Axial ligand tuning of a nonheme iron(IV)-oxo unit for hydrogen atom abstraction

The reactivities of mononuclear nonheme iron(IV)-oxo complexes bearing different axial ligands, [Fe^IV(O)(TMC)(X)]ⁿ⁺ [where TMC is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and X is NCCH ₃ (1-NCCH₃), CF₃COO- (1-OOCCF₃), or N₃^- (1-N₃)], and [Fe^IV(O)(TMCS)] ⁺ (1′-SR) (where TMCS is 1-mercaptoethyl-4,8,11-trimethyl-1,4, 8,11-tetraazacyclotetradecane), have been investigated with respect to oxo-transfer to PPh₃ and hydrogen atom abstraction from phenol O - H and alkylaromatic C - H bonds. These reactivities were significantly affected by the identity of the axial ligands, but the reactivity trends differed markedly. In the oxidation of PPh3, the reactivity order of 1-NCCH₃ > 1-OOCCF₃ > 1-N₃ > 1′-SR was observed, reflecting a decrease in the electrophilicity of iron(IV)-oxo unit upon replacement of CH₃CN with an anionic axial ligand. Surprisingly, the reactivity order was inverted in the oxidation of alkylaromatic C - H and phenol O - H bonds, i.e., 1′-SR > 1-N³ > 1-OOCCF₃ > 1-NCCH₃. Furthermore, a good correlation was observed between the reactivities of iron(IV)-oxo species in H atom abstraction reactions and their reduction potentials, E_p,c, with the most reactive 1′-SR complex exhibiting the lowest potential. In other words, the more electron-donating the axial ligand is, the more reactive the iron(IV)-oxo species becomes in H atom abstraction. Quantum mechanical calculations show that a two-state reactivity model applies to this series of complexes, in which a triplet ground state and a nearby quintet excited-state both contribute to the reactivity of the complexes. The inverted reactivity order in H atom abstraction can be rationalized by a decreased triplet-quintet gap with the more electron-donating axial ligand, which increases the contribution of the much more reactive quintet state and enhances the overall reactivity.