High Variety Of Coordination Modes Of Pi-Conjugated Phospholes In Dinuclear Rhenium Carbonyls. Fluxional Behavior Of Sigma,Pi-Complexes

A variety of dirhenium carbonyl complexes containing pi-conjugated phosphole derivatives were obtained from reaction of [Re-2(CO)(8)(CH3CN)(2)(with each of the following phospholes: 2,5-bis(2-thienyl)-1-phenylphosphole (btpp), 2,5-bis(2-pyridyl)-1-phenylphosphole (bpypp) and 1,2,5-triphenylphosphole (tpp). The pi-conjugated phospholes were found to behave as two-, four- or six-electron donor ligands via sigma or sigma-pi interactions with the metal centers, presenting bridging or chelating coordination modes as determined by spectroscopic methods and single crystal X-ray diffraction analysis. Metal-metal bond cleavage was evidenced when btpp was used, leading to a mono-substituted mononuclear complex. Variable-temperature H-1 NMR studies for sigma,pi-complexes showed a fluxional behavior due to the restricted rotation around the P-C and C-C bonds of the 1,2,5-trisubstituted phosphole ring.