Straightforward Access to Tetrametallic Complexes with a Square Array by Oxidative Dimerization of Organometallic Wires

The bimetallic ruthenium complex {Cp­(dppe)­Ru}₂(μ-CCCCCC) (<b>3</b>; Cp = cyclopentadienyl, dppe = 1,2-bis­(diphenylphosphino)­ethane) has been prepared and the molecular structure determined. The cyclic voltammogram of <b>3</b> is characterized by three reversible one-electron events with a large potential difference between the two first waves (Δ<i>E</i>° = 0.44 V), indicating the large thermodynamic stability of the MV (mixed-valence) <b>3</b>(PF₆), which can be considered as a class III MV complex. The complex <b>3</b>(PF₆) was quantitatively prepared by treatment of <b>3</b> with 1 equiv of [FeCp₂]­(PF₆) at −78 °C and characterized by EPR spectroscopy. Above −10 °C, solutions of <b>3</b>(PF₆) provide the asymmetric tetranuclear complex {cyclo-C­([Ru])­C­(CCCC­[Ru])­C­(CC­[Ru])­C­(CC­[Ru])}­(PF₆)₂ (<b>4</b>(PF₆)₂; [Ru] = Cp­(dppe)­Ru) which was formed regiospecifically and isolated in 92% yield. Thus, despite the thermodynamic stability of the monocation radical <b>3</b>(PF₆) as evidenced by the electrochemical data, and the extensively delocalized electronic structure, a slow dimerization reaction takes place, affording a stable, tetranuclear complex. The new compound was characterized by a single-crystal X-ray diffraction study, cyclic voltammetry, and multinuclear NMR, IR, UV–vis, and near-IR spectroscopy, and the data were analyzed with the support of quantum chemical investigations at the DFT level of theory. The regiospecificity of the dimerization reaction is controlled by a balance of steric and electronic factors, which favors intermolecular (C_α + C_γ) radical coupling