Axial Ligand and Solvent Effects on the O-O Bond Activation in Acylperoxo Complexes of [(Salen)MnIIIL]: MnIV versus MnV Oxo Species

摘要

The role of the axial ligand L ("push" effect) in the O-O bond cleavage in Mn-Salen based trans-acylperoxo complexes, [(Salen)Mn~(III)(RCO_3)L] (2) and [(Salen)Mn~(IV)(RCO_3)L]~+ (3), was investigated by the density functional (B3LYP) method for L = NH_3 and HCO_2~-. It was shown that coordination of an axial ligand L to the trans isomer of 2, 2-trans, substantially reduces the triplet-quintet energy gap from +12.4 to +1.8 and -0.3 kcal/mol, and consequently decreases the overall barrier for the O-O bond cleavage producing 2-trans-L-oxo species from 19.5 to 10.2 and 10.9 kcal/mol for L = none, HCO_2~-, and NH3, respectively. Because the O-O bond cleavage in 2-trans-L becomes slightly more favorable than that in the isomeric 2-cis acylperoxo complexes producing 2-cis-MnV-oxo species [Khavrutskii, I. V.; Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc. 2003, 125, 13879-13889], the axial ligand may be considered as a switch between cis- and trans-oxo species formation. The product 2-trans-L-oxo unlike 2-cis-MnV-oxo species is chameleon in nature and can be viewed either as MnV-oxo species associated with RCO_2~- (heterolytic cleavage) or as MnIV-oxo species associated with RCO_2 radical (homolytic cleavage). The mode of dissociation of this species into corresponding heterolytic or homolytic fragments is controlled by the nature of the axial ligand and the solvent employed.