Theoretical models on the Cu ₂O ₂ torture track: Mechanistic implications for oxytyrosinase and small-molecule analogues

Accurately describing the relative energetics of alternative bis(μ-oxo) and μ-η ²:η ² peroxo isomers of Cu ₂O ₂ cores supported by 0, 2, 4, and 6 ammonia ligands is remarkably challenging for a wide variety of theoretical models, primarily owing to the difficulty of maintaining a balanced description of rapidly changing dynamical and nondynamical electron correlation effects and a varying degree of biradical character along the isomerization coordinate. The completely renormalized coupled-cluster level of theory including triple excitations and extrȩmely efficient pure density functional levels of theory quantitatively agree with one another and also agree qualitatively with experimental results for Cu ₂O ₂ cores supported by analogous but larger ligands. Standard coupled-cluster methods, such as CCSD(T), are in most cases considerably less accurate and exhibit poor convergence in predicted relative energies. Hybrid density functionals significantly underestimate the stability of the bis(μ-oxo) form, with the magnitude of the error being directly proportional to the percentage Hartree-Fock exchange in the functional. Single-root CASPT2 multireference second-order perturbation theory, by contrast, significantly overestimates the stability of bis(μ-oxo) isomers. Implications of these results for modeling the mechanism of C-H bond activation by supported Cu ₂O ₂ cores, like that found in the active site of oxytyrosinase, are discussed.