Synthesis and Structural Characterization of a Homologous Series of Divalent-Metal Phosphonates, M^II(O₃PR).H₂O and M^II(HO₃PR)₂

Layered phosphonate salts of divalent metal ions (Mg, Mn, Zn, Ca, Cd) are precipitated by combining aqueous solutions of a phosphonic acid and a soluble metal salt. In this way compounds in the series M(O₃C_nH_2n+1).H₂O (M = Mg, Mn, Zn, = 1-12; M = Ca, Cd, = 1-4), M(O₃PC₆H₅-H₂O (M = Mg, Mn, Zn), Ca(HO₃PC₆H₅)₂, and Ca(HO₃PC_nH_2n+1)₂(n ≥ 5) were prepared. These compounds are insoluble in water at pH 7 but dissolve in dilute acid solutions. A preliminary X-ray structure for Mn(O₃PC₆H₅).H₂O, refined in the orthorhombic space group Pmn2₁with Z = 2 and a = 5.734 (5), b = 14.33 (3), and c = 4.945 (4) Å, is reported. The structure consists of layers of Mn atoms coordinated octahedrally by five phosphonate oxygen atoms and one water molecule. Each phosphonate group coordinates four Mn atoms, making a cross-linked Mn-O network. The phenyl groups lie above and below these approximately planar networks and make van der Waals contacts between layers. Compounds in the M(O₃PC_nH_2n+1.H₂O (= Mg, Mn, Zn) and M(O₃PC₆H₅).H₂O (M = Mg, Zn) series crystallize in Pmn2₁with a = 5.61-5.74 and c = 4.78-4.82 Å and with b varying according to the size of the alkyl or phenyl group. A structural model for these compounds is discussed.