Comparing the Activity of N-Heterocyclic Carbene Iron Complexes and Free Carbene Precursors in the Hydrosilylation of CO2

Piano-stool iron complexes with N-heterocyclic carbene (NHC) ligands have shown high potential in the activation and conversion of C═O bonds in aldehydes, ketones, and esters. Here we demonstrate the expansion of this activity to the functionalization of CO2 with amines to form N-formamide products in the presence of a silane (PhSiH3). This synthetically valuable utilization of CO2 as C1-synthon is catalyzed by iron complexes containing a variety of different NHC ligands. Structural variations of the NHC ligands did not reveal any correlation with catalytic activity, with imidazole and triazole-derived NHCs reaching similar conversions. However, the use of BF4– as counterion in cationic complexes, and also incorporating a Cp* instead of a Cp spectator ligand led to a marked decrease in catalytic activity. While NHC iron complexes reach mediocre activity (about 50% conversion at 5% catalyst loading in 24 h), the corresponding imidazolium and triazolium carboxylates as precursors of free carbenes display excellent catalytic activity with essentially full conversion after 1 h at 1 mol % loading. Experiments at variable iron complex loading as well as carbene trapping experiments with S8 together with spectroscopic investigations demonstrated a high stability of the Fe–CNHC bond during catalysis and lend strong support to a NHC iron unit as catalytically active species rather than free NHC resulting from Fe–NHC dissociation, even though free NHCs are excellent catalysts for this reaction. Optimized of reaction conditions, in particular the addition of another equivalent of silane allowed to reach high conversions of piperidine and CO2 to N-formylpiperidine using a triazolylidene iron complex as catalyst precursor.