Mixed chloride phosphine complexes of dirhenium cores. 1. New reactions and unprecedented structures involving trimethylphosphine

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, Re₂Cl₈^2-, 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) °). 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.