Tuning the structure of metal phosphonates using uncoordinating methyl group: syntheses, structures and properties of a series of metal diphosphonates

Eight new transition metal diphosphonates, namely, [Mn(H2L)(H2O)2][(H2O)2] (1), [Co(H2O)6][H2L][(H2O)2] (2), [Co(H2O)6][(H3L)2][(H2O)2] (3), [Ni(H2O)6][H2L][(H2O)2] (4), [Cu(H2L)][(H2O)2] (5), [Zn(H2O)6][(H3L)2][(H2O)2] (6), [Zn(H2L)(H2O)2] (7) and [Cd(H2L)(H2O)2] (8), have been synthesized hydrothermally from 2,5-dimethyl-1,4-phenylenediphosphonic acid (H4L) and thoroughly characterized using EA, IR, TGA, powder and single-crystal XRD, luminescence and magnetism methods. The single-crystal X-ray diffraction measurements indicate that the diphosphonate ligands adopt three kinds of coordination modes which have a huge influence on the structure formation. Compounds 1 and 8 are isostructural and feature 3D framework structures which are constructed by the connectivity of MO6 (M = Mn and Cd) octahedrons and tetradentate organic linkers. Compounds 2 and 4, 3 and 6 are also isostructural, but have isolated mononuclear structures. Compound 5 exhibits 2D layered structure, in which four-connected copper(II) centers are bridged by tetradentate diphosphonate ligands. Compound 7 has a 1D infinite chain structure which is constructed from ZnO4 tetrahedrons and bidentate bridging diphosphonate ligands. It is also found that compounds 6–8 display interesting luminescent properties, whereas 1 shows an antiferromagnetic property.