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

Mo-95 and C-13 NMR spectra of aqueous solutions containing molybdate a nd either DTPA or TTHA at pH values between 2.0 and 9.5 reflect the pr esence of two complexes of 1:1 (MoL) and 2:1 (Mo2L) stoichiometries fo r 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 met al center of the complexes consists of the MoO3 species, which is conf irmed by the presence of two O-17 NMR signals in the intensity ratio o f 1:2 from the corresponding oxo ligands which are trans to the nitrog en 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 o ne of the terminal IDA-type moieties of the ligands, whereas in the Mo 2L 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 l igand, L, free molybdate, Mo, and the MoL and Mo2L complexes. In gener al, for both 1:1 and 2:1 solution stoichiometries used, the free ligan d and the two complexes are present between pH 2.0 and about 8, Mo onl y appears above pH 6 and at pH 9 only L and Mo occur. The pH dependenc e of the C-13 complexation shifts reflects the protonation state and s ites 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 di astereomeric pairs of this complex, which are reflected in the presenc e of two sets of proton signals. Those spectra were assigned using two -dimensional COSY and J-resolved spectra. The vicinal coupling constan ts obtained were used to define the structures of the species present. Two-dimensional exchange spectra yielded the exchange mechanisms whic h operate in solution.