Crystal engineered supramolecular metal phosphonates: Crown ethers and iminodiacetates

Three classes of metal phosphonates will be described in this brief review. First, we describe the zirconium derivatives of N-(phosphonomethyl)iminodi acetic acid, H2O3PCH2N(CH2-COOH)(2). The acid itself is a zwitterion formed by transfer of a phosphonic acid proton to the nitrogen. Consequently, the acid forms a phosphonic acid dimer (O...O, 2.61 Angstrom). Each carboxyl g roup then hydrogen bonds with phosphonate oxygens to form a three-dimension al network of short hydrogen bonds. A linear chain compound is formed when the phosphonic acid was combined with Zr(IV), but the addition of phosphori c acid as a spacer molecule results in the formation of layered compounds. When limited amounts of phosphoric acid were added, the layer that formed w as of a new type embodying features of both the alpha- and gamma -type zirc onium phosphate layers. Increased addition of H3PO4 allowed the layers to r evert to the alpha -type. A series of N-(phosphonomethyl) aza-crown ethers were prepared and reacted with M(IV) and M(II) ions. With zirconium, the la yered compounds have structures similar to those of the N-(phosphonomethyl) iminodiacetic acid. However, with the divalent elements and Ce(IV), linear chain compounds resulted. These compounds have been described as macroscopi c leaflets since the crown ether portions resemble leaves bound to twigs. T he chains are either formed by covalently bonded metal phosphonate bridging or self-assembled hydrogen bonding of phosphonic acid dimers, which in tur n are linked by extremely short (2.45 Angstrom) chain-forming hydrogen bond s. These compounds show promise for use in separation science and catalysis and for design of a wide variety of structures with built-in physical prop erties.