Schiff bases functionalized with PPh2 and SPh groups and their Ni(II) and Pd(II) complexes: Synthesis, crystal structures and applications of a Pd complex for Suzuki–Miyaura Coupling

The reactions of 2-hydroxyacetophenone with NH2CH2CH2SPh and NH2CH2CH2PPh2 have resulted in Schiff bases having the chemical formulae 2-HO–C6H4C(CH3)N–CH2CH2Y (Y = SPh (L1) or PPh2 (L4)), which are potential (N, O, S) and (N, O, P) type ligands. Using an appropriate ketone, the Schiff base 2,4-(HO)2–C6H4C(CH3)N–CH2CH2SPh (L2) has also been synthesized in a similar fashion. L1, L2, L4 and the compounds obtained by borohydride reduction of L1 and L2, viz. 2-HO–C6H4CH(CH3)–NHCH2CH2Y [Y = SPh (L3) or PPh2 (L5)] 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 L4 and L5) NMR spectra of all the ligands and their Pd(II) and Ni(II) complexes have been found characteristic. Single crystal structures ofL1, L2, [PdCl(L1–H)] (1), [PdCl(L3–H)] (3) and [NiCl(L4–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(L2–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%.

Graphical abstractThe Schiff bases 2-HO–C6H4C(CH3)N–CH2CH2Y (Y = SPh (L1) or PPh2 (L4)) and 2,4-(HO)2-C6H4C(CH3)N–CH2CH2SPh (L2) have been synthesized, and the compounds obtained by the borohydride reduction of L1 and L2, 2-HO–C6H4CH(CH3)–NHCH2CH2Y (Y = SPh (L3) or PPh2 (L5)), have also been synthesized. The ligands L1–L5 coordinate in a uninegative tridentate mode (N, O−, S/P) with Pd(II) and Ni(II). Single crystal structures of L1, L2, [PdCl(L1–H)] (1), [PdCl(L3–H)] (3) and [NiCl(L4–H)] (6) have been solved. Pd(L2–H)Cl] has been immobilized on silica gel and used as a catalyst for Suzuki–Miyaura C–C coupling reactions.Figure optionsDownload full-size imageDownload as PowerPoint slide