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.