Strong Interfullerene Electronic Communication in a Bisfullerene−Hexarhodium Sandwich Complex

Reaction of Rh6(CO)12(dppm)2 (dppm = 1,2-bis(diphenylphosphino)methane) with 1.4 equiv. of
C60 in chlorobenzene at 120 °C affords a face-capping C60 derivative Rh6(CO)9(dppm)2(μ3-η2,η2,η2-C60) (1)
in 73% yield. Treatment of 1 with excess CNR (10 equiv., R = CH2C6H5) at 80 °C provides a
bisbenzylisocyanide-substituted compound Rh6(CO)7(dppm)2(CNR)2(μ3-η2,η2,η2-C60) (2) in 59% yield.
Reaction of 1 with excess C60 (4 equiv.) in refluxing chlorobenzene followed by treatment with 1 equiv. of
CNR at room temperature gives a bisfullerene sandwich complex Rh6(CO)5(dppm)2(CNR)(μ3-η2,η2,η2-C60)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 C60-localized electrochemical pathways up to 15- and 25-. 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 Rh6 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 Rh6 metal cluster spacer.