Influence of Copper Oxidation State on the Bonding and Electronic Structure of Cobalt-Copper Complexes

Heterobimetallic complexes that pair cobalt and copper were synthesized and characterized by a suite of physical methods, including X-ray diffraction, X-ray anomalous scattering, cyclic voltammetry, magnetometry, electronic absorption spectroscopy, electron paramagnetic resonance, and quantum chemical methods. Both Cu(II) and Cu(I) reagents were independently added to a Co(II) metalloligand to provide (py₃tren)CoCuCl (1-Cl) and (py₃tren)CoCu(CH₃CN) (2-CH₃CN), respectively, where py₃tren is the triply deprotonated form of N,N,N-tris(2-(2-pyridylamino)ethyl)amine. Complex 2-CH₃CN can lose the acetonitrile ligand to generate a coordination polymer consistent with the formula "(py₃tren)CoCu" (2). One-electron chemical oxidation of 2-CH₃CN with AgOTf generated (py₃tren)CoCuOTf (1-OTf). The Cu(II)/Cu(I) redox couple for 1-OTf and 2-CH₃CN is reversible at -0.56 and -0.33 V vs Fc⁺/Fc, respectively. The copper oxidation state impacts the electronic structure of the heterobimetallic core, as well as the nature of the Co-Cu interaction. Quantum chemical calculations showed modest electron delocalization in the (CoCu)⁺⁴ state via a Co-Cu σ bond that is weakened by partial population of the Co-Cu σ antibonding orbital. By contrast, no covalent Co-Cu bonding is predicted for the (CoCu)⁺³ analogue, and the d-electrons are fully localized at individual metals.