Electrochemical Reduction of [Ni(Mebpy)3]2+. Elucidation of the Redox Mechanism by Cyclic Voltammetry and Steady-State Voltammetry in Low Ionic Strength Solutions.

Bipyridine complexes of Ni are used as catalysts in a variety of reductive transformations. Here, the electroreduction of [Ni(Mebpy)₃]²⁺ (Mebpy = 4,4’-dimethyl-2,2’-bipyridine) in dimethylformamide is reported, with the aim of determining the redox mechanism and oxidation states of products formed under well-controlled electrochemical conditions. Results from cyclic voltammetry, steady-state voltammetry (SSV) and chronoamperometry demonstrate that [Ni(Mebpy)₃]²⁺ undergoes two sequential 1e reductions at closely separated potentials (E^0’₁ = -1.06 ± 0.01 V and E^0’₂ = -1.15 ± 0.01 V vs Ag/AgCl (3.4 M KCl)). Homogeneous comproportionation to generate [Ni(Mebpy)₃]⁺ is demonstrated in SSV experiments in low ionic strength solutions. The comproportionation rate constant is determined to be > 10⁶ M^-1s^-1, consistent with rapid outer-sphere electron transfer. Consequentially, on voltammetric time scales, the 2e reduction of [Ni(Mebpy)₃]²⁺ results in formation of [Ni(Mebpy)₃]¹⁺ as the predominant species released into bulk solution. We also demonstrate that [Ni(Mebpy)₃]⁰ slowly loses a Mebpy ligand (~10 s^-1).