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.

Keywords:

• Cobalt coordination chemistry
• Co-diimine complexes
• phenanthroline complexes
• redox chemistry
• crystallography

Acknowledgements

We thank Dr David Adam (University of Glasgow) for assistance with collecting the NMR spectra. MDS thanks the Royal Society for a University Research Fellowship (UF150104) and the University of Glasgow for funding. We thank Stephen Sproules and Jake McGuire (both University of Glasgow) for useful discussions. The data which underpin this work are available via Enlighten, the University of Glasgow’s open access online data repository, at https://doi.org/10.5525/gla.researchdata.449. CCDC 1556230 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.