Mononuclear manganese(III) complexes with imine/amine and phenolate ligation: X-ray structural, spectroscopic and electrochemical studies

Abstract

The tridentate Schiff base, formed by reacting salicylaldehyde with 2-dimethylaminoethyl amine, readily forms a 2:1 complex with manganese(III) in methanol where the Schiff base acts only as a bidentate ligand binding through the imine nitrogen and the phenolate oxygen thus occupying four coordination positions on the metal ion. The other two positions are occupied by two N-bonded thiocyanato groups. This complex (1) has been characterized by X-ray crystallography. This Schiff base, when reduced, acts simply as a tridentate ligand in the presence of a base forming a bis-complex (2) with the manganese(III) ion. Complex 2 has also been crystallographically characterized. The coordination geometry around manganese(III) in each is highly distorted octahedral consistent with a d⁴ ion showing strong Jahn-Teller distortion. In the cyclic voltammetric experiments at 298 K in DMF, complex 1 exhibits a cyclic response at E_f = -0.79 V (vs SCE) with ΔEp = 360 mV while 2 shows a quasi-reversible signal with E_f = -0.18 V (vs SCE) with ΔE_p = 110 mV both at the scan rate of 100 mV s^-1. The signal is due to the Mn(III)/Mn(II) couple. Solid state magnetic susceptibility experiments at 300 K indicate that both the complexes are high spin (S = 2) in nature. Electronic and infrared spectral data of the complexes are consistent with their structures.