Reusable Single Homogeneous Ir(III)–NHC Catalysts for Bidirectional Hydrogenation–Dehydrogenation of N-Heteroarenes in Water

Abstract

Bidirectional hydrogenation–dehydrogenation (BHD) catalysis is of utmost importance in the context of hydrogen storage and reutilization. There are several systems based on carbocycles/hydrogenated carbocycles and N-heterocycles/hydrogenated N-heterocycles available for storing H2 gas via hydrogenation and extracting it via the reverse dehydrogenation. However, single bidirectional catalysts to achieve both the hydrogenation and dehydrogenation reactions are rare, especially in water as the desired reaction solvent, and therefore developing such catalysts would lead to a significant development in the area. Moreover, the reuse of homogeneous aqueous-phase BHD catalysts in additional catalytic runs would practicalize a sustainable catalyst-utilization protocol. This work reports such a development where a water-soluble homogeneous iridium catalyst efficiently performed bidirectional hydrogenation–dehydrogenation of quinoxaline- and quinoline-based heterocycles in H2O under relatively mild conditions (1 atm H2, 50 °C for hydrogenation, and 100 °C for dehydrogenation), and it could be reused for running additional reaction cycles, thereby improving the net efficiency. Further, the catalyst was found to be durable as a 40 day-old stock solution of the catalyst in water still maintained high activity. A unique ligand platform consisting of uracil-containing abnormal N-heterocyclic carbene (aNHC) around the Cp*Ir center in the catalyst enabled the desired properties such as water-solubility (required for catalyst separation/reuse), metal–ligand bifunctional activation of H2 and hydrogenated N-heterocycles (during hydrogenation/dehydrogenation), and efficient hydride delivery (during hydrogenation/dehydrogenation). The mechanistic steps, as analyzed by computational studies, underscored that the activation and release of H2 were the rate-determining steps in the hydrogenation and dehydrogenation, respectively.