Synthesis Of N, N-dimethyl Aminomethane Bisphosphonic Acid As A Ligand, Metal Phosphonate Compounds, And Structural Studies

Metal phosphonates have received much attention in recent years. Among the various mono- or diphosphonate ligands studied so far, methylenediphosphonic acid and its derivatives are quite unique because they feature a close connection of two phosphonate moieties via one methylene group, which facilitate their synergic coordination ability of acting as a [CP2O6] unit rather than two [CPO3] units. As a result, they could show diversified coordination capabilities with metal ions and thus lead to the formation of new structural types.In this work, we choose N,N-dimethylaminomethane-1,1-diphosphonic acid (H4L) as the ligand, and eight novel metal diphosphonates based on such ligand were obtained by hydrothermal synthesis, which were further characterized by single crystal X-ray diffraction, powder X-ray diffraction, IR spectroscopy and thermogravimetric analysis.The eight metal diphosphonates are as follows: [Al₈Na₂(HL)₂(H₂L)₁₀(H₂O)₆]₃·20H₂O (1); [Fe₈Na₂(HL)₂(H₂L)₁₀(H₂O)₆]₃·22H₂O (2); [Cd(H₃L)₂] (3); [Pb(H₂L)] (4); [Ba₂(H₂L)₂] (5); [Zn₂(H₂L)₂]·2H₂O (6); [Sr₃(H₂L)2(H₃L)₂(H₂O)₂]·2H₂O (7); [Cd(H₃L)₂] (8).Complexes 1 and 2 are isostructural octanuclear metal phosphonate cages encapsulating two sodium(I) ions; complexes 3 and 4 feature one-dimensional chain array; complexes 5-7 exhibit two-dimensional layer structure; complex 8 features a three-dimensional netwok.The different structures in them are principally due to the various coordination modes of the N,N-dimethylaminomethane-1,1-diphosphonate ligand. And the two closely linked phosphonate moieties of the diphosphonate ligand exhibit a synergic coordination effect in these complexes, which can be promoted by large ionic radius.