Syntheses, structures, catecholase activity, spectroscopy and electrochemistry of a series of manganese(III) complexes: Role of auxiliary anionic ligand on catecholase activity

The work in this report presents the syntheses, characterization, catechol oxidase activity, electrochemistry and electrospray ionization mass spectroscopy (ESI-MS positive) of five mononuclear manganese(III) compounds [MnIIIL(OAc)(H2O)]·DMF (1), [MnIIIL(Cl)(H2O)]·2H2O (2), [MnIIIL(N3)(H2O)] (3), [MnIIIL(NCS)(H2O)] (4) and [MnIIIL(NCSe)(H2O)]·MeCN (5), derived from the Schiff base compartmental ligand N,N′-ethylenebis(3-ethoxysalicylaldimine) (H2L). All the five complexes are characterized by elemental analyses, FT-IR, UV–Vis spectra, molar conductance and single crystal X-ray structure determination. The structures of 1–5 show that these are mononuclear compounds in which the manganese(III) center occupies the N(imine)2O(phenoxo)2 compartment. In all structures, a dimer-of-mononuclear species is formed by bifurcated hydrogen bonding involving coordinated water molecule. The five mononuclear manganese(III) complexes 1–5 show catecholase activity with turnover (Kcat) numbers lying in the range 17.0–41.7 h−1 (for 1–5) in DMF and 16.9–137.3 h−1 (for 2–5) in MeCN. ESI-MS positive of 2–5 and their mixtures with 3,5-di-tert-butylcatechol have been recorded, revealing the existence of 3,5-di-tert-butylquinone containing species and a complex-substrate aggregate [MnIIIL(3,5-DTBCH)+H+]+. Cyclic voltammetric and square wave voltammetric studies of the compounds 1–5 reveal that these undergo quasireversible Mn(III)/Mn(II) redox couple with very close E½ values (from −200 to −213 mV).