Structure and Bonding in a Diamond-Shaped Tin Cluster Possessing a cyclo-Sn4 Core.

A tetrameric cluster composed entirely of (aryl)Sn units, [DMPSn]4 (DMP=2,6-dimesitylphenyl), has been prepared by reduction of [DMPSnCl]2 with a variety of reductants. This cluster was characterized in solution by multinuclear NMR spectroscopies, as well as in the solid-state by single crystal X-ray diffraction analysis. This species is stereochemically nonrigid in solution and possesses a cyclo-Sn4 core whose DMP substituents are equivalent at higher temperatures. The solid-state molecular structure is remarkably unsymmetrical and possesses a nearly planar cyclo-Sn4 core. The DMP substituents are arranged such that three are approximately coplanar, while one is nearly perpendicular to the cyclo-Sn4 core. Density functional theory calculations for a [PhSn]4 model system show that this distorted geometry about the cyclo-Sn4 core maximizes σ-bonding between the Sn centers in a manner reminiscent of trans-bent bonding in the heavy group 14 analogues of alkenes and alkynes.