Reduction of phosphine sulfides and selenides by samarium(

Abstract

The reductive action of the bulky SmII formamidinate complex [Sm(dippForm)2(thf)2] (dippForm− = HC(Ndipp)2 −, dipp = 2,6-diisopropylphenyl; thf = tetrahydrofuran) (1) on a series of phosphine chalcogenides (R3PE = Ph3PO, Ph3PS, Ph3PSe, n Bu3PS, n Bu3PSe) as well as Ph3AsS, has been investigated. The reactions are fast for the phosphine derivatives with E = Se and for Ph3AsS, and slow for the phosphine sulfides. They lead to mixtures of binuclear mono- and dichalcogenide complexes [(Sm(dippForm)2)2(µ-En )] (E = S (2), Se (3); n = 1, 2). The ratio of species with (µ-E) or (µ-η2:η2-E2) bridges depends on the nature of the chalcogenide reactant and its concentration, but even under its local excess the formation of the monochalcogenide is preferential. Both types of species form isostructural solid solutions. The dense outer packing of dippForm ligands in the complexes 2 and 3, leaving enough free space in the centre of the molecule, is thought to be the main reason for the geometrical similarity of mono- and dichalcogenides and for their ready co-crystallization. A reaction scheme is proposed, involving the coordination of the starting chalcogenide to the SmII centre with successive formation of E-centered transient radical species [Sm(dippForm)2(E˙)]. Contrary to the phosphine/arsine sulfides and selenides, the product of phosphine oxide coordination, [Sm(dippForm)2(OPPh3)] (4), was shown to be stable, while a similar complex with two phosphine oxide ligands, [Sm(dippForm)2(OPPh3)2] (5), was fortuitously crystallized at lower temperatures.