Binuclear manganese compounds of potential biological significance.: 1.: Syntheses and structural, magnetic, and electrochemical properties of dimanganese(II) and -(II,III) complexes of a bridging unsymmetrical phenolate ligand

Reactions of the unsymmetrical phenol ligand 2-(bis(2-pyridylmethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminomethyl)-4-methylphenol with Mn(OAc)(2).4H(2)O or Mn(H2O)(6)(ClO4)(2) in the presence of NaOBz affords the dimanganese(II) complexes l(CH3OH), [Mn-2(L)(OAc)(2)(CH3OH)](ClO4), and 2(H2O), [Mn-2(L)(OBz)(2)(H2O)]ClO4)(1) respectively. On the other hand, reaction of the ligand with hydrated manganese(III) acetate furnishes the mixed-valent derivative 3(H2O), [Mn-2(L)(OAc)(2)(H2O)](ClO4)(2). The three complexes have been characterized by X-ray crystallography. 1(CH3OH) crystallizes in the monoclinic system, space group P2(1)/C-1 with a = 10.9215(6) (A) over circle, b = 20.2318(12) (A) over circle, c = 19.1354(12) (A) over circle, alpha = 90degrees, beta = 97.5310(10)degrees, gamma = 90degrees, V = 4191.7 (A) over circle (3), and Z = 4. 2(H2O) crystallizes in the monoclinic system, space group P2(1)/n, with a = 10.9215(6) (A) over circle, b = 20.2318(12) (A) over circle, c = 19.1354(12) (A) over circle, alpha = 90degrees, beta = 97.5310(10)degrees, gamma = 90degrees, V = 4191.7 (A) over circle (3), and Z = 4. 3(H2O) crystallizes in the monoclinic system, space group P2(1)/c, with a = 11.144(6) (A) over circle, b = 18.737(10) (A) over circle, c = 23.949(13) (A) over circle, alpha = 90degrees, beta = 95.910(10)degrees, gamma = 90degrees, V = 4974(5) (A) over circle (3), and Z = 4. Magnetic measurements revealed that the three compounds exhibit very similar magnetic exchange interactions -J = 4.3(3) cm(-1). They were used to establish tentative magnetostructural correlations which show that for the dimanganese(II) complexes, -J decreases when the Mn-O-phenoxo distance increases as expected from orbital overlap considerations. For the dimanganese(II,III) complexes, crystallographic results show that the Mn-II-O-phenoxo and Mn-III-O-phenoxo bond lengths are inversely correlated. An interesting magneto-structural correlation is found between -J and the difference between these bond lengths, delta(Mn-O) = d(MnII-O) - d(MnIII-O): the smaller this difference, the larger -J. Electrochemical studies show that the mixed-valence state is favored in 1-3 by ca. 100 mV with respect to analogous complexes of symmetrical ligands, owing to the asymmetry of the electron density as found in the analogous diiron complexes.