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)<sub>2</sub>(NH<sub>2</sub>(CH<sub>2</sub>)<sub>6</sub>NH<sub>2</sub>)]<sub>n</sub> (<b>1</b>, Piv is Me<sub>3</sub>CCO<sub>2</sub><sup>−</sup>...
| Main Authors: | , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
MDPI AG
2022-12-01
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| Series: | International Journal of Molecular Sciences |
| Subjects: | |
| Online Access: | https://www.mdpi.com/1422-0067/24/1/215 |
| Summary: | A new 1D-coordination polymer [Co(Piv)<sub>2</sub>(NH<sub>2</sub>(CH<sub>2</sub>)<sub>6</sub>NH<sub>2</sub>)]<sub>n</sub> (<b>1</b>, Piv is Me<sub>3</sub>CCO<sub>2</sub><sup>−</sup> anion) was obtained, the mononuclear fragments {Co(O<sub>2</sub>CR)<sub>2</sub>} within which are linked by μ-bridged molecules of hexamethylenediamine (NH<sub>2</sub>(CH<sub>2</sub>)<sub>6</sub>NH<sub>2</sub>). For this compound, two different monoclinic <i>C</i>2/<i>c</i> (α-<b>1</b>) and <i>P</i>2/<i>n</i> (β-<b>1</b>) 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-<b>1</b>) of crystal <b>1</b> at 150 K corresponds to the triclinic space group <i>P</i>-1. As the temperature decreases, the structural phase transition (SPT) in the α-<b>1</b> and β-<b>1</b> 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 CoN<sub>2</sub>O<sub>4</sub> around the metal center at room temperature” → “pseudo-tetrahedral CoN<sub>2</sub>O<sub>2</sub> 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 <b>1</b>: 27% α-<b>1</b> phase, 61% γ-<b>1</b> phase. In addition, at 100 K, 12% of the new δ-<b>1</b> phase was detected, which was identified from the diffraction pattern at 260 K upon subsequent heating: the <i>a</i>,<i>b</i>,<i>c</i>-parameters and unit cell volume are close to the structure parameters of γ-<b>1</b>, and the values of the α,β,γ-angles are significantly different. Further heating leads to a phase transition from δ-<b>1</b> to α-<b>1</b>, which both coexist at room temperature. According to the DC magnetometry data, during cooling and heating, the χ<sub>M</sub><i>T</i>(<i>T</i>) curves for <b>1</b> form a hysteresis loop with ~110 K, in which the difference in the χ<sub>M</sub><i>T</i> values reaches 9%. Ab initio calculations of the electronic structure of cobalt(II) in α-<b>1</b> and γ-<b>1</b> have been performed. Based on the EPR data at 10 K and the ab initio calculations, the behavior of the χ<sub>M</sub><i>T</i>(<i>T</i>) curve for <b>1</b> was simulated in the temperature range of 2–150 K. It was found that <b>1</b> exhibits slow magnetic relaxation in a field of 1000 Oe. |
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| ISSN: | 1661-6596 1422-0067 |