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.