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

<jats:p> The reactivities of mononuclear nonheme iron(IV)–oxo complexes bearing different axial ligands, [Fe <jats:sup>IV</jats:sup> (O)(TMC)(X)] <jats:sup>n+</jats:sup> [where TMC is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and X is NCCH <jats:sub>3</jats:sub> (1-NCCH <jats:sub>3</jats:sub> ), CF <jats:sub>3</jats:sub> COO <jats:sup>−</jats:sup> (1-OOCCF <jats:sub>3</jats:sub> ), or N <jats:sub arrange="stack">3</jats:sub> <jats:sup arrange="stack">−</jats:sup> (1-N <jats:sub>3</jats:sub> )], and [Fe <jats:sup>IV</jats:sup> (O)(TMCS)] <jats:sup>+</jats:sup> (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 PPh <jats:sub>3</jats:sub> and hydrogen atom abstraction from phenol O <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cjs0807.gif"/> H and alkylaromatic C <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cjs0807.gif"/> H bonds. These reactivities were significantly affected by the identity of the axial ligands, but the reactivity trends differed markedly. In the oxidation of PPh <jats:sub>3</jats:sub> , the reactivity order of 1-NCCH <jats:sub>3</jats:sub> > 1-OOCCF <jats:sub>3</jats:sub> > 1-N <jats:sub>3</jats:sub> > 1′-SR was observed, reflecting a decrease in the electrophilicity of iron(IV)–oxo unit upon replacement of CH <jats:sub>3</jats:sub> CN with an anionic axial ligand. Surprisingly, the reactivity order was inverted in the oxidation of alkylaromatic C <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cjs0807.gif"/> H and phenol O <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cjs0807.gif"/> H bonds, i.e., 1′-SR > 1-N <jats:sub>3</jats:sub> > 1-OOCCF <jats:sub>3</jats:sub> > 1-NCCH <jats:sub>3</jats:sub> . Furthermore, a good correlation was observed between the reactivities of iron(IV)–oxo species in H atom abstraction reactions and their reduction potentials, <jats:italic>E</jats:italic> <jats:sub>p,c</jats:sub> , 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. </jats:p>