SYNTHESIS, STRUCTURE, AND CHARACTERIZATION OF A PHENOLATE-BRIDGED DIMANGANESE(II) COMPLEX OF A DINUCLEATING PHENOL LIGAND

The dinucleating phenol ligand LOH (LOH = 2,6-bis((bis(2-(2-pyridyl)ethyl)amino)methyl)phenol (1)) is reacted with Mn(ClO4)2.6H2O in the presence of excess acetate anion and a base (NEt3) in methanol to produce the dinuclear Mn(II)2 complex [Mn2(LO)(OAc)2]ClO4 (5) which crystallizes in the monoclinic space group P2(1)/n with unit cell dimensions a = 11.227 (3) angstrom, b = 21.815 (8) angstrom, c = 17.731 (7) angstrom, alpha = 90.00 (3)-degrees, beta = 102.45 (3)-degrees, and gamma = 90.00 (3)-degrees, with four molecules per unit cell. The structure shows that two acetate anions and a phenoxy oxygen bridge the manganese ions. The [Mn2(LO)(mu-OAc)2]+ cation has a noncrystallographic C2 axis with the two Mn(II) ions in a nearly equivalent distorted octahedron coordination environment. Cyclic voltammetry of 5 in acetonitrile shows a reversible peak at 0.73 V and an irreversible peak at 1.28 V (vs Ag/AgCl). The two redox potentials assigned to the Mn(II)2/Mn(II)Mn(III) and the Mn(II)Mn(III)/Mn(III)2 redox processes are significantly higher than the potentials assigned to the same processes in the analogous but mixed-valent Mn(II)Mn(III) complexes showing that ligand 1 stabilizes the lower oxidation states. This is attributed to the six-membered chelate rings in 5 vs five-membered rings found in the mixed-valent complexes. Complex 5 is the first example of a structurally characterized stable discrete dinuclear Mn(II)2 complex in this series of analogous complexes. The EPR spectrum of the crystals of 5 at 77 K shows six peaks with no hyperfine structures whereas in DMF/methanol frozen glass it shows a set of six peaks centered near g = 2 with an 11-line hyperfine structure (J = 43 G) superimposed and observable on the two lower field peaks. The hyperfine structure is attributed to the isotropic coupling to the nuclei of the two equivalent Mn(II) ions (total nuclear spin I = 5). A peak for the DELTA-M(s) = 2 forbidden transition at half-field and two outer flanking peaks are also observed. A superimposed six-peak hyperfine structure observed at g = 2 (J = 80 G) is due to impurity caused by monomeric Mn(II) species.