Engineering cobalt-copper complexes
Researchers using the ChemMatCARS 15-ID-B beamline at the APS have isolated and characterized the heterobimetallic (CoCu)4+ and (CoCu)3+ complexes. The researchers are seeking to engineer complexes with multiple transition metals for promoting multielectron reduction of small molecules
The copper II/I redox couple is prevalent in biology and underlies a wide range of enzymatic activities including electron transfer, O2 transport, substrate oxidation, and respiration. Multielectron cooperativity may also arise from metal–metal bonded complexes, where the electronic structure may be delocalized across multiple metals.
Hetero-bimetallic complexes pairing copper and cobalt were isolated in two redox states, Cu(II)Co(II) and Cu(I)Co(II). The intermetal distances in both redox states are less than 2.5 Å, suggesting the presence of a metal−metal interaction. The nature of the interaction is different and depends on the copper oxidation state.
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.
Reed J. Eisenhart, Rebecca K. Carlson, Laura J. Clouston, Victor G. YoungJr., Yu-Sheng Chen, Eckhard Bill, Laura Gagliardi and Connie C. Lu, “Influence of Copper Oxidation State on the Bonding and Electronic Structure of Cobalt–Copper Complexes,” Inorganic Chemistry, Article ASAP, DOI: 10.1021/acs.inorgchem.5b01950, Published Online November 9, 2015.