Multi-Electron Oxidation of Anthracene Derivatives by Nonheme Manganese(IV)-Oxo Complexes

Controlling Redox Potential of a Manganese(III)-Bis(hydroxo) Complex through Protonation and the Hydrogen-Atom Transfer Reactivity

A series of mononuclear manganese(III)-hydroxo and -aqua complexes, [MnIII(TBDAP)(OH)2]+ (1), [MnIII(TBDAP)(OH)(OH2)]2+ (2) and [MnIII(TBDAP)(OH2)2]3+ (3), were prepared from a manganese(II) precursor and confirmed using various methods including X-ray crystallography. Thermodynamic analysis showed that protonation from hydroxo to aqua species resulted in increased redox potentials (E1/2) in the order of 1 (-0.15 V) < 2 (0.56 V) < 3 (1.11 V), while pKa values exhibited a reverse trend in the order of 3 (3.87) < 2 (11.84). Employing the Bordwell Equation, the O-H bond dissociation free energies (BDFE) of [MnII(TBDAP)(OH)(OH2)]+ and [MnII(TBDAP)(OH2)2]2+, related to the driving force of 1 and 2 in hydrogen atom transfer (HAT), were determined as 75.3 and 77.3 kcal mol-1, respectively. It was found that the thermodynamic driving force of 2 in HAT becomes greater than that of 1 as the redox potential of 2 increases through protonation from 1 to 2. Kinetic studies on electrophilic reactions using a variety of substrates revealed that 1 is only weakly reactive with O-H bonds, whereas 2 can activate aliphatic C-H bonds in addition to O-H bonds. The reaction rates increased by 1.4 × 104-fold for the O-H bonds by 2 over 1, which was explained by the difference in BDFE and the tunneling effect. Furthermore, 3, possessing the highest redox potential value, was found to undergo an aromatic C-H bond activation reaction under mild conditions. These results provide valuable insights into enhancing electrophilic reactivity by modulating the redox potential of manganese(III)-hydroxo and -aqua complexes through protonation.

Iron-Catalyzed C-H Oxygenation Using Perchlorate Enabled by Secondary Sphere Hydrogen Bonds

Perchlorate (ClO4-) is a groundwater pollutant that is challenging to remediate. We report a strategy to use Fe(II) tris(2-pyridylmethyl)amine (TPA) complexes featuring appended aniline hydrogen bonds (H-bonds) to promote ClO4- reduction. These complexes facilitate oxygen atom transfer from ClO4- to PPh3 and C-H oxygenation reactions of organic substrates. Catalytic reactions using 15 mol % afforded excellent yields for oxygenation of anthracene and cyclic alkyl aromatics, and this methodology tolerates aryl halides as well as heterocycles containing either O, S, or N.

Synthetic Advances for Mechanistic Insights: Metal-Oxygen Intermediates with a Macrocyclic Pyridinophane System

ConspectusMetalloenzymes, which are proteins containing earth-abundant transition-metal ions as cofactors in the active site, generate various metal-oxygen intermediates via activating a dioxygen molecule (O2) to mediate vital metabolic functions, such as the oxidative metabolism of xenobiotics and the biotransformation of naturally occurring molecules. By replicating the active sites of metalloenzymes, many bioinorganic chemists have studied the geometric and electronic properties and reactivities of model complexes to understand the nature of enzymatic intermediates and develop bioinspired metal catalysts. Among the reported model complexes, nonporphyrinic macrocyclic ligands are the predominant coordination system widely used in stabilizing and isolating diverse metal-oxygen intermediates, which allows us to extensively investigate the physicochemical characteristics of the analogs of reactive intermediates of metalloenzymes. In particular, it has been reported that the ring size of the macrocyclic ligands, defined by the number of atoms in the macrocyclic ring, drastically affects the identity of the metal-oxygen intermediate. Thus, systematic modification of the macrocyclic ligands has been a great subject being examined in various inorganic fields.In this Account, we describe synthetic advances of a macrocyclic ligand system by introducing pyridine donors into a 12-membered tetraazamacrocyclic ligand (12-TMC) that initially has 4 amine donors. Interestingly, the backbone of the pyridinophane ligand with 2 pyridine and 2 amine donors in a 12-membered ring is shown to be much more folded than in other macrocyclic ligands, thereby allowing the axial and equatorial donors to separately control the electronic structure of metal complexes. Then, we looked over independent electronic and steric effects on metal-oxygen species with thorough physicochemical analysis. The NiIII-peroxo complexes exhibit nucleophilic reactivity dependent on the steric hindrance of the second coordination sphere. Furthermore, the C-H bond strength of the second coordination sphere has also been an important factor in determining the stability of MnIV-bis(hydroxo) intermediates. Electronic tuning on CoIII-hydroperoxo intermediates results in a trend between the electron-donating abilities of para-substituents on pyridine in the pyridinophane ligand and electrophilic reactivities, from which mechanistic insights into the metal-hydroperoxo species have been gained. Importantly, the metal-oxygen intermediates supported by the pyridinophane ligand system have revealed quite challenging chemical reactions, including dioxygenase-like nitrile activation by CoIII-peroxo intermediates and the oxidation of aldehyde and aromatic compounds by manganese-oxygen intermediates. Based on the fine substitution of donors, we have addressed that those novel reactions originated from the unique framework of the pyridinophane system incorporating spin-crossover behavior and high redox potentials of the metal-oxygen intermediates. These results will be valuable for the structure-activity relationship of metal-oxygen intermediates, giving a better understanding on the enzymatic coordination system where amino acid ligands vary for specific chemical reactions.

Nitrogen Atom Induced Contrast Effect on the Mechanofluorochromic Characteristics of Anthracene-Based Acceptor-Donor-Acceptor Fluorescent Molecules

The mechanofluorochromic (MFC) characteristics of anthracene-based acceptor-donor-acceptor (A-D-A) fluorescent molecules are explored through a comprehensive investigation of their photophysical behaviors. Six 9,10-diheteroarylanthracene derivatives with varying acceptor groups (pyridin-4-yl, pyridin-3-yl, pyridin-2-yl, pyrimidin-5-yl, pyrazinyl and quinoxalinyl) are synthesized and systematically characterized. The photophysical properties in both solution and solid-state are examined, revealing subtle yet significant influences of the spatial arrangement and number of nitrogen atoms within the acceptor group on fluorescence emission. Single-crystal structures of these compounds provide insights into their steric configurations and intermolecular packing modes, offering valuable insights into the fundamental mechanisms that underlie the observed MFC properties. This study illuminates the intricate interplay between MFC properties and the refined molecular structure, thus presenting promising avenues for the design and advancement of novel MFC materials.

A Contrasting Effect of Acid in Electron Transfer, Oxygen Atom Transfer, and Hydrogen Atom Transfer Reactions of a Nickel(III) Complex

There have been many examples of the accelerating effects of acids in electron transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. Herein, we report a contrasting effect of acids in the ET, OAT, and HAT reactions of a nickel(III) complex, [Ni^III(PaPy₃*)]²⁺ (1) in acetone/CH₃CN (v/v 19:1). 1 was synthesized by reacting [Ni^II(PaPy₃*)]⁺ (2) with magic blue or iodosylbenzene in the absence or presence of triflic acid (HOTf), respectively. Sulfoxidation of thioanisole by 1 and H₂O occurred in the presence of HOTf, and the reaction rate increased proportionally with increasing concentration of HOTf ([HOTf]). The rate of ET from diacetylferrocene to 1 also increased linearly with increasing [HOTf]. In contrast, HAT from 9,10-dihydroanthracene (DHA) to 1 slowed down with increasing [HOTf], exhibiting an inversely proportional relation to [HOTf]. The accelerating effect of HOTf in the ET and OAT reactions was ascribed to the binding of H⁺ to the PaPy₃* ligand of 2; the one-electron reduction potential (E_red) of 1 was positively shifted with increasing [HOTf]. Such a positive shift in the E_red value resulted in accelerating the ET and OAT reactions that proceeded via the rate-determining ET step. On the other hand, the decelerating effect of HOTf on HAT from DHA to 1 resulted from the inhibition of proton transfer from DHA^•+ to 2 due to the binding of H⁺ to the PaPy₃* ligand of 2. The ET reactions of 1 in the absence and presence of HOTf were well analyzed in light of the Marcus theory of ET in comparison with the HAT reactions.

Acid Catalysis in the Oxidation of Substrates by Mononuclear Manganese(III)-Aqua Complexes

Acids are known to enhance the reactivities of metal-oxygen intermediates, such as metal-oxo, -hydroperoxo, -peroxo, and -superoxo complexes, in biomimetic oxidation reactions. Although metal-aqua (and metal-hydroxo) complexes have been shown to be potent oxidants in oxidation reactions, acid effects on the reactivities of metal-aqua complexes have never been investigated previously. In this study, a mononuclear manganese(III)-aqua complex, [(dpaq^5NO2)Mn^III(OH₂)]²⁺ (1; dpaq^5NO2 = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-ylacetamidate with an NO₂ substituent at the 5 position), which is relatively stable in the presence of triflic acid (HOTf), is used in the investigation of acid-catalyzed oxidation reactions by metal-aqua complexes. As a result, we report a remarkable acid catalysis in the six-electron oxidation of anthracene by 1 in the presence of HOTf; anthraquinone is formed as the product. In the HOTf-catalyzed six-electron oxidation of anthracene by 1, the rate constant increases linearly with an increase of the HOTf concentration. Combined with the observed one-electron oxidation product, anthracene (derivative) radical cation, and the substitution effect at the 5 position of the dpaq ligand in 1 on the rate constants of the oxidation of anthracene, it is concluded that the oxidation of anthracene occurs via an acid-promoted electron transfer (APET) from anthracene to 1. The dependence of the rate constants of the APET from electron donors, including anthracene derivatives, to 1 on the driving force of electron transfer is also shown to be well fitted by the Marcus equation of outer-sphere electron transfer. To the best of our knowledge, this is the first example showing acid catalysis in the oxidation of substrates by metal(III)-aqua complexes.

The most reactive iron and manganese complexes with N-pentadentate ligands for dioxygen activation—synthesis, characteristics, applications

The iron and manganese complexes that activate oxygen atom play multiple role in technologically relevant reactions as well as in biological transformations, in which exist in different redox states. Among them, high-valent oxo intermediate seems to be the most important one. Iron, and/or manganese-based processes have found application in many areas, starting from catalysis and sustainable technologies, through DNA oxidative cleavage, to new substances useful in chemotherapeutic drugs. This review is not only the latest detailed list of uses of homogeneous N-pentadentate iron and manganese catalysts for syntheses of valuable molecules with huge applications in green technologies, but also a kind of "a cookbook", collecting "recipes" for the discussed complexes, in which the sources necessary to obtain a full characterization of the compounds are presented. Following the catalytic activity of metalloenzymes, and taking into account the ubiquity of iron and manganese salts, which in combination with properly designed ligands may show similarity to natural systems, the discussed complexes can find application as new anti-cancer drugs. Also, owing to ability of oxygen atom to exchange in reaction with H₂O, they can be successfully applied in photodriven reactions of water oxidation, as well as in chemically regenerated fuel cells as a redox catalyst.

Mechanistic dichotomies in redox reactions of mononuclear metal–oxygen intermediates

This review article focuses on various mechanistic dichotomies in redox reactions of metal–oxygen intermediates with the emphasis on understanding and controlling their redox reactivity from experimental and theoretical points of view.

Nickel(ii)dibenzotetramethyltetraaza[14]annulene complex immobilized on amino-functionalized TUD-1: an efficient catalyst for immediate and quantitative epoxidation of cyclohexene under ambient conditions

Nickel(ii)dibenzotetramethyltetraaza[14]annulene complex immobilized on amino-functionalized TUD-1 as a new nanocatalyst for spontaneous and quantitative epoxidation of cyclohexene under ambient conditions.

Redox Reactivity of a Mononuclear Manganese-Oxo Complex Binding Calcium Ion and Other Redox-Inactive Metal Ions

Mononuclear nonheme manganese(IV)-oxo complexes binding calcium ion and other redox-inactive metal ions, [(dpaq)Mn^IV(O)]⁺-M ⁿ⁺ (1-Mⁿ⁺, M ⁿ⁺ = Ca²⁺, Mg²⁺, Zn²⁺, Lu³⁺, Y³⁺, Al³⁺, and Sc³⁺) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate), were synthesized by reacting a hydroxomanganese(III) complex, [(dpaq)Mn^III(OH)]⁺, with iodosylbenzene (PhIO) in the presence of redox-inactive metal ions (M ⁿ⁺). The Mn(IV)-oxo complexes were characterized using various spectroscopic techniques. In reactivity studies, we observed contrasting effects of M ⁿ⁺ on the reactivity of 1-M ⁿ⁺ in redox reactions such as electron-transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. In the OAT and ET reactions, the reactivity order of 1-M ⁿ⁺, such as 1-Sc³⁺ ≈ 1-Al³⁺ > 1-Y³⁺ > 1-Lu³⁺ > 1-Zn²⁺ > 1-Mg²⁺ > 1-Ca²⁺, follows the Lewis acidity of M ⁿ⁺ bound to the Mn-O moiety; that is, the stronger the Lewis acidity of M ⁿ⁺, the higher the reactivity of 1-M ⁿ⁺ becomes. In sharp contrast, the reactivity of 1-M ⁿ⁺ in the HAT reaction was reversed, giving the reactivity order 1-Ca²⁺ > 1-Mg²⁺ > 1-Zn²⁺ > 1-Lu³⁺> 1-Y³⁺> 1-Al³⁺ ≈ 1-Sc³⁺; that is, the higher is Lewis acidity of M ⁿ⁺, the lower the reactivity of 1-M ⁿ⁺ in the HAT reaction. The latter result implies that the Lewis acidity of M ⁿ⁺ bound to the Mn-O moiety can modulate the basicity of the metal-oxo moiety, thus influencing the HAT reactivity of 1-M ⁿ⁺; cytochrome P450 utilizes the axial thiolate ligand to increase the basicity of the iron-oxo moiety, which enhances the reactivity of compound I in C-H bond activation reactions.

Aromatic hydroxylation of anthracene derivatives by a chromium(iii)-superoxo complex via proton-coupled electron transfer

Aromatic hydroxylation of anthracene by a mononuclear nonheme Cr(iii)-superoxo complex proceeds via the rate-determining proton-coupled electron transfer, followed by fast further oxidation to anthraquinone.

Long-Lived Photoexcited State of a Mn(IV)-Oxo Complex Binding Scandium Ions That is Capable of Hydroxylating Benzene

Photoexcitation of a Mn^IV-oxo complex binding scandium ions ([(Bn-TPEN)Mn^IV(O)]²⁺-(Sc(OTf)₃)₂) in a solvent mixture of trifluoroethanol and acetonitrile (v/v = 1:1) resulted in formation of the long-lived photoexcited state, which can hydroxylate benzene to phenol. The photohydroxylation of benzene by [(Bn-TPEN)Mn^IV(O)]²⁺-(Sc(OTf)₃)₂ was made possible by electron transfer from benzene to the long-lived ² E excited state of [(Bn-TPEN)Mn^IV(O)]²⁺-(Sc(OTf)₃)₂ to produce a benzene radical cation, which reacted with water as revealed by laser-induced transient absorption measurements.

Oxidation of Naphthalene with a Manganese(IV) Bis(hydroxo) Complex in the Presence of Acid

Naphthalene oxidation with metal-oxygen intermediates is a difficult reaction in environmental and biological chemistry. Herein, we report that a MnIV bis(hydroxo) complex, which was fully characterized by various physicochemical methods, such as ESI-MS, UV/Vis, and EPR analysis, X-ray diffraction, and XAS, can be employed for the oxidation of naphthalene in the presence of acid to afford 1,4-naphthoquinone. Redox titration of the MnIV bis(hydroxo) complex gave a one-electron reduction potential of 1.09 V, which is the most positive potential for all reported nonheme MnIV bis(hydroxo) species as well as MnIV oxo analogues. Kinetic studies, including kinetic isotope effect analysis, suggest that the naphthalene oxidation occurs through a rate-determining electron transfer process.

Involvement of high-valent manganese-oxo intermediates in oxidation reactions: realisation in nature, nano and molecular systems

Manganese plays multiple role in many biological redox reactions in which it exists in different oxidation states from Mn(II) to Mn(IV). Among them the high-valent manganese-oxo intermediate plays important role in the activity of certain enzymes and lessons from the natural system provide inspiration for new developments of artificial systems for a sustainable energy supply and various organic conversions. This review describes recent advances and key lessons learned from the nature on high-valent Mn-oxo intermediates. Also we focus on the elemental science developed from the natural system, how the novel strategies are realised in nano particles and molecular sites at heterogeneous and homogeneous reaction conditions respectively. Finally, perspectives on the utilisation of the high-valent manganese-oxo species towards other organic reactions are proposed.

Synthesis, Characterization, and Efficient Catalytic Activities of a Nickel(II) Porphyrin: Remarkable Solvent and Substrate Effects on Participation of Multiple Active Oxidants

A new nickel(II) porphyrin complex, [Ni^II (porp)] (1), has been synthesized and characterized by ¹ H NMR, ¹³ C NMR and mass spectrometry analysis. This Ni^II porphyrin complex 1 quantitatively catalyzed the epoxidation reaction of a wide range of olefins with meta-chloroperoxybenzoic acid (m-CPBA) under mild conditions. Reactivity and Hammett studies, H₂¹⁸ O-exchange experiments, and the use of PPAA (peroxyphenylacetic acid) as a mechanistic probe suggested that participation of multiple active oxidants Ni^II -OOC(O)R 2, Ni^IV -Oxo 3, and Ni^III -Oxo 4 within olefin epoxidation reactions by the nickel porphyrin complex is markedly affected by solvent polarity, concentration, and type of substrate. In aprotic solvent systems, such as toluene, CH₂ Cl₂ , and CH₃ CN, multiple oxidants, Ni^II -(O)R 2, Ni^IV -Oxo 3, and Ni^III -Oxo 4, operate simultaneously as the key active intermediates responsible for epoxidation reactions of easy-to-oxidize substrate cyclohexene, whereas Ni^IV -Oxo 3 and Ni^III -Oxo 4 species become the common reactive oxidant for the difficult-to-oxidize substrate 1-octene. In a protic solvent system, a mixture of CH₃ CN and H₂ O (95:5), the Ni^II -OOC(O)R 2 undergoes heterolytic or homolytic O-O bond cleavage to afford Ni^IV -Oxo 3 and Ni^III -Oxo 4 species by general acid catalysis prior to direct interaction between 2 and olefin, regardless of the type of substrate. In this case, only Ni^IV -Oxo 3 and Ni^III -Oxo 4 species were the common reactive oxidant responsible for olefin epoxidation reactions.