MULTINUCLEAR NMR-STUDIES OF SOME OXOMOLYBDENUM(VI) COMPLEXES WITH POLYAMINOCARBOXYLATES

Mo-95 and C-13 NMR spectra of aqueous solutions containing molybdate and either DTPA or TTHA at pH values between 2.0 and 9.5 reflect the presence of two complexes of 1:1 (MoL) and 2:1 (Mo2L) stoichiometries for both ligands. All complexes give one broad Mo-95 signal at 60-70 ppm relative to free molybdate, with linewidth 380-890 Hz which increases when the pH decreases. The Mo-95 NMR parameters indicate that the metal center of the complexes consists of the MoO3 species, which is confirmed by the presence of two O-17 NMR signals in the intensity ratio of 1:2 from the corresponding oxo ligands which are trans to the nitrogen and oxygen atoms of the ligand IDA-type moities that bind the metal center. The intensities and multiplicities of the C-13 signals of the complexes indicate that in the MoL complexes the metal center binds one of the terminal IDA-type moieties of the ligands, whereas in the Mo2L complexes two metal centers bind the two terminal IDA-type moieties, yielding a very symmetric species. The intensities of the Mo-95 and C-13 signals give the pH dependence of the concentration of the free ligand, L, free molybdate, Mo, and the MoL and Mo2L complexes. In general, for both 1:1 and 2:1 solution stoichiometries used, the free ligand and the two complexes are present between pH 2.0 and about 8, Mo only appears above pH 6 and at pH 9 only L and Mo occur. The pH dependence of the C-13 complexation shifts reflects the protonation state and sites of the ligand in the complexes. The binding of the MoO3 center to two nitrogen atoms and one carboxylate oxygen of EDDA leads to two diastereomeric pairs of this complex, which are reflected in the presence of two sets of proton signals. Those spectra were assigned using two-dimensional COSY and J-resolved spectra. The vicinal coupling constants obtained were used to define the structures of the species present. Two-dimensional exchange spectra yielded the exchange mechanisms which operate in solution.