Hemisynthesis, crystal structure and inhibitory effect of sesquiterpenic thiosemicarbazones and thiazolidin-4-ones on the corrosion behaviour of stainless steel in 1†M H2SO4 solution.

Treatment of thiosemicarbazones prepared from sesquiterpenes with ethyl 2-bromoacetate in the presence of sodium acetate afforded the corresponding thiazolidin-4-ones. The structures of all the newly synthesized compounds were established by considering spectral and single-crystal X-ray diffraction data. The title compound, ethyl 2-((Z)-2-{(Z)-[(1aR,5aR,9aS)-1,1-dichloro-1a,5,5,7-tetramethyl-1a,2,3,4,5,5a,8,9-octahydro-1H-benzo[a]cyclopropa[b][7]annulen-8-ylidene]hydrazono}-4-oxothiazolidin-3-yl)acetate, C23H31Cl2N3O3S, 5, crystallizes in the orthorhombic noncentrosymmetric space group P212121 with Z = 4. Within the molecule in the crystal structure, the cyclohexene ring has an envelope conformation and the cycloheptane ring, to which it is fused, has a boat conformation. In the crystal, molecules are linked by C-H...Cl hydrogen bonds forming chains propagating along the b-axis direction. The absolute configuration of the molecule in the crystal could be fully confirmed from anomalous dispersion effects [Flack parameter = -0.04 (2)]. Thiosemicarbazones 1 and 2 are efficient inhibitors for steel corrosion in 1 M H2SO4 solution, with a maximum efficiency of 92.28% at 10-3 M. Furthermore, thiosemicarbazone compounds were found to be more efficient than thiazolidin-4-one derivatives. In addition, cyclic voltammetry was used to characterize the tested molecules, as well to estimate the experimental value of the energy band gap.