Use of 1,4-bis(2,2,6,6-tetramethyl-1-oxyl-4-piperidyl)butane as a probe for studying acid sites

Article abstract of DOI:10.1007/s11172-007-0028-x

ESR spectroscopy was applied to study paramagnetic complexes of the nitroxyl biradical of 1,4-bis(2,2,6,6-tetramethyl-1-oxyl-4-piperidyl)butane formed with AlCl₃ in a toluene solution and resulted from the interaction with the acid sites on the SiO₂ and γ-Al ₂O₃ surface. This biradical in solution forms a complex with two AlCl₃ molecules, and a complex with two hydroxyl groups is formed on the SiO₂ surface. When the biradical is adsorbed on the γ-Al₂O₃ surface, complex formation is complicated because of steric hindrance preventing bidentate coordination. Springer Science+Business Media, Inc. 2007.

Full text of DOI:10.1007/s11172-007-0028-x

1
74
Russian Chemical Bulletin, International Edition, Vol. 56, No. 1, pp. 174—177, January, 2007
Brief Communications
Use of 1,4ꢀbis(2,2,6,6ꢀtetramethylꢀ1ꢀoxylꢀ4ꢀpiperidyl)butane
as a probe for studying acid sites
A. V. Fionovꢀ and A. F. Sadykov
Department of Chemistry, M. V. Lomonosov Moscow State University,
1
Leninskie Gory, 119992 Moscow, Russian Federation.
Fax: +7 (495) 939 4575. Eꢀmail: fionov@mail.ru
ESR spectroscopy was applied to study paramagnetic complexes of the nitroxyl biradical of
1
,4ꢀbis(2,2,6,6ꢀtetramethylꢀ1ꢀoxylꢀ4ꢀpiperidyl)butane formed with AlCl in a toluene solution
3
and resulted from the interaction with the acid sites on the SiO and γꢀAl O surface. This
2
2
3
biradical in solution forms a complex with two AlCl3 molecules, and a complex with two
hydroxyl groups is formed on the SiO surface. When the biradical is adsorbed on the γꢀAl O
2
2
3
surface, complex formation is complicated because of steric hindrance preventing bidentate
coordination.
Key words: 1,4ꢀbis(2,2,6,6ꢀtetramethylꢀ1ꢀoxylꢀ4ꢀpiperidyl)butane, paramagnetic comꢀ
plexes, aluminum chloride, silica, alumina, electron paramagnetic resonance.
Stable nitroxyl radicals, in particular, 2,2,6,6ꢀtetraꢀ
ecule of this biradical contains no other heteroatoms.
Two NOꢀcontaining moieties, which structurally resemble
TEMPO, are linked with each other through a mobile
aliphatic hydrocarbon chain.
methylpiperidineꢀ1ꢀoxyl (TEMPO), are widely used as
spin probes for studying acid properties of the surface of
heterogeneous catalysts and supports. Information on the
structure of electronꢀacceptor sites and their strength and
1
concentration can be obtained using nitroxyl radicals.
The nitroxyl radicals containing one >NO group are usuꢀ
ally adsorbed via oneꢀcenter mechanism. Twoꢀcenter adꢀ
sorption is often observed for the nitroxyl radicals conꢀ
taining another donor group in addition to >NO.1 Thereꢀ
fore, it seemed of interest to study 1,4ꢀbis(2,2,6,6ꢀ
tetramethylꢀ1ꢀoxylꢀ4ꢀpiperidyl)butane (thereafter biradiꢀ
cal 1) containing two >NO groups as a probe. These groups
are simultaneously radicals and electronꢀdonors. A molꢀ
,2
Biradical 1 has previously been studied as an efficient
inhibitor of radiationꢀchemical aging of nꢀoctane and
3
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 165—167, January, 2007.
066ꢀ5285/07/5601ꢀ0174 © 2007 Springer Science+Business Media, Inc.
1

Use of biradicals for studying acid sites
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
175
a paramagnetic probe for studying conformational changes
4
in Na,Kꢀdependent ATPase.
1
Experimental
The adsorbents used were γꢀAl O (trade mark Aꢀ1,
2
3
2″
Fe3+ content < 0.005 wt.%, S = 185 m g ) and SiO (S
3
2
–1
=
sp
2
sp
2
–1
30 m g ) prepared by hydrolysis of tetraethoxysilane (anaꢀ
2
lytical purity grade) followed by drying and calcination at 773 K
for 5 h.
Aluminum chloride was synthesized by the reaction of aluꢀ
minum metal with gaseous chlorine and purified by sublimation.
The specific surface was measured on a GKhꢀ1 gas meter by
the chromatographic method using lowꢀtemperature adsorption
3
″
3
(
77 K) of nitrogen (using an N (6 mol.%)—He mixture) folꢀ
2
lowed by desorption at room temperature (from the desorption
peak value).
1
´
The TEMPO radical was synthesized at our Laboratory usꢀ
5
ing a known procedure. TEMPO is represented by transparent
dark red prisms with strong camphor odor (m.p. 311 K).
6
Biradical 1 synthesized by an earlier published procedure was
2
3
´
´
kindly presented by A. I. Kokorin (N. N. Semenov Institute of
Chemical Physics, Russian Academy of Sciences). Toluene (reꢀ
agent grade) was purified by distillation and drying above sodium.
Samples of oxides were thermally treated (2 h in air and 2 h
under 10–5 Torr at 743 K) in glass ampules equipped with a side
branch with a sealed tube containing a glass ball. After the
treatment was completed, the ampule was sealed from the
vacuum setup, and the sample was transferred to the side branch,
which was then sealed in vacuo. Glass balls with a toluene soluꢀ
tion of biradical 1 (10–3 mol L ) were specially prepared. Then
the balls with biradical 1 and adsorbent were transferred to an
evacuated glass system with a branch, being simultaneously an
ESR tube, where the balls were broken with magnetic bullets.
The adsorbent and a solution of biradical 1 were mixed in the
ESR tube, which was then sealed off.
20 G
Fig. 1. ESR spectra of 10–3 M toluene solutions of biꢀ
radical 1 (1, 1´) and the complexes of biradical 1 (2, 2´)
and TEMPO (3, 3´) with aluminum chloride at 293 (1—3)
and 77 K (1´—3´). The model spectra are shown by dashed
lines (2″, 3″).
–1
similar magnetic resonance parameters is also observed
for the complex of TEMPO with AlCl (see Fig. 1). This
suggests that the both nitroxyl groups of biradical 1 form
complexes with AlCl in solution. Spin exchange in the
3
ESR spectra were recorded on REꢀ1306 and Bruker
EMXꢀ6/1 radiospectrometers at a frequency of 9.5 GHz (in the
Xꢀrange).
3
The ESR spectra were simulated using the SimFonia
program.7
complex of biradical 1 with AlCl is not manifested beꢀ
3
cause, most likely, collisions of the radical moieties stop
to occur. However, due to the dipoleꢀdipole broadening,
the ESR spectrum of the complex of biradical 1 has a
broader linewidth (6.9 G) than that in the case of the
TEMPO complex (3.4 G). For the TEMPO complex with
Results and Discussion
1
0
N
The spectra of the free biradical and complexes of
TEMPO and biradical 1 with aluminum chloride in a
toluene solution are shown in Fig. 1. The spectrum of
biradical 1 in solution at 293 K (see Fig. 1, curve 1)
AlCl in a CCl solution, the HFC constants are aiso =
3 4
Al
19.86 G and aiso = 8.80 G, being somewhat lower than
those measured in the present work. This is caused, most
likely, by the solvation of the >NO groups by toluene
1
1
exhibits five components with a successive spacing of
molecules (dipole moment 0.37 D), which is not maniꢀ
3
7
.9 G, which agrees with the literature data. This specꢀ
fested in a CCl solution (dipole moment is zero). The
4
trum pattern indicates spin exchange between the radical
spectra of the complexes of biradical 1 and TEMPO with
8
,9
N
moieties upon their collision. The aiso value (isotropic
AlCl at low temperature are also similar, which agrees
3
14
HFC constant with the N nucleus) is 15.8 G. The interꢀ
with the assumption on the formation of similar comꢀ
plexes by both >NO groups in biradical 1 (each group
N
action with AlCl increased the a
value to 20.7 G
3
iso
and resulted in the appearance of the HFS from the
Al nucleus (aiso = 9.6 G). The ESR spectrum with
coordinates with one AlCl molecule). Evidently, comꢀ
plex formation of biradical 1 and TEMPO in solution
3
2
7
Al

1
76
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
Fionov and Sadykov
It is most likely that the increased A||N value indicates
the formation of both >NO groups in biradical 1 with
the hydroxyl groups of the SiO surface. Measurement
2
of the dipoleꢀdipole broadening in the ESR spectrum
using a known procedure gives the distance between
1
2
9
the >NO groups equal to 1.32 nm, which is somewhat
shorter than that for free biradical 1 (1.40 nm). This result
is consistent with the assumption on the bidentate
character of interaction of biradical 1 with the silica gel
surface.
The spectrum of biradical 1 adsorbed on the Al O
2
3
surface is more complicated. A source of complexity can
be a superposition of two spectra. One spectrum correꢀ
sponds to the complex of biradical 1 with the surface
3
+
coordinately unsaturated Al cations and manifests the
HFS from the 27Al nucleus (nuclei). The second specꢀ
trum belongs to the complex of biradical 1 with the hydrꢀ
oxyl groups of the surface or with weaker Lewis acid sites
3
3
+
2
0 G
4
(Al ). The difference between the spectrum of biradical 1
adsorbed on the Al O surface and that of the complex
2
3
with AlCl in solution indicates steric hindrance preventꢀ
3
ing bidentate coordination.
Fig. 2. ESR spectra of biradical
1
adsorbed on the
The results obtained showed that biradical 1 can be
used as a probe for studying the surface of solid oxide
catalysts and adsorbents. Since the nitroxyl groups are
linked through a flexible aliphatic bridge, the both groups
can form a complex with the acid sites upon adsorption
on the surface, which makes it possible to estimate the
SiO (1, 2) and Al O (3, 4) surface recorded at 293 (1, 3) and
2
2
3
1
20 K (2, 4).
occurs similarly, and both >NO groups can simultaneously
form complexes.
distance between these sites (in the case of SiO ). Howꢀ
The spectrum of biradical 1 adsorbed on the SiO2
surface (Fig. 2) indicates restricted rotational mobility of
the probe compared to the spectrum of free biradical 1 in
solution. This is related, most likely, to the interaction of
the >NO groups with the hydroxyl groups of the surface.
2
ever, for analysis of the magnetic resonance parameters of
the surface complexes of probe molecules with the acid
sites, the use of biradical 1 demonstrates no substantial
advantages over the TEMPO monoradical, because the
ESR spectrum of biradical 1 is broadened due to the diꢀ
poleꢀdipole interaction.
1
It is known that the interaction of TEMPO with the
hydroxyl groups of the SiO surface increases the HFC
2
constant with the 14N nucleus (A ) from 34.6 G (in a
N
|
|
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 06ꢀ03ꢀ
toluene solution) to 38 G. In the case of TEMPO, the
N
A|| value can be determined by measuring a half of the
3
2830ꢀa), the Federal Agency on Science and Innovaꢀ
tions (State Contract No. 02.451.11.7012 of August 29,
005), and the Council on Grants of the President of the
distance between the ultimate extremes in the spectrum
recorded at low temperature (usually 77 K). It could be
expected that complex formation with the silanol groups
2
N
Russian Federation (Program of State Support for Leadꢀ
ing Scientific Schools of the Russian Federation, Grant
RIꢀ112/001/056).
would increase the A|| value in the case of biradical 1. In
fact, the distance between the ultimate components in the
spectrum of the complex of biradical 1 on the SiO surꢀ
2
face (see Fig. 2, curve 2) is longer than that for free
biradical 1 in a toluene solution (see Fig. 1, curve 4).
Exact measurement of A|| is difficult because of dipoleꢀ
References
N
dipole splitting D that bifurcates the ultimate components
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1
2
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2
9
the data according to which the biꢀ and monoradicals are
solvated similarly.

Use of biradicals for studying acid sites
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5
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[
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in revised form November 14, 2006