Syntheses, spectral, X-ray and DFT studies of 5-benzyl-N-phenyl-1,3,4-thiadiazol-2-amine, 2-(5-phenyl-1,3,4-thiadiazol-2-yl) pyridine and 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-1,3,4-thiadiazole obtained by Mn(II) catalyzed reactions

• Substituted thiosemicarbazide/thiohydrazide get cyclized into thiadiazole in the presence of manganese(II) salt.
• The compounds are stabilized through intramolecular and weak intermolecular hydrogen bonding.
• X-ray crystallographic results are well produced by DFT calculations.
• The HOMO and LUMO energies of the molecules are negative indicating that the all compounds are stable.
• Lower energy gap for the compound 2 indicates better NLO properties as compared to compounds 1 and 3.

New compounds 5-benzyl-N-phenyl-1,3,4-thiadiazol-2-amine (Bptha, 1), 2-(5-phenyl-1,3,4-thiadiazol-2-yl) pyridine (Pthp, 2) and 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-1,3,4-thiadiazole (Mtmth, 3) have been synthesized and characterized with the aid of elemental analyses, IR, NMR and single crystal X-ray data. The structure of compounds 1, 2 and 3 are stabilized via intramolecular as well as intermolecular hydrogen bonding and crystallize in monoclinic system with space group P 1, P21/n and P 1, respectively. During the course of reaction, the substituted thiosemicarbazide/thiohydrazide get cyclized into the corresponding thiadiazole in the presence of manganese(II) nitrate via loss of H2O to yield compounds 1 and 2. However condensation occurred in the case of 5-methyl-1,3,4-thiadiazole-2-thiol which yielded 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-1,3,4-thiadiazole (3) by loss of one mole of H2S from two moles of 5-methyl-1,3,4-thiadiazole-2-thiol in the presence of manganese(II) acetate. The geometry optimization has been performed using DFT method and geometrical parameters thus obtained for the compounds have been compared with their single crystal X-ray data. The negative values of HOMO and LUMO energies for the molecules indicate that they are stable. The electronic transition from the ground state to the excited state due to a transfer of electrons from the HOMO to LUMO levels is mainly associated with the π⋯π transition.