Manganese complexes of 1,3-bis(3-nitrosalicylideneamino)-propan-2-ol. Structure and magnetic and electrochemical properties

Abstract

Four complexes [Mn^II(3NO₂-Hsalpro)]1, [Mn^II₂(3NO₂-salpro)(O₂CMe)]·4MeOH 2, [Mn^III₂(3NO₂-salpro)₂(H₂O)]·H₂O 3 and [{Mn^III(3NO₂-salpro)}₂]·2dmf 4, [3NO₂-H₃salpro = 1,3-bis(3-nitrosalicylideneamino)propan-2-ol, dmf = dimethylformamide] have been synthesized and studied. The structure of 4 was solved by direct methods and refined to conventional agreement indices R=R′= 0.041. The molecular structure consists of discrete [{Mn^III(3NO₂-salpro)}₂] units and lattice-held dmf molecules. The co-ordination octahedron around each Mn^III in cludes three donors from each of the two pentadentate trianionic ligands. Both ligands asymmetrically bridge the Mn atoms affording an intra-dinuclear Mn ⋯ Mn′ separation of 3.223(1)Å. The symmetry-related manganese centres are in a distorted-octahedral environment. Infrared, ESR spectroscopic, magnetic susceptibility and electrochemical studies of 1–4 evidence a variety of structural types and nuclearities. Complex 1, resulting from the reaction of manganese(II) acetate with 3NO₂-H₃salpro, is characterized by the MnL stoichiometry while 2, resulting from the reaction of manganese(II) acetate with Na₃(3NO₂-salpro), is characterized by the Mn₂L stoichiometry. Two structurally different dinuclear manganese(III) complexes characterized by the same overall formulation [{Mn^III(3NO₂-salpro)(solvent)}₂] were obtained from the reaction of 1 with O₂. In non-dehydrated methanol, the asymmetric dinuclear species 3 is formed. When this complex is dissolved in dry dmf the water molecule is driven out of the manganese co-ordination sphere and a subsequent ligand environment reorganization affords 4. Variable-temperature magnetic susceptibility studies established the presence of isotropic magnetic exchange interactions between the manganese(III) centres of the dimeric species 3 and 4(J=–1.9 and –1.6 cm^–1, respectively) and crystalline field anisotropy of Mn^III(D=–2.6 and –2.5 cm^–1, respectively). Electrochemical studies indicated that the Mn^II₂, Mn^IIMn^III, Mn^III₂ and Mn^IIIMn^IV oxidation states are accessible for 4 in dmf–water.