Oxygen Atom Transfer Reactions from Isolated (Oxo)manganese(V) Corroles to Sulfides

Abstract

A series of five free-base corroles were metalated and brominated to form 10 manganese(III) corroles. Two of the free-base corroles and six manganese(III) corroles were analyzed by X-ray crystallography, including one complex that may be considered a transition-state analogue of oxygen atom transfer (OAT) from (oxo)manganese(V) to thioansisole. Oxidation by ozone allowed for isolation of the 10 corresponding (oxo)manganese(V) corroles, whose characterization by (1)H and (19)F NMR spectroscopy and electrochemistry revealed a low-spin and triply bound manganese-oxygen moiety. Mechanistic insight was obtained by investigating their reactivity regarding stoichiometric OAT to a series of p-thioanisoles, revealing a magnitude difference on the order of 5 between the β-pyrrole brominated (oxo)manganese(V) corroles relative to the nonbrominated analogues. The main conclusion is that the (oxo)manganese(V) corroles are legitimate OAT agents under conditions where proposed oxidant-coordinated reaction intermediates are irrelevant. Large negative Hammett ρ constants are obtained for the more reactive (oxo)manganese(V) corroles, consistent with expectation for such electrophilic species. The least reactive complexes display very little selectivity to the electron-richness of the sulfides, as well as a non-first-order dependence on the concentration of (oxo)manganese(V) corrole. This suggests that disproportionation of the original (oxo)manganese(V) corrole to (oxo)manganese(IV) and (oxo)manganese(VI) corroles, followed by substrate oxidation by the latter complex, gains importance when the direct OAT process becomes progressively less favorable.

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

“…22,23 However, the redox potential for PPh 3 is 2.08 V vs Cp 2 Co +/0 (converted to 2.97 V vs SCE), 56 and therefore one-electron oxidation of PPh 3 by the Mn IV (O) complex remains strongly endergonic, helping to rule out either a pure ET or ET-OT type mechanism. 30,57–58 …”

“…30,57,58,65,66 In addition, DFT calculations on two-electron atom-transfer reactions have suggested that spin state may also be an important factor. For example, DFT calculations performed on OAT of Mn V (O) corroles implicate a lower-barrier triplet state in the OAT reaction with thioanisole.…”

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

“…22−30 One of the few examples comes from Chang and co-workers, who showed that the reactivity of an Mn V (O) corrole toward styrene can be enhanced by 10-fold with an imidazole as an axial ligand. 23 The effects of axial donors on nonheme Mn(O) complexes such as Mn(O)(salen)(X) have been reported.…”

The Influence of Peripheral Substituent Modification on PV, MnIII, and MnV(O) Corrolazines: X-ray Crystallography, Electrochemical and Spectroscopic Properties, and HAT and OAT Reactivities

“…In contrast, the structures of metal-free corroles are significantly puckered due to the steric strain of the three pyrrolic N–H protons in the internal cavity. 2,66–71 …”