Phosphine-assisted synthesis of a nanostructured iridium catalyst for acceptorless dehydrogenation of alcohols and chemoselective hydrogenation of nitroarenes

Abstract

Ligand-assisted synthesis of noble metal nanoparticles has gained significant recent interest in the field of heterogeneous catalysis. In line with this, herein, we report the synthesis of a nanostructured iridium catalyst supported on silica via a one-pot wet chemical approach by treating IrCl₃ with commercially available phosphine-functionalized silica gel, without the use of an external reducing or stabilizing agent. The phosphine ligand attached to silica plays the dual role of stabilizing and reducing agents. High-resolution transmission electron microscopy (HR-TEM) analysis revealed the formation of uniformly sized iridium nanoparticles (Ir NPs ∼2 nm), consistently distributed throughout the silica surface without showing any signs of agglomeration. The nanomaterial was also characterized by other techniques such as SEM-EDX, XRD, ICP-AES, and XPS. To test the catalytic potential of the iridium-based material, oxidant-free dehydrogenation of primary alcohols to carboxylic acids, with concomitant liberation of two equivalents of molecular hydrogen, was chosen as the benchmark reaction. A range of primary alcohols, including usually less reactive aliphatic alcohols, can be converted to their corresponding carboxylates/acids in high yields, demonstrating excellent efficacy and a broad substrate scope, better than the state-of-the-art Ir-based heterogeneous catalysts reported to date for this transformation. In addition, the Ir-based nanomaterial was also explored as a catalyst for chemoselective hydrogenation of nitroarenes to aminoarenes. Moderate to excellent yields of the desired products were obtained with a diverse range of substrates. For both catalytic reactions, the catalyst was found to be recyclable for at least eight consecutive runs without significantly compromising yields.