PHOTOCHEMICAL GENERATION OF A REACTIVE RHENIUM(III) OXO COMPLEX AND ITS CURIOUS MODE OF CLEAVAGE OF DIOXYGEN

The rhenium(V) oxo oxalate complex (HBpz3)ReO(C2O4) (HBPZ3 = hydridotris(1-pyrazolyl)borato) has been synthesized in three steps from potassium perrhenate. It has been characterized spectroscopically and its molecular structure determined by X-ray crystallography. When irradiated with UV light, the oxo oxalate complex undergoes an internal redox reaction, predominantly losing carbon dioxide and generating a reactive rhenium complex. The characterization of the transient photoproduct as the rhenium(III) oxo complex (HBPZ3)Re(O) is inferred from its reactions with trapping reagents: for example, photolysis in the presence of phenanthrenequinone gives a rhenium-(V) oxo catecholate complex in good yield. (HBpz3)Re(O) also reacts with O2, yielding the rhenium(VII) complex (HBpz3)ReO3, formally the result of four-electron oxidation. Labeling studies show that only one oxygen atom in the product comes from O2, with the second deriving from an oxalate ligand. That unimolecular four-electron reduction of oxygen does not occur readily in this system, despite its great exothermicity, may be due to a general symmetry-imposed barrier to cleavage of O2 at a single metal center. Crystal data for (HBpz3)ReO(C2O4).0.5C6H6: a = 8.082 (2) angstrom, b = 9.125 (3) angstrom, c = 13.219 (3) angstrom, alpha = 84.03 (2)-degrees, beta = 74.26 (2)-degrees, gamma = 72.47 (2)-degrees, triclinic, P1BAR, Z = 2.