Photosubstitution of carbon monoxide in W(CO)6 by alkyne: NMR detection of thermally unstable alkyne tungsten(0) carbonyl complexes

Article abstract of DOI:10.1016/S0022-328X(00)00865-2

The NMR method has been used for the identification and characterisation of the unstable terminal alkyne complexes [W(CO)₅(η²-HCCR)], [W(CO)₄(η²-HCCR)₂], [W(CO)(η²-HCCR)₃], and v

Full text of DOI:10.1016/S0022-328X(00)00865-2

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 622 (2001) 74–83
Photosubstitution of carbon monoxide in W(CO)6 by alkyne:
NMR detection of thermally unstable alkyne tungsten(0) carbonyl
complexes
Teresa Szyman´ska-Buzar *, Krystyna Kern
Faculty of Chemistry, Uni6ersity of Wroc*law, F. Joliot-Curie 14, 50-383 Wroc*law, Poland
Received 28 August 2000; received in revised form 4 September 2000; accepted 19 October 2000
Abstract
The NMR method has been used for the identification and characterisation of the unstable terminal alkyne complexes
[W(CO)5(h2-HCꢀCR)], [W(CO)4(h2-HCꢀCR)2], [W(CO)(h2-HCꢀCR)3], and vinylidene derivatives [W(CO)5(CꢁCHR)] formed in
photochemical reactions of W(CO)6 and each of the alkynes HCꢀCH, HCꢀCMe and HCꢀCCMe3. The terminal alkyne carbonyl
complexes formed after sequential substitution of CO by alkyne were shown to be involved in the cyclotrimerisation of alkynes
to arenes, which were detected by NMR spectroscopy and GC-MS analysis in the photochemical reaction of W(CO)6 and each
terminal alkyne used. Benzene substitute adducts with W(CO)3 moiety were also observed by NMR in the above reactions.
© 2001 Elsevier Science B.V. All rights reserved.
Keywords: Tungsten; Carbonyl complexes; Alkyne complexes; NMR spectroscopy; Photolysis
plex [16,17] was observed. However, similar complexes
were not obtained with terminal alkynes. In photo-
chemical reaction of W(CO)6 with terminal alkynes, the
formation of the unstable [W(CO)5(h2-HCꢀCR)] com-
pound and its rearrangement to the vinylidene com-
1. Introduction
The photochemical and thermal reactions of W(CO)6
with alkynes lead to mixed carbonyl complexes of
tungsten with varying number of alkyne ligands (from
one to three) [1–11]. These alkyne carbonyl complexes
of tungsten(0) can be involved in the catalytic polymeri-
sation and cyclisation of alkynes [9,10]. Compounds
which are stable enough to be isolated and character-
ised in a pure state under normal conditions are of the
type [W(CO)(RCꢀCR%)3], R=R%=Et [2], Ph [3–5],
SMe [6], p-tolyl, i-Pr [8]; R=Ph, R%=Me [4] and
benzo-15-crown-5 [7]. Especially interesting reactivity
was observed for [W(CO)(PhCꢀCPh)3] in which the CO
ligand can be replaced by other two-electron donor
ligands: MeCN [12], PPh3 [12] and PMe3 [13,14]. In the
reaction of [W(CO)(PhCꢀCPh)3] with an excess of
PhCꢀCPh the coupling of alkyne molecules and the
formation of a cyclobutadiene [15] and a carbene com-
pound
[W(CO)5(ꢁCꢁCHR)],
initiating
the
polymerisation of terminal alkynes by W(CO)6 have
been postulated [9,10]. The first direct evidence for such
a transformation has been obtained by IR and UV–Vis
spectroscopy from matrix-isolation studies of alkyne
complexes formed in photochemical reaction of
W(CO)6 with terminal alkynes at 20 K [11]. There are
only a few examples of fully characterised terminal
alkyne-substituted tungsten(0) carbonyls. At low tem-
perature (ca. 210 K) it was possible to isolate thermally
unstable [W(CO)5(h2-HCꢀCPh] and [W(CO)5(h2-
HCꢀCCOOMe)] after substitution of CH2Cl2 by alkyne
in [W(CO)5(CH2Cl2)] [18]. The [W(CO)(HCꢀCPh)3] was
isolated in the reaction of [W(CO)3(NCEt)3] with
HCꢀCPh and was shown to exist as a mixture of
isomers in solution [19].
* Corresponding author. Tel.: +48-71-3204221; fax: +48-71-
3282348.
E-mail address: tsz@wchuwr.chem.uni.wroc.pl (T. Szyman´ska-
Buzar).
Because of our continuing interest in the catalytic
activity of W(CO)6 in the polymerisation and cyclisa-
tion of alkynes, it was our aim to investigate whether
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00865-2

T. Szyman´ska-Buzar, K. Kern / Journal of Organometallic Chemistry 622 (2001) 74–83
75
the catalytically important intermediate compounds
generated photochemically from W(CO)6 and terminal
alkynes and observed recently by IR measurements [11]
can be characterised by NMR spectroscopy.
lents were added using a microlitre syringe) and then
irradiated through quartz at ca. 270 K. After different
times of photolysis (0.5–4.5 h) the solvent was removed
under vacuum at ca. 270 K, the residual solid dissolved
in cold toluene-d8 or CDCl3 (0.7 cm3), and the resulting
solution transferred to the precooled NMR tube.
Recently, we demonstrated the first observation and
NMR spectral characterisation of thermally unstable
alkene-substituted tungsten carbonyls, viz. [W(CO)5(h2-
CH2ꢁCHR)], cis-[W(CO)4(h2-CH2ꢁCHR)2] (R=H, Me
and Et), mer-[W(CO)3(h2-CH2ꢁCH2)3] and fac-
[W(CO)3(h2-CH2ꢁCH2)3] formed in photochemical re-
actions of W(CO)6 or trans-[W(CO)4(h2-CH2ꢁCHR)2]
in alkene-saturated hydrocarbon solutions at tempera-
tures in the range 200–293 K [20,21]. NMR spec-
troscopy has proven very valuable in identifying and
spectroscopically characterising and monitoring the re-
active 18-electron species generated photochemically in
solution at low temperatures [20,21].
2.3. NMR tube reactions
The photochemical reactions were also followed di-
1
rectly by H-NMR measurements at low temperature.
In a typical experiment a solution of W(CO)6 (0.01 g,
0.03 mmol) in toluene-d8 or CDCl3 (0.7 cm3) was
saturated with ethyne or propyne gas for 2.5 min,
(tert-butylacetylene, 10 equivalents were added using a
microlitre syringe), and the tube was closed by a glass
cap. After cooling to ca. 200 K the tube was subjected
to broad-band UV–Vis irradiation. The tube was trans-
ferred to the precooled sample holder of the NMR
2. Experimental
1
spectrometer and the H-NMR spectrum of the solu-
2.1. Materials and method
tion recorded.
Manipulation of chemicals was carried out under
nitrogen using standard Schlenk techniques. Solvents
and liquid reagents were dried and distilled from CaH2
under nitrogen prior to use. Acetylene was passed
through cooled (ca. 200 K) n-hexane to remove the
acetone stabiliser. Propyne was used as supplied
(Aldrich, 99% grade). IR spectra were measured with
Nicolet FT-IR model 400 instrument. 1H- and 13C-
NMR (proton coupled and decoupled) spectra were
recorded with a Bruker AMX 300 instrument operating
at 300 and 75.5 MHz, respectively. All chemical shifts
2.4. IR and NMR data for [W(CO)3(p6-C6H3Me3)] (7)
The arene complexes [W(CO)3(h6-1,3,5-C6H3Me3)]
(7a) and [W(CO)3(h6-1,2,4-C6H3Me3)] (7b) were pre-
pared according to literature procedures [22] and char-
acterised by IR and NMR spectroscopy.
1
7a. IR (n-hexane): w(CO) 1974 s, 1901 s. H-NMR
(C6D5CD3, 294 K): lH 4.17 s (3CH), 1.88 s (3CH3);
(C6D5CD3, 243 K): lH 3.97 s (3CH), 1.82 s (3CH3);
(CDCl3, 294 K): lH 5.10 s (3CH), 2.46 s (3CH3);
1
1
are referenced to residual solvent protons for H-NMR
(CDCl3, 243 K): lH 5.16 s (3CH, JCH=174 Hz), 1.82
and to the chemical shift of the solvent for 13C-NMR.
In toluene-d8 solvent the methyl resonances at lH 2.10
in 1H-NMR and lC 20.4 in 13C-NMR were used as
internal standards. In CDCl3 solvent the proton reso-
s (3CH3, JCH=129 Hz). 13C{1H}-NMR (CDCl3, 294
1
K): lC 212.03 (3CO), 109.79 (3CMe3), 89.86 (3CH),
20.42 (3CH3); (CDCl3, 243 K): lC 212.52 (3CO), 109.66
(3CMe3), 90.28 (3CH), 20.46 (3CH3).
1
nance at lH 7.24 in H-NMR and lC 77.0 in 13C-NMR
7b. IR (n-hexane): w(CO) 1973 s, 1899. 1H-NMR
3
were used as internal standards. The photolysis source
was an HBO 200W high-pressure Hg lamp.
(C6D5CD3, 294 K): lH 4.77 d (1CH6, JH6H5=6.4 Hz),
4.49 d (1CH3, 4JH5H3=1.7 Hz), 4.41 dd (1CH5,
4
3JH6H5=6.4 Hz, JH5H3=1.7 Hz), 1.84 s, 1.81 s, 1.65 s
2.2. Photochemical reactions
(3CH3); (C6D5CD3, 243 K): lH 4.59 d (1CH6), 4.22 d
(1CH3), 4.21 dd; (CDCl3, 294 K): lH 5.34 d (1CH6),
5.10 d (1CH3), 5.00 dd (1CH5), 2.16 s, 2.10 s, 2.01 s
(3CH3); (CDCl3, 243 K): 5.67 d (1CH6, 3JH6H5=6.1
Hz, 1JCH=172 Hz), 5.39 d (1CH3, 4JH5H3=1.1 Hz,
1JCH=173 Hz), 5.27 dd (1CH5, 3JH6H5=6.1 Hz,
Photochemical reactions involving W(CO)6 and
alkyne (HCꢀCH, MeCꢀCH, BuCꢀCH, EtCꢀCEt and
t
MeCꢀCCꢀCMe) occuring in n-hexane solution at ca.
270 K were investigated by following the IR spectra of
the sample at room temperature. The products of these
reactions in toluene-d8 or CDCl3 solution were tracked
1
4JH5H3=1.1 Hz, JCH=174 Hz), 2.39 s, 2.33 s, 2.23 s
(3CH3, 1JCH=129 Hz). 13C{1H}-NMR (CDCl3, 293
K): lC 211.96 (3CO), 108.86, 104.31 (3CMe3), 94.76,
93.54, 89.72 (3CH), 20.20, 18.90, 18.69 (3CH3);
(CDCl3, 243 K): lC 212.57 (3CO), 109.96, 109.77,
104.81 (3CMe3), 95.77, 94.03, 90.14 (3CH), 20.21,
19.03, 18.88 (3CH3).
1
by H- and 13C-NMR measurements at temperatures in
the range 200–300 K. In a typical experiment, a solu-
tion of W(CO)6 (0.1 g, 0.28 mmol) in freshly distilled
n-hexane (50 cm3) was saturated with ethyne or
propyne gas for 5 min (tert-butylacetylene, 10 equiva-

76
T. Szyman´ska-Buzar, K. Kern / Journal of Organometallic Chemistry 622 (2001) 74–83
3. Results and discussion
lacetylene (2086, 1964 and 1946 cm−1) or propyne
(2086, 1960 and 1943 cm−1) complexes. Prolonged
irradiation causes a decrease in the intensity of these
bands as the subsequent photochemical reaction of
compound 1 but w(CO) bands of the secondary products
could not be observed in solution due to their low
concentration. The precipitate which is formed contains
several bands in the w(CO) region 2150–1800 cm−1 (Fig.
1c), indicating the formation of a mixture of compounds
unidentifiable here by IR. Investigations of the reaction
products by NMR at low temperature allowed us to
detect several compounds, the highest concentration
always being for the compound 1. Alkynepentacarbonyl-
tungsten(0) species could be fully characterised by NMR
(Table 1). Of the six alkyne adducts observed, the
internal alkyne complexes are much more stable than
terminal alkyne compounds.
3.1. Characterisation of [W(CO)5(p2-alkyne)] (1)
compounds
Broad-band photolysis of an n-hexane solution of
W(CO)6 in the presence of an excess of alkyne (HCꢀCR,
t
R=H, Me, Bu; EtCꢀCEt and MeCꢀCCꢀCMe) gives
rise initially to [W(CO)5(h2-alkyne)] (1), as was judged
by IR and 1H-NMR spectroscopic monitoring (Table 1).
The IR and NMR spectra of pentacarbonyl alkyne
compounds of tungsten are very similar and reveal a
similar structure for all complexes. Fig. 1 illustrates
representative IR spectral changes accompanying the
irradiation of W(CO)6 in the presence of alkyne. The IR
spectrum of the irradiated solution shows three w(CO)
absorption bands characteristic of
a
molecule
[W(CO)5L] with C4 symmetry. The p-acidity of alkyne
ligands is reflected in the w(CO) frequency. The w(CO)
vibrations for stronger p-acid ethyne complexes are
observed at higher frequency: a weak vibration (a1) at
2091 cm−1, a very strong one (e) at 1964 cm−1and a
strong one (a1) at 1951 cm−1, than for tert-buty-
In the 1H-NMR spectra of the terminal alkyne
[W(CO)5(h2-HCꢀCR)] compounds studied here, the
acetylenic proton resonance appears at ca. 4 ppm in
toluene-d8 solution and at ca. 5 ppm in CDCl3 solution
(Table 1). A similar chemical shift has been observed
previously by Templeton et al. for the acetylenic pro-
Table 1
1H- and 13C-NMR characteristics of [W(CO)5(h2-alkyne)] (1) compounds
Compound
Solvent T (K) 1H-NMR, lH (ppm), J (Hz)
13C-NMR lC(ppm), J (Hz)
2
[W(CO)5(HCꢀCH)]
C7D8
C7D8
293
243
4.47 s, JWH 4.4
4.07 s
Not observed
202.50 (1 CO), JWC 153; 195.72 (4 CO), JWC 125;
62.53 dd (ꢀCH), JCH 255, JCH 41, JWC 7.3
Not observed
203.16 (1 CO); 195.32 (4 CO); 65.37 s (ꢀCH)
Not observed
1
1
1
2
1
CDCl3 293
CDCl3 233
C7D8
5.83 s
5.86 s
4
2
[W(CO)5(HCꢀCCH3)]
293
3.79 q (CH), JHH 2.4, JWH 4.5;
2.05 d (CH3), JHH 2.4
3.46 q (CH), 1.86 d (CH3)
4
1
1
C7D8
243
202.03 (1 CO), JWC 156; 196.28 (4 CO), JWC 126;
2
2
72.94 dq (ꢀC), JCH 44.9, JC(CH3) 9.0; 52.51 dq (ꢀCH),
3
1
1
1JCH 255, JCH 3.6, JWC 7.2; 9.98 q (CH3), JCH 133
CDCl3 293
CDCl3 243
4.81 q (CH), 2.82 d (CH3)
4.83 q (CH), 2.83 d (CH3)
Not observed
202.47 (1 CO); 195.93 (4 CO); 74.03 (ꢀC); 53.64
(ꢀCH); 11.61 (CH3)
Not observed
2
[W(CO)5(HCꢀCC(CH3)3)] C7D8
293
243
4.24 s (CH), JWH 5.0; 1.19 s
(3CH3)
3.93 s (CH), 1.16 s (3CH3)
1
1
C7D8
203.30 (1 CO), JWC 145; 197.14 (4 CO), JWC 124; 96.26
(ꢀC); 54.41 (ꢀCH); 34.17 s (C(CH3)3); 31.10 (3 CH3),
1
1
CDCl3 243
5.19 s (CH), 1.44 s (3CH3)
203.48 (1 CO), JWC 146; 196.64 (4 CO), JWC 125; 97.28
2
3
1
dq (ꢀC), JCH 37.3, JCH 6.0, JWC 9.3; 54.60 d (ꢀCH),
1
1JCH 250, JWC 6.6; 34.03 (C(CH3)3); 31.45 dq (3 CH3),
3
1JCH 127, JCH 9.0
3
1
1
[W(CO)5(CH3CH2Cꢀ
CDCl3 243
CDCl3 243
CDCl3 243
2.80 q (CH2), JHH 7.4; 1.31 t
202.40 (1 CO), JWC 154; 196.27 (4 CO), JWC 126; 71.93
3
1
1
2
CCH2CH3)]
(CH3), JHH 7.4
s (ꢀC), JWC 7.2; 21.77 tq (CH2), JCH 133, JCH 4.5;
1 2
16.58 qt (CH3), JCH 129, JCH 4.5
[W(CO)5(C1H3C2ꢀC3
C4ꢀC5C6H3)]
s (C1H3), 2.19 s (C6H3)
2.99 s (C1H3)
204.05 (1CO), JWC 153; 195.78 (4CO), JWC 126; 91.65
(C2); 70.24 (C5); 65.13 (C4); 55.66 (C3); 14.73 (C1); 5.08
(C6)
1
1
1
1
[{W(CO)5}2(C1H3C2ꢀC3
C3ꢀC2C1H3)]
(1CO), JWC 148; 195.21 (4CO), JWC 126; 83.73 (C2),
1JWC 8.3; 60.40 (C3), JWC 5.5; 15.20 (C1)
1

T. Szyman´ska-Buzar, K. Kern / Journal of Organometallic Chemistry 622 (2001) 74–83
77
Fig. 1. w(CO) region of the IR absorption spectrum of W(CO)6 and ethyne reaction products formed after broad-band UV–Vis photolysis in
n-hexane solution at ca. 273 K: (a) after 10 min and (b) after irradiation for a further 10 min, showing the formation of [W(CO)5(h2-HCꢀCH)]
(bands denoted by 1); (c) IR spectrum (KBr disc) of the precipitate formed in the above photochemical reaction. Band denoted by an asterisk (*)
is due to W(CO)6.
tons of fac-[W(CO)3(dppe)(HCꢀCR)], R=H, n-Bu and
Ph, lH=5.78 (CD2Cl2), 4.49 (CD2Cl2) and 6.21 (ben-
zene-d6), respectively [23]. Such a small coordination
shift DlH from 2 to 4 ppm, is indicative of alkyne
behaving as a formal two-electron donor to tungsten
[24,25]. In good agreement with this description of

78
T. Szyman´ska-Buzar, K. Kern / Journal of Organometallic Chemistry 622 (2001) 74–83
2
1
bonding is the small value of ca. 5 Hz of JWH (Table
1).
to an alkyne ligand (lC:196 and JWC=126). A simi-
lar relation has been observed for the analogous alkene
pentacarbonyl complexes of tungsten [20,21]. This indi-
cates shorter and stronger tungsten–carbonyl bonds
trans to alkyne or alkene ligands than trans to carbonyl
ligands. By contrast, competition for a dp orbital of
tungsten causes an alkyne ligand trans to CO to interact
weakly with the tungsten atom. This was proved by
X-ray crystal structure analysis of [W(CO)5(h2-
HCꢀCPh)], where the W–C bond distance of the car-
The 13C-NMR spectra are the most informative in
showing resonances for the alkyne carbons. The chemi-
cal shifts lC of the acetylenic carbon atoms (ꢀCH) are
–
in the range 52 65 ppm and the acetylenic carbon
–
atoms (ꢀCR) are in the range 72 97 ppm (Table 1).
That means the 13C resonance signals of the acetylenic
carbon atoms (ꢀCH) of the free alkyne are shifted to
higher field (DlC:10) on coordination to the tung-
sten(0) centre in [W(CO)5(h2-HCꢀCR)]. Similar
acetylenic carbon resonances have been reported for
other tungsten(0) alkyne complexes (lC=79.8 and 65.2
for [W(CO)5(h2-HCꢀCPh)] [18] and lC=82.4 and 76.5
,
bonyl ligand trans to phenylacetylene of 1.969(10) A is
shorter than the W–C bond distance cis to the alkyne
,
ligand (2.05 A average) [18].
The relative concentration of photochemically gener-
ated compound 1 depends first of all on the time of
irradiation and temperature. Prolonged irradiation
leads to dissociation of the next molecules of CO and
the formation of a compound containing two or three
alkyne ligands (Scheme 1).
Thermally unstable compound 1 can decompose to
release the weakly bonded alkyne or isomerise to give
vinylidene species [W(CO)5(CꢁCHR)] (2) detected here
by 1H-NMR (see Section 3.2). It is known that the
vinylidene complex can be formed only by photochemi-
cal reaction of coordinated terminal alkynes such as
1
for [W(CO)3(dppe)(HCꢀCPh)] [23]). The JCH=255 Hz
and 2JCH=41 Hz are unprecedently large for
[W(CO)5(h2-HCꢀCH)] relative to 1JCH=249 Hz and
2JCH=49.3 Hz for free ethyne [26]. Similar coupling
constants are observed here for other alkyne pentacar-
bonyl compounds of tungsten(0) (Table 1). The cou-
pling constant of acetylenic carbon to tungsten
1JWC:7 Hz also reflects very weak tungsten–alkyne
bonding. All these NMR data are consistent with weak
coordination of alkyne ligands and the alkyne ligands
acting formally as two-electron donors [24,25].
t
Moreover, what we take to be a significant feature of
these alkyne pentacarbonyl complexes of tungsten(0) is
the value of carbonyl carbon resonances lC and tung-
sten–carbon coupling JWC (Table 1). The value of
lC:203 and JWC:150 for a carbonyl ligand trans to
HCꢀCH, MeCꢀCH and BuCꢀCH but the pentacar-
bonyl alkyne adducts formed by reaction of internal
alkynes such as EtCꢀCEt or MeCꢀCCꢀCMe are much
more stable. In photochemical reaction of W(CO)6 and
2,4-hexadiyne two adducts, with one and two pentacar-
bonyl moieties, were characterised by NMR (Table 1).
1
1
an alkyne ligand is higher than for a carbonyl ligand cis
Scheme 1.

T. Szyman´ska-Buzar, K. Kern / Journal of Organometallic Chemistry 622 (2001) 74–83
79
Fig. 2. The part of the 1H-NMR spectrum (300 MHz) in CDCl3 solution at 243 K of W(CO)6 and propyne reaction products formed after 2 h
broad-band UV–Vis photolysis in n-hexane solution at ca. 273 K. The proton signals are labelled as follows: (1) lH=4.83, [W(CO)5(h2-
HCꢀCMe)]; (2) lH=3.38, [W(CO)5(CꢁCHMe)]; (3) lH=3.28, {W(CO)5}2(m-CꢁCHMe)]. The signals due to compounds 1, 2 and 3 are shown at
scale expansion in the inset. The signals denoted by an asterisk (*) are due to 13C satellites.
[W(CO)3(dppe)(CꢁCHPh)], where the vinylidene proton
was observed at lH=4.42 in CDCl3 [31], i.e. much
lower than the acetylenic proton in its alkyne complex
counterparts (lH=6.76 in CD2Cl2 [23]). The highest
concentration of vinylidene species was achieved in the
reaction of propyne with W(CO)6. In this reaction the
intensity of the signal due to the vinylidene proton
(lH=3.38 in CDCl3) exceeded the intensity of the
acetylenic proton signal at lH=4.83 of [W(CO)5(h2-
HCꢀCCH3)] (Fig. 2). The lowest concentration of the
vinylidene species was observed in the reaction of tert-
butylacetylene. The intensity ratio between both vinyli-
dene signals changes with reaction time, but at the
begining the lower field signal is always more intense.
Only in propyne reaction, after prolonged irradiation
(3–4 h), did these two signals achieve almost equal
intensity (Fig. 2). The latter vinylidene proton signal
has been correlated with three carbon signals in the
13C-NMR spectrum: at lC 209.41, 205.13 and 197.07
(toluene-d8, 243 K) of ethyne reaction products and at
lC 210.61, 205.39 and 197.84 (CDCl3, 243 K) of
propyne reaction products (Fig. 3). However, in the
tert-butylacetylene reaction this kind of compound was
3.2. NMR characteristic of the 6inylidene species
The isomerisation of an h2-coordinated to a transi-
tion metal terminal alkyne into a vinylidene ligand is a
well known process [27–31]. In d6 octahedral complexes
this rearrangement is promoted by a repulsion between
filled dp of the metal and pÞ orbitals of the alkyne
ligand (4e-2-centre dp–pÞ conflict) [30,31].
During the course of the photochemical reaction of
W(CO)6 and terminal alkyne, two signals characteristic
of two different vinylidene ligands coordinated to the
1
tungsten centre have been observed in H-NMR spec-
tra, at a position at least 1 ppm lower than the
acetylenic proton signals of the h2-alkyne compound 1
(CꢁCH2: toluene-d8, 294 K, lH=2.84 and 2.81; 243 K
lH=2.61 and 2.59; CDCl3, 243 K, lH=3.67 and 3.39;
CꢁCHCH3: toluene-d8, 243 K, lH=2.70 and 2.51;
3
CDCl3, 243 K, lH=3.38 q, JHH=1 Hz, and 3.28 q,
3
3JHH=1 Hz, JWH=13 Hz, I=1/2, 27%; CꢁCHtBu:
CDCl3, 243 K, lH=3.44 and 3.41). Both signals shift
similarly with changes in the temperature and solvent.
1
These H-NMR data are comparable with those ob-
tained for the tungsten vinylidene complex

80
T. Szyman´ska-Buzar, K. Kern / Journal of Organometallic Chemistry 622 (2001) 74–83
formed only in trace amounts. The signal at l ca. 198 is
accompanied by 183W satellites (1JWC=64 Hz, I=1/2,
14.5%×2=29%), indicating bridging carbon between
two equivalent tungsten centres. The latter signal ap-
pears as a singlet in the proton-coupled 13C-NMR
spectrum, meaning that this carbon is not carrying the
proton and excludes the origin of this signal being the
bridging alkyne ligand. The carbon signal of the bridg-
ing carbonyl ligands in the tungsten carbonyl clusters
have always been observed about 40 ppm higher than
that for terminal carbonyls [32]. On the basis of this
observation, the formation of two vinylidene com-
plexes, the mononuclear [W(CO)5(CꢁCHR)](2) and the
dinuclear [{W(CO)5}2(m-CꢁCHR)] (3) has been postu-
lated (Scheme 1). Compound 3 is formed under photo-
chemical conditions, as the result of the interaction of
the coordinatively unsaturated pentacarbonyltungsten
moiety with the vinylidene ligand. There are many
example of compounds with bridging vinylidene ligands
reported in the literature [29,33–35], however there are
only a few reports of complexes containing the tungsten
pentacarbonyl moiety. One of them is [W2(CO)9(m-
C(OMe)CHꢁCH2)], formed in the interaction of the
a,b-unsaturated carbene pentacarbonyltungsten com-
plex with the W(CO)5 moiety, generated in a photo-
chemical reaction. In the NMR spectrum of this
compound the signal of a-carbon for the bridging
carbene ligand appears at lC 232.56, that is 77 ppm
lower than in its terminal carbene counterparts [36,37].
The chemical shift values of a-carbon observed for
other dimetallic bridging carbene complexes are in the
range 200–100 ppm which is at a much higher field
than for mononuclear compounds [38]. The concentra-
tion of the mononuclear vinylidene species 2 is too low
to detect the low-field resonance of the a-vinylidene
carbon but the signal of the b-carbon was observed at
lC 112.53 (CꢁCH2). The carbonyl carbon resonances
for the latter compound 2 were detected at lC 202.03
(1CO) and 195.76 (4CO), very close to the signals of the
ethyne compound 1 (Table 1).
In this way we were able to detect the rearrangement
products of [W(CO)5(h2-HCꢀCR)] complexes not only
by IR spectra of low-temperature matrix [11] but also
by NMR investigations.
3.3. Complexes of tungsten(0) with two or three alkyne
ligands
The photosubstution of two CO groups by alkyne
molecules in W(CO)6 can lead to cis-[W(CO)4(h2-
Fig. 3. The 13C{1H}-NMR spectrum (75.5 MHz) in CDCl3 solution at 243 K of the reaction products formed after 2 h broad-band UV–Vis
photolysis in n-hexane solution at ca. 273 K of W(CO)6 and propyne. The carbonyl region of the spectrum is shown at scale expansion in the
inset. The signals are labelled as follows: (1) [W(CO)5(h2-HCꢀCMe)]; (3) lC 198.28 (a-carbon of the bridging vinylidene ligand in {W(CO)5}2(m-
CꢁCHMe)]); (4) cis-[W(CO)4(h2-HCꢀCMe)2]; (5) trans-[W(CO)4(h2-HCꢀCMe)2]; (h) hexane; (0) W(CO)6. The signals denoted by an asterisk (*)
are due to 183W satellites.

T. Szyman´ska-Buzar, K. Kern / Journal of Organometallic Chemistry 622 (2001) 74–83
81
alkyne)2] (4) or trans-[W(CO)4(h2-alkyne)2] (5), but IR
spectra measured at room temperature did not signal
the appearance of the latter compounds (four bands or
one band in the carbonyl region, respectively [39]). The
reason is that such compounds are too labile. For the
same reason, compounds with three alkyne and three
carbonyl ligand are unknown. However, in the photo-
chemical reaction of terminal alkyne and W(CO)6 at
low temperature studied here by NMR, the acetylenic
proton and carbon signals, which could be assigned to
two-electron donor alkyne ligands in trans and cis-
bis(alkyne) compounds, have been detected. For ethyne
reaction products such signals appear at lH=6.96 and
5.20 in toluene-d8. For the latter signal tentatively
assigned to the cis isomer the coupling constants
tert-butyl group. It must be noted that bis(alkyne)
compounds 4 and 5 can be observed only at low
temperature and just after preparing the sample. Slow
decomposition of these compounds was observed even
at 243 K.
Although numerous bis(alkyne) complexes are
known, nearly all of these compounds contain the
metal in the d4 configuration and the two alkyne lig-
ands in mutually cis position [25]. The first tungsten(0)
d6 complex [W(CO)2(dppe)(DMAC)2] (DMAC=
dimethylacetylenecarboxylate) was prepared by Tem-
pleton et al. [23]. Recently Wang et al. [40–42] reported
a series of tungsten(0) and molybdenum(0) bis(alkyne)
complexes of the type [M(CO)2(NN)(alkyne)2], where
NN is a bidentate nitrogen ligand. In these compounds
two alkyne ligands are mutually trans and perpendicu-
lar and in 13C-NMR spectra display the acetylenic
carbon signals in the region 152–134 ppm [40–42].
The very well known thermodynamically stable
alkyne compounds contain three alkyne and one car-
bonyl ligand [2–7]. In these types of compounds each
alkyne is regarded as a 3.3 electron-donor ligand [24].
The characteristic feature of their 13C-NMR spectra is
the position of the acetylenic carbon resonances in the
region 200–160 ppm and the signal at lH ca. 10 ppm
for the acetylenic proton (ꢀCH) [8,12–17,19]. In termi-
nal alkyne–W(CO)6 reaction products investigated
here, such signals were detected. Two almost equal
intense signals at lH 10.56 (2JWH=7 Hz) and 9.77
(2JWH=9 Hz), with very close value of tungsten–hy-
drogen coupling were observed at 243 K in toluene-d8
solution of ethyne reaction products. When the temper-
ature is increased, the resonances broaden and at ca.
283 K merge into the baseline. The appearance of two
proton signals for [W(CO)(HCꢀCH)3] can be explained
by the structure of this type of compound [5], in which
a weaker interaction between the tungsten centre and
three equivalent acetylenic carbon atoms close to the
CO ligand was observed, than with other three
acetylenic carbon atoms.
2
1JCH=229 Hz and JWH=3.4 Hz were observed. The
acetylenic carbon signal was identified only for one
compound, most probably the cis isomer, at lC 73.62 as
1
1
a doublet, with JCH=229 Hz, JWC=14.5 Hz in tolu-
ene-d8. The acetylenic carbon signal (ꢀCH) of the cis-
bis(propyne) compound was observed at lC 73.15 d,
1JCH=226 Hz and the trans-bis(propyne) signal was
1
detected at lC 152.17 d (1JCH=196 Hz, JWC=34 Hz)
in CDCl3 at 243 K. In the same sample, signals due to
1
the bis(propyne) compounds in H-NMR spectra were
observed at lH 6.64 and 5.76, the latter tentatively
assigned to the cis isomer. For the bis(tert-buty-
lacetylene) compound 4 two signals of the acetylenic
protons were observed in toluene-d8 at very close posi-
tions, lH 6.05 and 6.03, and the intensity ratio ca. 1:3,
respectively. However, in CDCl3 solution these signals
are unresolved and only one resonance at lH 6.90
(2JWH=7.5 Hz) was detected. For these compounds
two acetylenic carbon signals (ꢀCH) were observed at
1
lC 102.18 (1JCH=214 Hz, JWC=28.7 Hz) and 95.23
1
(1JCH=200 Hz, JWC=12.6 Hz, toluene-d8). These two
acetylenic proton and carbon signals could be assigned
to two rotational isomers I and II, produced when two
identical unsymmetrical alkyne ligands are bound to a
W(CO)4 fragment in the mutually cis position and
parallel alignment of the CꢀC linkage.
For the trans isomer of bis(tert-butylacetylene) com-
pound 5 the acetylenic proton signal at lH 6.97 and the
acetylenic carbon signal at lC 148.10 (ꢀCH) were de-
tected in toluene-d8 at 243 K. However, we could not
detect the signals due to alkyne carbons carrying the
Due to the inequivalent ends of the terminal alkynes
in [W(CO)(HCꢀCR)3] (6) and different orientations of
alkyne ligands relative to the CO group, complexes of
this type can exist as a mixture of four isomers [19].
This leads to the observation of several acetylenic hy-
1
1
drogen signals in their H-NMR spectra. The H-NMR
spectrum of propyne reaction products at 243 K in
toluene-d8 solution displays four quartets (4JHH=1.4
Hz) at lH 11.08, 10.90, 9.08 and 9.04 in the integral
intensity ratio 1:3:3:14, respectively. The most intense
signal at lH 9.04 is accompanied by satellites due to 13C
(1JCH=196 Hz). Similar signals were observed in the
1H-NMR spectrum at 243 K in toluene-d8 solution of
tert-butylacetylene reaction products: lH 11.12 (1JCH
=
196 Hz), 10.44 (2JWH=5 Hz, 1JCH=201 Hz), 9.79
1
(2JWH=7 Hz) and 9.00 (2JWH=6 Hz, JCH=200 Hz)

82
T. Szyman´ska-Buzar, K. Kern / Journal of Organometallic Chemistry 622 (2001) 74–83
in the intensity ratio 13:8:1:4, respectively. In the 13C-
NMR spectrum, the acetylenic carbon signal was de-
tected at lC 172.71 as a doublet, with JCH:200 Hz
tons of h6-tri-t-butylbenzene ligand (lH 5.31 and 4.10,
respectively). In the 13C-NMR spectrum the signals due
to [W(CO)3(h6-C6H6)] were detected at lC 208.27
1
(ꢀCH), and a singlet at lC 188.00 (ꢀCtBu). Four reso-
nances at lC 226.58, 224.44, 218.16 and 217.92 with the
intensity ratio ca. 2:1:12:1, respectively, can be assigned
to the CO group in four isomers of [W(CO)-
(HCꢀCtBu)3]. The intensity ratio of the proton and
carbon signals changed as the sample grew older, but
the most intense was always the acetylenic proton sig-
nal at lH 11.12 and the carbonyl carbon signal at lC
218.16. These signals could be assigned to the most
stable isomer of [W(CO)(HCꢀCtBu)3] compound, con-
taining three chemically equivalent tert-butylacetylene
ligands, with the same orientation of the alkyne ligands
towards CO [43]. Simultaneously, with the disappear-
ance of [W(CO)(HCꢀCtBu)3] signals, the increase of the
signals characteristic of 1,3,5-tri-t-butylbenzene [44], at
lH 7.29 (HC) and 1.33 (tBu), lC 149.87 (CtBu) and
119.44 (CH) was observed in NMR spectra in CDCl3
solution. This indicates that the decomposition of the
labile [W(CO)(HCꢀCtBu)3] complex is the direct way in
which the substituted arenes are formed (Scheme 1).
(3CO) and 89.64, JCH=180 Hz (C6H6). Careful exam-
1
ination of specially synthesised compounds with
mesitylene and pseudocumene ligands (see Section 2.4)
allowed us to detect their formation in every photo-
chemical reaction of propyne and W(CO)6. The photo-
chemical reaction products analysed by NMR at 243 K,
just after reaction, contain the mesitylene and pseudoc-
umene complexes in the ratio ca. 1:5. This ratio changes
to ca. 1:3 as the sample is warmed and grows older.
This could suggest the higher stability of such alkyne
compounds, which after decomposition lead to the
formation of the h6-1,3,5-C6H3Me3 ligand then com-
pounds giving the h6-1,2,4-C6H3Me3 ligand.
According to a generally accepted cyclotrimerisation
mechanism [45–48], the reaction proceeds with the
photosubstitution of the CO ligands in W(CO)6 by the
molecules of alkyne and the formation of metallacycles.
Tungstenacyclopentadiene can be formed via oxidative
coupling of the two alkyne ligands in the cis-bis(alkyne)
complex and tungstenacycloheptatriene, as the result of
C–C bond formation between the three alkyne ligands
in the fac-tris(alkyne) complex. The reductive elimina-
tion reaction leads to the formation of h4-butadiene or
h6-arene ligands, respectively. In the photochemical
reaction investigated here of W(CO)6 and alkyne, com-
plexes of the type [W(CO)3(h6-arene)] (7) are observed,
indicating the formation of fac-[W(CO)3(h2-alkyne)3]
compounds as very labile intermediate species (Scheme
1).
3.4. Identification of [W(CO)3(p6-arene)] and free arene
The alkyne carbonyl complexes formed in a sequen-
tial photosubstitution of carbon monoxide in W(CO)6
by alkyne were shown to be involved in the cy-
clotrimerisation of alkynes to arenes: benzene,
trimethylbenzene and tri-t-butylbenzene, which were
detected by NMR spectroscopy in photochemical reac-
tions of W(CO)6 and each of the alkynes HCꢀCH,
HCꢀCMe and HCꢀCCMe3, respectively. In the case of
propyne reaction two different positional isomers of
trimethylbenzene (mesitylene and pseudocumene) were
detected in the ratio ca. 1:1.5 by GC-MS investigations
of the reaction products. The 1H-NMR spectrum
(CDCl3) displayed four aromatic protons resonances:
Although cyclotrimerisation of alkynes to arenes is
not uncommon in homogeneous catalysis by transition
metal compounds [45–48], to the best of our knowl-
edge, this is the first direct evidence for transformation
of h2-alkyne ligands coordinated to tungsten to h6-
arene ligands.
3
4
lH 7.01 d, JHH=7.9 Hz, lH 6.95 d, JHH=0.8 Hz, lH
3
4
6.90 dd, JHH=7.9 Hz, JHH=0.8 Hz and lH 6.80 s.
The first three signals are due to 1,2,4-trimethylbenzene
and the fourth (singlet) due to 1,3,5-trimethylbenzene.
In addition to signals assigned to the free arenes, in the
1H- and 13C-NMR spectra of the reaction mixture,
resonances due to the h6-arene ligand derived from an
alkyne cyclotrimerisation product (benzene, trimethyl-
benzene and tri-t-butylbenzene) coordinated to the tri-
carbonyltungsten moiety are readily developed. The
resonances of the h6-arene ligands are solvent depen-
dent. In CDCl3 solution the h6-arene ligand protons are
observed at lH ca. 1 ppm higher than in toluene-d8
solution (see Section 2.4). The six protons of the h6-
benzene ligand are observed at lH 5.36 in CDCl3 solu-
tion and at lH 4.25 in toluene-d8 solution (293 K). A
similar chemical shift was observed for the three pro-
4. Conclusions
Studies of alkyne reactions in the presence of tung-
sten carbonyls have attracted our interest, due in part
to the desire to find the intermediate compounds in
tungsten-based catalytic processes.
During the course of the photochemical reaction of
W(CO)6 and terminal alkyne, besides the 1H-NMR
signals due to the alkyne compound [W(CO)5(h2-
HCꢀCR)] (1), the signals characteristic of the vinylidene
ligand coordinated to the pentacarbonyl tungsten moi-
ety have been observed. This kind of compound is
responsible for initiating the polymerisation reaction of
terminal alkynes [9,10].
Photolysis of W(CO)6 in the presence of alkyne leads
to the generation of alkyne carbonyl complexes of

T. Szyman´ska-Buzar, K. Kern / Journal of Organometallic Chemistry 622 (2001) 74–83
83
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–
tungsten in which C C coupling of the alkyne ligands
has occurred. In addition to signals attributable to the
h2-alkyne ligand in alkyne carbonyl complexes of tung-
1
sten, resonances in the aromatic region of the H- and
13C-NMR spectra of reaction mixture are readily at-
tributable to the h6-arene ligand, derived from an
alkyne cyclotrimerisation product (benzene, trimethyl-
benzene and tri-t-butylbenzene) coordinated to the
W(CO)3 moiety.
Of interest is the formation of free arene observed by
1H- and 13C-NMR spectroscopy in the photochemical
reaction of W(CO)6 and HCꢀCH, HCꢀCMe and
HCꢀCtBu. That means that W(CO)6 shows acetylene
cyclotrimerisation activity under photochemical
conditions.
To the best of our knowledge this paper represents
the first study that has simultaneously detected the
thermally unstable h2-alkyne complexes and their rear-
rangement products, the h6-arene compounds of
tungsten.
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Acknowledgements
The authors thank Dr M. Kowalska and Mrs
Baczyn´ski for recording the NMR spectra.
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