Structural determinations, magnetic and EPR studies of complexes involving the Cr(OH)2Cr unit

Five heterometallic compounds with formulae [Ba(H2O)4Cr2(μ-OH)2(nta)2] · 3H2O (I), [M(bpy)2(H2O)2] [Cr2(OH)2(nta)2] · 7H2O, where M2+ = Zn, (II); Ni, (III); Co, (IV) and [Mn(H2O)3(bpy)Cr2(OH)2(nta)2] · (bpy) · 5H2O (V); bpy = 2,2′-bipyridine, (nta = nitrilotriacetate ion) have been prepared by reaction of I with the corresponding MII-sulfates in the presence of 2,2′-bipyridine. Substances I–V have been characterized by magnetic susceptibility measurements, EPR and X-ray determinations. I represents a 2D coordination polymer formed by coordination of centrosymmetrical dimeric chromium(III) units and Barium cations. The 10-coordinate Ba polyhedron is completed by four water molecules. Compounds II–IV are isostructural and consist of non-centrosymmetric dimeric anions [Cr2(μ-OH)2(nta)2]2−, complex cations [MII(bpy)2(H2O)2]2+ and solvate water molecules. The octahedral coordination of chromium atoms implies four donor atoms of the nta3− ligands and two bridging OH groups. Multiple hydrogen bonds of coordinated and solvate water molecules link anions and cations in a 3D network. A similar [Cr2(μ-OH)2(nta)2]2− unit is found in V. The bridging function is performed by a carboxylate oxygen atom of the nta ligand that leads to the formation of a trinuclear complex [Mn(bpy)(H2O)2Cr2(μ-OH)2(nta)2]. Experimental and calculated frequency and temperature dependences of EPR spectra of these compounds are presented. The fine structure appearing on the EPR spectra of compound V is analyzed in detail at different temperatures. It is established that the main part of the EPR signals is due to the transitions in the spin states of a spin multiplet with S = 2. Analyses of experimental and calculated spectra confirm the absence of interaction between metal ions (MII) and Cr-dimers in complexes III and IV and the presence of weak Mn–Cr interactions in V. The temperature dependence of magnetic susceptibilities for I–V was fitted on the basis of the expression derived from isotropic Hamiltonian including a bi-quadratic exchange term.