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 k nown for 125 years to exist in two crystalline phases; one phase (oran ge-red prisms) was reported, from solubility measurements, to be stabl e below 319 K and the other (yellow needles) above this temperature [R abe (1901). Z. Phys. Chem. 38, 175-184]. The transition temperature wa s confirmed by dilatometry [Cohen and Moesveld (1920). Z. Phys. Chi?m. 94, 450-464]. The crystal structure of the red prisms has been report ed [Herbstein, Kapon and Wielinski (1977). Acta Cryst. B33, 649-654] a nd that of the yellow needles is reported here. In both phases there a re stacks of anions with interposed cations, but the detailed arrangem ents 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 calo rimetry (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 meas urements give the enthalpy change of this endothermic conversion as 5. 0 kJ mol(-1). The high-temperature yellow phase decomposes before melt ing. Yellow crystals cooled slowly from above 423 K do not revert to t he red form. In the presence of a small amount of solvent (water), con version of the red into the yellow phase is visible at 333 K. Our expl anation is that the thermodynamic equilibrium temperature is indeed 31 9 K, that the forward transformation (red to yellow) in the dry state has such a large activation energy that it is fast enough for measurem ent by dynamic DSC only in the 423 K region, but that the rate is appr eciably increased by the presence of solvent. The reverse transformati on (yellow to red) occurs only below 319 K, where it is very slow in t he dry state but takes a few days in the presence of solvent. The deta ils of the structure determination (at 298 K) are: yellow laths (stabl e 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 = 1 5.431 (5), b = 15.830 (5), c = 3.951 (2) Angstrom, beta = 91.06 (10)de grees, 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 ha ve been measured for the red and yellow phases over the temperature ra nges 290-370 and 290-490 K, respectively.