Geometric and electronic structure of the heme-peroxo-copper complex [(F8TPP)FeIII-(O22-)-CuII(TMPA)](CIO4)

The geometric and electronic structure of the untethered heme-peroxo-copper model complex [(F8TPP)Fe-III-(O-2(2-))-Cu-II(TMPA)](ClO4) (1) has been investigated using Cu and Fe K-edge EXAFS spectroscopy and density functional theory calculations in order to describe its geometric and electronic structure. The Fe and Cu K-edge EXAFS data were fit with a (CuFe)-Fe-... distance of similar to 3.72 angstrom. Spin-unrestricted DFT calculations for the S-T = 2 spin state were performed on [(P)Fe-III-(O-2(2-))-Cu-II(TMPA)](+) as a model of 1. The peroxo unit is bound end-on to the copper, and side-on to the high-spin iron, for an overall mu-eta(1): 772 coordination mode. The calculated (CuFe)-Fe-... distance is similar to 0.3 angstrom longer than that observed experimentally. Reoptimization of [(P)Fe-III-(O-2(2-))-Cu-II(TMPA)](+) with a 3.7 angstrom (CuFe)-Fe-... constrained distance results in a similar energy and structure that retains the overall mu-eta(1):eta(2)-peroxo coordination mode. The primary bonding interaction between the copper and the peroxide involves electron donation into the half-occupied Cu d(z2) orbital from the peroxide pi(*)(sigma) orbital. In the case of the Fe-III-peroxide eta(2) bond, the two major components arise from the donor interactions of the peroxide pi(*)(sigma) and pi(*)(v) orbitals with the Fe d(xz) and d(xy) orbitals, which give rise to sigma and delta bonds, respectively. The pi(*)(sigma) interaction with both the half-occupied d(z2) orbital on the copper (eta(1)) and the d(xz) orbital on the iron (eta(2)), provides an effective superexchange pathway for strong antiferromagnetic coupling between the metal centers.