Platonic Relationships in Metal Phosphonate Chemistry: Ionic Metal Phosphonates

Phosphonate ligands demonstrate strong affnity 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.K.D.D. acknowledges the Research Committee of the University of Crete (Grant KA 10329) for financial suppor