Structures, magnetochemistry, spectroscopy, theoretical study, and catechol oxidase activity of dinuclear and dimer-of-dinuclear mixed-valence Mn(III)Mn(II) complexes derived from a macrocyclic ligand

Abstract

The work in this paper presents syntheses, characterization, magnetic properties (experimental and density functional theoretical), catecholase activity, and electrospray ionization mass spectroscopic (ESI-MS positive) studies of two mixed-valence dinuclear Mn(III)Mn(II) complexes, [Mn(III)Mn(II)L(μ-O2CMe)(H2O)2](ClO4)2·H2O·MeCN (1) and [Mn(III)Mn(II)L(μ-O2CPh)(MeOH)(ClO4)](ClO4) (2), and a Mn(III)Mn(II)Mn(II)Mn(III) complex, [{Mn(III)Mn(II)L(μ-O2CEt)(EtOH)}2(μ-O2CEt)](ClO4)3 (3), derived from the Robson-type macrocycle H2L, which is the [2 + 2] condensation product of 2,6-diformyl-4-methylphenol and 2,2-dimethyl-1,3-diaminopropane. In 1 and 2 and in two Mn(III)Mn(II) units in 3, the two metal centers are bridged by a bis(μ-phenoxo)-μ-carboxylate moiety. The two Mn(II) centers of the two Mn(III)Mn(II) units in 3 are bridged by a propionate moiety, and therefore this compound is a dimer of two dinuclear units. The coordination geometry of the Mn(III) and Mn(II) centers are Jahn-Teller distorted octahedral and distorted trigonal prism, respectively. Magnetic studies reveal weak ferro- or antiferromagnetic interactions between the Mn(III) and Mn(II) centers in 1 (J = +0.08 cm(-1)), 2 (J = -0.095 cm(-1)), and 3 (J1 = +0.015 cm(-1)). A weak antiferromagnetic interaction (J2 = -0.20 cm(-1)) also exists between the Mn(II) centers in 3. DFT methods properly reproduce the nature of the exchange interactions present in such systems. A magneto-structural correlation based on Mn-O bridging distances has been proposed to explain the different sign of the exchange coupling constants. Utilizing 3,5-di-tert-butyl catechol (3,5-DTBCH2) as the substrate, catecholase activity of all the three complexes has been checked in MeCN and MeOH, revealing that all three are active catalysts with Kcat values lying in the range 7.5-64.7 h(-1). Electrospray ionization mass (ESI-MS positive) spectra of the complexes 1-3 have been recorded in MeCN solutions, and the positive ions have been well characterized. ESI-MS positive spectrum of complex 1 in presence of 3,5-DTBCH2 has also been recorded, and a positive ion, [Mn(III)Mn(II)L(μ-3,5-DTBC(2-))](+), having most probably a bridging catecholate moiety has been identified.