Search

Adding HClO4 to [(BnTPEN)MnIII–OO]+ in MeOH generates a short-lived MnIII–OOH species, which converts to a putative MnV＝O species. The potent MnV＝O species in MeCN oxidizes the pendant phenyl ring of the ligand in an intramolecular fashion. The addition of benzene causes the formation of (BnTPEN)MnIII-phenolate. These findings suggest that high valent Mn species have the potential to catalyze challenging aromatic hydroxylation reactions. [ABSTRACT FROM AUTHOR]

TMC-anti and TMC-syn, the two topological isomers of [FeIV(O)(TMC)(CH3CN)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, or Me4cyclam), differ in the orientations of their FeIV=O units relative to the four methyl groups of the TMC ligand framework. The FeIV=O unit of TMC-anti points away from the four methyl groups, while that of TMC-syn is surrounded by the methyl groups, resulting in differences in their oxidative reactivities. TMC-syn reacts with HAT (hydrogen atom transfer) substrates at 1.3-to 3-fold faster rates than TMC-anti, but the reactivity difference increases dramatically in oxygen-atom transfer reactions. R2S substrates are oxidized into R²S=O products at rates 2-to-3 orders of magnitude faster by TMC-syn than TMC-anti. Even more remarkably, TMC-syn epoxidizes all the olefin substrates in this study, while TMC-anti reacts only with cis-cyclooctene but at a 100-fold slower rate. Comprehensive quantum chemical calculations have uncovered the key factors governing such reactivity differences found between these two topological isomers. [ABSTRACT FROM AUTHOR]

Lewis acid‐bound high valent Mn‐oxo species are of great importance due to their relevance to photosystem II. Here, we report the synthesis of a unique [(BnTPEN)Mn(III)−O−Ce(IV)(NO3)4]+ adduct (2) by the reaction of (BnTPEN)Mn(II) (1) with 4 eq. ceric ammonium nitrate. 2 has been characterized using UV/Vis, NMR, resonance Raman spectroscopy, as well as by mass spectrometry. Treatment of 2 with Sc(III)(OTf)3 results in the formation of (BnTPEN)Mn(IV)−O−Sc(III) (3), while HClO4 addition to 2 forms (BnTPEN)Mn(IV)−OH (4), reverting to 2 upon Ce(III)(NO3)3 addition. 2 can also be prepared by the oxidation of 1 eq. Ce(III)(NO3)3 with [(BnTPEN)Mn(IV)=O]2+ (5). In addition, the EPR spectroscopy revealed the elegant temperature‐dependent equilibria between 2 and Mn(IV) species. The binding of redox‐active Ce(IV) boosts electron transfer efficiency of 2 towards ferrocenes. Remarkably, the newly characterized Mn(III)−O−Ce(IV) species can carry out O‐atom and H‐atom transfer reactions. [ABSTRACT FROM AUTHOR]

An efficient electrochemical method for the selective oxidation of alcohols to their corresponding aldehydes/ketones using a biomimetic iron complex, [(bTAML)FeIII–OH2]−, as the redox mediator in an undivided electrochemical cell with inexpensive carbon and nickel electrodes using water as an oxygen source is reported. The substrate scope also includes alcohols that contain O and N heteroatoms in the scaffold, which are well tolerated under these reaction conditions. Mechanistic studies show the involvement of a high-valent FeV(O) species, [(bTAML)FeV(O)]−, formed via PCET (overall 2H+/2e−) from [(bTAML)FeIII–OH2]− at 0.77 V (vs. Fc+/Fc). Moreover, electrokinetic studies of the oxidation of C–H bonds indicate a second-order reaction, with the C–H abstraction by FeV(O) being the rate-determining step. The overall mechanism, studied using linear free energy relationships and radical clocks, indicates a "net hydride" transfer, leading to the oxidation of the alcohol to the corresponding aldehyde or ketone. When the reaction was carried out at pH > 11, the reaction could be carried out at a ∼500 mV lower potential than that at pH 8, albeit with reduced reaction rates. The reactive intermediate involved at pH > 11 is the corresponding one-electron oxidized [(bTAML)FeIV(O)]2− species. [ABSTRACT FROM AUTHOR]

Selective catalytic oxygenation of unactivated C–H bonds for a series of substrates by dioxygen using iron complexes was performed without the use of a co-reductant. Mechanistic studies indicate that the reaction proceeded via the autocatalytic formation of an oxoiron(V) intermediate, which brings high regioselectivity and stereoretention. [ABSTRACT FROM AUTHOR]

The oxidative cleavage of alkenes is a crucial step in synthesizing key organic molecules featuring carbonyl functional groups prevalent in natural products and pharmaceuticals. We introduce a photochemical method for heterogeneous C=C bond cleavage, employing photo-catalytically generated [(bTAML)FeIV-O-FeIV(bTAML)]- species (where bTAML stands for biuret-modified tetraamido macrocyclic ligand) in aqueous environments under gentle conditions. Leveraging the photosensitizing properties of Covalent Organic Frameworks (COFs) and their advantageous morphological traits as films, we enhance the reaction by closely associating the substrate with the catalyst. This study marks the inaugural demonstration of Fe2IV-µ-oxo radical cation and FeIV=O species facilitating alkene cleavage in water against a backdrop of a hydrophobic COF. Through comprehensive mechanistic studies, including control experiments, we confirm that these two high-valent iron oxo species collaborate to cleave alkenes, forming an intermediate epoxide. Our approach yields moderate to high success across various alkenes, displaying diverse functional groups (achieving up to 75% yield) with notable efficiency and selectivity towards aldehyde/ketone products. Moreover, the heterogeneous COF film, immobilizing (Et4N)2[FeIII(Cl)bTAML], exhibits exceptional recyclability, enduring up to four cycles., (© 2024 Wiley‐VCH GmbH.)

Adding HClO 4 to [(BnTPEN)Mn III -OO] + in MeOH generates a short-lived Mn III -OOH species, which converts to a putative Mn V O species. The potent Mn V O species in MeCN oxidizes the pendant phenyl ring of the ligand in an intramolecular fashion. The addition of benzene causes the formation of (BnTPEN)Mn III -phenolate. These findings suggest that high valent Mn species have the potential to catalyze challenging aromatic hydroxylation reactions.

TMC- anti and TMC- syn, the two topological isomers of [Fe IV (O)(TMC)(CH 3 CN)] 2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, or Me 4 cyclam), differ in the orientations of their Fe IV =O units relative to the four methyl groups of the TMC ligand framework. The Fe IV =O unit of TMC- anti points away from the four methyl groups, while that of TMC- syn is surrounded by the methyl groups, resulting in differences in their oxidative reactivities. TMC- syn reacts with HAT (hydrogen atom transfer) substrates at 1.3- to 3-fold faster rates than TMC- anti , but the reactivity difference increases dramatically in oxygen-atom transfer reactions. R 2 S substrates are oxidized into R 2 S=O products at rates 2-to-3 orders of magnitude faster by TMC- syn than TMC- anti . Even more remarkably, TMC- syn epoxidizes all the olefin substrates in this study, while TMC- anti reacts only with cis -cyclooctene but at a 100-fold slower rate. Comprehensive quantum chemical calculations have uncovered the key factors governing such reactivity differences found between these two topological isomers., Competing Interests: Competing interests statement:The authors declare no competing interest.

Lewis acid-bound high valent Mn-oxo species are of great importance due to their relevance to photosystem II. Here, we report the synthesis of a unique [(BnTPEN)Mn(III)-O-Ce(IV)(NO 3 ) 4 ] + adduct (2) by the reaction of (BnTPEN)Mn(II) (1) with 4 eq. ceric ammonium nitrate. 2 has been characterized using UV/Vis, NMR, resonance Raman spectroscopy, as well as by mass spectrometry. Treatment of 2 with Sc(III)(OTf) 3 results in the formation of (BnTPEN)Mn(IV)-O-Sc(III) (3), while HClO 4 addition to 2 forms (BnTPEN)Mn(IV)-OH (4), reverting to 2 upon Ce(III)(NO 3 ) 3 addition. 2 can also be prepared by the oxidation of 1 eq. Ce(III)(NO 3 ) 3 with [(BnTPEN)Mn(IV)=O] 2+ (5). In addition, the EPR spectroscopy revealed the elegant temperature-dependent equilibria between 2 and Mn(IV) species. The binding of redox-active Ce(IV) boosts electron transfer efficiency of 2 towards ferrocenes. Remarkably, the newly characterized Mn(III)-O-Ce(IV) species can carry out O-atom and H-atom transfer reactions., (© 2023 Wiley-VCH GmbH.)

Water has been recognized as an excellent solvent for maneuvering both the catalytic activity and selectivity, especially in the case of heterogeneous catalysis. However, maintaining the active catalytic species in their higher oxidation states (IV/V) while retaining the catalytic activity and recyclability in water is an enormous challenge. Herein, we have developed a solution to this problem using covalent organic frameworks (COFs) to immobilize the (Et 4 N) 2 [Fe III (Cl)bTAML] molecules, taking advantage of the COF's morphology and surface charge. By using the visible light and [Co III (NH 3 ) 5 Cl]Cl 2 as a sacrificial electron acceptor within the COF, we have successfully generated and stabilized the [(bTAML)Fe IV -O-Fe IV (bTAML)] - species in water. The COF backbone simultaneously acts as a porous host and a photosensitizer. This is the first time that the photochemically generated Fe 2 IV -μ-oxo radical cation species has demonstrated high catalytic activity with moderate to high yield for the selective oxidation of the unactivated C-H bonds, even in water. To enhance the catalytic activity and achieve good recyclability, we have developed a TpDPP COF film by transforming the TpDPP COF nanospheres. We have achieved the regio- and stereoselective functionalization of unactivated C-H bonds of alkanes and alkenes (3°:2° = 102:1 for adamantane with the COF film), which is improbable in homogeneous conditions. The film exhibits C-H bond oxidation with higher catalytic yield (32-98%) and a higher degree of selectivity ( cis / trans = 74:1; 3°:2° = 100:1 for cis -decalin).

An efficient electrochemical method for the selective oxidation of C-H bonds of unactivated alkanes (BDE ≤97 kcal mol -1 ) and C[double bond, length as m-dash]C bonds of alkenes using a biomimetic iron complex, [(bTAML)Fe III -OH 2 ] - , as the redox mediator in an undivided electrochemical cell with inexpensive carbon and nickel electrodes is reported. The O-atom of water remains the source of O-incorporation in the product formed after oxidation. The products formed upon oxidation of C-H bonds display very high regioselectivity (75 : 1, 3° : 2° for adamantane) and stereo-retention (RC ∼99% for cyclohexane derivatives). The substrate scope includes natural products such as cedryl acetate and ambroxide. For alkenes, epoxides were obtained as the sole product. Mechanistic studies show the involvement of a high-valent oxoiron(v) species, [(bTAML)Fe V (O)] - formed via PCET (overall 2H + /2e - ) from [(bTAML)Fe III -OH 2 ] - in CPE at 0.80 V ( vs. Ag/AgNO 3 ). Moreover, electrokinetic studies for the oxidation of C-H bonds indicate a second-order reaction with the C-H abstraction by oxoiron(v) being the rate-determining step., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)