Uric acid salts of magnesium: crystal and molecular structures and thermal analysis of two phases of Mg(C 5H 3N 4O 3) 2 · 8H 2O

Abstract

Two structurally different phases of a uric acid salt of magnesium, Mg(hydrogenurate) 2 · 8H 2O, have been prepared by crystallization from solution at pH = 7.5–8.0 and were investigated by x-ray crystallography, thermal analysis, and ir spectroscopy. Both phases are monoclinic, space group P2 1/c with a = 9.573(2), b = 14.627(3), c = 7.170(1) Å, β = 101.91(1)° (phase I) and a = 10.397(2), b = 14.306(3), c = 6.732(1) Å, β = 104.64(2)° (phase II). The crystal structures of both phases (R = 0.053 and 0.051, respectively) contain isolated octahedral [Mg(H 2O) 6] 2+ cations, hydrogenurate monoanions, and two molecules of water of crystallization per formula unit. The structural formula representing these facts is [Mg(H 2O) 6] (hydrogenura-te) 2·2H 2O. The tautomeric form of the hydrogenurate molecule corresponds to the tri-keto form of uric acid deprotonated at N(3). Differences in bond length between uric acid and the hydrogenurate molecule may be described in terms of three additional resonance structures distributing the formal negative charge at N(3) within the pyrimidine (but not the imidazole) ring. Deprotonation at N(3) significantly decreases the internal C-N-C angle at N(3). Alternating pairs of medium-strong intermolecular N-HO hydrogen bonds lead to infinite chains of hydrogenurate molecules extending along the b axis of the unit cells in both phases. The main difference between the two phases lies in their stacking pattern of the hydrogenurate molecules. Infrared data confirm the hydrogen bonding characteristics resulting from the crystal structure analysis. Thermogravimetric measurements and differential scanning calorimetry data show that the dehydration of both phases occurs in two distinct steps with Mg(hydrogenurate) 2.6H 2O as an intermediate phase. The first dehydration step (−2H 2O) is a topotactic reaction with three-dimensional preservation of the main structure elements of the octahydrate in the structure of the hexahydrate.