Effect of coordination ability of axial ligands on the stability and catalytic activity of chloro(tetramesitylporphyrinato)manganese(III) and lifetime of Mn-oxo intermediate

Significant hydrogen-bonding effect on the reactivity of high-valent manganese(V)–oxo porphyrins in C–H bond activation: A DFT study

The stereo electronic effects as well as hydrogen bonding effects of imidazole, pyridine and 2,6-dimethylpyridine as N-donor axial ligands on the C–H oxidation activity of high-valent manganese(V)–oxo meso-tetraphenylporphyrin (TPP) and meso-tetrakis(pentaflourophenyl)porphyrin (TPFPP), are investigated by DFT calculations. The electronic and steric properties of axial donors and porphyrin ligands affected on the activation energy of cyclohexane hydroxylation as well as the Mn–O bond strength of the oxo species in transition state. Imidazole with the strong [Formula: see text]-donating ability and the least steric hindrance showed greater co-catalytic activity than those of pyridine and in particularly hindered 2,6-Me2 pyridine in the presence of simple [(TPP)MnO][Formula: see text]. Nevertheless, the C–H bond activation by hindered and electron-deficient perfluorinated catalyst [(TPFPP)MnO][Formula: see text] is in the order of pyridine >2, 6-Me2 pyridine >imidazole. AIM analysis showed hydrogen bonding (HB) between the C–H [Formula: see text] bonds of pyridine (C[Formula: see text]-H of ring) and 2,6-Me2Py (C[Formula: see text]-H of methyl groups) with ortho-C–F bond of phenyl rings of TPFPP in Mn[Formula: see text]O species (C–H…..F–C hydrogen bond) which might be responsible for this unusual behavior. These results are supported by natural bond orbital (NBO) analysis.

Effect of L-cysteine on the oxidation of cyclohexane catalyzed by manganeseporphyrin

Effect of L-cysteine as the cocatalyst on the oxidation of cyclohexane by tert-butylhydroperoxide (TBHP) catalyzed by manganese tetraphenylporphyrin (MnTPP) has been investigated. The results showed that L-cysteine could moderately improve the catalytic activity of MnTPP and significantly increase the selectivity of cyclohexanol. Different from imidazole and pyridine, the L-cysteine may perform dual roles in the catalytic oxidation of cyclohexane. Besides as the axial ligand for MnTPP, the L-cysteine could also react with cyclohexyl peroxide formed as the intermediate to produce alcohol as the main product.

Crystal structure, magnetic and catalytic oxidation properties of manganese(iii) tetrakis(ethoxycarbonyl)porphyrin

5,10,15,20-tetrakis(ethoxycarbonyl)porphyrin manganese(iii) chloride (Mn^IIITECPCl) existed as a Mn–O coordinated dimer. Mn^IIITECPCl gave high yield in oxidation of styrene by using TBHP or PhIO oxidant and was found recyclable.

Dramatic influence of an anionic donor on the oxygen-atom transfer reactivity of a Mn(V) -oxo complex

Addition of an anionic donor to an Mn(V) (O) porphyrinoid complex causes a dramatic increase in 2-electron oxygen-atom-transfer (OAT) chemistry. The 6-coordinate [Mn(V) (O)(TBP8 Cz)(CN)](-) was generated from addition of Bu4 N(+) CN(-) to the 5-coordinate Mn(V) (O) precursor. The cyanide-ligated complex was characterized for the first time by Mn K-edge X-ray absorption spectroscopy (XAS) and gives MnO=1.53 Å, MnCN=2.21 Å. In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN(-) complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e(-) -reduced Mn(III) (CN)(-) complex. Kinetic measurements reveal a dramatic rate enhancement for OAT of approximately 24 000-fold versus the same reaction for the parent 5-coordinate complex. An Eyring analysis gives ΔH(≠) =14 kcal mol(-1) , ΔS(≠) =-10 cal mol(-1)  K(-1) . Computational studies fully support the structures, spin states, and relative reactivity of the 5- and 6-coordinate Mn(V) (O) complexes.

Oxygen-atom transfer reactivity of axially ligated Mn(V)-oxo complexes: evidence for enhanced electrophilic and nucleophilic pathways

Addition of anionic donors to the manganese(V)–oxo corrolazine complex Mn^V(O)(TBP₈Cz) has a dramatic influence on oxygen-atom transfer (OAT) reactivity with thioether substrates. The six-coordinate anionic [Mn^V(O)(TBP₈Cz)(X)]⁻ complexes (X = F^–, N₃^–, OCN^–) exhibit a ∼5 cm^–1 downshift of the Mn–O vibrational mode relative to the parent Mn^V(O)(TBP₈Cz) complex as seen by resonance Raman spectroscopy. Product analysis shows that the oxidation of thioether substrates gives sulfoxide product, consistent with single OAT. A wide range of OAT reactivity is seen for the different axial ligands, with the following trend determined from a comparison of their second-order rate constants for sulfoxidation: five-coordinate ≈ thiocyanate ≈ nitrate < cyanate < azide < fluoride ≪ cyanide. This trend correlates with DFT calculations on the binding of the axial donors to the parent Mn^V(O)(TBP₈Cz) complex. A Hammett study was performed with p-X-C₆H₄SCH₃ derivatives and [Mn^V(O)(TBP₈Cz)(X)]⁻ (X = CN^– or F^–) as the oxidant, and unusual “V-shaped” Hammett plots were obtained. These results are rationalized based upon a change in mechanism that hinges on the ability of the [Mn^V(O)(TBP₈Cz)(X)]⁻ complexes to function as either an electrophilic or weak nucleophilic oxidant depending upon the nature of the para-X substituents. For comparison, the one-electron-oxidized cationic Mn^V(O)(TBP₈Cz^•+) complex yielded a linear Hammett relationship for all substrates (ρ = −1.40), consistent with a straightforward electrophilic mechanism. This study provides new, fundamental insights regarding the influence of axial donors on high-valent Mn^V(O) porphyrinoid complexes.