Photocatalytic asymmetric epoxidation of trans-stilbene with manganese–porphyrin/graphene-oxide nanocomposite and molecular oxygen: axial ligand effect

An efficient, visible light-driven manganese–porphyrin photocatalyst was developed for the asymmetric epoxidation of trans-stilbene by molecular oxygen under mild conditions.

Singlet versus Triplet Reactivity in an Mn(V)-Oxo Species: Testing Theoretical Predictions Against Experimental Evidence

Discerning the factors that control the reactivity of high-valent metal-oxo species is critical to both an understanding of metalloenzyme reactivity and related transition metal catalysts. Computational studies have suggested that an excited higher spin state in a number of metal-oxo species can provide a lower energy barrier for oxidation reactions, leading to the conclusion that this unobserved higher spin state complex should be considered as the active oxidant. However, testing these computational predictions by experiment is difficult and has rarely been accomplished. Herein, we describe a detailed computational study on the role of spin state in the reactivity of a high-valent manganese(V)-oxo complex with para-Z-substituted thioanisoles and utilize experimental evidence to distinguish between the theoretical results. The calculations show an unusual change in mechanism occurs for the dominant singlet spin state that correlates with the electron-donating property of the para-Z substituent, while this change is not observed on the triplet spin state. Minimum energy crossing point calculations predict small spin-orbit coupling constants making the spin state change from low spin to high spin unlikely. The trends in reactivity for the para-Z-substituted thioanisole derivatives provide an experimental measure for the spin state reactivity in manganese-oxo corrolazine complexes. Hence, the calculations show that the V-shaped Hammett plot is reproduced by the singlet surface but not by the triplet state trend. The substituent effect is explained with valence bond models, which confirm a change from an electrophilic to a nucleophilic mechanism through a change of substituent.

Oxygen-atom transfer reactivity of axially ligated Mn(V)-oxo complexes: evidence for enhanced electrophilic and nucleophilic pathways

Addition of anionic donors to the manganese(V)–oxo corrolazine complex Mn^V(O)(TBP₈Cz) has a dramatic influence on oxygen-atom transfer (OAT) reactivity with thioether substrates. The six-coordinate anionic [Mn^V(O)(TBP₈Cz)(X)]⁻ complexes (X = F^–, N₃^–, OCN^–) exhibit a ∼5 cm^–1 downshift of the Mn–O vibrational mode relative to the parent Mn^V(O)(TBP₈Cz) complex as seen by resonance Raman spectroscopy. Product analysis shows that the oxidation of thioether substrates gives sulfoxide product, consistent with single OAT. A wide range of OAT reactivity is seen for the different axial ligands, with the following trend determined from a comparison of their second-order rate constants for sulfoxidation: five-coordinate ≈ thiocyanate ≈ nitrate < cyanate < azide < fluoride ≪ cyanide. This trend correlates with DFT calculations on the binding of the axial donors to the parent Mn^V(O)(TBP₈Cz) complex. A Hammett study was performed with p-X-C₆H₄SCH₃ derivatives and [Mn^V(O)(TBP₈Cz)(X)]⁻ (X = CN^– or F^–) as the oxidant, and unusual “V-shaped” Hammett plots were obtained. These results are rationalized based upon a change in mechanism that hinges on the ability of the [Mn^V(O)(TBP₈Cz)(X)]⁻ complexes to function as either an electrophilic or weak nucleophilic oxidant depending upon the nature of the para-X substituents. For comparison, the one-electron-oxidized cationic Mn^V(O)(TBP₈Cz^•+) complex yielded a linear Hammett relationship for all substrates (ρ = −1.40), consistent with a straightforward electrophilic mechanism. This study provides new, fundamental insights regarding the influence of axial donors on high-valent Mn^V(O) porphyrinoid complexes.

Solvent effects on the catalytic activity of manganese(III) corroles

Four Mn ( III ) corroles that differ in their electronic environments based on meso-substitution by pentafluorophenyl and phenyl groups were synthesized and characterized by spectroscopic techniques. Utilization of these Mn ( III ) corroles for styrene oxidation using iodosylbenzene (PhIO) as oxygen source in toluene, dichloromethane (DCM), DMF, DMAc, THF and DMSO revealed a remarkable effect of solvent on the catalytic activity. Furthermore, the transformation of Mn ( III ) corroles into their corresponding Mn ( V )-oxo corroles, and subsequent treatment with styrene also indicated that more electron-deficient Mn ( V )-oxo corroles exhibit higher reactivity in toluene and DCM, while less electron-deficient Mn ( V )-oxo corroles exhibit higher reactivity in DMF and DMAc. A significant difference in the observed rates of reaction suggest that the catalytic oxidation of styrene by manganese corroles may proceed through different pathways, and is strongly solvent-dependent.

Reactivity of (oxo)manganese(V) corroles in one-electron redox state: insights from conceptual DFT and transition state calculations

The electronic and structural features of (oxo)manganese(V) corroles ( Mn ( V )-oxo(cor)) in different oxidation states and their oxygen atom transfer (OAT) reactivity towards dimethyl sulfide (DMS) have been investigated by density functional theory (DFT) calculations. Three Mn ( V )-oxo corroles with different β-substituents ( Br (1), H (2), CH 3(3)) were choosen as model complexes. The results revealed that Mn ( V )-oxo corrole bearing electron-withdrawing substituent ( Br ) is more reactive than those having electron-donating substituents. Moreover, the conceptual DFT descriptor ω for Mn ( V )-oxo corrole in one-electron oxidized state is always larger than the neutral or one-electron reduced state. These calculations for the transition state involved in OAT reaction also indicated that the radical cation formed after one-electron oxidation, Mn ( V )-oxo(cor+•), is the most reactive specie, while the neutral [ Mn ( V )-oxo(cor)] in the triplet states are more reactive than those in the singlet states.