Selective Oxidation of Glycerol via Acceptorless Dehydrogenation Driven by Ir(I)-NHC Catalysts

Abstract

Iridium(I) compounds featuring bridge-functionalized bis-NHC ligands (NHC = N-heterocyclic carbene), [Ir(cod)(bis-NHC)] and [Ir(CO)2(bis-NHC)], have been prepared from the appropriate carboxylate- or hydroxy-functionalized bis-imidazolium salts. The related complexes [Ir(cod)(NHC)2]+ and [IrCl(cod)(NHC)(cod)] have been synthesized from a 3-hydroxypropyl functionalized imidazolium salt. These complexes have been shown to be robust catalysts in the oxidative dehydrogenation of glycerol to lactate (LA) with dihydrogen release. High activity and selectivity to LA were achieved in an open system under low catalyst loadings using KOH as a base. The hydroxy-functionalized bis-NHC catalysts are much more active than both the carboxylate-functionalized ones and the unbridged bis-NHC iridium(I) catalyst with hydroxyalkyl-functionalized NHC ligands. In general, carbonyl complexes are more active than the related 1,5-cyclooctadiene ones. The catalyst [Ir(CO)2{(MeImCH2)2CHOH}]Br exhibits the highest productivity affording TONs to LA up to 15,000 at very low catalyst loadings.

Homogeneous vs. heterogeneous catalysts for acceptorless dehydrogenation of biomass-derived glycerol and ethanol towards circular chemistry

“…Furthermore, the reaction rate is highly sensitive to temperature, emphasizing that these parameters must be well-controlled in making conclusions about the structure–activity relationships revealed by these examples. Jiménez & Perez-Torrente et al 20 synthesized series of Ir( i ) complexes featuring bridge-functionalized bis-NHC ligands, which have shown to be robust catalysts for dehydrogenation process of glycerol to LAC with dihydrogen release. When used in an open system with low catalyst loadings and KOH as a base, these catalysts achieved high levels of activity and selectivity towards LAC.…”

Homogeneous and Heterogeneous Catalysis of Glucose to Lactic Acid and Lactates: A Review

“…Indeed, used in the presence of a soluble base, these catalysts offer almost quantitative LA yields from sorbitol 95 and 93% yield from glycerol. 96 NHCs are strong σ-electron donors, which increases the electron density around the iridium complex. 97 They also enhance the steric bulkiness of the complex, while avoiding clustering of iridium.…”

“…N-Heterocyclic carbenes (NHCs), however, display extraordinary activity when used to self-support iridium complexes. Indeed, used in the presence of a soluble base, these catalysts offer almost quantitative LA yields from sorbitol and 93% yield from glycerol . NHCs are strong σ-electron donors, which increases the electron density around the iridium complex .…”