Chelation of Vanadium(V) by Difluoromethylene Bisphosphonate, a Structural Analogue of Pyrophosphate

The structural and functional analogy between difluoromethylene bisphosphonate (CF₂PP) and pyrophosphate (PP_i) is investigated in a reaction with V(V) in the form of vanadate. The reaction of CF₂PP with vanadate was investigated using 1.00 M KCl as supporting electrolyte over the ranges 3 ≤ [CF₂PP] ≤ 60 mM and 2.06 ≤ pH ≤ 11.80. ⁵¹V, ¹⁹F, and ³¹P NMR spectroscopic studies showed that a 1:1 species was formed with an H⁺-dependent formation constant of 110 M^-1 at pH 7.22. Results of solution experiments and ab initio calculations are consistent with CF₂PP coordinating V(V) in a bidentate manner, as previously reported for PP_i. Below pH 4, a minor complex forms, which is consistent with a 1:2 stoichiometry. This complex was also observed with pyrophosphate. The X-ray crystal structure of the monoprotonated difluoromethylene bisphosphonate anion (H[CF₂PP]^3-)toludine complex is presented. The H[CF₂PP]^3- anion crystallized in the triclinic space group <i>P</i>1̄ with <i>a</i> = 12.7629(7) Å, <i>b</i> = 13.3992(7) Å, <i>c</i> = 17.1002(9) Å, and <i>V</i> = 2584.4(2) ų, and <i>Z</i> = 2. Sheets of the layers of anions are connected through a network of H-bonds and separated by a layer of toludine cations. The structural features are investigated, and the CF₂PP anion was found to be longer and wider than the corresponding PP_i. Given the larger size of this anion compared to PP_i, the chelation affinity upon CF₂ substitution was found to be 4−5-fold reduced at neutral pH.