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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

Electronic Supplementary Material (ESI) for Organic and Biomolecular ChemistryThis journal is © The Royal Society of Chemistry 2011

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


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


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.


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.


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).


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.


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.


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


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


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-



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-



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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