Strong interfullerene electronic communication in a bisfullerene-hexarhodium sandwich complex

Reaction of Rh-6(CO)(12)(dPPM)(2) (dppm = 1,2-bis(diphenylphosphino) methane) with 1.4 equiv. of C-60 in chlorobenzene at 120 degreesC affords a face-capping C-60 derivative Rh-6(CO)(9)(dPPM)(2)(mu(3)-eta(2),eta(2),eta(2)-C-60) (1) in 73% yield. Treatment of 1 with excess CNR (10 equiv., R = CH2C6H5) at 80 degreesC provides a bisbenzylisocyanicle-substituted compound Rh-6(CO)(7)(dppm)(2)(CNR)(2)(mu(3)-eta(2),eta(2),eta(2)-C-60) (2) in 59% yield. Reaction of 1 with excess C-60 (4 equiv.) in refluxing chlorobenzene followed by treatment with 1 equiv. of CNR at room temperature gives a bisfullerene sandwich complex Rh-6(CO)(5)(dppm)(2)(CNR)(mu(3)-eta(2),eta(2),eta(2)-C-60)(2) (3) in 31% yield. Compounds 1, 2, and 3 have been characterized by spectroscopic and microanalytical methods as well as by X-ray crystallographic studies. Electrochemical properties of 1, 2, and 3 have been examined by cyclic voltammetry. The cyclic voltammograms (CVs) of 1 and 2 show two reversible one-electron redox waves, a reversible one-step two-electron redox wave, and a reversible one-electron redox wave, respectively, within the solvent cutoff window. This observation suggests that compounds 1 and 2 undergo similar C-60-localized electrochemical pathways up to 1(5-) and 2(5-). Each redox wave of 2 appears at more negative potentials compared to that of 1 because of the donor effect of the benzylisocyanide ligand. The CV of compound 3 reveals six reversible well-separated redox waves due to strong interfullerene electronic communication via the Rh-6 metal cluster bridge. The electrochemical properties of 1, 2, and 3 have been rationalized by molecular orbital calculations using the density functional theory (DFT) method. In particular, the molecular orbital (MO) calculation reveals significant contribution of the metal cluster center to the unoccupied molecular orbitals in 3, which is consistent with the experimental result of strong interfullerene electronic communication via the Rh-6 metal cluster spacer.