No abstract

The insertion of carbon dioxide into the Mn−O bond of fac-(CO)3(dppe)MnOCH3 (1) was observed to occur instantaneously at −78 °C by in situ infrared spectroscopy. The product of carboxylation of 1, fac-(CO)3(dppe)MnOC(O)OCH3 (2), underwent decarboxylation with a first-order rate constant of 1.49 × 10-4 s-1 at 23 °C. The kinetic parameters for this process were determined by trapping the intermediate produced upon CO2 extrusion, complex 1, with COS to provide the very stable fac-(CO)3(dppe)MnSC(O)OCH3 (3) derivative. The structure of 3 was determined by single-crystal X-ray diffraction analysis, establishing the presence of the Mn−S bond.

Metal–superoxo species are typically proposed as key intermediates in the catalytic cycle of dioxygen activation by metalloenzymes involving different transition metal cofactors. In this regard, while a series of Fe–, Co–, and Ni–superoxo complexes have been reported to date, well-defined Mn–superoxo complexes remain rather rare. Herein, we report two mononuclear Mn III –superoxo species, Mn(BDPP)(O 2 •– ) ( 2 , H 2 BDPP = 2,6-bis((2-( S )-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Mn(BDP Br P)(O 2 •– ) ( 2′ , H 2 BDP Br P = 2,6-bis((2-( S )-di(4-bromo)phenylhydroxyl-methyl-1-pyrrolidinyl)methyl)pyridine), synthesized by bubbling O 2 into solutions of their Mn II precursors, Mn(BDPP) ( 1 ) and Mn(BDP Br P) ( 1′ ), at −80 °C. A combined spectroscopic (resonance Raman and electron paramagnetic resonance (EPR) spectroscopy) and computational study evidence that both complexes contain a high-spin Mn III center ( S Mn = 2) antiferromagnetically coupled to a superoxo radical ligand ( S OO• = 1/2), yielding an overall S = 3/2 ground state. Complexes 2 and 2′ were shown to be capable of abstracting a H atom from 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPO-H) to form Mn III –hydroperoxo species, Mn(BDPP)(OOH) ( 5 ) and Mn(BDP Br P)(OOH) ( 5′ ). Complexes 5 and 5′ can be independently prepared by the reactions of the isolated Mn III -aqua complexes, [Mn(BDPP)(H 2 O)]OTf ( 6 ) and [Mn(BDP Br P)(H 2 O)]OTf ( 6′ ), with H 2 O 2 in the presence of NEt 3 . The parallel-mode EPR measurements established a high-spin S = 2 ground state for 5 and 5′ .

Two dinickel mimics, [LNi2(DMF)4](ClO4)3 () and [L'Ni2(CH3CN)4](ClO4)3 (), for the active site of urease supported by a disubstituted benzoate polydentate ligand were synthesized and fully characterized, subsequently addition of urea afforded two urea adducts, [LNi2(urea)4](ClO4)3 () and [L'Ni2(urea)4](ClO4)3 ().

Systematic investigations on H atom transfer (HAT) thermodynamics of metal O 2 adducts is of fundamental importance for the design of transition metal catalysts for substrate oxidation and/or oxygenation directly using O 2 . Such work should help elucidate underlying electronic-structure features that govern the OO–H bond dissociation free energies (BDFEs) of metal-hydroperoxo species, which can be used to quantitatively appraise the HAT activity of the corresponding metal-superoxo complexes. Herein, the BDFEs of two homologous Co III - and Mn III -hydroperoxo complexes, 3-Co and 3-Mn , were calculated to be 79.3 and 81.5 kcal/mol, respectively, employing the Bordwell relationship based on experimentally determined p K a values and redox potentials of the one-electron-oxidized forms, 4-Co and 4-Mn . To further verify these values, we tested the HAT capability of their superoxo congeners, 2-Co and 2-Mn , toward three different substrates possessing varying O–H BDFEs. Specifically, both metal-superoxo species are capable of activating the O–H bond of 4-oxo-TEMPOH with an O–H BDFE of 68.9 kcal/mol, only 2-Mn is able to abstract a H atom from 2,4-di- tert -butylphenol with an O–H BDFE of 80.9 kcal/mol, and neither of them can react with 3,5-dimethylphenol with an O–H BDFE of 85.6 kcal/mol. Further computational investigations suggested that it is the high spin state of the Mn III center in 3-Mn that renders its OO–H BDFE higher than that of 3-Co , which features a low-spin Co III center. The present work underscores the role of the metal spin state being as crucial as the oxidation state in modulating BDFEs.