Trialkylstibine Complexes of Boron, Aluminum, Gallium, and Indium Trihalides: Synthesis, Properties, and Bonding

The
reaction of BX3 (X = Cl, Br, I) with SbR3 (R
= Et,iPr) in anhydrous hexane or toluene produced
moisture-sensitive [BX3(SbR3)], whose stability
increases with halide: Cl < Br < I. Unstable [BF3(SbR3)] species were also characterized spectroscopically
but lose BF3 very readily. The [MX3(SbR3)] (M = Al, Ga, In) complexes were isolated from hexane solution;
the Ga and In complexes can be recrystallized unchanged from CH2Cl2, but [AlX′3(SbR3)] (X′ = Br, I) undergoes X′/Cl exchange with the solvent.
The new complexes were characterized by IR and multinuclear NMR spectroscopy
(1H, 13C­{1H}, 11B, 27Al, 71Ga, 115In, as appropriate). X-ray
crystal structures are reported for [BX3(SbR3)] (X = Br, I; R = Et, iPr; X = Cl, R = iPr),
[AlI3(SbiPr3)], [GaX3(SbR3)] (X = Cl, I; R = iPr), [GaBr3(SbEt3)], and [InX3(SbR3)] (X = Cl, I; R =
Et, iPr); all contain pseudotetrahedral geometries at both
the group 13 and antimony atoms, and comparisons with phosphine and
arsine analogues are discussed. The donor–acceptor bond lengths
are unexceptional, but coordination of the stibine is accompanied
by a significant widening of the C–Sb–C angles and a
small reduction in C–Sb distances. DFT calculations confirm
these results from significant changes in the antimony 5s and 5p contributions
to the Sb–C bonds, with corresponding increases in the 5p character
of the antimony-based “lone pair” on coordination. Intermolecular
hypervalent Cl···Sb interactions are present in the
two [InCl3(SbR3)] complexes but absent in the
others.