Novel ONN pyrazolyl-imine and imidazolyl-imine pincer palladium complexes as heck coupling catalysts

Five pyrazolyl and imidazolyl compounds, 2,4-di-tert-butyl-6-[2-pyrazol-1- yl-ethylimino-methyl]-phenol (L1), 2,4-di-tert-butyl- 6-{[2-(3,5-dimethyl- pyrazol-1-yl)-ethylimino]-methyl}-phenol (L2), 2,4-di-tert-butyl-6-{[2-(1H- imidazol-4-yl)-ethylimino]- methyl}-phenol (L3), 4-tert-butyl-2-{[2-(1H- imidazole-4-yl)-ethylimino]-methly}-phenol (L4) and 2-{[2-(1H-imidazole-4-yl)- ethylimino]-methly}-phenol (L5) were synthesized by condensation of the appropriate 2-hydroxybenzaldehyde and the corresponding alkylamine. Reactions of L1-L5 with either [Pd(NCMe) ₂Cl ₂] or [Pd(COD)MeCl] led to in situ deprotonation of the phenolic-OH proton to afford the pincer palladium complexes [Pd(L1)Cl] (1), [Pd(L1)Me] (2), [Pd(L2)Cl] (3), [Pd(L3)Cl] (4), [Pd(L4)Cl] (5) and [Pd(L5)Cl] (6). The tridentate coordination modes of the ligands were confirmed by the solid state structures of 1, 2, 3 and 4.H ₂O. Complexes 1-6 catalyzed the Heck coupling reactions of iodobenzene and butylacrylate. In addition, complex 4 catalyzed the Heck coupling reaction of butyl acrylate and bromobenzene; giving conversions as high as 70 %.