Palladium(II)-complexes of bi- and tri-dentate phosphine ligands: precursor for palladium-phosphorous nanoparticles and activity towards Suzuki-Miyaura coupling

Crystalline Pd7P3 and Pd3P0.8 nanoparticles (NPs) formed when palladium(II)-complexes (1 and 2) of (P, N) ligand [3-(diphenylphosphino)propan-1-amine] L1 and (P, N, O-) ligand [2-(((3-(diphenylphosphino)propyl)imino)-(phenyl)methyl)phenol] L2 were thermolyzed in a mixture of oleylamine (1 g) and oleic acid (6 mL), respectively. These NPs were characterized by powder-XRD, SEM and TEM. L1 and L2 and their Pd(II)-complexes (1 and 2) were characterized by multinuclear NMR. The complexes were also characterized with single crystal X-ray diffraction. The L1 coordinates with Pd(II) in a bidentate mode through P and N forming a six-membered chelate ring in 1. The ligand L2 coordinates with Pd in a monoanionic tridentate (P, N, O-) mode, which results in the formation of two six-membered chelate rings. The geometry around Pd in the two complexes is nearly square planar. The Pd-P bond lengths in 1 and 2 were found to be 2.225(2) and 2.234(16)angstrom, respectively. The Pd7P3 and Pd3P0.8 nano-phases, and their precursor complexes 1 and 2, were explored for their activity for catalyzing Suzuki-Miyaura coupling. The NPs were found to be less active as a catalyst for Suzuki coupling than the parent complexes. Both 1 and 2 showed good catalytic activity for Suzuki-Miyaura coupling of aryl bromides, which followed the order 2 > 1, i.e. it increases with dentate character. The catalysis with complexes 1 and 2 occurs via the formation of Pd-NPs associated with a ligand or its fragments as evidenced by TEM and SEM studies. NPs obtained from 2 have a higher percentage of P, which probably makes them more efficient. The Hg and PPh3 poisoning tests support the molecular nature of the catalysis. Three phase tests carried out with 1 and Pd7P3 reveal that the catalytic action of the complexes, as well as the NPs, involves both homogeneous and heterogeneous processes.

Automated summary

Crystalline Pd7P3 and Pd3P0.8 nanoparticles were obtained by thermolysis of palladium(II)-complexes of two different ligands. These nanoparticles showed lower catalytic activity than the parent complexes in Suzuki-Miyaura coupling reactions. The catalysis involved the formation of Pd-NPs associated with ligands or their fragments. Three phase tests revealed that both the complexes and the nanoparticles had both homogeneous and heterogeneous catalytic processes.