Bioinspirierte Nicht‐Häm‐Eisenoxidationskatalyse: Beteiligung von Oxoeisen(V)‐Oxidantien an der Spaltung starker C‐H‐Bindungen

Characterization of a Proposed Terminal Iron(III) Nitride Intermediate of Nitrogen Fixation Stabilized by a Trisphosphine-Borane Ligand

Terminal iron nitrides (Fe≡N) have been proposed as intermediates of Fe-mediated nitrogen fixation, and well-defined synthetic iron nitrides have been characterized in high oxidation states, including FeIV , FeV , and FeVI . This study reports the generation and low temperature characterization of a terminally bound iron(III) nitride, P3 B Fe(N) (P3 B =tris(o-diisopropylphosphinophenyl)borane), which is a proposed intermediate of iron-mediated nitrogen fixation by the P3 B Fe-catalyst system. CW- and pulse EPR spectroscopy (HYSCORE and ENDOR), supported by DFT calculations, help to define a 2 A ground state electronic structure of this C3 -symmetric nitride species, placing the unpaired spin in a sigma orbital along the B-Fe-N vector; this electronic structure is distinct for an iron nitride. The unusual d5 -configuration is stabilized by significant delocalization (≈50 %) of the unpaired electron onto the axial boron and nitrogen ligands, with a majority of the spin residing on boron.

Chemoselective Oxyfunctionalization of Functionalized Benzylic Compounds with a Manganese Catalyst

Whilst allowing for easy access to synthetically versatile motifs and for modification of bioactive molecules, the chemoselective benzylic oxidation reactions of functionalized alkyl arenes remain challenging. Reported in this study is a new non-heme Mn catalyst stabilized by a bipiperidine-based tetradentate ligand, which enables methylene oxidation of benzylic compounds by H2 O2 , showing high activity and excellent chemoselectivity under mild conditions. The protocol tolerates an unprecedentedly wide range of functional groups, including carboxylic acid and derivatives, ketone, cyano, azide, acetate, sulfonate, alkyne, amino acid, and amine units, thus providing a low-cost, more sustainable and robust pathway for the facile synthesis of ketones, increase of complexity of organic molecules, and late-stage modification of drugs.

Reactivity vs. Selectivity of Biomimetic Catalyst Systems of the Fe(PDP) Family through the Nature and Spin State of the Active Iron-Oxygen Species

Catalytic approaches to late-stage creation of new C-O bonds, especially via oxygenation of particular C-H groups in complex organic molecules, provide challenging tools for the synthesis of biologically active compounds and candidate drugs. In the last decade, significant efforts were invested in designing bioinspired iron based catalyst systems, capable of conducting selective oxidations of organic compounds. The key role of the oxygen-transferring high-valent iron-oxygen species in selective oxygenation is now well established; the next logical step would be gaining insight into the factors governing the oxidation chemo- and stereoselectivity, in relation to the peculiarities of their electronic structure, which would allow introducing the desired level of predictability into those catalytic transformations. In this Personal Account we analyze recent data on the reactivity of bioinspired formally oxoiron(V) catalytically active sites toward organic substrates having C=C and C(sp³ )-H groups. While the majority of reported oxoiron(V) active species are low-spin (S=1/2) complexes, the presence of strong electron-donating groups (NR₁ R₂ ) in the ligand backbone favors the high-spin (S=3/2) ground state. Remarkably, the high-spin perferryl species exhibit higher chemo-, regio-, and stereoselectivity in the oxidations than their low-spin counterparts, thus witnessing the significance of these subtle electronic effects for the selectivity of oxidations conducted by bioinspired catalysts of the Fe(PDP) family.

A Combined Spectroscopic and Computational Study on the Mechanism of Iron-Catalyzed Aminofunctionalization of Olefins Using Hydroxylamine Derived N-O Reagent as the "Amino" Source and "Oxidant"

Herein, we study the mechanism of iron-catalyzed direct synthesis of unprotected aminoethers from olefins by a hydroxyl amine derived reagent using a wide range of analytical and spectroscopic techniques (Mössbauer, Electron Paramagnetic Resonance, Ultra-Violet Visible Spectroscopy, X-ray Absorption, Nuclear Resonance Vibrational Spectroscopy, and resonance Raman) along with high-level quantum chemical calculations. The hydroxyl amine derived triflic acid salt acts as the “oxidant” as well as “amino” group donor. It activates the high-spin Fe(II) (S_t = 2) catalyst [Fe(acac)₂(H₂O)₂] (1) to generate a high-spin (S_t = 5/2) intermediate (Int I), which decays to a second intermediate (Int II) with S_t = 2. The analysis of spectroscopic and computational data leads to the formulation of Int I as [Fe(III)(acac)₂-N-acyloxy] (an alkyl-peroxo-Fe(III) analogue). Furthermore, Int II is formed by N–O bond homolysis. However, it does not generate a high-valent Fe(IV)(NH) species (a Fe(IV)(O) analogue), but instead a high-spin Fe(III) center which is strongly antiferromagnetically coupled (J = −524 cm^–1) to an iminyl radical, [Fe(III)(acac)₂-NH·], giving S_t = 2. Though Fe(NH) complexes as isoelectronic surrogates to Fe(O) functionalities are known, detection of a high-spin Fe(III)-N-acyloxy intermediate (Int I), which undergoes N–O bond cleavage to generate the active iron–nitrogen intermediate (Int II), is unprecedented. Relative to Fe(IV)(O) centers, Int II features a weak elongated Fe–N bond which, together with the unpaired electron density along the Fe–N bond vector, helps to rationalize its propensity for N-transfer reactions onto styrenyl olefins, resulting in the overall formation of aminoethers. This study thus demonstrates the potential of utilizing the iron-coordinated nitrogen-centered radicals as powerful reactive intermediates in catalysis.

Low-Spin and High-Spin Perferryl Intermediates in Non-Heme Iron Catalyzed Oxidations of Aliphatic C-H Groups

The selectivity patterns of iron catalysts of the Fe(PDP) family in aliphatic C-H oxidation with H₂ O₂ have been studied (PDP=N,N'-bis(pyridine-2-ylmethyl)-2,2'-bipyrrolidine). Cyclohexane, adamantane, 1-bromo-3,7-dimethyloctane, 3,7-dimethyloctyl acetate, (-)-acetoxy-p-menthane, and cis-1,2-dimethylcyclohexane were used as substrates. The studied catalyst systems generate low-spin (S=1/2) oxoiron(V) intermediates or high-spin (S=3/2) oxoiron(V) intermediates, depending on the electron-donating ability of remote substituents at the pyridine rings. The low-spin perferryl intermediates demonstrate lower stability and higher reactivity toward aliphatic C-H groups of cyclohexane than their high-spin congeners, according to the measured self-decay and second-order rate constants k₁ and k₂ . Unexpectedly, there appears to be no uniform correlation between the spin state of the oxoiron(V) intermediates, and the chemo- and regioselectivity of the corresponding catalyst systems in the oxidation of the considered substrates. This contrasts with the asymmetric epoxidations by the same catalyst systems, in which case the epoxidation enantioselectivity increases when passing from the systems featuring the more reactive low-spin perferryl intermediates to those with their less reactive high-spin congeners.

C-H Activation by an Iron-Nitrido Bis-Pocket Porphyrin Species

High-valent iron-nitrido species are nitrogen analogues of iron-oxo species which are versatile reagents for C-H oxidation. Nonetheless, C-H activation by iron-nitrido species has been scarcely explored, as this is often hampered by their instability and short lifetime in solutions. Herein, the hydrogen atom transfer (HAT) reactivity of an Fe porphyrin nitrido species (2 c) toward C-H substrates was studied in solutions at room temperature, which was achieved by nanosecond laser flash photolysis (LFP) of its Fe^III -azido precursor (1 c) supported by a bulky bis-pocket porphyrin ligand. C-H bonds with bond dissociation enthalpies (BDEs) of up to ≈84 kcal mol^-1 could be activated, and the second-order rate constants (k₂ ) are on the order of 10² -10⁴  s^-1  m^-1 . The Fe-amido product formed after HAT could further release ammonia upon protonation.

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

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 H2 O2 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.