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

Abstract

Spectroscopic and computational studies of the mono-oxo-bridged complex [Mn(III)2O(OAc)2(Me3tacn)2]2+ (μ-O,Me3tacn, where Me3tacn 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 μ-O,Me3tacn. Both the spectroscopic and computational data indicate that the mixed π/σ Mn(xz)−O−Mn(z2) 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 = −2JS1S2) results from a near cancellation of ferromagnetic and a...