Structural studies of tetrazoles. Crystal and molecular structure and ab initio calculations of 1-phenyl-1H-tetrazole-5-thiolate, as its [diaqua(18-crown-6)sodium] salt: An anionic tetrazole free of direct metal interactions

The structure of the anionic heterocycle obtained by deprotonation of 1-phenyl-1H-tetrazole-5-thione was determined by X-ray diffractometry. The counterion is [Na(18-crown-6)(H2O)2]+. There are no direct interactions between the metal and the tetrazole, while hydrogen bonds between sodium ion coordinated water molecules and neighboring azole anions give cohesion to the crystal. The thiotetrazolato ion displays extensive π-delocalization, with all members of the ring and the exocyclic sulfur contributing to it; however, no interannular conjugation between the phenyl and azole rings is found. The structure of the heterocycle is best described as a resonance hybrid. The geometric parameters of the metal-free tetrazole could be used as the input of ab initio MO calculations at the 6-31G*//3-21G* level, which supported the proposal that the negative charge is mainly concentrated on the N4-C5-S portion of the tetrazolate. Some small but significant and consistent bond length differences are found in the uncoordinated tetrazole with respect to metal-bonded analogues. The coordination behavior of the crown ether, with five oxygens bonded to sodium in the complex cation, is rare.