Nature exhibits numerous examples of challenging reactions such as oxidation of strong C–H bonds and activation of molecular oxygen and nitrogen. These reactions are made possible because of biological catalysts called metalloenzymes. For example, the manganese lipoxygenase (MnLOX) uses molecular oxygen to catalyse the oxidation of polyunsaturated fatty acids, and manganese superoxide dismutase (MnSOD) catalyses the decomposition of the harmful superoxide radical anion. In these reactions the manganese shuttles between MnIII/MnII oxidation states and the MnIII state is believed to be stabilized by a hydroxo ligand derived from solvent. The X-ray crystal structure of these enzymes suggests presence of hydrogen bond between the hydroxo ligand and a cis bound carboxylate ligand. However, the role of this hydrogen bond on the reactivity of Mn(III)-hydroxo species in the active sites of the enzyme remains ambiguous. Therefore, to understand the influence of this hydrogen bond interaction on the thermodynamic properties of the Mn(III)-hydroxo species and their reactivity, model manganese complexes supported on pentadentate ligand scaffold differing by one functional group were designed. This simple change in the ligand scaffold permits the intramolecular hydrogen bonding with the hydroxo ligand in one complex. The presence of this hydrogen bond interaction in the model complex revealed 100-fold rate enhancement in oxidation of O–H bond. Furthermore, attempts have been made to synthesize Mn(III)-aqua complex from the corresponding Mn(III)-hydroxo complexes to better understand the relation between the thermodynamic properties and reactivity of the Mn(III)-hydroxo complexes in oxidation of phenols. These Mn(III)-hydroxo and Mn(III)-aqua complexes have been characterized by spectroscopic techniques; such as, electronic absorption (UV-vis), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR). DFT computations have also been used to determine the thermodynamic properties of the Mn(III)-hydroxo and the corresponding Mn(III)-aqua complexes which closely match with the structural parameters, electronic absorption data and bond dissociation free energies of the O–H bond of Mn(III)-hydroxo complexes.

Effect of hydrogen bond interactions on the properties and hydrogen atom abstraction reactivity of the Mn(III)-hydroxo complexes

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