Kinetic and Spectroscopic Investigation of Structural Influences on Hydrogen-atom Transfer Reactivity of N5-Ligated Oxomanganese(IV) Complexes

Abstract

High valent iron- and manganese-oxo intermediates perform important hydrogenatom transfer (HAT) reactions in both biological and synthetic systems. There has been extensive characterization of HAT reactivity of enzymatic and synthetic oxoiron(IV) species, and these complexes have been shown to cleave a wide range of C-H bonds. Although there is no evidence for this type of reactivity by manganese enzymes in nature, several synthetic oxomanganese(IV) species are also capable of activating C-H bonds. However, whereas oxoiron(IV) species have been extensively characterized, the factors that influence HAT reactivity of oxomanganese(IV) species is poorly understood. In an effort to further understand the structure-reactivity relationship of oxomanganese(IV) species, a series of novel oxomanganese(IV) species with subtle perturbations to the equatorial ligand field of the neutral, pentadentate supporting ligand were prepared. Extensive spectroscopic studies, including structural characterization by X-ray absorption spectroscopy (XAS), revealed very minor geometric and electronic structure changes. Kinetic studies of C-H bond containing substrates offered insight into the effects of the ligand perturbations on the reactivity of these oxomanganese(IV) species. Electron donation from the supporting ligand decreases the rate of C-H activation. Conversely, weakening the ligand field through the use of a bulkier ligand dramatically speeds up HAT reactivity, allowing for the reaction with the strong C-H bonds of cyclohexane at rates exceeding that of oxoiron(IV) species with similar ligands. Further, it was discovered that the reaction with cyclohexane could be carried out catalytically in the presence of excess oxidant. The products of catalytic cyclohexane oxidation, cyclohexanol and cyclohexanone, provided evidence of the ability of this oxomanganese(IV) species to perform substrate hydroxylation. Lastly, peroxomanganese(III) species are proposed to be important intermediates in many biological systems. However, in both biological and synthetic systems, the factors that determine the cleavage of the O-O versus Mn-O bonds are not well understood. A novel peroxomanganese(III) species was prepared using electrochemically generated superoxide. The electrochemical reduction of this species was probed to further understand the effects of supporting ligand, which has an amide group trans to the peroxo.