Structural rearrangements in triple-decker-like complexes with mixed group15/16 ligands: Synthesis and characterization of the redox couple [Cp-2*Fe2As2Se2]/[Cp-2*Fe2As2Se2](+) (Cp*=C5Me5)

The reaction of As4Se4 with stoichiometric amounts of [Cp-2*Fe-2(CO)(4)] (C p* = C5Me5) in boiling toluene forms [Cp-2*Fe2As2Se2] (1) in good yield, X- ray crystallography shows 1 to have a triple-decker structure which compris es a tetraatomic mu,eta (4:4)-As2Se2 ligand. Density functional theory (DFT ) and extended Huckel molecular orbital (EHMO) calculations confirm that th e As2Se2 ligand behaves as a four-electron pi donor. Oxidation of 1 with eq uimolar amounts of [(C5H5)(2)Fe]PF6, Br-2 and I-2, respectively, gave compo unds 2-4. According to X-ray crystallographic investigations that were carr ied 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 dista nce (Deltad(Fe-Fe)> 0.3 Angstrom) and weakening of the As-As bond length (( Deltad(As-As) > 0.3 Angstrom) suggests the formal presence of two diatomic AsSe ligands and a Fe-Fe bond. DFT and EHMO calculations confirm that an el ectron is removed from an occupied Fe-Fe orbital of antibonding character d uring oxidation, All molecular orbitals lower their energies upon oxidation , but the energy drop is relatively small for those involving the As-As bon d. An additional structural feature in 4 consists of an electronic interact ion of the iodide with both As atoms which suggests a formally neutral ion pair. Electrochemical studies confirm that the oxidation of 1 is a reversib le one-electron process with E-1/2 = +0.07 V (in THF). These studies also r eveal that 4 dissociates in polar solvents, such as THF into [1](+) and I-, which is followed by transformation into 1 and I-3.