Subtle Structural changes in (Cu^IIL)₂Mn^II complexes to induce heterometallic cooperative catalytic oxidase activities on phenolic substrates (H₂L = Salen type unsymmetrical Schiff base)

Abstract

A new Cu(II) complex of an asymmetrically dicondensed Schiff base (H₂L = N-(2-hydroxyacetophenylidene)-N′-salicylidene-1,3-propanediamine) derived from 1,3-propanediamine, salicylaldehyde, and o-hydroxyacetophenone has been synthesized. Using this complex, [CuL] (1), as a metalloligand, two new trinuclear Cu-Mn complexes, [(CuL) ₂Mn(N₃)(H₂O)](ClO₄)•H₂O (2) and [(CuL)₂Mn(NCS)₂] (3), have been prepared. Single-crystal structural analyses reveal that complexes 2 and 3 both have the same bent trinuclear {(CuL)₂Mn}²⁺ structural unit in which two terminal bidentate square-planar (CuL) units are chelated to the central octahedral Mn(II) ion. This structural similarity is also evident from the variable-temperature magnetic susceptibility measurements, which suggest that compounds 2 and 3 are both antiferromagnetically coupled with comparable exchange coupling constants (−21.8 and −22.3 cm^–1, respectively). The only difference between 2 and 3 lies in the coordination around the central Mn(II) ion; in 3, two SCN^– groups are coordinated to the Mn(II), leaving a neutral complex, but in 2, one N₃^– group and one H₂O molecule are coordinated to give a positively charged species. The presence of such a labile H₂O coligand makes 2 catalytically active in mimicking two well-known polynuclear copper proteins, catecholase and phenoxazinone synthase. The turnover numbers (k_cat) for the aerial oxidation of 3,5-di-tert-butylcatechol and o-aminophenol are 1118 and 6581 h^–1, respectively, values which reflect the facility of the heterometallic catalyst in terms of both efficiency and catalytic promiscuity for aerial dioxygen activation. The mechanisms of these biomimetic oxidase reactions are proposed for the first time involving any heterometallic catalyst on the basis of mass spectral analysis, EPR spectroscopy, and cyclic voltammetry. The evidence of the intermediates indicates possible heterometallic cooperative activity where the substrates bind to a Mn(II) center and Cu(II) plays the role of an electron carrier for transformation of the phenolic substrates to their respective products with the reduction of aerial dioxygen.