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) (3; Cp = cyclopentadienyl, dppe = 1,2-bis(diphenylphosphino)ethane) has been prepared and the molecular structure determined. The cyclic voltammogram of 3 is characterized by three reversible one-electron events with a large potential difference between the two first waves (ΔE° = 0.44 V), indicating the large thermodynamic stability of the MV (mixed-valence) 3(PF6), which can be considered as a class III MV complex. The complex 3(PF6) was quantitatively prepared by treatment of 3 with 1 equiv of [FeCp2](PF6) at −78 °C and characterized by EPR spectroscopy. Above −10 °C, solutions of 3(PF6) provide the asymmetric tetranuclear complex {cyclo-C([Ru])C(CCCC[Ru])C(CC[Ru])C(CC[Ru])}(PF6)2 (4(PF6)2; [Ru] = Cp(dppe)Ru) which was formed regiospecifically and isolated in 92% yield. Thus, despite the thermodynamic stability of the monocation radical 3(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 (Cα + Cγ) radical coupling.