Efficient transfer hydrogenation of carbonate salts from glycerol using water-soluble iridium N-heterocyclic carbene catalysts

Abstract

The transfer hydrogenation of CO₂ and carbonates from biomass-derived alcohols, such as glycerol, to afford formic and lactic acid is a highly attractive path to valorizing two waste streams, and is significantly more thermodynamically favorable than direct carbonate hydrogenation. Expanding on our seminal report of the first homogeneous catalyst for this process, here we show that thermally-robust and water-soluble Ir(I) and Ir(III) N-heterocyclic carbene (NHC) complexes with sulfonate-functionalized wingtips are highly prolific and robust catalysts for carbonate transfer hydrogenation from glycerol, requiring no additives in aqueous media. The most prolific catalyst of the nine examined, [Ir(NHC-Ph-SO₃⁻)₂CO₂]Na (cat 7), effectively facilitates the reaction at low catalyst loading (10 ppm) at 150 °C using microwave or conventional heating. The cation of the carbonate salt significantly impacts catalytic activity, with highest activity observed with Cs₂CO₃ (27 850 and 13 350 TONs for lactate and formate respectively in 6 hours, compared to 15 400 and 8120 with K₂CO₃). Catalytic amounts of Cs⁺ were found to significantly enhance activity with K₂CO₃. Catalyst 7 is even more prolific with conventional heating under a positive N₂ pressure, reaching TOFs of >3000 h⁻¹ and >2100 h⁻¹ respectively for lactate and formate with K₂CO₃. The high activity of this catalyst compared to non-sulfonated and cyclooctadiene analogs is attributed to a combination of catalyst solubility in aqueous media and presence of π-acceptor carbonyl ligands. A catalytic mechanism is proposed for 7 involving O–H oxidative addition of glycerol, β-hydride elimination, bicarbonate dehydroxylation, insertion and reductive elimination.