Structural, Magnetic and Electrochemical Studies of a New Series of Macrocyclic Mononuclear and Binuclear Manganese(III) and Unusually Stable Manganese(II) Complexes

Abstract

The synthesis of several mononuclear and binuclear manganese(III) complexes from 3,4:9,10-dibenzo-1,12[{N,N'-bis(4-methyl-1-oxa)-6,6'-formyl}-2-benzyldiaza]-5,8-dioxacyclotetradecane have been described. The X-ray crystal structure of the manganese(III) complex [MnL2a(H2O)(ClO4)]·(ClO4)2·3H2O·CH3OH revealed a six coordinate manganese atom with octahedral geometry. All four phenyl rings were found to lie almost in one plane with the manganese atom slightly lifted from it due to Jahn-Teller distortion. Room temperature magnetic studies of the mononuclear complexes gave μ eff values around 5B.M. Electrochemical studies of mononuclear complexes showed irreversible reduction in the range -0.15 to -0.4V for the Mn(III)/Mn(II) redox couple and quasi-reversible oxidation around +0.8V for the Mn(III)/Mn(IV) species. Binuclear complexes showed two quasi-reversible reductions in the -0.3 and -0.7V regions corresponding to Mn(III)Mn(III)/Mn(III)Mn(II) and Mn(III)Mn(II)/Mn(II)Mn(II) couples, respectively. The oxidation process showed two waves around +0.6 and +0.9V; the former is due to the formation of a Mn(III)Mn(IV) complex and the latter to a Mn2(IV,IV) complex (the latter showed more reversibility in the given scan time). Rare types of mono and binuclear manganese(II) complexes consisting of bridging phenolic oxygens were synthesized under special conditions. The complexes showed an ESR signal centred around 3200 G. Room temperature magnetic studies of the mononuclear manganese(II) complexes gave a μ eff value of 6.1B.M., close to the spin-only value for high spin Mn(II). Electrochemical studies of the mononuclear complex showed quasi reversible oxidation around 1 V and the binuclear complex showed unusual two, one-electron oxidations at high positive potentials (1.2 to 1.7 V).