Building layered structures from hydrogen bonded molecular units and 1D metal phosphonate chains: synthesis, characterization and crystal structures of N,N′-dimethyl-N,N′-ethylenediamine-bis(methylenephosphonic acid), its Ni(II) and Pb(II) complexes

The diphosphonic acid, N,N′-dimethyl-N,N′-ethylenediamine-bis(methylenephosphonic acid) (H4L) 1, along with its Ni(II) and Pb(II) complexes, have been synthesized and structurally determined. The structure of the diphosphonic acid features a 2D layer built from the molecular units interconnected by hydrogen bonds between phosphonate oxygen atoms and/or nitrogen atoms. Upon complexing with the Ni(II) ion, the diphosphonic acid acts as a tetradentate chelating ligand (2N, 2O), forming Ni(H2L)(H2O)22 molecular units, such units are also interconnected into a hydrogen bonded double layer via hydrogen bonds between uncoordinated phosphonate oxygen atoms. The ligand in Pb(H2L) 3 also chelates with a Pb(II) ion tetradentately, however, neighboring Pb(H2L) units are interconnected into a one-dimensional chain along the c-axis by a pair of μ2-chelating and bridging phosphonate oxygen atoms. Such chains are further interconnected into a 3D network via hydrogen bonds between uncoordinated phosphonate oxygen atoms. These structures demonstrate the important role of hydrogen bonds in the chemistry of divalent metal phosphonates.