Metal Chelation Versus Internal Hydrogen Bonding of The α-hydroxy Carboxylate Group

This study analyzes metal chelation of the α-hydroxy carboxylate group and the effect of the replacement of the hydroxyl group by fluorine in some alkali-metal citrate salts. The structure of potassium citrate monohydrate shows that, unlike the case for sodium or many other salts of citrate, the metal ion does not span the α-hydroxy carboxylate group. In this case, as for free citric acid, the hydrogen atom from the hydroxyl group forms a hydrogen bond to an oxygen atom of the central carboxylate group. When no such hydroxyl hydrogen atom is available for hydrogen bonding, as in potassium 3-fluorodeoxycitrate, where the hydroxyl group is replaced by fluorine, the metal spans the α-fluoro carboxylate group so that the carbon-bound fluorine is part of the coordination sphere of the metal ion. This analysis involved crystal structure analyses by X-ray and neutron diffraction of tripotassium citrate monohydrate with unit cell dimensions a ═ 13.795 (1), b ═ 11.772 (1), c ═ 7.092 (2) Å; β ═ 112.57 (2)°; space group P2/a. In addition, X-ray diffraction studies were done on potassium 3-fluoro-3-deoxycitrate monohydrate, unit cell dimensions a ═ 8.062 (2), b ═ 10.215 (1), c ═ 6.832 (1) Å; α ═ 96.27 (1), β ═ 93.66 (1), γ ═ 69.00 (1)°; space group P1