Degradation pathways of a highly active iron(iii) tetra-NHC epoxidation catalyst

Abstract

Elucidation of different decomposition pathways of a highly active tetradentate iron–NHC epoxidation catalyst reveals direct carbene oxidation to be the decisive cause of catalyst degradation.

Cyclic iron tetra N-heterocyclic carbenes: synthesis, properties, reactivity, and catalysis

“…This finding has been explained by the oxidation of the solvent, MeCN, to form This journal is © The Royal Society of Chemistry 2023 glycolonitrile followed by decomposition to gaseous formaldehyde and HCN. 72,74 Indeed, when using t BuCN, PhCN or acetone as solvents bearing less activated C-H bonds, a m 2oxo bridged diiron(III) complex 44 is obtained (67% yield for acetone, Scheme 12). 74,77 The above described reactivity hasso far -not been reported for other m 2 -oxo diiron complexes.…”

Organometallic 3d transition metal NHC complexes in oxidation catalysis

“…Besides the formation of a μ 2 -oxo bridged Fe III -O-Fe III dimer, methylene bridge C-H oxidation is described. 100,101 For NHC supported iron catalysts, irreversible ligand protonation at the carbene C-donors with subsequent dissociation and/or oxidation of the ligand could be possible. 100 As mentioned before, degeneration over the Fe III -O-Fe III dimer can be excluded due to the regeneration with Lewis acids.…”

“…By HR-ESI-MS, an intermediate degradation species consisting of a mono-oxidized ligand still coordinating to an Fe( iii ) centre was found and correlates with DFT calculations. 101…”

The first macrocyclic abnormally coordinating tetra-1,2,3-triazole-5-ylidene iron complex: a promising candidate for olefin epoxidation

“…1,[7][8][9][10] Furthermore, the addition of Lewis acids to the catalytic reaction proofed to reactivate μ 2 -oxodiiron(III) Fe III -O-Fe III species, which has been identified as important deactivation product. 11,47 With the addition of 0.10 eq. Sc(OTf ) 3 to the reaction under standard conditions at 20 °C and 0.50 mol% of 2, the reaction is completed after 30 min and a 97% conversion is reached with a cyclooctene oxide yield of 93% (Fig.…”

“…S32 †). The carbene carbon signal is significantly high field shifted, compared to imidazole-2-ylidene iron complexes (δ( 13 C)C NHC = 205-194 ppm) 12,13,15,20,22,47 and especially its imidazole counterpart 1 Fe[cCCCC] im , where the carbene signals appear at δ( 13 C) = 205 ppm. 22 The observed 13 C shifts indicates a high electron density environment at the iron centre, as expected for a strong σ-donation.…”