3 research outputs found
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<sup>III</sup>-alkylperoxo complexes having been reported. In the present study,
we describe the formation and properties of two new Mn<sup>III</sup>-alkylperoxo complexes, namely, [Mn<sup>III</sup>(OO<sup><i>t</i></sup>Bu)Â(dpaq)]<sup>+</sup> and [Mn<sup>III</sup>(OO<sup><i>t</i></sup>Bu)Â(dpaq<sup>2Me</sup>)]<sup>+</sup>, which utilize the anionic, amide-containing pentadentate
dpaq ligand platform. These complexes were generated by reacting the
corresponding Mn<sup>II</sup> precursors with a large excess of <sup><i>t</i></sup>BuOOH at −15 °C in MeCN. In both
cases, the corresponding mononuclear Mn<sup>III</sup>-hydroxo complexes
[Mn<sup>III</sup>(OH)Â(dpaq)]<sup>+</sup> and [Mn<sup>III</sup>(OH)Â(dpaq<sup>2Me</sup>)]<sup>+</sup> are observed as intermediates
en route to the Mn<sup>III</sup>-alkylperoxo adducts. These new Mn<sup>III</sup>-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<sup>III</sup>-alkylperoxo adducts, the Mn<sup>III</sup> 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<sup>III</sup>-alkylperoxo bonding that is dominated by σ-interactions
between the alkylperoxo Ï€<sub>ip</sub>*Â(O–O) orbital
and the Mn d<sub><i>z</i><sup>2</sup></sub> 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<sup>IV</sup>-oxo adducts has been extensively investigated, studies of analogous
Mn<sup>IV</sup>-oxo systems are less common. There are several recent
reports of Mn<sup>IV</sup>-oxo complexes, supported by neutral pentadentate
ligands, capable of cleaving strong C–H bonds at rates approaching
those of analogous Fe<sup>IV</sup>-oxo species. In this study, we
provide a thorough analysis of the HAT reactivity of one of these
Mn<sup>IV</sup>-oxo complexes, [Mn<sup>IV</sup>(O)Â(2pyN2Q)]<sup>2+</sup>, 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<sup>IV</sup>-oxo complexes with similar ligands. In the presence of excess oxidant
(iodosobenzene), cyclohexane oxidation by [Mn<sup>IV</sup>(O)Â(2pyN2Q)]<sup>2+</sup> is catalytic, albeit with modest turnover numbers. Because
the rate of cyclohexane oxidation by [Mn<sup>IV</sup>(O)Â(2pyN2Q)]<sup>2+</sup> 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<sup>IV</sup>-oxo and Fe<sup>IV</sup>-oxo reactivity are discussed
Steric and Electronic Influence on Proton-Coupled Electron-Transfer Reactivity of a Mononuclear Mn(III)-Hydroxo Complex
A mononuclear
hydroxomanganeseÂ(III) complex was synthesized utilizing the N<sub>5</sub> amide-containing ligand 2-[bisÂ(pyridin-2-ylmethyl)]Âamino-<i>N</i>-2-methyl-quinolin-8-yl-acetamidate (dpaq<sup>2Me</sup> ). This complex is similar to previously reported [Mn<sup>III</sup>(OH)Â(dpaq<sup>H</sup>)]<sup>+</sup> [<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<sup>III</sup>(OH)Â(dpaq<sup>2Me</sup>)]<sup>+</sup> gives
rise to a 0.1 Ã… elongation in the Mn–NÂ(quinoline) distance
relative to [Mn<sup>III</sup>(OH)Â(dpaq<sup>H</sup>)]<sup>+</sup>. Similar bond elongation is observed in the corresponding MnÂ(II)
complex. In MeCN, [Mn<sup>III</sup>(OH)Â(dpaq<sup>2Me</sup>)]<sup>+</sup> 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><sub>2</sub> of 3.9(3) M<sup>–1</sup> s<sup>–1</sup>). Using enthalpies and entropies of activation from variable-temperature
studies of TEMPOH oxidation by [Mn<sup>III</sup>(OH)Â(dpaq<sup>2Me</sup>)]<sup>+</sup> (Δ<i>H</i><sup>‡</sup> = 5.7(3) kcal<sup>–1</sup> M<sup>–1</sup>; Δ<i>S</i><sup>‡</sup> = −41(1) cal M<sup>–1</sup> K<sup>–1</sup>), it was determined that [Mn<sup>III</sup>(OH)Â(dpaq<sup>2Me</sup>)]<sup>+</sup> oxidizes TEMPOH ∼240
times faster than [Mn<sup>III</sup>(OH)Â(dpaq<sup>H</sup>)]<sup>+</sup>. The [Mn<sup>III</sup>(OH)Â(dpaq<sup>2Me</sup>)]<sup>+</sup> 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<sup>III</sup>(OH)Â(dpaq<sup>2Me</sup>)]<sup>+</sup> displays, at best, modest
rate enhancement relative to [Mn<sup>III</sup>(OH)Â(dpaq<sup>H</sup>)]<sup>+</sup>. A combination of density function theory (DFT)
and cyclic voltammetry studies establish an increase in the Mn<sup>III</sup>/Mn<sup>II</sup> reduction potential of [Mn<sup>III</sup>(OH)Â(dpaq<sup>2Me</sup>)]<sup>+</sup> relative to [Mn<sup>III</sup>(OH)Â(dpaq<sup>H</sup>)]<sup>+</sup>, which gives rise to a
larger driving force for CPET for the former complex. Thus, more favorable
thermodynamics for [Mn<sup>III</sup>(OH)Â(dpaq<sup>2Me</sup>)]<sup>+</sup> 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<sup>2Me</sup> 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<sup>III</sup>(OH)
species during a CPET reaction