Crystal and molecular structures of 1,1-bis(methylthio)-4-(2-pyridyl)-2,3,5-triaza-1,3-pentadiene and its 5-phenyl derivative

Structures of the two dimethylthiomethylene-2-pyridinecarboxamide hydrazones have been examined spectroscopially and by X-ray diffraction to establish the dominant tautometric form and conformation. The two crystal structures are very similar with approximately planar conformation and intramolecular hydrogen bond between 4-imine group as hydrogen donor and pyridyl N atom as an acceptor. Several significant differences found may be easily explained by phenyl substitution in one of the compounds. The benzene ring in compound 2 is twisted by about 40o in relation to the mean molecular plane due to packing forces. An interesting feature of the structures is the deviation of the pyridyl ring by 11-14o from the mean 2,3,5-triaza-1,3-pentadiene plane in the two structures, despite the intramolecular hydrogen bond spanning the two fragments. It agrees with the elongation of C(4)-pyridine bond to 1.488 A, which corresponds with the lack of conjugation between p electrons of the pyridine ring and a lone pair at adjacent N(4) atom.