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RESEARCH PRODUCT

Structural Rearrangements in Triple-Decker-Like Complexes with Mixed Group 15/16 Ligands: Synthesis and Characterization of the Redox Couple [Cp2*Fe2As2Se2]/[Cp2*Fe2As2Se2]+ (Cp*=C5Me5)

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The reaction of As4Se4 with stoichiometric amounts of [Cp*Fe2(CO)4] (Cp* = C5Me5) in boiling toluene forms [Cp2*Fe2As2Se2] (1) in good yield. X-ray crystallography shows 1 to have a triple-decker structure which comprises a tetraatomic mu,eta4:4-As2Se2 ligand. Density functional theory (DFT) and extended Huckel molecular orbital (EHMO) calculations confirm that the As2Se2 ligand behaves as a four-electron pi donor. Oxidation of 1 with equimolar amounts of [(C5H5)2Fe]PF6, Br2 and I2, respectively, gave compounds 2-4. According to X-ray crystallographic investigations that were carried out on 2 and 4, the oxidation state has a considerable influence on the structure of the Fe2As2Se2 core: significant shortening of the Fe-Fe distance (deltad(Fe-Fe)> 0.3 A) and weakening of the As-As bond length ((deltad(As-As) > 0.3 A) suggests the formal presence of two diatomic AsSe ligands and a Fe-Fe bond. DFT and EHMO calculations confirm that an electron is removed from an occupied Fe-Fe orbital of antibonding character during oxidation. All molecular orbitals lower their energies upon oxidation, but the energy drop is relatively small for those involving the As-As bond. An additional structural feature in 4 consists of an electronic interaction of the iodide with both As atoms which suggests a formally neutral ion pair. Electrochemical studies confirm that the oxidation of 1 is a reversible one-electron process with E(1/2)= +0.07 V (in THF). These studies also reveal that 4 dissociates in polar solvents, such as THF, into [1]+ and I-, which is followed by transformation into 1 and I3.