Gold Nanoparticle-Catalyzed Solvent Switchable Selective Partial Reduction of Nitrobenzene to N-Phenylhydroxylamine and Azoxybenzene

Abstract

Impregnation of phosphine-decorated polymer-immobilized ionic liquid with the tetrachloroaurate anion results in reduction of the gold(III) to gold(I) with concomitant oxidation of the phosphine to its oxide. In situ reduction of the resulting precursor, AuCl@O = PPh₂-PEGPIILS, generated the corresponding O = PPh₂-PEGPIIL-stabilized AuNPs, AuNP@O = PPh₂-PEGPIILS, which is a highly active and selective catalyst for the solvent-dependent partial reduction of nitrobenzene to N-phenylhydroxylamine in water and azoxybenzene in ethanol. The initial TOFs are comparable to those obtained with gold nanoparticles generated by reduction of tetrachloroaurate-impregnated phosphine oxide-decorated polymer-immobilized ionic liquid AuCl₄@O = PPh₂-PEGPIILS, i.e., the activity and selectivity profiles do not appear to depend on whether the AuNPs are generated from Au(III) or in situ-generated Au(I). In stark contrast, gold nanoparticles prepared by NaBH₄ reduction of AuCl@PPh₂-PEGPIILS based on gold(I) confined in phosphine-modified polymer-immobilized ionic liquid gave markedly lower initial TOFs. The use of dimethylamine borane (DMAB) as the hydrogen donor resulted in a substantial and dramatic enhancement in activity for reductions conducted in water compared with NaBH₄ and the initial TOF of 20,400 mol nitrobenzene converted mol Au⁻¹ h⁻¹ obtained with AuNPs generated in situ from AuCl₄@O = PPh₂-PEGPIILS is among the highest to be reported for the metal nanoparticle catalyzed selective reduction of nitrobenzene to N-phenylhydroxylamine; this is a significant improvement on existing protocols, which should enable the partial selective reduction of nitroarenes to be conducted in water with a low catalyst loading under extremely mild conditions.