Multi-dimensional systems built from dichromate anions - Syntheses, crystal structures, and magnetic properties

Three novel compounds with bridging dichromate anions with the formulae [Fe(4,4'-bpy)(2)Cr2O7](n) (1), [Co(4,4'-bpy)(2)Cr2O7](n) (2), and [Ni(dpa)(2)Cr2O7].H2O (3) (4,4'-bpy = 4,4'-bipyridine, dpa = 2,2'-dipyridylamine) have been synthesized and characterized. The isostructural compounds I and 2 belong to the monoclinic space group C2/c and have been shown to be three-dimensional polymers in which the dichromate anions connect neighboring layers formed by [M(4,4'-bpy)(2)](2+) (M = Fe, Cc) units. Compound 3 is a metallacrown type compound, crystallizing in the monoclinic space group P2(1)/n. Two chromate anions link two Ni-II centers forming a dinuclear entity. The uncoordinated oxygen atom of the dichromate anion forms a hydrogen bond with the N-H unit of dpa resulting in a two-dimensional network. Measurements of magnetic susceptibilities indicate the presence of antiferromagnetic interactions in 1 and 2 whereas the exchange coupling between the Ni-II ions in 3 is ferromagnetic. The IR and UV/Vis spectra of the above complexes have also been investigated. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).