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. 2015 Mar 11;137(9):3330-7.
doi: 10.1021/ja512584r. Epub 2015 Feb 26.

Lewis acid-induced change from four- to two-electron reduction of dioxygen catalyzed by copper complexes using scandium triflate

Saya Kakuda  1 , Clarence J Rolle, Kei Ohkubo, Maxime A Siegler, Kenneth D Karlin, Shunichi Fukuzumi
Affiliations

Lewis acid-induced change from four- to two-electron reduction of dioxygen catalyzed by copper complexes using scandium triflate

Saya Kakuda et al. J Am Chem Soc. .

Abstract

Mononuclear copper complexes, [(tmpa)Cu(II)(CH3CN)](ClO4)2 (1, tmpa = tris(2-pyridylmethyl)amine) and [(BzQ)Cu(II)(H2O)2](ClO4)2 (2, BzQ = bis(2-quinolinylmethyl)benzylamine)], act as efficient catalysts for the selective two-electron reduction of O2 by ferrocene derivatives in the presence of scandium triflate (Sc(OTf)3) in acetone, whereas 1 catalyzes the four-electron reduction of O2 by the same reductant in the presence of Brønsted acids such as triflic acid. Following formation of the peroxo-bridged dicopper(II) complex [(tmpa)Cu(II)(O2)Cu(II)(tmpa)](2+), the two-electron reduced product of O2 with Sc(3+) is observed to be scandium peroxide ([Sc(III)(O2(2-))](+)). In the presence of 3 equiv of hexamethylphosphoric triamide (HMPA), [Sc(III)(O2(2-))](+) was oxidized by [Fe(bpy)3](3+) (bpy = 2,2-bipyridine) to the known superoxide species [(HMPA)3Sc(III)(O2(•-))](2+) as detected by EPR spectroscopy. A kinetic study revealed that the rate-determining step of the catalytic cycle for the two-electron reduction of O2 with 1 is electron transfer from Fc* to 1 to give a cuprous complex which is highly reactive toward O2, whereas the rate-determining step with 2 is changed to the reaction of the cuprous complex with O2 following electron transfer from ferrocene derivatives to 2. The explanation for the change in catalytic O2-reaction stoichiometry from four-electron with Brønsted acids to two-electron reduction in the presence of Sc(3+) and also for the change in the rate-determining step is clarified based on a kinetics interrogation of the overall catalytic cycle as well as each step of the catalytic cycle with study of the observed effects of Sc(3+) on copper-oxygen intermediates.

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Figures

Figure 1
Figure 1
(a) UV-vis spectral changes observed in the two-electron and four-electron reduction of O2 (0.5 mM) by Fc* (2.0 mM) with Sc(OTf)3 (2.0 mM) catalyzed by 1 (40 μM) in acetone at 298 K. (b) Time courses of absorbance at 780 nm due to Fc*+ in the two-electron and four-electron reduction of O2 (0.5 mM) by Fc* (2.0 mM) catalyzed by 1 (40 μM) in the presence of Sc(OTf)3 (2.0 mM) and HOTf (40 mM), respectively.
Figure 2
Figure 2
Plot of absorbance at 780 nm due to Fc*+ vs concentration of Sc(OTf)3 in the two-electron reduction of O2 (2.5 mM) by Fc* (2.0 mM) with Sc(OTf)3 (0–2.0 mM) catalyzed by 1 (40 μM) in acetone at 298 K.
Figure 3
Figure 3
EPR spectrum observed after addition of [FeIII(bpy)3]3+ and HMPA (30 mM) to an N2-saturated acetone solution of [Sc3+(O22−)]+, which was produced by the two-electron reduction of O2 (11 mM) by Fc* in the presence of [(tmpa)CuII](ClO4)2 (1) (10 μM) and Sc(OTf)3 (10 mM) in acetone at 298 K. The g value is 2.011, confirming the production of the known HMPA-Sc-superoxide complex.
Figure 4
Figure 4
(a) Time profiles of formation of Fc*+ monitored by absorbance at 780 nm (ε = 500 M−1 cm−1) in the two-electron reduction of O2 by Fc* (2.0 mM) with Sc(OTf)3 (10 mM) catalyzed by 1 (2–10 μM) in saturated ([O2] = 11 mM) acetone at 298 K. Inset: First-order plots. (b) Plot of kobs vs [1] for the two-electron reduction of O2 by Fc* (2.0 mM) in the presence of Sc(OTf)3 (10 mM) in acetone at 298 K. (c) Plot of kobs vs [O2] for the two-electron reduction of O2 by Fc* (2.0 mM) catalyzed by 1 (2.0 μM) in saturated ([O2] = 11 mM) acetone at 298 K. (d) Plot of kobs vs [Sc(OTf)3] for the two-electron reduction of O2 by Fc* (2.0 mM) with Sc(OTf)3 (5–25 mM) catalyzed by 1 (2 μM) in saturated ([O2] = 11 mM) acetone at 298 K.
Figure 5
Figure 5
EPR spectra of [(tmpa)CuII]2+ 1 (0.10 mM) (black line) measured at 77 K during the catalytic two-electron reduction of O2 (11 mM) by Fc* (2 mM) with Sc(OTf)3 (10 mM) at 298 K (red line). EPR parameters of [(tmpa)CuII]2+: g⊥ = 2.21, |A⊥| = 100 G, g// = 2.00, |A//| = 64 G.
Figure 6
Figure 6
(a,c) UV-vis absorption spectral change in the reaction of [(tmpa)CuI]+ (2.0 mM) with O2 in O2-saturated acetone (black), followed by addition of Sc(OTf)3 (2.0 mM) to the resulting solution at 213 K (red) at (a) 0–4 s and (c) 4–10 s time delays. (b) Absorption time profiles at 394 nm due to the Sc3+-bound peroxo complex, [(tmpa)CuII(O2)Sc(OTf)3]2+ and 520 nm due to [(tmpa)CuII(O2)CuII(tmpa)]2+. Sc(OTf)3 was added at 2 s time delay.
Figure 7
Figure 7
Time courses of absorbance at 780 nm due to Fc*+ in the two-electron reduction of O2 (11 mM) by Fc* (2.0 mM) with metal triflates [Sc(OTf)3, Yb(OTf)3, Y(OTf)3, Lu(OTf)3, Mg(OTf)2, and Ca(OTf)2] (2.0 mM) catalyzed by 1 (40 μM) in acetone at 298 K.
Figure 8
Figure 8
Displacement ellispoid plot (50% probability level)of one crystallographically independent cation of [CuII(BzQ)(H2O)2](ClO4)2 • 2.33(C3H6O) BzQCuII; the two remaining cations, the ClO4− counteranions, and the lattice acetone solvent molecules have been omitted for clarity. Selected bond distances: Cu1-N1A,1.9950(14) Å; Cu1-N2A, 2.0494(14) Å; Cu1-N3, 1.9958(14) Å; Cu1-O1W1, 2.1934(12) Å Cu1-O1W2 2.0002 (13). Selected bond angles: N1A-Cu1-N3A, 165.60(6)°; N1A-Cu1-N2A, 83.21(6)°; N2A-Cu1-N3A, 83. 82.39(6)°; N1A-Cu1-O1W1, 90.69(5)°; O1W1-Cu1-O1W2, 108.97(5)°; N2A-Cu1-O1W2, 141.95(6)°.
Figure 9
Figure 9
(a) Plot of the initial rate of formation of Me2Fc+ vs [Me2Fc] in the two-electron reduction of O2 by Me2Fc with Sc(OTf)3 (10 mM) catalyzed by 2 (0.2 mM) in saturated ([O2] = 11 mM) acetone at 298 K. (b) Plot of the initial rate of formation of Me2Fc+ vs [Sc(OTf)3] in the two-electron reduction of O2 by Me2Fc (10 mM) with Sc(OTf)3 catalyzed by 2 (0.20 mM) in saturated ([O2] = 11 mM) acetone at 298 K. (c) Plot of kobs vs [O2] for the two-electron reduction of O2 by Me2Fc (10 mM) with Sc(OTf)3 (10 mM) catalyzed by 2 (0.2 mM) in acetone at 298 K. (d) Plot of kobs vs [2] for the two-electron reduction of O2 by Me2Fc (10 mM) with Sc(OTf)3 (10 mM) catalyzed by 2 in saturated ([O2] = 11 mM) acetone at 298 K.
Figure 10
Figure 10
EPR spectra of [[(BzQ)CuII]2+] (2) (0.10 mM) (black line) observed at 77 K, [[(BzQ)CuI]+] (0.05 mM) produced during the catalytic reduction of oxygen (2.2 mM) in the presence of Me2Fc (10 mM) and Sc(OTf)3 (10 mM) (red line).
Figure 11
Figure 11
(a) UV-vis spectral changes observered upon the addition of 2 eq of HOTf (4.0 mM) to the mixture of the μ-η2-η2-(side-on) peroxo dinuclear copper(II) complex and the bis-μ-oxo dinuclear copper(III) complex. (b) Absorption time profiles at 394 nm due to the addition of HOTf (4.0 mM) to to the mixture of the μ-η2-η2-(side-on) peroxo dinuclear copper(II) complex and the bis-μ-oxo dinuclear copper(III) complex. (c) UV-vis spectral changes observered upon the addition of Sc(OTf)3 (4.0 mM) to the mixture of the μ-η2-η2-(side-on) peroxo dinuclear copper(II) complex and the bis-μ-oxo dinuclear copper(III) complex.
Scheme 1
Scheme 1
Scheme 2
Scheme 2
Scheme 3
Scheme 3
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