The role of solvent in the stepwise electrochemical oxidation of nickelocene to the nickelocenium dication

The anodic electrochemistry of nickelocene, 1, first probed in ethanolic media by Cotton et al. in 1954, has been examined in 12 different nonaqueous solvents or solvent mixtures using [NBu4][TFAB] (TFAB = [B(C6F5)4]−) as the supporting electrolyte. Two quasi-Nernstian couples were observed for 1/1+ and 1+/12+ with a potential separation (ΔE1/2) that is consistent with a modified Born model for low-donor solvents varying in dielectric constant (ε) from 5.6 (chlorobenzene) to 37.3 (nitromethane). In stronger–donor solvents such as acetonitrile, benzonitrile, and THF, the oxidation of nickelocenium ion to the dication 12+ is coupled to chemical follow-up reactions, reversible in the cases of the nitriles, which are likely to involve solvent coordination to the nickel center. The observed complexity of the anodic behavior shows that much of the previous literature on the second oxidation of nickelocene has given oversimplified conclusions. Variations of up to 600 mV in ΔE1/2 for nickelocene can be achieved by solvent variations when using the weakly-coordinating electrolyte anion [B(C6F5)4]−.