Spectroscopic and Structural Characterization of Mn(III)-Alkylperoxo Complexes Supported by Pentadentate Amide-Containing Ligands

Manganese-alkylperoxo species have been proposed as important intermediates in certain enzymatic pathways and are presumed to play a key role in catalytic substrate oxidation cycles involving manganese catalysts and peroxide oxidants. However, structural and spectroscopic understanding of these intermediates is very limited, with only one series of synthetic Mn^III-alkylperoxo complexes having been reported. In the present study, we describe the formation and properties of two new Mn^III-alkylperoxo complexes, namely, [Mn^III(OO^<i>t</i>Bu)­(dpaq)]⁺ and [Mn^III(OO^<i>t</i>Bu)­(dpaq^2Me)]⁺, which utilize the anionic, amide-containing pentadentate dpaq ligand platform. These complexes were generated by reacting the corresponding Mn^II precursors with a large excess of ^<i>t</i>BuOOH at −15 °C in MeCN. In both cases, the corresponding mononuclear Mn^III-hydroxo complexes [Mn^III(OH)­(dpaq)]⁺ and [Mn^III(OH)­(dpaq^2Me)]⁺ are observed as intermediates en route to the Mn^III-alkylperoxo adducts. These new Mn^III-alkylperoxo complexes were characterized by electronic absorption, infrared, and Mn K-edge X-ray absorption spectroscopies. Complementary density functional theory calculations were also performed to gain insight into their bonding and structural properties. Compared to previously reported Mn^III-alkylperoxo adducts, the Mn^III centers in these complexes exhibit significantly altered primary coordination spheres, with a strongly donating anionic amide nitrogen located trans to the alkylperoxo moiety. This results in Mn^III-alkylperoxo bonding that is dominated by σ-interactions between the alkylperoxo π_ip*­(O–O) orbital and the Mn d_<i>z</i>² orbital

Relationship between Hydrogen-Atom Transfer Driving Force and Reaction Rates for an Oxomanganese(IV) Adduct

Hydrogen atom transfer (HAT) reactions by high-valent metal-oxo intermediates are important in both biological and synthetic systems. While the HAT reactivity of Fe^IV-oxo adducts has been extensively investigated, studies of analogous Mn^IV-oxo systems are less common. There are several recent reports of Mn^IV-oxo complexes, supported by neutral pentadentate ligands, capable of cleaving strong C–H bonds at rates approaching those of analogous Fe^IV-oxo species. In this study, we provide a thorough analysis of the HAT reactivity of one of these Mn^IV-oxo complexes, [Mn^IV(O)­(2pyN2Q)]²⁺, which is supported by an N5 ligand with equatorial pyridine and quinoline donors. This complex is able to oxidize the strong C–H bonds of cyclohexane with rates exceeding those of Fe^IV-oxo complexes with similar ligands. In the presence of excess oxidant (iodosobenzene), cyclohexane oxidation by [Mn^IV(O)­(2pyN2Q)]²⁺ is catalytic, albeit with modest turnover numbers. Because the rate of cyclohexane oxidation by [Mn^IV(O)­(2pyN2Q)]²⁺ was faster than that predicted by a previously published Bells–Evans–Polanyi correlation, we expanded the scope of this relationship by determining HAT reaction rates for substrates with bond dissociation energies spanning 20 kcal/mol. This extensive analysis showed the expected correlation between reaction rate and the strength of the substrate C–H bond, albeit with a shallow slope. The implications of this result with regard to Mn^IV-oxo and Fe^IV-oxo reactivity are discussed

Steric and Electronic Influence on Proton-Coupled Electron-Transfer Reactivity of a Mononuclear Mn(III)-Hydroxo Complex

A mononuclear hydroxomanganese­(III) complex was synthesized utilizing the N₅ amide-containing ligand 2-[bis­(pyridin-2-ylmethyl)]­amino-<i>N</i>-2-methyl-quinolin-8-yl-acetamidate (dpaq^2Me ). This complex is similar to previously reported [Mn^III(OH)­(dpaq^H)]⁺ [<i>Inorg. Chem.</i> <b>2014</b>, 53, 7622–7634] but contains a methyl group adjacent to the hydroxo moiety. This α-methylquinoline group in [Mn^III(OH)­(dpaq^2Me)]⁺ gives rise to a 0.1 Å elongation in the Mn–N­(quinoline) distance relative to [Mn^III(OH)­(dpaq^H)]⁺. Similar bond elongation is observed in the corresponding Mn­(II) complex. In MeCN, [Mn^III(OH)­(dpaq^2Me)]⁺ reacts rapidly with 2,2′,6,6′-tetramethylpiperidine-1-ol (TEMPOH) at −35 °C by a concerted proton–electron transfer (CPET) mechanism (second-order rate constant <i>k</i>₂ of 3.9(3) M^–1 s^–1). Using enthalpies and entropies of activation from variable-temperature studies of TEMPOH oxidation by [Mn^III(OH)­(dpaq^2Me)]⁺ (Δ<i>H</i>^‡ = 5.7(3) kcal^–1 M^–1; Δ<i>S</i>^‡ = −41(1) cal M^–1 K^–1), it was determined that [Mn^III(OH)­(dpaq^2Me)]⁺ oxidizes TEMPOH ∼240 times faster than [Mn^III(OH)­(dpaq^H)]⁺. The [Mn^III(OH)­(dpaq^2Me)]⁺ complex is also capable of oxidizing the stronger O–H and C–H bonds of 2,4,6-tri-<i>tert</i>-butylphenol and xanthene, respectively. However, for these reactions [Mn^III(OH)­(dpaq^2Me)]⁺ displays, at best, modest rate enhancement relative to [Mn^III(OH)­(dpaq^H)]⁺. A combination of density function theory (DFT) and cyclic voltammetry studies establish an increase in the Mn^III/Mn^II reduction potential of [Mn^III(OH)­(dpaq^2Me)]⁺ relative to [Mn^III(OH)­(dpaq^H)]⁺, which gives rise to a larger driving force for CPET for the former complex. Thus, more favorable thermodynamics for [Mn^III(OH)­(dpaq^2Me)]⁺ can account for the dramatic increase in rate with TEMPOH. For the more sterically encumbered substrates, DFT computations suggest that this effect is mitigated by unfavorable steric interactions between the substrate and the α-methylquinoline group of the dpaq^2Me ligand. The DFT calculations, which reproduce the experimental activation free energies quite well, provide the first examination of the transition-state structure of mononuclear Mn^III(OH) species during a CPET reaction