A structural study of bisphosphonate metal complexes. Alkaline earth metal complexes of (dichloromethylene)bisphosphonic acid P,P′-diethyl ester

Abstract

The complexation properties of an ester derivative of clodronate with divalent metal cations Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ has been studied using the methods of structural chemistry and spectroscopy. A dipiperidinium salt and four alkaline earth metal complexes of the (dichloromethylene)bisphosphosphonic acid P,P′-diethyl ester: (C₅H₁₀NH₂)₂Cl₂C(PO₃Et)₂ (1), [MgCl₂C(PO₃Et)₂(H₂O)₃]_n (2), [(H₂O)₃Ca{Cl₂C(PO₃Et)₂}₂Ca(H₂O)₃] (3), [(H₂O)₃Sr{Cl₂C(PO₃Et)₂}₂Sr(H₂O)₃] (4) and [(H₂O)₂Ba{Cl₂C(PO₃Et)₂}₂Ba(H₂O)₂]_n (5) were prepared and characterised. The crystal structures were determined with the single crystal X-ray diffraction technique. The asymmetric unit of compound 1 contains two piperidinium cations and one (dichloromethylene)bisphosphosphonic acid P,P′-diethyl ester anion, which are bound to each other by hydrogen bonding via amine-N⁺H₂. Compound 2 is polymeric, having two independent magnesium atoms with octahedral geometry. Calcium and strontium complexes 3 and 4 are isomorphous and dimeric with seven coordinated metal atoms connected by two Cl₂C(PO₃Et)₂²⁻ ligands. In the asymmetric unit of complex 5, two adjacent barium atoms are joined by two Cl₂C(PO₃Et)₂²⁻ ligands. The dimeric units are bridged through the oxygen atoms of Cl₂C(PO₃Et)₂²⁻ and aqua ligands, forming polymeric chains. The compounds were characterised by infrared spectroscopy and ³¹P NMR spectroscopy. The aqueous solubility of the diethyl ester was defined in the presence of Ca²⁺ cations.