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)(H2 O)] (1), [Co(4-mobp)(H2 O)] (2), [Co(3-mopp)(H2 O)] (3), and [Co(3-mobp)(H2 O)] (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 [Co4 O4 ] rhombi are capped by the PO3 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.