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

Abstract

Crystalline Pd₇P₃ and Pd₃P_0.8 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) Å, respectively. The Pd₇P₃ and Pd₃P_0.8 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.