Stereoelectronic Effects on Molecular Geometries and State-Energy Splittings of Ligated Monocopper Dioxygen Complexes

Cramer, C. J.; Gour, J. R.; Kinal, A.; Wloch, M.; Piecuch, P.; Moughal Shahi, A. R.; Gagliardi, L.
J. Phys. Chem. A 2008, 112, 3754.

The relative energies of side-on vs end-on binding of molecular oxygen to a supported Cu(I) species, and the singlet vs triplet nature of the ground electronic state, are sensitive to the nature of the supporting ligands, and in particular depend upon their geometric arrangement relative to the O₂ binding site. Highly correlated ab initio and density functional theory electronic structure calculations demonstrate that optimal overlap (and oxidative charge transfer) occurs for a side-on geometry, which is favored by ligands that raise the energy, thereby enhancing resonance, of the filled Cu d_xz orbital that hybridizes with the in-plane π* orbital of O₂. Conversely, ligands that raise the energy of the filled Cu d_z2 orbital foster a preference for end-on binding as this is the only mode that permits good overlap with the in-plane O₂ π*. Because overlap of Cu d_z2 with O₂ π* is reduced compared to overlap of Cu d_xz with the same O₂ orbital, resonance is also reduced leading to generally more stable triplet states relative to singlets in the end-on geometry compared to the side-on, where singlet ground states become more easily accessible once ligands are stronger donors. Biradical Cu(II)-O₂ superoxide character in the electronic structure of the supported complexes leads to significant challenges for accurate quantum chemical calculations that are best addressed by exploiting the spin-purified M06L local density functional, single-reference completely renormalized coupled-cluster theory, or multi-reference second-order perturbation theory, all of which provide predictions that are qualitatively and quantitatively consistent with one another.

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