Polar layered coordination polymers incorporating triazacyclononane-triphosphonate metalloligands

Reactions of metalloligands M-III(notpH(3)) (M = Fe, Co and notpH(6) = 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid)) with Zn(OAc)(2) under hydrothermal conditions resulted in new metal phosphonates Zn2Fe(notp)Cl(H2O) (1) and ZnCo(notpH)(H2O)2H(2)O (2). They crystallize in polar space groups P6(3) (for 1) and Pca2(1) (for 2), respectively, and exhibit layer structures in which the inorganic layers are separated by the organic groups of the notp ligands. However, the layer topologies of the two compounds are quite different. In 1, the layer contains 6-membered rings composed of one {FeN3O3} octahedron, one {Zn1O(3)Cl}, one {Zn2O(4)} and three {PO3C} tetrahedra via corner-sharing connections, while in 2, the layer contains 10-membered rings composed of two {CoO3N3} octahedra, three {ZnO4} and five {PO3C} tetrahedra via vertex-sharing connections. Dielectric measurements on single crystals of 2 confirmed the presence of high dielectric anisotropy. Proton conductivity measurements revealed that the proton conduction is more favourable in 2 due to the presence of a continuous hydrogen bond network in this compound.