Catalytic activity of palladium(II) complexes with tridentate nitrogen ligands in the hydrogenation of alkenes and alkynes

Abstract

A series of palladium(II) complexes with the nitrogen ligands N-pyridin-2-yl- N′-pyridin-2-ylmethylene-hydrazine ( HL 1 ), and N, N-dimethyl- N′-pyridin-2-ylmethylene-ethane-1,2-diamine, ( L 2 ) have been synthesised and their catalytic activity in the hydrogenation of alkenes and alkynes in mild conditions ( P H 2 =1 atm, T=40 °C) has been studied. Ligand L 2 behaves as tridentate in the complexes Pd( L 2 )Cl 2 ( 4) and [Pd( L 2 )Cl](OTf) ( 5), whose catalytic activity is negligible in the hydrogenation of the tested substrates (styrene and phenylacetylene). The complexes Pd( L 1 )Cl ( 1) and Pd( L 1 )(OAc) ( 2) show a good catalytic activity towards styrene and phenylacetylene under homogeneous conditions. The high insolubility of the complex Pd 3( L 1 ) 2Cl 4 ( 3) prevents its use as catalyst in homogeneous hydrogenations; however, it has been employed in heterogeneous conditions, showing a very good chemo- and stereo-selectivity in the semi-hydrogenation of alkynes (phenylacetylene, diphenylacetylene and 4-octyne).