Ligand-Induced Tuning of the Oxidase Activity of μ-Hydroxidodimanganese(III) Complexes Using 3,5-Di-tert-butylcatechol as the Substrate: Isolation and Characterization of Products Involving an Oxidized Dioxolene Moiety.

Oxidase activities of a μ-hydroxidodimanganese(III) system involving a series of tetradentate capping ligands H2LR1,R2 with a pair of phenolate arms have been investigated in the presence of 3,5-di-tert-butylcatechol (H2DBC) as a coligand cum-reductant. The reaction follows two distinctly different paths, decided by the substituent combinations (R1 and R2) present in the capping ligand. With the ligands H2Lt-Bu,t-Bu and H2Lt-Bu,OMe, the products obtained are semiquinonato compounds [MnIII(Lt-Bu,t-Bu)(DBSQ)]·2CH3OH (1) and [MnIII(Lt-Bu,OMe)(DBSQ)]·CH3OH (2), respectively. In the process, molecular oxygen is reduced by two electrons to generate H2O2 in the solution, as confirmed by iodometric detection. With the rest of the ligands, viz., H2LMe,Me, H2Lt-Bu,Me, H2LMe,t-Bu, and H2LCl,Cl, the products initially obtained are believed to be highly reactive quinonato compounds [MnIII(LR1,R2)(DBQ)]+, which undergo a domino reaction with the solvent methanol to generate products of composition [MnIII(LR1,R2)(BMOD)] (3-6) involving a nonplanar dioxolene moiety, viz., 3,5-di-tert-butyl-3-methoxy-6-oxocyclohexa-1,4-dienolate (BMOD-). This novel dioxolene derivative is formed by a Michael-type nucleophilic 1,4-addition reaction of the methoxy group to the coordinated quinone in [MnIII(LR1,R2)(DBQ)]+. During this reaction, molecular oxygen is reduced by four electrons to generate water. The products have been characterized by single-crystal X-ray diffraction analysis as well as by spectroscopic methods and magnetic measurements. Density functional theory calculations have been made to address the observed influence of the secondary coordination sphere in tuning the two-electron versus four-electron reduction of dioxygen. The semiquinone form of the dioxolene moiety is stabilized in compounds 1 and 2 because of extended electron delocalization via participation of the appropriate metal orbital(s).