Oxygen-Atom Transfer Reactivity of Axially Ligated Mn(V)–Oxo Complexes: Evidence for Enhanced Electrophilic and Nucleophilic Pathways

Abstract

Addition of anionic donors to the manganese(V)–oxo corrolazine complex MnV(O)(TBP8Cz) has a dramatic influence on oxygen-atom transfer (OAT) reactivity with thioether substrates. The six-coordinate anionic [MnV(O)(TBP8Cz)(X)]− complexes (X = F–, N3–, OCN–) exhibit a ∼5 cm–1 downshift of the Mn–O vibrational mode relative to the parent MnV(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 MnV(O)(TBP8Cz) complex. A Hammett study was performed with p-X-C6H4SCH3 derivatives and [MnV(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 [MnV(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 MnV(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 MnV(O) porphyrinoid complexes.

Strong Inhibition of O-Atom Transfer Reactivity for MnIV(O)(π-Radical-Cation)(Lewis Acid) versus MnV(O) Porphyrinoid Complexes

“…32 In other work, Mn V (O)(TBP 8 Cz) was modified through attachment of anionic axial donors (X = CN − , F − ) which dramatically enhanced OAT reactivity for thioether substrates. 33 However, the kinetics for the oxidation of phosphine substrates by Mn V (O) corrolazine has not been reported. Herein stopped-flow UV-vis spectroscopy was used to measure the rate constants of O-atom transfer between Mn V (O)(TBP 8 Cz) and a wide range of triarylphosphine derivatives.…”

“…A color change to a dark green solution typical of [Mn III (TBP 8 Cz)(F)] − (λ max = 428, 471, 680 nm) was noted. 33 The solution was then concentrated under vacuum and re-dissolved in CD 2 Cl 2 :CD 3 CN (500 μL; 10:1 v/v) and immediately analyzed by 31 P{ 1 H} NMR (85% H 3 PO 4 external standard). The delay time (D1) was set to 150 s to allow for complete relaxation of the 31 P nucleus.…”

Photocatalytic Oxygenation of Substrates by Dioxygen with Protonated Manganese(III) Corrolazine

“…Coordination of OTf- provides a possible mechanism for enhancing catalytic reactivity through a proposed Mn V (O)(TBP 8 Cz) intermediate. 44,45,51 In our previous report on proton-assisted catalysis, we suggested a mechanism that involved a short-lived tripseptet excited state of the monoprotonated Mn III complex as a key intermediate. This mechanism was based on prior photochemical characterization of the parent Mn III complex.…”

The Role of Nonheme Transition Metal-Oxo, -Peroxo, and -Superoxo Intermediates in Enzyme Catalysis and Reactions of Bioinspired Complexes

“…100 Hence, if a triplet spin state is involved in the reaction mechanism then a spin state crossing from the closed-shell singlet spin state to one of the possible triplet spin states must happen along the reaction mechanism for substrate activation and ideally before the rate determining reaction barrier. To this end, we investigated thioanisole sulfoxidation by [Mn(O)(H 8 Cz)(CN)] -with substrates with a range of para-substituents and measured and calculated rate constants for the reaction.…”