Platonic relationships in metal phosphonate chemistry: Ionic metal phosphonates

Abstract

Phosphonate ligands demonstrate strong affinity for metal ions. However, there are several cases where the phosphonate is found non-coordinated to the metal ion. Such compounds could be characterized as salts, since the interactions involved are ionic and hydrogen bonding. In this paper we explore a number of such examples, using divalent metal ions (Mg2+, Ca2+, Sr2+ and Ni2+) and the phosphonic acids: p-aminobenzylphosphonic acid (H2PABPA), tetramethylenediamine-tetrakis(methylenephosphonic acid) (H8TDTMP), and 1,2-ethylenediphosphonic acid (H4EDPA). The compounds isolated and structurally characterized are [Mg(H2O)6]·[HPABPA]2·6H2O, [Ca(H2O)8]·[HPABPA]2, [Sr(H2O)8]·[HPABPA]2, [Mg(H2O)6]·[H6TDTMP], and [Ni(H2O)6]·[H2EDPA]·H2O. Also, the coordination polymer {[Ni(4,4’-bpy)(H2O)4]·[H2EDPA]·H2O}n was synthesized and characterized, which contains a bridging 4,4’-bipyridine (4,4’-bpy) ligand forming an infinite chain with the Ni2+ cations. All these compounds contain the phosphonate anion as the counterion to charge balance the cationic charge originating from the metal cation.