Factors Affecting Hydrogen Atom Transfer Reactivity of Metal-Oxo Porphyrinoid Complexes

Abstract

There has been considerable interest in hydrogen atom transfer (HAT) reactions mediated by metal/oxygen species because of their central role in metalloenzyme function as well as synthetic catalysts. This Account focuses on our progress in synthesizing high-valent metal-oxo and metal-hydroxo porphyrinoid complexes and determining their reactivities in a range of HAT processes. For these studies we have utilized corrolazine and corrole ligands, which are a ring-contracted subclass of porphyrinoid compounds designed to stabilize high-valent metal complexes. The high-valent manganese complex Mn^V(O)(TBP₈Cz) (TBP₈Cz = octakis(4- tert-butylphenyl)corrolazine^3-) provided an early example of a well-characterized low-potential oxidant that can still be effective at abstracting H atoms from certain C-H/O-H bonds. Approximating the thermodynamics of the HAT reactivity of the Mn^V(O) complex and related species with the help of a square scheme approach, in which HAT can be formally separated into proton (p K_a) and electron transfers ( E°), indicates that affinity for the proton (i.e., the basicity) is a key factor in promoting HAT. Anionic axial ligands have a profound influence on the HAT reactivity of Mn^V(O)(TBP₈Cz), supporting the conclusion that basicity is a critical parameter in determining the reactivity. The influence of Lewis acids on Mn^V(O)(TBP₈Cz) was examined, and it was shown that both the electronic structure and reactivity toward HAT were significantly altered. High-valent Cr(O), Re(O), and Fe(O) corrolazines were prepared, and a range of HAT reactions were studied with these complexes. The chromium and manganese complexes form a rare pair of structurally characterized Cr^V(O) and Mn^V(O) species in identical ligand environments, allowing for a direct comparison of their HAT reactivities. Although the Cr^V(O) species was the better oxidant as measured by redox potentials, the Mn^V(O) species was significantly more reactive in HAT oxidations, pointing again to basicity as a key determinant of HAT reactivity. The iron complex, Fe^IV(O)(TBP₈Cz^+•), is an analogue of the heme enzyme Compound I intermediate, and was found to be mildly reactive toward H atom abstraction from C-H bonds. In contrast, Re^V(O)(TBP₈Cz) was inert toward HAT, although one-electron oxidation to Re^V(O)(TBP₈Cz^+•) led to some interesting reactivity mediated by the π-radical-cation ligand alone. Other ligand modifications, including peripheral substitution as well as novel alkylation of the meso position on the Cz core, were examined for their influence on HAT. A highly sterically encumbered corrole, tris(2,4,6-triphenylphenyl)corrole (ttppc), was employed for the isolation and structural characterization of the first Mn^IV(OH) complex in a porphyrinoid environment, Mn^IV(OH)(ttppc). This complex was highly reactive in HAT with O-H substrates and was found to be much more reactive than its higher-oxidation-state counterpart Mn^V(O)(ttppc), providing important mechanistic insights. These studies provided fundamental knowledge on the relationship between structure and function in high-valent M(O) and M(OH) models of heme enzyme reactivity.

Figure 1.

Reactivity of Mn^V(O)(TBP₈Cz) with phenol derivatives, showing the postulated Mn^IV(OH) intermediate.

Figure 2.

(a) UV–vis changes for the reaction between Fc* and Mn^V(O)(TBP₈Cz) in PhCN. (b) Spectral titration showing a 2:1 stoichiometry of electron transfer.

Figure 3.

Plots of the observed pseudo-first-order rate constants (k_obs) for the reaction of NADH analogues with Mn^V(O)(TBP₈Cz).

Figure 4.

(left) Calculated potential energy profiles for reactions of singlet-state [Mn^V(O)(TBP₈Cz)(X)] (X = none (1), F (1-F), CN (1-CN)) with DHA; TS = transition state; Int = intermediate; Prod = product). (right) Transition state structures for the initial HAT step, with selected bond distances in angstroms, bond angles in degrees, and imaginary frequencies in wavenumbers. (bottom) Mechanism of HAT with DHA as the substrate. Adapted with permission from ref . Copyright 2010 John Wiley & Sons.

Figure 5.

(a) Crystal structure of Cr^V(O)(TBP₈Cz). (b) Cyclic voltammogram of Cr^V(O)(TBP₈Cz) in CH₂Cl₂.

Figure 6.

(a) Time-resolved UV–vis spectral changes for the reaction of Cr^V(O)(TBP₈Cz) with excess TEMPOH. (b) Plot of the absorbance at 653 nm vs time for the reaction in (a). Inset: dependence of the first-order rate constant on [TEMPOH] and the best-fit line.

Figure 7.

(a) Plot of log k versus BDE for the reaction of Fe^IV(O)(TBP₈Cz^+•) with C–H substrates. (b) Second-order plots for xanthene (green circles) and 9,10-d₂-xanthene (blue diamonds).

Figure 8.

(top) HAT steps relating Mn(ttppc) complexes and (bottom) their crystal structures.

Figure 9.

(a) Reaction of Mn^V(O)(ttppc) and 2,4-DTBP. (b) Time-resolved UV–vis spectral changes for the reaction between Mn^V(O)(ttppc) and 2,4-DTBP. Inset: change in absorbance vs time for the growth of Mn^III(ttppc) (660 nm) (green circles). (c) Plots of k_obs versus [2,4-DTBP-OH] (black circles) and [2,4-DTBP-OD] (red squares). (d) Reaction of Mn^IV(OH)(ttppc) and 2,4-DTBP. (e) Time-resolved UV–vis spectral changes for the reaction between Mn^IV(OH)(ttppc) and 2,4-DTBP. Inset: change in absorbance vs time for the growth of Mn^III(ttppc) (660 nm) (green circles). (f) Plots of k_obs versus [2,4-DTBP-OH] (black circles) and [2,4-DTBP-OD] (red squares). Reproduced from ref . Copyright 2018 American Chemical Society.

Figure 10.

(a) Hammett and (b) Marcus plots for the reaction of Mn^IV(OH)(ttppc) and 4-X-2,6-DTBP (X = OMe, Me, tBu, H). Reproduced from ref . Copyright 2018 American Chemical Society.

Scheme 1.

Hydrogen Atom Transfers in CYP and the OEC^{a a}Reproduced from ref . Copyright 2018 American Chemical Society.

Scheme 2.

HAT in Metal–Oxo Systems

Scheme 3.

Corrolazine Synthesis

Scheme 4.

Possible Mechanisms of H-Atom Abstraction by a Mn^V(O) Complex^{a a}Reproduced from ref . Copyright 2018 American Chemical Society.

Scheme 5.

Square Scheme for HAT to Mn^V(O)(TBP₈Cz)

Scheme 6.

Mechanism of Electron Transfer from [Fe(C₅H₄Me)₂] to Mn^V(O)(TBP₈Cz)

Scheme 7.

Mechanism of Hydride Transfer from NADH Analogues (AcrHR) to Mn^V(O)(TBP₈Cz)

Scheme 8.

Reversible Binding of Lewis/Bronsted Acids Stabilizing Mn^IV(O–LA)(TBP₈Cz^+•)^{a a}Reproduced from ref . Copyright 2016 American Chemical Society.

Scheme 9.

HAT Reactivities of Mn^V(O)(TBP₈Cz), Mn^V(O)(tpfc), and Lewis Acid Adducts

Scheme 10.

Generation of Fe^IV(O)(TBP₈Cz^+•) Using Different Oxidants

Scheme 11.

HAT Reactivity of Re^V(O)(TBP₈Cz^+•) with DHA

Scheme 12.

Influence of Ligand Modification on the HAT Reactivity of Re^V(O)(TBP₈Cz)