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

A series of palladium(II) complexes with the nitrogen ligands N-pyridin-2-yl-N'-pyridin-2-ylmethylene-hydrazine (HL1), 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-H2 = 1 atm, T = 40degreesC) 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)(2)Cl-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). (C) 2002 Elsevier Science B.V. All rights reserved.