Redox-inactive metal ions play pivotal roles in regulating the reactivities of high-valent metal-oxo species in a variety of enzymatic and chemical reactions. A mononuclear non-heme Mn(IV)-oxo complex bearing a pentadentate N5 ligand has been synthesized and used in the synthesis of a Mn(IV)-oxo complex binding scandium ions. The Mn(IV)-oxo complexes were characterized with various spectroscopic methods. The reactivities of the Mn(IV)-oxo complex are markedly influenced by binding of Sc(3+) ions in oxidation reactions, such as a ~2200-fold increase in the rate of oxidation of thioanisole (i.e., oxygen atom transfer) but a ~180-fold decrease in the rate of C-H bond activation of 1,4-cyclohexadiene (i.e., hydrogen atom transfer). The present results provide the first example of a non-heme Mn(IV)-oxo complex binding redox-inactive metal ions that shows a contrasting effect of the redox-inactive metal ions on the reactivities of metal-oxo species in the oxygen atom transfer and hydrogen atom transfer reactions.

A mononuclear non-heme manganese(IV)-oxo complex has been synthesized and characterized using various spectroscopic methods. The Mn(IV)-oxo complex shows high reactivity in oxidation reactions, such as C-H bond activation, oxidations of olefins, alcohols, sulfides, and aromatic compounds, and N-dealkylation. In C-H bond activation, the Mn(IV)-oxo complex can activate C-H bonds as strong as those in cyclohexane. It is proposed that C-H bond activation by the non-heme Mn(IV)-oxo complex does not occur via an oxygen-rebound mechanism. The electrophilic character of the non-heme Mn(IV)-oxo complex is demonstrated by a large negative ρ value of -4.4 in the oxidation of para-substituted thioanisoles.

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One and two scandium ions (Sc3+) are bound strongly to nonheme manganese(IV)–oxo complexes, [(N4Py)MnIV(O)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) and [(Bn-TPEN)MnIV(O)]2+ (Bn-TPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)-1,2-diaminoethane), to form MnIV(O)–(Sc3+)1 and MnIV(O)–(Sc3+)2 complexes, respectively. The binding of Sc3+ ions to the MnIV(O) complexes was examined by spectroscopic methods as well as by DFT calculations. The one-electron reduction potentials of the MnIV(O) complexes were markedly shifted to a positive direction by binding of Sc3+ ions. Accordingly, rates of the electron transfer reactions of the MnIV(O) complexes were enhanced as much as 107–fold by binding of two Sc3+ ions. The driving force dependence of electron transfer from various electron donors to the MnIV(O) and MnIV(O)–(Sc3+)2 complexes was examined and analyzed in light of the Marcus theory of electron transfer to determine the reorganization energies of electron transfer. The smaller reorganization energies and much more positive reduction potentials of the MnIV(O)–(Sc3+)2 complexes resulted in remarkable enhancement of the electron-transfer reactivity of the MnIV(O) complexes. Such a dramatic enhancement of the electron-transfer reactivity of the MnIV(O) complexes by binding of Sc3+ ions resulted in the change of mechanism in the sulfoxidation of thioanisoles by MnIV(O) complexes from a direct oxygen atom transfer pathway without metal ion binding to an electron-transfer pathway with binding of Sc3+ ions.

Enzymatic reactions that involve C-H bond activation of alkanes by high-valent iron-oxo species can be explained by the rebound mechanism (RM). Hydroxylation reactions of alkane substrates effected by the reactive compound I (Cpd I) species of cytochrome P450 enzymes are good examples. There was initially little doubt that the rebound paradigm could be carried over in the same form to the arena of synthetic nonheme high-valent iron-oxo or other metal-oxo complexes. However, the active reaction centres of these synthetic systems are not well-caged, in contrast to the active sites of enzymes; therefore, the relative importance of the radical dissociation pathway can become prominent. Indeed, accumulating experimental and theoretical evidence shows that introduction of the non-rebound mechanism (non-RM) is necessary to rationalise the different reactivity patterns observed for synthetic nonheme complexes. In this tutorial review, we discuss several specific examples involving the non-RM while making frequent comparisons to the RM, mainly from the perspective of computational chemistry. We also provide a technical guide to DFT calculations of RM and non-RM and to the interpretations of computational outcomes.