Enhanced Electron-Transfer Reactivity of Nonheme Manganese(IV)–Oxo Complexes by Binding Scandium Ions

Abstract

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.

Synthetic High‐Valent M–O–X Oxidants

“…This allowed for spectroscopic characterisation, which, together with DFT computations, confirmed structures with an Mn IV –O–Sc III core in both compounds, and with the second Sc 3+ ion in 24c bound in the second coordination sphere through bridging OTf groups. The complex [Mn IV (O)(Bn‐TPEN)(OTf)] 2+ ( 25a ) displayed similar behaviour, leading to the formation of adducts 25b – c (Figure ) with one and two Sc 3+ ions respectively …”

Redox-inactive metal ions modulate the reactivity and oxygen release of mononuclear non-haem iron(III)–peroxo complexes

“…Binding of redox-inactive metal ions to high-valent metal–oxo complexes was demonstrated recently in synthetic non-haem metal–oxo complexes of iron(IV), manganese(IV) and cobalt(IV) 18–25 , and the crystal structure of a Sc 3+ -bound [(TMC)Fe IV (O)] 2+ (TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) complex was determined by X-ray crystallography 18 . It has been shown that reactivities of metal(IV)–oxo complexes are markedly affected by binding redox-inactive metal ions in various oxidation reactions 21–25 .…”

“…It has been shown that reactivities of metal(IV)–oxo complexes are markedly affected by binding redox-inactive metal ions in various oxidation reactions 21–25 . More recently, it has been reported that a non-haem iron(III)–peroxo complex, [(TMC)Fe III (O 2 )] + (ref.…”

Redox-inactive metal ions promoted the catalytic reactivity of non-heme manganese complexes towards oxygen atom transfer

“…9 For BF 3 as Lewis acid, Lau also even found that it can dramatically improve the oxidation of alkanes and arylalkanes by KMnO 4 through BF 3 binding to the Mn VII =O group. 15,16 Collins also found that redoxinactive metal ions does not improve olefin epoxidation by their (TAML)Mn V (O) analogue but accelerate triphenylphosphine oxygenation. 11 The dioxygen activation triggered by redoxinactive metal ions was also observed by Que, Nam and Fukuzumi with Fe(TMC) complex.…”