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

The reaction between Re₂(CO)₈(μ-H)(μ-η¹,η²-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 Re₂(CO)₈(bmf) (2) as the sole observable product in good yield after 24 h. Re₂(CO)₈(bmf) has been isolated and characterized in solution by IR and NMR (³¹P and ¹H) spectroscopies, and the molecular structure of Re₂(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 Re₂(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 σ bond and electron occupation of the π^∗ system associated with the furanone ring of Re₂(CO)₈(bmf), respectively. The electrochemical behavior is discussed relative to the nature of the HOMO and LUMO levels in the model compound Re₂(CO)₈(H₄-bmf), whose HOMO and LUMO compositions have been established by extended Hückel MO calculations. The electrochemistry and MO data from Re₂(CO)₈(bmf) are contrasted with the structurally similar dirhenium compound Re₂(CO)₈(bma).