Spontaneous generation of stable pnictinyl radicals from "jack-in-the-box" dipnictines: A solid-state, gas-phase, and theoretical investigation of the origins of steric stabilization

The molecular structures of the stable phosphinyl and arsinyl radicals, (.)PnR(2) [Pn = P (2): As (4); R = CH(SiMe3)(2)], have been determined by gas-phase electron diffraction (GED) in conjunction with ab initio molecular orbital calculations. The X-ray crystal structures of the corresponding dipnictines, the "dimers", R(2)PnPnR(2) [Pn = P (1), As (3)], and the chloro derivatives R(2)PnCl [Pn = P (5), As (6)] have also been determined. Collectively, these structural investigations demonstrate that large distortions of the ligands attached to Pn occur when the pnictinyl radicals unite to form the corresponding dipnictine dimers. Principally, it is the shape and flexibility of the CH(SiMe3)2 ligands; that permit the formation of the P-P and As-As bonds in 1 and 3, respectively. However, theoretical studies indicate that in the process of pnictinyl radical dimerization to form 1 and 3, both molecules accumulate substantial amounts of potential energy and are thus primed to spring apart upon release from the solid state by melting, dissolution, or evaporation. The insights gleaned from these unusual systems have permitted a deeper understanding of the functioning of sterically demanding substituents.