A ferro- and antiferro-magnetically coupled binuclear manganese(II,III) complex of an orthofunctionalised <i>para</i>-naphthoquinone: Synthesis and characterization by thermogravimetric, thermochromic, cyclic voltammetric, magnetic and EPR studies

Abstract

Synthesis and characterization of manganese (II, III) dimer with O-propionyl oxime of ortho-functionalised <i>para-</i>naphthoquinone-phthiocol(I), are described. The structural features within the inner coordination sphere are consistent with Mn(III) and Mn(II) centres, together with one of the independent ligands are coordinated in catecholate (CAT), and semiquinone (SQ), forms respectively. The thermochromic behaviour of complex and magnetic susceptibility data indicate effective charge transfers between metal and ligand. The latter studies are interpreted in terms of Heisenberg's isotropic model. Here both ferromagnetic (<i>j=</i>42 cm^-1) and antiferromagnetic (J=-27 cm^-1) exchanges occur at lower and higher temperatures with effective spin pairs (S=2,2) and (S=5/2, 3/2) respectively. The electron transport via SQ form of ligand results in such effective spin pairs at variable temperatures. The room temperature and glassy sample EPR spectra exhibit the Class-II mixed-valence nature of complex, indicating localized electron interaction with Mn(II) centre. The six hyperfine spin-allowed transitions are flanked by ten satellite lines. The line positions of allowed and forbidden doublets are computed with coupling constant A =100 G. The zero field splitting and quadrupole splittings are estimated as 220 G and 0.55 G respectively. In cyclic voltammograms, irreversible oxidation peaks at + 1.195 V and +0.65 V together with reduction peak at +0.45 V are due to Mn (III) → Mn (IV) oxidation and Mn(II) (SQ)₂/ Mn(III) (CAT)₂ redox species in dimer respectively.