Group 6 Metal Complexes with Sulfur-Functionalized Triazolylidene Ligands: Synthesis, Characterization and Reactivity

This thesis describes the synthesis and characterization of new Group 6 metal complexes of chromium, molybdenum and tungsten, bearing new mesoionic 1,2,3-triazolylidene (MIC) ligands functionalized with sulfur groups. First, bromine-functionalized triazolium salt was synthesized following a two-step procedure previously reported in the literature. Subsequently, nucleophilic substitution reactions with thiols, namely propane-2-thiol and thiophenol, afforded two new triazolium salts. These compounds were fully characterized by NMR spectroscopy (1H, 13C, 19F) and mass spectrometry (ESI-MS). The propane-2-thiol-functionalized triazolium pro-ligand was reacted with the corresponding Group 6 metal hexacarbonyl precursors (M = Cr, Mo, W) in the presence of a strong base, leading to the in situ generation of the triazolylidene ligand and its subsequent coordination to the metal center. This synthetic approach afforded a new family of tetracarbonyl chromium, molybdenum and tungsten complexes, bearing bidentate propane-2-thiol-functionalized triazolylidene ligand. All new metal complexes were characterized by NMR spectroscopy (1H, 13C), FT-IR spectroscopy and elemental analysis (EA). Additionally, the molecular structures of the new molybdenum and tungsten complexes were confirmed by single-crystal X-ray diffraction (SC-XRD) analysis. The catalytic activity of the newly synthesized molybdenum and tungsten complexes was investigated in the acceptorless dehydrogenative coupling (ADC) and borrowing hydrogen (BH) reactions of an amine and an alcohol, using 4-methylbenzylamine and benzyl alcohol as model substrates. Their performance in homogeneous catalysis was compared with that of molybdenum and tungsten tetracarbonyl complexes bearing a bis-triazolylidene ligand, which were prepared following the procedures previously described in the literature by the group of Royo.