Alkyne oligomerization catalyzed by molybdenum(0) complexes

Article abstract of DOI:10.1016/S0022-328X(02)01114-2

Three new molybdenum(0) complexes, [Mo(CO)₃(Hpz)₃] (1), [Mo(CO)₂(Hpz)₂(DMAD)₂] (2), (DMAD = dimethyl acetylenedicarboxylate) and [Mo(CO)₃(1-Me-imidazole)₃] (3) were synthesized and characterized. Their activity and selectivity in alkyne cyclotrimerization and co-trimerization reactions was investigated. The molecular structures of 1 and 2 have been determined by unconventional powder and standard single-crystal diffraction methods, respectively. 1 consists of a pseudo-octahedral complex of C₃ symmetry, with the ligands in fac disposition; complex 2, of idealized C₂ symmetry, is obtained by substitution of one CO and one pyrazole in 1 by two DMAD ligands, and shows the rare trans configuration of π-bound acetylenic moieties.

Full text of DOI:10.1016/S0022-328X(02)01114-2

Journal of Organometallic Chemistry 649 (2002) 173–180
www.elsevier.com/locate/jorganchem
Alkyne oligomerization catalyzed by molybdenum(0) complexes
G. Attilio Ardizzoia a,b,*, S. Brenna a,b, G. LaMonica a,b, A. Maspero a,b,
N. Masciocchi b,c,
*
a Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, Uni6ersita` di Milano, Via Venezian 21, 20133 Milan, Italy
b Dipartimento di Scienze Chimiche, Fisiche e Matematiche, Uni6ersita` dell’Insubria, Via Valleggio 11, 22100 Como, Italy
c Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Uni6ersita` di Milano, Via Venezian 21, 20133 Milan, Italy
Received 31 August 2001; accepted 5 December 2001
Abstract
Three new molybdenum(0) complexes, [Mo(CO)3(Hpz)3] (1), [Mo(CO)2(Hpz)2(DMAD)2] (2), (DMAD=dimethyl acetylenedi-
carboxylate) and [Mo(CO)3(1-Me-imidazole)3] (3) were synthesized and characterized. Their activity and selectivity in alkyne
cyclotrimerization and co-trimerization reactions was investigated. The molecular structures of 1 and 2 have been determined by
unconventional powder and standard single-crystal diffraction methods, respectively. 1 consists of a pseudo-octahedral complex of
C3 symmetry, with the ligands in fac disposition; complex 2, of idealized C2 symmetry, is obtained by substitution of one CO and
one pyrazole in 1 by two DMAD ligands, and shows the rare trans configuration of p-bound acetylenic moieties. © 2002
Published by Elsevier Science B.V.
Keywords: Molybdenum; Cyclotrimerization; Catalysis; Powder diffraction; Crystal structure
EtOOCꢀCꢁCꢀH) and dimethyl acetylenedicarboxylate
(DMAD, CH3OOCꢀCꢁCꢀCOOCH3). To this goal, we
employed [Mo(CO)6] as starting material; partial ligand
substitution with pyrazole and its analogues was later
performed, in search of an improved catalyst activity
and satisfactory products yields. Therefore, this paper
report on the synthesis and characterization of these
catalysts, as well as on the selectivity of the cyclotrimer-
ization reactions derived therefrom.
1. Introduction
In the last decades, the alkyne cyclotrimerization
reaction forming benzene derivatives has been widely
investigated [1,2], particularly because, through the for-
mation of CꢀC bonds, new interesting products, later
susceptible of further transformations, can be prepared.
Several catalytic systems, based on cobalt [3], nickel [4],
palladium [5] and rhodium [6] species, can promote this
reaction, in heterogeneous or homogeneous conditions.
Examples of alkyne polymerization and cyclotrimeriza-
tion by molybdenum(II) complexes, in the presence of
Lewis acids [7], or by heterobimetallic systems contain-
ing MoꢀSn [8] or MoꢀGe [9] bonds are also known.
Polymerization reactions of phenylacetylene catalyzed
by molybdenum(0) complexes such as [Mo(CO)6]/
PhOH [10] or by [ArM(CO)3], (M=Cr, Mo, W) [11],
have also been reported.
2. Results and discussion
When a suspension of [Mo(CO)6] in toluene under
nitrogen at 60 °C was treated with excess ethyl propio-
late (EP), the two possible products of cyclotrimeriza-
tion, 1,2,4- and 1,3,5-tricarboethoxybenzene, were
catalytically formed in an approximate 2:1 ratio (see
Table 1); thus the 1,3,5-isomer is obtained in higher
yields than that foreseen from purely statistical consid-
erations. Indeed, in agreement with the mechanism
proposed for alkynes cyclotrimerization [1], a (1,2,4)-
versus (1,3,5)-isomer ratio of 3:1 is expected [12]. Thus,
steric effects seem to be at work, as confirmed by the
Recently, we began investigating the activity of
molybdenum(0) species in this reaction, using ‘electron-
poor’
alkynes
like
ethyl
propiolate
(EP,
* Corresponding authors. Fax: +39-031-2386119.
E-mail address: attilio.ardizzoia@uninsubria.it (G.A. Ardizzoia).
0022-328X/02/$ - see front matter © 2002 Published by Elsevier Science B.V.
PII: S0022-328X(02)01114-2

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G.A. Ardizzoia et al. / Journal of Organometallic Chemistry 649 (2002) 173–180
observation that [Mo(CO)6] does not promote the
oligomerization of diphenylacetylene, which contains
two bulky, and rigid, aryl groups next to the reactive
site.
sharp absorption of medium intensity, centered at 3410
cm−1, attributable to the NꢀH stretching of the pyra-
zole molecules and three strong absorptions at 1897,
1777, 1730 cm−1, due to the carbonyls stretching. The
pattern of these bands is in agreement with a fac
geometry about the metal centers [14], as later confi-
rmed by our X-ray powder diffraction analysis (re-
ported below).
Catalytic cyclotrimerization of EP was effective even
in the presence of water. Moisture often causes the
decomposition of the intermediate metalla-cyclopenta-
diene species, and only recently a catalytic system active
in aqueous medium has been reported [13]. Our studies
showed that the activity of [Mo(CO)6] was not inhibited
by H2O, its efficiency being apparently unaltered. On
the contrary, the presence of molecular oxygen in the
reaction vessel dramatically lowered the yield of ben-
zene derivatives down to about 5%, and the formation
of a black precipitate, constituted by insoluble species
perhaps derived from alkyne oligo- and poly-meriza-
tion, was observed. Similar results were observed em-
ploying dimethyl acetylenedicarboxylate (DMAD).
With the aim to improve reaction yields and selectiv-
ities, we decided to induce CO substitution on the
starting [Mo(CO)6] complex, thus modifying the steric
and the electronic properties at the metal center. As
initial choice, we selected pyrazole (Hpz), a s-donor
ligand which was expected to increase the electron
density at the active site (Table 1).
1H-NMR spectrum of 1 (acetone-d6, RT) showed
four signals: a singlet at 12 ppm, attributed to the NꢀH
proton (disappearing after treatment with D2O) and
three singlets in the aromatic region, centered at 7.83,
7.21 and 6.37 ppm, respectively (the pyrazole CꢀH
protons). The separation observed for the protons in 3
and 5 position on the heteroaromatic ring, which was
higher than that reported in literature [15], is probably
due to the different influence of the metal center over
the considered protons.
If [Mo(CO)3(Hpz)3] was suspended in toluene with a
1:15 excess of alkyne (EP or DMAD), cyclotrimeriza-
tion products were quickly formed. It was found neces-
sary to operate at 60 °C in order to facilitate partial
pyrazole dissociation and, consequently, alkyne co-or-
dination and completion of the reaction. Unfortunately,
the presence of dissociated pyrazole molecules pro-
moted undesired side reactions, giving rise to the for-
When [Mo(CO)6] was suspended in heptane at 95 °C
and treated with excess pyrazole, a pale-yellow precipi-
tate suddenly formed, which, on the basis of elemental
analysis and spectroscopic data, was formulated as
[Mo(CO)3(Hpz)3] (1). The IR spectrum of 1 shows a
mation of both cis and trans isomers of
a
pyrazole-alkyne olefinic adduct, via a nucleophilic at-
tack to the activated acetylenic triple bond Eq. (1) [16].
Table 1
Product distribution in EP cyclotrimerization
The presence of molecular oxygen reduces the yield to 5%, the percentage distribution of isomers remaining essentially constant (see text).

G.A. Ardizzoia et al. / Journal of Organometallic Chemistry 649 (2002) 173–180
175
(1)
Differently, when compound 1 was dissolved at room
temperature in carbon disulfide (CS2) in the presence of
DMAD, a pale yellow product was obtained. On the
basis of elemental analysis and spectroscopic data, it
was formulated as [Mo(CO)2(Hpz)2(DMAD)2] (2),
derived by the substitution of one carbonyl and one
pyrazole ligands by two DMAD molecules. As men-
tioned above, the reaction of 1 with DMAD in toluene
solution affords mainly cyclotrimerization products,
even at room temperature; thus, isolation of 2 in CS2
speaks for a relevant solvent effect. Additionally, we
proved that [Mo(CO)6], in CS2 and in presence of
excess DMAD, did not afford any reaction, thus indi-
cating that pyrazole is indeed necessary to promote
(further) ligand substitution during the formation of 2.
The IR spectrum (nujol mulls) of [Mo(CO)2(Hpz)2-
(DMAD)2] shows the NꢀH stretching of the pyrazole
ligands at 3376 cm−1 and two sharp carbonyl stretch-
ing bands at 2011 and 1950 cm−1 characteristic of
cis-Mo(CO)2 fragments, as observed in a number of
analogue species [17].
Again, as previously noted for [Mo(CO)3(Hpz)3], there
was a large separation between the 3 and 5 H reso-
1
nances. In addition, a signal at 3.77 ppm due to
COOCH3 groups of the alkynes is also present. When
the spectrum was recorded at −90 °C (acetone-d6) the
spectral features of the pyrazole protons did not change
significantly; differently, splitting of the COOCH3 sin-
glet of the co-ordinated DMAD molecules (3.70 and
3.76 ppm) occurs. At low temperatures, the fluxional
behavior of the acetylenic ligands was probably pre-
vented, causing the differentiation of the swinging CH3
groups [17a]. The detailed RT molecular structure of 2
was obtained by conventional single crystal X-ray
analysis.
When 2 was suspended in toluene at 60 °C with a
tenfold excess of EP, the formation of different cy-
clotrimerization products was observed (Scheme 1).
Apart from the obvious presence of 1,2,4- and 1,3,5-tri-
carboethoxybenzenes (m/z 294) derived from EP cy-
clotrimerization, EP/DMAD co-trimerization products
were formed, containing EP/DMAD fragments in 2:1
(m/z 338) and 1:2 ratio (m/z 382); moreover, the forma-
tion of hexacarbomethoxy benzene ([C(COOMe)]6) was
noted (m/z 426). When performing the same reaction
(catalytically promoted by [Mo(CO)2(Hpz)2(DMAD)2])
1H-NMR of 2 (acetone-d6, RT) exhibits a singlet at
12 ppm attributable to the pyrazole NꢀH protons and
three signals at 7.82, 7.47 and 6.35, respectively, as-
signed to 3, 5 and 4 protons of the heteroaromatic ring.
Scheme 1. Products distribution in the cyclotrimerization of EP catalyzed by complex 2. (a) Stoichiometric amounts; (b) catalytic amounts.

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G.A. Ardizzoia et al. / Journal of Organometallic Chemistry 649 (2002) 173–180
with excess DMAD (rather than EP), as expected, only
[C(COOMe)]6 was formed. Unfortunately, also employ-
ing 2 as catalyst, the formation of alkyne-pyrazole
by-products was observed (Scheme 1).
In order to avoid this problem, we decided to employ
a different s-donor ligand, which could not further act
as nucleophile, such as a N-substituted pyrazole (or
similar). However, we decided to use N-methyl-imida-
zole (1-Meim), which is more easily available than the
corresponding N-methylpyrazole and essentially identi-
cal in terms of s-donor ability and steric hindrance.
[Mo(CO)6] reacts with excess 1-Meim in heptane at
95° giving place to the pale yellow [Mo(CO)3(1-Meim)3]
(3), derivative. The IR spectrum shows three carbonyl
stretching bands at 1883, 1754 and 1732 cm−1. A
comparison with CO bands in 1 reveals the effect of the
alkyl residue on the heterocyclic nitrogen: as a conse-
quence of the enhanced s-donor properties of the lig-
and, resulting in an increased electron density on
molybdenum, greater back-donation to the carbonyl
ligands takes place with a lowering of the frequency of
1
the stretching modes. H-NMR (acetone-d6, RT) evi-
denced a strong pseudo-triplet centered at 3.76 ppm,
due to the different (but very close) coupling constant
values with the C(2)ꢀH and C(5)ꢀH of the methyl
protons. For the same reason, the aromatic protons
give place to a convoluted series of signals in the
7.80–6.60 ppm region.
Fig. 1. Top: molecular drawing for [Mo(CO)3(Hpz)3], with partial
labeling scheme. Bottom: crystal packing, viewed down [001], show-
ing NꢀH···O interactions (fragmented lines). Relevant bond distances
,
(A) and angles (°), with estimated S.D.’s in parentheses: MoꢀN1
2.317(9), C1ꢀMoꢀC1% 84.2(7), N1ꢀMoꢀN1% 83.5(6), cis-C1ꢀMoꢀN1
96.1(7), trans-C1ꢀMoꢀN1 179.5(1), NꢀH···O 2.753, intermolecular
p–p interaction: B···B 3.63 (B=pyrazole center of mass).
Table 2
Among the Mo(0) species tested during this work,
compound 3 resulted to be the most active in the
catalytic alkyne cyclotrimerization reaction (see Table
2). This can be tentatively explained as follows: (i) the
methyl residue on N(1) increases electron density on the
metal, making it a better candidate in the formation
MꢀC bonds with electron-poor acetylenes, such as EP
and DMAD; (ii) the presence of the N-alkyl residue
prevents the formation of by-products by nucleophilic
attack of the nitrogen lone pair on the EP/DMAD
triple bond.
Cyclotrimerization products formed by employing complex 3 as
active species
2.1. Crystal structure of [Mo(CO)3(Hpz)3] (1)
This complex, which lies on a crystallographic three-
fold axis, consists of a molecular species bearing three
carbonyls and three pyrazoles in the octahedral co-ordi-
nation mode. A schematic drawing of this complex is
reported in Fig. 1, whose caption includes some rele-
vant geometrical parameters. Thus, such Mo(0) species
closely resembles already known octahedral complexes
of facial geometry, such as Mo(CO)3(3LN) [18] (3LN=a
saturated polycyclic ligand with three donor N atoms)
or, to a higher extent, Mo(CO)3(4-Me-pyridine)3 [19].
Consistently, the MoꢀN1 distance observed here,

G.A. Ardizzoia et al. / Journal of Organometallic Chemistry 649 (2002) 173–180
177
,
2.317(9) A, well compares to the values found for the
[20]. Within each acetylenic ligands, CꢁC distances
,
,
last cited species (2.30–2.33 A) [19]. As shown in the
(1.28 A) and heavily bent (ca. 139–146°) CꢀCꢁC angles
bottom part of Fig. 1, where the overall packing is
depicted, several intermolecular hydrogen bonds are
present in the solid state (NꢀH···O 2.753 A), which are
further accompanied by relevant p–p interactions be-
tween pyrazole rings across inversion centers of the −1
type (the distance between their centers of mass being
ca. 3.6 A). This particular disposition of the hetero-
cyclic ring may further be stabilized by antiparallel
dipolar interactions.
well agree with literature data and, in particular, closely
match those reported for the [Mo(CO)2(en)(DMAD)]
analogue [17b]. However, given the paucity of struc-
tural data available for this class of compounds, we
believe that the 6ery small variations observed in the
actual geometrical values cannot be taken as evidence
of ‘disagreement between X-ray and IR spectral data’
[17b].
,
,
2.2. Crystal structure of [Mo(CO)2(Hpz)2(DMAD)2] (2)
3. Conclusions
This complex, of idealized C2 symmetry, consists of a
molecular species bearing two mutually cis carbonyls
and two pyrazoles trans to them, the remaining co-ordi-
nation sites of a pseudo-octahedral geometry being
occupied by two DMAD molecules, bound through
their CꢁC p electrons, trans to each other. An ORTEP
drawing of this complex, viewed ca. down its idealized
twofold axis, is reported in Fig. 2. A number of com-
plexes bearing two substituted acetylenes (ac*) have
been structurally characterized in the past, mostly be-
longing to the ML4(ac*)2 type with cis-ac* units, with
nearly parallel CꢁC vectors [20]. Only a few reports on
trans ac* moieties in ML4(ac*)2 species have appeared,
all complexes of this type showing nearly orthogonal
CꢁC bonds [17]. Consistently, also Mo(CO)2(Hpz)2-
(DMAD)2 shows this latter feature, which can been
attributed to the presence of d-p bonds through two
competing mutually orthogonal d orbitals. Similar ef-
fects have been observed for other trans p-acid ligands,
such CO2, O2 and olefins [21]. Accordingly, rather long
We reported here the synthesis and characterization
of some molybdenum(0) derivatives, as well as their
catalytic activity and selectivity in the alkyne cyclo-
trimerization and cyclo-co-trimerization reactions.
Since it has been verified that the nature of ancillary
ligands plays an important role in tailoring the catalyst
activity, we plan to extend this work by pursuing a
systematic investigation of the steric and electronic
influence of substituents on the heterocyclic rings, in a
number of catalytic systems, with particular emphasis
on CꢀC and CꢀN bond formation.
4. Experimental
4.1. General procedures
All solvents used were purified according to standard
procedures and kept under nitrogen atmosphere.
Molybdenum hexacarbonyl, pyrazole, (Aldrich) and N-
methyl-imidazole (Merck) were used without further
purification. The alkynes employed in catalytic reac-
tions were taken from sealed bottles kept at −25 °C
and their purity grade were confirmed by GC–MS
control analysis. If not otherwise specified, all reactions
were performed under an inert atmosphere of dry nitro-
gen. Elemental analyses (C,H,N) were carried out at the
Microanalysis Laboratory of the University of Milan.
IR spectra were recorded on a BIO-RAD FTS 7,
1H-NMR were recorded on a Bruker 300. Quantitative
analysis of products were performed on a Shimadzu
GC-17A gas chromatograph fitted with a 30 m (0.25
mm) capillary column coupled with a Shimadzu MS
QP5000 instrument.
,
MoꢀC distances (avg. 2.16 A) are observed for the
acetylenic carbons, if compared with those cited in ref.
4.2. Syntheses
Fig. 2. ORTEP drawing of the molecular structure of [Mo(CO)2(Hpz)2-
,
(DMAD)2]. Relevant bond distances (A) and angles (°), with esti-
4.2.1. Synthesis of [Mo(CO)3(Hpz)3] (1)
mated S.D.’s in parentheses: MoꢀC1 2.005(2), MoꢀC2 2.008(2),
C1ꢀO1 1.135(3), C2ꢀO2 1.135(3), MoꢀC1ꢀO1 175.9(2), MoꢀC2ꢀO2
177.6(2), MoꢀN1 2.292(2), MoꢀN3 2.280(3), MoꢀC9 2.158(2),
MoꢀC10 2.178(2), C9ꢀC10 1.277(3), MoꢀC15 2.166(2), MoꢀC16
2.164(2), C15ꢀC16 1.281(3), CꢁCꢀC 139.4(2)ꢀ146.4(2) [avg. 142].
To a solution of [Mo(CO)6] (700 mg, 2.65 mmol) in
heptane (20 ml), pyrazole (Hpz, 1.44 g, 21.2 mmol) was
added. The solution was stirred at 95 °C for 8 h.
During this time, a pale-yellow solid formed. The solid

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G.A. Ardizzoia et al. / Journal of Organometallic Chemistry 649 (2002) 173–180
was then filtered under nitrogen, washed with hot hep-
tane and dried in a slow nitrogen flow. 73% yields. IR
(nujol) 1897, 1777, 1730 cm−1. Anal. Found: C, 37.44;
H, 2.67; N, 21.61. Calc. for C12H9N6O3Mo: C, 37.50;
H, 3.12; N, 21.87%. Upon exposure to air (few hours),
powders of 1 turn violet; however, we proved by pow-
der diffraction and IR spectroscopy that only surface
oxidation takes place, the spectral features and diffrac-
tion pattern remaining substantially unchanged.
the two phases were separated and 6 ml of wet toluene
were transferred to a Schlenk tube and degassed under
nitrogen. Then, [Mo(CO)6] (75 mg, 0.284 mmol) and
EP (100 ml, d=0.968, 0.988 mmol) were added to the
solution and heated to 60 °C for 24 h under a bleed of
wet nitrogen.
4.3.3. Cyclotrimerization of ethyl propiolate (EP) by
[Mo(CO)6]/O2
To a suspension of [Mo(CO)6] (75 mg, 0.284 mmol)
in toluene (6 ml) previously degassed with molecular
oxygen, ethyl propiolate was added (100 ml, d=0.968,
0.988 mmol). The suspension, kept under O2 atmo-
sphere, was heated to 60 °C for 8 h.
4.2.2. Synthesis of [Mo(CO)2(Hpz)2(DMAD)2] (2)
(DMAD=dimethyl acetylenedicarboxylate)
To a suspension of [Mo(CO)3(Hpz)3] (120 mg, 0.31
mmol) in CS2 (5 ml), DMAD (600 ml, 4.88 mmol), was
added under stirring. After 3 h, the mixture was filtered
off and the residue washed with diethyl ether, affording
a yellow compound, stable in the solid state, but not in
solution (84% yields). Crystals suitable for an X-ray
determination were obtained by slow diffusion of di-
ethyl ether in a dichloromethane solution of the com-
pound. IR (nujol): 2011, 1950 cm−1. Anal. Found: C,
42.12; H, 3.41; N, 9.56. Calc. for C20H20N4O10Mo: C,
41.96; H, 3.50; N, 9.79%.
4.3.4. Alkyne cyclotrimerization in the presence of
[Mo(CO)3(Hpz)3] (1), [Mo(CO)2(Hpz)2(DMAD)2] (2) or
[Mo(CO)3(1-Me-im)3] (3)
In a typical experiment, the appropriate complex was
dissolved in toluene (5 ml) and treated with an excess of
alkyne (Mo:alkyne ratio 1:10) at 60 °C for 5 h. All
reactions were monitored by means of GC–MS, and
found complete within 8 h.
4.3.5. X-ray powder diffraction analysis [22] of
[Mo(CO)3(Hpz)3] (1)
4.2.3. Synthesis of [Mo(CO)3(1-Me-im)3] (3)
1-Methyl imidazole (1.5 ml, d=1.03, 18.8 mmol),
dissolved in 0.5 ml of degassed heptane, was added to a
solution of [Mo(CO)6] (600 mg, 2.27 mmol) in heptane
(20 ml). The solution was kept at 95 °C for 4 h; it was
then slowly cooled to room temperature (r.t.) and hep-
tane was removed under reduced pressure. The yellow
residue was treated with degassed methanol (20 ml) and
then filtered over a septum. The complex was dried
under a nitrogen flow. 83% yields. Anal. Found: C,
45.16; H, 4.44; N, 19.28. Calc. for C15H18N6O3Mo: C,
45.25; H, 4.22; N, 19.72%.
The gently ground powders were cautiously de-
posited in the hollow of an aluminium holder equipped
with a zero background plate (supplied by The Gem
Dugout, State College, PA). Diffraction data (Cu–Ka,
,
u=1.5418 A) were collected on a vertical scan PW1820
diffractometer, equipped with parallel (Soller) slits, a
secondary beam curved graphite monochromator, a
Na(Tl)I scintillation detector and pulse height amplifier
discrimination. The generator was operated at 40 KV
and 40 mA. Slits used: divergence 1.0°, antiscatter 1.0°
and receiving 0.2 mm. Nominal resolution for the
present set-up is 0.14° 2q (FWHM) for the Si(111) peak
at 28.44° (2q). A long overnight scan was performed
with 5B2qB105°, with t=10 s and D2q=0.02°. In-
dexing, using TREOR [23], of the low angle diffraction
peaks suggested a trigonal cell of approximate dimen-
4.3. Catalytic reactions
4.3.1. Cyclotrimerization of dimethyl
acetylenedicarboxylate (DMAD) and ethyl propiolate
(EP) by [Mo(CO)6]
,
sions a=12.69, c=16.54 A {M(20)=38 [24]; F(20)=
In a Schlenk tube containing degassed toluene (5 ml)
[Mo(CO)6] (120 mg, 0.454 mmol) was added under
stirring. The system was thermostated at 70 °C obtain-
ing a clear solution. The appropriate alkyne, previously
degassed with nitrogen, was then added (Mo:alkyne
molar ratio 1:18). The reaction was monitored by GC–
MS analysis, which revealed the presence of the cy-
clotrimerization products. The complete conversion of
the starting alkyne is attained after about 8 h.
57 [25] (0.006, 64)}. Systematic absences indicated,
(
among others, R3 as the probable space group, later
confirmed by successful solution and refinement. Struc-
ture solution was initiated by using EXPO [26], which
afforded the location of the metal atom and of some
lighter atoms. Difference Fourier syntheses and geomet-
rical modeling later afforded approximate co-ordinates
for the remaining non-hydrogen atoms. The final refine-
ments were performed with the aid of the GSAS suite of
programs [27], by imposing geometric constraints to the
pyrazole rings (CꢀC and CꢀN were given average litera-
4.3.2. Cyclotrimerization of ethyl propiolate (EP) by
[Mo(CO)6] in the presence of H2O
In a 50 ml flask, a solution of toluene (10 ml) and
water (10 ml) was stirred vigorously for 15 min. Then
,
ture values of 1.38 A, and internal ring angles fixed at
108°). Soft restraints were also applied to MoꢀC and
,
CꢀO distances [2.0 and 1.15 A, respectively]. The peak

G.A. Ardizzoia et al. / Journal of Organometallic Chemistry 649 (2002) 173–180
179
Fig. 3. Rietveld refinement plot for [Mo(CO)3(Hpz)3], with peak markers and difference plot at the bottom.
shapes were best described by the Thompson/Cox/
Hastings formulation [28] of the pseudo Voigt function,
with GV and LY set to zero. The background function
was described by a cosine Fourier series, while system-
atic errors were corrected with the aid of a sample-dis-
placement angular shift; a single (refinable) isotropic
displacement parameter [Uiso(Mo)] was assigned to
with a 25% extension at each end for background
determination. Three standard reflections were mea-
sured at regular intervals and showed no decay of the
scattering power over the data collection period. Data
were corrected for Lorentz, polarization and absorption
(c-scan) effects. Structure solution by Direct (SIR97)
and conventional Fourier difference methods; refine-
ment by full-matrix least-squares using SHELX97 and
the physical constants tabulated therein. Anisotropic
thermal parameters were assigned to all non-hydrogen
atoms. The hydrogen atoms contribution to the scatter-
ing factors was included in the last cycles in the riding
model.
molybdenum, while lighter atoms U’s were arbitrarily
2
,
given [Uiso(Mo)+0.02] A values. The contribution of
the hydrogen atoms to the scattered intensity was ne-
glected. Scattering factors, corrected for real and imagi-
nary anomalous dispersion terms were taken from the
internal library of GSAS
.
Crystal data: C9H8MoN4O3, fw 316.13 g mol−1
;
Crystal data: C20H20MoN4O10, fw 572.34 g mol−1
;
(
,
(
trigonal, R3, a=12.6982(3), c=16.5385(7) A, U=
triclinic, P1, a=8.660(1), b=10.320(1), c=13.822(3)
3
2309.4(1) A , Z=6, zcalc=1.364 g cm−3; Rp and
A, h=81.95(1), i=76.69(1), k=85.82(1)°, U=
,
,
3
1189.3(3) A , Z=2, zcalc=1.598 g cm−3; R1 and
wR2=0.025 and 0.059, respectively, for 4167 indepen-
dent reflections collected in the 3BqB25° range.
,
R
wp=0.085 and 0.111, respectively, for 4500 data
points (588 independent reflections, RF=0.095) col-
lected in the 15B2qB105° range. Fig. 3 shows the
final Rietveld refinement plot for [Mo(CO)3(Hpz)3].
5. Supplementary material
4.3.6. Single crystal X-ray structure determination of
[Mo(CO)2(Hpz)2(DMAD)2] (2)
Crystallographic data for the structural analyses re-
ported above have been deposited with the Cambridge
Crystallographic Data Center, CCDC no. 167916, and
167917, for compounds 1 and 2, respectively. Copies of
this information may be obtained free of charge from:
The Director, CCDC, 12 Union Road, Cambridge,
CB2 1EZ UK (Fax: (int. code) +44-1223-336-033;e-
mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
A yellow prismatic crystal of approximate dimen-
sions 0.30×0.12×0.08 mm was mounted on a tip of a
glass fiber and put onto a goniometer head. The inten-
sity data were collected at r.t. on an Enraf–Nonius
CAD4 automated diffractometer. A least-squares fit of
25 randomly oriented intense reflections with q ranging
from 10 to 15° provided the unit cell parameters.
Intensities were collected using a variable scan-range

180
G.A. Ardizzoia et al. / Journal of Organometallic Chemistry 649 (2002) 173–180
G. La Monica, N. Masciocchi, M. Moret, J. Organomet. Chem.
Acknowledgements
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We thank the Italian Consiglio Nazionale delle
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