E- or Z-Isomers Arising from the Geometries of Ligands in the Mercury Complex of 2-(Anthracen-9-ylmethylene)-N-phenylhydrazine Carbothioamide

[Image: see text] An anionic mercury(II) complex of 2-(anthracen-9-ylmethylene)-N-phenylhydrazine carbothioamide (HATU) and two isomers of a neutral mercury(II) complex of the anion of the same ligand (ATU) were reported. The anionic complex [Hg(HATU)(2)Cl(2)]·CH(2)Cl(2) had a monodentate HATU ligand (a neutral form of the ligand) and chloride ligands. The two conformational isomers were of the neutral mercury(II) complex Hg(ATU)(2)·2DMF. The two isomers were from the E or Z geometry of the ligands across the conjugated C=N–N=C–N scaffold of the coordinated ligand. The two isomers of the complex were independently prepared and characterized. The spectroscopic properties of the isomers in solution were studied by (1)H NMR as well as fluorescence spectroscopy. Facile conversion of the E-isomer to the Z-isomer in solution was observed. Density functional theory (DFT) calculations revealed that the Z-isomer of the complex was stable compared to the E-isomer by an energy of 14.35 kJ/mol; whereas, E isomer of the ligand was more stable than Z isomer by 8.37 KJ/mol. The activation barrier for the conversion of the E-isomer to the Z-isomer of the ligand was 167.37 kJ/mol. The role of the mercury ion in the conversion of the E-form to the Z-form was discussed. The mercury complex [Hg(HATU)(2)Cl(2)]·CH(2)Cl(2) had the E-form of the ligand. Distinct photophysical features of these mercury complexes were presented.