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.