Studies on binuclear hydroxo/carboxylato-bridged manganese(III) complexes and a mononuclear manganese(III) complex involving salen type ligands

Synthesis of six hydroxo-bridged binuclear manganese(III) complexes of formulae [MnL-X-MnL](ClO4) [X = OH (1-6)] along with a mononuclear manganese(III) complex (7) [Mn(L)(L')(MeOH)(2)] [HL' = 2-(2-hydroxy-phen-yl) benzimidazole] and two carboxylate-bridged binuclear manganese(III) complexes ( 8) and ( 9) are described. The complexes have been characterized by the combination of i.r., u.v.-vis spectroscopy, magnetic moments and by their redox properties. The electronic spectra of all the complexes exhibit almost identical features consisting of two d-d bands at ca. 550 and 600 nm, one MLCT band at ca. 400 nm, together with two pi-pi(*) intra-ligand transitions at ca. 250 nm and ca. 300 nm. Room temperature magnetic data range from mu = 2.7-3.0 BM indicates some super-exchange between the binuclear metal centers via bridging hydroxo/carboxylato groups. The X-ray crystal structure of the binuclear complex (5) revealed that it has a symmetric (MnN2O2)-N-III core bridged by a hydroxyl group. The X-ray analysis of the mononuclear complex (7) showed that the manganese-center possesses a distorted octahedral geometry. Electrochemical properties of hydroxo-bridged manganese(III) complexes (1-6) show identical features consisting of an irreversible and a quasi-reversible reduction corresponding to the Mn-2(III) -> (MnMnIII)-Mn-II -> (MnMnII)-Mn-II couples in the voltammogram. It was found that electron withdrawing substituents on the ligand result in easier reduction. Complex (7) displays an irreversible reduction at 0.08 V and a reversible oxidation at 0.45V assignable to the Mn-III -> Mn-II reduction and Mn-III -> Mn-IV oxidation, respectively. The carboxylate-bridged compound (8) exhibits two irreversible oxidations at 0.4 and 0.6 V, probably due to Mn-2(III) -> (MnMnIV)-Mn-III -> Mn-IV Mn-IV oxidations and shows a quasi-reversible reductive wave at -0.85 V, tentatively assigned to Mn-2(III) -> (MnMnIII)-Mn-II reduction.