Highly Active Pd Nanocatalysts Regulated by Biothiols for Suzuki Coupling Reaction

A series of biothiols involving N-acetyl-l-cysteine (NAC), penicillamine (PEN), homocysteine (Hcys) and cysteine (Cys), were employed as nucleation templates to synthesize Pd nanocatalysts for Suzuki coupling reaction. It is the first report to comparatively study the physicochemical properties of Pd nanoparticles synthesized by biothiols with different chemical structures. Through the characterizations of UV–Vis, FT-IR, TEM, XPS and HPLC, it is demonstrated that Pd(II) precursors can coordinate with biothiols mainly through thiol, amino/amido and carboxyl groups. NAC molecules are capable of generating uniform Pd nanoparticles with the size distribution from 2.2 to 3.3 nm. At the [PdCl42−]/[biothiol] of 6, NAC- and Hcys-stabilized Pd displays spherical particles with the average size of 3.3 and 3.6 nm respectively, whereas Cys- and PEN-capped Pd exhibit worm-like morphology with poor dispersion. NAC-capped Pd nanoparticles with the mean diameter of 3.3 nm and the Pd0 proportion of 63%, show highly active in Suzuki cross-coupling between iodobenzene and phenylboronic acid, with the turn over frequency (TOF) of 38625 mol biphenyl (mol Pd × h)−1 in EtOH/water = 1:2 at 60 °C. This work paves a promising way to construct highly active Pd nanocatalysts, with a facile modulation of the morphology, the particle size and the charge property via different biothiols.