Excited-state exchange coupling in bent Mn(III)-O-Mn(III) complexes:: Dominance of the π/σ superexchange pathway and its possible contributions to the reactivities of binuclear metalloproteins

Spectroscopic and computational studies of the mono-ore-bridged complex [Mn(III)(2)O(OAc)(2)(Me(3)tacn)(2)](2+) (mu-O,Me(3)tacn, where Me(3)tacn denotes 1,4,7-trimethyl-1,4,7-triazacyclononane) are presented and discussed. The polarized single-crystal absorption spectra exhibit marked changes with increasing temperature between 10 and 300 K, in particular a significant red-shift and a broadening of the dominant feature in the visible spectral region. These data serve as the basis for evaluating density functional theory calculations to obtain quantitative molecular orbital descriptions of the dominant Mn-O-Mn superexchange pathways in mu-O,Me(3)tacn. Both the spectroscopic and computational data indicate that the mixed pi/sigma Mn(xz)-O-Mn(z(2)) superexchange pathway (where x and z are in the Mn-O-Mn plane and z is oriented along the Mn-O vector) produces the key contribution to exchange coupling. The weak ferromagnetic coupling in the ground state (J = +9 cm(-1), H = -2JS(1)S(2)) results from a near cancellation of ferromagnetic and antiferromagnetic contributions. Upon d --> d excitation ferromagnetic pathways are converted into antiferromagnetic pathways, leading to sizable antiferromagnetic exchange interactions in the ligand-field excited states. This gives rise to the observed red-shifts of the corresponding absorption bands with increasing temperature (i.e., population of thermally accessible ground-state spin sublevels with S < 4). To obtain deeper insight into the origin of the ferromagnetic ground-state coupling a valence-bond configuration interaction model is modified to include metal-to-metal charge-transfer excited states involving the unoccupied Mn d orbitals. It is shown that the contribution to J from a superexchange pathway can be used to estimate the value of the corresponding electronic coupling matrix element H-AB for electron transfer. Possible implications of our results for the binuclear metalloproteins manganese catalase, hemerythrin, and ribonucleotide reductase are discussed.