Studies of a dinuclear manganese complex with phenoxo and bis-acetato lBridging in the Mn-2(II,II) and Mn-2(II,III) states: Coordination structural shifts and oxidation state control in bridged dinuclear complexes

摘要

The dinucleating ligand, 2,6-bis {[((2-(2-pyridyl)ethyl)(2-pyridylmethyl)-amino]-methyl}-4-methylphenol) ((LOH)-O-1) reacts with Mn(ClO4)(2)center dot 6H(2)O to form the dinuclear complex [Mn-2(II,II)((LO)-O-1)(mu-OOCCH3)(2)]ClO4 (1). The electrolytic oxidation of 1 at 0.7 V (vs Ag/AgCl) produces the mixed valent complex [Mn-2(II,III)((LO)-O-1)(mu-OOCCH3)(2)](ClO4)(2) (1ox) quantitatively, while electrolysis at 0.20 V converts 1ox back to 1. X-ray crystallographic structures show that both 1 and 1ox are dinuclear complexes in which the two manganese ions are each in distorted octahedral coordination environments bridged by the phenoxo oxygen and two acetate ions. The structural changes that occur upon the oxidation 1 to 1ox suggest an extended pi-bonding system involving the phenoxo ring C-O-phenoxo-Mn(II)-N-pyridyl chain. In addition, as 1 is oxidized to 1ox, the rearrangements in the coordination sphere resulting from the oxidation of one Mn(II) ion to Mn(III) are transmitted via the bridging Mn-O-phenoxo bonds and cause structural changes that render the site of the second manganese ion unfit for the +3 state and hence unstable to reduction. Thus the electrolytic oxidation of 1ox in acetonitrile at 1.20 V takes up slightly greater than 1 F of charge/mol of 1ox, but the starting complex, 1ox, is recovered, showing the instability of the Mn-2(III,III) state that is formed with respect to reduction to 1ox. Variable-temperature magnetic susceptibility measurements of 1 and 1ox over the temperature range from 1.8 to 300 K can be modeled with magnetic coupling constants J = -4.3 and -4.1 cm(-1), respectively showing the weak antiferromagnetic coupling between the two manganese ions in each dinuclear complex, which is commonly observed among similar phenoxo- and bis-1,3-carboxylato-bridged dinuclear Mn-2(II,II) and Mn-2(II,III) complexes.