Platonic Relationships in Metal Phosphonate Chemistry: Ionic Metal Phosphonates

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 (H(2)PABPA), tetramethylenediamine-tetrakis(methylenephosphonic acid) (H8TDTMP), and 1,2-ethylenediphosphonic acid (H(4)EDPA). The compounds isolated and structurally characterized are [Mg(H2O)(6)][HPABPA](2)6H(2)O, [Ca(H2O)(8)][HPABPA](2), [Sr(H2O)(8)][HPABPA](2), [Mg(H2O)(6)][H6TDTMP], and [Ni(H2O)(6)][H(2)EDPA]H2O. Also, the coordination polymer {[Ni(4,4'-bpy)(H2O)(4)][H(2)EDPA]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.