A polar cobalt(II) phosphonate Co(bamdpH2)(H2O) (1) [bamdpH4 = (benzylazanediyl)bis(methylene)diphosphonic acid] is reported. It shows a linear chain structure. The neighboring chains are connected by moderately strong hydrogen bonds forming a supramolecular layer. The interlayer spaces are filled with the organic groups of the phosphonate ligands. Compound 1 displays the coexistence of single-chain magnet behavior and canted antiferromagnetism below 2.8 K. Moreover, the magnetic dynamics is strongly dependent on the synthetic methods, a phenomenon that has not been documented before.
Successive phase transitions of three kinds of simple 1:1 organic salts of laurylammonium (LA)-phenyl (3-pyridyl) phosphate derivative were examined in terms of thermal properties, single crystal X-ray structural analyses, powder X-ray diffractions, and dielectric responses, where the phosphate anion was chemically modified from phenylphosphate (1) and 3-pyridylphosphate (2) to m-fluorophosphate (3). All 1:1 simple organic salts showed the successive solid-solid and solid-smectic A (SmA) phase transition with high thermal stability. Isostructural alternate cation-anion layer was observed in LA-1 and LA-2, and the packing structure of LA-3 was different from those of the former salts. The L-shaped cation-anion conformation in the molecular assemblies was transformed to the rod-like conformation through the phase transition to SmA phase, where both conformations coexisted in the intermediate solid phase of LA-2 and LA-3. The DSC, PXRD, and dielectric responses of LA-2 showed the antiferroelectric-paraelectric phase transition couple with the flip-flop motion of 3-pyridyl ring along the long axis of the molecule. On the contrary, such molecular motion of the phenyl ring did not show the dielectric phase transition due to no dipole change during molecular rotation of phenyl ring. The motional freedom of m-fluorophenyl ring in LA-3 was completely suppressed by the steric hindrance from the neighboring anions in the absence of dielectric ordering.
By introducing the polar methoxy group into phenyl- or benzyl-phosphonate ligands, four cobalt phosphonates with layered structures are obtained, namely, [Co(4-mopp)(H O)] (1), [Co(4-mobp)(H O)] (2), [Co(3-mopp)(H O)] (3), and [Co(3-mobp)(H O)] (4), where 4- or 3-moppH is (4- or 3-methoxyphenyl)phosphonic acid and 4- or 3-mobpH is (4- or 3-methoxybenzyl)phosphonic acid. Compounds 1, 2, and 4 crystallize in the polar space groups Pmn2 or Pna2 , whereas compound 3 crystallizes in the centrosymmetric space group P2 /n. The layer topologies in the four structures are similar and can be viewed as perovskite type, where the edge-sharing [Co O ] rhombi are capped by the PO C groups. The phenyl and MeO groups in compounds 1-3 are heavily disordered, whereas that in 4 is ordered. Structural comparison based on the data at 296 and 123 K reveals distinct dynamic motion of the organic groups in compounds 1 and 2. The fluctuation of the polar MeO groups in these two compounds is confirmed by dielectric relaxation measurements. In contrast, the fluctuation of polar groups in compounds 3 and 4 is not evident. Interestingly, the dehydrated samples of 3 and 4 (i.e., 3-de and 4-de) exhibit one-step and two-step phase transitions associated with the motion of polar organic groups, as proven by DSC and dielectric measurements. The magnetic properties of compounds 1-4 are investigated, and strong antiferromagnetic interactions are found to mediate between the magnetic centers through μ-O(P) and O-P-O bridges.
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