Palladium complexes of 2-formylpyridine thiosemicarbazone and two related ligands: Synthesis, structure and, spectral and catalytic properties

• 2-Formylpyridine thiosemicarbazone and two related ligands (HL-R) are chosen.
• The thiosemicarbazone ligands react with [Pd(PPh3)2Cl2].
• Mixed-ligand complexes are formed.
• NNS-mode of binding is observed without any rotation around the imine bond.
• The palladium complexes efficiently catalyze C–C and C–N coupling reactions.

Thiosemicarbazones of 2-formylpyridine, 2-aceylpyridine and 2-benzoylpyridine, abbreviated in general as HL-R (where H depicts the acidic hydrogen and R the fragment (R = H, Me and Ph) linked to the imine-carbon) react with [Pd(PPh3)2Cl2] in refluxing ethanol in the presence of triethylamine to afford complexes of the type [Pd(L-R)(PPh3)]Cl (1, R = H; 2, R = Me, 3, R = Ph). Structures of the complexes 1 and 3 have been determined by X-ray crystallography, and the structure of complex 2 has been optimized by DFT. In all three the thiosemicarbazone ligand binds to the metal center as a monoanionic tridentate NNS-donor forming two adjacent five-membered chelate rings, the triphenylphosphine occupies the fourth coordination site on palladium. Complexes 1–3 display intense absorptions in the visible and ultraviolet regions, which have been analyzed by TDDFT calculations. All the complexes are found to efficiently catalyze Suzuki, Heck and Sonogashira type C–C cross-coupling, and C–N coupling reactions of aryl halides with primary and secondary amines. All the catalytic reactions are found to proceed under ligand-free condition.

Reaction of 2-formylpyridine thiosemicarbazone and two related ligands (HL-R) with [Pd(PPh3)2Cl2] affords the mixed-ligand complexes 1–3 with a NNS-mode of binding from the thiosemicarbazones. These complexes have shown efficient catalytic efficiency towards C–C and C–N coupling reactions of different types.Figure optionsDownload as PowerPoint slide