Molecular and Electronic Structures of a Series of Dinuclear CoII Complexes Varied by Exogeneous Ligands: Influence of π‐Bonding on Redox Potentials

Abstract

AbstractFour dinuclear CoII complexes have been synthesized and structurally characterized with the dinucleating ligand susan (susan=4,7‐dimethyl‐1,1,10,10‐tetra(2‐pyridylmethyl)‐1,4,7,10‐tetraazadecane) varying in the exogeneous ligands: [(susan){CoII(CH3CN)2}2](PF6)4, [(susan){CoIICl}2](ClO4)2, [(susan){CoIIBr}2](ClO4)2, and [(susan){CoII(μ‐OH)CoII}](ClO4)3. The CoII ions are six‐coordinate with CH3CN ligands and five‐coordinate with anionic ligands. The electronic absorption spectra reflect the differences in the electronic structures, not only between the six‐ and five‐coordinate complexes, but also between the five‐coordinate complexes. These variations are also reflected in the magnetic properties with the highest orbital angular momentum contribution in six‐coordinate [(susan){CoII(CH3CN)2}2](PF6)4. The lower symmetry in the five‐coordinate complexes reduces the orbital angular momentum contribution. The hydroxo‐bridged complex [(susan){CoII(μ‐OH)CoII}](ClO4)3 exhibits an additional antiferromagnetic interaction. The electrochemical characterization reveals that the π‐acceptor ligand CH3CN facilitates not only reduction from CoII to CoI but also oxidation to CoIII, while the π‐donor ligands Br−, Cl−, and μ‐OH− impede both reduction to CoI and oxidation to CoIII. [(susan){CoII(CH3CN)2}2](PF6)4 and [(susan){CoIICl}2](ClO4)2 show electrocatalytic proton reduction at a potential of ≈−1.9 V vs Fc+/Fc that is associated with a ligand‐centered reduction.