Dynamic Motion of Organic Ligands in Polar Layered Cobalt Phosphonates

By introducing the polar methoxy group into phenyl- or benzyl-phosphonate ligands, four cobalt phosphonates with layered structures are obtained, namely, [Co(4-mopp)(H₂O)] (1), [Co(4-mobp)(H₂O)] (2), [Co(3-mopp)(H₂O)] (3), and [Co(3-mobp)(H₂O)] (4), where 4- or 3-moppH₂ is (4- or 3-methoxyphenyl)phosphonic acid and 4- or 3-mobpH₂ is (4- or 3-methoxybenzyl)phosphonic acid. Compounds 1, 2, and 4 crystallize in the polar space groups Pmn2₁ or Pna2₁, whereas compound 3 crystallizes in the centrosymmetric space group P2₁/n. The layer topologies in the four structures are similar and can be viewed as perovskite type, where the edge-sharing [Co₄O₄] rhombi are capped by the PO₃C 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.