Chelation of vanadium(V) by difluoromethylene bisphosphonate, a structural analogue of pyrophosphate.

The structural and functional analogy between difluoromethylene bisphosphonate (CF 2 PP) and pyrophosphate (PP i ) is investigated in a reaction with V(V) in the form of vanadate. The reaction of CF 2 PP with vanadate was investigated using 1.00 M KCI as supporting electrolyte over the ranges 3 < [CF 2 PP]≤ 60 mM and 2.06 < pH s 11.80. 51 V, 19 F, and 31 P 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[CF 2 PP] 3 -)-toludine complex is presented. The H[CF 2 PP] 3 - anion crystallized in the triclinic space group P1 with a = 12.7629(7) A, b = 13.3992-(7) A, c = 17.1002(9) A, and V= 2584.4(2) A 3 , and Z= 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.