Sonogashira (Cu and amine free) and Suzuki coupling in air catalyzed via nanoparticles formed in situ from Pd(II) complexes of chalcogenated Schiff bases of 1-naphthaldehyde and their reduced forms

The reaction of 1-naphthaldehyde with 2-(phenylthio/seleno) ethylamine afforded air- and moisture-insensitive Schiff bases: C10H7-1-CH=N-CH2CH2EPh (L1: E = S; L2: E = Se). Then, on treatment with NaOAc and Li2PdCl4, palladacycles, [Pd(L1-H/L2-H)Cl] (1/2) were formed at room temperature, in which L1/L2 are ligated as an unsymmetric (C-, N, E) pincer. The reduction of >C=N bonds of L1 and L2 with sodium borohydride gave C10H7-1-CH2NH-CH2CH2EPh (L3: E = S; L4: E = Se). The reactions of L3/L4 at room temperature, similar to those of L1/L2, resulted in the formation of complex [Pd(L3/L4)Cl-2] (3/4), in which the ligand is coordinated in a bidentate (N, E) mode. The yield of all the complexes was >85%. Characterization by HR-MS, H-1, C-13{H-1} and Se-77{H-1} NMR spectra of L1-L4 and their complexes 1-4 were performed. The structures of L1 and 1-4 were established with single-crystal X-ray diffraction. In all the complexes, the geometry of palladium was distorted square planar. The Pd-S bond distances in 1 and 3 were 2.426(12) and 2.259(2) angstrom, respectively, whereas Pd-Se bond lengths (angstrom) were 2.523(11) (2) and 2.369(10) (4) angstrom. The catalytic activities of 1-4 were explored for copper-and amine-free Sonogashira and Suzuki-Miyaura coupling (SMC) of aryl halides under aerobic conditions. The amount of catalyst required for achieving good conversion was 0.01 and 0.05 mol% for SMC and Sonogashira coupling, respectively. The conversion of some substrates reached a maximum in 1 and 2 h for Sonogashira coupling and SMC, respectively. The palladacycles as catalysts gave good conversion efficiency. The generation of palladium-containing nanoparticles (NPs) during both coupling reactions was observed. These were isolated and HR-TEM studies were performed on them and revealed their size as similar to 2-7 nm. The SEM-EDX analysis indicated the presence of organochalcogen ligands or their fragments in the samples. They independently catalyzed both reactions. Therefore, the role of 1-4 in catalysis undoubtedly exists. For Sonogashira coupling, the formation and role of such Pd-based NPs under aerobic conditions were observed for the first time. The complexes 1-4 showed the potential for reuse, as in the eighth cycle, conversion dropped by only 20%.