S1
Supporting Information
for
One-Electron Oxidation of Ferrocenes by Short-Lived N-oxyl Radicals.
Structural Effects on the Intrinsic Electron Transfer Reactivities of N-oxyl
Radicals
Enrico Baciocchi, Massimo Bietti, Claudio D’Alfonso, Osvaldo Lanzalunga, Andrea Lapi and
Michela Salamone
Figure S1: Cyclic voltammogram of N-oxyl radicals 1-4 in MeCN S2
Figure S2: Time-resolved absorption spectra of the dicumyl peroxide/NHQO system S3
Figures S3-S6: Dependence of kobs for the decay of CumO• on the concentration of 1H-4H S4
Figure S7: Time-resolved absorption spectra of the dicumyl peroxide/NHBO/ethylferrocene
carboxylate system S6
Figure S8. Plots of kobs against [FcX] for the reactions of the MINO radical S7
Figure S9. Plots of kobs against [FcX] for the reactions of the QONO radical S7
Figure S10. Plots of kobs against [FcX] for the reactions of the BONO radical S8
Determination of the reorganization energy S9
Figure S11. Marcus Plot for the reactions of substituted ferrocenes with MINO S11
Figure S12. Marcus Plot for the reactions of substituted ferrocenes with BQNO S11
Figure S13. Marcus Plot for the reactions of substituted ferrocenes with BONO S12
References for Supporting Information S13
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S2
E (V vs Ag/AgCl)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
i (m
A)
-15
-10
-5
0
5
10
E (V vs Ag/AgCl)
0.2 0.4 0.6 0.8 1.0 1.2 1.4
i (μ A
)
-200
-150
-100
-50
0
50
100
E (V vs Ag/AgCl)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
i (m
A)
-30
-20
-10
0
10
20
E (V vs Ag/AgCl)
0.4 0.6 0.8 1.0 1.2
i (m
A)
-250
-200
-150
-100
-50
0
Figure S1. Cyclic voltammogram of 2 mM solutions of N-hydroxyderivatives 1H-4H (2 mM) in
MeCN at 25 °C in the presence of collidine (4 mM), containing Bu4NBF4 (0.1 M) as the supporting
electrolyte. Voltage sweep rate 500 mV/s-1
.N
NN
O
O.N
N
O
O
.N O
O
O
.N O
O
O
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S3
λ (nm)
350 400 450 500 550 600 650
Δ A
0.00
0.01
0.02
0.03
0.04
0.05
Figure S2. Transient absorption spectra measured 96 ns (empty circles), 416 ns (filled triangles),
1.0 μs (empty squares) and 3.0 μs (filled circles) after 355 nm laser excitation of a solution of
dicumyl peroxide (0.9 M) and NHQO (2.5 mM) in CH3CN at 25 °C under N2
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S4
[NHSI] (M)
0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030
k obs
(s-1
)
7.0e+5
8.0e+5
9.0e+5
1.0e+6
1.1e+6
1.2e+6
1.3e+6
kH=1.6 x 108 M-1s-1
r ²=0.994
Figure S3. Dependence of kobs on the concentrations of NHSI for the hydrogen atom abstraction
from NHSI by the cumyloxyl radical in CH3CN at 25 °C.
[NHMI] (M)
0.000 0.002 0.004 0.006 0.008
k obs
(s-1
)
6.0e+5
7.0e+5
8.0e+5
9.0e+5
1.0e+6
1.1e+6
kH=4.3x107 M-1s-1
r ²=0.982
Figure S4. Dependence of kobs on the concentrations of NHMI for the hydrogen atom abstraction
from NHMI by the cumyloxyl radical in CH3CN at 25 °C.
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S5
[NHBQ] (M)
0.000 0.001 0.002 0.003 0.004 0.005
k obs
(s-1
)
7e+5
8e+5
8e+5
9e+5
9e+5
1e+5
1e+6
kH=3.6x107 M-1s-1
r ²=0.996
Figure S5. Dependence of kobs on the concentrations of NHBQ for the hydrogen atom abstraction
from NHBQ by the cumyloxyl radical in CH3CN at 25 °C.
[NHBO] M)
0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008
k obs
(s-1
)
7.5e+5
8.0e+5
8.5e+5
9.0e+5
9.5e+5
1.0e+6
kH = 3.6x107M-1s-1
r ² = 0.989
Figure S6. Dependence of kobs on the concentrations of NHBO for the hydrogen atom abstraction
from NHBO by the cumyloxyl radical in CH3CN at 25 °C.
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S6
λ (nm)
350 400 450 500 550 600 650
Δ OD
0.00
0.02
0.04
0.06
0.08
0.10
Figure S7. Time-resolved absorption spectra observed after 355 nm LFP of an N2-saturated CH3CN
solution (T = 25 °C) containing dicumyl peroxide (1 M), NHBO (5.0 mM) and ethylferrocene
carboxylate (0.1 mM), at 1.4 μs (empty circles), 4.0 μs (filled triangles), 31 μs (empty squares)
and 300 μs (filled circles).
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S7
[FcX] (M)
0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008
k obs
(s-1
)
0
10000
20000
30000
40000
50000
60000
FcCH2OH
FcEt
FcH
FcMe2
Figure S8. Plots of the observed rate constant (kobs) against [FcX] for the reactions of the MINO
radical measured in argon-saturated MeCN solution at T = 25 °C, following the buildup of the
ferrocenium ions.
[FcX] (M)
0.0000 0.0002 0.0004 0.0006 0.0008 0.0010 0.0012 0.0014
k obs
(s-1
)
0
1e+5
2e+5
3e+5
4e+5
5e+5
FcMe2 FcEt
FcH
FcCH2OH
FcCONH2
Figure S9. Plots of the observed rate constant (kobs) against [FcX] for the reactions of the BQNO
radical measured in argon-saturated MeCN solution at T = 25 °C, following the buildup of the
ferrocenium ions.
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S8
[FcX] (M)
0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006
k obs
(s-1
)
0.0
5.0e+4
1.0e+5
1.5e+5
2.0e+5
FcCONH2
FcCO2Et
FcCOPh
FcCOMe
Figure S10. Plots of the observed rate constant (kobs) against [FcX] for the reactions of the BONO
radical measured in argon-saturated MeCN solution at T = 25 °C, following the decay of the BONO
radical.
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S9
Determination of the reorganization energies
The second-order rate constants (ket) for the electron transfer process from the substitued ferrocenes
FcX to N-oxyl radicals 1-4 were treated according to eq 1, which combines the Eyring and Marcus
equation. S1
( )RTG
et Zek λλ
4' 2°Δ+−
= (1)
Z is taken as 6×1011, λ is the reorganization energy representing the energy associated with
adjustments in the nuclear geometries and solvent shells of the interacting species which are
required to make possible the transfer of the electron from the ferrocene donors to the acceptor N-
oxyl radicals 1-4 and ΔG°' is the standard free energy for the same step corrected for the
electrostatic interaction arising from the charge variation in the reactants upon electron transfer. Δ
G°' values are calculated from the ΔG° values reported in Table S1. The ΔG° values have then be
corrected for the electrostatic free energy change due to the change in Z1 and Z2 upon electron
transfer on the basis of eq 2 to give the ΔG°' value to introduce in the eq 1.
ΔG°' = ΔG° + (Z1-Z2-1)(e2f/Dr12) (2)
In eq 2, e is the electron charge, Z1 and Z2 are the charge numbers of the reactants, r12 is the distance
between N-oxyl radicals 1-4 and the ferrocenes in the encounter complex (taken as 7.0 Å for SINO
and MINO radicals and 7.4 Å for QONO and BONO radicals),S2 D is the dielectric constant of the
solvent (35 for CH3CN), and f is a factor depending upon the ionic strength (f ≅ 1 for μ → 0). The
values of ΔG°’ evaluated as described above are listed in Table S1. These values refer to reactions
carried out at 25 °C.
Nonlinear least squares curve fitting of the experimental data has been carried out using the
reorganization energy λ as adjustable parameter. The best fit of the experimental data with the curve
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S10
calculated by eq 1 (Figure 7 and Figures S10-S12) are obtained for λ values of 43.6 kcal mol-1 (1),
49.3 kcal mol-1 (2), 36.0 kcal mol-1 (3) and 28.4 kcal mol-1 (4).
Table S1. Standard free energy (ΔG°) and standard free energy corrected for the electrostatic
contribution (ΔG°´) for the electron transfer from ferrocenes (FcX) to N-oxyl radicals 1-4 at 25 °C
in CH3CN.
SINO (1) MINO (2) QONO (3) BONO (4) FcX
ΔG° ΔG°´ ΔG° ΔG°´ ΔG° ΔG°´ ΔG° ΔG°´
FcH -11.7 -13.1 -8.2 -9.6 -9.9 -11.2
FcMe2 -14.4 -15.8 -11.0 -12.4 -12.7 -14.0
FcEt -13.3 -14.7 -9.8 -11.2 -11.5 -12.8
FcCH2OH -11.9 -13.3 -8.4 -9.8 -10.1 -11.4
FcCONH2 -7.7 -9.1 -6.0 -7.3 -4.4 -5.7
FcCO2Et -6.1 -7.5 -2.8 -4.1
FcCOMe -2.5 -3.8
FcCOPh -1.9 -3.2
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S11
ΔG°'
-25 -20 -15 -10 -5
lnk e
t
10
12
14
16
18
20
22
24
Figure S11. Diagram of lnket vs ΔG°´ for the reactions of substituted ferrocenes with MINO. The
solid circles correspond to the experimental values; the curve is calculated by nonlinear least-
squares fit to eq 1.
ΔG°'
-16 -14 -12 -10 -8 -6
ln k
et
16
17
18
19
20
21
22
23
λ = 36.0 kcal mol-1
r2= 0.943
Figure S12. Diagram of lnket vs ΔG°´ for the reactions of substituted ferrocenes with QONO. The
solid circles correspond to the experimental values; the curve is calculated by nonlinear least-
squares fit to eq 1.
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S12
ΔG°' (kcal mol-1)
-15 -10 -5 0 5
ln k
et
16
18
20
22
24
λ = 27.9 kcal/mol
Figure S13. Diagram of ln ket vs ΔG°´ for the reactions of substituted ferrocenes with BONO.
The solid circles correspond to the experimental values; the curve is calculated by nonlinear least-
squares fit to eq 1.
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S13
References for Supporting Information
S1. Eberson, L. Electron Transfer Reactions in Organic Chemistry; Springler Verlag: Berlin
Heidelberg, 1986; Chapter 3.
S2. The radius of ferrocene (2.6 Å) is taken from ref. S3. The radii of SINO and MINO (4.4 Å) and
QONO and BONO (4.8 Å) were calculated from the volume of the N-oxyl radicals given by the
Hyperchem program.
S3. Carlson, B. W.; Miller, L. L.; Neta, P.; Grodkowski, J. J. Am. Chem. Soc., 1984, 103, 7233.
S4. Baciocchi, E.; Bietti, M.; Gerini, M. F.; Lanzalunga O. J. Org. Chem. 2005, 70, 5144-5149.
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