Catalytic Oxidation of 3,5-Di-tert-butylcatechol by a Series of Mononuclear Manganese Complexes: Synthesis, Structure, and Kinetic Investigation

The manganese compounds [Mn(bpia)(OAc)(OCH3)](PF6) (1), [Mn(bipa)(OAc)(OCH3)](PF6) (2), [Mn(bpia)(Cl)2](ClO4) (3), [Mn(bipa)(Cl)2](ClO4) (4), [Mn (Hmimppa)(Cl)2]·CH3OH (5), and [Mn(mimppa)(TCC)]·2CHCl3 (6) (bpia = bis(picolyl)(N-methylimidazole-2-yl)amine; bipa = bis(N-methylimidazole-2-yl) (picolyl)amine; Hmimppa = ((1-methylimidazole-2-yl)methyl)((2-pyridyl)methyl)(2-hydroxyphenyl)amine; TCC = tetrachlorocatechol) were synthesized and characterized by various techniques such as X-ray crystallography, mass spectrometry, IR, EPR, and UV/vis spectroscopy, cyclic voltammetry, and elemental analysis. 1 and 2 crystallize in the triclinic space group P (No. 2), 4 and 6 crystallize in the monoclinic space group P21/n (No. 14), and 5 crystallizes in the orthorhombic space group Pna21. Complexes 1-4 are structurally related to the proposed active site of the manganese-dependent extradiol-cleaving catechol dioxygenase exhibiting an N4O2 donor set (1 and 2) or N4Cl2 donor set (3 and 4). Cyclic voltammetric data show that the substitution of oxygen donor atoms with chloride causes a shift of redox potentials to more positive values. These compounds show high catalytic activity regarding the oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone exhibiting saturation kinetics at high substrate concentrations. The turnover numbers kcat = (86 (plus-minus) 7) h^-1 (1), kcat = (101 (plus-minus) 4) h^-1 (2), kcat = (230 (plus-minus) 4) h^-1 (3), and kcat = (130 (plus -minus) 4) h^-1 (4) were determined from the double reciprocal Lineweaver-Burk plot. Compounds 5 and 6 can be regarded as structural and electronic Mn analogues for substituted forms of Fe-containing intradiol-cleaving catechol dioxygenases. To our knowledge 5 is the first mononuclear Mn(II) compound featuring an N3OCl2 donor set.