Highly Efficient Microwave-Accelerated Transfer Hydrogenation for CO₂-Derived Carbonate Valorization by Ru(II)-Protic NHC Pincer Catalysts

Abstract

The sustainable valorization of carbon dioxide and its derivatives into fuels and chemicals remains a major challenge in catalysis. Transfer hydrogenation (TH) using renewable polyols, such as glycerol, offers an attractive pathway that simultaneously enables the reduction of CO₂ derivatives and the upgrading of glycerol to value-added products. Herein, we report the synthesis of robust Ru(II)-protic-N-heterocyclic carbene (pNHC) (Ru9-10) bearing bipyridine ligands, prepared through an efficient microwave-accelerated protocol. Comprehensive characterization by multinuclear NMR spectroscopy and single-crystal X-ray diffraction confirmed their structure. The newly developed complexes exhibit remarkable catalytic activity in the TH of inorganic carbonates with glycerol under microwave conditions, affording formate and lactate as the major products. Among them, Ru10, featuring a mixed protic-NHC/bipyridine ligand environment, emerged as the most efficient, delivering record turnover frequencies exceeding 9.2 × 10⁴ h^-1 and turnover numbers approaching 1.1 × 10⁴ under mild conditions (microwave heating at 50 °C for 7 min). The catalytic activity is significantly affected by the nature of the cation in the carbonate salt . Cs₂CO₃ exhibits the highest activity, delivering TOF of 5.8 × 10⁵ h^-1 for lactate and 1.6 × 10⁵ h^-1 for formate, whereas K₂CO₃ in the presence of 10% Cs₂CO₃ shows comparable performance, with TOF values of 5.3 × 10⁵ h^-1 and 1.4 × 10⁵ h^-1, respectively for lactate and formate production, under the same conditions. Control studies highlight the formation of hydride intermediates confirmed by spectroscopic analysis. Notably, microwave irradiation markedly enhances reaction rates compared to conventional thermal protocols, providing a faster and scalable route to carbonate hydrogenation. Mechanistic investigations, supported by NMR spectroscopy and mass spectrometry analyses, provide direct evidence for Ru-H and Ru-formate intermediates, substantiating a metal-mediated pathway involving β-hydride elimination and hydride transfer to carbonate. This study underscores the potential of Ru(II)-pNHC pincer catalysts as a general platform for CO₂-derived carbonate valorization, simultaneously advancing hydrogen storage, liquid organic hydrogen carrier (LOHC) production, and glycerol upgrading.