ESR Spectra of Superoxide Anion−Scandium Complexes Detectable in Fluid Solution

Abstract

The most definitive detection of superoxide anion O 2 -, which is of considerable importance in living systems, 1 has been made by the ESR spectrum which gives a characteristic two-line highly anisotropic signal due to the large spin-orbit coupling of oxygen in frozen media containing aprotic solvents or on various solid oxide surfaces at 77 K. 2-4 However, the disadvantage of the ESR detection of O 2 -is the significant line broadening resulting from fast relaxation because of the quenching of the orbital angular momentum by fluctuating solvent interactions or orbital degeneracy which has precluded the detection in fluid solution particularly at higher temperatures. 5,6 Thus, no ESR spectra of O 2 -have so far been reported in fluid solution at room temperature. 7 We report herein that the isotropic ESR spectra of O 2 -can be obtained successfully in solution at temperatures much higher than the melting point of aprotic solvents even at 60°C when O 2 -forms a 1:1 complex with scandium ion. The superhyperfine splitting due to the scandium nucleus and the hyperfine coupling of oxygen enriched in 17 O provide valuable information about the structure of the O 2 --Sc 3+ complex in solution.The dimeric 1-benzyl-1,4-dihydronicotinamide [(BNA) 2 ], which can act as a unique two-electron donor, 8 is used as an electron source to reduce O 2 to O 2 -photochemically. As shown in Scheme 1, the photoinduced electron transfer from the singlet excited state of (BNA) 2 to O 2 gives (BNA) 2 •+ and O 2 -, followed by a fast cleavage of the C-C bond of the dimer to produce N-benzylnicotinamide radical (BNA • ) and BNA + . 8,9 The subsequent second electron transfer from BNA • to O 2 occurs rapidly, judging from the oxidation potential of BNA • (E 0 ox vs SCE ) -1.08 V) 10 lower than the reduction potential of O 2 (E 0 red ) -0.8 V). 11 Thus, once photoinduced electron transfer from (BNA) 2 to O 2 occurs, 2 equiv of O 2 -molecules are produced. 12 When an oxygen-saturated propionitrile solution containing (BNA) 2 (1.7 × 10 -3 M) is irradiated with a high-pressure mercury lamp, O 2 -formed photochemically is detected by the ESR spectrum in frozen propionitrile at 77 K which shows a wellknown signal with g | ) 2.072 and g ⊥ ) 2.009. 5 When the temperature is raised above the frozen temperature (e.g., -60°C), no ESR signal can be detected due to the line-broadening resulting from the fast relaxation, although the signal appears clearly again by lowering the temperature to 77 K.Addition of scandium triflate (Sc(OTf) 3 ) and 3 equiv of hexamethylphosphoric triamide (HMPA) ligand to the (BNA) 2 -O 2 system, however, results in the appearance of the sharp isotropic ESR signal under irradiation of the light in fluid solution at 25°C as shown in Figure 1, where oxygen is enriched in 17 O to provide valuable information about the inequivalency of oxygen nuclei in the O 2 -complex. 13,14 The clear eight-line isotropic spectrum at the center is ascribed to the superhyperfine coupling of O 2 -with the 7 / 2 nuclear spin of the scandium nucle...

Quantitative Evaluation of Lewis Acidity of Metal Ions Derived from theg Values of ESR Spectra of Superoxide: Metal Ion Complexes in Relation to the Promoting Effects in Electron Transfer Reactions

“…OÀO À ± Sc 3 is indicated by the hyperfine splitting of two different 17 O atoms (I 5 2) in which the electron spin is more localized at the terminal oxygen (60 %). [8] This is confirmed by the DFT (density function theory) calculation using the spin-restricted B3LYP functional and the 6-311 G(3d,3p) basis set for the open shell O 2 .À ± Sc 3 , which gives more localized spin density at the terminal oxygen (65 %) (see Experimental Section). [17] The (Table 1).…”

“…Superoxide ion is formed by the photoinduced reduction of O 2 by the dimeric 1-benzyl-1,4-dihydronicotinamide [(BNA) 2 ] in acetonitrile (MeCN), [8] which can act as a unique two electron donor, [9] as shown in Equation (1).…”

Lewis Acid-Induced Change from Four- to Two-Electron Reduction of Dioxygen Catalyzed by Copper Complexes Using Scandium Triflate

“…The g value (2.0112) and and superhyperfine coupling constant due to scandium ( I =7/2; a Sc = 3.82 G) are the same as those reported previously. 38 The end-on coordination of O 2 •− to Sc 3+ is indicated by inequivalent a ( 17 O) values (14 and 17 G) 38 and supported by optimized geometry calculations by an unrestricted Hartree-Fock (UHF) SCF optimization using the 6-311++G** basis set. 39 In contrast to the case for this superoxo complex, DFT calculations suggest that the side on structure of the peroxo complex ([Sc 3+ (O 2 2− )] + ) is more stable than an end-on coordinated peroxo-scandium species (Figure S2 in SI).…”

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

“…It was shown previously that photoirradiation of an O 2 -saturated solution containing redox-inactive metal ions and (BNA) 2 produces superoxo complexes of these redox-inactive metal ions (equation (1)) 30,31 . Interestingly, when the photoirradiation reaction was performed with Ca(CF 3 SO 3 ) 2 and (BNA) 2 in the presence of [(TMC)Fe II ] 2+ in CH 3 CN at −20 °C, we observed the formation of an intermediate ( 1-Ca 2+ ) with an absorption band at λ max = 670 nm (Fig.…”