Influence of ancillary ligands on preferential geometry and biomimetic catalytic activity in manganese(III)-catecholate systems: A combined experimental and theoretical study

Abstract

The present report describes the synthesis and structural characterizations of six new manganese(III) complexes with redox-active tetrachlorocatecholate ligand in the presence of different ancillary ligands (pyridines and imidazole). X-ray crystal structure analysis reveals that the geometry of manganese(III) centres in 1 and 2 is essentially square pyramidal, while it is discrete octahedron in compounds 3-6. These preferential structural diversities in these systems have been critically analysed by theoretical calculations. Remarkably, the characterization of both π⋯π stacking interactions and MnMn bonds in the supramolecular dimeric aggregates in the solid state in 1 and 2 by means of the Bader's theory of "atoms in molecules" (AIM) is quite interesting as that nicely corroborates the experimental fact. All the complexes are active toward the phenoxazinone synthase like activity and the detailed kinetic analysis was performed to get better insight into their catalytic efficiency. Electrochemical property of these complexes as well as different donor property of the ancillary ligands clearly establish that the ease of reduction of the metal centre i.e., the catalytic ability is favoured when the metal centre is bonded to the electron deficient pyridyl systems. EPR spectroscopy and theoretical study are further helpful to get insight into origin of the catalytic activity in these compounds. The present report overall highlights that tuning of the geometry and catalytic activity of manganese(III) complexes with tetrachlorocatecholate ligand can be attained by the introduction of different substitutions in ancillary pyridine ligands.

Keywords: Biomimetic catalysis; Crystal structure; DFT calculations; Geometrical preferences; Manganese-tetrachlorocatecholate complex.