MODULATING DIOXYGEN ACTIVATION AND OXYGEN ATOM TRANSFER REACTIVITY OF MANGANESE CORROLAZINES

DIOXYGEN ACTIVATION;porphyrinoids;oxygen-atom-transfer;manganese;Chemistry

Neu, Heather MarieGoldberg, David P. ;

Johns Hopkins University

关键词: DIOXYGEN ACTIVATION;    porphyrinoids;    oxygen-atom-transfer;    manganese;    Chemistry;   

Others  :  https://jscholarship.library.jhu.edu/bitstream/handle/1774.2/44616/NEU-DISSERTATION-2015.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: JOHNS HOPKINS DSpace Repository

【 摘 要 】

Heme enzymes are widespread in nature and catalyze a wide variety of organic transformations. High-valent metal-oxo species are implicated as reactive intermediates in many of these enzymatic systems and it is of interest to understand how nature modulates and controls the reactivity of these intermediates.For example, in heme enzymes, the reactivity of the high-valent metal-oxo species can be tuned by the coordination of axial ligands to the metal center. It has been proposed that the anionic cysteine donor in Cytochrome P450 plays an important role in tuning the electronic features and reactivity of the high-valent iron-oxo intermediate (Compound-I). Our approach is to synthesize well defined porphyrinoid model complexes that stabilize high-valent metal-oxo species and allow us to tune their reactivity through a variety of structural and electronic modification including the addition of anionic donors and Brønsted acids. Chapter 1 provides a brief introduction to metalloenzymes, porphyrinoid model complexes and the earlier work on stabilization of manganese-oxo species with porphyrins and corroles. The last part of the chapter presents the synthesis of manganese(III) andmanganese(V)-oxo corrolazines and the unique properties of both complexes. Chapter 2 presents the effect of the addition of an anionic donor ligand (CN–) on the two-electron oxygen-atom-transfer (OAT) chemistry of the manganese(V)-oxo corrolazine [MnV(O)(TBP8Cz)].The six-coordinate [MnV(O)(TBP8Cz)(CN)]– complex was characterized by Mn K-edge X-ray absorption spectroscopy (XAS). Fitting of the XAS data revealed a short Mn–O bond at 1.53 Å and a sixth ligand bound at 2.21 Å from the metal center. Reaction of the six-coordinate [MnV(O)(TBP8Cz)(CN)]– complex with thioether substrates reveal a dramatic rate enhancement for OAT of 24,000-fold over the parent five-coordinate complex. Computational studies and an Eyring analysis fully support the relative reactivity of the five- and six-coordinate MnV(O) complexes. In Chapter 3, the OAT reactivity for a range of six-coordinate complexes, [(TBP8Cz)MnV(O)(X)]– (where X = CN–, F–, OCN–, NO3–, N3–, or SCN–), is presented. The six-coordinate complexes with X = F−, N3−, OCN− were examined by resonance Raman spectroscopy revealing a ∼5 cm−1 downshift of the Mn−O vibrational mode relative to the parent 5-coordinate complex. These results provide further evidence that the anionic donors are coordinated trans to the terminal oxo ligand, which induces a subtle weakening of the Mn−O vibrational mode. Examining the OAT kinetics for all of the six-coordinate [MnV(O)(TBP8Cz)(X)]– complexes versus the parent five-coordinate complex gave the following trend for the influence of the anionic donors: none ≈ SCN− ≈ NO3− < OCN− < N3− < F− ≪ CN−. The six-coordinate [MnV(O)(TBP8Cz)(X)]−(X = CN− or F−) complexes were reacted with para-X-C6H4SCH3 derivatives and revealed unusual ;;V-shaped” Hammett plots. These results are rationalized based upon a change in mechanism that hinges on the ability of the six-coordinate complexes to function as either an electrophile or weak nucleophile depending upon the nature of the substrate. Previously it was shown that photoirradiation (λ > 400 nm) of MnIII(TBP8Cz) in the presence of excess hexamethylbenzene (HMB) in benzonitrile (PhCN) under ambient conditions led to the selective oxidation of HMB to pentamethylbenzyl alcoholand production of MnV(O)(TBP8Cz).In Chapter 4, it will be shown that the addition of protons (H+ source: H+[B(C6F5)4]−) transforms this stoichiometric reaction into a catalytic one. The addition of one and two equivalents of protons to MnIII(TBP8Cz) formed two new species that were unambiguously characterized by X-ray diffraction as the mono-protonated [MnIII(H2O)(TBP8Cz(H))]+, and the diprotonated [MnIII(H2O)(TBP8Cz(H)2)]2+, respectively.The monoprotonated MnIII complex is catalytically active, but upon further protonation to the diprotonated species, the reactivity with O2 is shut down. Low-temperature spectroscopic methods (UV-vis and NMR) revealed that the related MnV(O)(TBP8Cz) is also protonated at the same remote site at −60°C, but undergoes valence tautomerization to [MnIV(O)(TBP8Cz•+)(H)]+ upon warming. Chapter 5 presents the photocatalytic oxygenation of substrates with trifluoromethanesulfonic acid. It was found that by changing the acid from H+[B(C6F5)4]− to trifluoromethanesulfonic acid (HOTf) high turn-overs were seen with a greater selectivity. The site of protonation with HOTf of monoprotonated [MnIII(TBP8Cz(H))]+ and diprotonated [MnIII(TBP8Cz(H)2)]2+ complexes were identified by X-ray diffraction and were found to be the same as those with H+[B(C6F5)4]−. Femtosecond laser flash photolysis measurements of [MnIII(TBP8Cz(H))]+ and [MnIII(TBP8Cz(H)2)]2+ in the presence of O2 revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state of [MnIII(TBP8Cz(H))]+ can react with O2 to produce the putative [MnIV(O2•–)(TBP8Cz(H))]+, whereas the tripseptet excited state of [MnIII(TBP8Cz(H)2)]2+ exhibits no reactivity toward O2. In Chapter 6 the synthesis of a new corrolazine and subporphyrazine will be presented.Also, the synthesis and NMR characterization of manganese(V)-oxo 10-(4-cyanophenyl)-5,15-(2,5-dichlorophenyl)corrole and the x-ray emission spectroscopy (XES) spectra of MnIII(TBP8Cz) and MnV(O)(TBP8Cz) complexes will be reported.