Deeper Understanding of Mononuclear Manganese(IV)–Oxo Binding Brønsted and Lewis Acids and the Manganese(IV)–Hydroxide Complex

Abstract

Binding of Lewis acidic metal ions and Brønsted acid at the metal–oxo group of high-valent metal–oxo complexes enhances their reactivities significantly in oxidation reactions. However, such a binding of Lewis acids and proton at the metal–oxo group has been questioned in several cases and remains to be clarified. Herein, we report the synthesis, characterization, and reactivity studies of a mononuclear manganese­(IV)–oxo complex binding triflic acid, {[(dpaq)­MnIV(O)]–HOTf}+ (1–HOTf). First, 1–HOTf was synthesized and characterized using various spectroscopic techniques, including resonance Raman (rRaman) and X-ray absorption spectroscopy/extended X-ray absorption fine structure. In particular, in rRaman experiments, we observed a linear correlation between the Mn–O stretching frequencies of 1–HOTf (e.g., νMn–O at ∼793 cm–1) and 1–M n+ (M n+ = Ca2+, Zn2+, Lu3+, Al3+, or Sc3+) and the Lewis acidities of H+ and M n+ ions, suggesting that H+ and M n+ bind at the metal–oxo moiety of [(dpaq)­MnIV(O)]+. Interestingly, a single-crystal structure of 1–HOTf was obtained by X-ray diffraction analysis, but the structure was not an expected Mn­(IV)–oxo complex but a Mn­(IV)–hydroxide complex, [(dpaq)­MnIV(OH)]­(OTf)2 (4), with a Mn–O bond distance of 1.8043(19) Å and a Mn–O stretch at 660 cm–1. More interestingly, 4 reverted to 1–HOTf upon dissolution, demonstrating that 1–HOTf and 4 are interconvertible depending on the physical states, such as 1–HOTf in solution and 4 in isolated solid. The reactivity of 1–HOTf was investigated in hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reactions and then compared with those of 1–M n+ complexes; an interesting correlation between the Mn–O stretching frequencies of 1–HOTf and 1–M n+ and their reactivities in the OAT and HAT reactions is reported for the first time in this study.

Lewis acid improved dioxygen activation by a non-heme iron(ii) complex towards tryptophan 2,3-dioxygenase activity for olefin oxygenation

“…Clearly, the electrostatic interaction of Lewis acid with the active oxygen species made it more electrophilic in oxygenation, thus disclosing a more negative ρ value in the Hammett plot from a stronger Lewis acid. In particular, using the binding energies (Δ E ) of the metal ions with a superoxide anion (O 2 ˙ − ) as the indicator of its corresponding Lewis acid strength, which was introduced by Fukuzumi, 21 a linear correlation of styrene conversion with the strength of Lewis acid was observed (Fig. 5).…”

Brønsted Acids Promote Olefin Oxidations by Bioinspired Nonheme CoIII(PhIO)(OH) Complexes: A Role for Low-Barrier Hydrogen Bonds

“…This acceleration was attributed to the enhancement of one-electron reduction energy from 0.51 V (without acid) to 1.43 V vs SCE of the reactions (Scheme c) . These roles for Brønsted acids have also been found in other catalytic systems. − …”

Aliphatic and Aromatic C–H Bond Oxidation by High-Valent Manganese(IV)-Hydroxo Species

“…Several mid- and high-valent metal-hydroxo complexes have been synthesized and spectroscopically characterized. However, most synthetic metal-hydroxo intermediates have been assigned as sluggish oxidants that react with relatively weak C–H and O–H bonds of substrates to date. ,,− Thus, strong C–H bond activation by metal-hydroxo adducts is challenging.…”

“…This observed value is comparable to those of Mn(IV)-OH complexes of which Mn−O frequency is spectroscopically measured in the range from 660 to 682 cm −1 . 41,54,55,66 Based on the UV−vis and Raman analyses, a manganese(IV)-bis(hydroxo) intermediate, [Mn IV (CHDAP)-(OH) 2 ] 2+ (2′, see Figure 2), was strongly suggested, but further spectroscopic characterization could be precluded due to its short lifetime caused by intramolecular C−H bond activation (vide infra). The ESI-MS experiments detect 2′ only indirectly as the reduced [Mn III (CHDAP)(OH) 2 ] + complex upon addition of water to the reaction mixture (see Figure S34 and the accompanying discussion).…”

“…Recently, metal-hydroxo species have also been recognized as critical intermediates in the catalytic event of metalloenzymes such as lipoxygenase (LOX). ,− Mn-LOX enzyme is proposed to oxidize C–H bonds of polyunsaturated fatty acids, where Mn III -hydroxo species performs the rate-determining hydrogen atom abstraction. Several mid- and high-valent metal-hydroxo complexes have been synthesized and spectroscopically characterized.…”