Asymmetric Epoxidation with H₂O₂ by Manipulating the Electronic Properties of Non-heme Iron Catalysts

A non-heme iron complex that catalyzes highly enantioselective epoxidation of olefins with H₂O₂ is described. Improvement of enantiomeric excesses is attained by the use of catalytic amounts of carboxylic acid additives. Electronic effects imposed by the ligand on the iron center are shown to synergistically cooperate with catalytic amounts of carboxylic acids in promoting efficient O–O cleavage and creating highly chemo- and enantioselective epoxidizing species which provide a broad range of epoxides in synthetically valuable yields and short reaction times

Zirconium Hydrazides as Metallanitrene Synthons: Release of Molecular N₂ from a Hydrazinediido Complex Induced by Oxidative N–N Bond Cleavage

The N–N bond in the zirconium hydrazinediido(2−) complex [Zr­(N₂^TBSN_py)­(NNPh₂)­(py)] (<b>1</b>) is readily cleaved by one-electron oxidation. Reacting [Zr­(N₂^TBSN_py)­(NNPh₂)­(py)] (<b>1</b>) with 0.5 molar equiv of iodine led to the release of molecular N₂ and yielded the mixed diphenylamido/iodo complex [Zr­(N₂^TBSN_pyNPh₂)­(I)] (<b>2</b>). Exposure of hydrazinediide <b>1</b> to an excess of iodine resulted in further oxidation of the diphenylamido ligand, yielding the diiodo complex <b>3</b> and tetraphenylhydrazine. Similar reactivity was observed in the reaction of <b>1</b> with diphenyl diselenide and diaryl disulfides, which reacted to give the corresponding diphenylamido/arylchalcogenido complexes [Zr­(N₂^TBSN_pyNPh₂)­(SePh)] (<b>4a</b>) and [Zr­(N₂^TBSN_py)­(NPh₂)­(SAr)] (Ar = Ph (<b>4b</b>), C₆F₅ (<b>4c</b>)) along with N₂. The reactions were also carried out on an NMR scale with a ¹⁵N_α-labeled hydrazido complex (<b>1-</b>^<b>15</b><b>N</b>). In all cases a single ¹⁵N NMR resonance at 310.16 ppm, assigned to ¹⁵N₂, indicated the formation of dinitrogen from the N_α atom in the hydrazide. A crossover labeling experiment employing a 1:1 mixture of <b>1</b> and ¹⁵N_α-labeled <b>1-</b>^<b>15</b><b>N</b> revealed that the isotope distribution is, as expected, statistical 1:2:1 (¹⁴N₂: ^14/15N₂: ¹⁵N₂), which is consistent with a reaction pathway involving a dinuclear intermediate in the dinitrogen-forming step. Complex <b>1</b> reacted with N₂O to give a mixture of two compounds, the bis­(diphenylamido) complex <b>6</b> and the doubly bridged μ-oxo complex <b>7</b>. In contrast, reaction of <b>1</b> with 1 molar equiv of pyridinium <i>N</i>-oxide only gave the doubly bridged μ-oxo complex <b>7</b> along with 2,2′-bipyridine and diphenylamine

Highly Stereoselective Epoxidation with H₂O₂ Catalyzed by Electron-Rich Aminopyridine Manganese Catalysts

Fast, efficient, and highly stereoselective epoxidation with H₂O₂ is reached by manganese coordination complexes with e-rich aminopyridine tetradentate ligands. It is shown that the electronic properties of these catalysts vary systematically with the stereoselectivity of the O-atom transfer event and exert fine control over the activation of hydrogen peroxide, reducing the amount of carboxylic acid co-catalyst necessary for efficient operation

Highly Stereoselective Epoxidation with H₂O₂ Catalyzed by Electron-Rich Aminopyridine Manganese Catalysts

Fast, efficient, and highly stereoselective epoxidation with H₂O₂ is reached by manganese coordination complexes with e-rich aminopyridine tetradentate ligands. It is shown that the electronic properties of these catalysts vary systematically with the stereoselectivity of the O-atom transfer event and exert fine control over the activation of hydrogen peroxide, reducing the amount of carboxylic acid co-catalyst necessary for efficient operation

Toward the Understanding of the Structure–Activity Correlation in Single-Site Mn Covalent Organic Frameworks for Electrocatalytic CO₂ Reduction

The encapsulation of organometallic complexes into reticular covalent organic frameworks (COFs) represents an effective strategy for the immobilization of molecular electrocatalysts. In particular, well-defined polypyridyl Mn sites embedded into a crystalline COF backbone (COFbpyMn) were found to exhibit higher selectivity and activity toward electrochemical CO2 reduction compared to the parent molecular derivative noncovalently immobilized on carbon electrodes. In situ mechanistic studies revealed that the electronic and steric features of the reticular framework strongly affect the redox mechanism of the Mn sites, stabilizing the formation of a mononuclear Mn(I) radical anion intermediate over the most common off-cycle Mn0–Mn0 dimer. Herein, we report the study of a Mn-based COF (COFPTMn), introducing a larger phenanthroline building block, to explore how tuning the structural and electronic properties of the lattice may affect the catalytic CO2 reduction performance and the mechanism at the molecular level of the reticular system. The Mn sites encapsulated into the reticular COFPTMn exhibited a remarkable enhancement in the intrinsic catalytic CO2 reduction activity at near-neutral pH compared to that of the corresponding noncovalently immobilized molecular derivative. On the other hand, the poor crystallinity and porosity of COFPTMn, likely introduced by the lattice expansion and spatial dynamics of the phenanthroline linker, were found to limit its catalytic performances compared to those of the bipyridyl COFbpyMn analogue. ATR-IR spectroelectrochemistry revealed that the higher spatial mobility of the Mn sites does not completely suppress the Mn0–Mn0 dimerization upon the electrochemical reduction of the Mn sites at the COFbpyMn. This work highlights the positive role of the reticular structure of the material in enhancing its catalytic activity versus that of its molecular counterpart and provides useful hints for the future design and development of efficient reticular frameworks for electrocatalytic applications

Oxidant-Free Au(I)-Catalyzed Halide Exchange and C_sp2–O Bond Forming Reactions

Au has been demonstrated to mediate a number of organic transformations through the utilization of its π Lewis acid character, Au­(I)/Au­(III) redox properties or a combination of both. As a result of the high oxidation potential of the Au­(I)/Au­(III) couple, redox catalysis involving Au typically requires the use of a strong external oxidant. This study demonstrates unusual external oxidant-free Au­(I)-catalyzed halide exchange (including fluorination) and C_sp2–O bond formation reactions utilizing a model aryl halide macrocyclic substrate. Additionally, the halide exchange and C_sp2–O coupling reactivity could also be extrapolated to substrates bearing a single chelating group, providing further insight into the reaction mechanism. This work provides the first examples of external oxidant-free Au­(I)-catalyzed carbon–heteroatom cross-coupling reactions

Triggering the Generation of an Iron(IV)-Oxo Compound and Its Reactivity toward Sulfides by Ru^II Photocatalysis

The preparation of [Fe^IV(O)­(MePy₂tacn)]²⁺ (<b>2</b>, MePy₂tacn = <i>N</i>-methyl-<i>N</i>,<i>N</i>-bis­(2-picolyl)-1,4,7-triazacyclononane) by reaction of [Fe^II(MePy₂tacn)­(solvent)]²⁺ (<b>1</b>) and PhIO in CH₃CN and its full characterization are described. This compound can also be prepared photochemically from its iron­(II) precursor by irradiation at 447 nm in the presence of catalytic amounts of [Ru^II(bpy)₃]²⁺ as photosensitizer and a sacrificial electron acceptor (Na₂S₂O₈). Remarkably, the rate of the reaction of the photochemically prepared compound <b>2</b> toward sulfides increases 150-fold under irradiation, and <b>2</b> is partially regenerated after the sulfide has been consumed; hence, the process can be repeated several times. The origin of this rate enhancement has been established by studying the reaction of chemically generated compound <b>2</b> with sulfides under different conditions, which demonstrated that both light and [Ru^II(bpy)₃]²⁺ are necessary for the observed increase in the reaction rate. A combination of nanosecond time-resolved absorption spectroscopy with laser pulse excitation and other mechanistic studies has led to the conclusion that an electron transfer mechanism is the most plausible explanation for the observed rate enhancement. According to this mechanism, the in-situ-generated [Ru^III(bpy)₃]³⁺ oxidizes the sulfide to form the corresponding radical cation, which is eventually oxidized by <b>2</b> to the corresponding sulfoxide

Spectroscopic and DFT Characterization of a Highly Reactive Nonheme Fe^V–Oxo Intermediate

The reaction of [(PyNMe₃)­Fe^II(CF₃SO₃)₂], <b>1</b>, with excess peracetic acid at −40 °C generates a highly reactive intermediate, <b>2b</b>(PAA), that has the fastest rate to date for oxidizing cyclohexane by a nonheme iron species. It exhibits an intense 490 nm chromophore associated with an <i>S</i> = 1/2 EPR signal having <i>g</i>-values at 2.07, 2.01, and 1.94. This species was shown to be in a fast equilibrium with a second <i>S</i> = 1/2 species, <b>2a</b>(PAA), assigned to a low-spin acylperoxoiron­(III) center. Unfortunately, contaminants accompanying the <b>2</b>(PAA) samples prevented determination of the iron oxidation state by Mössbauer spectroscopy. Use of MeO-PyNMe₃ (an electron-enriched version of PyNMe₃) and cyclohexyl peroxycarboxylic acid as oxidant affords intermediate <b>3b</b>(CPCA) with a Mössbauer isomer shift δ = −0.08 mm/s that indicates an iron­(V) oxidation state. Analysis of the Mössbauer and EPR spectra, combined with DFT studies, demonstrates that the electronic ground state of <b>3b</b>(CPCA) is best described as a quantum mechanical mixture of [(MeO-PyNMe₃)­Fe^V(O)­(OC­(O)­R)]²⁺ (∼75%) with some Fe^IV(O)­(^•OC­(O)­R) and Fe^III(OOC­(O)­R) character. DFT studies of <b>3b</b>(CPCA) reveal that the unbound oxygen of the carboxylate ligand, O2, is only 2.04 Å away from the oxo group, O1, corresponding to a Wiberg bond order for the O1–O2 bond of 0.35. This unusual geometry facilitates reversible O1–O2 bond formation and cleavage and accounts for the high reactivity of the intermediate when compared to the rates of hydrogen atom transfer and oxygen atom transfer reactions of Fe^III(OC­(O)­R) ferric acyl peroxides and Fe^IV(O) complexes. The interaction of O2 with O1 leads to a significant downshift of the Fe–O1 Raman frequency (815 cm^–1) relative to the 903 cm^–1 value predicted for the hypothetical [(MeO-PyNMe₃)­Fe^V(O)­(NCMe)]³⁺ complex