The use of bulky ligands for the stabilization of group 15 radicals and cations

Abstract

A variety of bulky ligands have been utilized for the stabilization of group 15 radicals and cations. The molecular structures of the stable phosphinyl and arsinyl radicals, • PnR2 (Pn = P, As; 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", R2PnPnR2, and the corresponding chloro derivatives R2PnCl have also been determined. Collectively, these gas-phase and solid-state structural investigations demonstrate that large distortions of the ligands attached to phosphorous and arsenic occur when the pnictinyl radicals unite to form the corresponding dipnictine dimers. Theoretical studies indicate that in the process of pnictinyl radical dimerization, the 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 first example of group 15-containing “constrained geometry” complexes, [Me2Si(η-C5Me4)(N-t-Bu)]PnCl (Pn = P, As, Sb), have been prepared and isolated. In the case of the chlorophosphine, [Me2Si(η-C5Me4)(N-t-Bu)]PCl, an unprecedented shift of the µ-SiMe2 group from the ipso to the α-position on the Me4C5 ring occurs. Treatment of [Me2Si(η-C5Me4)(N-t-Bu)]PnCl (Pn = P, Sb) with AlCl3 results in chloride ion abstraction and formation of the pnictocenium salt, {[Me2Si(η-C5Me4)(N-t-Bu)]Pn}{AlCl4}. The structures of these salts were established by X-ray crystallography. The chloropnictines (η-C5Me5)2PnCl (Pn = As, Sb, Bi) have been synthesized and structurally characterized, and exhibit the expected periodic trend toward increased multihapto bonding with increasing atomic number. The arsenocenium and stibocenium tetrachloroaluminates salts were prepared by chloride anion abstraction from the precursor chlorides and the structures of the cations in these salts were found to be different than those in previously reported examples.