5590(5) Å, and is shorter than the AueAu bond
distance found in the SnB11H11-bridged digold dianion of com-
pound 5, AueAu ¼ 2.625(1) Å (Scheme 2) [10]. The PeAueAui angle
in 6, 170.74(5)ꢀ, is almost linear and is similar to that in 5,
178.78(4)ꢀ. Mingos described the structure of digold molecule
Ph3AuAuPPh3 7 which has no bridging ligands [18]; the AueAu
distance given for 7 is 2.76 Å. Although the identity of 7 has recently
MAS spectrum of 6 does show a singlet at
d
¼ 121.01 in the solid
state. This is consistent with the solid state structure having
equivalent tin atoms. The 119Sn CP-MAS spectrum of Sn2Ph6 in the
solid state shows a singlet at
d
¼ ꢁ142.1.
There are very few examples of complexes containing bridging
SnPh3 ligands. Bridging SnPh3 groups were found across the
unsaturated MeM bond of the dinuclear transition metal com-
plexes [M2Cp2(
[23]; across a GeeGe edge of the complex polyhedral anion
[Ge9(
-SnPh3)]3ꢁ [24]; and across a BeB edge of the polyhedral
borane B5H8(2,3- -SnPh3) [25]. It is unusual to have a SnPh3
m
-SnPh3)(
m
-PCy2)(CO)2], M ¼ Mo [22], M ¼ W
m
m
group bridging two metal atoms because the tin atom has only
one unpaired valence electron. Clearly some sort of multicenter
delocalized bonding can be anticipated for the AueSn bonding in
the cluster of 6.
been questioned [19], Bertrand recently reported
a related
unbridged digoldedicarbene complex (CAAC)AuAu(CAAC),
8
CAAC ¼ cyclic(alkyl)(amino)carbene [20] that contains a very short
AueAu bond, 2.5520(6) Å having nearly linear CeAueAu angles,
173.8(2)ꢀ and 171.3(3)ꢀ. The shortest reported AueAu bond dis-
tances are for the complexes [Au2(hpp)2Cl2], AueAu ¼ 2.4752(9) Å,
[(PhCO2)6Au4(hpp)2Ag2], AueAu ¼ 2.4473(19) Å, hpp ¼ 3,4,6,7,8,9-
hexahydro-pyrimido[1,2-a]pyrimidinate [21]. Compound 6 con-
tains two independent AueSn distances, Au1eSn1 ¼ 2.8207(16) Å
and Au1eSn1i ¼ 2.9038(6) Å that are statistically different in
length. This is probably due to the rotational conformation of the
SnPh3 group. These distances are much longer than that to
In order to understand the AueAu and AueSn bonding in 6,
geometry-optimized DFT molecular orbital calculations were per-
formed. Selected molecular orbitals (MOs) that show the metal-
emetal orbital interactions in the Au2Sn2 ring are shown in Fig. 2.
The LUMO (ꢁ2.21 eV) is a
atoms. The HOMO (ꢁ4.30 eV) is a delocalized four center e two
electron -type orbital that shows the nature of the goldetin
p-type MO delocalized across all four
s
bonding interactions, but it has a node along the AueAu vector.
Because of this node, this orbital makes no significant contribution
to direct bonding between the two gold atoms. The HOMO-1
(ꢁ5.69 eV) is a symmetric two electron bond that is composed
principally of d-orbitals from the Au atoms and with small contri-
butions from atomic p-orbitals on the two Sn atoms. This orbital
confirms the existence of a significant direct AueAu
s-bonding
interaction. By using conventional electron counting procedures,
the AueSn bonding in 6 would be best described schematically by
the diagram A where the four dashed lines represent the four pairs
of AueSn interactions represented by the HOMO, formally 1/4 of a
bond for each AueSn interaction, and the solid line between the
two Au atoms is formally an AueAu single bond that is represented
by the HOMO-1.
The complete DFT analysis also reveals three low-lying orbitals
that show favorable overlaps that could be interpreted as
Fig. 1. An ORTEP diagram of the molecular structure of [(AuPPh3)(m-SnPh3)]2, 6,
showing 30% thermal ellipsoid probability.

R.D. Adams et al. / Journal of Organometallic Chemistry 795 (2015) 40e44
43
Fig. 2. Selected molecular orbitals with calculated energies in electron volts (eV) for the LUMO, HOMO, HOMO-1, HOMO-62, HOMO-74 and HOMO-75 of 6.
supplementary metalemetal bonding. These are the HOMO-62, the
HOMO-74 and HOMO-75. The HOMO-62 shows goldetin bonding
energies they probably do not make major contributions to the total
of the metalemetal bonding (Tables 2 and 3).
interactions and the HOMO-74 and HOMO-75 both show
laps derived from Au d-orbitals, see Fig. 2, but because of their low
p
-over-
Curiously, a number of years ago Schmidbaur reported the
compound Au(PPh3) (GeCl3), 9 which was shown to be a dimer in
the solid state and contained a significant, but long AueAu
Table 2
Selected intramolecular distances and angles for
6 from the X-ray structural
analysis.a
Atom
Atom
Distance (Å)
Atom
Atom
Atom
Angle (ꢀ)
Table 3
Au1i
Sn1i
Au1i
53.083(13)
124.613(15)
170.74(5)
Interatomic distances for the DFT geometry-optimized structure of 6.
Au1
Au1
Au1
Au1
Sn1
Sn1
Sn1i
Au1i
P1
2.8207(6)
2.9038(6)
2.5590(5)
2.3336(17)
5.069(1)
Au1
Sn1
P1
Sn1
Au1
Au1
Atom
Atom
Distance (Å)
Atom
Atom
Atom
Angle (ꢀ)
Au1
Au1
Au1
Au1
Sn1
Sn1i
Au1i
P
2.806
2.835
2.552
2.375
Au1
Sn1
Sn1
Au1
Au1i
Sn1i
53.7
124.1
Sn1i
a
Estimated standard deviations in the least significant figure are given in pa-
rentheses. i ¼ symmetry related position.

44
R.D. Adams et al. / Journal of Organometallic Chemistry 795 (2015) 40e44
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Conclusions
Compound 6 is a dimer of the formula unit “Au(PPh3)(SnPh3)”.
The molecule is held together by a strong direct AueAu s-bonding
interaction and two bridging SnPh3 ligands. The AueSn bonding is
best described by a four center e two electron bond having a node
along the AueAu vector.
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Acknowledgments
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This research was supported by the following grants from the
National Science Foundation: CHE-1111496 and CHE-1048629.
Many thanks to Professor John Fackler for helpful discussions.
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Appendix A. Supplementary data
CCDC 1035628 for compound 6 contains the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Appendix B. Supplementary data
[14] SAINTþ, Version 6.2a, Bruker Analytical X-ray Systems, Inc., Madison, WI,
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[16] ADF2013; SCM Theoretical Chemistry, Vrije Universiteit, Amsterdam, The
Netherlands, http://www.scm.com.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2015.01.021.
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