A 13C and 183W NMR study of d-glycero-d-gulo-heptonic acid as a multisite ligand in tungstate complexes

The formation of complexes (x, y, z) between x tungstate ions, y d-glycero-d-gulo-heptonate ions, and z protons was studied in aqueous solution by 13C and 183W NMR spectroscopy. The overall equilibrium constant for the complex-forming reaction was obtained by a competition method, showing that a (2,2,2) species prevails at pH 4–5. Depending on the pH, various species are formed that involve chelating oxygen atoms originating from hydroxyl groups only, or from the carboxyl and the α-hydroxyl groups. In acidic medium, three types of complexes are formed: a (2,1,2) complex at the arabino site (O-4,5,6,7), two complexes at the O-1,2 site which have either (1,2,2) or (2,2,2) compositions, and a mixed 5:2 complex involving O-1,2,4,5,6,7 of each ligand. In alkaline medium, the carboxylate group is not involved in chelation and a single (2,1,1) complex of the pentadentate ligand (O-2,3,4,5,6) was characterized.