Preparation, spectroscopic properties, and dynamic behavior of a μ-oxo dirhenium(III) complex [Re2(μ-O)Cl2(tpa)2](PF6)2 (tpa=tris(2-pyridylmethyl)amine)

An oxo-bridged dirhenium(III,III) complex of tris(2-pyridylmethyl)amine (tpa) and its one-electron oxidized (III,IV) species, [Re2(μ-O)Cl2(tpa)2]3+,4+ have been prepared. They are new members to a series of rhenium tpa complexes in various oxidation states. X-ray structural determination of the Re2(III,IV) complex revealed practically linear ReORe bridge (178(1)°) with short ReO distances of 1.85(2) Å indicative of some multiple bonded character. The 1H NMR spectrum disclosed the relatively slow rotation around the ReORe axis in CH3CN solution in the timescale of 1H NMR. The complex undergoes two consecutive reversible one electron oxidations Re2(III,III)/(III,IV) and Re2(III,IV)/(IV,IV) at E1/2=0.23 and 0.90 V vs Ag/AgCl, respectively. Strong visible absorption bands are observed for the Re2(III,III) species at 448 (ε=31 160) and 563 nm (19 550) which are tentatively assigned to MLCT transitions. A unique oxidation product, Re2(μ-O)(O)2Cl2(bpaO2)2 (bpaO2H=1,3-bis(2-pyridyl)-2-aza-propanedione) has also been isolated and its crystal structure was determined. The complex is dirhenium(V) species with linear OReOReO moiety. Ligand tpa has been oxidized to ketone with simultaneous dissociation of one of the 2-pyridylmethyl arms.