Mixed Chloride Phosphine Complexes of Dirhenium Cores. 1. New Reactions and Unprecedented Structures Involving Trimethylphosphine

Abstract

The unusual 1,2,7-isomer of Re2Cl5(PMe3)3 (1) has been used as a starting material to prepare previously unknown dirhenium complexes. One-electron reduction of 1 by cobaltocene followed by nonredox substitution of the resulting anionic species with PMe3 led to the formation of a triply bonded 1,2,7,8-Re2Cl4(PMe3)4 (2). In the crystal structure of 2, phosphine ligands on both metal centers exhibit a cis arrangement with a P−Re−P angle of 93.3° in contrast to the well-known type of 1,3,6,8-isomers with a trans arrangement of the monodentate phosphines connected to each metal atom. Complex 2 represents the first example of an isomer of this type in the large M2X4(PR3)4 class of compounds (M = Re, Tc, W, Mo; X = Cl, Br, I; PR3 = monodentate phosphine). The one-electron oxidation product of 1 cocrystallized with one molecule of tetrabutylammonium chloride afforded Re2Cl6(PMe3)2 (3), for which all previous synthetic attempts had failed. This quadruply bonded complex exhibits an unusual 3-fold disorder of the Re2 unit with equal populations for all three orientations. We also report that when the “classic” reaction of octachlorodirhenate anion, Re2Cl82-, with trimethylphosphine is carried out in benzene at room temperature the reduction processes do not occur and the product is a novel paramagnetic complex, Re2Cl6(PMe3)4 (4), which does not have a metal−metal bond (the Re−Re separation is 3.8476(4) Å). Another interesting feature of compound 4 is that the PMe3 ligands have a cis disposition at each rhenium center and are located in the same plane as the metal atoms and bridging chlorine ligands. For such a ligand arrangement the molecule of 4 is the only example of a nonmetal−metal-bonded dinuclear compound with monodentate phosphine ligands.