Probing the effects of steric bulk on the solution-phase behaviour and redox chemistry of cobalt-diimine complexes

Abstract

Cobalt-diimine complexes are important structural and redox-active elements in supramolecular assemblies. However, functionalisation of the diimine ligand adjacent to the N-donor atoms can affect dramatically the types of Co-diimine complexes that can form and their redox activity. Herein, we compare the solution phase and redox chemistry of Co(II) complexes with 1,10-phenanthroline, 5,5′-dimethyl-2,2′-bipyridine and 2,9-dimethyl-1,10-phenanthroline (neocuproine). In acetonitrile solutions containing Co(NO3)2 and neocuproine, the dominant species is the mono-diimine complex [Co(neocuproine)(NO3)(CH3CN)2]+. This complex cannot be oxidised, either electrochemically nor with iodine. We rationalise this behaviour by considering the steric constraints placed upon the metal centre by the bulky methyl substituents on the neocuproine ligand. Furthermore, from solutions of [Co(neocuproine)(NO3)(CH3CN)2]+, crystals of formula [Co(neocuproine)2(NO3)]+·[Co(neocuproine)(NO3)3]− can be obtained. We believe that this work will guide the development of Co-diimine supramolecular assemblies by highlighting the extent to which substituents close to the N-donor atoms affect which species form in solution, and their likely redox activity.