Pushing the limits of activity and stability: the effects of Lewis acids on non-heme iron–NHC epoxidation catalysts

Dyckhoff, Florian; Schlagintweit, Jonas F.; Reich, Robert M.; Kühn, Fritz E.

doi:10.1039/d0cy00631a

Cited by 25 publications

(37 citation statements)

References 34 publications

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“…Recently, we reported a heme‐analogous iron(II) complex bearing a methylene‐bridged 16‐membered macrocyclic tetra‐NHC ligand (cCCCC; Figure 1, center) and its iron(III) derivative [8] . This unique ligand induces unprecedented activity in olefin epoxidation catalysis of the respective Fe III complex (TOF up to 180 000 h −1 without additives; TOF up to 415 000 h −1 in presence of strong Lewis acids), [8b, 9] considerably exceeding that of the optimized homogeneous benchmark system methyltrioxorhenium (MTO, TOF up to 40 000 h −1 ) [10] . Both iron and rhenium containing catalysts utilize H 2 O 2 as oxidant [8b, 10] .…”

Section: Methodsmentioning

confidence: 99%

Activation of Molecular Oxygen by a Cobalt(II) Tetra‐NHC Complex**

Schlagintweit

Altmann

Böth

et al. 2020

Chemistry A European J

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The first dicobalt(III) μ2‐peroxo N‐heterocyclic carbene (NHC) complex is reported. It can be quantitatively generated from a cobalt(II) compound bearing a 16‐membered macrocyclic tetra‐NHC ligand via facile activation of dioxygen from air at ambient conditions. The reaction proceeds via an end‐on superoxo intermediate as demonstrated by EPR studies and DFT. The peroxo moiety can be cleaved upon addition of acetic acid, yielding the corresponding CoIII acetate complex going along with H2O2 formation. In contrast, both CoII and CoIII complexes are also studied as catalysts to utilize air for olefin and alkane oxidation reactions; however, not resulting in product formation. The observations are rationalized by DFT‐calculations, suggesting a nucleophilic nature of the dicobalt(III) μ2‐peroxo complex. All isolated compounds are characterized by NMR, ESI‐MS, elemental analysis, EPR and SC‐XRD.

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Section: Methodsmentioning

confidence: 99%

Activation of Molecular Oxygen by a Cobalt(II) Tetra‐NHC Complex**

Schlagintweit

Altmann

Böth

et al. 2020

Chemistry A European J

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“…Compared to 1 , complex 2 and its Fe(III) derivative, bearing a tetradentate macrocyclic NHC ligand cCCCC has been standing out with particularly high activity (turnover frequencies up to 183,000 h −1 ) and in comparison to other iron catalysts high turnover numbers (TON up to 4,300). Here again, the activity‐rising‐effect of additives (largely doubling TOFs to values up to 410,000 h −1 ) by applying Sc(OTf) 3 , Ce(OTf) 4 , or Fe(ClO 4 ) 3 ⋅ H 2 O on complex 2 is of high relevance in epoxidation catalysis [20] . These reference points showcase again the competitive potential of iron complexes over more expensive metal catalysts [4] …”

Section: Introductionmentioning

confidence: 83%

“…The presence of additives is supposed to facilitate the O−O bond heterolysis towards the formation of a highly electrophilic active species [18a,19] . The suppressing effect of Sc 3+ on the deactivation pathway leading to an oxo‐bridged Fe III −O−Fe III species of a non‐heme iron‐NHC‐complex epoxidation reaction is also another essential point with regard to a significant increase of the turnover number [20] …”

Section: Introductionmentioning

confidence: 99%

The Effect oftransAxial Isocyanide Ligands on Iron(II) Tetra‐NHC Complexes and their Reactivity in Olefin Epoxidation

et al. 2021

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The performance of trans axially substituted mono‐(2 a) and bis(tert‐butylisocyanide) (2 b) complexes derived from the highly active bio‐inspired iron(II) (pre‐ )catalyst 2 containing an equatorial macrocyclic tetra N‐heterocyclic carbene in homogenous olefin epoxidation catalysis is reported. H2O2 is used as oxidant in combination with the Lewis acid Sc(OTf)3 as additive resulting in a considerable improvement of catalytic activity. In contrast to other iron epoxidation catalysts, the introduction of π‐accepting isocyanide ligands does not improve the catalytic performance of 2 a and 2 b posed by cyclic voltammetry. However, besides their lower activity, a high temperature tolerance of both compounds is found as a unique feature for iron NHC epoxidation catalysts.

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“…16,21,29,30 These studies lead one to consider a purely organometallic analogue 1 of the classical Fe-porphyrin V as a promising electrocatalyst for CO2 reduction (Scheme 1). Iron complexes supported by macrocyclic tetracarbene ligand scaffolds have been intensively studied in recent years, but the focus has been on their use in oxidation/oxygenation [31][32][33][34][35][36] and aziridination chemistry [37][38][39] as well as the isolation of high-valent and biorelevant reactive intermediates. [40][41][42][43][44] In contrast, the reductive chemistry of such "organometallic heme analogues" 35,45 has hardly been studied so far.…”

Section: Introductionmentioning

confidence: 99%

Selective Electrocatalytic CO2 Reduction to CO by an NHC-Based Organometallic Heme Analogue

Massie¹,

Schremmer²,

Rüter³

et al. 2020

Preprint

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Molecular first-row transition metal complexes for electrocatalytic CO2 reduction mostly feature N-donor supporting ligands, iron porphyrins being among the most prominent catalysts. Introducing N-heterocyclic carbene (NHC) ligation has previously shown promising effects for some systems, yet the application of NHC iron complexes for electrochemical CO2 reduction has so far remained unreported. Herein we show that the macrocyclic tetracarbene iron complex [LFe(NCMe)2](OTf)2 (1), which can be described as an organometallic heme analogue, mediates selective electrocatalytic CO2-to-CO conversion with a faradaic efficiency of over 90% and a very high initial observed catalytic rate constant (kobs) of 7,800 s−1. Replacement of an axial MeCN ligand by CO significantly increases the catalyst stability and turnover number, while the rate of catalysis decreases only slightly (kobs = 3,100 s−1). Ferrous complexes with one or two axial CO ligands, [LFe(NCMe)(CO)](OTf)2 (1-CO) and [LFe(CO)2](OTf)2 (1-(CO)2), have been isolated and fully characterized. Based on linear sweep voltammogram (LSV) spectroelectro-IR (SEC-IR) studies for 1 and 1-CO, both under N2 and CO2 atmosphere, a mechanistic scenario in anhydrous acetonitrile is proposed. It involves two molecules of CO2 and results in CO and CO32− formation, whereby the first CO2 binds to the doubly reduced, pentacoordinated [LFe0(CO)] species. This work commences the exploration of the reductive chemistry by the widely tunable macrocyclic tetracarbene iron motif, which is topologically similar to hemes but electronically distinct as the strongly s-donating and redox inactive NHC scaffold leads to metal-centered reduction and population of the exposed dz² orbital, in contrast to ligand-based orbitals in the analogous porphyrin systems.

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Pushing the limits of activity and stability: the effects of Lewis acids on non-heme iron–NHC epoxidation catalysts

Abstract: Tetradentate iron–NHC complexes exhibit unprecedented activity (TOF: 410 000 h−1) in the epoxidation of cis-cyclooctene by addition of Lewis acids.

Cited by 25 publications

References 34 publications

Activation of Molecular Oxygen by a Cobalt(II) Tetra‐NHC Complex**

Activation of Molecular Oxygen by a Cobalt(II) Tetra‐NHC Complex**

The Effect oftransAxial Isocyanide Ligands on Iron(II) Tetra‐NHC Complexes and their Reactivity in Olefin Epoxidation

Selective Electrocatalytic CO2 Reduction to CO by an NHC-Based Organometallic Heme Analogue

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