Synthesis and X-ray Crystal Structure of 2 and 4-Trifluoromethyl Substituted Phenyl Semicarbazone and Thiosemicarbazone

Abstract

NMR and single crystal X-ray structure data for four structurally similar semicarbazones and thiosemicarbazones were compared. In solution, proton NMR showed considerable variation in their chemical shift values especially for the NH2 protons. In the case of the semicarbazones this peak appeared as a broad singlet with an integration ratio of two while for the thiosemicarbazones the amino group showed two distinct singlets with marked chemical shift differences. This is attributed to the differences in the canonical forms of the thiosemicarbazone amino group and the semicarbazone analogue. Additionally, we provide evidence that the 2-trifluoromethyl phenyl substituted semicarbazone (2) formed an intermolecular hydrogen bond with one of the hydrogens of the NH2 group while this was totally absent in the thiosemicarbazone. We explain this by the restricted rotation of the CN bond in the thiosemicarbazone due to its double bond character compared to the less restricted rotation in semicarbazone compound. NMR and single crystal X-ray structure data for four structurally similar semicarbazones and thiosemicarbazones were compared for their hydrogen bonding characteristics.