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(O)(TBPCz) (TBPCz = octakis(4- tert-butylphenyl)corrolazine) 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(O) complex and related species with the help of a square scheme approach, in which HAT can be formally separated into proton (p K) 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(O)(TBPCz), supporting the conclusion that basicity is a critical parameter in determining the reactivity. The influence of Lewis acids on Mn(O)(TBPCz) 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(O) and Mn(O) species in identical ligand environments, allowing for a direct comparison of their HAT reactivities. Although the Cr(O) species was the better oxidant as measured by redox potentials, the Mn(O) species was significantly more reactive in HAT oxidations, pointing again to basicity as a key determinant of HAT reactivity. The iron complex, Fe(O)(TBPCz), 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(O)(TBPCz) was inert toward HAT, although one-electron oxidation to Re(O)(TBPCz) 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(OH) complex in a porphyrinoid environment, Mn(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(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.