Crystal Engineered Supramolecular Metal Phosphonates: Crown Ethers and Iminodiacetates
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Three classes of metal phosphonates will be described in this brief review. First, we describe the zirconium derivatives of N-(phosphonomethyl)iminodiacetic acid, H2O3PCH2N(CH2COOH)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 (0···0, 2.61 Å). Each carboxyl group then hydrogen bonds with phosphonate oxygens to form a three-dimensional network of short hydrogen bonds. A linear chain compound is formed when the phosphonic acid was combined with Zr(IV), but the addition of phosphoric acid as a spacer molecule results in the formation of layered compounds. When limited amounts of phosphoric acid were added, the layer that formed was of a new type embodying features of both the α- and γ-type zirconium phosphate layers. Increased addition of H3PO4 allowed the layers to revert to the α-type. A series of N-(phosphonomethyl) aza-crown ethers were prepared and reacted with M(IV) and M(II) ions. With zirconium, the layered 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 macroscopic leaflets since the crown ether portions resemble leaves bound to twigs. The chains are either formed by covalently bonded metal phosphonate bridging or self-assembled hydrogen bonding of phosphonic acid dimers, which in turn are linked by extremely short (2.45 Å) chain-forming hydrogen bonds. These compounds show promise for use in separation science and catalysis and for design of a wide variety of structures with built-in physical properties.
author list (cited authors)
Clearfield, A., Sharma, C., & Zhang, B.