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

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-butylpheny1)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 (pK(a)) and electron transfers (E degrees), 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(+center dot)), 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(+center dot)) led to some interesting reactivity mediated by the pi-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.