Kinetic Control of Zinc Cyamelurate Crystal Formations

The dynamic combinatorial library (DCL) of cyamelurates consists of 17 tautomers of cyameluric acid and their ions, as well as metal aqua complexes. That leads to a variety of compounds in the Zn(NO3)2 – K3(C6N7O3) – H2O system. All compounds are obtained with impurity amorphous and crystalline phases, which complicates their study. In this work, metastable phases of zinc, nickel, and iron cyamelurates were obtained and their crystal structures were determined for the first time. Kinetic control of chemical reactions allowed us to isolate two zinc cyamelurate metastable phases. The metastable compound K2Zn(C6N7O3H)2∙8H2O is formed immediately after mixing the precursor solutions, and after two hours, it is converted to another metastable compound Zn(C6N7O3H)∙5H2O. Due to hydrate isomerization, after 12 hours in the mother liquor, the latter transforms into a thermodynamically stable compound of the same composition. The features of the synthesis and crystalline structure of zinc cyamelurates can be interpreted from the standpoint of the nonclassical nucleation theory. The crystal structures of the synthesized compounds K2Zn(C6N7O3H)2∙8H2O, Zn(C6N7O3H)∙5H2O, and Zn(C6N7O3H2)2∙8H2O allowed us to conclude that decomposition of supersaturated zinc solution leads to the formation of double electric micelles (DEM). They can be considered nanoreactors playing an important role in crystal nucleation.