Epoxidation of Olefins Catalyzed by a Molecular Iron <i>N</i>‐Heterocyclic Carbene Complex: Influence of Reaction Parameters on the Catalytic Activity

Kück, Jens W.; Raba, Andreas; Markovits, Iulius I. E.; Cokoja, Mirza; Kühn, Fritz E.

doi:10.1002/cctc.201402063

Cited by 57 publications

(34 citation statements)

References 39 publications

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“…152 The ferrous complex [Fe II (71a)(CH3CN)2](PF6)2 was the first organometallic iron compound to be used as olefin epoxidation catalyst and also exhibits high efficiency in the oxygenation alkanes with H2O2 as the oxidant. 153,154 Under ambient conditions and at low catalyst loadings cyclohexane is oxidised with up to 32 turnovers and high selectivity towards the formation of cyclohexanol and cyclohexyl hydroperoxide ((A + H)/K = 19). Substitution of one of the axial acetonitrile units by a phosphine or isonitrile ligand stabilises the catalyst under oxidising conditions without altering the selectivity, which translates to increased turnovers.…”

Section: Iron Nonheme Complexes Bearing C- O-and S-donor Ligandsmentioning

confidence: 99%

“…125 As already mentioned for alkane oxidation reactions, iron-NHC complex [Fe II (71a)(CH3CN)2](PF6)2 was also applied for the oxygenation of aromatic substrates with H2O2 as the oxidant. 153,190 In the presence of 1 mol% catalyst benzene and toluene are converted to phenols in yields of 11.2 and 14.7% respectively. In the case of toluene a high selectivity for aromatic instead of benzylic oxidation is observed.…”

Section: Iron Nonheme Complexes Bearing C- O-and S-donor Ligandsmentioning

confidence: 99%

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Molecular iron complexes as catalysts for selective C–H bond oxygenation reactions

2015

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The selective oxygenation of C-H bonds is a promising and interesting task for both academia and chemical industry. Inspired by the efficiency and selectivity of naturally occurring enzymes, iron heme and nonheme complexes have proven to be valuable candidates for the development of environmentally friendly catalysts as alternative to traditional systems. This feature article summarises developments of the last decade regarding oxygenation reactions of aliphatic and aromatic substrates with various oxidants catalysed by molecular iron complexes. With a special focus on the catalytic performance of the systems, both the potential and the limitations as well as mechanistic aspects are discussed in some detail.

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Section: Iron Nonheme Complexes Bearing C- O-and S-donor Ligandsmentioning

confidence: 99%

Section: Iron Nonheme Complexes Bearing C- O-and S-donor Ligandsmentioning

confidence: 99%

Molecular iron complexes as catalysts for selective C–H bond oxygenation reactions

2015

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“…Selected known NHC/N-donor proligands are shown in Figure 1 and range from tri-and tetradentate macrocyclic systems, [H 2 L 1 ] 2+28 or [H 2 L 2 ] 2+ , 29 to pentadentate NHC/py 4 ligands [HL 3 ] + , 30 tetradentate bis(NHC)s [H 2 L 4 ] 2+ , 31 tridentate NHC/py 2 ligands [HL 5 ] + , 32 and pyridine-or pyrimidine-substituted imidazolium salts like [HL 6 ] + . 32b, 33 As an example of the beneficial use of such hybrid ligands in iron chemistry, we note that the ferric complex of the open chain NHC/pyridine ligand L 4b has been shown to serve as a catalyst for aromatic hydroxylation 34 and olefin epoxidation 35 reactions.…”

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Iron Complexes of a Macrocyclic N-Heterocyclic Carbene/Pyridine Hybrid Ligand

et al. 2015

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The tetradentate ligand system L 1 combines two Nheterocyclic carbene (NHC) and two pyridine donor functions in a macrocyclic scaffold. Its iron(II) complex [FeL 1 (MeCN) 2 ](PF 6 ) 2 (1) has been synthesized and fully characterized. The macrocyclic ligand in 1 is puckered and shows a significant barrier for ring inversion (ΔH ⧧ = 15.1 kcal mol −1 , and ΔS ⧧ = −4.7 cal mol −1 K −1 ). Axial ligands in 1 can be readily substituted to give heteroleptic [FeL 1 (CO)(MeCN)](PF 6 ) 2 (2) or neutral [FeL 1 (N 3 ) 2 ] (3). The strong ligand field of the NHC/pyridine hybrid ligand imparts low-spin states (S = 0) for all iron(II) complexes 1−3. Mossbauer data reflect the asymmetric electronic situation that results from the strongly covalent Fe−C NHC bonds in the basal plane constituted by the macrocyclic ligand L 1 . Oxidation of 1 has been monitored by UV−vis spectro-electro chemistry, and the resulting iron(III) complex [FeL 1 (MeCN) 2 ](PF 6 ) 3 (4) has been isolated after chemical oxidation. SQUID and Mossbauer data have shown an S = 1 / 2 ground state for 4, and X-ray crystallographic analyses of 1 and 4 revealed that structural parameters of the {FeL 1 } core are basically invariant with respect to changes in the metal ion's oxidation state. Density functional theory calculations support the experimental findings. The combined structural, spectroscopic, and electrochemical data for 1 with its {C 2 N 2 } hybrid ligand L 1 allowed for useful comparison with the related iron(II) complex that has a macrocyclic {C 4 } tetracarbene ligand. N-Heterocyclic carbenes (NHCs) have attracted a great deal of attention over the past two decades. NHCs, most of them imidazol-2-ylidenes, are now established as a prime ligand class in organometallic chemistry, with numerous applications in homogeneous catalysis.

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“…7−9 Examples of remarkable reactivity were obtained on the basis of the use of chelating, polydentate NHC ligands, 10−17 including oxidation catalysis with NHC/pyridine hybrid ligands. 18,19 Recently, it could be shown that replacing acetonitrile ligands of the catalytically active iron(II) NCCN complex has a strong influence on the electronic structure of the metal center. 20 However, the literature lacks comprehensive studies on the influence of NHC/pyridine hybrid ligands with different NHC/pyridyl ratios on the oxidation potential of the metal.…”

Section: ■ Introductionmentioning

confidence: 99%

NHC Versus Pyridine: How “Teeth” Change the Redox Behavior of Iron(II) Complexes

et al. 2015

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A series of octahedral iron(II) complexes with tetradentate NHC/pyridine hybrid ligands containing up to three pyridyl units was designed to study the influence of NHC and pyridine donors on the electronic structure of the metal center. Structural analysis of the iron complexes by NMR spectroscopy and single-crystal X-ray diffraction reveals different coordination modes of the ligand depending on the linkage of the different donor moieties. The oxidation potentials of all complexes correlate linearly with the number of NHC moieties coordinated to iron, as shown by cyclic voltammetry. The influence, although minor, of structural properties on the oxidation potential and (in one case) the influence of the oxidation state of the coordination geometry of the hybrid ligand are also demonstrated. ■ INTRODUCTIONN-heterocyclic carbene (NHC) ligands have become a multifunctional tool in organometallic chemistry and homogeneous catalysis, as steric and electronic properties can be easily influenced by modification of the substituents. 1−6 During the past decade, iron NHC complexes have especially received increasing attention. 7−9 Examples of remarkable reactivity were obtained on the basis of the use of chelating, polydentate NHC ligands, 10−17 including oxidation catalysis with NHC/pyridine hybrid ligands. 18,19 Recently, it could be shown that replacing acetonitrile ligands of the catalytically active iron(II) NCCN complex has a strong influence on the electronic structure of the metal center. 20 However, the literature lacks comprehensive studies on the influence of NHC/pyridine hybrid ligands with different NHC/pyridyl ratios on the oxidation potential of the metal. Such manipulations would allow fine-tuning of the metal center without blocking labile binding sites.Several alkylene-bridged tetradentate NHC/pyridine hybrid ligands are known (Figure 1). Ligands of the general types A (NCCN) and B and C (both CNNC) as well as their arylated derivatives 21−27 count for open-chain structures, while E fits into the class of macrocyclic ligands. 28 Three types of alkylenebridged tetra-NHC ligands (CCCC) have been described so far: open-chain (D), 29 macrocyclic (F), 30−32 and mixed aryl-or heteroatom-bridged ligands. 33−38 The coordination chemistry of the aforementioned acyclic type A di-NHC ligand derivatives has been well investigated. Complexes are known for group 10 metals, 21,22,25,26,39−41 coinage metals, 22,42 cobalt, 41 ruthenium, 43 and iron. 41,44 Only one example of class B is known; it is a highly rigid ligand and does not permit mononuclear, chelated metal complexes, with only a silver complex being reported. 23 The more flexible skeletal structure of type C offers chelating abilities, 24,27,45 although the reported palladium complex is strongly distorted with N−Pd−N and N−Pd−C angles of 80°in comparison to C−Pd−C angles of 120°. 24 Acyclic tetra-NHC ligands (D) are highly flexible, and complexes are known for coinage metals, group 10 metals, and iron. 29,46 For macrocyclic ligands of type E and F a broad r...

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Epoxidation of Olefins Catalyzed by a Molecular Iron N‐Heterocyclic Carbene Complex: Influence of Reaction Parameters on the Catalytic Activity

Cited by 57 publications

References 39 publications

Molecular iron complexes as catalysts for selective C–H bond oxygenation reactions

Molecular iron complexes as catalysts for selective C–H bond oxygenation reactions

Iron Complexes of a Macrocyclic N-Heterocyclic Carbene/Pyridine Hybrid Ligand

NHC Versus Pyridine: How “Teeth” Change the Redox Behavior of Iron(II) Complexes

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