Ground-State Electronic and Magnetic Properties of a mu_3-Oxo-Bridged Trinuclear Cu(II) Complex: Correlation to the Native Intermediate of the Multicopper Oxidases

摘要

The ground-state electronic and magnetic properties of one of the possible structures of the trinuclear Cu~(II) site in the native intermediate (NI) of the multicopper oxidases,the mu_3-oxo-bridged structure,are evaluated using the C_3-symmetric Cu_3~(II) complex,mu_3O.mu_3O is unique in that no ligand,other than the oxo,contributes to the exchange coupling.However,mu_3O has a ferromagnetic ground state,inconsistent with that of NI.Therefore,two perturbations have been considered: protonation of the mu_3-oxo ligand and relaxation of the mu_3-oxo ligand into the Cu_3 plane.Notably,when the oxo ligand is sufficiently close to the Cu_3 plane (<0.3 A),the ground state of mu_3O becomes antiferromagnetic and can be correlated to that of NI.In addition,the ferromagnetic ~4A ground state of mu_3O is found from variable-temperature EPR to undergo a zero-field splitting (ZFS) of 2D=-5.0 cm~(-1),which derives from the second-order anisotropic exchange.This allows evaluation of the sigma-to-pi excited-state exchange pathways and provides experimental evidence that the orbitally degenerate ~2E ground state of the antiferromagnetic mu_3O would also undergo a ZFS by the first-order antisymmetric exchange that has the same physical origin as the anisotropic exchange.The important contribution of the mu_3-oxo bridge to the ground-to-ground and ground-to-excited-state superexchange pathways that are responsible for the isotropic,antisymmetric,and anisotropic exchanges are discussed.