Metal complexes of α-hydroxyiminophosphonic acid derivatives. Preparation and crystal structures of calcium and cadmium complexes of methyl (E)-α-hydroxyiminobenzylphosphonate

Both Ca²⁺ and Cd²⁺ react with methyl (E)-α-hydroxyiminobenzylphosphonate in aqueous solution to yield insoluble polymeric species which have been characterized by single-crystal X-ray diffraction studies. The two isomorphic compounds crystallize in the monoclinic system in the space group P2₁/_c. Unit-cell parameters: Ca²⁺, a = 13.169(1), b = 8.623(1), c = 19.161 (2) Å, β = 90.93(1)°, Z = 4; Cd²⁺, a = 13.010(1), b = 8.666(1), c = 18.906(2) Å, β = 91.84(1)°, Z = 4. The metal ions are seven-co-ordinate with a distorted pentagonal bipyramidal geometry generated by two bidentate, two unidentate phosphonate ligands, and one water molecule. The ligands act simultaneously in a chelating bidentate mode (through the oxyimino nitrogen and the phosphonate oxygen) and in a bridging mode. The bond distances and angles are similar to those observed for the calcium complexes of bisphosphonate drugs.