A comparison between nitroxide and hydrazyl free radicals in selective alcohols oxidation

Article abstract of DOI:10.1016/j.chemphys.2017.03.011

Literature data are abundant in oxidations catalyzed by nitroxides free radicals, while hydrazyls free radicals are very seldom encountered. In this work we made a comparison between some nitroxides and hydrazyl free radicals towards the ability to selectively oxidize activated alcohols to the corresponding carbonyl derivatives. As nitroxides we used 2,2,6,6-tetramethylpirrolidin-N-oxide (TEMPO) and phtalimide-N-oxyl (PINO) free radicals, while as hydrazyls were used 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2-p-nitrophenyl-1-picrylhydrazyl (DN-DPPH). The differences in the physico-chemical properties between nitroxides and hydrazyls were evaluated by UV–vis, EPR and cyclic voltammetry. For DN-DPPH the X-ray crystal structure was resolved. It has been shown that a nitroxide radical is a far better catalyst in such aerobic oxidations comparatively with a hydrazyl one. The explanation of these results consists firstly in a different mechanism involved in the oxidation procedures and secondly in the variation of the oxidation potentials of nitroxides comparatively with hydrazyls.

Full text of DOI:10.1016/j.chemphys.2017.03.011

Accepted Manuscript
A comparison between nitroxide and hydrazyl free radicals in selective alcohols
oxidation
Ahmed J. Shakir, Augustin M. Madalan, Gabriela Ionita, Stelian Lupu, Cecilia
Lete, Petre Ionita
PII:
S0301-0104(17)30182-9
DOI:
http://dx.doi.org/10.1016/j.chemphys.2017.03.011
CHEMPH 9765
Reference:
Chemical Physics
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9 March 2017
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26 March 2017
Please cite this article as: A.J. Shakir, A.M. Madalan, G. Ionita, S. Lupu, C. Lete, P. Ionita, A comparison between
nitroxide and hydrazyl free radicals in selective alcohols oxidation, Chemical Physics (2017), doi: http://dx.doi.org/
10.1016/j.chemphys.2017.03.011
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A comparison between nitroxide and hydrazyl free radicals in selective
alcohols oxidation
Ahmed J. Shakir,1 Augustin M. Madalan,1 Gabriela Ionita,2 Stelian Lupu,3 Cecilia Lete,2
Petre Ionita 1,2*
1 University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry,
Biochemistry and Catalysis, Panduri 90-92, Bucharest, Romania. E-mail: pionita@icf.ro
2 Institute of Physical Chemistry, Spl. Independentei 202, Bucharest, Romania
3University Politehnica Bucharest, Polizu 1, Bucharest, Romania
Abstract
Literature data are abundant in oxidations catalyzed by nitroxides free radicals, while
hydrazyls free radicals are very seldom encountered. In this work we made a comparison
between some nitroxides and hydrazyl free radicals towards the ability to selectively oxidize
activated alcohols to the corresponding carbonyl derivatives. As nitroxides we used 2,2,6,6-
tetramethylpirrolidin-N-oxide (TEMPO) and phtalimide-N-oxyl (PINO) free radicals, while as
hydrazyls were used 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2-p-nitrophenyl-1-
picrylhydrazyl (DN-DPPH). The differences in the physico-chemical properties between
nitroxides and hydrazyls were evaluated by UV-Vis, EPR and cyclic voltammetry. For DN-
DPPH the X-ray crystal structure was resolved. It has been shown that a nitroxide radical is a
far better catalyst in such aerobic oxidations comparatively with a hydrazyl one. The
explanation of these results consists firstly in a different mechanism involved in the oxidation
procedures and secondly in the variation of the oxidation potentials of nitroxides
comparatively with hydrazyls.
Keywords:
TEMPO; DPPH; PINO; Free radical; Oxidation
1

1. Introduction
Selective oxidation of primary and secondary alcohols to the corresponding aldehydes
and ketones is one of the most problematic reactions in organic chemistry [1]. A classical
procedure usually involves transition metal ions in at least stoichiometric quantities, which
eventually leads to the following two important issues: i) separation of the desired product
and ii) management of the large quantities of transition metal ion by-products.
As a consequence of these major difficulties, there is a continuous interest in the
improvement of such procedures or in the developing of new ones [2-4]. A better approach
should avoid the environmental negative matter and diminish the work-up protocol, thus
making possible large-scale or even industrial applications.
Currently, there are under consideration novel catalytic methods that involve clean
oxidants like air, hydrogen peroxide or sodium hypochlorite and non-metallic catalysts [5,6].
A promising and nowadays well documented procedure makes use of stable or persistent free
radicals as catalyst and of air as final oxidant [7].
In the last decade nitroxides stable free radicals were successfully used as organo-
catalyst for a wide range of oxidations, including here those considered green (i.e. using air as
oxidant, solvent free reactions, high selectivity, mild conditions, etc.) [8-10]. Among
nitroxides, 2,2,6,6-tetramethylpirrolidin-N-oxide (TEMPO) and phtalimide-N-oxyl (PINO)
free radicals are most known and employed in such reactions (Fig. 1), due to their
effectiveness [11-13].
Other stable free radicals, which also have oxidant capacities, are hydrazyl ones, like
2,2-diphenyl-1-picrylhydrazyl free radical (well known in literature as DPPH), and its
congener 2,2-p-nitrophenyl-1-picrylhydrazyl (DN-DPPH, Fig. 1). However, up to date, there
are very few papers containing data about involving such stable hydrazyl free radicals as
catalyst in oxidation reactions [14, 15].
O2N
O
O2N
O2N
N
O
N
N
.
NO2
N
O
N
N
.
NO2
O
O2N
O2N
TEMPO
PINO
Nitroxides
O2N
Hydrazyls
DN-DPPH
DPPH
Figure 1. Chemical structure of TEMPO, PINO, DPPH and DN-DPPH
2

Although there are available numerous publications of quantitative data about the
efficiency of TEMPO or PINO in this type of catalytic oxidation reactions, no direct
comparison between them is available; in addition, many literature data reports on different
reaction conditions [13, 14]. In an attempt to compare the catalytic properties of TEMPO with
PINO, in our work we used as substrates three activated alcohols (benzyl alcohol, 2-
phenylethanol and diphenylcarbinol, Table 1) and several co-catalyst (different NOx
generating systems and sodium hypochlorite); moreover, we introduced in this comparative
study, for the first time, the use of the stable hydrazyl free radical DN-DPPH, as it is shown
below.
2. Results and Discussion
2.1 General remarks, synthesis and comparison of the free radicals characteristics
All the free radicals used in this study are known in literature. TEMPO and DN-DPPH
are indefinitely stable free radicals under usual conditions (meaning that they do not
decompose in time, dimerize nor react with atmospheric oxygen), while PINO is considered a
persistent free radical, as is decomposing slowly in time (therefore it has to be prepared just
before use; it is simply obtained in situ by oxidation of N-hydroxyphtalimide (NHPI) with
different reagents, like lead dioxide, lead tetra-acetate, nitrogen dioxide, etc. [11]).
Because DN-DPPH is not a commercially available product, our first step in our work
was to synthesize it starting from DPPH. This reaction is easily performed in a biphasic
system by reacting DPPH (dissolved in dicloromethane, DCM) with nitrogen dioxide
generated by an aqueous mixture of sodium nitrite and diluted hydrochloric acid [16]. DPPH
is a good scavenger of nitrogen dioxide, leading finally to the desired product in about 90%
yields (see Experimental part).
By chance, we obtained good quality crystals of DN-DPPH and therefore this
compound was also characterized also by X-Ray diffraction. DN-DPPH crystallizes in the P-1
triclinic space group and the asymmetric unit contains two DN-DPPH molecules and two
DCM solvent molecules. The two crystallographically independent molecules of DN-DPPH
have similar conformations (Fig. 2). The nitro groups of the p-nitrophenyl moieties lie almost
in the planes of the phenyl rings. The dihedral angles formed by these groups with the mean
planes of the corresponding phenyl rings are: 3.41° (N6O7O8), 3.85° (N7O9O10), 3.25°
3

(N13O17O18) and 16.68° (N14O19O20). The dihedral angles formed by the ortho nitro
groups with the mean planes of phenyl rings in the picrylhydrazyl fragments are significant
higher compared with that formed by the para groups: 55.45° (N2O3O4), 35.47° (N3O5O6),
48.07° (N9O13O14), 32.20° (N10O15O16), respectively 11.08° (N1O1O2) and 14.17°
(N8O11O12). It is worth to mention that the N–N bond distance in DPPH is shorter than in
substituted hydrazine derivatives [17-19]. The bond lengths for the two DN-DPPH molecules
are gathered in Supplementary Material (SM), Table S1. Fig. 2 shows the crystal structure of
DN-DPPH (DCM molecules were removed for clarity, but the packing diagram in the crystal
structure along with more pictures are shown in SM, Fig. S1-S3).
Figure 2. View of the two crystallographic independent DN-DPPH molecules
Usually, the best method to characterize free radicals is by electron paramagnetic
resonance (EPR). For TEMPO and PINO, the spectra showed the expected triplet, due to the
interaction of the unpaired electron with the 14N nucleus; the hyperfine coupling constants are
aN = 15.7 G for TEMPO, while for PINO aN 4.8= G. DPPH give five lines, as the two
hyperfine coupling constants are very close (aN1 = 9.0 G, aN2 = 8.9 G). A very different
spectrum is recorded for DN-DPPH, showing the interaction of the unpaired electron with two
different 14N nucleus (aN1 = 10.9 G, aN2 = 6.9 G). The EPR spectra are shown as Fig. S4 in
SM.
Another interesting feature of these free radicals is their colour. UV-Vis spectra of
TEMPO showed an absorption band at λmax = 440 nm, PINO at λmax = 380 nm, DPPH at λmax
= 515 nm, and DN-DPPH at λmax = 475 nm. The UV-Vis spectra are shown as Fig. S5 in SM.
It is worth to mention that there is a big change in colour of the free radical compared with the
4

colour of the reduced counterpart, therefore this property can be used even as a naked-eye
indicator of a reaction progress.
Because these free radicals will be used in oxidation reactions, it was worth to find out
their oxidation potential Eox, and this was achieved by employing cyclic voltammetry (CV).
The electrochemical behavior of the employed compounds was investigated in acetonitrile by
Pt electrode using cyclic voltammetry (CV) at various potential scan rates ranging from 50 to
500 mV/s. Figure S6 from SM displays the CVs recorded for TEMPO, PINO, DPPH and DN-
DPPH compounds, at a scan rate v of 100 mV/s. The shape of the CV recorded for TEMPO
shows one redox wave (reversible oxidation-reduction process), while the DPPH and DN-
DPPH compounds display two redox waves. In the case of PINO, the CV shape indicates a
quasi reversible redox wave followed by an irreversible oxidation wave. The scan rate study
has shown that the redox processes for all compounds are diffusion-controlled, according to
the slope values of the log(ip) vs. log(v) plots which were ca. 0.5.
The values obtained vs. Ag/AgCl were 0.54 V for TEMPO, 0.59 V for PINO, 0.08 V
for DPPH and 0.39 V for DN-DPPH (similar values are presented in literature [20]).
2.2 Free radicals as oxidant catalysts
Due to our previous experience in selective oxidations of alcohols using TEMPO
derivatives as catalysts [10], in this work we chosen as co-catalysts different NOx generating
systems; besides, we used also sodium hypochlorite, as this system was one of the first used
in literature in such catalytic oxidation reactions [21, 22]. Thus, the co-catalyst named A was a
mixture of sodium nitrite and acetic acid, as co-catalyst B was used nitrosonium
tetrafluoroborate, co-catalyst C was sodium hypochlorite while co-catalyst D was gaseous
nitrogen dioxide (see also Table 1).
Typically, the oxidation reactions of the chosen alcohols (Table 1) were performed at
room temperature in air atmosphere, using a ration of 10% mol free radical and 20% mol co-
catalyst. The yields of oxidations were quantified by 1H-NMR and the values obtained are
compiled in Table 1.
5

Table 1. Yields of oxidation
Entry
Catalyst
Co-catalyst
Alcohol
Yields (%)
TEMPO
PINO
63
9
1
OH
2
DN-DPPH
TEMPO
PINO
DN-DPPH
TEMPO
PINO
DN-DPPH
TEMPO
PINO
DN-DPPH
TEMPO
PINO
DN-DPPH
TEMPO
PINO
DN-DPPH
TEMPO
PINO
DN-DPPH
TEMPO
PINO
DN-DPPH
TEMPO
PINO
0
3
NaNO2/
CH3COOH
39
10
0
28
16
25
100
4
30
100
44
16
100
95
25
100
16
9
100
38
7
100
28
53
100
0
4
OH
5
6
(A)
OH
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
OH
NO+BF4
-
OH
(B)
OH
OH
OH
NaClO/
KBr
(C)
OH
DN-DPPH
TEMPO
PINO
DN-DPPH
TEMPO
PINO
DN-DPPH
TEMPO
PINO
OH
OH
1
16
10
0
100
0
NO2
(D)
OH
DN-DPPH
25
As a general rule, the first thing that emerges from the Table 1 is that TEMPO is a
much better catalyst compared with PINO and DN-DPPH. For all alcohols used and also for
all used co-catalysts, TEMPO performed better, reaching quantitative oxidation yields for
nitrosonium tetrafluoroborate and sodium hypochlorite co-catalyst (Entries 10, 13, 16, 19, 22,
25); moreover, in the case of nitrogen dioxide as co-catalyst the same yields were found for
6

benzyl alcohol and diphenylcarbinol (Entries 28 and 34, respectively). However, PINO seems
to be particularly good for oxidation of diphenylcarbinol in the presence of nitrosonium
tetrafluoroborate (yield 95%, Entry 17). Compared with the other two free radicals, DN-
DPPH gave usually the lowest yields (DPPH has not been used in this procedure as it is
converted to DN-DPPH by nitrogen dioxide [16]).
2.3 Mechanisms of oxidation
The high differences between the oxidation yields obtained by different free radicals
might be attributed to different oxidation capacity and also to different reaction mechanisms.
As mentioned before, the highest Eox values have been recorded for TEMPO (0.54 V) and
PINO (0.59 V), comparatively with DPPH (0.08 V) and DN-DPPH (0.39 V), and therefore
this can explain the better performance of nitroxides comparatively with hydrazyls. Regarding
the mechanisms of oxidation, there are two ways of action, one for nitroxide (I) type free
radicals and one for hydrazyl (II). Fig. 3 shows these mechanistic pathways in a simplified
manner.
ox
N+
O
N O
N
N
.
-
(-e
)
H
OH
II
H
OH
I
O
O
ox
ox
N
OH
N
N
H
Figure 3. The two proposed mechanisms of oxidation, involving a nitroxide (I) and a
hydrazyl (II) free radical
For the type I mechanism, the now classical pathway involving oxoammonium salt is
taking place [14, 23]. Thus, the nitroxide radical (TEMPO or PINO) is oxidized via one
electron transfer to the more powerful oxoammonium oxidant, and this oxidizes the alcohol to
the corresponding aldehyde or ketone. The hydroxylamine formed is easily re-oxidized even
by air to the nitroxide radical, thus closing the catalytic cycle. Using NHPI instead of
TEMPO, the same pathway is followed, as the oxidant (nitrogen dioxide or sodium
hypochlorite) re-generates in situ the PINO nitroxide free radical.
7

A simpler catalytic cycle is taking place using DN-DPPH (type II mechanism). A
hydrazyl free radical is able to directly extract hydrogens atom from the alcohol, leading
finally to the desired oxidation product. While the H-atoms are extracted (this depends on
bond dissociation energy [24, 25]), DN-DPPH is reduced to the corresponding hydrazine, and
this is re-oxidized in the reaction mixture (i.e. by nitrogen dioxide) to the starting hydrazyl
free radical. It is worth to mention that in both mechanisms the final oxidant is the oxygen
from air, nitrogen dioxide having its own catalytic cycle, as well it is documented in literature
[10-14].
3. Experimental
3.1 Chemicals, materials and methods
All chemicals, solvents and materials were purchased from Sigma-Aldrich, Acros or
Chimopar and used as received.
UV-Vis spectra were recorded on a UVD-3500 spectrophotometer at room
temperature using 1 cm quartz cells and acetonitrile as solvent.
EPR spectra were recorded in acetonitrile at room temperature on a Jeol JES FA 100
apparatus. NMR spectra were recorded in deuterated chloroform on a Bruker Fourier
apparatus at 300 MHz.
Cyclic voltammetry (electrochemical measurements) were performed using a
potentiostat/galvanostat Autolab 302N connected to a PC running the software GPES
(Ecochemie, Utrecht, The Netherlands), in an electrochemical cell (Metrohm) with a three
electrodes system. A Pt disk electrode (diameter of 3 mm) was used as working electrode, a
glassy carbon rode was the auxiliary electrode, and a Ag/AgCl electrode was used as
reference electrode. The potential of Ag/AgCl electrode was measured against the
ferrocene/ferricinium redox couple (Fc/Fc+) potential, which was of 0.27 V. The cyclic
voltammograms (CVs) were recorded in acetonitrile containing the free radical at a
concentration of 2 mmol and lithium percholate (0.1 M) as supporting electrolyte, at various
potential scan rates (v) ranging from 50 to 500 mV/s. Before the measurements, the surface of
the working electrode was polished with alumina powder (0.1 µm) and then sonicated for 10
min in deionized water and, finally, throughly washed with acetone. High purity Ar (5.0,
Linde Romania) was bubbled through the solution before the measurements, and an Ar
blanket was maintained over the solution during the electrochemical experiments. All
electrochemical experiments were performed at room temperature.
8

X-ray diffraction measurements were performed on a STOE IPDS II diffractometer,
operating with Mo-Kα (λ = 0.71073 Å) X-ray tube with graphite monochromator. The
structure was solved by direct methods and refined by full-matrix least squares techniques
based on F2. The non-H atoms were refined with anisotropic displacement parameters.
Calculations were performed using SHELX-2014 crystallographic software package. A
summary of the crystallographic data and the structure refinement for crystal of DN-DPPH
are given in Table 2. CCDC reference number: 1526824.
Table 2. Crystallographic data, details of data collection and structure refinement parameters
for the stable free radical DN-DPPH
Compound
DN-DPPH
C19H12Cl2N7O10
569.26
Chemical formula
M (g mol-1)
Temperature, (K)
2002)
Wavelength, (Å)
0.71073
Crystal system
Triclinic
P-1
Space group
a(Å)
12.2010(13)
14.4162(15)
14.6132(15)
113.119(8)
93.134(8)
96.763(8)
2333.1(4)
4
b(Å)
c(Å)
α(°)
β(°)
γ(°)
V(Å3)
Z
Dc (g cm-3)
1.621
µ (mm-1)
0.350
F(000)
1156
Goodness-of-fit on F2
Final R1, wR2 [I>2σ(I)]
R1, wR2 (all data)
Largest diff. peak and hole (eÅ-3)
0.971
0.0557, 0.1409
0.0944, 0.1600
0.865, -0.740
9

3.2 Synthesis of DN-DPPH
Attention: toxic nitrogen dioxide evolves during reaction and a fume hood with good
ventilation is required. To 1 g DPPH (this can be also easily synthesized, see [16]) dissolved
into 50 mL DCM was added 50 mL of diluted hydrochloric acid (1 M) and under vigorous
stirring was added from time to time about 100 mg sodium nitrite (about 20 times during 8 h),
and the mixture left overnight. Next day another portion of sodium nitrite was added and the
stirring continued for 1 h (for the water solution, the pH was checked from time to time to be
acidic- if not, this should be corrected by adding small amount of acid). The organic layer was
separated (it is possible that DN-DPPH to precipitate partially), dried over anhydrous sodium
sulphate and the solvent removed. The residue was dissolved in DCM (about 100 mL) to
which 10 g of lead dioxide together with 5 g of anhydrous sodium sulphate was added; the
mixture was vigorously stirred for 1 h, filtered off and the solvent removed. DN-DPPH is thus
obtained as dark solid crystals. Yield ~90%.
3.3 General procedure for the oxidation of alcohols
To 0.5 mmole of one of the chosen alcohols, dissolved in 5 mL of DCM, was added
10% mol of the free radical as catalyst (TEMPO, DN-DPPH or NHPI- as precursor of PINO)
and one of the co-catalysts A-D, and the mixture was stirred at room temperature for 24h
under air. As co-catalyst A was used a mixture of 20% mol sodium nitrite in 5 mL of water
and 0.2 mL of acetic acid; co-catalyst B consisted in 20% mol of nitrosonium
tetrafluoroborate; co-catalyst C was a mixture of 0.5 mL sodium hypochlorite (5%) and 10 mg
of potassium bromide [19] dissolved in 5 mL of water; co-catalyst D was gaseous nitrogen
dioxide (5 mL) bubbled slowly into DCM solution. After completion of the reaction, the
solution was filtered of using a small cotton pad and the solvent was removed using a rotavap.
The residue was dissolved in 1 mL of deuterated chloroform and the NMR spectrum recorded.
The yields of the reactions were calculated using the integral values obtained from 1H-NMR
spectra.
10

4. Conclusions
By comparing the stable nitroxide TEMPO free radical with the persistent nitroxide
PINO and stable hydrazyl DN-DPPH it can be concluded that TEMPO performed much better
in selective alcohol oxidation processes, in specific conditions being able to quantitatively
oxidize activated alcohols to the corresponding aldehydes or ketones. Two distinct
mechanisms of oxidations are involved for the nitroxide and hydrazyl radicals. In addition, the
different ability of oxidation can be correlated with their oxidation potential, as measured by
cyclic voltammetry.
Acknowledgements. This work was partially supported by a grant from the Romanian
National Authority for Scientific Research, CNCS-UEFISCDI.
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17. Williams, D. E.; J. Am. Chem. Soc. 1967, 89, 4280.
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12

Supplementary Material (SM)
A comparison between nitroxide and hydrazyl radicals as catalyst in alcohols oxidation
A. J. Shakir, A. M. Madalan, G. Ionita, S. Lupu, C. Lete, P. Ionita
Table S1. Selected bond distances (Å) for the DN-DPPH molecule
N1-O2 = 1.220(4)
N1-O1 = 1.228(4)
N2-O4 = 1.212(4)
N2-O3 = 1.213(4)
N3-O6 = 1.223(4)
N3-O5 = 1.225(4)
N4-N5 = 1.340(3)
N6-O8 = 1.219(4)
N6-O7 = 1.222(4)
N7-O10 = 1.214(4)
N7-O9 = 1.215(4)
N8-O12 = 1.218(4)
N8-O11 = 1.224(4)
N9-O13 = 1.216(3)
N9-O14 = 1.220(4)
N10-O16 = 1.219(4)
N10-O15 = 1.227(4)
N11-N12 = 1.349(3)
N13-O18 = 1.214(4)
N13-O17 = 1.217(4)
N14-O20 = 1.218(5)
N14-O19 = 1.223(5)
C1-C2 = 1.368(5)
C1-C6 = 1.381(5)
C1-N1 = 1.461(4)
C2-C3 = 1.372(4)
C3-C4 = 1.413(4)
C3-N2 = 1.458(4)
C4-N4 = 1.379(4)
C4-C5 = 1.411(4)
C5-C6 = 1.378(4)
C5-N3 = 1.467(4)
C7-C8 = 1.392(4)
C7-C12 = 1.393(4)
C7-N5 = 1.422(4)
C8-C9 = 1.375(5)
C9-C10 = 1.384(4)
C10-C11 = 1.383(4)
C10-N6 = 1.465(4)
C11-C12 = 1.379(4)
C13-C14 = 1.379(4)
C13-C18 = 1.396(4)
C13-N5 = 1.422(4)
C14-C15 = 1.379(4)
C15-C16 = 1.383(5)
C16-C17 = 1.375(5)
C16-N7 = 1.475(4)
C17-C18 = 1.382(4)
C19-C20 = 1.371(5)
C19-C24 = 1.376(5)
C19-N8 = 1.465(4)
C20-C21 = 1.370(4)
C21-C22 = 1.408(4)
C21-N9 = 1.463(4)
C22-N11 = 1.381(4)
C22-C23 = 1.410(4)
C23-C24 = 1.379(5)
C23-N10 = 1.470(4)
C25-C30 = 1.382(4)
C25-C26 = 1.393(4)
C25-N12 = 1.433(4)
C26-C27 = 1.371(5)
C27-C28 = 1.380(4)
C28-C29 = 1.381(4)
C28-N13 = 1.469(4)
C29-C30 = 1.382(4)
C31-C32 = 1.397(4)
C31-C36 = 1.404(4)
C31-N12 = 1.412(4)
C32-C33 = 1.371(5)
C33-C34 = 1.389(5)
C34-C35 = 1.369(5)
C34-N14 = 1.467(4)
C35-C36 = 1.378(5)
13

Figure S1. Perspective view of one DN-DPPH molecule
Figure S2. Packing diagram in crystal structure of DN-DPPH, view along the
crystallographic b axis
14

Figure S3. Packing diagram in crystal structure of DN-DPPH, view of the supramolecular
dimers formed by π−π interactions
Figure S4. EPR spectra of TEMPO (a), PINO (b), DPPH (c) and DN-DPPH (d), recorded at
room temperature in acetonitril (scale range 100 G).
(a)
(b)
(d)
(c)
15

Figure S5. UV-Vis spectra of TEMPO (a), PINO (b), DPPH (c), and DN-DPPH (d).
(a)
(b)
(d)
(c)
16

Figure S6. Cyclic voltammetry of TEMPO (a), PINO (b), DPPH (c), and DN-DPPH (d).
E
=0.54V
8.0x10-5
OX
1.2x10-5
8.0x10-6
E
=0.99V
OX2
PINO
4.0x10-5
TEMPO
4.0x10-6
0.0
EOX1=0.59V
0.0
-4.0x10-5
-4.0x10-6
-8.0x10-6
E
=0.43V
red
E
=0.45V
0.0
0.4
0.8
red1
E / V
0.0
0.4
0.8
1.2
E / V
6.0x10-5
6.0x10-5
4.0x10-5
E
=0.62V
OX2
E
=0.91V
OX2
E
=0.088V
OX1
EOX1=0.39V
3.0x10-5
DPPH
DNDPPH
2.0x10-5
0.0
0.0
-3.0x10-5
-6.0x10-5
-2.0x10-5
-4.0x10-5
E
=0.53V
red1
Ered1=0.82V
E
=0.004V
red2
E
=0.29V
red2
-0.4
0.0
0.4
0.8
1.2
-0.4
0.0
0.4
E / V
0.8
1.2
E / V
17

Graphical Abstract
TEMPO 0.59 V
DN-DPPH 0.39 V
ox
N
N
N+
O
N O
.
-
(-e
)
H
OH
II
H
OH
I
O
O
ox
ox
N
OH
N
N
H
The two proposed mechanisms of alcohol oxidation, involving a nitroxide (I) and a hydrazyl
(II) free radical
18

Highlights
•
•
•
Properties of nitroxide free radicals compared with hydrazyl free radicals.
Nitroxide radicals better catalysts in selective aerobic oxidations.
Distinct oxidation mechanism for nitroxide and hydrazyl.
19