The Influence of Peripheral Substituent Modification on P(V), Mn(III), and Mn(V)(O) Corrolazines: X-ray Crystallography, Electrochemical and Spectroscopic Properties, and HAT and OAT Reactivities

Systematic Electronic Tuning on the Property and Reactivity of Cobalt-(Hydro)peroxo Intermediates

A series of cobalt(III)-peroxo complexes, [Co^III(R₂-TBDAP)(O₂)]⁺ (1^R2; R₂ = Cl, H, and OMe), and cobalt(III)-hydroperoxo complexes, [Co^III(R₂-TBDAP)(O₂H)(CH₃CN)]²⁺ (2^R2), bearing electronically tuned tetraazamacrocyclic ligands (R₂-TBDAP = N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)-p-R₂-pyridinophane) were prepared from their cobalt(II) precursors and characterized by various physicochemical methods. The X-ray diffraction and spectroscopic analyses unambiguously showed that all 1^R2 compounds have similar octahedral geometry with a side-on peroxocobalt(III) moiety, but the O-O bond lengths of 1^Cl [1.398(3) Å] and 1^OMe [1.401(4) Å] were shorter than that of 1^H [1.456(3) Å] due to the different spin states. For 2^R2, the O-O bond vibration energies of 2^Cl and 2^OMe were identical at 853 cm^-1 (856 cm^-1 for 2^H), but their Co-O bond vibration frequencies were observed at 572 cm^-1 for 2^Cl and 550 cm^-1 for 2^OMe, respectively, by resonance Raman spectroscopy (560 cm^-1 for 2^H). Interestingly, the redox potentials (E_1/2) of 2^R2 increased in the order of 2^OMe (0.19 V) < 2^H (0.24 V) < 2^Cl (0.34 V) according to the electron richness of the R₂-TBDAP ligands, but the oxygen-atom-transfer reactivities of 2^R2 showed a reverse trend (k₂: 2^Cl < 2^H < 2^OMe) with a 13-fold rate enhancement at 2^OMe over 2^Cl in a sulfoxidation reaction with thioanisole. Although the reactivity trend contradicts the general consideration that electron-rich metal-oxygen species with low E_1/2 values have sluggish electrophilic reactivity, this could be explained by a weak Co-O bond vibration of 2^OMe in the unusual reaction pathway. These results provide considerable insight into the electronic nature-reactivity relationship of metal-oxygen species.

Oriented External Electric Fields Regurating the Reaction Mechanism of CH4 Oxidation Catalyzed by Fe(IV)-Oxo-Corrolazine: Insight from Density Functional Calculations

Methane is the simplest alkane and can be used as an alternative energy source for oil and coal, but the greenhouse effect caused by its leakage into the air is not negligible, and its conversion into liquid methanol not only facilitates transportation, but also contributes to carbon neutrality. In order to find an efficient method for converting methane to methanol, CH₄ oxidation catalyzed by Fe(IV)-Oxo-corrolazine (Fe(IV)-Oxo-Cz) and its reaction mechanism regulation by oriented external electric fields (OEEFs) are systematically studied by density functional calculations. The calculations show that Fe(IV)-Oxo-Cz can abstract one H atom from CH₄ to form the intermediate with OH group connecting on the corrolazine ring, with the energy barrier of 25.44 kcal mol⁻¹. And then the product methanol is formed through the following rebound reaction. Moreover, the energy barrier can be reduced to 20.72 kcal mol⁻¹ through a two-state reaction pathway. Furthermore, the effect of OEEFs on the reaction is investigated. We found that OEEFs can effectively regulate the reaction by adjusting the stability of the reactant and the transition state through the interaction of electric field-molecular dipole moment. When the electric field is negative, the energy barrier of the reaction decreases with the increase of electric intensity. Moreover, the OEEF aligned along the intrinsic Fe‒O reaction axis can effectively regulate the ability of forming the OH on the corrolazine ring by adjusting the charges of O and H atoms. When the electric field intensity is −0.010 a.u., the OH can be directly rebounded to the CH₃· before it is connecting on the corrolazine ring, thus forming the product directly from the transition state without passing through the intermediate with only an energy barrier of 17.34 kcal mol⁻¹, which greatly improves the selectivity of the reaction.

Oxidative versus basic asynchronous hydrogen atom transfer reactions of Mn(III)-hydroxo and Mn(III)-aqua complexes

Hydrogen atom transfer (HAT) of metal-oxygen intermediates such as metal-oxo, -hydroxo and -superoxo species have so far been studied extensively. However, HAT reactions of metal-aqua complexes have yet to be...

Amphiphilicity in Oxygen Atom Transfer Reactions of Oxobis(iminoxolene)osmium Complexes

Oxobis(iminoxolene)osmium(VI) compounds (^Rap)₂OsO (^Rap = 2-(4-RC₆H₄N)-4,6-^tBu₂C₆H₂O) are readily deoxygenated by phosphines and phosphites to give five-coordinate (^Rap)₂Os(PR'₃) or six-coordinate (^Rap)₂Os(PR'₃)₂. Structural data indicate that this net two-electron reduction is accompanied by apparent oxidation of the iminoxolene ligands due to their greater ability to engage in π donation to the reduced deoxy form of the osmium complex. In (^Rap)₂Os(PR'₃)₂, the HOMO is a ligand-based combination of the iminoxolene redox-active orbitals, while the LUMO is a highly covalent metal-iminoxolene π* orbital. In the trans isomer, the HOMO is required to be ligand-localized by symmetry, while in the cis isomer, the ligands adopt a conformation that minimizes metal-ligand π* interactions in the HOMO. Kinetic studies indicate that the deoxygenations involve the rate-determining attack of the phosphorus(III) reagent on the five-coordinate oxo complexes. Varying the substituents of the aryl groups on the iminoxolene ligands or on the triarylphosphines has little effect on the rate of oxygen atom transfer, with the best correlation shown between oxygen atom transfer rates and the HOMO-LUMO gap of the oxo complexes. This suggests that the osmium oxo group shows a balance between electrophilic and nucleophilic character in its oxygen atom transfer reactions with phosphorus(III) reagents.

Fluorescence properties and quantum-chemical modeling of tert-butyl-substituted porphyrazines: Structural and ionization effect

Synthesis and identification of tetrakis-[5,6-bis(4-tert-butylphenyl)pyrazino] porphyrazine, tetra-(4-tert-butyl)phthalocyanine and octakis-(4-tert-butylphenyl)porphyrazine were carried out. Spectrophotometric method was used to study the spectral, acidic and fluorescence properties of the synthesized compounds. It was determined that the synthesized tert-butyl-substituted porphyrazines exhibit a high sensitivity of fluorescence to the molecule ionization. To understand the features of the spectral properties the geometry optimization and an analysis of energy levels and localization of highest occupied and lowest unoccupied molecular orbitals of the studied compounds were performed on the basis of density functional theory with the BP86 functional and the def2-TZVP basis set. The effect of substituents in molecular fragments of the macrocycle on the acidic and electro-optical properties of the studied compounds is revealed. Materials with pH-tunable fluorescence were designed.

Structure and Reactivity of (μ-Oxo)dimanganese(III,III) and Mononuclear Hydroxomanganese(III) Adducts Supported by Derivatives of an Amide-Containing Pentadentate Ligand

Mononuclear Mn^III-hydroxo and dinuclear (μ-oxo)dimanganese(III,III) complexes were prepared using derivatives of the pentadentate, amide-containing dpaq ligand (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate). Each of these ligand derivatives (referred to as dpaq^5R) contained a substituent R (where R = OMe, Cl, and NO₂) at the 5-position of the quinolinyl group. Generation of the Mn^III complexes was achieved by either O₂ oxidation of Mn^II precursors (for [Mn^II(dpaq^5OMe)]⁺ and [Mn^II(dpaq^5Cl)]⁺ or PhIO oxidation (for [Mn^II(dpaq^5NO₂)]⁺). For each oxidized complex, ¹H NMR experiments provided evidence of a water-dependent equilibrium between paramagnetic [Mn^III(OH)(dpaq^5R)]⁺ and an antiferromagnetically coupled [Mn^IIIMn^III(μ-O)(dpaq^5R)₂]²⁺ species in acetonitrile, with the addition of water favoring the Mn^III-hydroxo species. This conversion could also be monitored by electronic absorption spectroscopy. Solid-state X-ray crystal structures for each [Mn^IIIMn^III(μ-O)(dpaq^5R)₂](OTf)₂ complex revealed a nearly linear Mn-O-Mn core (angle of ca. 177°), with short Mn-O distances near 1.79 Å, and a Mn···Mn separation of 3.58 Å. X-ray crystallographic information was also obtained for the mononuclear [Mn^III(OH)(dpaq^5Cl)](OTf) complex, which has a short Mn-O(H) distance of 1.810(2) Å. The influence of the 5-substituted quinolinyl moiety on the electronic properties of the [Mn^III(OH)(dpaq^5R)]⁺ complexes was demonstrated through shifts in a number of ¹H NMR resonances, as well as a steady increase in the Mn^III/II cyclic voltammetry peak potential in the order [Mn^III(OH)(dpaq^5OMe)]⁺ < [Mn^III(OH)(dpaq)]⁺ < [Mn^III(OH)(dpaq^5Cl)]⁺ < [Mn^III(OH)(dpaq^5NO₂)]⁺. These changes in oxidizing power of the Mn^III-hydroxo adducts translated to only modest rate enhancements for TEMPOH oxidation by the [Mn^III(OH)(dpaq^5R)]⁺ complexes, with the most reactive [Mn^III(OH)(dpaq^5NO₂)]⁺ complex showing a second-order rate constant only 9-fold larger than that of the least reactive [Mn^III(OH)(dpaq^5OMe)]⁺ complex. These modest rate changes were understood on the basis of density functional theory (DFT)-computed p K_a values for the corresponding [Mn^II(OH₂)(dpaq^5R)]⁺ complexes. Collectively, the experimental and DFT results reveal that the 5-substituted quinolinyl groups have an inverse influence on electron and proton affinity for the Mn^III-hydroxo unit.

Factors Affecting Hydrogen Atom Transfer Reactivity of Metal-Oxo Porphyrinoid Complexes

There has been considerable interest in hydrogen atom transfer (HAT) reactions mediated by metal/oxygen species because of their central role in metalloenzyme function as well as synthetic catalysts. This Account focuses on our progress in synthesizing high-valent metal-oxo and metal-hydroxo porphyrinoid complexes and determining their reactivities in a range of HAT processes. For these studies we have utilized corrolazine and corrole ligands, which are a ring-contracted subclass of porphyrinoid compounds designed to stabilize high-valent metal complexes. The high-valent manganese complex Mn^V(O)(TBP₈Cz) (TBP₈Cz = octakis(4- tert-butylphenyl)corrolazine^3-) provided an early example of a well-characterized low-potential oxidant that can still be effective at abstracting H atoms from certain C-H/O-H bonds. Approximating the thermodynamics of the HAT reactivity of the Mn^V(O) complex and related species with the help of a square scheme approach, in which HAT can be formally separated into proton (p K_a) and electron transfers ( E°), indicates that affinity for the proton (i.e., the basicity) is a key factor in promoting HAT. Anionic axial ligands have a profound influence on the HAT reactivity of Mn^V(O)(TBP₈Cz), supporting the conclusion that basicity is a critical parameter in determining the reactivity. The influence of Lewis acids on Mn^V(O)(TBP₈Cz) was examined, and it was shown that both the electronic structure and reactivity toward HAT were significantly altered. High-valent Cr(O), Re(O), and Fe(O) corrolazines were prepared, and a range of HAT reactions were studied with these complexes. The chromium and manganese complexes form a rare pair of structurally characterized Cr^V(O) and Mn^V(O) species in identical ligand environments, allowing for a direct comparison of their HAT reactivities. Although the Cr^V(O) species was the better oxidant as measured by redox potentials, the Mn^V(O) species was significantly more reactive in HAT oxidations, pointing again to basicity as a key determinant of HAT reactivity. The iron complex, Fe^IV(O)(TBP₈Cz^+•), is an analogue of the heme enzyme Compound I intermediate, and was found to be mildly reactive toward H atom abstraction from C-H bonds. In contrast, Re^V(O)(TBP₈Cz) was inert toward HAT, although one-electron oxidation to Re^V(O)(TBP₈Cz^+•) led to some interesting reactivity mediated by the π-radical-cation ligand alone. Other ligand modifications, including peripheral substitution as well as novel alkylation of the meso position on the Cz core, were examined for their influence on HAT. A highly sterically encumbered corrole, tris(2,4,6-triphenylphenyl)corrole (ttppc), was employed for the isolation and structural characterization of the first Mn^IV(OH) complex in a porphyrinoid environment, Mn^IV(OH)(ttppc). This complex was highly reactive in HAT with O-H substrates and was found to be much more reactive than its higher-oxidation-state counterpart Mn^V(O)(ttppc), providing important mechanistic insights. These studies provided fundamental knowledge on the relationship between structure and function in high-valent M(O) and M(OH) models of heme enzyme reactivity.

meso-N-Methylation of a porphyrinoid complex: activating the H-atom transfer capability of an inert ReV(O) corrolazine

The selective alkylation of a single meso-N atom of a corrolazine macrocycle is reported. Alkylation has a dramatic impact on the physicochemical properties of Re^V(O)(TBP₈Cz). New electron-transfer and hydrogen-atom-transfer reactivity is also seen for this complex, including one-electron reduction, which gives an air-stable 19π-electron aromatic radical complex.

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

Heme proteins utilize the heme cofactor, an iron porphyrin, to perform a diverse range of reactions including dioxygen binding and transport, electron transfer, and oxidation/oxygenations. These reactions share several key metalloporphyrin intermediates, typically derived from dioxygen and its congeners such as hydrogen peroxide. These species are composed of metal-dioxygen, metal-superoxo, metal-peroxo, and metal-oxo adducts. A wide variety of synthetic metalloporphyrinoid complexes have been synthesized to generate and stabilize these intermediates. These complexes have been studied to determine the spectroscopic features, structures, and reactivities of such species in controlled and well-defined environments. In this Review, we summarize recent findings on the reactivity of these species with common porphyrinoid scaffolds employed for biomimetic studies. The proposed mechanisms of action are emphasized. This Review is organized by structural type of metal-oxygen intermediate and broken into subsections based on the metal (manganese and iron) and porphyrinoid ligand (porphyrin, corrole, and corrolazine).

Azaporphyrin phosphorus(v) complexes: synthesis, structure, and modification of optical properties

Azaporphyrinoids, such as phthalocyanines (Pcs), tetraazaporphyrins (TAPs), and tetrabenzotriazacorroles (TBCs), are some of the most well-known and successful artificial dyes and pigments in modern material chemistry.

High-Valent Manganese-Oxo Valence Tautomers and the Influence of Lewis/Brönsted Acids on C-H Bond Cleavage

The addition of Lewis or Brönsted acids (LA = Zn(OTf)₂, B(C₆F₅)₃, HBAr^F, TFA) to the high-valent manganese-oxo complex Mn^V(O)(TBP₈Cz) results in the stabilization of a valence tautomer Mn^IV(O-LA)(TBP₈Cz^•+). The Zn^II and B(C₆F₅)₃ complexes were characterized by manganese K-edge X-ray absorption spectroscopy (XAS). The position of the edge energies and the intensities of the pre-edge (1s to 3d) peaks confirm that the Mn ion is in the +4 oxidation state. Fitting of the extended X-ray absorption fine structure (EXAFS) region reveals 4 N/O ligands at Mn-N_ave = 1.89 Å and a fifth N/O ligand at 1.61 Å, corresponding to the terminal oxo ligand. This Mn-O bond length is elongated compared to the Mn^V(O) starting material (Mn-O = 1.55 Å). The reactivity of Mn^IV(O-LA)(TBP₈Cz^•+) toward C-H substrates was examined, and it was found that H^• abstraction from C-H bonds occurs in a 1:1 stoichiometry, giving a Mn^IV complex and the dehydrogenated organic product. The rates of C-H cleavage are accelerated for the Mn^IV(O-LA)(TBP₈Cz^•+) valence tautomer as compared to the Mn^V(O) valence tautomer when LA = Zn^II, B(C₆F₅)₃, and HBAr^F, whereas for LA = TFA, the C-H cleavage rate is slightly slower than when compared to Mn^V(O). A large, nonclassical kinetic isotope effect of k_H/k_D = 25-27 was observed for LA = B(C₆F₅)₃ and HBAr^F, indicating that H-atom transfer (HAT) is the rate-limiting step in the C-H cleavage reaction and implicating a potential tunneling mechanism for HAT. The reactivity of Mn^IV(O-LA)(TBP₈Cz^•+) toward C-H bonds depends on the strength of the Lewis acid. The HAT reactivity is compared with the analogous corrole complex Mn^IV(O-H)(tpfc^•+) recently reported (J. Am. Chem. Soc. 2015, 137, 14481-14487).