Structure and Reactivity of Nonporphyrinic Terminal Manganese(IV)–Hydroxide Complexes in the Oxidative Electrophilic Reaction

Abstract

High-valent transition metal–hydroxide complexes have been proposed as essential intermediates in biological and synthetic catalytic reactions. In this work, we report the single-crystal X-ray structure and spectroscopic characteristics of a mononuclear nonporphyrinic MnIV–(OH) complex, [MnIV(Me3-TPADP)­(OH)­(OCH2CH3)]2+ (2), using various physicochemical methods. Likewise, [MnIV(Me3-TPADP)­(OH)­(OCH2CF3)]2+ (3), which is thermally stable at room temperature, was also synthesized and characterized spectroscopically. The MnIV–(OH) adducts are capable of performing oxidation reactions with external organic substrates such as C–H bond activation, sulfoxidation, and epoxidation. Kinetic studies, involving the Hammett correlation and kinetic isotope effect, and product analyses indicate that 2 and 3 exhibit electrophilic oxidative reactivity toward hydrocarbons. Density functional theory calculations support the assigned electronic structure and show that direct C–H bond activation of the MnIV–(OH) species is indeed possible.

Electrocatalytic Atom Transfer Radical Addition with Turbocharged Organocopper(II) Complexes

“…The ligand L1, including its analogues bearing various Nalkyl groups and pyridyl ring substituents, forms 6-coordinate distorted octahedral complexes with Mn IV , 26 Fe II , 15,17,27 Fe III , 27,28 Co II , 29 Co III , 25 Ni II (high-spin), 30 and Ni III . 30 Interestingly, and also found here, Cu II only forms 5coordinate complexes with L1 and its derivatives.…”

Mn(II) Polypyridyl Complexes: Precursors to High Valent Mn(V)=O Species and Inhibitors of Cancer Cell Proliferation

“…Literature highlights Mn IV =O species show Raman band in the range of 700-750 cm À 1 (e. g. [(BQCN)Mn IV =O] 2 + (where BQCN = N,N'-dimethyl-N,N'-bis(8-quinolyl)cyclohexane-1,2-diamine) at 707 cm À 1 ), [28] while few reported cases for Mn V =O to exhibit the band around 755 cm À 1 and Mn V � O's above 900 cm À 1 (Table S4). [6,15,[29][30][31][32] The appearance of the band at 840 cm À 1 in 2 a, however, is distinct and a plausible candidate for a terminal Mn=O stretch in the complex. Thus, to verify whether the 840 cm À 1 band is originated from a Mn=O, the rRaman experiments were performed in the presence of 18 O-labeled water.…”

“…From Figure 3, it is evident that 2 a and 2 b share several common bands, indicating the presence of m ‐Cl‐salicylate coordinated to the Mn centre. Literature highlights Mn IV =O species show Raman band in the range of 700–750 cm −1 (e. g. [(BQCN)Mn IV =O] 2+ (where BQCN= N,N′ ‐dimethyl‐ N,N′ ‐bis(8‐quinolyl)cyclohexane‐1,2‐diamine) at 707 cm −1 ), [28] while few reported cases for Mn V =O to exhibit the band around 755 cm −1 and Mn V ≡O's above 900 cm −1 (Table S4) [6,15,29–32] . The appearance of the band at 840 cm −1 in 2 a , however, is distinct and a plausible candidate for a terminal Mn=O stretch in the complex.…”

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

“…In biomimetic chemistry, the aim is to develop bioinspired and environmentally benign catalysts by utilizing reactive metal-oxygen species. − The C–H bond activation by synthetic high-valent metal-oxo species has been intensively investigated through detailed experimental and theoretical studies. For instance, high-valent iron and manganese-oxo species carry out C–H functionalization of aliphatic hydrocarbons, which have C–H bond dissociation energy in the range of 77–99.5 kcal/mol. ,,, The generally accepted mechanism in the hydroxylation is the hydrogen atom transfer (HAT), where the initial hydrogen abstraction step is followed by fast oxygen rebound of substrate radical species. , In some cases, after rate-determining HAT, the dissociation pathway of substrate radical species is preferred rather than the oxygen-rebound process. , There are a few examples of electrophilic oxidation reactions of anthracene by metal-oxo complexes .…”

“…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.…”