Synthesis, X-ray diffraction structure, and redox chemistry of Re2(CO)8(bmf)

The reaction between Re-2(CO)(8)(mu-H)(mu-eta(1),eta(2)-CH=CHBu) (1) and the diphosphine ligand 3,4-bis(diphenylphosphino)-5-methoxy-2(5H)-furanone (bmf) proceeds at room temperature to give the dirhenium compound Re-2(CO)(8)(bmf) (2) as the sole observable product in good yield after 24 h. Re-2(CO)(8)(bmf) has been isolated and characterized in solution by IR and NMR (P-31 and H-1) spectroscopies, and the molecular structure of Re-2(CO)(8)(bmf) has been determined by X-ray crystallography. The solid-state structure of 2 reveals a bridging bmf ligand that spans the Re=Re vector and whose Re(CO)(4) moieties exhibit a staggered rotational geometry about the equatorial phosphine and CO groups. The redox properties of Re-2(CO)(8)(bmf) have been explored by cyclic voltammetry in CH2Cl2, where an irreversible oxidation was found at E-p(a) = 1.04 V, along with a reversible, one-electron reduction at E-1/2 = -1.23 V. These redox couples have been assigned to the oxidation of the Re=Re sigma bond and electron occupation of the pi(*) system associated with the furanone ring of Re-2(CO)(8)(bmf), respectively. The electrochemical behavior is discussed relative to the nature of the HOMO and LUMO levels in the model compound Re-2(CO)(8)(H-4-bmf), whose HOMO and LUMO compositions have been established by extended Huckel MO calculations. The electrochemistry and MO data from Re-2(CO)(8)(bmf) are contrasted with the structurally similar dirhenium compound Re-2(CO)(8)(bma).