A water-soluble Ir-NHC complex is demonstrated as an efficient, durable, reusable, and sustainable alternative catalyst for transfer-hydrogenation of quinolines and related N-heteroarenes with (buffered) formic acid.

Abstract

Transfer hydrogenation of N-heteroarenes was successfully achieved using a water-soluble half-sandwich Cp*Ir-based catalyst containing a uracil-based bifunctional abnormal NHC ligand, using HCOOH/HCOONa buffer solution as the hydrogen source. Reduction of N-heteroarenes was shown to be highly pH-dependent, and an acidic pH = 3.0 was found to be suitable for the best activity. The catalyst showed excellent functional group compatibility and high turnover number (up to 10,400), with catalyst loadings as low as 0.005 mol%. Finally, we demonstrated the catalyst's efficacy and applicability toward reusable and repetitive transfer hydrogenation using liquid/liquid extraction methodology, which underscored the significance of sustainable usage of noble metal catalysts in such transformations.