engleski

A New Heterometallic (Ni^2+ and Cr^3+) Complex – Crystal Structure and Spectroscopic Characterization

The design of new molecular solids with desirable physical properties is often based on the controlled supramolecular aggregation of molecular entities. In molecular solids with localized paramagnetic centres, supramolecular aggregates often serve as pathways for the magnetic exchange interactions. In a complete characterization of these systems (besides crystal structure analysis), various spectroscopic methods take a very important part. The first idea on coordination geometry is often obtained by UV/Vis spectroscopy, whereas the evidences on magnetic properties of individual metal centres of these species, provided by the EPR measurements have an exceptionally significant role for a better understanding of their magnetic behaviour. As a continuation of our research work on polynuclear transition metal complexes, a new heterometallic complex of the composition [Ni(bpy)_3]_2[Cr(C_2O_4)_3]NO_3• 10H_2O (bpy = 2, 2'-bipyridine) has been synthesized. The prepared compound has been characterized by means of chemical and TG/DTA analysis, IR, UV/Vis and EPR spectroscopy and by single crystal X-ray diffraction method. Due to the rigidity of the didendate bipyridine and oxalate ligands, two symmetry independent [Ni(bpy)_3]^2+ cations as well as the [Cr(C_2O_4)_3]^3– anion possess a trigonally distorted octahedral geometry. Analysis of crystal packing reveals a specific type of supramolecular contact comprising four bipyridine ligands from two neighbouring [Ni(bpy)_3]^2+ cations – quadruple aryl embrace (QAE) contact. The electronic spectrum has shown the superposition of bands characteristic for both the nickel(II) and chromium(III) six-coordinated ions. The EPR spectra have been recorded on a single crystal as well as on a powdered sample of the prepared complex. During the measurements the crystal was rotated round three arbitrary, mutually perpendicular axes. In the complex investigated, two kinds of paramagnetic species are present: Ni^2+ with a spin S = 1 and Cr^3+ with S = 3/2. In the frequency range examined, from the room to the liquid helium temperature, Ni2+ has been EPR silent and only the transition for Cr^3+ has been observed.

Heterometallic; Crystal structure; EPR Spectroscopy

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