Enhanced electron-transfer reactivity of nonheme manganese(IV)-oxo complexes by binding scandium ions

One and two scandium ions (Sc³⁺) are bound strongly to nonheme manganese(IV)-oxo complexes, [(N4Py)Mn^IV(O)]²⁺ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) and [(Bn-TPEN)Mn ^IV(O)]²⁺ (Bn-TPEN = N-benzyl-N,N′,N′-tris(2- pyridylmethyl)-1,2-diaminoethane), to form Mn^IV(O)-(Sc ³⁺)₁ and Mn^IV(O)-(Sc³⁺)₂ complexes, respectively. The binding of Sc³⁺ ions to the Mn ^IV(O) complexes was examined by spectroscopic methods as well as by DFT calculations. The one-electron reduction potentials of the Mn ^IV(O) complexes were markedly shifted to a positive direction by binding of Sc³⁺ ions. Accordingly, rates of the electron transfer reactions of the Mn^IV(O) complexes were enhanced as much as 10 ⁷-fold by binding of two Sc³⁺ ions. The driving force dependence of electron transfer from various electron donors to the Mn ^IV(O) and Mn^IV(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 Mn^IV(O)-(Sc³⁺)₂ complexes resulted in remarkable enhancement of the electron-transfer reactivity of the Mn ^IV(O) complexes. Such a dramatic enhancement of the electron-transfer reactivity of the Mn^IV(O) complexes by binding of Sc³⁺ ions resulted in the change of mechanism in the sulfoxidation of thioanisoles by Mn^IV(O) complexes from a direct oxygen atom transfer pathway without metal ion binding to an electron-transfer pathway with binding of Sc ³⁺ ions.