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 (L-1) or PPh2 (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)-C6H4C(CH3)=N-CH2CH2SPh (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-C6H4CH(CH3)-NHCH2CH2Y [Y = SPh (L 3) or PPh2 (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)] and [NiCl(L-4 -H)] (6) have been solved. The bond distances are: Pd-N, 2.005(3)-2.042(7) angstrom; Pd-S, 2.257(2)-2.2644(11) angstrom and Pd-Cl, 2.3091(11)-2.321(2) angstrom. The Ni-P, Ni-N and Ni-Cl bond lengths in 6 are 2.1324(8) 1.896(2) and 2.1940(7) angstrom 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%. (C) 2008 Elsevier Ltd. All rights reserved.