CRYSTALLOGRAPHY OF METAL PICRATES .2. CRYSTAL-STRUCTURE OF YELLOW THALLIUM(I) PICRATE - RELATIONS AMONG VARIOUS METAL(I) PICRATE PHASES

Thallium(I) picrate, Tl+.C6H2N3O7- is an organic salt which has been known for 125 years to exist in two crystalline phases; one phase (orange-red prisms) was reported, from solubility measurements, to be stable below 319 K and the other (yellow needles) above this temperature [Rabe (1901). Z. Phys. Chem. 38, 175-184]. The transition temperature was confirmed by dilatometry [Cohen and Moesveld (1920). Z. Phys. Chi?m. 94, 450-464]. The crystal structure of the red prisms has been reported [Herbstein, Kapon and Wielinski (1977). Acta Cryst. B33, 649-654] and that of the yellow needles is reported here. In both phases there are stacks of anions with interposed cations, but the detailed arrangements are different (for example, the shortest Tl+...O distance is 0.2 Angstrom less in the yellow polymorph than in the red). The transition between the two phases is a single crystal of red to polycrystalline yellow, showing some preferred orientation. Differential scanning calorimetry (DSC) and X-ray powder diffraction show that the red phase is converted into the yellow at 423 K in the absence of solvent. DSC measurements give the enthalpy change of this endothermic conversion as 5.0 kJ mol(-1). The high-temperature yellow phase decomposes before melting. Yellow crystals cooled slowly from above 423 K do not revert to the red form. In the presence of a small amount of solvent (water), conversion of the red into the yellow phase is visible at 333 K. Our explanation is that the thermodynamic equilibrium temperature is indeed 319 K, that the forward transformation (red to yellow) in the dry state has such a large activation energy that it is fast enough for measurement by dynamic DSC only in the 423 K region, but that the rate is appreciably increased by the presence of solvent. The reverse transformation (yellow to red) occurs only below 319 K, where it is very slow in the dry state but takes a few days in the presence of solvent. The details of the structure determination (at 298 K) are: yellow laths (stable above 319 K): M(r) = 432.47, lambda(Mo K alpha) = 0.71069 Angstrom, F(000) = 784, monoclinic, mu(Mo K alpha) = 16.9 mm(-1), P2(1)/a, a = 15.431 (5), b = 15.830 (5), c = 3.951 (2) Angstrom, beta = 91.06 (10)degrees, V = 965.0 alpha(3), Z = 4, D-m = 2.993 [290 K, Rabe (1901)], D-x = 2.978 g cm(-3), R(int) = 0.0268, R(F)= 0.0394, wR = 0.0388 [based on 1701 independent reflections with F>3 sigma(F)]. Cell dimensions have been measured for the red and yellow phases over the temperature ranges 290-370 and 290-490 K, respectively.