Photochemical generation and kinetic studies of a putative porphyrin-ruthenium(V)-oxo species

Photochemical generation and reactivity of a new phthalocyanine-manganese-oxo intermediate

The first phthalocyanine-manganese-oxo intermediate was successfully generated by visible-light photolysis of chlorate or nitrite manganese(III) precursors, and its reactivity towards organic substrates was kinetically probed and compared with other related porphyrin-metal-oxo intermediates.

Kinetics of chromium(V)-oxo and chromium(IV)-oxo porphyrins: Reactivity and mechanism for sulfoxidation reactions

In this work, chromium(IV)-oxo porphyrins [Cr^IV(Por)(O)] (2) (Por = porphyrin) were produced either by oxidation of [Cr^III(Por)Cl] (1) with iodobenzene diacetate or visible light photolysis of porphyrin‑chromium(III) chlorates. Subsequent oxidation of 2 with silver perchlorate gave chromium(V)-oxo porphyrins [Cr^V(Por)(O)](ClO₄) (3) in three porphyrin ligands, including 5,10,15,20-tetramesitylporphyrin(TMP, a), 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin(TDFPP, b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPFPP, c). Complexes 2 and 3 reacted with thioanisoles to produce the corresponding sulfoxides, and their kinetics of sulfoxidation reactions with a series of aryl methyl sulfides(thioanisoles) were studied in organic solutions. Chromium(V)-oxo porphyrins are several orders of magnitudes more reactive than chromium(IV)-oxo species, and representative second-order rate constants (k_ox) for the oxidation of thioansole are (0.40 ± 0.01) M^-1 s^-1 (3a), and (2.82 ± 0.20) × 10² M^-1 s^-1 (3b), and (2.20 ± 0.01) × 10³ M^-1 s^-1 (3c). The order of reactivity for 2 and 3 follows TPFPP > TDFPP > TMP, in agreement with the electrophilic nature of metal-oxo complexes. Hammett analyses indicate significant charge transfer in the transition states for oxidation of para-substituted thioanisoles by [Cr^V(Por)(O)]⁺. The ρ⁺ constants are -1.69 for 3a, -2.63 for 3b, and - 2.89 for 3c, respectively, mirror values found previously for related metal-oxo species. A mechanism involving the electrophilic attack of the Cr^V-oxo at sulfides to form a sulfur cation intermediate in the rate-determining step is suggested. Competition studies with chromium(III) porphyrin chloride and PhI(OAc)₂ gave relative rate constants for oxidations of competing thioanisoles that closely match ratios of absolute rate constants from chromium(V)-oxo species, which are true oxidants under catalytic conditions.

Computational Evaluation of Potential Molecular Catalysts for Nitrous Oxide Decomposition

Nitrous oxide (N₂O) is a potent greenhouse gas (GHG) with limited use as a mild anesthetic and underdeveloped reactivity. Nitrous oxide splitting (decomposition) is critical to its mitigation as a GHG. Although heterogeneous catalysts for N₂O decomposition have been developed, highly efficient, long-lived solid catalysts are still needed, and the details of the catalytic pathways are not well understood. Reported herein is a computational evaluation of three potential molecular (homogeneous) catalysts for N₂O splitting, which could aid in the development of more active and robust catalysts and provide deeper mechanistic insights: one Cu(I)-based, [(CF₃O)₄Al]Cu (A-1), and two Ru(III)-based, Cl(POR)Ru (B-1) and (NTA)Ru (C-1) (POR = porphyrin, NTA = nitrilotriacetate). The structures and energetic viability of potential intermediates and key transition states are evaluated according to a two-stage reaction pathway: (A) deoxygenation (DO), during which a metal-N₂O complex undergoes N-O bond cleavage to produce N₂ and a metal-oxo species and (B) (di)oxygen evolution (OER), in which the metal-oxo species dimerizes to a dimetal-peroxo complex, followed by conversion to a metal-dioxygen species from which dioxygen dissociates. For the (F-L)Cu(I) activator (A-1), deoxygenation of N₂O is facilitated by an O-bound (F-L)Cu-O-N₂ or better by a bimetallic N,O-bonded, (F-L)Cu-NNO-Cu(F-L) complex; the resulting copper-oxyl (F-L)Cu-O is converted exergonically to (F-L)Cu-(η²,η²-O₂)-Cu(F-L), which leads to dioxygen species (F-L)Cu(η²-O₂), that favorably dissociates O₂. Key features of the DO/OER process for (POR)ClRu (B-1) include endergonic N₂O coordination, facile N₂ evolution from LR'u-N₂O-RuL to Cl(POR)RuO, moderate barrier coupling of Cl(POR)RuO to peroxo Cl(POR)Ru(O₂)Ru(POR)Cl, and eventual O₂ dissociation from Cl(POR)Ru(η¹-O₂), which is nearly thermoneutral. N₂O decomposition promoted by (NTA)Ru(III) (C-1) can proceed with exergonic N₂O coordination, facile N₂ dissociation from (NTA)Ru-ON₂ or (NTA)Ru-N₂O-Ru(NTA) to form (NTA)Ru-O; dimerization of the (NTA)Ru-oxo species is facile to produce (NTA)Ru-O-O-Ru(NTA), and subsequent OE from the peroxo species is moderately endergonic. Considering the overall energetics, (F-L)Cu and Cl(POR)Ru derivatives are deemed the best candidates for promoting facile N₂O decomposition.

Visible light generation of high-valent metal-oxo intermediates and mechanistic insights into catalytic oxidations

High-valent metal-oxo complexes play central roles as active oxygen atom transfer (OAT) agents in many enzymatic and synthetic oxidation catalysis. This review focuses on our recent advances in application of photochemical approaches to probe the oxidizing metal-oxo species with different metals and macrocyclic ligands. Under visible light irradiation, a variety of important metal-oxo species including iron-oxo porphyrins, manganese-oxo porphyrin/corroles, ruthenium-oxo porphyrins, and chromium-oxo salens have been successfully generated. Kinetical studies in real time have provided mechanistic insights as to the reactivity and reaction pathways of the metal-oxo intermediates in their oxidation reactions. In photo-induced ligand cleavage reactions, metals in n⁺ oxidation state with the oxygen-containing ligands bromate, chlorate, or nitrites were photolyzed. Homolytic cleavage of the O-X bond in the ligand gives (n + 1)⁺ oxidation state metal-oxo species, and heterolytic cleavage gives (n + 2)⁺ oxidation state metal-oxo species. In photo-disproportionation reactions, reactive Mⁿ⁺¹-oxo species can be formed by photolysis of μ-oxo dimeric Mⁿ⁺ complexes with the concomitant formation of M^n-1 products. Importantly, the oxidation of M^n-1 products by molecular oxygen (O₂) to regenerate the μ-oxo dimeric Mⁿ⁺ complexes in photo-disproportionation reactions represents an attractive and green catalytic cycle for the development of photocatalytic aerobic oxidations.

Multielectron C–H photoactivation with an Sb(v) oxo corrole

An Sb(v) bis-μ-oxo corrole dimer performs photochemical multielectron C–H activation, oxidising toluene to benzaldehyde in a four-electron process.

Formation and kinetic studies of manganese(IV)-oxo porphyrins: Oxygen atom transfer mechanism of sulfide oxidations

Visible light irradiation of photo-labile porphyrin-manganese(III) chlorates or bromates (2) produced manganese(IV)-oxo porphyrins [Mn^IV(Por)(O)] (Por = porphyrin) (3) in three porphyrin ligands. The same oxo species 3 were also formed by chemical oxidation of the corresponding manganese(III) precursors (1) with iodobenzene diacetate, i.e. PhI(OAc)₂. The systems under study include 5,10,15,20-tetra(pentafluorophenyl)porphyrin‑manganese(IV)-oxo (3a), 5,10,15,20-tetra(2,6-difluorophenyl)porphyrin‑manganese(IV)-oxo (3b), and 5,10,15,20-tetramesitylporphyrin‑manganese(IV)-oxo (3c). As expected, complexes 3 reacted with thioanisoles to produce the corresponding sulfoxides and over-oxidized sulfones. The kinetics of oxygen atom transfer (OAT) reactions of these generated 3 with aryl sulfides were studied in CH₃CN solutions. Second-order rate constants for sulfide oxidation reactions are comparable to those of alkene epoxidations and activated CH bond oxidations by the same oxo species 3. For a given substrate, the reactivity order for the manganese(IV)-oxo species was 3a > 3b > 3c, consistent with expectations on the basis of the electron-withdrawing capacity of the porphyrin macrocycles. Free-energy Hammett analyses gave near-linear correlations with σ values, indicating no significant positive charge developed at the sulfur during the oxidation process. The mechanistic results strongly suggest [Mn^IV(Por)(O)] reacts as a direct OAT agent towards sulfide substrates through a manganese(II) intermediate that was detected in this work. However, an alternative pathway that involves a disproportionation of 3 to form a higher oxidized manganese(V)-oxo species may be significant when less reactive substrates are present. The competition product studies with the Hammett correlation plot confirmed that the observed manganese(IV)-oxo species is not the true oxidant for the sulfide oxidations catalyzed by manganese(III) porphyrins with PhI(OAc)₂.

New Functionalized Polycycles Obtained by Photocatalytic Oxygenation Using Mn(III) Porphyrins in Basic Media

According to our earlier observations, the products of photocatalytic oxygenations of furan and thiophene derivatives of benzobicyclo[3.2.1]octadiene with anionic and cationic manganese(III) porphyrin at pH = 7 strongly depended on the type and position of the heteroatom in the aromatic ring, as well as the charge of the photocatalyst. Hence, a significant pH increase (to 10) in these systems offered a reasonable tool to affect the diversity and yields of the oxygenation products. They were quantitatively separated by TLC and identified with NMR analyses. The results clearly indicated that the increase of HO− concentration, in most cases, considerably changed the product yield, e.g., enhanced it to 70% for the hydroxy-furyl derivative. Accordingly, the selectivity of the oxygenation of the furan compound could be improved in this way. In the case of one thienyl compound, however, even an additional product appeared, while the yields of the products of the other thiophene derivative (with cationic catalyst) decreased to zero, suggesting the application of lower pH for preparative purposes. The pH effects indicate that oxygenation reactions in these systems involve more photochemically generated oxidative agents, e.g., •OH and (P)Mn(V)=O), the role of which is affected by the pH increase in various ways.

Arylruthenium(III) Porphyrin-Catalyzed C-H Oxidation and Epoxidation at Room Temperature and [RuV(Por)(O)(Ph)] Intermediate by Spectroscopic Analysis and Density Functional Theory Calculations

The development of highly active and selective metal catalysts for efficient oxidation of hydrocarbons and identification of the reactive intermediates in the oxidation catalysis are long-standing challenges. In the rapid hydrocarbon oxidation catalyzed by ruthenium(IV) and -(III) porphyrins, the putative Ru(V)-oxo intermediates remain elusive. Herein we report that arylruthenium(III) porphyrins are highly active catalysts for hydrocarbon oxidation. Using catalyst [Ru^III(TDCPP)(Ph)(OEt₂)] (H₂TDCPP = 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin), the oxidation of C-H bonds of various hydrocarbons with oxidant m-CPBA at room temperature gave alcohols/ketones in up to 99% yield within 1 h; use of [ ⁿBu₄N]IO₄ as a mild alternative oxidant avoided formation of lactone from cyclic ketone in C-H oxidation, and the catalytic epoxidation with up to 99% yield and high selectivity (no aldehydes as side product) was accomplished within 5 min. UV-vis, electrospray ionization-mass spectrometry, resonance Raman, electron paramagnetic resonance, and kinetic measurements and density functional theory calculations lend evidence for the formation of Ru(V)-oxo intermediate [Ru^V(TDCPP)(O)(Ph)].

An investigation of ligand effects on the visible light-induced formation of porphyrin–iron(iv)-oxo intermediates

Depending on the structure of the porphyrin ligands, the visible light photolysis of porphyrin–iron(iii) bromates produced iron(iv)-oxo radical cations or iron(iv)-oxo porphyrins, permitting direct kinetic studies of their oxidation reactions.

Synthesis and catalytic activity of μ-oxo ruthenium(IV) porphyrin species to promote amination reactions

This work describes the synthesis of ruthenium(IV) [Formula: see text]-oxo porphyrin complexes of general formula [RuIV(TPP)(X)]2O which have been applied as catalysts in nitrene transfer reactions using aryl azides (ArN[Formula: see text] as nitrene sources. Collected data indicated that the catalytic efficiency of [RuIV(TPP)(OCH[Formula: see text]]2O was comparable to that of RuII(TPP)CO because of their analogous reactivity towards aryl azides to give the same catalytically active bis-imido species RuVI(TPP)(ArN)2. The reaction of [RuIV(TPP)(OCH[Formula: see text]]2O with Ph3CN3 or (CH[Formula: see text]SiN3 afforded [RuIV(TPP)(N[Formula: see text]]2O which was fully characterised, its molecular structure was also determined by single crystal X-ray analysis.

A Synthetic Oxygen Atom Transfer Photocycle from a Diruthenium Oxyanion Complex

Three new diruthenium oxyanion complexes have been prepared, crystallographically characterized, and screened for their potential to photochemically unmask a reactive Ru-Ru═O intermediate. The most promising candidate, Ru2(chp)4ONO2 (4, chp = 6-chloro-2-hydroxypyridinate), displays a set of signals centered around m/z = 733 amu in its MALDI-TOF mass spectrum, consistent with the formation of the [Ru2(chp)4O](+) ([6](+)) ion. These signals shift to 735 amu in 4*, which contains an (18)O-labeled nitrate. EPR spectroscopy and headspace GC-MS analysis indicate that NO2(•) is released upon photolysis of 4, also consistent with the formation of 6. Photolysis of 4 in CH2Cl2 at room temperature in the presence of excess PPh3 yields OPPh3 in 173% yield; control experiments implicate 6, NO2(•), and free NO3(-) as the active oxidants. Notably, Ru2(chp)4Cl (3) is recovered after photolysis. Since 3 is the direct precursor to 4, the results described herein constitute the first example of a synthetic cycle for oxygen atom transfer that makes use of light to generate a putative metal oxo intermediate.

Visible light-induced formation of corrole-manganese(V)-oxo complexes: Observation of multiple oxidation pathways

Two manganese(V)-oxo corroles [Mn^V(Cor)O] that differ in their electronic environments were produced by visible light irradiation of highly photo-labile corrole-manganese(IV) bromates. The corrole ligands under study include 5,10,15-tris(pentafluorophenyl)corrole (TPFC), and 5,10,15-triphenylcorrole (TPC). The kinetics of oxygen transfer atom (OAT) reactions with various organic reductants by these photo-generated Mn^V(Cor)O were also studied in CH₃CN and CH₂Cl₂ solutions. Mn^V(Cor)O exhibits remarkable solvent and ligand effect on its reactivity and spectral behavior. In the more electron-deficient TPFC system and in the polar solvent CH₃CN, Mn^V(Cor)O returned Mn^III corrole in the end of oxidation reactions. However, in the less polar solvent CH₂Cl₂ or in the less electron-deficient TPC system, Mn^IV product was formed instead of Mn^III. Furthermore, with the same substrates and in the same solvent, the order of reactivity of Mn^V(Cor)O was TPC>TPFC, which is inverted from that expected based on the electron-demand of corrole ligands. Our spectral and kinetic results in this study provide compelling evidence in favor of multiple oxidation pathways, where Mn^V(Cor)O may serve as direct two-electron oxidant or undergo a disproportionation reaction to form a manganese(VI)-oxo corrole as the true oxidant. The choice of pathways is strongly dependent on the nature of the solvent and the corrole ligand.

Ligand control in the photochemical generation of high-valent porphyrin-iron-oxo derivatives

Visible-light irradiation of photo-labile bromate porphyrin-iron(III) salts gave iron(IV)-oxo porphyrin radical cations (compound I model) or the neutral iron(IV)-oxo porphyrin (compound II model), depending on the electronic structure of porphyrin ligands.