Cyclotrimerization and linear oligomerization of phenylacetylene on the nickel(I) monocyclopentadienyl complex CpNi(PPh3)2

Article abstract of DOI:10.1134/S002315840706002X

The reactions of the CpNi(PPh₃)₂ monocyclopentadienyl complex with phenylacetylene and diphenylacetylene in toluene have been studied by ESR. When an alkyne is in twofold molar excess over nickel, it substitutes rapidly for PPh₃

Full text of DOI:10.1134/S002315840706002X

ISSN 0023-1584, Kinetics and Catalysis, 2007, Vol. 48, No. 6, pp. 778–784. © MAIK “Nauka /Interperiodica” (Russia), 2007.
Original Russian Text © V.V. Saraev, P.B. Kraikivskii, A.I. Vilms, S.N. Zelinskii, A.Yu. Yunda, E.N. Danilovtseva, A.S. Kuzakov, 2007, published in Kinetika i Kataliz, 2007, Vol. 48,
No. 6, pp. 834–840.
Cyclotrimerization and Linear Oligomerization
of Phenylacetylene on the Nickel(I) Monocyclopentadienyl
Complex CpNi(PPh3)2
V. V. Saraeva, P. B. Kraikivskiia, A. I. Vilmsa, S. N. Zelinskiia,b,
A. Yu. Yundaa, E. N. Danilovtsevab, and A. S. Kuzakova
a Irkutsk State University, Irkutsk, 664003 Russia
b Institute of Limnology, Siberian Branch, Russian Academy of Sciences, Irkutsk, 664033 Russia
e-mail: peter10@list.ru
Received October 26, 2006
Abstract—The reactions of the CpNi(PPh3)2 monocyclopentadienyl complex with phenylacetylene and diphe-
nylacetylene in toluene have been studied by ESR. When an alkyne is in twofold molar excess over nickel, it
substitutes rapidly for PPh3 ligands to form the bisalkyne π complex CpNi(η2-C2PhR)2, where R = H or Ph. In
the case of phenylacetylene, two structural isomers of the Ni(I) π complex have been identified. Irreversible
clustering occurs in the system as time passes. When phenylacetylene is in excess, it oligomerizes actively at
ambient temperature. The composition of the oligomerization products depends substantially on the reaction
temperature: at T = 20°C, the main product is 1,2,4-triphenylbenzene (97% of the conversion products); at T =
40°C, the main products are linear oligomers with an average molecular weight of 1050. The formation and
stabilization of active complexes in the system take place when the substrate is in excess. Phenylacetylene tri-
merization and linear oligomerization schemes in which the Ni(I) monocyclopentadienyl complex stabilized by
substrate molecules is the active species are suggested.
DOI: 10.1134/S002315840706002X
INTRODUCTION
the cyclotrimerization and linear oligomerization
of PhA.
Nickelocene NiCp2 in combination with organolith-
ium compounds (LiR) is an efficient catalyst of alkyne
oligomerization and polymerization [1–3]. The cyclo-
pentadienyl group is exchanged for an alkyl group dur-
ing catalyst formation to form the unstable 16-electron
complex {CpNiR} [4, 5], which is converted into the
highly reactive monocyclopentadienyl intermediate
{CpNi} by the cross-coupling [6, 7] or homolytic
decomposition of alkyl groups [8]. The {CpNi} inter-
mediate exists in solution in dimeric and polymeric
forms. A number of nickel clusters were isolated and
characterized [9]. Based on systematic studies [1, 2],
the {CpNiR} and {CpNiH} species stabilized by
alkyne molecules were assumed to be reactive in poly-
merization and cyclotrimerization, respectively.
EXPERIMENTAL
All procedures were carried out in purified argon
using Schlenk techniques. Glass Schlenk filters were
used in the filtration of precipitates. All prepared and
synthesized reagents were stored in sealed tubes under
argon.
Toluene, hexane, and tetrahydrofuran (Merck) were
distilled from sodium metal under argon prior to use.
Commercial triphenylphosphine (PPh3), triethyl
phosphite (P(OEt)3), and diphenylacetylene (Merck)
were used as received.
Phenylacetylene was purified according to a stan-
dard procedure [11] and was distilled in vacuo prior to
the experiment. Commercial diethylethoxyaluminum
Et2AlOEt was distilled in vacuo.
Previously, we demonstrated that catalyst formation
in the presence of stabilizing ligands (L = PPh3,
P(OEt)3, bipy/2, COD/2) affords Ni(I) mononuclear
complexes with the general formula CpNiL2 in the sys-
tem [10].
The NiCp2 and Ni(PPh3)4 complexes were synthe-
sized according to procedures described in [12, 13].
ESR spectra were recorded on a CMS-8400 spec-
In order to elucidate the role of the Ni(I) mononu- trometer (operating frequency of 9.6 GHz) at the nitro-
clear complexes in the nickelocene-based catalysts of gen boiling point. The diphenylpicrylhydrazyl free rad-
alkyne oligomerization, we studied the reactions of the ical (DPPH) and Mn2+ in MgO were used to calibrate
CpNi(PPh3)2 complex with phenylacetylene (PhA) and the scan ranges of the spectrometer. The ESR spectra
diphenylacetylene (DPhA) and its catalytic activity in were simulated according to our program [14] in which
778

CYCLOTRIMERIZATION AND LINEAR OLIGOMERIZATION OF PHENYLACETYLENE
779
3
1
2
3'
2'(Ä)
2'(B)
4
2'(3Ä + 1B)
290
325
360 290
Magnetic field, mT
325
360
2
Fig. 1. (1–4) Experimental and (2', 3') simulated ESR spectra of the Ni(I) complexes: (1) CpNi(PPh ) , (2) CpNi(η -C PhH) , (3)
3 2
2
2
2
–2
CpNi(η -C Ph ) , and (4) CpNi(P(OEt) ) . The solvent is toluene, C = 10 mol/l, and T = 77 K.
2
2 2
3 2
Ni
the hyperfine coupling (HFC) is truncated to second-
The molecular weight of the oligomers was deter-
order terms and the principal axes of the g tensor and mined by the isopiestic method in a toluene solution
HFC tensors coincide.
using azobenzene as the standard [15].
NMR spectra were recorded at 25°C on a VXR-
500S spectrometer (Varian).
RESULTS AND DISCUSSION
The CpNi(PPh3)2 complex, which was synthesized
by counter disproportionation between nickelocene and
the phosphine complex Ni(PPh3)4 in toluene,
Mass spectra were obtained on an Agilent 5973N-
6890 mass spectrometer with electron impact and
chemical ionization coupled with a gas chromatograph
(GC-MS). Mass spectra were identified using a data-
base from the NIST Library of Mass Spectra and Sub-
sets.
NiCp2 + Ni(PPh3)4
2CpNi(PPh3)2,
(I)
gives an intense ESR signal in which the hyperfine
structure (HFS) from two nonequivalent 31P nuclei is
well resolved at T = 77 K [10] (Fig. 1, curve 1). The
nonequivalence of the coordinated phosphine ligands is
due to the vibronic interaction in the Ni(I) complex,
whose ground state is pseudodegenerate.
After the addition of phenylacetylene to the solution
(PhA/Ni = 2), the initial ESR signal disappears imme-
diately and two overlapped signals appear (Fig. 1,
curves 2(Ä), 2(B)). Their ESR parameters are similar,
and no HFC from 31P is observed. The ratio between the
intensities of the new ESR signals (3 : 1) is independent
of the amount of phenylacetylene introduced into the
solution.
Catalytic reactions involving phenylacetylene were
carried out as follows. Toluene, a weighed sample of
the nickel complex, and the other reactants were suc-
cessively loaded into a temperature-controlled reactor
in an argon atmosphere. The concentration of the nickel
complex was 1 × 10–4 mol/l. Freshly distilled pheny-
lacetylene was added under vigorous stirring. ESR
spectra were recorded after one procedure or another,
depending on the purpose of the experiment. The
amount of reacted phenylacetylene was determined by
GLC on a GALS-311 gas chromatograph with a flame-
ionization detector (50-m-long capillary column, Api-
ezon). After the reaction, the phenylacetylene conver-
If diphenylacetylene is introduced into the solution
sion products were precipitated by adding excess etha- instead of phenylacetylene (DPhA/Ni = 2), the initial
nol, reprecipitated from toluene, and dried to constant signal disappears as well, but only one signal, without
weight in vacuo.
HFS (Fig. 1, curve 3), appears instead of the two sig-
KINETICS AND CATALYSIS Vol. 48 No. 6 2007

780
SARAEV et al.
ESR data for the CpNiL2 complexes
No.
1
Complex
gx
gy
gz
Ax, mT
Ay, mT
Az, mT Reference
CpNi(PPh3)2
2.030
2.047
2.119
12.4(P1)
11.6(P2)
12.0(P1)
11.4(P2)
10.9(P1)
9.35(P2)
[10]
2
CpNi(η2-C2PhH)2
Isomer A
2.040
2.033
2.035
2.032
2.061
2.061
2.061
2.032
2.221
2.206
2.206
2.093
–
–
–
Isomer B
CpNi(η2-C2Ph2)2
3
4
–
–
–
CpNi(P(OEt)3)2
20.1(P1)
22.0(P2)
20.1(P1)
22.0(P2)
18.9(P1)
20.5(P2)
[10]
nals. The experimental ESR parameters of the Ni(I) from the complex containing no phosphine ligands was
complexes refined by simulation of the ESR spectra are
listed in the table. The presence of two signals in the
case of phenylacetylene indicates that the new para-
magnetic complex, without phosphine ligands, can
exist as two isomers.
detected.
Let us consider the possible structures of the para-
magnetic nickel alkyne complexes using DPhA as an
example and known diamagnetic alkyne complexes of
Co(I) and Co(III) as analogues [16–29]. These struc-
tures are the bisdiphenylacetylene π complex CpNi(η2-
C2Ph2)2 (I), the metallocyclic σ complex CpNi(1,4-σ-
C4Ph4) (II), and the tetraphenylcyclobutadiene π com-
Attempts to detect the Ni(I) complex with one phos-
phine ligand as an intermediate product of consecutive
alkyne substitution for phosphine ligands by decreasing
the amount of alkyne introduced were unsuccessful:
only a mixture of signals from the initial complex and plex CpNi(η4-C4Ph4) (III):
Ph
Ph
Ph Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ni
Ni
Ni
I
II
III
According to quantum chemical calculations, the
thermodynamic stability of the analogous Co com-
plexes increases in the order I < II < III [30]. However,
structure III seems improbable for the paramagnetic Ni
complex because the ESR parameters of the DPhA
complex do not coincide with the data reported for the
intermediate complex detected in the chemical reduc-
tion of the cationic complex [CpNi(η4-C4Ph4)]PF6 with
sodium amalgam in THF (g1 = 2.143, g2 = 2.035, g3 =
1.993) [31]. Structure II also seems improbable
because mononuclear metallocyclic Co(III) complexes
are stabilized only as CpCo(PPh3)(1,4-σ-C4R4) phos-
phine complexes [17, 20–23, 25, 27]. The known met-
allocyclic Ni(III) complexes involving no stabilizing
ligands exist only in dinuclear or polynuclear form [9,
32, 33]. Therefore, the bisdiphenylacetylene π complex
CpNi(η2-C2Ph2)2 (I) seems to be the most likely struc-
ture. In the case of phenylacetylene, the presence of two
isomers is likely due to the existence of the π complex
CpNi(η2-C2PhH)2 as cis (IÄ) and trans (IB) isomers:
Ph Ph
Ni
Ph H
Ni
H
H
H
Ph
IA
IB
Further investigation is necessary to assign the ESR
signals to particular isomers of the π complex. The sig-
nal with the largest g tensor components is convention-
ally assigned to the isomer A.
The formation of the bisphenylacetylene π complex
can be represented as follows:
CpNi(PPh3)2 + PhC≡CH
(II)
CpNi(PPh3)(η2-C2PhH) + PPh3,
2CpNi(PPh3)(η2-C2PhH)
CpNi(η2-C2PhH)2 + CpNi(PPh3) .
(III)
2
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CYCLOTRIMERIZATION AND LINEAR OLIGOMERIZATION OF PHENYLACETYLENE
781
Integral intensity, arb. units
According to experimental data, the unstable mixed-
ligand complex CpNi(PPh3)(η2-C2PhH), which results
from the substitution of PhA for coordinated PPh3, dis-
proportionates rapidly into a bisphenylacetylene
π complex and the initial Ni(I) bistriphenylphosphine
complex.
25000
20000
15000
10000
5000
The ESR signals from the Ni(I) phenylacetylene
complex decrease in time (Fig. 2) and disappear com-
pletely in 30 min. In the case of diphenylacetylene, the
ESR signal intensity changes so slowly that the signal
is still detected after 24 h.
The noticeable decrease in the intensity of the ESR
signals in the case of phenylacetylene likely arises both
from the clustering of the Ni(I) complexes, which is
characteristic of Ni(I) monocyclopentadienyl com-
plexes, and from the change in the oxidation state of the
transition metal due to the oxidative addition of pheny-
lacetylene and metallocycle formation. For instance, a
number of di-, tri-, and tetranuclear nickel clusters were
described in the literature. They were isolated from a
0
6
12
18
24
30
Time, min
Fig. 2. Time variation of the intensity of the ESR signals from
the CpNi(η -C PhH) complex (PhA/Ni = 2, T = 20°C).
2
2
2
reaction mixture prepared from nickelocene NiCp2, H3CC≡CCH3 in tetrahydrofuran and were character-
methyllithium
LiCH3,
and
dimethylacetylene ized by X-ray diffraction [9, 32, 33]:
CpNi
CpNi
CH3
C
H3C
CH3
CpNi
CH
Ni
3
C C
Ni
C
C
CpNi
NiCp
CpNi
Ni
H3CC CCH3
CpNi
NiCp
C
Ni
Cp
Ni
Ni
CH
3
Cp
Cp
CH3
Cp
It is noteworthy that the processes that occur in the For example, at T = 20°C, the major reaction product is
system considered are irreversible, as is indicated by 1,2,4-triphenylbenzene (97% of the conversion prod-
the fact that no ESR signals are detected after triethyl ucts), whereas the major reaction products at T = 40°C
phosphite, a reactive ligand, is introduced into the are linear oligomers with an average molecular weight
system (P(OEt)3/Ni = 3) at the end of the experiment. of 1050 (9–10 PhA molecules). The product of phenyl-
If P(OEt)3 is introduced into a system in which the acetylene cyclotrimerization was identified by mass
1
ESR signals from the phenylacetylene complex are spectrometry and NMR: H NMR (CDCl3): δ 7.84,
still detected, the signals immediately transform into 7.77, 7.49, 7.38 ppm; 13C NMR (CDCl3): δ 141.5,
a stable signal from the CpNi(P(OEt)3)2 complex 141.1, 140.9, 140.5, 140.3, 139.5 (quaternary carbon
(Fig. 1, curve 4), whose ESR spectrum was described atoms), 129–126 ppm (CH atoms). These NMR data
in detail [10].
are identical to the data reported for 1,2,4-triphenylben-
zene [34].
If excess phenylacetylene is added to the
CpNi(PPh3)2 complex (PhA/Ni = 200) in toluene, the
active oligomerization of the alkyne begins after a short
induction period. The PhA consumption curves at three
reaction temperatures (20, 30, and 40°C) are shown in
Fig. 3. The maximum PhA conversion is 77% (at T =
30°C).
The linear oligomerization products isolated from
the reaction medium give a weak symmetric ESR signal
characteristic of organic semiconductors with a devel-
oped polyconjugated system [35]. The NMR spectra of
the linear products confirm the presence of conjugated
double bonds in the oligomer chain (13C NMR
(CDCl3): δ 151–148.5, 138–132, 109–110 ppm).
The intensity of the ESR signals from the CpNi(η2-
C2PhH)2 complex decreases rapidly during the induc-
tion period preceding the active stage of phenylacety-
Note that, taken separately, the starting Ni(II) and
Ni(0) complexes used in the synthesis of CpNi(PPh3)2
via reaction (I) exhibit no activity in phenylacetylene
oligomerization under the conditions examined.
The composition of the PhA oligomerization prod- lene oligomerization, so the oligomerization process
ucts depends substantially on the reaction temperature. occurs in the absence of any ESR signals. However, if
KINETICS AND CATALYSIS Vol. 48 No. 6 2007

782
SARAEV et al.
the catalytic poison triethyl phosphite is introduced into
the system (P(OEt)3/Ni = 3) during oligomerization, the
reaction ceases immediately and an intense signal from
CpNi(P(OEt)3)2 appears. If triethyl phosphite is intro-
duced at the end of the oligomerization reaction, no
ESR signals appear. Therefore, during oligomerization,
the nickel present in the system is either in the Ni(I)
state (e.g., as dinuclear structures) or in the high-spin
state Ni(III) (e.g., as tetrahedral structures character-
ized by short spin–lattice relaxation times, which
readily turn into mononuclear Ni(I) complexes upon
the substitution of triethyl phosphite for coordinated
phenylacetylene). Since the known schemes of alkyne
Since the complete substitution of phenylacetylene
for PPh3 in the CpNi(PPh3)2 complex occurs already at
a twofold molar excess of PhA over nickel, the role of
PPh3 in PhA oligomerization on CpNi(PPh3)2 is negli-
gible under the conditions examined. It follows from
this conclusion that the behavior of the individual
CpNi(PPh3)2 complex in excess phenylacetylene is
equivalent to the behavior of the Ni(I) monocyclopen-
tadienyl complexes in the catalytic system NiCp2/LiR
in phenylacetylene oligomerization.
Based on systematic studies of the oligomerization
trimerization on various transition metal complexes of a wide variety of alkynes on the catalytic system
include metallocyclopentadiene formation steps [36–
39], it seems most likely that most of the nickel is in the
Ni(III) state during phenylacetylene oligomerization.
NiCp2/LiR, it was assumed that the Ni(II) complexes
{CpNiR} and {CpNiH} stabilized by alkyne molecules
are active in the polymerization and cyclotrimerization,
respectively, of the monomers [1, 2]. In our system,
which is based on the Ni(I) monocyclopentadienyl
complex CpNi(PPh3)2, the formation of the {CpNiR}
and {CpNiH} species is not so evident as in the
NiCp2/LiR catalytic system. Therefore, although we do
not rule out the participation of these Ni(II) complexes
in phenylacetylene oligomerization, we suggest an
alternative mechanism involving the Ni(I) and Ni(III)
monocyclopentadienyl complexes (scheme), which is
in best agreement with experimental data:
Note that a mixture of CpNi(PPh3)2 with a twofold
excess of PhA held for 0.5 h in toluene exhibits no oli-
gomerization activity after excess phenylacetylene is
added. Therefore, the formation and, what is most
important, stabilization of the active complexes in the
system take place when phenylacetylene is in excess, as
was noted in earlier publications [1, 2]. The lower limit
of the substrate excess necessary to maintain the nickel
complexes in their active state is 50–60 molar parts, as
was estimated from the amount of unreacted phenyl-
acetylene (Fig. 3).
Ph
Ph
Ph
Ni
Ph
Ni H
Ph
H
H
H
Ph
Ph
Ni
H
Ä
B
Ph
Ph
(H)Ph
Ph(H)
Ph
Ph
Ni
Ni H
Ph
Ph
H
H
Ph
Ph
Ph
(H)Ph
H
Ph(H)
Ph
Scheme.
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CYCLOTRIMERIZATION AND LINEAR OLIGOMERIZATION OF PHENYLACETYLENE
783
Amount of phenylacetylene in system,
ACKNOWLEDGMENTS
mol PhA/mol Ni
This work was supported by the President of the
Russian Federation Grants Council (grant no. MK-
2066.2005.3) and RFBR/CRDF (USA) (grant no. 07-
03-91123).
250
200
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