Redox reactions between phosphines (R3P; R = nBu, Ph) or carbene (iPr2IM) and chalcogen tetrahalides ChX4 (iPr2IM=2,5-diisopropylimidazole-2-ylidene; Ch = Se, Te; X = Cl, Br)

The reactions between chalcogen tetrahalides (ChX(4); Ch = Se, Te; X = Cl, Br) and the neutral donors (Bu3P)-Bu-n, Ph3P, or the N-heterocyclic carbene, 2,5-diisopropylimidazole-2-ylidene ((Pr2IM)-Pr-i), have been investigated. In cases involving a phosphine, the chemistry can be understood in terms of a succession of two-electron redox reactions, resulting in reduction of the chalcogen center (e.g., Se(IV) --> Se(II)) and the oxidation of phosphorus to the [R3P-X] cation (P(III) --> P(V)). The stepwise reduction of Se(IV) --> Se(II) --> Se(0) --> Se(-II) occurs upon the successive addition of stoichiometric equivalents of Ph3P to SeCl4, which can readily be monitored by P-31{H-1} NMR spectroscopy. In the case of reacting SeX4 With (Pr2IM)-Pr-i, a similar two-electron reduction of the chalcogen is observed and there is the concomitant production of a haloimidazolium hexahaloselenate salt. The products have been comprehensively characterized, and the solid-state structures of [R3PX][SeX3] (9), [Ph3PCl](2)[TeCl6] (10), (Pr2IM)-Pr-i-SeX2 (11), and [(Pr2IM)-Pr-i-Cl](2)[SeCl6] (12) have been determined by X-ray diffraction analysis. These data all support two electron redox reactions and can be considered in terms of the formal reductive elimination of X-2, which is sequestered by the Lewis base.