Change in the Electronic Structure of the Cobalt(II) Ion in a One-Dimensional Polymer with Flexible Linkers Induced by a Structural Phase Transition.

A new 1D-coordination polymer [Co(Piv)(NH(CH)NH)] (, Piv is MeCCO anion) was obtained, the mononuclear fragments {Co(OCR)} within which are linked by μ-bridged molecules of hexamethylenediamine (NH(CH)NH). For this compound, two different monoclinic 2/ (α-) and 2/ (β-) phases were found at room temperature by single-crystal X-ray diffraction analysis, with a similar structure of chains and their packages in unit cells. The low-temperature phase (γ-) of crystal at 150 K corresponds to the triclinic space group -1. As the temperature decreases, the structural phase transition (SPT) in the α- and β- crystals is accompanied by an increase in the crystal packing density caused by the rearrangements of both H-bonds and the nearest ligand environment of the cobalt atom ("octahedral CoNO around the metal center at room temperature" → "pseudo-tetrahedral CoNO at 150 K"). The SPT was confirmed by DSC in the temperature range 210-150 K; when heated above 220 K, anomalies in the behavior of the heat flow are observed, which may be associated with the reversibility of SPT; endo effects are observed up to 300 K. The SPT starts below 200 K. At 100 K, a mixture of phases was found in sample : 27% α- phase, 61% γ- phase. In addition, at 100 K, 12% of the new δ- phase was detected, which was identified from the diffraction pattern at 260 K upon subsequent heating: the ,,-parameters and unit cell volume are close to the structure parameters of γ-, and the values of the α,β,γ-angles are significantly different. Further heating leads to a phase transition from δ- to α-, which both coexist at room temperature. According to the DC magnetometry data, during cooling and heating, the χ() curves for form a hysteresis loop with ~110 K, in which the difference in the χ values reaches 9%. Ab initio calculations of the electronic structure of cobalt(II) in α- and γ- have been performed. Based on the EPR data at 10 K and the ab initio calculations, the behavior of the χ() curve for was simulated in the temperature range of 2-150 K. It was found that exhibits slow magnetic relaxation in a field of 1000 Oe.