Oligomerization and polymerization of alkynes catalyzed by rhodium(I) pyrazolate complexes

Article abstract of DOI:10.1016/S1381-1169(03)00315-7

Three new Rh(I) derivatives containing the dcmpz ligand (Hdcmpz: 3,5-dicarbomethoxypyrazole) have been prepared and employed as catalysts or catalyst precursors in the cyclotrimerization of terminal and internal alkynes and in the polymerization of ethyne

Full text of DOI:10.1016/S1381-1169(03)00315-7

Journal of Molecular Catalysis A: Chemical 204–205 (2003) 333–340
Oligomerization and polymerization of alkynes catalyzed by
rhodium(I) pyrazolate complexes
G. Attilio Ardizzoiaa,∗, Stefano Brennaa, Sergio Ceninib, Girolamo LaMonicaa,
Norberto Masciocchia, Angelo Masperoa
Dipartimento di Scienze Chimiche, Fisiche e Matematiche, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy
a
b
Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Università di Milano, via Venezian 21, 20133 Milano, Italy
Received 5 September 2002; received in revised form 11 February 2003; accepted 15 February 2003
Dedicated to Professor Renato Ugo on the occasion of his 65th birthday
Abstract
Three new Rh(I) derivatives containing the dcmpz ligand (Hdcmpz: 3,5-dicarbomethoxypyrazole) have been prepared and
employed as catalysts or catalyst precursors in the cyclotrimerization of terminal and internal alkynes and in the polymerization
of ethyne to polyacetylene.
© 2003 Elsevier Science B.V. All rights reserved.
Keywords: Rhodium; Alkynes; Cyclotrimerization; Polymerization; Pyrazolate complexes
1. Introduction
ing of the reaction paths by permitting the isolation of
some possible intermediates [11–13].
Several studies have been carried out in the last
years in order to test the catalytic activity of some
pyrazolate-bridged complexes in various reactions.
These studies concerned the discovery of new catalytic
systems as well as the elucidation of the function of the
complexes during the catalytic cycle. In the catalytic
mechanism involving group VIII metals, oxidative
addition and reductive elimination have a predomi-
nant role. As they usually cause a dramatic distortion
in the coordination geometry of the metallic center,
we decided to use a particular ligand, 3,5-pyrazoledi-
carboxylic acid dimethyl ester (Hdcmpz), which, as
demonstrated by earlier studies performed on pyrazo-
late copper(I) and copper(II) complexes [14], can co-
ordinate to a metal with the ester carbonyl oxygen of
COOMe substituents, thus affording an extra-stabili-
zation of the complexes. In the present work, we de-
scribe the capability of the new pyrazolato rhodium(I)
Eighty-two years after the discovery by Berthelot
in 1866 that acetylene thermally trimerizes to ben-
zene in low yield at high temperature (>400 ◦C) [1],
Reppe and Sweckendiek reported that this reaction is
catalyzed in solution near room temperature by nickel
complexes [2]. Since then, alkyne cyclotrimerization
has become one of the most intensely studied synthet-
ically useful transformations [3–7]. This reaction has
long been known to be catalyzed by a wide range of
organo-transition metal compounds both in heteroge-
neous and homogeneous phase [8–10]. Organo-cobalt
and rhodium derivatives are among the most widely
used catalysts and have contributed to the understand-
∗
Corresponding author. Tel.: +39-031-2386440;
fax: +39-031-2386119.
E-mail address: attilio.ardizzoia@uninsubria.it (G.A. Ardizzoia).
1381-1169/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1381-1169(03)00315-7

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G.A. Ardizzoia et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 333–340
complexes [Rh(dcmpz)(PPh3)(C2H4)]2 and [Rh(dcm-
pz)(PPh3)3] to act as catalyst precursors for the cy-
clotrimerization of terminal and internal alkynes.
(1.97 mmol) were added and gas evolution was noted.
After 1 h stirring, the precipitate was filtered, washed
with diethyl ether and dried in vacuum (89% yield).
The compound was kept under nitrogen atmosphere
at 0 ◦C. Analysis: Found C 56.69, H 4.80, N 4.41%;
Calculated for C54H52N4O8P2Rh2 C 56.25, H 4.51, N
4.86%.
2. Experimental
2.1. General procedures
2.4. Synthesis of [Rh(dcmpz)(PPh3)3], 3
All solvents used in the reactions were purified
according to the standard procedures and were kept
under nitrogen atmosphere. If not otherwise spec-
ified, all reactions were performed under an inert
atmosphere of dry nitrogen. [Rh(C2H4)2Cl]2 and
3,5-dicarbomethoxypyrazole were prepared accord-
ing to literature procedures [15,16]. The alkynes
employed in catalytic reactions were taken from new
bottles (Sigma–Aldrich) kept at −25 ◦C and their
purity grade was confirmed by GC–MS analysis. El-
emental analysis (C, H, N) were carried out at The
Microanalytical Laboratory of the University of Mi-
lan. IR spectra were recorded on a Bio-Rad FTS 7,
1H-NMR and 31P-NMR were recorded on a Bruker
400 Avance. Quantitative analysis of products were
performed on a Shimadzu GC-17A gas chromato-
graph fitted with a 30 m (0.25 mm) capillary column
coupled with a Shimadzu MS QP5000 instrument.
Naphtalene was employed as internal standard.
To a solution of 0.300 g (0.438 mmol) of [Rh-
(dcmpz)(C2H4)2]2, 1, in 5 ml of toluene, 1.148 g of
PPh3 (4.38 mmol) were added, causing rapid evolution
of gas. After 2 h stirring, the suspension was filtered
off and the precipitate washed with diethyl ether and
dried in vacuum. The compound was kept under ni-
trogen at 0 ◦C. (93% yield). Analysis: Found C 68.87,
H 5.14, N 2.62%; Calculated for C61H52N2O4P3Rh
C 68.28, H 4.85, N 2.61%.
2.5. Alkynes dimerization and cyclotrimerization
In a typical experiment, 10 mg of complex (2 or 3)
were dissolved in 6 ml of degassed toluene obtaining a
clear solution. The appropriate alkyne was then added
under stirring in a unique step (Rh/alkyne, 1:100 molar
ratio). The evolution of the reaction was monitored by
GC–MS analysis.
2.2. Synthesis of [Rh(dcmpz)(C2H4)2]2, 1
2.6. Acetylene polymerization catalyzed by 2
To a suspension of [RhCl(C2H4)2]2 (0.500 g,
1.28 mmol) in 10 ml of degassed methanol, 0.590 g of
Hdcmpz (3.21 mmol) and 0.200 ml of triethylamine
were added. The suspension immediately turned
red and was kept at room temperature for 1 h and
then filtered off. The precipitate was washed with
methanol and dried in vacuum (78% yield). Analy-
sis: Found C 38.79, H 4.44, N 8.13%; Calculated for
C22H30N4O8Rh2 C 38.60, H 4.38, N 8.19%.
0.050 g of 2 (0.087 mmol Rh) were dissolved in
15 ml of acetonitrile under nitrogen. The solution was
thermostated at 60 ◦C and then acetylene was bubbled
through the solution at a constant rate (about one bub-
ble every 2 s). After 5 h, the system was purged with
dinitrogen and 0.046 g of PPh3 (0.175 mmol) were
added. The suspension was then filtered under nitrogen
and the black solid washed with acetonitrile and dried
under vacuum giving 2.7 g of cis/trans-polyacetylene
mixture. The solid was kept at 0 ◦C under nitrogen at-
mosphere.
Crystals suitable for X-ray structural determination
were obtained by slow diffusion of n-hexane through
a toluene solution of the complex.
2.7. Cis/trans isomerization of polyacetylene
2.3. Synthesis of [Rh(dcmpz)(PPh3)(C2H4)]2, 2
In a Schlenk tube, 1 g of the previously synthe-
sized polymeric mixture was heated under nitrogen at
140 ◦C for 1 h by means of an oil-bath. The resulting
0.450 g of 1 (0.658 mmol) were dissolved in 5 ml of
degassed toluene. To the red solution, 0.517 g of PPh3

G.A. Ardizzoia et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 333–340
335
derivative was shown to be pure trans-polyacetylene
(IR evidences, see text).
wR2: 0.038 and 0.056 for 4736 data (Rint = 0.024)
collected in the 2 < θ < 26◦ range; GoF = 1.00. Fur-
ther crystallographic data and final fractional atomic
coordinates (excluding structure factors) for 1 have
been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication number
CCDC 192388. Copies of the data can be obtained free
of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44-1223-336-033;
e-mail: deposit@ccdc.cam.ac.uk).
3. Results and discussion
3.1. Synthesis
When a suspension of the dinuclear [RhCl(C2H4)2]2
in methanol was treated with a small excess of
Hdcmpz in the presence of triethylamine, a red
precipitate was formed, which, on the basis of ele-
Complex 1 belongs to a series of pyrazolato-bridged
dirhodium complexes of general formula [Rh(pz∗)L2]
(Hpz∗: pyrazole, 3,5-dimethylpyrazole; L: ␲-acid lig-
ands) originally prepared by Usón et al. [17]. The
IR spectrum of 1 (nujol mull) shows two strong
1
mental analysis, IR and H-NMR spectroscopy, was
formulated as [Rh(dcmpz)(C2H4)2]2, 1 (Hdcmpz:
3,5-dicarbomethoxypyrazole), as later confirmed by
a X-ray single crystal diffraction (Fig. 1) (Crystal
absorptions, respectively, at 1737 and 1706 cm−1
,
data for 1: C22H40N4O8Rh2, fw = 684.32 g mol−1
,
due to the ν(CO) of the COOMe substituents
on the pyrazole ring. In the 1H-NMR spectrum
(CD2Cl2, RT), a doublet centered at 3.21 ppm was
assigned to olefinic protons, while signals at 3.82
and 6.79 ppm were associated to COOCH3 and the
C(4)–H proton on 3,5-dicarbomethoxy pyrazole, res-
pectively.
monoclinic, P21/n, a = 12.348(2), b = 18.297(4),
c = 12.834(2) Å, β = 115.22(1); V = 2623.2(8)
Å3, Z = 4; ρc = 1.733 g cm−3; µ = 1.31 mm−1
;
F(0 0 0) = 1376; 20256 reflections collected using
graphite-monochromatized Mo K␣ radiation on a
SMART Bruker automated diffractometer; R1 and
Fig. 1. ORTEP drawing of the [Rh(dcmpz)(C2H4)2]2, 1, molecule. Thermal ellipsoids are drawn at the 30% probability level, hydrogen
atoms of arbitrary size. Dashed bonds connect Rh–C atoms.

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Scheme 1.
3.2. Reactions of [Rh(dcmpz)(C2H4)2]2 with PPh3
ands coupled with the metal centers [1J (103Rh-31P)
= 172 Hz]. As 31P-NMR does not permit to discrim-
inate between the two possible cis/trans (related to
the binuclear core) isomers of complex 2, we ten-
tatively assign to 2 the trans geometry, in analogy
with the geometry of similar derivatives reported in
the literature [18], and taking into account the lesser
steric demand of the trans derivative with respect the
cis one, rendering the former the thermodynamically
favorite species.
A toluene solution of complex 1 treated with PPh3
afforded two different species, depending on the
Rh/PPh3 ratio (Scheme 1). When a 1:1.5 Rh/PPh3
ratio was used, an orange precipitate, formulated,
on the basis of elemental analysis and spectroscopic
data, as [Rh(dcmpz)(PPh3)(C2H4)]2, 2, was formed.
The IR spectrum of 2 (nujol mull) shows, besides
the expected absorptions related to the presence
of phenyl groups (900–600 cm−1), two carbonyl
stretching at 1723 and 1705 cm−1, assigned to the
COOMe substituents. The 31P-NMR registered in
CDCl3 at room temperature shows a doublet centered
at 45.10 ppm indicating two equivalent PPh3 lig-
When the reaction was carried out in the presence
of a larger excess of PPh3, namely a Rh/PPh3 ratio
of 1:5, the complete substitution of the ethylene lig-
ands takes place and the formation of the mononuclear
[Rh(dcmpz)(PPh3)3], 3, species occurs. The IR spec-

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337
trum of 3 presents the COOMe carbonyl adsorptions
at 1730 and 1707 cm−1 (nujol mull). Complex 3 can
also be easily obtained by reacting 2 with excess PPh3
(see Scheme 1).
complexes, such as [(triphos)Rh(Cl)(C2H4)] [12] and
[(COD)Rh(X)(Ph2PC6H4-3-SO3Na)] [13].
Species 2 was active both with internal (DMAD)
and with terminal alkynes (ethyl propiolate). A partic-
ular trend was noted with phenylacetylene as substrate;
apart from the expected 1,2,4- and 1,3,5-triphenyl ben-
zenes, the formation of head-to-head and head-to-tail
dimers took place, the latter being predominant
versus cyclotrimerization products. The presumed
mechanism for this dimerization, different from
that proposed for alkyne trimerization, probably in-
volves the formation of a vinylidene species which
subsequently undergoes acetylene insertion and
reductive elimination. A similar mechanism can
3.3. Catalysis: cyclotrimerization of alkynes
catalyzed by 2 and 3
Species 2 promoted alkyne cyclotrimerization
in very mild conditions (toluene, RT) with high
yields and selectivity. For example, the reaction with
dimethylacetylene dicarboxylate (DMAD) showed a
turnover frequency of about 8000 h−1, to be com-
pared with those reported in literature for other Rh(I)
Table 1
Catalytic activity of species 2 and 3
Alkyne
Products
Selectivity (%)
2
3
≡≡
MeOOC–C C–COOMe
100
100
≡≡
H–C C–COOEt
72
28
14
71
29
≡≡
H–C C–Ph
traces
3
traces
≡≡
=
Ph–C C–CH CH(Ph)
52
31
82
17
≡≡
=
Ph–C C–C(Ph) CH2

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G.A. Ardizzoia et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 333–340
be proposed for the polymerization of acetylene,
atoms in the two complexes. The bulky phosphine lig-
ands in species 3 are likely to prevent the formation
of the metallacycle involved in alkyne cyclotrimeriza-
tion mechanism, thus favoring the dimerization reac-
tion through a less hindered vinylidene species.
≡≡
HC CH, to give poly-acetylene, also catalyzed by
[Rh(dcmpz)(PPh3)(C2H4)]2 (vide infra).
As previously cited, even species 3 was active
in alkynes cyclotrimerization and oligomerization.
A comparison of the catalytic activity of species
2 and 3 is reported in Table 1. Products distribu-
tion in phenylacetylene oligomerization was rather
different using the two catalysts. The dinuclear
[Rh(dcmpz)(PPh3)(C2H4)]2, 2, afforded 17% of
trisubstituted benzenes, dimers being the major prod-
ucts, but with the mononuclear [Rh(dcmpz)(PPh3)3],
3, the dimerization was even further favored, and ben-
zene derivatives were detected only in traces. Similar
results were reached with species 2 as catalyst in
the presence of a small amount of free PPh3 in the
reaction mixture.
3.4. Acetylene polymerization catalyzed by
[Rh(dcmpz)(PPh3)(C2H4)]2
Since Ziegler’s studies on aluminium-alkyls [19],
acetylene polymerization has always represented an
important purpose in organo-metallic chemistry [20].
The interest is related to the possibility to generate
semi-conducting species, i.e. trans-polyacetylene,
later susceptible of chemical doping with formation
of conducting solids of metallic lustre.
When gaseous acetylene was bubbled through an
acetonitrile suspension of [Rh(dcmpz)(PPh3)(C2H4)]2,
2, a dark precipitate formed, in a temperature-depen-
These different selectivities could probably derive
from the different steric hindrance around the rhodium
Scheme 2. Proposed mechanism for acetylene polymerization.

G.A. Ardizzoia et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 333–340
339
dent rate, which increased with increasing temper-
ature. Consequently, all polymerization tests were
performed at 60 ◦C. The black precipitate was not
easily recovered. The addition of triphenylphosphine
(PPh3/Rh, 1:1 molar ratio) to the reaction medium
generates an amorphous (XRPD evidences), but
easily filterable, species. This was recognized (IR
evidences) as a mixture of cis and trans isomers
of polyacetylene. Indeed, in the region of the C–H
out of plane bending, two strong absorptions (740
and 1010 cm−1) are present, attributable to cis- and
trans-polyacetylene isomers, respectively. According
to an empirical equation reported in literature [21]
from the area of the absorption bands, is possible to
obtain the cis/trans ratio in the mixture, namely 3:1
in our experiments. Heating the solid to 140 ◦C under
nitrogen turned the mixture completely into the trans
form, thermodynamically more stable, as confirmed
by the presence in the IR spectrum of heated samples
ized by standard spectroscopic techniques and single
crystal X-ray diffraction. Starting from 1, by reaction
with triphenylphosphine, other two new rhodium(I)
pyrazolate complexes, [Rh(dcmpz)(PPh3)(C2H4)]2,
2 and [Rh(dcmpz)(PPh3)3], 3, were synthesized and
characterized. Complexes 2 and 3 showed high activ-
ity in catalytic oligomerization and cyclotrimerization
of both terminal and internal alkynes. In particular,
with monosubstituted alkynes, products distribution
deeply differentiates from that based on purely statis-
tical calculations.1 Interesting results occurred with
phenylacetylene and acetylene itself, which under-
went oligomerization or polymerization, respectively.
As the nature of ancillary ligands probably plays
an important role on the catalytic activity of these
species, a systematic study about electronic and steric
influence of aryl- and alkyl-phosphines, different from
PPh3, will be conducted.
of a single C–H adsorption at 1010 cm−1
.
The mechanism of polymerization (Scheme 2)
is probably similar to that proposed for acetylene
polymerization by titanium(I) compounds [22], the
Acknowledgements
We thank the Italian Consiglio Nazionale delle
Ricerche (CNR) and MIUR for funding. We also
thank Prof. M. Moret, University of Milano-Bicocca,
for helpful discussions.
≡≡
first step being the coordination of HC CH to the
rhodium center, followed by oxidative addition to
form an acetylide–hydride species. 1H-NMR evi-
dences confirmed the presence of hydridic species
in the quenched polymerization reaction. Subse-
quently, a vinylidene species is formed, which under-
goes the continuous insertion of acetylene molecules
generating the polymeric chain. In the absence of
triphenylphosphine, this highly conjugated polymer
can probably act as a ligand for rhodium centers,
giving place to a kind of supported catalyst. The
presence of PPh3 hampers the formation of the an-
chored species, freeing the polymer as an easily
recoverable species. Since addition of PPh3 can in-
differently occur at the beginning (under acetylene
flow) or at the end of the polymerization reaction
(under nitrogen), we can safely exclude participa-
tion of the free phosphine into the reaction mecha-
nism.
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