Paramagnetic nickel(I) complexes and their role in the catalytic dimerization of norbornadiene

Article abstract of DOI:10.1134/S0023158408040046

The kinetics of the formation of a paramagnetic nickel(I) complex from bis(η³-allyl)nickel under conditions of catalytic norbornadiene dimerization is reported. It is demonstrated by ESR and GLC that the concentrations of Ni(I), norbornadiene and its pentacyclic dimers change in the same way. It might be inferred from this finding that Ni(I) is involved in the catalytic process as an intermediate. However, experiments on model systems have not confirmed this assumption. At the same time, they have not ruled out the participation of the paramagnetic complex in side catalytic reactions. The presence of Ni(I) in the reaction system is connected with the presence of free norbornadiene there. Hypotheses as to the probable structure and formation mechanism of the paramagnetic Ni(I) are suggested.

Full text of DOI:10.1134/S0023158408040046

ISSN 0023-1584, Kinetics and Catalysis, 2008, Vol. 49, No. 4, pp. 479–483. © MAIK “Nauka /Interperiodica” (Russia), 2008.
Original Russian Text © Ya.Ya. Otman, O.S. Manulik, V.R. Flid, 2008, published in Kinetika i Kataliz, 2008, Vol. 49, No. 4, pp. 502–506.
Paramagnetic Nickel(I) Complexes and Their Role
in the Catalytic Dimerization of Norbornadiene
Ya. Ya. Otman, O. S. Manulik, and V. R. Flid
Moscow State Academy of Fine Chemical Technology, Moscow, 119571 Russia
e-mail: vitaly-flid@yandex.ru
Received November 9, 2007
Abstract—The kinetics of the formation of a paramagnetic nickel(I) complex from bis(η3-allyl)nickel under
conditions of catalytic norbornadiene dimerization is reported. It is demonstrated by ESR and GLC that the con-
centrations of Ni(I), norbornadiene and its pentacyclic dimers change in the same way. It might be inferred from
this finding that Ni(I) is involved in the catalytic process as an intermediate. However, experiments on model
systems have not confirmed this assumption. At the same time, they have not ruled out the participation of the
paramagnetic complex in side catalytic reactions. The presence of Ni(I) in the reaction system is connected with
the presence of free norbornadiene there. Hypotheses as to the probable structure and formation mechanism of
the paramagnetic Ni(I) are suggested.
DOI: 10.1134/S0023158408040046
Due to the steady progress in physical methods of
analysis, it is increasingly becoming possible to detect
and control the conversion of intermediates directly
during the catalytic reaction. However, elucidating the
role of intermediate complexes in catalysis often poses
serious difficulties. For example, in a number of pro-
cesses catalyzed by nickel compounds, such as the
dimerization, oligomerization, and polymerization of
alkenes, dienes, and alkynes, paramagnetic Ni(I) com-
plex species were detected by ESR [1–8]. It was sug-
gested that these compounds are typical of nickel-com-
plex catalysts and that both paramagnetic and diamag-
netic alkyl or hydride nickel complexes stabilized by
the substrate can be the reactive species in the above
reactions [9, 10]. Nevertheless, the available data do not
suggest any unambiguous conclusion regarding the
participation of the paramagnetic species in the cata-
lytic reaction.
Nickel tetracarbonyl was obtained from a nickel
chloride solution in methanol at the atmospheric pres-
sure of CO. The reducing agent was an iron metal
powder. The reaction was performed for 12 h at 20°ë
in the presence of small amounts of Na2S. Ni(ëé)4
yield: 72%.
Nickel metal was evaporated onto a frozen norbor-
nadiene (NBD) matrix (77 K) using a rotary vacuum
setup with electrodes mounted inside. Nickel evapora-
tion was carried out at 0.001 Pa. After the completion
of the process, the temperature was raised to –5°ë and
the dark red solution was poured out into the reactor to
be used in kinetic studies.
All manipulations involved in the synthesis and
storage of the complexes and the transfer of the com-
plexes into the reactor were carried out in vacuo
(0.1 Pa) or in dry, thoroughly purified argon. The com-
plexes were stored in vacuumized tubes at 77 K or were
synthesized immediately before the experiment.
The starting nickel compounds—bis(η3-allyl)nickel
and nickel tetracarbonyl—were synthesized by stan-
dard procedures [11, 12].
Norbornadiene (Aldrich, 99%) was distilled from
sodium metal before use.
Toluene, benzene, and meta-xylene (special purity
grade) were distilled from sodium metal in the presence
of benzophenone before use.
Bis(η3-allyl)nickel was obtained by reacting anhy-
drous NiBr2 with C3H5MgCl in diethyl ether
(NiBr2/C3H5MgCl = 1 : 2 mol/mol) at –5 to –10°ë in
vacuo (0.1 Pa) or in dry, thoroughly purified argon. All
chemicals were purified before experiments. The yel-
lowish orange reaction solution was concentrated
using a low-temperature Vigreaux column, and the
product was cooled to –80°ë. Bis(η3-allyl)nickel
crystallized as yellow needles. The complex was sub-
limed in vacuo and was recrystallized from n-pentane
at –80°ë. Yield: 63%.
Kinetic experiments were carried out in tempera-
ture-controlled pumped 25-ml reactors fitted with a
sampler. Kinetic data were processed using the DynaFit
program (version 3.28.024). Analysis of the reaction
mixture and reaction monitoring were carried out by
gas chromatography (Kristall-2000 chromatograph
with DB-35, SPB-20, and Supelco β-Dex capillary col-
umns; an Al2O3-packed column; a flame ionization
detector; and a thermal conductivity detector).
479

480
OTMAN et al.
(‡)
(b)
10 mT
Fig. 1. ESR spectra of the Ni(I) complex resulting from the interaction between Ni(ë ç ) and norbornadiene: (a) T = 298 K (g =
3
5 2
2.066); (b) T = 77 K (g = 2.002, g = 2.021, g = 2.166). The solvent is meta-xylene.
1
2
3
ESR studies of solutions were carried out in stan-
dard quartz tubes (Bruker) fitted with a vacuum valve.
Norbornadiene and meta-xylene were place in a tube
(total volume of 0.30 ml), and bis(η3-allyl)nickel
(0.005–0.05 g) was transferred into this tube by recon-
densation. ESR spectra were recorded on an ER-200
spectrometer (Bruker) operating at 9.7 Hz. A capillary
with a crystalline standard (2,2-diphenyl-1-picrylhy-
drazyl, DPPH) was in the resonator of the spectrometer
throughout the experiment. The signal from DPPH
served as a g-factor reference and was used in the mea-
surements of the relative concentration of paramagnetic
centers in the samples.
2
I
II
(I)
+
+
III
IV
The shape and magnetic parameters of the ESR
spectrum (Fig. 1) and a comparison of this spectrum
with literature data [17–20] indicate that both the iso-
tropic and anisotropic components are due to nickel(I)
with the electronic configuration 3d9 and the spin
moment 1/2. The formation of the paramagnetic
nickel(I) complex was detected for the first time in a
system free of common stabilizing ligands and Brøn-
sted or Lewis acids [21, 22]. Moreover, upon the intro-
duction of such compounds (phosphines, phosphites,
and amines of various structures), the paramagnetic
signals disappear. It is difficult to carry out a systematic
study of the nickel(I) complex because of its extremely
low concentration in the reaction solution (the fraction
of Ni(I) is 10–4–10–6 of the total nickel content of the
system).
Reaction products were identified by GC/MS and
NMR spectroscopy. Mass spectra were recorded using
an Agilent 6890 GC instrument with an MSD detector
(5973 N) at an ionization energy of 50 eV. The ioniza-
tion current was 100 mA, and the source temperature
was 150°ë. NMR spectra were recorded on a Bruker
DPX-300 spectrometer (1H, 300 MHz, CDCl3 or
C6D6; 13C, 70 MHz, CDCl3). The norbornadiene dimer
structure was identified with reference to the literature
[13–15].
Elemental and quantitative analyses were carried
out using a Hitachi S-800 electron microscope with an
energy-dispersive LZ-5 detector (Oxford Instruments).
The accuracy of weight fraction determination was at
least 0.01%.
The structure of the allyl ligand (η3-allyl, 2-methyl-
η3-allyl, or 1-methyl-η3-allyl) has no effect on the ESR
spectrum of the Ni(I) complex. This indicates that the
unpaired electron is almost completely localized on the
metal atom.
Since the Ni(I) complex is in a system in which a
catalytic reaction takes place, we consider it necessary
to study the formation kinetics of the paramagnetic spe-
cies.
An analysis of the material balance and kinetic data
indicates that a stoichiometric reaction between Ni(all)2
and several norbornadiene molecules occurs initially,
accompanied by the replacement of allyl ligands in the
RESULTS AND DISCUSSION
In the catalytic homodimerization of norbornadiene
(reaction (I)) using the η3-allyl nickel complexes Ni(η3-
‡ll)2 (η3-all = C3H5, 1-ëç3ë3ç4, 2-ëç3ë3ç4) as cata-
lyst precursors, we detected the formation of a para-
magnetic nickel(I) complex [16].
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PARAMAGNETIC NICKEL(I) COMPLEXES AND THEIR ROLE
481
6
5
4
3
2
1
4
2
3
1
2
3
0
20
40
60
80 200 600 1000 1400 1800
Time, min
Fig. 2. Variation of the concentrations of norbornadiene, its dimers, and the paramagnetic nickel complex during the kinetic run at
333 K: (1) norbornadiene, (2) pentacyclic dimers of norbornadiene, (3) Ni(I) without addition of Ni(C H ) ([Ni(C H ) ] =
7
8 2
3 5 2
0.050 mol/l), and (4) Ni(I) upon addition of Ni(C H ) ([Ni(C H ) ] = [Ni(C H ) ] = 0.025 mol/l).
7
8 2
3
5 2
7 8 2
complex (stoichiometric norbornadiene allylation) [23, (Fig. 2). In addition, the induction period of catalytic
24]. At 25°ë, the reaction is almost complete in a few norbornadiene dimerization is considerably shorter.
minutes.
Symbatic changes in the concentrations of paramag-
netic nickel, norbornadiene, and its pentacyclic dimers
The curves plotted in Fig. 2 show how the concen-
(II–IV) are also typical of the subsequent stages of the
reaction.
trations of norbornadiene, its dimers, and the paramag-
netic Ni(I) complex vary during the reaction. These
data were obtained by GC and ESR. The reaction time
The steady-state portion of curve 3 in Fig. 2 corre-
was counted from the instant bis(η3-allyl)nickel was sponds to the catalytic process. The Ni(I) concentration
added to the norbornadiene solution. The paramagnetic decreases during this process. The Ni(I) disappearance
nickel complex concentration curve is S-shaped. The rate is much higher when the reaction is conducted in
formation of the complex takes place during the induc- the liquid phase (in the presence of a solvent) than
tion period of norbornadiene cyclodimerization, the when the reaction is conducted in the norbornadiene
main catalytic process. After the induction period, the bulk (dimer II, which is the main product under these
Ni(I) concentration reaches a steady-state level and conditions, is a solid compound). When the solvent and
almost does not change while there is free norborna- the unreacted norbornadiene are removed during the
diene in the reaction system. At the end of the induction reaction, the paramagnetic complex incorporated into
period, Ni(η3-all)2 in the system is almost completely structure II is stable in the solid state at room tempera-
absent (according to GC data, ~100% material balance ture in the absence of oxygen over several days.
is achieved for the products containing allyl groups). In
The symbasis of the kinetic curves characterizing
spite of this, it is not impossible that the Ni(C3H5)2
the changes of the concentrations of norbornadiene, its
complex is present in large excess relative to Ni(I).
dimers, and the paramagnetic nickel complex during
the reaction suggests that the complex can be an inter-
Our earlier detailed investigation of the formation of
mediate in the catalytic dimerization of norbornadiene.
the true catalyst of norbornadiene cyclodimerization
indicated the formation of the complex Ni(NBD)2. The
Obviously, the nickel (I) complex includes one or
preparation and isolation of this complex is described several norbornadiene molecules, which act as stabiliz-
in [25, 26]. The simultaneous presence of Ni(NBD)2 ers. The dependence of the relative amount of paramag-
and Ni(all)2 in the system is a necessary condition for netic species on the allyl structure in the initial complex
Ni(I) formation. This conclusion was drawn from the indicates that the formation of Ni(I) may involve the
following experiment. Upon mixing of separately pre- allyl ligand and its relative concentration increases as
pared solutions of these complexes in meta-xylene, the the positive charge on the Ni atom decreases [27]. For
Ni(I) formation curve shows no induction period example, in the reactions of norbornadiene with homo-
KINETICS AND CATALYSIS Vol. 49 No. 4 2008

482
OTMAN et al.
ligand η3-allyl nickel complexes, namely, Ni(ë3ç5)2, sor. No paramagnetic species in were detected in this
Ni(1-ëç3ë3ç4)2, and Ni(2-ëç3ë3ç4)2, under identi- reaction system.
cal conditions, the steady-state relative concentrations
of paramagnetic nickel differ substantially. At 298 K,
these concentrations are, respectively, 9 × 10–5, 3 ×
10−4, and 1 × 10–6 of the total nickel concentration. As
was shown above, when norbornadiene is in excess,
each of the three bis-η3-allyl nickel complexes is
almost quantitatively converted into the same norbor-
nadiene complex Ni(C7H8)2. It is natural that the kinet-
ics of the catalytic cyclodimerization of norbornadiene
is absolutely identical for precursors with different allyl
ligands.
In the second case, the initial complex was nickel
tetracarbonyl, Ni(CO)4. As the reaction solution con-
sisting of norbornadiene, the solvent, and a catalytic
amount of Ni(CO)4 was heated to 353 K, the complete
removal of CO from the coordination sphere of the
metal and Ni(NBD)2 formation took place. The solution
was cooled to room temperature, and the catalytic reac-
tion that occurred in the system was investigated by
ESR. As in the first case, no paramagnetic species were
detected in the system. The catalytic activity of the sys-
tem was comparable with the activity of the Ni(all)2-
based system. Evidently, the catalytic reaction (I) does
not involve the paramagnetic Ni(I) complex.
The nickel(I) complex may have structure V, and a
possible way of its formation is the counter dispropor-
tionation reaction between Ni(all)2 and Ni(NBD)2:
The results of this study do not allow us to make a
substantiated conclusion as to the structure of this com-
plex. It is possible to claim, however, that, if the para-
magnetic Ni(I) complex contains an allyl fragment, its
participation in the catalytic cycle of the main reaction
is quite unlikely. If the Ni(I) complex has a hydride
nature, it can participate in the catalytic process as an
intermediate, but the contribution from the hydride
component to the overall norbornadiene cyclodimeriza-
tion mechanism will be insignificant. Apparently, Ni(0)
and Ni(I) compounds catalyze different processes, and
a detailed structural study of all products of norborna-
diene homodimerization, including the products form-
ing in minor amounts, is required to understand the role
of the paramagnetic nickel complex in catalysis. At the
same time, it is beyond question that the formation of
the Ni(I) complexes under the conditions examined is
due to the unique ability of norbornadiene to stabilize
unstable compounds in valent states unnatural for them.
Ni +
Ni
(II)
Ni
Ni
2
Ni
V
The formation of Ni(I) only via the reaction between
Ni(NBD)2 and Ni(all)2 seems unlikely because of the
high reactivity of the latter.
The paramagnetic Ni(I) complex may also be of
hydride nature (VI) and be formed, for example, from
V according to the reaction
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research, grant nos. 06-03-33091 and 08-03-
00743.
(C3H5)Ni(C7H8) + nC7H8
(III)
H–Ni(C7H8)n + C10H12.
VI
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