Lewis acid effects on donor-acceptor associations and redox reactions: Ternary complexes of heteroaromatic N-oxides with boron trifluoride and organic donors

Article abstract of DOI:10.1039/b9nj00205g

Structural and spectral features of the novel supramolecular [D·NXO·BF₃] complexes formed via simultaneous n-coordination of a Lewis acid (BF₃) and π-complexation of organic donors (D) to polyfunctional nitrosubstituted N-oxide molecules (NXO = 4-nitropyridine-N-oxide or 4-nitroquinoline-N-oxide) are reported. X-Ray studies of [pyrene·NPO·BF₃]·CH₂Cl ₂ and [(pyrene)₂·NQO·BF₃] salts revealed that the Lewis acid is coordinated to the oxygen atom of the N-oxide group with O-B distance of ～1.52 similar to that in the separate [NXO·BF₃] adducts. Aromatic rings of N-oxide molecules in the ternary systems are π-stacked with pyrene moieties, and their interplanar separations of ～3.35 are common for conventional charge-transfer complexes. In dichloromethane, associations between [NXO·BF₃] adduct and organic donors are characterized by higher formation constants (and charge-transfer bands of [D·NXO·BF₃] complexes are red-shifted) as compared to the complexes between the same donors and NXO acceptors. Spectral data and electrochemical measurements point out enhanced acceptor abilities of [NXO·BF₃] adducts relative to the separate N-oxides which correspond to ～0.5 V positive shift of reduction potentials of acceptors. Synergetic oxidation of strong organic donors by [NXO·BF₃] dyads (which results in the formation of the corresponding cation radicals and products of transformation of both BF ₃ and NXO components) is discussed.

Full text of DOI:10.1039/b9nj00205g

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/njc | New Journal of Chemistry
Lewis acid effects on donor–acceptor associations and redox reactions:
ternary complexes of heteroaromatic N-oxides with boron trifluoride
and organic donorsw
Yakov P. Nizhnik,zJianjiang Lu, Sergiy V. Rosokha*y and Jay K. Kochiz
Received (in Montpellier, France) 20th May 2009, Accepted 28th July 2009
First published as an Advance Article on the web 22nd September 2009
DOI: 10.1039/b9nj00205g
Structural and spectral features of the novel supramolecular [DꢀNXOꢀBF
simultaneous n-coordination of a Lewis acid (BF ) and p-complexation of organic donors (D) to
polyfunctional nitrosubstituted N-oxide molecules (NXO = 4-nitropyridine-N-oxide or
-nitroquinoline-N-oxide) are reported. X-Ray studies of [pyreneꢀNPOꢀBF ]ꢀCH Cl and
3
] complexes formed via
3
4
3
2
2
[
(pyrene) ꢀNQOꢀBF ] salts revealed that the Lewis acid is coordinated to the oxygen atom of the
˚
2
3
N-oxide group with O–B distance of B1.52 A similar to that in the separate [NXOꢀBF
3
] adducts.
Aromatic rings of N-oxide molecules in the ternary systems are p-stacked with pyrene moieties,
˚
and their interplanar separations of B3.35 A are common for conventional charge-transfer
complexes. In dichloromethane, associations between [NXOꢀBF
3
] adduct and organic donors are
characterized by higher formation constants (and charge-transfer bands of [DꢀNXOꢀBF
3
]
complexes are red-shifted) as compared to the complexes between the same donors and NXO
acceptors. Spectral data and electrochemical measurements point out enhanced acceptor abilities
of [NXOꢀBF ] adducts relative to the separate N-oxides which correspond to B0.5 V positive
3
shift of reduction potentials of acceptors. Synergetic oxidation of strong organic donors by
[
NXOꢀBF
3
] dyads (which results in the formation of the corresponding cation radicals and
and NXO components) is discussed.
products of transformation of both BF
3
1
. Introduction
and thus, in favorable modulation of the overall thermo-
4
dynamics of the electron-transfer step.
Lewis acid catalysis is a common method of activation of
various organic reactions, in particular chemical transformations
In order to clarify the mechanistic role of a Lewis acid in a
redox process, it is essential to determine its binding site and
its effects on reactants, intermediates and/or products. Of
particular interest in this respect is a Lewis acid binding
with the key intermediate of intermolecular electron-transfer
1
–3
involving an electron-transfer step.
Catalysis of the latter
usually occurs via (a) an electron abstraction from a donor by
a Lewis acid, (b) assembly of Lewis acids and bases together
with metal ions that undergo redox process, and (c) formation
of Lewis acid adducts with oxidizing or reducing agents (which
5
processes, a precursor complex. We have shown recently
that analysis of the spectral and structural features of
the (donor–acceptor) precursor complexes provides important
insight into the potential energy surfaces (and thus mechanism)
3
enhances their reactivity). Besides, Fukuzumi and co-workers
reported recently that addition of a Lewis acid (metal ions)
4
accelerates drastically electron-transfer-activated reactions.
6
for the various organic self- and cross-exchange redox reactions.
This was assigned primarily to the coordination of the Lewis
acid with the anionic products of the redox process which
resulted in the positive shift of redox potential of acceptors,
Thus, experimental evaluation of the corresponding characteristics
of associates involving a donor, an acceptor and a Lewis acid
is expected to provide valuable information for the elucidation
of the catalytic redox processes. However, quantitative spectral
and structural data on these ternary complexes are lacking (in
spite of the fact that some indications of the Lewis acid
enhancement of the acceptor abilities of nitrobenzenes in
Department of Chemistry, University of Houston, Houston,
TX 77204, USA
w Electronic supporting information (ESI) available: IR, UV-Vis
spectral data and cyclic voltammograms of NXOs, BF and their
3
mixtures, pyrene–NXO stacking in X-ray structures, resonance con-
tributors to N-oxide structure. CCDC 742367, 742368 and
their association with organic donors were reported more than
7
0 years ago). In search for the donor–acceptor–Lewis acid
5
743147–743149. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b9nj00205g
components for ternary supramolecular associates, we turn to
the nitrosubstituted N-oxides (designated hereinafter as NXO,
where X = Q for 4-nitroquinoline-N-oxide and X = P for
4-nitropyridine-N-oxide, see Chart 1).
zCurrent address: Department of Molecular Biology, Biological and
Organic Chemistry, Petrozavodsk State University, Pr. Lenina 33,
Petrozavodsk, 185910, Russia. E-mail: yakov_nizhnik@mail.ru
y Current address: Department of Biological, Chemical and Physical
Sciences, Roosevelt University, 430 S. Michigan Ave, Chicago, IL
60605, USA. Tel: +1 847-670-1595, Fax: +1 847-619-8555. E-mail:
srosokha@roosevelt.edu.
z Deceased, August 9, 2008.
This choice is determined by the dual nature of nitro-
substituted N-oxides in Chart 1, which contain both
aromatic parts with mild p-acceptor properties appropriate
for the charge-transfer complexation with organic donors and
This journal is ꢁc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 2317–2325 | 2317

View Article Online
2
.2 Measurements and computations
UV-Vis measurements were carried out with a HP-845 UV-Vis
spectrophotometer. Formation constants and extinction
coefficients were evaluated via the Benesi–Hildebrand and
2
0
Drago procedures, and/or by minimizing differences between
experimental and calculated absorbances of solutions at
2
1
Chart 1
different concentrations of reactants as described earlier. Cyclic
voltammetry (CV) was performed with a BAS 100A electro-
chemical analyzer equipped with a platinum electrode under an
N-oxide groups suitable for Lewis acid coordination. Indeed,
21
argon atmosphere with Bu BF supporting electrolyte. IR
4
4
NQO acceptor forms charge-transfer complexes with DNA,
8
proteins and their components, and other donors, and this
spectra of neat crystalline materials were recorded on a Nicolet
0 DX FT spectrometer using single-reflection HATR (Smart
1
complexation was considered as a major factor in the
9
biological activity of 4-nitroquinoline-N-oxide. [Note that
Miracle). GC-MS studies were carried out with an Agilent 6890
GC system with a 5973 mass selective detector. Frontier orbitals
NQO represents a model carcinogen for studies of the
11
mechanism of carcinogenicity and mutagenicity, apoptosis,
22
10
were evaluated via single-point B3LYP/6-311* Gaussian 98
computation using Cube = (55, orbital) option and atomic
coordinate from the corresponding X-ray structure.
as well for the evaluation of the anticarcinogenic properties of
various substances, e.g. polyphenols from green tea and
1
fermented milk.] Furthermore, it was shown that S
2
N
Ar
2
.3 Crystallization—(pyrene)
2
ꢀNPO
reactions of N-oxides (which are important precursors in the
1
heterocyclic chemistry ) are drastically accelerated in the
3
7
0 mg of 4-nitropyridine-N-oxide (0.50 mmol) and 202 mg of
1
4
pyrene (1.0 mmol) were dissolved in 2 mL of dichloromethane.
The resulting solution was cooled to ꢂ78 1C, which led to
formation of yellow plates suspended at the surface of the
liquid and a small amount of microcrystalline material on the
bottom of the vial. After separation of the microcrystals by
centrifugation, the remaining solution containing floating
plates was filtered, and the yellow crystals were dried in
air. Single-crystal X-ray study revealed 2 : 1 pyrene–NPO
stoichiometry of this salt (see crystallographic parameters and
details of structure refinements in Table 1). IR data for this
salt (together with those for separate NQO and pyrene) are
listed in Table S1 in the ESI.w
presence of Lewis acids. UV-Vis/IR spectral data revealed
the formation of complexes between various N-oxides and
boron trifluoride, but their structural features remained
1
5
ambiguous.
In the current work, we report structural and spectral
1
6
features of the novel ternary complexes
formed via
simultaneous n-coordination of boron trifluoride (Lewis acid)
and p-complexation of organic donors to polyfunctional NQO
and NPO molecules, and discuss how coordination of a Lewis
acid affects optical and thermal electron-transfer processes in
these donor–acceptor systems.
2
.4 NPOꢀBF
00 mL (1.6 mmol) of Et
40 mg (1.0 mmol) of 4-nitropyridine-N-oxide in dichloro-
3
2
. Experimental
2
1
2
OꢀBF
3
were added to the solution of
2
.1 Materials
methane. It resulted in the precipitation of a colorless crystal-
line material, which was filtered, washed with 1 mL of
Commercially available tetracyanoquinodimethane, tetra-
fluorotetracyanoquinodimethane, p- and o-chloranils, p- and
o-bromanils, p-fluoranil, tetracyanoethylene, p-benzoquinone,
2 5
dichloromethane and dried in a desiccator over P O powder
and paraffin oil (see crystallographic and IR data in Table 1
2
,3-dicyano-p-benzoquinone, 2,6-dichloro-p-benzoquinone,
,3-dichloro-5,6-dicyano-p-benzoquinone, 2,3-dibromo-5,6-
and Table S1 in ESI,w respectively).
2
dicyano-p-benzoquinone acceptors, as well as hexamethyl-
benzene, hexaethylbenzene, pentamethylbenzene, naphthalene,
p-dimethoxybenzene, anthracene, pyrene, 9,10-dimethyl-
anthracene, octamethylbiphenylene and tetrathiafulvalene
2
.5 NQOꢀBF
63 mL (0.50 mmol) of Et
95 mg (0.50 mmol) of 4-nitroquinoline-N-oxide in 3 ml of
CH Cl . Evaporation of the major portion of the solvent in a
desiccator (over paraffin oil and P ) led to formation of
3
2
OꢀBF
3
were added to the solution of
(
TTF) donors were used without additional purification.
2
2
Tetracyanopyrazine was a gift from Dr J. F. Neumer (DuPont
Co). Phenothiazine (PTZ) was recrystallized from toluene,
and tetramethyl-p-phenylenediamine (TMPD) was sublimed
2 5
O
large yellow needles. The rest of the solvent (B0.5 mL) was
decanted, and crystals were washed with diethyl ether (Table 1
and Table S2 in ESIw).
in vacuo. Solutions of BF
passing gaseous BF through dry acetonitrile. Concentrations
of BF in acetonitrile and in the commercially available BF
Et O solutions were determined by their titration as described
3
in acetonitrile were prepared by
3
2
.6 PyreneꢀNPOꢀBF ꢀCH Cl
3
2
2
3
3
ꢀ
2
30 mL (0.24 mmol) of Et
28 mg (2.0 mmol) of NPO and 81 mg (4.0 mmol) of pyrene in
2
OꢀBF
3
were added to the solution of
1
7
earlier. 4-Nitroquinoline-N-oxide (NQO) and 4-nitropyridine-
N-oxide (NPO) were synthesized from quinoline and
1
3 mL of dichloromethane. It resulted in a red-brown coloration
3,18
pyridine N-oxides as described earlier.
Dichloromethane
of the solution. In few minutes, colorless crystals of NQOꢀBF
3
and acetonitrile were purified according to the published
1
procedures.
complex were formed and the coloration almost disappeared.
9
This heterogeneous system was kept at ꢂ30 1C, and large
2
318 | New J. Chem., 2009, 33, 2317–2325
This journal is ꢁc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009

View Article Online
Table 1 Crystallographic parameters and the details of the structure refinements
Compound
(Pyrene)
2
ꢀNPO
NPOꢀBF
BF
3
PyreneꢀNPOꢀBF
16BCl
3
ꢀCH
2
Cl
2
NQOꢀBF
BF
3
(Pyrene)
2
ꢀNQOꢀBF
3
Formula
FW
C
37
H
24
N
2
O
3
C
5
H
4
3
N
2
O
3
C
22
H
2
F
3
N
2
O
3
C
9
H
6
3
N
O
2 3
82 52 2 6 4 6
C H B F N O
544.58
Triclinic
ꢀ
P1
207.91
Orthorhombic
Pca2
495.08
Monoclinic
P2 /c
257.97
Orthorhombic
P2
662.45
Monoclinic
P2
Crystal system
Space group
1
1
1
2 2
1 1
1
˚
a/A
8.1578(16)
8.2030(16)
10.141(2)
89.462(4)
76.889(4)
80.215(4)
651.0(2)
1
1.389
0.089
9011
3789
2386
11.996(3)
6.0475(13)
11.156(2)
90.00
90.00
90.00
809.3(3)
4
1.706
0.177
7360
2319
1643
0.0493
0.0919
1.004
7.0332(14)
10.3535(19)
29.588(6)
90.00
91.623(5)
90.00
2153.7(7)
4
1.527
5.4823(9)
12.217(2)
15.168(3)
90.00
90.00
90.00
1015.9(3)
4
1.687
0.159
6559
2956
2073
0.0504
0.0954
1.012
14.799(3)
8.1970(14)
25.036(5)
90.00
90.744(5)
90.00
3036.8(10)
4
1.449
˚
b/A
˚
c/A
a/1
b/1
g/1
3
˚
V/A
Z
ꢂ3
d
calcd/g cm
ꢂ1
m/mm
n measd
0.356
0.103
24 464
6358
3107
0.0784
0.1912
1.028
31 925
14 640
7477
0.0802
0.1716
1.057
n ind
n obs (I Z 2s(I))
R (I Z 2s(I))
0.0572
0.1502
1.022
R
GOF
w
(I Z 2s(I))
dark-red crystals were formed after 1 week (Table 1 and
Table S1, ESIw). Exposure of the dark-red crystals to air
resulted in their decomposition (in 10–15 min) and formation
of a white residue.
2
.7 (Pyrene)
2
ꢀNQOꢀBF
3
3
3
3
0 mL (0.24 mmol) of Et OꢀBF were added to the solution of
2
3
6 mg (2.0 mmol) of NQO and 81 mg (4.0 mmol) of pyrene in
mL of acetonitrile. The resulting red-brown solution was
cooled to ꢂ30 1C. This led to formation of reddish-black
crystals together with a small amount of microcrystalline
yellow sediment. After one week, the latter disappeared and
only large reddish-black crystals remained in the solution
Fig. 1 Molecular structure of the [NPOꢀBF
3
] (left) and [NQOꢀBF
3
]
(
right) adducts (Note: ellipsoids are drawn at 50% probability level).
(
Table 1 and Table S2 in ESIw).
Analysis of the molecular structures of N-oxides in the
] adducts (Table 2) indicates that
[NPOꢀBF
3
] and [NQOꢀBF
3
2
.8 X-Ray Crystallography
˚
Lewis acid binding leads to significant elongation (B0.075 A)
of the N1–O1 bonds, as compared to the corresponding values
Intensity data for X-ray crystallographic analysis were
collected at 173 K with a Bruker SMART Apex diffractometer
in the isolated NXO molecules. Besides, BF -coordinated
3
˚
using MoKa radiation (l = 0.71073 A). The structures
were solved by direct methods and refined by full matrix
moieties are characterized by less bond length variation within
pyridine rings, by longer C3–NO bonds and by larger nitro
2
2
3
least-squares treatment. Crystallographic details for the
binary and ternary salts of NQO or NPO N-oxides with BF3
and/or pyrene donor are listed in Table 1.
group twist angles (Table 2). Such structural changes indicate
that Lewis acid coordination results in some decrease of the
26
quinonoidal character of N-oxide molecules.
3
.1.B Donor–acceptor complexes of NPO with pyrene.
3
. Results and discussion
Cooling or evaporation of solutions containing NXO acceptors
and organic donors (various alkybenzenes, naphthalenes,
anthracenes, etc.) led to crystallization of separate starting
materials, except for the solutions containing NPO and
pyrene. The latter afforded yellow crystals, and their X-ray
measurements showed stacks of interchanging pyrene and
NPO molecules. These stacks are separated by additional
arenes, so the overall NPO to donor ratio is 1 : 2 (Fig. 2).
All three components lie on inversion centers in this structure
(which results in disorder for the NPO molecule).
3
.1 Structural characterization of binary and ternary
complexes
3
.1.A Binary [NXO, BF
graphy of binary [NPOꢀBF
showed coordination of BF
3
] complexes. X-Ray crystallo-
] and [NQOꢀBF ] complexes
to the N-oxide group (Fig. 1)
3
3
3
˚
with B–O distances of B1.52 A (Table 2), which are in the
same range as those in the boron trifluoride adducts with
4
various n-donors. The dihedral C5–N1–O1–B1 angle of
2
7
8.4(3)1 in [NPOꢀBF ] and the C5–N1–O1–B1 angle of 81.
In this structure, NPO moieties are located in the ‘‘cavities’’
formed by four pyrene molecules (two of which are almost
parallel, and two are nearly perpendicular to NPO plane).
3
3
6
(3)1 in [NQOꢀBF ] point out the sp hybridization of oxygen
3
2
5
atoms of the N-oxide in these complexes.
This journal is ꢁc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 2317–2325 | 2319

View Article Online
a
˚
Table 2 Selected bond distances (A) and angles (1) for NPO and NQO molecules
b
c
d
e
3
Bond lengths and angles
NPO
NPOꢀBF
3
(Pyrene)
2
ꢀNPO
(Pyrene)
2
ꢀNPOꢀBF
3
NQO
NQOꢀBF
3
PyreneꢀNQOꢀBF
N1–O1
N1–C1
N1–C5
1.291(2)
1.370(2)
1.369(2)
1.458(2)
1.230(2)
1.223(2)
1.377(3)
1.387(3)
1.388(2)
1.377(3)
—
1.368(3)
1.350(4)
1.340(4)
1.478(3)
1.218(4)
1.222(4)
1.371(4)
1.376(4)
1.378(4)
1.377(4)
1.524(5)
3.39
1.343(18)
1.306(14)
1.376(10)
1.51(2)
1.162(17)
1.252(14)
1.380(10)
1.331(12)
1.397(8)
1.391(10)
—
1.375(3)
1.338(4)
1.342(4)
1.484(4)
1.209(4)
1.227(4)
1.376(4)
1.371(4)
1.376(4)
1.364(4)
1.518(4)
2.12
1.293(4)
1.342(4)
1.420(4)
1.464(4)
1.236(4)
1.222(4)
1.385(5)
1.360(5)
1.427(4)
1.420(4)
—
1.370(3)
1.320(3)
1.384(3)
1.352(5)/1.358(5)
1.331(6)/1.307(6)
1.386(5)/1.385(6)
1.470(6)/1.492(6)
1.219(5)/1.181(6)
1.216(5)/1.179(6)
1.376(7)/1.391(7)
1.361(6)/1.358(7)
1.427(6)/1.404(7)
1.407(6)/1.417(7)
1.511(6)/1.517(7)
39.53/11.67
f
e
e
e
C3–NO
N2–O2
N2–O3
C1–C2
C2–C3
C3–C4
2
1.480(3)
1.220(3)
1.231(3)
1.393(3)
1.354(3)
1.415(3)
g
f
f
f
C4–C5
O1–B1
1.420(3)
1.513(3)
38.47
79.47
h
+
+
NO
2
–Pyr
NOB–Pyr
0
—
2.81
—
16.73
—
i
75.70
79.35
72.66/39.38
a
b
See Fig. 1 for atom labeling. Ref. 27. Disordered NPO molecule. Ref. 28. Two crystallographically independent molecules. Or N1–C9
c
d
e
f
g
h
for NQO). Or C4–C9 (for NQO). Dihedral angle between O2–N2–O3 and pyridine planes. Dihedral angle between N1–O1–B and pyridine
i
(
planes.
Fig. 2 Fragment of crystal structure of the (pyrene)
2
ꢀNPO.
Fig. 3 Fragment of crystal structure of pyreneꢀNPOꢀBF
3
ꢀCH
2 2
Cl salt
with light blue line indicating contacts shorter than the sum of the
van der Waals radii (solvent molecules are omitted for clarity).
Interplanar separations between nearly parallel NPO and
˚
pyrene moieties (B3.32 A) are usual for charge-transfer
salts, but lateral donor–acceptor shift significantly decreases
2
9
X-Ray measurement of the dark-red crystals formed in the
their overlap (Fig. S1 in ESIw). The distances between carbon
solutions containing NQO, BF
3
, and pyrene also revealed
˚
atoms of NPO and perpendicular pyrene planes (B3.52 A),
infinite stacks of [NQOꢀBF ] adducts interchanging with
3
and between pyrene hydrogens and neighboring pyrene planes
˚
pyrene moieties. [Note that there are two crystallographically
independent pyrenes and two independent NQO moieties
within these stacks, see Table 2.] These stacks form layers,
which are separated by additional aromatic molecules (Fig. 4).
As such, these crystals are characterized by 2 : 1 : 1 pyrene :
(
B2.722 and B2.636 A) point to some C–H–p (T-shaped)
interactions in these crystals.
3
.1.C Structural characterization of ternary complexes of
N-oxides with BF and pyrene. Cooling of the solutions con-
3
NQO : BF
stacked and inter-stack pyrenes are somewhat analogous to
that in (pyrene)
3
ratio, and T-shaped C–H–p-interactions between
taining mixtures of pyrene, N-oxide and Lewis acid resulted in
isolation of the supramolecular associates containing all three
components (see Experimental section for details). In particular,
dark-red crystals of pyreneꢀNPOꢀBF ꢀCH Cl solvate showed
2
ꢀNPO salt.
Pyrene and N-oxide molecules in the stacks form two
distinct donor–acceptor dyads. One of them shows two short
intermolecular carbon–carbon contacts (light blue lines in
3
2
2
vertical stacks in which BF -coordinated N-oxides interchange
3
˚
Fig. 4) of 3.260(7) and 3.360(7) A, and the other shows
with pyrenes (Fig. 3), and NPO moieties are located over the
˚
separations of 3.363(6) and 3.379(6) A.
center of neighboring donors (see Fig. S1 in ESIw). The closest
donor–acceptor CꢀꢀꢀC contacts within these stacks are
Thus, X-ray crystallographic studies of the supramolecular
˚
.351(4) and 3.380(4) A (i.e. slightly less than their van der
3
pyreneꢀNXOꢀBF
p-bonded to the aromatic fragments of N-oxide moieties. This
adduct LUMO
3
assemblies show that organic donors are
˚
Waals separation of 3.40 A).
In general, structural characteristics of NPO moieties and
is consistent with the fact that the NPOꢀBF
3
their bonding with BF
3
are similar to those in the NPOꢀBF
3
(which is critical for interaction with the donor) is located
predominantly on the NPO moiety, and its shape is very
similar to the LUMO shape of isolated NPO N-oxide
(Fig. 5). [Note, however, that disordered NPO molecules in
adduct (Table 2). It is noticeable that p-interaction of NPO
with pyrene leads to further elongation of the N1–O1
and C3–NO2 bond lengths, and thus to a decrease of the
quinonoidal character of the N-oxide as compared to the
analogous isolated NPO molecule.
(pyrene)
2
ꢀBF
3
salt in Fig. 2 are stacked over the periphery of
-coordinated NPO and NQO
the pyrene donor, while BF
3
2
320 | New J. Chem., 2009, 33, 2317–2325
This journal is ꢁc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009

View Article Online
ꢂ1
and afforded formation constant of KDA = 1.2 M . Similar
bands in the visible range were observed when other strong
donors (phenothiazine or tetrathiafulvalene) were added to the
solution of either NQO or NPO acceptors. In comparison,
addition of a weaker anthracene donor to the solution of NPO
resulted in appearance of a broad shoulder on the low-energy
tail of the strong band of the acceptor (Fig. 6B). Intensity of
this absorption (measured at 450 nm) was consistent with
the formation of a 1 : 1 [pyrene, NPO] complex, and its
deconvolution into Gaussian components (see Fig. S2 in ESIw)
produced an approximate position of the new band maximum
at lmax E 425 nm. An analogous broad shoulder on the
low-energy tail of NPO was observed with pyrene (Fig. S3 in
ESIw). Formation constants for the complexes of NPO with
strong organic donors are listed in Table 3. [Note that an
intense absorption band of NQO at l = 386 nm hindered
quantitative analysis of complexation of this acceptor with
most of the donors.]
Fig. 4 Fragment of crystal structure of the (pyrene)
2
ꢀNQOꢀBF
3
with
light blue line indicating contact shorter than the sum of the van der
Waals radii.
UV-Vis measurements of the solutions containing NXO
N-oxides and one of the weaker donors (e.g. alkylbenzenes
or naphthalenes) did not reveal any new absorption bands.
molecules in Fig. 3 and 4 are stacked essentially over the center
of the donor, see Fig. S1 in ESIw.] As such, these ternary
associates can be considered as donor–acceptor complexes in
which the properties of the N-oxide acceptors are modulated
by the coordinated Lewis acid. To evaluate how such
coordination affects the thermodynamics of donor–acceptor
interactions and thermal/optical electron-transfer processes, we
turn to electronic spectroscopic study in solutions, as follows.
29
This observation accords with Mulliken’s theory which
predicts that absorption bands (if any) for complexes with
weaker donors are expected at higher energies, where they are
apparently overshadowed by absorption of NXO acceptors
and/or organic donors.
3.2.B Binary acceptor–Lewis acid complexes. Earlier
studies established that interaction of N-oxides with boron
trifluoride results in formation of the 1 : 1 complex according
3
.2 Electronic spectroscopy of binary and ternary complexes
15
to eqn (2).
of 4-nitrosubstituted N-oxides
KLA
ꢂ!
3
.2.A Binary donor–acceptor complexes. Pale-yellow
NXO þBF3 ꢂ½NXO ꢀBF3ꢃ
ð2Þ
solutions of NPO and NQO N-oxides in dichloromethane
show intense absorption bands at l = 344 and 386 nm,
respectively (Table S3 in ESIw). Addition of the strong donor
tetramethyl-p-phenylenediamine (TMPD) to either of these
solutions resulted in the appearance of new absorption bands
around 500 nm. In particular, addition of TMPD to solution
of NPO led to the appearance of a band at 517 nm (Fig. 6A).
In agreement with the reported data, our spectral measurements
showed that addition of BF to solutions of NPO or NQO in
3
acetonitrile results in disappearance of the lowest-energy
bands of N-oxides and rise of new (blue-shifted) bands of
[
NXOꢀBF ] complex (Fig. 7, and Table S3 and Fig. S4 in
3
ESIw).
Quantitative analysis of the spectral changes confirmed the
stoichiometry in eqn (2) and afforded high values of complex
2
0,21
Quantitative analysis
of the absorption intensity at various
concentrations of components indicated that this band is
related to formation of a 1 : 1 [TMPD, NPO] complex (eqn (1))
3
ꢂ1
formation constants of KLA = 2.8 ꢄ10 and 1.7 ꢄ
0 M with NPO and NQO, respectively. Similar studies in
dichloromethane showed that equilibria in eqn (2) are
M
3
ꢂ1
1
KDA
ꢂ!
TMPD þNPO ꢂ½TMPD; NPOꢃ
ð1Þ
4
ꢂ1
shifted to the right with KLA 4 10
complete coordination of N-oxide molecules with BF
M
, and essentially
occurs if
3
significant excess of Lewis acid is added to 1–10 mM solutions
of NXOs.
3
.2.C Ternary donor– acceptor–Lewis acid complexes.
Addition of anthracene donor to the dichloromethane solution
containing NPO N-oxide and excess of boron trifluoride led
to appearance of a new band with a distinct maximum at
lmax = 511 nm (Fig. 8). This band is red-shifted relative to the
absorption of the [anthracene, NPO] complex in Fig. 6B. In
view of essentially complete binding of NPO molecules with
Fig. 5 Frontier orbital shape for NPO N-oxide and its complex
with BF
BF in the solutions containing excess of Lewis acid (and since
3
3
.
further addition of boron trifluoride did not affect spectra), we
This journal is ꢁc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 2317–2325 | 2321

View Article Online
Fig. 6 Spectral changes accompanying addition of TMPD (A) and anthracene (B) donors to the dichloromethane solutions of NPO acceptor.
Concentrations of components: (A) 0.2 M NPO and from 0.0 to 1.2 M TMPD. (B) 0.12 M NPO and from 0.0 to 0.14 M anthracene.
Table 3 Spectral characteristics and formation constants for com-
plexes between various donors and NPO or NPOꢀBF
3
acceptors
(
CH Cl , 22 1C)
2
2
Acceptor
NPO
NPOꢀBF
3
1/2 a
ꢂ1
ꢂ1
Donor
E
ox
/V
K
DA/M
lmax (e)
K
DAL/M
lmax (e)
b
b
b
b
TMPD
PTZ
Anthracene +1.09
+0.12
+0.59
1.2
1.0
0.3
0.3
518 (110)
480 (55cd)
c
c
425 1.0
0.7
511 (430)
500 (550)
f
Pyrene
+1.16
—
a
b
From ref. 6. Cation radical absorption was observed (see text).
c
Band maximum was overshadowed by NPO absorption, and K was
d
estimated from absorption changes at l = 450 nm.
estimated from spectral deconvolution. In comparison, KDA
lmax was
e
=
=
ꢂ1
f
0
.5 M for complex of pyrene with NQO. In comparison, l max
Fig. 8 Spectral changes accompanying addition of anthracene donor
ꢂ1
5
10 nm and KDAL = 1.1 M for complex of pyrene with [NQOꢀBF
3
].
(from 0.0 to 0.14 M) to the dichloromethane solutions containing
0
.015 M of NPO and 0.030 M of BF
3
. [Note: dotted line represents a
spectrum of NPOꢀBF adduct.]
3
Addition of other aromatic donors to the dichloromethane
solutions containing NPO and BF3 resulted in appearance
of similar absorption bands in the visible range indicating
formation of the complexes analogous to those in eqn (3).
Mulliken correlation between the absorption band energies
(
ECT) of these complexes and the oxidation potentials of the
donors in Fig. 9 (with correlation coefficient of 0.97) confirms
2
9
the charge-transfer nature of the [DꢀNPOꢀBF
3
] associates.
Analogous charge-transfer absorption bands in the visible
range were observed in electronic spectra of the solutions
containing the same donors and [NQOꢀBF
3
] adducts. In
comparison, addition of the very strong TMPD donor to the
solution containing mixture of either NPO or NQO and BF3
Fig. 7 Spectral changes accompanying addition of BF
.5 mM solution of NQO in acetonitrile).
3
(from 0 to the
resulted in appearance of the spectrum with peaks at 567 and
+
0
ꢅ
6
17 nm characteristic of corresponding TMPD
6
cation
radicals. Similar formation of cation radicals was observed
upon addition of tetrathiafulvalene (TTF) or phenothiazine
assigned the new band at l = 511 nm to a complex between
donor and [NPOꢀBF ] adduct:
3
(
PTZ) donors to the solutions containing mixtures of N-oxide
KDAL
ꢂ!
(NPO or NQO) and BF Lewis acid (vide infra).
3
anthracene þ½NPO ꢀBF3ꢃ ꢂ½anthracene ꢀNPO ꢀBF3ꢃ
ð3Þ
Thus, UV-Vis spectral measurements indicated that
complexation of N-oxide acceptor with Lewis acid results in
increased stability of their complexes with organic donors and
significant red shift of the charge-transfer absorption bands of
their complexes (which allow measurements of these bands in
the systems with relatively weak donors) and/or in oxidation
of the strong donors. These data point out significant increase
Quantitative treatment of the dependence of this band
intensity on the concentrations of donor, acceptor and Lewis
acid was consistent with equilibrium in eqn (3), and it afforded
the equilibrium constant: KDAL = [anthraceneꢀNPOꢀBF ]/
3
ꢂ1
[
anthracene] [NPOꢀBF ] = 1.0 M (Table 3).
3
2
322 | New J. Chem., 2009, 33, 2317–2325
This journal is ꢁc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009

View Article Online
measurements of its binary [donor, NQO] complexes and,
thus, their spectral comparison with corresponding ternary
complexes.]
Alternative estimations of acceptor strengths of the
[
NXOꢀBF ] adducts were obtained using Mulliken correlations
3
for complexes of pyrene and anthracene donors with various
conventional organic acceptors. Indeed, charge-transfer
associates between either of these donors and many organic
2
9,31
acceptors are well known in the literature.
Absorption
band energies of these complexes (which were reported
earlier or measured in current work, see Table S5 in ESIw)
demonstrate typical Mulliken correlations with reduction
potentials of acceptors (Fig. 10). The placement of the energies
of absorption bands of [pyreneꢀNPOꢀBF ] and [pyreneꢀNQOꢀBF ]
3
3
complexes (2.43 and 2.32 eV, respectively) onto the
Mulliken correlation for pyrene donor (as illustrated in
Fig. 9 Mulliken correlation between energies of absorption bands
of [DꢀNPOꢀBF ] complexes and oxidation potentials of donors. D =
pentamethylbenzene (1), hexamethylbenzene (2), hexaethylbenzene
Fig. 10A) afforded reduction potentials of [NPOꢀBF ] and
3
3
[NQOꢀBF ] adducts of ꢂ0.38 V and ꢂ0.33 V, respectively.
3
The same procedure with [anthraceneꢀNPOꢀBF
anthraceneꢀNQOꢀBF ] associates in Fig. 10B led to similar
] and [NQOꢀBF
3
]
and
(
(
3), naphthalene (4), 1,4-dimethoxybenzene (5), pyrene (6), anthracene
7), 9,10-dimethoxyanthracene (8), 9,10-dimethylanthracene (9), and
[
3
numbers of ꢂ0.37 and ꢂ0.29 V for [NPOꢀBF
3
3
]
octamethylbiphenylene (10) (see Table S4 (ESIw) for details).
adducts, respectively. These values are shifted by B0.5 V in a
positive direction as compared to the reduction potentials of
the isolated NPO and NQO acceptors, which is in reasonable
agreement with the electrochemical evaluations based on CV
data for NPO–BF and NQO–BF mixtures.
of acceptor strength of [NXOꢀBF
3
] adducts, as compared to
the isolated NXO N-oxide, which was further evaluated, as
follows.
3
3
3
.3 Electrochemical and spectral evaluations of the acceptor
3.4 Synergetic oxidation of organic donors in solution with
NQO–BF or NQO–BF pairs
strength of [NXOꢀBF ] adducts
3
3
3
Cyclic voltammetry studies (see Fig. S5 in ESIw) reveal that
NQO and NPO N-oxides are characterized in dichloro-
methane by quasi-reversible reduction potentials of 0.81 and
Spectral studies of dichloromethane solutions of strong
TMPD, TTF or phenothiazine donors and either NQO or
NPO N-oxides revealed absorption bands of corresponding
charge-transfer complexes, and no indication of redox products
0
.87 V vs. SCE, respectively, which point to moderate acceptor
strengths of these molecules. Similar measurements of
the dichloromethane solution of boron trifluoride revealed
irreversible reduction waves at about ꢂ0.5 V vs. SCE
(vide supra). Addition of BF to these solutions led to their
3
intense coloration and UV-Vis measurements showed appearance
of the characteristic spectra of corresponding cation radicals.
For example, Fig. 11 demonstrates that the visible range of the
spectrum of 1 mM dichloromethane solution of phenothiazine
(
Fig. S3, ESIw). In comparison, cyclic voltammograms of
dichloromethane solutions containing mixtures of BF
3
and either NPO or NQO N-oxides show three irreversible
reduction waves (Fig. S5, ESIw). In each case, the first of these
waves appears at about ꢂ0.3 V vs. SCE, i.e. it is shifted in a
positive direction as compared to the position of reduction
donor and either 5 mM of NPO or 10 mM of BF is essentially
3
clear at l 4 550 nm (dash line). However, addition of a third
component resulted in the fast appearance of the characteristic
spectrum of phenathiazine cation radical (red line). At
waves of isolated BF
3
, NPO or NQO acceptors. We tentatively
higher concentrations of PTZ, NPO and BF , their reactions
3
+
resulted in precipitation of black PTZ BF4 salt and
ꢅ
ꢂ
assigned the waves at ꢂ0.3 V vs. SCE to the irreversible
3
] adducts. However, the irreversible
0
0
reduction of [NXOꢀBF
3
GC-MS analysis indicated formation of 4,4 -diazopyridine
0
character of the electrochemical processes in the solutions
containing NXO–BF3 mixtures hindered quantitative
N,N -dioxide. Spectral titration (see Fig. S6 in ESIw for
details) suggested that this reaction occurs according to eqn (4):
analysis of effects of BF coordination. As such, we turned
3
to the spectral data for the evaluation of acceptor strength of
[
NXOꢀBF
According to Mulliken and Person, absorption band
energies of charge-transfer complexes of the same donor are
3
] adducts.
2
9
directly related to the acceptor strengths. Thus, the red shift
of the charge-transfer band maxima of [anthraceneꢀNPOꢀBF
3
]
associate (511 nm or 2.43 eV) relative to that of [anthraceneꢀ
ð4Þ
Similar results were obtained in the solutions containing NQO
NPO] complexes (B425 nm or 2.92 eV) points out that the
acceptor strength of the [NPOꢀBF ] adduct is enhanced by
3
B0.5 V as compared to the NPO molecule. [Note that the
intense 386 nm absorption band of NQO acceptor hinders
N-oxide, BF3 and phenothiazine. These data indicate that
simultaneous presence of BF and NXO facilitates oxidation of
3
This journal is ꢁc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 2317–2325 | 2323

View Article Online
Fig. 10 Mulliken correlations for the complexes of various acceptors with pyrene (A) and anthracene (B): (a) 1,3,5-trinitrobenzene;
(
b) tetracyanopyrazine; (c) 2,6-dichloro-p-benzoquinone; (d) tropylium; (e) fluoranil; (f) p-bromanil; (g) p-chloranil; (h) o-bromanil; (i) o-chloranil;
j) tetracyanoethylene; (k) tetracyanoquinodimethane; (l) 2,3-dicyano-p-benzoquinone; (m) dibromodicyano-p-benzoquinone; (n) 2,3-dichloro-5,6-
(
dicyano-p-benzoquinone; (o) tetrafluorotetracyanoquinodimethane (see Table S5 in ESIw for details).
Fig. 11 (A) Spectra of dichloromethane solutions of 1 mM of PTZ with either 10 mM BF
of NPO and 10 mM of BF (dash line) and (B) time dependence of 745 nm absorption of the solutions of 1 mM of PTZ in dichloromethane upon
addition of: 10 mM of BF (solid line), 10 mM of BF and 5 mM of NPO (dash line).
3
or 5 mM of NPO (solid line) and 1 mM of PTZ, 5 mM
3
3
3
+
ꢅ
+
ꢅ
ꢂ
ꢅ
3
]
phenothiazine to the corresponding PTZ cation radical, and
this process is accompanied by complex transformation of both
Lewis acid N-oxides. [Note that rigorous study of redox process
the intermediate formation of the separated PTZ –[NPOꢀBF
pair. These data suggest that follow-up reactions proceed
directly from the successor complex for electron transfer
as shown in Chart 2, (where D = PTZ, TMPD or TTF
2 2
stoichiometries requires isolation of BF –O–BF , etc., and is
beyond the scope of current work.] Similar appearance of cation
radical spectra was observed in the ternary systems containing
donors, and A = [[NXOꢀBF
3
] adducts]). Stabilization of the
successor complex by electrostatic interaction and/or
electronic coupling within the ion-radical pair apparently
decrease endergonicity of the electron-transfer step and
NPO, BF and TTF or TMPD donors, as well as in the mixtures
3
containing NQO, BF3 and either phenothiazine or TTF or
TMPD donor. As such, this chemical process (as well as
analogous reaction with TTF and TMPD) can be described as
synergetic oxidation of strong organic donors by NXO acceptor
and Lewis acid.
a
thus attenuate the barrier DG for the overall process,
as compared to the formation of the separate D + A
+
ꢂ
6
,32
pair.
Comparison of the oxidation potential of phenothiazine
(
0.59 V vs. SCE) with reduction potentials of [NXOꢀBF
3
]
adducts (about ꢂ0.3 V vs. SCE) indicates that PTZ +
+
ꢅ
ꢂ
ꢅ
[
NXOꢀBF
3
] - PTZ
+ [NXOꢀBF
3
]
electron transfer
ꢂ1
is strongly endergonic (DG
4 20 kcal M ). Thus, the
ET
free-energy gain necessary for the facile proceeding of
phenothiazine oxidation is, in fact, provided by the follow-
up reactions. Furthermore, the fast proceeding of the reaction
(
t1/2 E 10 s) indicates that the rate-limiting activation barrier
for the whole process is lower than the energy associated with
Chart 2
2
324 | New J. Chem., 2009, 33, 2317–2325
This journal is ꢁc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009

View Article Online
4
. Conclusions
16 The term ‘‘ternary’’ hereinafter refers to a number of chemically
distinct components in the complex, i.e. an amphoteric N-oxide
(
7 A. B. Burg and J. H. Bickerton, J. Am. Chem. Soc., 1945, 67, 2261.
8 E. Ochiai, J. Org. Chem., 1953, 18, 534.
9 D. D. Perrin, W. L. Armarego and D. R. Perrin, Purification of
Laboratory Chemicals, Pergamon, New York, 1980.
0 H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc., 1949, 71,
2703; R. S. Drago, Physical Methods in Chemistry, Saunders Co,
Philadelphia, 1977.
1 S. V. Rosokha and J. K. Kochi, J. Am. Chem. Soc., 2001, 123,
8985.
2 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski,
J. A. Montgomery, Jr., R. E. Stratmann, J. C. Burant,
S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin,
M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi,
R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford,
J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui,
K. Morokuma, P. Salvador, J. J. Dannenberg, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski,
J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox,
T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen,
M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-Gordon,
E. S. Replogle and J. A. Pople, GAUSSIAN 98 (Revision A.11),
Gaussian, Inc., Pittsburgh, PA, 2001.
3
NXO), a p-donor (e.g. pyrene) and a Lewis acid (v-acceptor: BF ).
Spectral studies of the binary and ternary complexes involving
N-oxides, BF and organic donors, as well as their structural
1
1
1
3
measurements (showing p-complexation of donor to the
aromatic segment of the N-oxide moiety) and frontier orbital
2
calculations (which revealed the LUMO of NXOꢀBF
3
adducts
concentrating on N-oxide), indicate that optical and thermal
electron transfer in these systems occurs from the donor to the
N-oxide moiety. Acceptor strengths of the N-oxides are
considerably enhanced by the Lewis acid coordination, and
this enhancement is vital for the synergetic oxidation of
organic donors. While the electron-transfer step in such
ternary systems still remains quite endergonic even with
strong organic donors, the overall redox process is apparently
facilitated by free-energy gain from follow-up reactions,
as well as by formation of donor–acceptor associates and
ion-radical pairs.
2
2
Acknowledgements
We thank the R. A. Welch Foundation for financial support.
2
2
3 G. M. Sheldrick, SADABS (Ver. 03), 2000; G. M. Sheldrick,
¨
SHELXS 97, University of Gottingen, Germany, 1997.
4 H. Feinberg, I. Columbus, S. Cohen, M. Rabnovitz, H. Selig and
G. Shoham, Polyhedron, 1993, 12, 2913; S.-Z. Zhang, S. Sato,
E. Horn and N. Furukava, Heterocycles, 1998, 48, 227.
References
1
Lewis acid in organic synthesis, ed. H. Yamamoto, Wiley-VCH,
Verlag, 2000.
2
25 With sp -hybridized oxygen, this dihedral angle is expected to be
close to zero (Fig. S7 in ESIw). Note that use of the N–O–B angle
as an indicator of the oxygen hybridization is hindered by its
sensitivity to steric factors (Table S6 in ESIw).
26 The NPO molecule can be presented as a hybrid of two major
resonance contributors, i.e. (A) benzenoid structure with nearly
equivalent bond lengths in the aromatic ring, as well as single N–O
2
Selectivity in Lewis Acid Promoted Reactions, ed. M. Sanelli and
J.-M. Pons, CRC Press, Boca Raton, 1995.
3
4
A. Corma and H. Garcia, Chem. Rev., 2002, 102, 3837.
S. Fukuzumi, Org. Biomol. Chem., 2003, 1, 609; J. Yuasa,
T. Suenobu and S. Fukuzumi, ChemPhysChem, 2006, 7, 942;
J. Yuasa, S. Yamada and S. Fukuzumi, Chem.–Eur. J., 2008, 14,
1866.
N. Sutin, Prog. Inorg. Chem., 1983, 30, 441.
S. V. Rosokha and J. K. Kochi, Acc. Chem. Res., 2008, 41, 641;
S. V. Rosokha, D. Sun and J. K. Kochi, J. Phys. Chem. B, 2007,
and C–NO
2
bonds and (B) quinonoid structures with two double/
5
6
four single bonds in the ring, as well as double N–O and C–NO
bonds (Fig. S8 in ESIw). The higher contribution of the quinonoid
structure results in larger bond length variation in the ring, longer
2
111, 6555; S. V. Rosokha and J. K. Kochi, J. Am. Chem. Soc.,
2
N–O and C–NO bonds and larger nitro group twist angle (and
2
007, 129, 3683.
vice versa). Quantitatively, it can be evaluated as: q = 1/n
S{Dd (NPO)/Dd (Q)}, where Dd refers to the difference of a
i i
i
7
8
H. C. Brown and M. Grayson, J. Am. Chem. Soc., 1953, 75, 6285.
T. Okano, A. Takadate and T. Kano, Jpn. J. Cancer Res., 1969, 60,
particular bond in the NPO molecule and in the hypothetical
i
557; A. V. Ryzhakov, Russ. J. Gen. Chem., 1997, 67, 808.
S. A. Winkle and I. Tinoko, Biochemistry, 1979, 18, 3833.
benzenoid structure, Dd (Q)—the difference of the same bond in
9
0 S. Miyamoto, Y. Yasui, M. Kim, S. Sugie, A. Murakami,
the benzenoid and quinonoid structures, and n is a number of
bonds (so q = 0 and q = 1 correspond to benzenoid structure A
and to quinonoid structure B, respectively). Compare: J. M. Lu,
I. S. Neretin, S. V. Rosokha and J. K. Kochi, J. Am. Chem. Soc.,
2006, 128, 16708.
1
1
R. Ishigamori-Suzuki and T. Tanaka, Carcinogenesis, 2007, 29,
418; M. Itjima, K. Mihara, T. Kondo, T. Tsuji, C. Ishioka and
M. Namba, Int. J. Cancer, 1996, 66, 698; R. Mirzayans, S. Bashir,
D. Murray and M. C. Paterson, Carcinogenesis, 1999, 20, 941.
1 H. B. Han, Q. Z. Pan, B. L. Zhang, J. Li, X. M. Deng, Z. X. Lian
and N. Li, Toxicology, 2007, 230, 151; C. E. Ogburn, J. Oshima,
M. Poot, R. Chen, K. A. Gollahon, P. S. Rabinovitch and
G. M. Martin, Hum. Genet., 1997, 101, 121; R. A. Schoop,
R. J. Baatenburg de Jong and M. N. Noteborn, Cancer Biol. Ther.,
27 P. Coppens and M. S. Lehmann, Acta Crystallogr., Sect. B: Struct.
Crystallogr. Cryst. Chem., 1976, 32, 1777.
28 Y. P. Nizhnik, S. V. Rosokha, J. J. Lu and J. K. Kochi, Cryst-
EngComm, 2009, DOI: 10.1039/b905845a.
29 R. S. Mulliken and W. B. Person, Molecular Complexes, Wiley,
New York, 1969; Molecular Association, ed. R. Foster, Academic
Press, New York, 1975, vol. 1.
2008, 7, 1368.
1
2 P. Srinisavan, S. Suchalatha, P. V. A. Babu, R. S. Devi,
S. Narayan, K. E. Sabitha, D. Shymala and S. Chennam, Chem.-
Biol. Interact., 2008, 172, 224; C. Matar, S. S. Nadathur,
A. T. Bakalinsky and J. Goulet, J. Dairy Res., 1997, 80, 1965.
3 E. Ochiai, Aromatic Amine Oxides, Elsevier, Amsterdam, 1967.
4 A. V. Ryzhakov and L. L. Rodina, Heterocycles, 2008, 75, 2367;
A. V. Ryzhakov, V. P. Andreev and L. L. Rodina, Heterocycles,
30 The increase of concentration of BF
increase of the wave at ꢂ0.3 V and a decrease of the intensity of
the ꢂ0.8 V wave in the cyclic voltammograms of the NXO–BF
solutions, which accords with the increase of the fraction of BF
3
resulted in an intensity
3
3
-
1
1
coordinated N-oxide. However, the irreversible character of the
redox processes hindered rigorous analysis and assignment of these
data.
2003, 60, 419; V. P. Andreev and Y. P. Nizhnik, Russ. J. Org.
Chem., 2001, 37, 141.
31 V. Ganesan, S. V. Rosokha and J. K. Kochi, J. Am. Chem. Soc.,
2003, 125, 2559.
1
5 Y. P. Nizhnik, V. P. Andreev and B. Z. Belashev, Russ. J. Org.
Chem., 2008, 44, 1824; V. P. Andreev and A. V. Ryzhakov, Chem.
Heterocycl. Compd., 1994, 30, 938.
32 S. V. Rosokha and J. K. Kochi, J. Org. Chem., 2002, 67, 1727;
S. V. Rosokha, S. M. Dibrov, T. Y. Rosokha and J. K. Kochi,
Photochem. Photobiol. Sci., 2006, 5, 914.
This journal is ꢁc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 2317–2325 | 2325