Rational Design of Sterically Hindered and Unsymmetrical NpyNimOph Pincer-Type Ligands and Their Palladium(II) Complexes: Catalytic Applications in Suzuki-Miyaura Reaction and Allylation of Aldehydes

Palladium(II) complexes 1-4 [Pd(L-1 to L-4)Cl] derived from pincer-type tridentate ligands having NpyNimOph donors were synthesized from ligands (LH)-H-1 to (LH)-H-4 (where (LH)-H-1=(2-phenyl-2-(pyridin-2-ylmethyl)hydrazono)methyl)phenol, (LH)-H-2 =[2-((2-phenyl-2-(pyridinyl-2-ylmethyl) hydrazono) methyl) phenol], (LH)-H-3=[2-((2-benzyl-2-phenylhydrazono) methyl)-4,6-di-tert-butylphenol], and (LH)-H-4=[2-((2-benzyl-2-phenylhydrazono)methyl)-6-(tert-butyl)-4-methoxy phenol where H stands for dissociable proton)in quantitative yields. Synthesized ligands (LH)-H-1 to (LH)-H-4 and corresponding palladium(II) complexes 1-4 were characterized by NMR, IR and HR-MS spectroscopic studies. Molecular structures of ligand (LH)-H-4 and complexes 1 and 3 were determined by X-ray crystallography. Complexes 1-4 were utilized as catalysts to investigate Suzuki-Miyaura cross-coupling reaction upon catalyst loading 0.01 mol%. These catalysts also catalysed allylation of aldehydes upto 95% yield upon catalyst loading 0.5 mol% with broad substrate scopes. Theoretical and experimental investigation was performed to speculate reaction pathway of Suzuki-Miyaura cross-coupling reaction. A reaction model has also proposed for allylation of aldehydes the presence of allyltributylstannane.