Carbon-13 Fourier transform nuclear magnetic resonance study of gallium citrate in aqueous solution

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

“…The 1 H and 13 C NMR spectra (400 and 100.6 MHz, respectively) show features in common with certain of the 1 H spectra reported by Glickson et al [20] and the 13 C spectra reported by Chang et al, [21] with the expected better resolution here resulting from the use of higher field spectro- (Table 1) shows two sets of citrate resonances in an approximate ratio 45:55; the slightly weaker group signals occurring at δ ϭ 43.1 (ϪCH 2 Ϫ), 74.7 (ϾCϽ), 175.5 (terminal ϪCH 2 CO 2 ), and 180.6 (central CO 2 ) are assigned to citrate in a GaϪcitrate complex, whereas the slightly stronger signals at δ ϭ 43.6, 73.7, 173.9 and 177.2 are assigned to free citrate. The assignment between free and complexed citrate for these two groups of signals is confirmed by the 13 C shift difference between the two carbonyl signals (3.3 ppm) from a solution of citric acid alone, measured at pH ϭ 1.95.…”

“…All solution state NMR spectroscopic data on the complex, both from this study and earlier work [20,21] show that in solution the terminal ϪCH 2 CO 2 groups in the gallium complex are equivalent. The X-ray structure of the 1:2 complex indicates that in solution there should be at the very least two distinct environments for the ϪCH 2 CO 2 groups -two of the groups are bound through oxygen to the gallium, and the other two are not directly bound.…”

“…In the intermediate pH range many minor signals appeared in both the 1 H and 13 C spectra which are probably due to various oligomeric complex species. [20,21] These minor signals were not apparent in either 1 H or 13 C spectra measured at extremes of pH (1.75 and 9.4).…”

“…The earlier solution-state studies [20,21] used samples prepared by the equilibration of solutions of gallium nitrate and citric acid, in contrast to the samples used here which were prepared by direct dissolution of the crystalline gallium citrate complex 1. However, it is clear that the main features of the solution structure are similar.…”

“…They concluded that 1:1 citrate/metal complexes were formed in strongly acidic media, whereas in near neutral solutions the stoichiometry of the complexation was more difficult to determine, although polymeric species were indicated. In a subsequent study of the aqueous gallium nitrate/citric acid system, Chang et al [21] used both 1 H and 13 C NMR spectroscopy to show the existence of both 1:1 and 2:1 citrate/ gallium complexes (Ga n Cit n and Ga n Cit 2n ), and for approximately neutral solutions concluded that n is a small integer. Apparent conditional (pH ϭ 5.80) stability constants for the stepwise complexation were estimated as K 1 ϭ 48 Ϯ 1  Ϫ1 and K 2 ϭ 13 Ϯ 1  Ϫ1 .…”