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

Cobalt-diimine complexes have been used as structural and redox-active elements in a number of supramolecular assemblies. Frequently, it is necessary to functionalise the diimine ligand in order to incorporate it into a larger ensemble, and this can have a dramatic effect on 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), and show that in solutions containing Co(II) nitrate and neocuproine, the dominant species that forms is the mono-diimine complex [Co(neocuproine)(NO3)(CH3CN)2]+. The mono-neocuproine Co(II) complex is resistant to oxidation, either electrochemically or 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]+, we isolate (and determine the structure of) crystals of formula [Co(neocuproine)2(NO3)]+·[Co(neocuproine)(NO3)3]− for the first time. We believe that this work will help to guide the development of Co-diimine supramolecular assemblies by highlighting the extent to which substituents close to the N-donor atoms can affect which species form in solution, and their likely redox activity.