Monomeric versus dimeric structures in ternary complexes of manganese(II) with derivatives of benzoic acid and nitrogenous bases: structural details and spectral properties

Adducts formed by [Mn(2,6-dmb)2(H2O)3]n · nH2O, 2,6-dmb=2,6-dimethoxybenzoate(1–), Mn(2,4-dhb)2 · 8H2O, Mn(2,5-dhb)2 · 4H2O or Mn(2,6-dhb)2 · 8H2O, dhb=dihydroxybenzoate(1–), and 2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (Me2bpy) or 4,7-dimethyl-1,10-phenanthroline (Me2phen) were isolated in the solid state and characterised by IR, EPR and thermogravimetry. Two of them, [Mn(2,6-dhb)2(bpy)2] (1) and [Mn2(2,6-dmb)4(Me2Phen)2(H2O)2] · 2EtOH (2), were studied by single crystal X-ray diffraction. The adduct 1 is mononuclear and consists of hexa-co-ordinate manganese(II) ions bound to two bipyridine and two 2,6-dihydroxybenzoate ligands in a cis-octahedral arrangement. The complex 2 exhibits a dinuclear structure in which two manganese(II) ions share two carboxylate groups adopting a rather uncommon single-atom bridging mode. The results allow us to conclude that weak, e.g., hydrogen bonding and stacking interactions govern the type of structure, monomeric or dimeric. The spectral features of the complexes are discussed. In particular, the solid-state EPR features of the complexes are interpreted in terms of D, E and Hmax, the high-field resonance. For the monomeric species, the higher is the D value, the higher is Hmax.