The role of deprotonation of the ligand on the structures of metal phosphonates: synthesis, characterization and crystal structures of two new metal diphosphonates with a 1D double chain and a 2D layer structure

Abstract

Reactions of N-methyl-iminobis(methylenephosphonic acid), CH₃N(CH₂PO₃H₂)₂, (H₄L) with divalent metal nitrates in aqueous solution afforded two new metal diphosphonates with different structures. Zn(H₂L)(H₂O) (1) features a 1D double chain built from ZnO₄ tetrahedra cross-linked by bridging phosphonate groups and ligands. Each zinc cation is tetrahedrally coordinated by three phosphonate oxygen atoms from three ligands and an aqua ligand. These double chains are interconnected into a <111> layer through N–H⋯O and O–H⋯O hydrogen bonds. In Cd(H₃L)₂·2H₂O (2) the Cd(II) ion is octahedrally coordinated by six phosphonate oxygen atoms from four ligands, two of them in a bidentate and two in a unidentate fashion. Each CdO₆ octahedron is further linked to four neighboring CdO₆ octahedra through four bridging phosphonate groups, resulting in a two-dimensional metal phosphonate (002) layer. These layers are held together by strong hydrogen bonds between uncoordinated phosphonate oxygen atoms. The effect of the extent of deprotonation of phosphonic acids on the type of complex formed is discussed.