Substitution kinetics of W(CO)5(η2-bis(trimethylsilyl)ethyne) with triphenylbismuthine

Article abstract of DOI:10.1016/j.jorganchem.2006.03.036

The labile complex W(CO)₅(η²-btmse) undergoes replacement of bis(trimethylsilyl)ethyne, btmse, by triphenylbismuthine in cyclohexane solution at an observable rate in the temperature range of 35-50 °C yielding almost solely W(CO)₅(BiPh₃) as the final product. The kinetics of this substitution reaction was studied in cyclohexane solution by quantitative FT-IR spectroscopy. The substitution reaction obeys a pseudo-first-order kinetics with respect to the concentration of the starting complex. The observed rate constant, k_obs, was determined at four different temperatures and three different concentrations of the entering ligand BiPh₃ in the range 16.8-65.4 mM. From the evaluation of kinetic data a possible reaction mechanism was proposed in which the rate determining step is the cleavage of metal-alkyne bond in the complex W(CO)₅(η²-btmse). A rate law was derived from the proposed mechanism. From the dependence of k_obs on the entering ligand concentration, the rate constant k₁ for the rate determining step was estimated at all temperatures. The activation enthalpy (106 ± 2 kJ mol^-1) and the activation entropy (111 ± 6 J K^-1 mol^-1) were determined for this rate determining step from the evaluation of k₁ values at different temperatures. The large positive value of the activation entropy is consistent with the dissociative nature of reaction. The large value of the activation enthalpy, close to the calculated tungsten-alkyne bond dissociation energy, also supports this dissociative rate-determining step of the substitution reaction.

Full text of DOI:10.1016/j.jorganchem.2006.03.036

Journal of Organometallic Chemistry 691 (2006) 3267–3273
www.elsevier.com/locate/jorganchem
Substitution kinetics of W(CO)5(g2-bis(trimethylsilyl)ethyne)
with triphenylbismuthine
*
¨
Ercan Bayram, Saim Ozkar
Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
Received 27 February 2006; received in revised form 27 March 2006; accepted 27 March 2006
Available online 25 April 2006
Abstract
The labile complex W(CO)5(g2-btmse) undergoes replacement of bis(trimethylsilyl)ethyne, btmse, by triphenylbismuthine in cyclohex-
ane solution at an observable rate in the temperature range of 35–50 ꢀC yielding almost solely W(CO)5(BiPh3) as the final product. The
kinetics of this substitution reaction was studied in cyclohexane solution by quantitative FT-IR spectroscopy. The substitution reaction
obeys a pseudo-first-order kinetics with respect to the concentration of the starting complex. The observed rate constant, kobs, was deter-
mined at four different temperatures and three different concentrations of the entering ligand BiPh3 in the range 16.8–65.4 mM. From the
evaluation of kinetic data a possible reaction mechanism was proposed in which the rate determining step is the cleavage of metal–alkyne
bond in the complex W(CO)5(g2-btmse). A rate law was derived from the proposed mechanism. From the dependence of kobs on the
entering ligand concentration, the rate constant k1 for the rate determining step was estimated at all temperatures. The activation
enthalpy (106 2 kJ molꢀ1) and the activation entropy (111 6 J Kꢀ1 molꢀ1) were determined for this rate determining step from
the evaluation of k1 values at different temperatures. The large positive value of the activation entropy is consistent with the dissociative
nature of reaction. The large value of the activation enthalpy, close to the calculated tungsten–alkyne bond dissociation energy, also
supports this dissociative rate-determining step of the substitution reaction.
ꢁ 2006 Elsevier B.V. All rights reserved.
Keywords: Tungsten; Carbonyl; Bis(trimethylsilyl)ethyne; Triphenylbismuthine; Kinetics; Substitution; FT-IR spectroscopy
1. Introduction
tene [3]. In the case of tricyclohexylphosphine, the forma-
tion of mono and disubstitution products has been
observed [4]. This lability of W(CO)5(g2-btmse) makes it
a useful starting material for the syntheses of some inac-
cessible tungsten-carbonyl derivatives, similar to how
Cr(CO)5(g2-Z-cyclooctene) serves as a convenient Cr(CO)5
source [5]. These results motivated us to examine the kinet-
ics of the substitution reaction of W(CO)5(g2-btmse) with
triphenylbismuthine for which there is relatively little work
has been reported either on simple synthesis or substitution
reactions [6]. Although substituted group 6 metal–carbonyl
complexes containing ligands having group 15 and 16
donor atoms have been widely studied [7] and remains
one of the most active areas of coordination chemistry
[8], the reported coordination chemistry of triarylbismu-
thine is rather meager due to the fact that it has very weak
nucleophilic or donor character [9]. Furthermore, the
Pentacarbonyl(g2-bis(trimethylsilyl)ethyne)tungsten(0),
W(CO)5(g2-btmse), has been shown to be stable enough to
be isolated as yellow crystals from the hydrocarbon solu-
tion [1]. Although this complex survives in solution even
in the absence of btmse at room temperature under argon
atmosphere at room temperature, it is labile towards
ligand substitution. It can undergo both CO and alkyne
substitution depending on the incoming ligand [2]. For
example, it gives only alkyne substitution product with eth-
ene and 1,3-butadiene, while the btmse and one CO ligand
are concomitantly replaced by two molecules of E-cyclooc-
*
Corresponding author. Fax: +90 312 210 1280.
¨
E-mail address: sozkar@metu.edu.tr (S. Ozkar).
0022-328X/$ - see front matter ꢁ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.03.036

¨
E. Bayram, S. Ozkar / Journal of Organometallic Chemistry 691 (2006) 3267–3273
3268
bismuth–carbon bond breaks easily in the presence of
many metal centres [10]. Thus, the coordination of triphe-
nylbismuthine to a transition metal requires milder condi-
tion. Using the labile complex, W(CO)5(g2-btmse) provides
a versatile synthetic route to the carbonyl–tungsten com-
plexes of BiPh3. The reaction between W(CO)5(g2-btmse)
and BiPh3 yields solely the alkyne substitution product,
W(CO)5(BiPh3). The latter complex was also isolated from
the reaction of photogenerated W(CO)5(thf) with triphe-
nylbismuthine in tetrahydrofuran (thf) and identified [8].
Herein, we report the results of a kinetic study on the
ligand substitution reaction between pentacarbonyl(g2-
bis(trimethylsilyl)ethyne)tungsten(0) and triphenylbismu-
thine in cyclohexane solution. This is the first kinetic study
of substitution reaction of a group 6 metal penta-
carbonylalkyne complex with a bismuthine ligand. The
ligand substitution reaction could be followed quantita-
tively by FT-IR spectroscopy.
The kinetic measurements were recorded in cyclohexane
solutions in an IR cell with a 0.20 mm path length with cal-
cium fluoride windows. Since the substitution reaction does
not proceed at an observable rate at room temperature, the
mixture of reactants were stirred in a Schlenk tube under
nitrogen atmosphere for 1–2 min to obtain a homogeneous
mixture of the reactants. It was then transferred into an IR
liquid sample cell, which is placed inside the cell holder of
the thermostat in which the rest of the reaction mixture is
maintained. The reaction rates were determined by follow-
ing the disappearance of the highest frequency peak of the
reactant at 2080.1 cmꢀ1, because it is the most distinct peak
that does not overlap with any peak of the product, and the
growth of the highest frequency peak of the product at
2074.1 cmꢀ1, which also remains well resolved throughout
the reaction. Thus, the material balance could also be
checked at any point of conversion. The observed rate con-
stants were obtained as a result of the graphical evaluation
of the data for the substitution reactions in the temperature
range of 35–50 ꢀC by an increment of 5 ꢀC.
2. Experimental
Kinetic experiments were also performed by varying the
concentration of BiPh3 (16.8, 33.7, and 67.4 mM, corre-
sponding to 5-, 10- and 20-fold the initial concentration
of the complex) at all the aforementioned temperatures,
35, 40, 45 and 50 ꢀC, in order to study the dependence of
the observed rate constants on the concentration of enter-
ing ligand. Moreover, kinetic experiments were also con-
ducted by varying the concentration of btmse (0, 16.8,
and 33.7 mM, corresponding to 0-, 5- and 10-fold the ini-
tial concentration of the complex) at 45 ꢀC in order to
study the dependence of the observed rate constants on
the concentration of leaving ligand.
All of reactions and manipulations were carried out
either in vacuum or under a dry and oxygen free nitrogen
atmosphere. Solvents were distilled after refluxing over
metallic sodium or phosphorous pentoxide for 3–4 days
and stored under nitrogen until used. Hexacarbonyltung-
sten(0) and bis(trimethylsilyl)ethyne were purchased from
Aldrichꢂ, triphenylbismuthine was purchased from Flukaꢂ
and used without further purification. The thermal reac-
tions and other treatments of organometallic compounds
such as purification and crystallisation were followed by
taking IR spectra at appropriate time intervals. NMR spec-
tra were recorded on a Bruker DPX 400 spectrometer
The IR molar extinction coefficients of W(CO)5(g2-
btmse) and W(CO)5(BiPh3) were determined from the stan-
dard solutions prepared by dissolving the known amount of
pure complexes in cyclohexane. A calibration curve was
obtained by plotting absorbance at the highest frequency
peak of the reactant or product versus concentration of the
respective complex in the range 2–10 and 20–100 mM,
respectively. The slope of these lines gave molar extinction
coefficients as e = 2578 mol Lꢀ1 cmꢀ1 at 2080.1 cmꢀ1
for W(CO)5(g2-btmse) and e = 7320 mol Lꢀ1 cmꢀ1 at
2074.1 cmꢀ1 for W(CO)5(BiPh3).
1
(400.132 MHz for H, and 100.613 MHz for 13C). TMS
was used as internal reference for 1H and 13C NMR chem-
ical shifts. Infrared spectra of the complexes were recorded
from their cyclohexane solutions on a Nicolet 510 FT-IR
Spectrophotometer using Omnic software. Photochemical
reactions were carried out in an immersion-well apparatus
[11] (solidex glass, k > 280 nm) by using a Hanau TQ 150
high-pressure mercury lamp, which was cooled by circulat-
ing water. The thermal substitution reactions were per-
formed in the Specac Variable Temperature Cell in
combination with a circulating thermostat bath (Heto CB
11e).
3. Results and discussion
W(CO)5(g2-btmse) [1] and W(CO)5(BiPh3) [8] were pre-
pared according to procedures given in the literature.
The complex W(CO)5(g2-btmse) (1), prepared and
identified as described in the literature [1], reacts with tri-
phenylbismuthine, BiPh3 at temperatures in the range of
35–50 ꢀC to yield the complex W(CO)5(BiPh3) (2) as the
sole substitution product according to the following
equation
2.1. Kinetic measurements
The substitution rate of btmse in W(CO)5(g2-btmse)
with BiPh3 in cyclohexane was determined by using quan-
titative FT-IR spectroscopy at various temperatures. The
thermal substitution reactions were performed in the IR
cell with the Specac Automatic Temperature Controller
P/N 20120 to maintain a constant temperature throughout
the reaction.
WðCOÞ5ðg2-btmseÞ þBiPh3
1
D
! WðCOÞ5ðBiPh3Þ þbtmse
ð1Þ
2

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E. Bayram, S. Ozkar / Journal of Organometallic Chemistry 691 (2006) 3267–3273
3269
The product complex 2 was also isolated and identified
by IR and 13C NMR spectroscopy [8].
In this thermal substitution reaction of the labile com-
plex 1 with BiPh3, only the replacement of the btmse ligand
occurs. In all experiments, W(CO)6 was formed in small
amount as monitored by the growing absorption band at
1982 cmꢀ1 (however, totally less than 5%). The formation
of W(CO)6 may be due to the decomposition of both the
reactant and product complexes. The product complex 2
is known to be slightly unstable in solution at temperatures
above 0 ꢀC due to the weak coordination ability of bismuth
[8]. Also very small amount of decomposition of the reac-
tant complex 1 may be anticipated since the detachment
of the alkyne ligand generates the reactive intermediate
W(CO)5. However, the W(CO)5 fragment will be attacked
by BiPh3 which is in large excess in solution. Hence, forma-
tion of W(CO)6 in small amount may be mostly attributed
to the decomposition of the product complex 2.
The logarithmic plot of concentration of the reactant
versus time gives a straight line for the aforementioned
reaction in the presence of triphenylbismuthine in large
excess at all temperatures, as illustrated in Fig. 2 for
the reaction at 45 ꢀC starting with [W(CO)5(g2-
btmse)]0 = 3.37 mM and [BiPh3]0 = 65.4 mM. This indi-
cates that the displacement of btmse in 1 obeys the
pseudo-first-order kinetics with a correlation constant
greater than 0.99. The slope of the straight line gives the
observed rate constant, kobs (sꢀ1), for the pseudo-first-
order thermal substitution reaction at the respective tem-
perature. The kobs values obtained for the substitution of
btmse in 1 by triphenylbismuthine at various temperatures
and initial concentration of BiPh3 are listed in Table 1.
It has been shown that rapid and complete conversion of
1 into W(CO)6 occurs upon treatment of an alkane solution
of 1 with carbon monoxide and this reaction is significantly
The kinetics of the thermal substitution reaction in
cyclohexane solution were studied by using the quantitative
FT-IR spectroscopy at temperatures between 35 and 50 ꢀC.
The IR spectrum initially shows five prominent absorption
bands at 2080.1, 1988.3, 1960.4, 1953.1, and 1938.6 cmꢀ1
due to the CO stretching along with a weak feature at
1906.0 cmꢀ1 due to the C„C stretching for W(CO)5(g2-
btmse). These bands were gradually replaced by the new
absorptions at 2074.1, 1949.3, and 1943.2 cmꢀ1 due to
the CO stretching for the corresponding product
W(CO)5(BiPh3) in the course of the displacement reaction.
The gradual change in the IR spectrum during the reaction
of 1 with BiPh3 at 45 ꢀC in cyclohexane is depicted in Fig. 1
as an example. The observation of nice isosbestic points
indicates an almost straightforward conversion of the reac-
tant into the product without any side or subsequent reac-
tions [12], except the formation of W(CO)6 in small
amount. Another point emerging from the inspection of
the IR spectra at first glance is that the highest frequency
v(CO) bands of the reactant and product do not overlap
and well resolved throughout the substitution reaction.
Therefore, these two IR bands were selected to follow the
consumption of the reactant and the growth of the prod-
uct, respectively.
Although one expects three IR active m(CO) modes,
2A1 + E (the B2 mode is IR inactive), for the product com-
plex 2 based on the C4v symmetry of W(CO)5 skeleton [13],
only two m(CO) bands have been given in the full character-
isation paper reporting a coincidence of A1 and E modes
[8]. However, in our study we also observed the A1 mode
as a shoulder to the E band at around 1943 cmꢀ1
.
0.50
0.45
0.40
0. 35
0.30
0.25
0.20
0.15
0. 10
0.05
-0.00
2100
2000
1900
Wavenumbers (cm-1
)
Fig. 1. The FT-IR spectra taken in the course of the thermal substitution reaction of W(CO)5(g2-btmse) with BiPh3 in cyclohexane solution at 45 ꢀC with
the initial concentrations of [W(CO)5(g2-btmse)]0 = 3.37 mM and [BiPh3]0 = 67.4 mM.

¨
E. Bayram, S. Ozkar / Journal of Organometallic Chemistry 691 (2006) 3267–3273
3270
0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
-1
0
500
1000
1500
2000
2500
3000
3500
Time (s)
Fig. 2. Logarithmic plot of the reacting complex concentration versus time for the pseudo-first-order substitution of btmse in W(CO)5(g2-btmse) with
BiPh3 at 45 ꢀC in cyclohexane. The initial concentrations are [W(CO)5(g2-btmse)]0 = 3.37 mM and [BiPh3]0 = 67.4 mM.
Table 1
1, or BiPh3 forming the product complex 2, as shown in
Scheme 1.
Observed rate constants (kobs · 105) for the pseudo-first-order thermal
substitution reaction of W(CO)5(g2-btmse) with BiPh3 in cyclohexane
Assuming a steady-state approximation for the interme-
solution at different temperatures in the presence of BiPh3 in various
diate, W(CO)5(solvent), the rate law corresponding to pro-
concentration
posed mechanism can be derived as
[BiPh3]0 (mM)
35 ꢀC
40 ꢀC
45 ꢀC
50 ꢀC
½WðCOÞ5ðg2-btmseÞꢂ
16.8
33.7
67.4
3.95
4.02
4.10
7.43
7.71
8.45
12.97
14.52
14.64
26.43
28.20
29.20
ꢀ
¼ kobs½WðCOÞ5ðg2-btmseÞꢂ
ð2Þ
ð3Þ
dt
k1k2½BiPh3ꢂ
kobs
¼
k
ꢀ1
The initial concentration of 1 is [W(CO)5(g2-btmse)]0 = 3.37 mM.
½btmseꢂ þ k2½BiPh3ꢂ
The concentration of entering ligand, BiPh3 plays an
important role on the rate of reaction via the observed rate
constant. Fig. 3a shows how the observed rate constant
varies with the concentration of BiPh3 at 45 ꢀC. As the
concentration of BiPh3 increases, kobs increases and
approaches a saturation value at high concentrations of
BiPh3 at all temperatures. This limiting value is the rate
constant k1 for the displacement of btmse from the com-
plex W(CO)5(g2-btmse)
retarded if carried out in the presence of an excess of btmse
[1,2]. It is most likely that a dissociative mechanism
involves the facile reversible loss of the alkyne ligand yield-
ing the W(CO)5 species as an intermediate which will be
attacked by a nucleophile present in the solution. The ques-
tion whether a W(CO)5(solvent) molecule or coordinatively
unsaturated W(CO)5 complex is the species involved in the
further reaction has been addressed through studies of
mechanism and catalysis employing photogenerated inter-
mediates [14]. Mechanistic studies have shown that the
replacement of the solvent from W(CO)5(solvent) interme-
diate is much slower (microsecond timescale) [15] than the
attack of W(CO)5 by solvent (ꢁ3 ps) [16]. Therefore, it is
conceivable that W(CO)5(solvent) is formed as intermedi-
ate upon the reversible, thermal detachment of btmse in
W(CO)5(g2-btmse). This intermediate undergoes solvent
replacement by either btmse yielding the starting complex
k1k2½BiPh3ꢂ
lim kobs ¼ lim
¼ k1
ð4Þ
½BiPh3ꢂ!1
k
½btmseꢂ þ k2½BiPh3ꢂ
ꢀ1
Due to the limited solubility of triphenylbismuthine in
cyclohexane, it is not possible to increase the BiPh3 concen-
tration beyond the 20-fold excess. However, Eq. (3) can be
rearranged into a form (Eq. (5)), which will permit us to
estimate the k1 value from the present data obtained by
working at available concentration of BiPh3
Scheme 1. Mechanism proposed for the substitution reaction of W(CO)5(g2-btmse) with triphenylbismuthine in cyclohexane solution.

¨
E. Bayram, S. Ozkar / Journal of Organometallic Chemistry 691 (2006) 3267–3273
3271
20
16
12
8
4
0
0
20
40
60
80
[BiPh3] (mM)
(a)
12000
8000
4000
0
0
100
200
300
400
[BiPh3]-1 (M-1)
(b)
Fig. 3. (a) The variations in the observed rate constant kobs with the concentration of the entering ligand, BiPh3, for the substitution of btmse in
W(CO)5(g2-btmse) with BiPh3 at 45 ꢀC in cyclohexane. [W(CO)5(g2-btmse)]0 = 3.37 mM. (b) 1/kobs versus 1/[BiPh3] graph for the same reaction.
ꢀ
ꢁ
1
1
k
½btmseꢂ
tration (Table 3). Obviously, the addition of free btmse
inhibits the formation of the W(CO)5(BiPh3) complex,
thus, lowers kobs values. According to the mechanism pro-
posed, two potential ligands, BiPh3 and btmse, compete
with each other for the attachment to the W(CO)5-moiety
in the intermediate complex, W(CO)5(solvent). However,
the higher coordination ability of btmse and yet its higher
ꢀ1
¼
þ
ꢃ
ð5Þ
kobs k1
k1k2 ½BiPh3ꢂ
The k1 values obtained by employing 1/k1 versus
1/[BiPh3] graph for all temperatures are given in Table 2.
In order to study the effect of concentration of the leav-
ing ligand, btmse, reaction was performed by adding free
btmse in large excess at 45 ꢀC. As expected from Eq. (3),
the kobs value decreases with the increasing btmse concen-
Table 3
kobs Values depending on the initial concentration of btmse for the
substitution reaction of W(CO)5(g2-btmse) with BiPh3 at 45 ꢀC in
cyclohexane
Table 2
k1 Values for the rate determining step in the substitution of btmse in
W(CO)5(g2-btmse) with BiPh3 evaluated from the graph of 1/kobs versus
1/[BiPh3]
kobs · 105 (sꢀ1
)
[btmse] (mM)
Temperature (ꢀC)
k1 · 105 (sꢀ1
)
12.97 0.65
9.06 0.45
5.51 0.28
0
16.8
33.7
35
40
45
50
4.02 0.20
8.96 0.45
14.98 0.78
30.30 1.52
The initial concentrations of reactants are [W(CO)5(g2-btmse)]0 =
3.37 mM and [BiPh3]0 = 33.7 mM.

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E. Bayram, S. Ozkar / Journal of Organometallic Chemistry 691 (2006) 3267–3273
3272
-2
-3
-4
-5
3.04
3.12
3.2
3.28
(1/T)x103 (K-1)
Fig. 4. Eyring plot for the rate determining step of the substitution of btmse in W(CO)5(g2-btmse) by BiPh3 at different temperatures in cyclohexane.
concentration favor the coordination of btmse to the
W(CO)5-moiety over BiPh3. Thus, the formation of 2 is
inhibited by increasing concentration of the leaving btmse
ligand (see Fig. 4).
The calculated k1 values at four different temperatures
are used to calculate the activation parameters of the disso-
ciation of btmse from the complex 1 and the formation of
the solvated complex, W(CO)5(solvent). The activation
enthalpy and entropy were found to be 106 2 kJ molꢀ1
and 111 6 J Kꢀ1 molꢀ1, respectively. Although associa-
tive mechanism in the transition state may be expected
due to the tungsten metal centre with large atomic size
4. Conclusion
Although W(CO)5(g2-btmse) can undergo replacement
of alkyne and/or CO ligand depending on the entering
group, its substitution reaction with BiPh3 yields almost
solely W(CO)5(BiPh3) in cyclohexane solution at an
appreciable rate in the temperature range of 35–50 ꢀC.
The kinetics of this ligand substitution reaction was stud-
ied by using quantitative FT-IR spectroscopy. A first-
order dependence of reaction rate on the concentration
of W(CO)5(g2-btmse) was observed under the pseudo-
first-order conditions. The results of the kinetic experi-
ments directed us to propose a mechanism for the substi-
tution of the btmse ligand by triphenylbismuthine
involving the reversible loss of alkyne as the rate deter-
mining step. The dissociative nature of the reaction may
be attributed to the presence of quite bulky entering
and leaving groups which inhibit the possible formation
of seven-coordinate complex in transition state.
which may allows possible seven-coordination, large posi-
¼
tive entropy of activation (DS = 111 6 J Kꢀ1 molꢀ1
)
was observed implying a dissociative mechanism in transi-
tion state. For a dissociative reaction like the displacement
of btmse from the complex 1, the enthalpy of activation is
expected to be close (ideally equal) to the metal–ligand
bond dissociation enthalpy [17]. Unfortunately, there exists
no report on the tungsten–alkyne bond dissociation
enthalpy in hydrocarbon solution. Only the tungsten–acet-
ylene bond dissociation enthalpy (105.9 kJ molꢀ1) has been
calculated in the gas phase [18].
The rate constants kꢀ1 and k2 for the other two individ-
ual steps have not been reported so far in the literature.
Their determination would require the performance of
the reaction between the W(CO)5(solvent) complex, photo-
generated by using flash photolysis, and the respective
ligand. However, their ratio kꢀ1/k2 could be estimated
from the slope of the 1/kobs versus 1/[BiØ3] line by employ-
ing the k1 value obtained from the intercept of the line and
using the known concentration of ligands. The estimated
values of kꢀ1/k2 are in the range of 0.3–0.5. It is not sur-
prising that the kꢀ1 and k2 rate constants do not differ from
each other significantly, as both of them are dealing with
the attachment of a highly reactive 16-electron intermedi-
ate W(CO)5 by potential ligands btmse or BiØ3.
Acknowledgement
Partial support of this work by Turkish Academy of Sci-
ences is gratefully acknowledged.
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