Gold-silane and gold-stannane complexes: Saturated molecules as σ-acceptor ligands

Article abstract of DOI:10.1002/anie.200905391

Gold gets a grip: Spectroscopic, structural and computational results provide evidence for unprecedented Au→Si and Au→Sn interactions in gold (l) complexes derived from diphosphino-substituted silane and stannane ligands (see scheme).

Full text of DOI:10.1002/anie.200905391

Communications
DOI: 10.1002/anie.200905391
s-Acceptor Ligands
Gold–Silane and Gold–Stannane Complexes: Saturated Molecules as
s-Acceptor Ligands**
Pauline Gualco, Tzu-Pin Lin, Marie Sircoglou, Maxime Mercy, Sonia Ladeira,
Ghenwa Bouhadir, Lisa M. Pꢀrez, Abderrahmane Amgoune, Laurent Maron,*
Franꢁois P. Gabbaꢂ,* and Didier Bourissou*
The discovery that saturated molecules may form s com-
plexes by side-on coordination of a s bond to a transition
metal represents a major breakthrough in transition-metal
chemistry.[1–3] Over the years, considerable progress has been
made in the understanding of this bonding situation. The
coordination and activation of s bonds involving Group 14
elements (E = C, Si, Ge, Sn, Pb) is at the forefront of
developments in this area. An increasing variety of complexes
A[3–5] and B[6,7] (Scheme 1) featuring side-on coordinated
ꢀ
ꢀ
s(E H) and s(E E) bonds have been isolated, and the key
factors governing the delicate balance between dissociation
and oxidative addition have been progressively identified.
The common feature and believed prerequisite for the
coordination of saturated molecules free of lone pairs to
Scheme 1. Complexes A–C featuring Group 14 saturated molecules
coordinated to transition metals (E=C, Si, Ge, Sn, Pb).
transition metals is the superposition of ligand!metal
donation (from a filled s orbital of the ligand to an empty
d orbital of the metal) and metal!ligand back-donation
(from a filled d orbital of the metal to an empty s* orbital of
the ligand).
Aiming at identifying new types of metal–ligand inter-
actions, and stimulated by our work on Group 13 Lewis acids
as acceptor ligands,[8–10] we recently became interested in
complexes of type C, in which a saturated Group 14 element
could behave as an end-on, s-acceptor ligand toward a
transition metal. Heavier Group 14 elements such as silicon
and tin are known to readily form hypervalent compounds
through donor!acceptor interactions with organic Lewis
bases.[11–13] A related situation is envisioned in complexes C,
with a transition metal acting as a Lewis base. Such donor!
acceptor interactions between transition metals and silanes or
[*] M. Mercy, Dr. L. Maron
Universitꢀ de Toulouse, INSA, UPS, LPCNO
135 avenue de Rangueil, 31077 Toulouse (France)
and
CNRS, LPCNO UMR 5215, 31077 Toulouse (France)
E-mail: laurent.maron@irsamc.ups-tlse.fr
T.-P. Lin, Dr. L. M. Pꢀrez, Dr. F. P. Gabbaꢁ
Department of Chemistry, Texas A&M University
College Station, TX 77843 (USA)
E-mail: gabbai@mail.chem.tamu.edu
P. Gualco, Dr. M. Sircoglou, Dr. G. Bouhadir, Dr. A. Amgoune,
Dr. D. Bourissou
Universitꢀ de Toulouse, UPS, LHFA
118 route de Narbonne, 31062 Toulouse (France)
and
CNRS, LHFA UMR 5069, 31062 Toulouse (France)
Fax: (+33)5-6155-8204
E-mail: dbouriss@chimie.ups-tlse.fr
stannanes have been invoked in a few highly strained
[14]
ꢀ
complexes on the basis of relatively short M E distances.
Furthermore, the presence of a PdII!SnIV dative bond was
recently evidenced structurally and theoretically in a palla-
dastannatrane cage complex supported by four methimazolyl
groups.[18] Herein, we report the straightforward synthesis and
complete characterization of three gold complexes supported
by diphosphino silane and stannane ligands. The presence of
metal!silane and metal!stannane interactions in these
complexes has been substantiated spectroscopically, structur-
ally, and theoretically, thus providing unambiguous evidence
for the existence of complexes of type C.
S. Ladeira
Structure Fꢀdꢀrative Toulousaine en Chimie Molꢀculaire, Toulouse
(France)
[**] Financial support from the Centre National de la Recherche
Scientifique, the Universitꢀ de Toulouse, the Agence Nationale de la
Recherche (ANR-06-BLAN-0034), the National Science Foundation
(CHE-0646916), and the Welch Foundation (A-1423) is gratefully
acknowledged. We thank: the Unidade de RaiosX of the Universi-
dade de Santiago de Compostela for the X-ray diffraction analyses of
4 and 5; Christopher Cummins (MIT) for useful discussions
regarding the use of the Dgrid program; the Laboratory for
Molecular Simulation at Texas A&M University for providing
software and computation resources. L.M. thanks the Institut
Universitaire de France and D.B. thanks the COST action CM0802
PhoSciNet for supporting this work.
From our previous studies on Group 13 Lewis acids,[8] the
use of two phosphine buttresses ligated by ortho-phenylene
spacers was considered as a good compromise in order to
support, but not impose, the coordination of the Group 14
element. We thus targeted the two complexes [o-
{(iPr2P)C6H4}2E(Ph)FAuCl] 2 (E = Si) and 4 (E = Sn). The
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905391.
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Chemie
diphosphino silane ligand 1 was prepared by treating the o-
lithiated diisopropylphenylphosphine with PhSiCl3 and then
with HF·collidine (collidine = 2,4,6-trimethylpyridine).[19] The
chlorine–fluorine exchange imparts higher stability to the
ligand. By allowing 1 to react with [AuCl(SMe2)], the desired
gold complex 2 was obtained in 88% yield as an analytically
pure white solid (Scheme 2). Complex 2 exhibits a single
which evolves from approximately tetrahedral in the free
ligand 1 (S(C-Si-C) 327.68) to slightly distorted trigonal-
bipyramidal in complex 2 (S(C-Si-C) 353.18 and F-Si-Au
ꢀ
166.11(12)8). Moreover, the Si F bond is noticeably elon-
gated upon coordination (from 1.600(2) ꢀ in the free ligand 1
to 1.635(3) ꢀ in complex 2), as observed in fluorosilane
adducts with organic Lewis bases.[11,20]
To confirm and further demonstrate the ability of
saturated heavier Group 14 elements to behave as s-
acceptor ligands, the related tin complex 4 was then
targeted. The Lewis acidity of Group 14 compounds
towards organic Lewis bases increases on going from
silicon to tin.[23,24] The diphosphino stannane ligand 3
was readily prepared according to a slightly different
procedure,[19] the chlorine–fluorine exchange at tin
being performed in this case with KF in DMF at reflux
(Scheme 2). Upon reaction with [AuCl(SMe2)], the
desired complex 4 was isolated in 87% yield as a white
solid. The single resonance in the 31P NMR spectrum,
observed at 79 ppm with the expected tin satellites
(3J117/119Sn-P) = 53/56 Hz), indicates the symmetric coor-
dination of the two phosphorus atoms. Consistently,
the signal in the 119Sn NMR spectrum appears as a
triplet at ꢀ147 ppm (3J119Sn-P = 56 Hz). This chemical
shift is at distinctly lower frequency than those of tetracoor-
dinate Ar3SnF derivatives (d = ꢀ65 to ꢀ85 ppm)[25] but
approaches those reported for related nitrogen adducts (d =
ꢀ195 to ꢀ200 ppm).[26] This finding suggests an increase in the
coordination number of the tin atom and thus the presence of
a gold!stannane interaction.
Scheme 2. Synthesis and coordination of the diphosphino silane and stannane
ligands 1 and 3.
signal by 31P NMR spectroscopy (d = 57.1 ppm), indicating
the symmetric coordination of the two phosphorus atoms. The
signal in the 29Si NMR spectrum (d = ꢀ21.4 ppm) is shifted by
d = 16 ppm from that of the free ligand 1 (d = ꢀ5.2 ppm). This
shift to lower frequency parallels those typically observed for
pentacoordinate neutral silicon adducts[11,20] and strongly
suggests the presence of a gold!silane interaction in 2.
According to X-ray diffraction analysis (Figure 1a), com-
plex 2 adopts a discrete mononuclear structure in the solid
state. The gold center is surrounded by the chlorine and the
two phosphorus atoms organized in a quasi-planar arrange-
ment halfway between trigonal and T-shape geometries (P-
Au-P 140.14(5)8). The silicon atom also comes close to the
This hypothesis was confirmed by X-ray diffraction
(Figure 1b). Despite the larger size of tin than silicon, the
ꢀ
Au Sn separation in complex 4 (2.891(2) ꢀ) is shorter than
ꢀ
the Au Si distance in 2 (3.090(2) ꢀ), and only slightly exceeds
the sum of the covalent radii (2.75 ꢀ). The tin center adopts a
quasi-ideal trigonal-bipyramidal coordination. The F and Au
atoms define the apical axis (F-Sn-Au 176.27(9)8), while the
three C atoms occupy the basal positions (S(C-Sn-C) 359.98).
ꢀ
metal center (Au Si 3.090(2) ꢀ). This distance is beyond the
sum of covalent radii (2.47 ꢀ)[21] but is well within the sum of
the van der Waals radii (4.20 ꢀ),[22] suggesting the presence of
a gold!silane interaction. This unusual bonding situation is
further supported by the geometry around the silicon atom,
The Sn F bond length (2.018(3) ꢀ) significantly surpasses
ꢀ
those of tetracoordinate Ar3SnF compounds (1.96–1.97 ꢀ)[25]
and falls in the same range as their nitrogen adducts
(2.02 ꢀ).[26] These geometric features indicate that tin is
more strongly bound to gold than silicon.[27] In turn, this
finding suggests that the higher Lewis acidity of tin toward
organic Lewis bases can be extended to metallobases.
Furthermore, the geometry around the gold center in com-
plex 4 approaches square-planar (P-Au-P 159.91(6)8 and Cl-
Au-Sn 151.36(4)8), whereas tetracoordinate gold(I) com-
plexes usually adopt tetrahedral arrangements. This situation
is reminiscent of that observed in diphosphino borane gold
complexes.[8d] Interestingly, the chlorostannane complex [o-
{(iPr2P)C6H4}2Sn(Ph)ClAuCl] (5)[19] displays a dative Au!Sn
interaction as well, with similar magnitude to that encoun-
tered in 4. This comparison provides further evidence for
metal!stannane interaction and demonstrates that the
coordination of saturated heavier Group 14 elements is also
possible with nonfluorinated moieties.
Figure 1. Molecular views of complexes 2 (a) and 4 (b) in the solid
state (hydrogen atoms omitted for clarity). Selected bond lengths [ꢂ]
and angles [8]: 2: P1–Au 2.323(2), P2–Au 2.305(2), Au–Cl 2.577(2), Si–
F 1.635(3), Au–Si 3.090(2); P1-Au-P2 140.14(5), Cl-Au-Si 141.54(4), Au-
Si-F 166.11(12). 4: P1–Au 2.334(2), P2–Au 2.330(2), Au–Cl 2.601(2),
Sn–F 2.018(3), Au–Sn 2.891(2); P1-Au-P2 159.91(6), Cl-Au-Sn
151.36(4), Au-Sn-F 176.27(9).
To gain a deeper insight into these unusual Au!silane
and Au!stannane interactions, DFT calculations were car-
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Communications
ried out on the actual complexes 2, 4, and 5.[19,28] The
optimized geometries nicely match those determined crystal-
lographically. In particular, the geometries around Au, Si, and
ꢀ
ꢀ
Sn, including the key Au Si and Au Sn separations, are
reliably reproduced. Interestingly, structural optimizations
carried with the ADF program in the absence of the zero-
Scheme 3. The two possible pathways for the oxidative addition of
ꢀ
Csp3 X bonds (X=halogen, sulfonate, etc.) to transition metals.
order regular approximation (ZORA) lead to significantly
[19]
ꢀ
ꢀ
elongated Au Si and Au Sn separations.
This trend
reflects the importance of relativistic effects, which destabi-
lize the 5d orbitals and thus increase the donor ability of the
gold center. Natural bond orbital (NBO) analyses identify in
all three complexes as donor–acceptor interaction between
the gold center and the heavier Group 14 element. The
heavier Group 14 elements, the nucleophilic attack of the
metal can be frozen, and stable compounds that mimic their
carbon congeners can be isolated.
corresponding delocalization energy DENBO is about three Experimental Section
times greater for tin (4: 22.8 kcalmolꢀ1 and 5: 26.6 kcalmolꢀ1)
than for silicon (2: 7.6 kcalmolꢀ1), in agreement with the
conclusions drawn from the metric data. The bonding
situation in complexes 2, 4, and 5 was further assessed by
atoms in molecules (AIM) calculations (Figure 2). The
All reactions and manipulations were carried out in an atmosphere of
dry argon using standard Schlenk techniques.
2: A solution of 1 (79 mg, 0.15 mmol) in dichloromethane (1 mL)
was added to a suspension of [AuCl(SMe2)] (44 mg, 0.15 mmol) in
dichloromethane (1 mL) at ꢀ788C. After warming to room temper-
ature, volatile components were
removed in vacuo, and complex 2 was
isolated as a white powder (98 mg,
88%). Colorless crystals suitable for
X-ray crystallography were obtained
from dichloromethane/pentane (3:7)
at room temperature. 31P{1H} NMR:
d = 57.1 ppm (s); 29Si NMR: d =
ꢀ21.4 ppm
(d,
1JSi-F = 259 Hz);
19F{1H} NMR: d = ꢀ135.8 ppm (s).
+
ꢀ
HRMS (ESI + ) calcd for [M Cl]
(C30H41P2FSiAu+): 707.2102, found:
707.2113.
Figure 2. Calculated electron density maps for complexes 2 (a) and 4 (b) with relevant bond paths
and bond critical points. The isopropyl substituents at phosphorus are omitted, and the o-phenylene
linkers are simplified for clarity.
4:
A
solution of
3 (102 mg,
0.17 mmol)
in dichloromethane
(1 mL) was added to a suspension of
[AuCl(SMe2)] (50 mg, 0.17 mmol) in
dichloromethane (1 mL) at ꢀ788C.
After warming to room temperature, volatile components were
removed in vacuo, and complex 4 was isolated as a white powder
(123 mg, 87%). Colorless crystals suitable for X-ray crystallography
were obtained from dichloromethane/pentane (3:7) at ꢀ308C.
electron density maps show the presence of bond paths
between Au and Si and Au and Sn with localized bond critical
points (BCPs). The electron density 1(r) at the BCP is higher
for complexes
4
(3.50 ꢁ 10ꢀ2 ebohrꢀ3) and
5
(3.56 ꢁ
31P{1H} NMR: d = 79.1 ppm (d, JP–117Sn = 53.4 Hz; JP–119Sn = 55.7 Hz,
3
3
10ꢀ2 ebohrꢀ3) than for 2 (2.13 ꢁ 10ꢀ2 ebohrꢀ3), corroborating
the stronger binding of tin than silicon to gold. Furthermore,
charge depletion at the BCP is consistent with the closed-
4JP-F = 14.9 Hz); 119Sn{1H} NMR: d = ꢀ147.8 ppm (td, JSn-F
=
1
2110.3 Hz, 3JSn-P = 55.7 Hz); 19F{1H} NMR: d = ꢀ189.6 ppm (t, JF–
117Sn = 2016.6 Hz; 1JF–119Sn = 2110.3 Hz; 4JF-P = 14.9 Hz). HRMS (ESI + )
calcd for [MꢀCl]+ (C30H41P2SnFAu+): 799.1355, found: 799.1316.
1
ꢀ
shell, donor–acceptor nature of the Au E interactions.
To date, the variety of complexes derived from saturated
molecules free of lone pairs has been limited to s complexes.
The Au!silane and Au!stannane interactions exhibited by
complexes 2, 4, and 5 provide unambiguous evidence for an
alternative coordination mode, in which the saturated
Group 14 element behaves as a s acceptor ligand toward
the transition metal. Accordingly, the concept of s acceptor
ligands appears to be much more general than previously
anticipated.[29] Moreover, compounds 2, 4, and 5 are novel
neutral hypervalent derivatives of heavier Group 14 ele-
ments. The donation from a metal center, as illustrated herein
with gold, opens access to hypervalent derivatives featuring a
transition metal as a substituent. Importantly, the geometric
and electronic features of complexes 2, 4, and 5 parallel those
predicted computationally for the transition-state structures
ꢀ
Received: September 25, 2009
Revised: October 20, 2009
Published online: November 24, 2009
Keywords: density functional calculations ·
.
hypervalent compounds · silicon · tin · s-acceptor ligands
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