Schiff bases functionalized with PPh 2 and SPh groups and their Ni(II) and Pd(II) complexes: Synthesis, crystal structures and applications of a Pd complex for Suzuki–Miyaura Coupling

Abstract

The reactions of 2-hydroxyacetophenone with NH 2CH 2CH 2SPh and NH 2CH 2CH 2PPh 2 have resulted in Schiff bases having the chemical formulae 2-HO–C 6H 4C(CH 3) N–CH 2CH 2Y (Y = SPh ( L 1) or PPh 2 ( L 4)), which are potential (N, O, S) and (N, O, P) type ligands. Using an appropriate ketone, the Schiff base 2,4-(HO) 2–C 6H 4C(CH 3) N–CH 2CH 2SPh ( L 2) has also been synthesized in a similar fashion. L 1, L 2, L 4 and the compounds obtained by borohydride reduction of L 1 and L 2, viz. 2-HO–C 6H 4CH(CH 3)–NHCH 2CH 2Y [Y = SPh ( L 3) or PPh 2 ( L 5)] have been explored as ligands. In their complexes with Pd(II) and Ni(II), all the five ligands coordinate in a uninegative tridentate mode (N, O −, S/P). The proton, carbon-13 and phosphorus-31 (in the case of L 4 and L 5) NMR spectra of all the ligands and their Pd(II) and Ni(II) complexes have been found characteristic. Single crystal structures of L 1, L 2, [PdCl( L 1–H)] ( 1), [PdCl( L 3–H)] ( 3) and [NiCl( L 4–H)] ( 6) have been solved. The bond distances are: Pd–N, 2.005(3)–2.042(7) Å; Pd–S, 2.257(2)–2.2644(11) Å and Pd–Cl, 2.3091(11)–2.321(2) Å. The Ni–P, Ni–N and Ni–Cl bond lengths in 6 are 2.1324(8) 1.896(2) and 2.1940(7) Å respectively. The geometries of complexes 1, 3 and 6 have been found to be square planar. [Pd( L 2–H)Cl] has been immobilized on silica gel and used as a catalyst for Suzuki–Miyaura C–C coupling reactions of benzeneboronic acid with bromobenzene and substituted bromobenzenes. The yields are between 75% and 90%.