Electronic Structure of a Low-Spin Heme/Cu Peroxide Complex: Spin-State and Spin-Topology Contributions to Reactivity

This study details the electronic structure of the heme peroxo copper adduct {[(F(8))Fe(DCHIm)]center dot O(2)center dot[Cu(AN)]}(+) (LS(AN)) in which O(2)(2-) bridges the metals in a mu-1,2 or end-on configuration. LS(AN) is generated by addition of coordinating base to the parent complex {[(F(8))Fe]-O(2)-[Cu(AN)]}(+) (HS(AN)) in which the O(2)(2-) bridges the metals in an mu-eta(2):eta(2) or side-on mode. In addition to the structural change of the O(2)(2-) bridging geometry, coordination of the base changes the spin state of the heme fragment (from S = 5/2 in HS(AN) to S = 1/2 in LS(AN)) that results in an antiferromagnetically coupled diamagnetic ground state in LS(AN). The strong ligand field of the porphyrin modulates the high-spin to low-spin effect on Fe-peroxo bonding relative to nonheme complexes, which is important in the O-O bond cleavage process. On the basis of DFT calculations, the pound state of LS(AN) is dependent on the Fe-O-O-Cu dihedral angle, wherein acute angles (