Relation of Structural Changes to Electron-Transfer Parameters in Fulvalenediyl Dirhodium Complexes Demonstrating Quasi-Reversible Two-Electron Voltammetry^†

The oxidations of two fulvalenediyl (Fv) dirhodium complexes [(Fv)Rh2(CO)4, 1, and (Fv)Rh2(CO)2(μ-dppm), 2] have been studied by voltammetry, coulometry, IR spectroelectrochemistry, UV−VIS spectroscopy, and X-ray crystallography. Both complexes undergo net two-electron oxidations to dications having a Rh−Rh bond. The M−M bond distances are 2.837(1) Å in 22+ and 4.511(1) Å in 2. Quasi-reversible cyclic voltammetric responses were exploited over a range of sweep rates to give CV curves successfully fit to an EE mechanism by digital simulations: for 1, E°‘1 = −0.10 V (vs Fc), ks1 = 0.013 cm s-1, (1 − α1) = 0.66, E°‘2 = −0.245 V, ks2 = 0.044 cm s-1, α2 = 0.5; for 2, E°‘1 = −0.76 V, ks1 = 0.035 V, (1 − α1) = 0.75, E°‘2 = −0.776 V, ks2 > 0.2 cm s-1, α2 = 0.5. Complementary results were obtained for the reduction of the dications 12+ or 22+. The effect of the bridging fulvalenediyl ligand is to retard the oxidation process by an amount consistent with the calculated barrier to rotation around the C−C bond of the η5,η5‘-C10H8 ligand. In both complexes the 0/1+ redox process is slower than the 1+/2+ process. The electron transfer rates are consistent with a progressive increase in the Rh−Rh bond order in the 0/1+/2+ charged complexes rather than full M−M bond formation and cleavage in a single step.