Stabilization of a Dialkylstannylene by Unusual B−H···Sn γ-Agostic-Type Interactions. A Structural, Spectroscopic, and DFT Study

The reaction between SnCl2 and the lithium salt [[{nPr2P(BH3)}(Me3Si)CCH2]Li(THF)2]2 yields the
cyclic dialkylstannylene [{nPr2P(BH3)}(Me3Si)CCH2]2Sn (6) as a 1:1 mixture of rac and meso
diastereomers. Fractional crystallization from n-hexane gives samples of pure rac-6 and meso-6 which
have been studied by X-ray crystallography, UV/visible, infrared, and multi-element NMR spectroscopy.
In the solid state, rac-6 exhibits two short agostic-type B−H···Sn contacts, one to each BH3 group,
whereas meso-6, in which both BH3 groups lie on the same side of the heterocycle, exhibits a single
B−H···Sn contact. Multi-element and variable-temperature NMR spectroscopy suggests that these contacts
persist in solution, resulting in unusual 119Sn chemical shifts of 587 and 787 ppm for the rac and meso
diastereomers, respectively, significantly upfield in comparison to other dialkylstannylenes. Variable-temperature 1H, 31P{1H}, and 11B{1H} NMR studies reveal dynamic equilibria for the meso diastereomer
involving competitive binding of the two BH3 groups to the electron-deficient tin center and restricted
rotation about the P−C bond of the free phosphine−borane group. DFT studies confirm that the B−H···Sn
contacts are an integral part of the structures; the HOMO and LUMO of both stereoisomers are comprised
of an orbital of essentially 5s character on tin and an orbital of predominantly 5p character on tin
perpendicular to the plane of the heterocycle, respectively. There is significant donation of electron density
from the B−H σ-bonds to the vacant 5p orbital on tin; these B−H···Sn interactions stabilize the electron-deficient Sn(II) center by 30 kcal mol-1 for meso-6 and 40 kcal mol-1 for rac-6.