The pH-controlled hydrothermal synthesis and crystal structures of two zinc N,N′-piperazinebis(methylenephosphonate) frameworks

Abstract

An exploration of the reactions of N,N′-piperazinebis(methylenephosphonic acid), H₄L, with zinc salts has led to the isolation of two new framework zinc phosphonates. ZnLH₂·H₂O (I) is isostructural with previously reported manganese(II) and cobalt(II) analogues, and consists of infinite ‘zinc phosphate’ chains bridged into three dimensions via the organic moieties. The resulting framework encloses large channels in which the loosely bound H₂O resides. The H₂O is lost reversibly at around 160 °C, without framework collapse. Zn₂L (II) has a novel framework structure, prepared at an initial pH > 7, which consists of two-dimensional ‘zinc phosphate’ sheets, comprising both four- and eight-membered –Zn–O–P– rings, which are also linked into three dimensions via the organic groups. In both cases, the zinc centre is tetrahedral; in I coordination is by oxygen atoms from four different phosphonate groups, whereas in II the additional deprotonation of the ligand allows coordination via three oxygen atoms plus the amine nitrogen atom.