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

Conspectus 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)(TBP 8 Cz) (TBP 8 Cz = 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)(TBP 8 Cz), supporting the conclusion that basicity is a critical parameter in determining the reactivity. The influence of Lewis acids on Mn V (O)(TBP 8 Cz) 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)(TBP 8 Cz +• ), 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)(TBP 8 Cz) was inert toward HAT, although one-electron oxidation to Re V (O)(TBP 8 Cz +• ) 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.