Addition of the Lewis acid Zn(2+) to (TBP(8)Cz)Mn(V)(O) induces valence tautomerization, resulting in the formation of [(TBP(8)Cz(+•))Mn(IV)(O)-Zn(2+)]. This new species was characterized by UV-vis, EPR, the Evans method, and (1)H NMR and supported by DFT calculations. Removal of Zn(2+) quantitatively restores the starting material. Electron-transfer and hydrogen-atom-transfer reactions are strongly influenced by the presence of Zn(2+).

Heme proteins utilize the heme cofactor, an iron porphyrin, to perform a diverse range of reactions including dioxygen binding and transport, electron transfer, and oxidation/oxygenations. These reactions share several, key metalloporphyrin intermediates, typically derived from dioxygen and its congeners such as hydrogen peroxide, and which are comprised of metal-dioxygen, metal-superoxo, metal-peroxo, and metal-oxo adducts. A wide variety of synthetic metalloporphyrinoid complexes have been synthesized to generate and stabilize these intermediates, and then employed to determine the spectroscopic features, structures, and reactivities of such species in controlled and well-defined environments. In this review, we summarize recent findings on the reactivity of these species with common porphyrinoid scaffolds employed for biomimetic studies. The proposed mechanisms of action are emphasized. The review is organized by structural type of metal-oxygen intermediate, and broken into subsections based on the metal (manganese, iron) and porphyrinoid ligand (porphyrin, corrole, corrolazine).

Addition of anionic donors to the manganese(V)–oxo corrolazine complex MnV(O)(TBP8Cz) has a dramatic influence on oxygen-atom transfer (OAT) reactivity with thioether substrates. The six-coordinate anionic [MnV(O)(TBP8Cz)(X)]− complexes (X = F–, N3–, OCN–) exhibit a ∼5 cm–1 downshift of the Mn–O vibrational mode relative to the parent MnV(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 MnV(O)(TBP8Cz) complex. A Hammett study was performed with p-X-C6H4SCH3 derivatives and [MnV(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 [MnV(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 MnV(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 MnV(O) porphyrinoid complexes.

CONSPECTUS A large class of heme and nonheme metalloenzymes utilize O2 or its derivatives (e.g. H2O2) to generate high-valent metal-oxo intermediates for performing challenging and selective oxidations. Due to their reactive nature, these intermediates are often short-lived and very difficult to characterize. Synthetic chemists have sought to prepare analogous metal-oxo complexes with ligands that impart enough stability to allow for their characterization and an examination of their inherent reactivity. The challenge in designing these molecules is to achieve a balance between their stability, which should allow for their in situ characterization or isolation, and their reactivity, in which they can still participate in interesting chemical transformations. This review focuses on our recent efforts to generate and stabilize high-valent manganese-oxo porphyrinoid complexes, and tune their reactivity in the oxidation of organic substrates. Dioxygen can be used to generate a high-valent MnV(O) corrolazine (MnVO(TBP8Cz)) by irradiation of MnIII(TBP8Cz) with visible light in the presence of a C–H substrate. Quantitative formation of the MnV(O) complex occurs with concomitant selective hydroxylation of the benzylic substrate hexamethylbenzene. Addition of a strong H+ donor converted this light/O2/substrate reaction from a stoichiometric to a catalytic process with modest turnovers. The addition of H+ likely activates a transient MnV(O) complex to achieve turnover, whereas in the absence of H+, the MnV(O) complex was an unreactive, “dead-end” complex. Addition of anionic donors to the MnV(O) complex also leads to enhanced reactivity, with a large increase in the rate of 2-electron oxygen-atom-transfer (OAT) to thioether substrates. Spectroscopic characterization (Mn K-edge X-ray absorption and resonance Raman spectroscopies) revealed that the anionic donors (X−) bind to the MnV ion to form six-coordinate [MnV(O)(X)]− complexes. An unusual “V-shaped” Hammett plot for the oxidation of para-substituted thioanisole derivatives suggested that six-coordinate [MnV(O)(X)]− complexes can act as both electrophiles or nucleophiles, depending on the nature of the substrate. Oxidation of the MnV(O) corrolazine resulted in the in situ generation of an MnV(O) π-radical cation complex, [MnV(O)(TBP8Cz•+)]+, which exhibited more than a 100-fold rate increase in the oxidation of thioethers. The addition of Lewis acids (LA: ZnII, B(C6F5)3) to the closed-shell, diamagnetic MnV(O)(TBP8Cz) stabilized a paramagnetic valence tautomer MnIV(O)(TBP8Cz•+):LA, which was characterized as a second π-radical cation complex by NMR, EPR, UV-vis, and high resolution CSI-MS. The MnIV(O)(TBP8Cz•+):LA complexes are able to abstract H• from phenols and exhibit a rate enhancement of up to ∼100-fold over the parent MnV(O) valence tautomer. In contrast, a large decrease in rate is observed for OAT for the MnIV(O)(TBP8Cz•+):LA complexes. The rate enhancement for HAT may derive from the higher redox potential for the π-radical cation complex, while the large rat...

The visible light-driven, catalytic aerobic oxidation of benzylic C-H bonds was mediated by an MnIII corrolazine complex. To achieve catalytic turnovers, a strict selective requirement for the addition of protons was established. The resting state of the catalyst was unambiguously characterized by X–ray diffraction as [MnIII(H2O)(TBP8Cz(H))]+, in which a single, remote site on the ligand is protonated. If two remote sites are protonated, however, reactivity with O2 is shut down. Spectroscopic methods revealed that the related MnV(O) complex is also protonated at the same remote site at −60 °C, but undergoes valence tautomerization upon warming.

Isomorphous crystals of MnV(O) and CrV(O) corrolazines were characterized by single crystal X-ray diffraction. Reactivity studies with H-atom donors and separated PCET reagents show a dramatic difference in H-atom abstracting abilities for these two complexes. The implied large difference in driving force is opposite to the trend in redox potentials, indicating that basicity is a key factor in determining the striking difference in reactivity for two metal-oxo species in identical ligand environments.

The oxygen atom transfer (OAT) reactivity of two valence tautomers of a MnV(O) porphyrinoid complex was compared. The OAT kinetics of MnV(O)(TBP8Cz) (TBP8Cz = octakis(p-tert-butylphenyl)corrolazinato3−) reacting with a series of triarylphosphine (PAr3) substrates were monitored by stopped-flow UV-vis spectroscopy, and revealed second-order rate constants ranging from 16(1) to 1.43(6) × 104 M−1 s−1. Characterization of the OAT transition state analogs MnIII(OPPh3)(TBP8Cz) and MnIII(OP(o-tolyl)3)(TBP8Cz) was carried out by single-crystal X-ray diffraction (XRD). A valence tautomer of the closed-shell MnV(O)(TBP8Cz) can be stabilized by the addition of Lewis and Brønsted acids, resulting in the open-shell MnIV(O)(TBP8Cz•+):LA (LA = ZnII, B(C6F5)3, H+) complexes. These MnIV(O)(π-radical-cation) derivatives exhibit dramatically inhibited rates of OAT with the PAr3 substrates (k = 8.5(2) × 10−3 − 8.7 M−1 s−1), contrasting the previously observed rate increase of H-atom transfer (HAT) for MnIV(O)(TBP8Cz•+):LA with phenols. A Hammett analysis showed that the OAT reactivity for MnIV(O)(TBP8Cz•+):LA is influenced by the Lewis acid strength. Spectral redox titration of MnIV(O)(TBP8Cz•+):ZnII gives Ered = 0.69 V vs SCE, which is nearly +700 mV above its valence tautomer MnV(O)(TBP8Cz) (Ered = −0.05 V). These data suggest that the two-electron electrophilicity of the Mn(O) valence tautomers dominate OAT reactivity and do not follow the trend in one-electron redox potentials, which appear to dominate HAT reactivity. This study provides new fundamental insights regarding the relative OAT and HAT reactivity of valence tautomers such as MV(O)(porph) versus MIV(O)(porph•+) (M = Mn or Fe) found in heme enzymes.

The addition of Lewis or Brönsted acids (LA = Zn(OTf)2, B(C6F5)3, HBArF, TFA) to the high-valent manganese—oxo complex MnV(O)(TBP8Cz) results in the stabilization of a valence tautomer MnIV(O-LA)(TBP8Cz•+). The ZnII and B(C6F5)3 complexes were characterized by manganese K-edge X-ray absorption spectroscopy (XAS). The position of the edge energies and the intensities of the pre-edge (1s to 3d) peaks confirm that the Mn ion is in the +4 oxidation state. Fitting of the extended X-ray absorption fine structure (EXAFS) region reveals 4 N/O ligands at Mn−Nave = 1.89 Å and a fifth N/O ligand at 1.61 Å, corresponding to the terminal oxo ligand. This Mn−O bond length is elongated compared to the MnV(O) starting material (Mn−O = 1.55 Å). The reactivity of MnIV(O-LA)(TBP8Cz−+) toward C−H substrates was examined, and it was found that H• abstraction from C−H bonds occurs in a 1:1 stoichiometry, giving a MnIV complex and the dehydrogenated organic product. The rates of C−H cleavage are accelerated for the MnIV(O-LA)(TBP8Cz•+) valence tautomer as compared to the MnV(O) valence tautomer when LA = ZnII, B(C6F5)3, and HBArF, whereas for LA = TFA, the C−H cleavage rate is slightly slower than when compared to MnV(O). A large, nonclassical kinetic isotope effect of kH/kD = 25–27 was observed for LA = B(C6F5)3 and HBArF, indicating that H-atom transfer (HAT) is the rate-limiting step in the C−H cleavage reaction and implicating a potential tunneling mechanism for HAT. The reactivity of MnIV(O-LA)(TBP8Cz•+) toward C−H bonds depends on the strength of the Lewis acid. The HAT reactivity is compared with the analogous corrole complex MnIV(O−H)(tpfc•+) recently reported.