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

摘要

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-O_2CMe)(H _2O)_2](ClO_4)_2·H _2O·MeCN (1) and [Mn~(III)Mn~(II)L(μ-O _2CPh)(MeOH)(ClO_4)](ClO_4) (2), and a Mn ~(III)Mn~(II)Mn~(II)Mn~(III) complex, [{Mn ~(III)Mn~(II)L(μ-O_2CEt)(EtOH)}_2(μ- O_2CEt)](ClO_4)_3 (3), derived from the Robson-type macrocycle H_2L, 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 (J_1 = +0.015 cm~(-1)). A weak antiferromagnetic interaction (J_2 = -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-DTBCH_2) 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 K _(cat) 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-DTBCH_2 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.