Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Organometallics
Article
of such complexes. In this paper, we present a series of results
that addresses these questions and objectives.
RESULTS AND DISCUSSIONS
■
Synthesis, Structure, and Bonding of Cationic
Stiborane−Gold Complexes. In this study, we decided to
investigate pincer complexes of the general formula [(o-(R2P)-
C6H4)2SbPh]AuCl that differ by the nature of the phosphine
substituents R. We have previously synthesized the ligand (o-
(Ph2P)-C6H4)2SbPh (LPh) and studied its interaction with
precious metals such as palladium and platinum,5b,c but the
synthesis of the corresponding gold complex had not been
attempted. We have now observed that LPh reacts with 1 equiv
of (tht)AuCl in CH2Cl2 at 0 °C to afford the desired complex 1
as an air-stable, yellow powder in 80% yield (Scheme 2).
Figure 1. Crystal structure of 2. Displacement ellipsoids are scaled to
the 50% probability level. Hydrogen atoms and the lattice solvent
molecules are omitted for clarity. Selected bond lengths (Å) and angles
(deg) for 2: Au−Sb 2.8608(5), Au−Cl1 2.5376(11), Au−P1
2.3161(11), Au−P2 2.3172(12), Sb−C13 2.132(4), Sb−O1
2.078(3), Sb−O2 2.079(3); P1−Au−P2 148.74(4), P1−Au−Cl1
104.71(4), P2−Au−Cl1 98.14(4), Sb−Au−Cl1 150.96(3), C13−
Sb−Au 163.99(12), O1−Sb−O2 79.29(11), C1−Sb−C7 100.20(16).
Scheme 2. Synthesis of 1, 2, and 3[PF6]
catecholate ligand lies in a plane approximately perpendicular
to the CPh−Sb−Au axis, with the two oxygen atoms positioned
trans from the ortho-phenylene linkers. The strain imposed by
the five-membered SbO2C2 ring leads to an acute O1−Sb−O2
angle of 79.29(11)°. By contrast, the C1−Sb−C7 angle of
100.20(16)° involving the two ortho-phenylene linkers is
relaxed by about ∼10° when compared to the ideal value of
90°. In the absence of an Au→Sb interaction, the sum of the
P1−Au−P2, P1−Au−Cl1, and P2−Au−Cl1 angles should be
equal to 360°. However, the sum of these angles in 2 is equal to
351.6°, suggesting some degree of electron donation from gold
to antimony. The structural differences observed between 2 and
C may originate from steric effects or even weak interactions
between the peripheral ligands. We speculate that, in the case of
2, the observed structure is stabilized by weak interactions
occurring between the phosphorus-bound phenyl group and
the tetrachlorophenylene ring of the catecholate ligand.10 Such
an arene−arene stabilizing interactions would be absent with
the di-iso-propyl phosphine units, leading to the conformation
observed in C.
Next, we decided to convert 2 into a potentially catalytically
active species by abstraction of the gold-bound chloride. Salt
3[PF6] was generated by treating 2 with 1 equiv of TlPF6 in
CH2Cl2/CH3CN at room temperature. The product was
obtained as a dark orange crystalline powder in 80% yield.
Complex 3[PF6] gives rise to a 31P NMR resonance at 56.34
ppm in CH2Cl2 and 56.09 in CHCl3. However, the use of the
more coordinating ligand CH3CN produces a peak at 61.86
ppm, suggesting solvent coordination to the gold center.
Addition of excess 4-dimethylaminopyridine (DMAP) to a
CH2Cl2 solution of 3[PF6] results in a 31P NMR resonance a
67.63 ppm, again suggesting coordination of DMAP to the gold
atom. However, no notable changes in the 31P NMR resonance
of 3[PF6] were induced by addition of phenylacetylene,
indicating that this substrate is too weakly basic to shift the
equilibrium in favor of the corresponding complex. We also
generated the di-iso-propylphosphino derivative 4[PF6] starting
from the previously reported complex C (Scheme 3). Salt
4[PF6] was also obtained as a dark orange powder that showed
a single 31P NMR peak at 81.86 ppm in CDCl3. Interestingly,
Complex 1 has been characterized by 31P NMR spectrosco-
py, which showed a sharp peak at 40.92 ppm. The production
of complex 1 was often accompanied by the formation of the
chlorostibine complex [(o-(Ph2P)-C6H4)2SbCl]AuCl, which
was identified by its characteristic 31P NMR signal at 38 ppm.
Formation of this derivative suggests that complex 1 is unstable
in chlorinated solvents.
Although we failed to crystallize 1, we found that it cleanly
reacts with o-chloranil in dichloromethane to afford the
corresponding tetrachlorocatecholatostiborane gold complex 2
(Scheme 2). Complex 2 was obtained in a 60% yield as a bright
orange crystalline solid after purification by column chromatog-
raphy over silica. The 31P NMR spectrum of the crude product
shows a resonance at 83.03 ppm. Although this compound was
characterized by elemental analysis, its low solubility did not
1
allow for reliable H and 13C NMR data to be obtained. When
compared to complex 1, the 31P NMR resonance of complex 2
is shifted downfield by ∼45 ppm, in line with an increased
oxidation of the AuSb core.7a,b
The crystal structure of complex 2 has been determined
(Figure 1). The Au−Sb bond distance of 2.8608(5) Å is
considerably longer than that in the analogous complex C
(2.6833(3) Å). This longer distance may be correlated to the
fact that the position trans to the gold atom is occupied by a
phenyl ligand (CPh−Sb−Au = 163.99(12)°) rather than an
electron-withdrawing oxygen atom as in the case of C. The
B
DOI: 10.1021/acs.organomet.7b00654
Organometallics XXXX, XXX, XXX−XXX