Investigation of 1,3,5-Triaza-7-phosphaadamantane-Stabilized Silver Nanoparticles as Catalysts for the Hydration of Benzonitriles and Acetone Cyanohydrin

Abstract

A straightforward synthesis of water-soluble silver nanoparticles stabilized by PTA (1,3,5-triaza-7-phosphaadamantane, a water-soluble phosphine ligand) ligands was developed. The nanoparticles were thoroughly characterized by ultraviolet–visible spectroscopy, 31P nuclear magnetic resonance spectroscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy. The effectiveness of the Ag–PTA nanoparticles as catalysts for the hydration of nitriles to amides in water under mild conditions was explored using a series of substituted benzonitriles and cyanohydrins. In comparison to all previously investigated homogeneous catalysts, the Ag–PTA system excels at cyanohydrin hydration, including acetone cyanohydrin hydration. Cyanohydrins are in equilibrium with small amounts of cyanide, and experiments revealed that the Ag–PTA nanoparticles disassemble in the presence of cyanide. The catalyst solution, which is proposed to contain a soluble Ag(CN)n1–n complex (with n likely equal to 2), remained unpoisoned even in the presence of 10 equiv of cyanide. It is suggested that no cyanide poisoning occurs because the Ag(I) complex is labile. Overall, the Ag–PTA catalyst system (a) is not poisoned by cyanide, (b) catalyzes hydration reactions under mild conditions (in air and at relatively low temperatures), (c) is easily synthesized from cheap starting materials, and (d) can hydrate heteroaromatics in good yields. The recognition of the importance of labile metal cyanide bonding represents an important step forward in catalyst design for improving the catalytic hydration of acetone cyanohydrin.