Solid and Solution State NMR Spectra and the Structure of the Gallium Citrate Complex (NH4)3[Ga(C6H5O7)2]·4H2O

Abstract

A gallium citrate complex (NH4)3[Ga(C6H5O7)2]·4H2O (1) has been isolated from an aqueous mixture of gallium nitrate and citric acid at pH = 9. In the solid state 1 is monomeric and includes the near centrosymmetric ion [Ga(C6H5O7)2]3− in the asymmetric unit, with each tridentate citrate coordinating to gallium through an alkoxide oxygen, an oxygen of the central carboxyl group and an oxygen of a terminal carboxyl group. The second terminal carboxyl group is not bound to the metal centre. The two alkoxide oxygen−gallium distances in the asymmetric unit are significantly shorter than the four carboxyl oxygen−gallium distances, indicating a stronger bond for the former. The number and intensities of the resolved resonances in the solid state 13C MAS/NMR spectrum are consistent with the X-ray structure. In aqueous solution near neutral pH (5.5 to 6.4) the complex partially dissociates to give an equilibrium of the 1:1 and 1:2 gallium/citrate species together with free citrate. Two sets of resonances are observed in both the 1H and 13C NMR spectra of solutions of the complex and these correspond to bound and free citrate; there is no resolution of resonances for the 1:1 and 1:2 species. The relative strength of the alkoxide oxygen−gallium bond in the metal-bound citrate ligand leads to a slow intermolecular chemical exchange situation (rate < ca. 12 s−1) between the bound and free ligand. The carboxylate oxygen−gallium bonds are quite labile and relatively rapid intramolecular chemical exchange occurs (rate > ca. 700 s−1) between the metal-bound and pendant terminal carboxyl groups, averaging their 1H and 13C NMR signals. The diffusion coefficients for the free and complexed citrate measured from the 1H NMR spectra do not appear to be complicated by chemical exchange effects and, as expected, the diffusion coefficient for the complexed ligand is smaller than for the free ligand. In solution in the intermediate pH range (4.8 to 6.4) a new 71Ga signal is observed at ca. δ = 27 (Δν1/2 ≈︁ 10.6 kHz) which is assigned to the gallium citrate complexed species.