Ab Initio Studies on Organophosphorus Compounds. 5. Interactions of Dianionic Bisphosphonate Compounds with Magnesium and Calcium

Bisphosphonates are characterized by a P−C−P backbone angle and two phosphonic acid groups bonded to the same carbon atom. Two clodronate-type compounds, dianionic methylenebisphosphonate (MBP) and (dichloromethylene)bisphosphonate (Cl2MBP), were selected for study. Molecular geometries and energetics were calculated, and their interactions with cationic magnesium and calcium ions were investigated in the gas phase by ab initio molecular orbital methods. Energetically different metal-binding sites were identified. The metal-binding models were extended through the addition of six explicit water molecules to the complexes. Intermolecular hydrogen bonds were formed in the water sphere. The explicit models of MBP−Ca−water and MBP−Mg−water complexes were completed by adding a water medium produced by the self-consistent reaction field (SCRF) method. The 3-21G(*) basis set was used in all the ab initio MO calculations, and relaxed molecular geometries are reported for the studied complexes. Model reactions were used to study the energetics of the various complexes.