Solid and solution state NMR spectra and the structure of the gallium citrate complex (NH4)(3)[Ga(C6H5O7)(2)]center dot 4H(2)O

A gallium citrate complex (NH4)(3)[Ga(C6H5O7)(2)].4H(2)O (1) has been isola ted from an aqueous mixture of gallium nitrate and citric acid at pH = 9. I n the solid state 1 is monomeric and includes the near centrosymmetric ion [Ga(C6H5O7)(2)](3-) in the asymmetric unit, with each tridentate citrate co ordinating to gallium through an alkoxide oxygen, an oxygen of the central carboxyl group and an oxygen of a terminal carboxyl group. The second termi nal carboxyl group is not bound to the metal centre. The two alkoxide oxyge n-gallium distances in the asymmetric unit are significantly shorter than t he four carboxyl oxygen-gallium distances, indicating a stronger bond for t he former, The number and intensities of the resolved resonances in the sol id state C-13 MAS/NMR spectrum are consistent with the X-ray structure. In aqueous solution near neutral pH (5.5 to 6.4) the complex partially dissoci ates to give an equilibrium of the 1:1 and 1:2 gallium/citrate species toge ther with free citrate. Two sets of resonances are observed in both the H-1 and C-13 NMR spectra of solutions of the complex and these correspond to b ound 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 exch ange situation (rate < ca. 12 s(-1)) between the bound and free Ligand. The carboxylate oxygen-gallium bonds are quite labile and relatively rapid int ramolecular chemical exchange occurs (rate > ca. 700 s(-1)) between the met al-bound and pendant terminal carboxyl groups, averaging their H-1 and C-13 NMR signals. The diffusion coefficients for the free and complexed citrate measured from the H-1 NMR spectra do not appear to be complicated by chemi cal exchange effects and, as expected, the diffusion coefficient for the co mplexed Ligand is smaller than for the free Ligand. In solution in the inte rmediate pH range (4.8 to 6.4) a new Ga-71 signal is observed at ca. delta = 27 (Delta nu (1/2) approximate to 10.6 kHz) which is assigned to the gall ium citrate complexed species.