Dimerization of terminal alkynes catalyzed by chloro(η5-pentadienyl) bis(triphenylphosphine)ruthenium(II) and kinetics of phosphine substitution

Article abstract of DOI:10.1016/j.jorganchem.2006.12.031

Exchange of PMe₂Ph for PPh₃ in (η⁵-pentadienyl)ruthenium{bis(triphenylphosphine)}chloride, (η⁵-C₅H₇)Ru(PPh₃)₂Cl (1) under first order conditions proceeds rapidly in TH

Full text of DOI:10.1016/j.jorganchem.2006.12.031

Journal of Organometallic Chemistry 692 (2007) 1716–1725
www.elsevier.com/locate/jorganchem
Dimerization of terminal alkynes catalyzed by
chloro(g5-pentadienyl) bis(triphenylphosphine)ruthenium(II)
and kinetics of phosphine substitution
*
Matthew Daniels, Rein U. Kirss
Department of Chemistry, Northeastern University, Boston, MA 02115, United States
Received 21 November 2006; received in revised form 22 December 2006; accepted 22 December 2006
Available online 9 January 2007
Abstract
Exchange of PMe2Ph for PPh3 in (g5-pentadienyl)ruthenium{bis(triphenylphosphine)}chloride, (g5-C5H7)Ru(PPh3)2Cl (1) under first
order conditions proceeds rapidly in THF at room temperature. A pseudo-first order rate constant of 17 2 · 10ꢀ4 sꢀ1 is obtained for
the reaction at 21 ꢀC. The rate constant is essentially independent of the phosphine concentration. The activation parameters, DHꢀ
= 16.1 0.4 kcal molꢀ1 and DSꢀ = ꢀ16 1 cal Kꢀ1 molꢀ1 differ from those reported for phosphine exchange in CpRu(PPh3)2Cl (2)
and (g5-indenyl)Ru(PPh3)2Cl (3). The reaction of 1 with PMe2Ph is about 70 times faster than the reaction of 2 at 30 ꢀC and some
40 times faster than the reaction of 3 at 20 ꢀC. (g5-C5H7)Ru(PPh3)2Cl(1) is more active than the ruthenium(II) complexes 2, 3, and
TpRu(PPh3)2Cl (4) in the catalytic dimerization of terminal alkynes with nearly quantitative conversion of PhCCH and FcCCH at ambi-
ent temperature in 24 h. The enhanced substitution rate is accompanied by >50% conversion of phenylacetylene to oligomeric products.
Reaction of 1 with NaPF6 in acetonitrile yields the cationic ruthenium(II) complex [(g5-C5H7)Ru(PPh3)2(CH3CN)][PF6] (7). The latter
complex is much less active in reactions with phenylacetylene than 1 but avoids the formation of oligomeric products.
ꢁ 2007 Elsevier B.V. All rights reserved.
Keywords: Pentadienyl ruthenium compounds; Phosphine exchange; Kinetics; Catalytic dimerization of alkynes
1. Introduction
The potential for ring slippage from g5 to g3 in pentadienyl
metal complexes raises the possibility of novel reactivity in
pentadienyl metal complexes. Such differences between the
C5H7 and Cp ligands are expected to influence the respec-
tive catalytic properties of compounds, such as (g5-
C5H7)Ru(PPh3)2Cl (1) and CpRu(PPh3)2Cl (2).
Several decades of research on pentadienyl transition
metal compounds have revealed significant differences in
the steric and electronic properties of the g5-pentadienyl
(C5H7) and g5-cyclopentadienyl (C5H5 or Cp) ligands in
transition metal compounds [1]. The two ligands are iso-
electronic, however, the pentadienyl ligand (cone angle
180ꢀ) [2] is sterically more demanding than a Cp ligand
(cone angle 145ꢀ) [3]. The pentadienyl ligand is also both
a better acceptor and a better donor than the Cp ligand
[1]. Calculations predict that g5 to g3 conversions in the
C5H7 ligand should be easier than in a C5H5 ligand [1].
Ru
Cl
Ph P
PPh
3
3
1
The ruthenium(II) complexes CpRu(PPh3)2Cl (2), (g5-
indenyl)Ru(PPh3)2Cl (3), and TpRu(PPh3)2Cl (4) [4] are
all active as catalysts in a number of carbon–carbon bond
*
Corresponding author. Tel.: +1 617 373 4513; fax: +1 617 373 8795.
E-mail address: rkirss@neu.edu (R.U. Kirss).
0022-328X/$ - see front matter ꢁ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.12.031

M. Daniels, R.U. Kirss / Journal of Organometallic Chemistry 692 (2007) 1716–1725
1717
forming reactions [5]. One well-explored reaction is the cat-
alytic dimerization of terminal alkynes in the presence of 2,
4 and other ruthenium(II) complexes [6]. Three dimeric
products, Z- and E-1,4-substituted-1-buten-3-yne (‘‘head-
to-head’’ dimers, Fig. 1) as well as a ‘‘head-to-tail’’ dimer,
2,4-substituted-1-buten-3-yne, are typically observed in
ruthenium catalyzed reactions of terminal alkynes. In some
cases, a 1,4-substituted butatriene (a cumulene, R(H)C@
C@C@C(H)R), is formed. Changing the ligands around
the ruthenium center affects the product distribution and
reaction rate. For example, TpRu(PPh3)2Cl catalyzed
dimerization of phenylacetylene yields a 9:89 ratio of Z
to E-1,4-diphenyl-1-buten-3-yne [6a] while the CpRu-
(PPh3)2Cl catalyzed reaction yields a 11:10 ratio of Z to
E products [6b]. The Z:E ratio increases to 67:33 when (g5-
dihydropentalenyl)Ru(PPh3)2Cl is substituted for CpRu-
(PPh3)2Cl [6b]. The conversion rate of phenylacetylene is
in the order TpRu(PPh3)2Cl > (g5-dihydropentalenyl)
Ru(PPh3)2Cl > CpRu(PPh3)2Cl under similar conditions
[6].
Dissociation of one phosphine ligand from the 18-elec-
tron ruthenium(II) complexes 1–4 is necessary to open a
coordination site for substrates in the catalytic process
and is often invoked as the first step in catalytic reactions
involving 2–4 [6a]. The rates of phosphine exchange in
CpRu(PPh3)2Cl and (g5-indenyl)Ru(PPh3)2Cl have been
reported [7]. The ‘‘indenyl effect’’ [8] has been invoked to
explain the faster rate of phosphine substitution in (g5-
indenyl)Ru(PPh3)2Cl. Qualitatively, the substitution of tri-
phenylphosphine in (g5-C5H7)Ru(PPh3)2Cl (1) by phos-
phines is rapid [9], however, the kinetics of phosphine
substitution have not been reported. In this paper we
report on the kinetics of phosphine substitution in 1 and
on the catalytic activity of 1 in the dimerization of terminal
alkynes. This reaction is selected as a model given the sig-
nificant literature available for comparison of our results
with those using other of ruthenium(II) catalysts. We have
found both a faster rate of phosphine substitution in 1 rel-
ative to both 2 and 3 as well as an enhancement in the rate
of dimerization of terminal alkynes catalyzed by 1 relative
to 2–4. The formation of Z- and E-1,4-substituted-1-buten-
3-ynes, however, is often accompanied by significant oligo-
merization in the presence of 1.
2. Experimental
All compounds described in this work were handled
using Schlenk techniques or a M.I. Braun glove-box under
purified argon or nitrogen atmospheres [10]. RuCl3 Æ xH2O
was purchased from Alfa Inorganics, Inc. Tertiary phos-
phines, PMePh2, PMe2Ph, PEt3, and PCy3, were obtained
from Strem Chemical, Inc. and used as received. Alkynes
t
(PhCCH, TMSCCH, BuCCH, C4H9CCH) were distilled
prior to use. Solvents were purified by refluxing over Na/
benzophenone (toluene, tetrahydrofuran, benzene, hexane,
pentane) or P2O5 (dichloromethane) and distilled prior to
use. Benzene-d6 (Cambridge Isotope Laboratories) was
purified by refluxing over Na/benzophenone and distilled
prior to use. Ruthenium(II) compounds (g5-C5H7)Ru-
(PPh3)2Cl (1), CpRu(PPh3)2Cl (2), (g5-C9H7)Ru(PPh3)2Cl
(3), (g5-C5H7)Ru(PPh3)(PMe2Ph)Cl (5), and ethynylferro-
cene were prepared by literature procedures [9,11,12].
Elemental analyses (C, H) were performed by Desert Ana-
lytics, Inc. Tucson, AZ.
NMR spectra were recorded at 300 MHz for 1H,
75.4 MHz for 13C{1H}, and 121.4 MHz for 31P{1H} on a
Varian XL300 spectrometer. Proton chemical shifts are
reported relative to residual protons in the solvent
(C6D5H at d 7.15 ppm, CDHCl2 at 5.24 ppm, CD3CO-
CD2H at d 2.05 ppm, or CD2HCN at d 1.97 ppm relative
to TMS at 0.00 ppm). Carbon chemical shifts are reported
relative to solvent (C6D6 at d 128.0 ppm or CD3CN at
1.7 ppm). Phosphorus chemical shifts are reported relative
to external 85% H3PO4 at 0.0 ppm.
Electrochemical measurements were made under nitro-
gen on a BAS 100 B/W electrochemical workstation using
a 1 · 10ꢀ3 M solution of 1 in dry CH2Cl2, 0.1 M nBu4NPF6
as supporting electrolyte at a scan rate of 100 mV/s. The
working electrode was a 3 mm Pt disk with a Pt wire as
auxiliary electrode. A silver wire was used as a reference
electrode with all potentials referenced to SCE. The oxida-
tion potential of 1 is measured as 0.46 V relative to SCE.
R
R
R
R
E
Z
Head-to-tail
R
R
H
R
R
H
R = phenyl, ferrocenyl, tbutyl, trimethylsilyl
C
C
C
C
cumulene
Fig. 1. Structures for isomeric dimers of terminal alkynes.

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M. Daniels, R.U. Kirss / Journal of Organometallic Chemistry 692 (2007) 1716–1725
2.1. Kinetic measurements for reaction of 1 with PMe2Ph,
PMePh2, and PEt3
with the reported spectral parameters listed below. The %
conversion was determined by integration of resonances
for RCCH and the dimeric products in the H NMR spec-
1
A 40 mM stock solution of 1 was prepared by dissolving
144 mg (0.20 mol) of 1 in 4.5 mL THF and 0.5 mL C6D6.
The C6D6 served as a lock signal for the spectrometer.
Samples for the kinetic experiments were prepared by add-
ing 40 lL PMe2Ph (0.28 mmol) to 500 lL aliquots of the
stock solution of 1 (40 mM in 1 and 400 mM in PMe2Ph)
in screw capped 5 mm NMR tubes (Norell, Inc) in the
glove-box immediately prior the start of the experiment.
The start time was recorded. Samples were inserted into
the NMR probe as soon as possible after mixing, usually
within 2 min. The rate of substitution of PPh3 by PMe2Ph
in 1 was measured by monitoring the increase in the dou-
blet for (g5-C5H7)Ru(PPh3)(PMe2Ph)Cl (5) at 11.8 ppm
(JPP = 27 Hz) over time relative to the broad singlet for
(g5-C5H7)Ru(PPh3)2Cl at 28.8 ppm. Kinetic measurements
below room temperature used constant temperature baths
(ice at 0 ꢀC and salt/ice mixtures, ꢀ15 ꢀC) to cool the
probe. Samples for lower temperature measurements were
pre-cooled (0 ꢀC or ꢀ15 ꢀC) before insertion into the
NMR probe to ensure rapid equilibration in the probe.
The actual temperature in the probe was determined from
the frequency difference between the CH3OH and CH3OH
protons. Three independent measurements of the substitu-
tion rate were made at each temperature to determine the
rate constant for the reaction. Activation parameters were
determined using the Eyring equation (three independent
measurements of kobs at each of four temperatures). Errors
were calculated using statistical methods described in stan-
dard analytic chemistry textbooks [13]. An additional series
of experiments were performed using PMe2Ph concentra-
tions of 310 and 210 mM. Three independent measure-
ments of the substitution rate were made at each
phosphine concentration to determine the effect of phos-
phine concentration on the reaction rate.
tra. The amounts of oligomeric and polymeric products
were estimated by comparison of the integrated area of
the vinyl protons assigned to dimeric products to the inte-
grated area of the protons assigned to the R groups. The
1
relevant H resonances for the products in C6D6 are:
Z-PhCH@CHCCPh [9]: d 5.75 d (JHH = 11.7 Hz), 6.38
(JHH = 11.7 Hz).
E-PhCH@CHCCPh [9]: d 6.27 d (JHH = 16.2 Hz), 7.01
d (J = 16.2 Hz).
CH2@C(Ph)(CCPh) [9] : d 5.66 (JHH = 1 Hz), d 5.72
(JHH = 1 Hz).
The aryl protons of all three products overlap in the
region 6.5–7.5 ppm.
Z-FcCH@CHCCFc [14] : d 4.16 s (5H), 4.27 s (5H), 4.25
t (2H), 4.31 t (2H), 4.50 t (2H), 4.85 t (2H), 5.63 d
(J = 11.4 Hz, 1H, FcCH@CHCCFc), 6.24 d (J = 11.4 Hz,
1H, FcCH@CHCCFc), E-FcCH@CHCCFc: d 4.15 s
(5H), 4.25 s (5H), 4.20 t (2H), 4.36 t (2H), 4.42 t (2H),
4.62 t (2H), 6.02 d (J = 16.0 Hz, 1H), 6.82 d (J =
16.0 Hz, 1H).
E/Z-Me3SiCH@CHCCSiMe3: d 6.25 d (J = 15 Hz), 6.01
d (J = 15 Hz). The ratio of the two geometric (E and Z)
1
isomers cannot be determined by H NMR as the vinyl
protons of the products overlap [9].
CH2@C(SiMe3)CCSiMe3 [9]: d 6.14 d (J @ 3.3 Hz), 5.55
d (J = 3.3 Hz).
Resonances assigned to the TMS (Me3Si) groups are
observed as singlets between 0.1 and 0.15 ppm.
Z-Me3CCH@CHCCMe3 [9]: d 5.45 d (J = 11.7 Hz,
Me3CCH@CHCCCMe3), 5.55 d (J = 11.7 Hz, Me3CCH
@CHCCMe3).
The same procedure was used to determine the rate con-
stants for reactions of 1 (40 mM) with PMePh2 and PEt3
(400 mM). The product ratios were determined by integra-
tion of doublet resonances at 19.6 ppm (JPP = 31 Hz, (g5-
C5H7)Ru(PPh3)(PMePh2)Cl) and 26.6 (JPP = 27 Hz, (g5-
C5H7)Ru(PPh3)(PEt3)Cl) to the broad singlet for (g5-
C5H7)Ru(PPh3)2Cl at 28.8 ppm. No reaction was observed
at ambient temperature or upon refluxing solutions of (g5-
C5H7)Ru(PPh3)2Cl (40 mM) with PCy3 (400 mM) for 60 h.
E-Me3CCH@CHCCMe3: d 5.59 d (J = 16 Hz, Me3CCH
@CHCCCMe3), 6.14
d
(J = 16 Hz, Me3CCH@
CHCCMe3).
CH2@C(CMe3)CCMe3 [9]: d 5.11 d (J = 2.1 Hz, 1H,
CH), 5.33 d (J = 2.1 Hz, 1H, CH).
(Me3C)HC@C@CH(CMe3) [9]: d 5.51 s.
Resonances assigned to the tBu(Me3C) groups are
observed as singlets between 1.0 and 1.3 ppm.
Two broad signals at 29 and 37 ppm in the 31P NMR
spectrum assigned to (g5-C5H7)Ru(PPh3)2Cl [6] are
replaced by resonances at ꢀ4.6 ppm assigned to PPh3
and a singlet at 25.5 ppm at the end of the experiment.
2.2. 1H NMR studies of the dimerization of terminal alkynes
catalyzed by (g5-C5H7)Ru(PPh3)2Cl (1), (g5-Indenyl)Ru-
(PPh3)2Cl (3), and (g5-C5H7)Ru(PPh3)(PMe2Ph)Cl (5)
Solutions of 1, 3, or 5 (3–7 mol%), alkyne (0.3–0.7 M
PhCCH, FcCCH, TMSCCH, tBuCCH, or C4H9CCH)
and PPh3 (0–270 mM) in C6D6 were prepared under an
argon atmosphere. The reactions were followed by 1H
and 31P NMR spectroscopy. Products were identified by
comparison of the chemical shifts of the observed products
2.3. Separation and characterization of the products from
reactions of phenylacetylene and ethynylferrocene
2.3.1. Reaction of phenylacetylene in benzene at 80 ꢀC
A yellow solution of 167 mg (0.23 mmol, 4.5 mol%) 1
and 550 lL (5 mmol) phenylacetylene in 25 mL C6H6 was

M. Daniels, R.U. Kirss / Journal of Organometallic Chemistry 692 (2007) 1716–1725
1719
refluxed under nitrogen for 2 h. The solution darkens to a
brown color in less than a minute. After cooling to ambient
temperature, solvent was evaporated under vacuum. The
crude product was extracted with 25 mL refluxing ether,
cooled, and filtered to remove a brown insoluble ruthe-
nium-containing product (41 mg). The yellow-orange fil-
trate was evaporated to dryness. Chromatography on
silica did not result in good separation of the products,
yielding 451 mg (88% yield based on phenylacetylene) of
a mixture of dimers, oligomers, and polymers. Chromatog-
raphy of the latter mixture on alumina using a 10:1 mixture
of hexane:CH2Cl2 gave a better separation, yielding 110 mg
(24% based on phenylacetylene) of Z/E-1,4-diphenyl-1-
butene-3-yne (8:1 ratio) and 61 mg (12%) of phenylacety-
lene oligomers. A brown product adsorbed on the column
could not be eluted with CH2Cl2 or methanol. These prod-
ucts are presumed to be polymeric products derived from
phenylacetylene and should account for the remaining
products (estimated to be ꢁ280 mg, 52% based on products
collected after chromatography on silica above).
Changing the eluent to benzene and finally to methanol
yielded 69 mg (20% yield based on ethynylferrocene) of a
brown solid with a 1H NMR spectrum similar to the previ-
ous product.
2.4. Synthesis of (g5-pentadienyl)rutheniumbis
(triphenylphosphine)(acetonitrile) hexafluorophosphate (7)
A slurry of 250 mg (0.34 mmol) 1 and 180 mg (1.1 mmol)
NaPF6 in 20 mL acetonitrile was stirred for 24 h under
nitrogen. The yellow starting material, 1, slowly dissolves
forming a light yellow solution and leaving a white precip-
itate. The mixture was filtered through a thin pad of Celite
and the white solid washed with 5 mL of acetonitrile. The
volume of the combined filtrate and washings was reduced
to 2 mL under vacuum. Upon cooling a yellow solid precip-
itates. The solid was collected by filtration and washed twice
with 8 mL of diethyl ether and once with 8 mL hexane. Yel-
low [(g5-C5H7)Ru(PPh3)2(CH3CN)][PF6] (7) (237 mg, 79%
yield) was isolated after drying in vacuum. The compound
darkens upon heating above 150 ꢀC without melting.
Anal. Calc. for C43H40F6NP3Ru: C, 58.77; H, 4.59.
Found: C, 59.06; H, 4.79%.
2.3.2. Reaction of phenylacetylene in benzene at ambient
temperature
A yellow solution of 175 mg (0.24 mmol, 5 mol%) 1 and
550 lL (5 mmol) phenylacetylene in 25 mL C6H6 was stir-
red under nitrogen for 24 h at ambient temperature. The
solution slowly darkens to a brown color over 30 min. Sol-
vent was evaporated under vacuum and the residue
extracted with 25 mL of hot diethylether. Chromatography
of the extract on alumina yielded 230 mg of E/Z-1,4-diphe-
nyl-1-butene-3-yne (45% based on phenylacetylene) and
79 mg of phenylacetylene oligomers (15%) for an overall
yield of 60% based on phenylacetylene used in the reaction.
The ratio of Z- to E-1,4-diphenyl-1-butene-3-yne was
determined to be 8:1 by 1H NMR spectroscopy. As before,
a brown product adsorbed on the column and could not be
eluted with CH2Cl2 or methanol. These products are pre-
sumed to be polymeric products derived from phenylacety-
lene and should account for the remaining products.
1H (CD3COCD3): ꢀ0.60 m (1H), ꢀ0.44 d (J = 10.5 Hz,
1H), 1.59 s (3H, CH3CN), 2.69 (J = 9 Hz, 1H), 2.97 d
(J = 9 Hz, 1H), 5.06 m (1H), 5.38 m (1H), 5.57 m (1H),
7.15–7.6 m (30H, C6H5).
1H (CD3CN): ꢀ0.63 m (1H), ꢀ0.50 d (J = 9.6 Hz, 1H),
2.18 s (3H, CH3CN), 2.60 (J = 9.6 Hz, 1H), 2.88 d
(J = 9.6 Hz, 1H), 4.88 m (1H), 5.22 m (1H), 5.43 m (1H),
7.15–7.6 m (30H, C6H5).
1H (CD2Cl2): ꢀ0.30 m (1H), ꢀ0.15 d (J = 12 Hz, 1H),
1.58 s (CH3CN, 3 H), 2.87 d (J = 12 Hz, 1H), 3.17 d
(J = 12 Hz, 1H), 5.23 m (1H), 5.58 m (1H), 5.70 br s
(1H), 7.4, 7.5–7.9 m (C6H5).
13C (CD3CN): 44.70, 55.91, 92.22, 97.95 d (JCP
11 Hz), 106.42, 129.38 d (JCP = 10 Hz), 130.11 d (JCP
=
=
10 Hz), 131.26, 131.86, 134.48 d (JCP = 10 Hz), 135.39 d
(JCP = 10 Hz), 136.0 d (JCP = 37 Hz).
31P (CD3CN): 32.9 d (JPP = 24 Hz), 48.4 d (JPP
24 Hz), 62.4 heptet (JPF = 1006 Hz).
=
2.3.3. Reaction of ethynylferrocene in benzene at 80 ꢀC
A solution of 336 mg (1.6 mmol) ethynylferrocene and
93 mg (0.13 mmol, 7.5 mol%) 1 in 25 mL benzene was stir-
red overnight at ambient temperature. The orange color of
ethynylferrocene changes to a dark red. After 24 h at room
temperature (or 3 h at reflux) the solvent was evaporated
from the reaction mixture under vacuum, yielding a red
oil. The crude products were chromatographed on silica
using a 2:1 mixture of petroleum ether to benzene. Two
red-orange fractions were collected. The first yielded
171 mg (51% yield based on ethynylferrocene) of an 8.5:1
mixture of Z-1,3-diferrocenyl-1-buten-3-yne to E-1,3-dif-
errocenyl-1-buten-3-yne identified by comparison of the
1H NMR spectrum with literature data [14]. The second
fraction yielded 88 mg (26% yield based on ethynylferro-
2.5. Dimerization of phenylacetylene catalyzed by
[(g5-C5H7)Ru(PPh3)2(CH3 CN)][PF6]
A pale yellow solution of 120 lL (1.1 mmol, 44 mM)
phenylacetylene and 48 mg (0.055 mmol, 5 mol%) 7 in
25 mL THF was stirred at ambient temperature for 6 h
with no observable color changes. The mixture was
refluxed for 16 h yielding a darker, yellow-brown solution.
Solvent was evaporated under vacuum and the oily residue
chromatographed on alumina as described above for the
dimerization of phenylacetylene in the presence of 1.
53 mg (47%) of a 4:1 mixture of Z:E 1,4-diphenyl-1-
buten-3-yne was isolated. There were no dark residues left
on the column as in the case of the (g5-C5H7)Ru(PPh3)2Cl
catalyzed reaction.
1
cene) of a red oil. The H NMR spectrum of this oil con-
sists of a complex multiplet between 4 and 5 ppm.

1720
M. Daniels, R.U. Kirss / Journal of Organometallic Chemistry 692 (2007) 1716–1725
2.6. NMR experiments
surements (ꢁ30–40 min per experiment). Formation of 6
is seen in samples allowed to stand at ambient temperatures
for several hours. Below room temperature (5 and 0 ꢀC),
the reaction is much slower and provides linear plots of
the ln[1] as a function of time over several half-lives. The
first order rate constants for reactions of 1 with PMe2Ph,
PMePh2, and PEt3 are listed in Table 1. There is little
dependence of the rate constant on the nature of the phos-
phine although the bulky tertiary phosphine PCy3 failed to
react even after refluxing for 60 h. The rate of substitution
of PPh3 in 1 is at least 70 times faster than in 2 and almost
40 times faster than in 3 at 20 ꢀC. Activation parameters
for the reaction of 1 with PMe2Ph are also listed in Table
1. The reaction rate at 20 ꢀC is essentially independent of
the phosphine concentration for three different concentra-
tions (400, 310, and 210 mM) of PMe2Ph.
Solutions of 7 (5 mol%) and phenylacetylene (0.7–
1.2 M) in CD3CN or CD2Cl2 were prepared under an
argon atmosphere. The reactions were followed by 1H
spectroscopy. Product ratios and % conversion were deter-
mined by integration of resonances assigned to phenylacet-
ylene, Z-1,4-diphenyl-1-buten-3-yne and E-1,4-diphenyl-1-
buten-3-yne. Conversion of phenylacetylene was deter-
mined by comparing the integrated area for the terminal
proton in phenylacetylene with the integrated area for the
vinyl protons of the products. No reaction is observed in
either solvent after 24 h at ambient temperature. In
CD3CN, 24% conversion to a 2.5:1 ratio of Z:E 1,4-diphe-
nyl-1-buten-3-yne is observed after 5 h at 80 ꢀC, increasing
to 48% after 26 h of reaction with no change in the product
ratio. In CD2Cl2, 33% conversion to a 3.5:1 ratio of Z:E
1,4-diphenyl-1-buten-3-yne is observed after 5 h at reflux
increasing to 43% conversion after 26 h of reaction. The
product ratio remains constant. Integration of the aryl
and vinyl resonances yields a 5:1 ratio suggesting that no
oligomerization has taken place.
3.2. Catalytic activity of (g5-C5H7)Ru(PPh3)2Cl in the
dimerization of phenylacetylene
A significant increase in catalytic activity is observed
when (g5-C5H7)Ru(PPh3)2Cl (1) is used as the catalyst
(5 mol%) in the dimerization of phenylacetylene (0.5 M in
benzene) relative to 2 and 4 under identical conditions.
3. Results
1
When followed by H NMR spectroscopy, conversion of
phenylacetylene was complete after 90 min at 80 ꢀC com-
pared to 24 h or more for the other ruthenium complexes
(Table 2). In fact, reaction at ambient temperature leads
to complete conversion after 24 h. The products identified
in the reaction mixture by 1H NMR and confirmed by
gc/ms are: Z-1,4-diphenyl-1-buten-3-yne and E-1,4-diphe-
nyl-1-buten-3-yne along with trace amounts of 2,4-diphe-
nyl-1-buten-3-yne (the ‘‘head-to-tail’’ dimer). The Z-
isomer is characterized by well-resolved doublets at d
5.75 (JHH = 11.7 Hz) and d 6.38 (JHH = 11.7 Hz) while
the E-isomer has a distinct resonance at d 6.27 (J =
3.1. Kinetics of phosphine substitution
Under pseudo-first order conditions (ꢁ10 equiv.
PMe2Ph) the conversion of 1 to (g5-C5H7)Ru(PPh3)-
(PMe2Ph)Cl (5) in benzene (Eq. (1)) is essentially complete
upon mixing at ambient temperature and leads to mixtures
of 5 and (g5-C5H7)Ru(PMe2Ph)2Cl (6) upon standing. The
reaction between 5 and PMe2Ph in C6D6 forming 6 is
slower than the conversion of 1 to 5 but still has a half-life
of less than 2 min at ambient temperature making it unsuit-
able for our study. Similar mixtures are reported in reac-
tions of (g5-C9H7)Ru(PPh3)2Cl (3) with PMe2Ph in
benzene [7]. Rapid reactions were also observed between
1 and PEt3 or PMePh2 in benzene as determined by both
1H and 31P NMR spectroscopy
Table 1
Observed rate constants for the reaction of (g5-C5H7)Ru(PPh3)2Cl (1),
CpRu(PPh3)2Cl (2), and (g5-C9H7)Ru(PPh3)2Cl (3) with tertiary
phosphines
PMe2Ph
Reaction
T (ꢀC)
kobs
(sꢀ1 · 104)
DHꢀ
DSꢀ
(kcal molꢀ1
)
(cal Kꢀ1 molꢀ1
)
Ru
Ru
ð1Þ
1 + PMe2Pha
0
5
21
2.0 0.3
2.7 0.1
Cl
Ph3P
PPh3
Cl
Ph3P
PMe2Ph
1
16.3 1.9
17.0 1.8b
18.0 1.8c
24.8 1.3
14.8 0.4
15.8 0.2
0.25 0.02
0.050 0.005
0.42 0.02
5
Good kinetic data can be collected for the reaction of 1
with PMe2Ph, PMePh2, and PEt3 at 20 ꢀC and below in
THF/C6D6 mixtures (90/10 v/v). The reaction is quite
rapid at ambient temperature, reaching completion after
ꢁ30 min, however, reproducible data could be collected
using concentrated solutions to reduce data acquisition
times. The monosubstituted product, (g5-C5H7)Ru(PPh3)-
(PMe2Ph)Cl (5), is the only product present at the end of
each kinetic run. There is no evidence for the formation
of (g5-C5H7)Ru(PMe2Ph)2Cl (6) during the kinetic mea-
25
16.1 0.4
ꢀ16
1
a
1 + PMePh2 22 0.2
a
1 + PEt3
22 0.2
2 + PMe2Phd 30.0
d
2 + PMePh2 20.6
3 + PMe2Phd 20.0
29
26
1
1
17
11
2
2
a
40 mM in 1, 400 mM in phosphine in THF/C6D6 (90/10 v/v).
b
c
[PMe2Ph] = 310 mM.
[PMe2Ph] = 210 mM.
Ref. [7].
d

M. Daniels, R.U. Kirss / Journal of Organometallic Chemistry 692 (2007) 1716–1725
1721
Table 2
Product distribution from ruthenium catalyzed dimerization of terminal alkynes
R
R
R
R
R
R
catalyst
•
•
R
R
a
b
c
R
d
Substrate
PhCCH
Catalysta
T (ꢀC)
t (h)
% Conversion
% ag
% bg
% c
% d
1b
5
20
80
80
80
110
80
80e
80f
18
24
1.5
24
15
20
1.5
26
16
94
100
24
80
98
100
43
47
40 (40)
22 (21)
11
64
9
25
33
(38)
5 (5)
3 (3)
10
16
89
21
10
(9)
5
3
3
2c
3
4d
5
4
7
FcCCH
1
20
24
74
57
17
TMSCCH
1
80
80
110
6
20
20
64
70
94
23
64
21
17
2c
4d
49h
77
tBuCCH
1
80
80
110
19
24
20
85
0
11
30
10
39
1
6
2c
4d
a
Reactions in C6D6 unless other indicated.
b
c
d
e
f
Remaining products appear to be oligomers and polymers.
see Ref. [6a].
In toluene, see Ref. [9].
CD3CN.
In THF.
g
h
Ratios from integration of 1H NMR spectra of reaction mixtures. Values in parentheses represent isolated yields.
The resonances for E- and Z-isomers overlap. A combined yield is reported.
16.2 Hz) [9]. The phenyl resonances and the remaining
vinyl resonances overlap with those assigned to other prod-
ucts in solution. Two doublets at d 5.66 (JHH = 1 Hz) and d
5.72 (JHH = 1 Hz) are assigned to the head-to-tail dimer,
2,4-diphenyl-1-buten-3-yne [9]. A consistent ratio of 8:1:1
for the Z:E:head-to-tail dimer is observed after 24 h of
reaction at ambient temperature and 80 ꢀC when conver-
sion of phenylacetylene is complete at all temperatures.
Integrated peak areas of the aryl resonances relative to
the resonances assigned to the vinyl protons of E- and Z-
1,4-diphenyl-1-buten-3-yne are consistent with the partial
oligomerization or polymerization of phenylacetylene.
The latter products account for about 50% of the reaction
products at ambient temperature in C6D6 after 24 h and
increase to about 75% in reactions at 80 ꢀC after 90 min.
Analysis of the reaction mixtures by gc/ms reveals the pres-
ence of isomeric phenylacetylene dimers (m/e = 204) and
additional products with m/e = 306, corresponding to tri-
mers of phenylacetylene as the volatile products. In a sep-
arate experiment using toluene as the solvent, essentially no
reaction takes place after two weeks in a ꢀ18 ꢀC freezer but
80% conversion is observed when the sample is stored in a
refrigerator at 5 ꢀC for 24 h. Integration of the aryl and
vinyl resonances reveals that oligomerization occurs even
at 5 ꢀC.
When the reaction is carried out on a preparative scale
followed by chromatography of the products on alumina,
isolated yields of E- and Z-1,4-diphenyl-1-butene-3-yne
decrease from 50% for ambient temperature reactions to
25% for reactions at 80 ꢀC. These yields are comparable
to those observed in the NMR experiments. The NMR
spectra of the other products (orange to red oils) are char-
acterized by a very broad resonance between 6.8 and
7.8 ppm, and appear to be consistent with formation of
oligomeric (PhC@CH)n. Accurate yields of these products
could not be determined as significant amounts of products
absorb strongly on alumina preventing their isolation and
characterization. These products may be higher oligomers
or polymers of phenylacetylene. For comparison, we note
that in the absence of ruthenium-containing catalysts, ther-
mal polymerization accounts for 13% of the products after
13 h at 110 ꢀC in toluene [15].
The catalytic activity of (g5-indenyl)Ru(PPh3)2Cl (3) in
the dimerization of phenylacetylene was investigated under
the same conditions used in reactions where 1 was the cat-
alyst. The dimerization of phenylacetylene in the presence

1722
M. Daniels, R.U. Kirss / Journal of Organometallic Chemistry 692 (2007) 1716–1725
of 3 is 80% complete after 20 h at 80 ꢀC. The products were
identified as a 4:1 ratio of Z:E-1,4-diphenyl-1-butene-3-
ynes. There appears to be minimal oligomerization of the
alkyne in this case. When 5 mol% (g5-C5H7)Ru(PPh3)(P-
Me2Ph)Cl (5) is used as a catalyst nearly 100% conversion
is observed after 90 min of reaction at 80 ꢀC with forma-
tion of nearly equal amounts of Z- and E-1,4-diphenyl-1,
3-buten-1-ynes along with smaller amounts of the ‘‘head-
to-tail’’ dimer. Integration of the aryl resonances suggests
about 50% of the phenylacetylene is converted to oligo-
meric and polymeric products in the latter reaction.
When additional phenylacetylene is introduced to the
NMR tube containing 1 after conversion of alkyne is com-
plete, catalytic dimerization continues but at a diminished
rate. Whereas conversion of phenylacetylene (0.5 M,
20 equiv.) is complete after 2 h at 80 ꢀC, significant amounts
of unreacted alkyne remain 4 h after adding another
20 equiv. of phenylacetylene and heating at 80 ꢀC. These
observations suggest a change in the catalytically active spe-
cies. The fate of 1 in the reactions with PhCCH was exam-
ined by 31P NMR. The two broad signals at 29 and 37 ppm
assigned to 1 [6] disappear after 30 min of reaction at 80 ꢀC
with the appearance of a singlet at ꢀ4.6 ppm assigned to
PPh3. No additional resonances are detected in the 31P
NMR spectrum until the conversion of phenylacetylene is
complete (90 min at 80 ꢀC). At this point, a singlet at
25.5 ppm begins to grow in intensity. The same changes
are observed at ambient temperature but the disappearance
of the resonances for the staring material is slower. A brown
powder isolated from the catalytic reactions has a single res-
onance in the 31P spectrum at 25.5 ppm. Resonances char-
acteristic of Ca for ruthenium vinylidene species (e.g.
TpRuCl(PPh3)@C@C(Ph)H [10]) between 345 and
355 ppm have not been detected in the 13C NMR spectrum
of the ruthenium-containing product. Despite repeated
attempts, this material has resisted our efforts at growing
crystals suitable for X-ray diffraction. A pure sample for
elemental analysis has similarly not been obtained.
The reaction rate and product ratio change in the pres-
ence of 1 and excess PPh3. At constant temperature
(22 ꢀC), catalyst concentration (25 mM, 5 mol%), and reac-
tion time (24 h) the conversion of phenylacetylene
(500 mM) decreases from 94% (in the absence of 1 equiv.
of additional PPh3) to 19% in the presence of 10 equiv. of
PPh3. A concurrent increase in the amount of oligomeric
and polymeric species is observed as the concentration of
PPh3 increases. Adding 1 equiv. of PCy3 to 0.5 M PhCCH
containing 5 mol% 1, however, has no effect on either the
rate of the reaction or the product ratio.
The pentadienyl ruthenium complex 1 is also active as a
catalyst for the dimerization of ethynylferrocene, trimethyl-
silylacetylene, and tbutylacetylene (Table 2). The dimeriza-
tion of ethynylferrocene proceeds smoothly at ambient
temperature with 74% conversion after 24 h at ambient
temperature. Complete conversion is achieved by heating
for 2 h at 80 ꢀC. NMR spectra of the reaction mixtures
are consistent with the formation of mostly Z-1,4-di(ferr-
ocenyl)-1-buten-3-yne with lesser amounts of the E-isomer.
Z-1,4-Di(ferrocenyl)-1-buten-3-yne can be isolated in
ꢁ50% yield from reaction of ethynylferrocene with
5 mol% 1. The remaining products remain poorly charac-
terized but appear to be oligomers or polymers of ethynyl-
1
ferrocene based on H NMR spectra.
In contrast to phenylacetylene and ethynylferrocene,
t
neither trimethysilylacetylene nor butylacetylene react at
ambient temperature in the presence of 1. The dimerization
of trimethylsilylacetylene in the presence of 3 mol% of 1
proceeds with 64% conversion (by 1H NMR) at 80 ꢀC after
6 h and yields a 1.8 to 1 mixture of 1, 3-bis(trimethylsilyl)-
3-buten-1-yne to E/Z-1,4-bis(trimethylsilyl)-1-buten-3-yne.
In the latter case, the resonances for the vinyl protons of
the E and Z isomers overlap, preventing the determination
of an Z:E ratio. Dimerization of tbutylacetylene in the pres-
ence of 4.5 mol% of 1 is slower, requiring 19 h of reaction
at 80 ꢀC to achieve 85% conversion. By 1H NMR, the
products consist of a mixture of 2,4-di(tbutyl)-1-buten-3-
yne, Z-1,4-ditbutyl-1-buten-3-yne and a cumulene, 1,4-
di(tbutyl)butatriene, in a 2.4:1.8:1 ratio. In neither case is
there evidence for oligomerization of the alkyne. 1-Hexyne
fails to react under these conditions.
A less dramatic rate enhancement for alkyne dimeriza-
tion is observed at elevated temperatures in reactions cata-
lyzed by 1 relative to reactions catalyzed by 2–4. Reaction
of tbutylacetylene in the presence of 5 is 85% complete after
19 h at 80 ꢀC compared to 10% conversion after 68 h at
110 ꢀC [9] in the presence of 4. No reaction is observed
when 2 is used as the catalyst. It is difficult to detect a clear
increase in reaction rate with trimethylsilylacetylene for 1
relative to 2 and 4. Trimethylsilylacetylene (5 mol% 1)
reacts rapidly at 80 ꢀC with 64% conversion after 6 h com-
pared to 70% conversion after 20 h (80 ꢀC) in the presence
of 5 mol% of 2 and 94% conversion after 20 h (110 ꢀC) in
the presence of 2 mol% 4.
3.3. Synthesis and catalytic activity of [(g5-C5H7)-
Ru(PPh3)2(CH3CN)][PF6]
The reaction between 1 and NaPF6 in acetonitrile leads
to solvolysis of the Ru–Cl bond forming [(g5-C5H7)Ru-
(PPh3)2(CH3CN)][PF6] (7) in excellent yield. Satisfactory
elemental analysis has been obtained for this new com-
pound. The cationic ruthenium(II) complex 7 is soluble
in acetonitrile, THF, CH2Cl2, and acetone (although sol-
ubility in the latter solvent is lower than in the others).
The 1H NMR spectrum of 7 is characterized by seven
well-resolved sets of resonances for the C5H7 ligand indi-
cating a static structure for 7, rather than the fluxional
structure reported for 1 at ambient temperature [6]. The
spectroscopic data for 7 are consistent with a solution
structure in which one PPh3 ligand lies under the open
edge of the pentadienyl ligand. Such an arrangement ren-
ders the CH2 groups at the termini of the pentadienyl
ligand inequivalent and accounts for the seven resonances
for the C5H7 ligand.

M. Daniels, R.U. Kirss / Journal of Organometallic Chemistry 692 (2007) 1716–1725
1723
Phosphine substitution in both 2 and 3 is proposed to
occur by a dissociative pathway based on the activation
parameters [7]. It was suggested that the substitution reac-
tions of 3 are faster as the indenyl ligand is a better donor
than the Cp ligand, stabilizing the 16 electron intermediate
that results from dissociation of PPh3. Electrochemical
measurements of the oxidation potentials of 2 (0.56 V)
and 3 (0.45 V) support this interpretation. The pentadienyl
ligand in 1 (cone angle 180ꢀ) [2] is significantly larger than
that of the Cp ligand in 2 (136ꢀ) [3] and the oxidation
potential of 1 (0.45 V) is similar to 3. The increased rate
of phosphine substitution in 1 compared to 2 can be
explained on both electronic and steric grounds. The better
donor ligand in 1 stabilizes the 16-electron intermediate
while the larger size of the C5H7 ligand promotes dissocia-
tion of PPh3. It seems reasonable to assume that the cone
angle of the indenyl ligand lies somewhere in between the
values for Cp and C5H7. The rate enhancement for phos-
phine substitution in 1 relative to 3 may reflect steric effects
as 3 and 1 have nearly identical oxidation potentials. The
possibility of an g5 to g3 conversion of the pentadienyl
ligand can not be ruled out. There is precedent in the liter-
ature for the latter process in pentadienyl metal complexes.
For example, refluxing (g5-C5H7)Mn(CO)3 with PMe2Ph
in cyclohexane produces (g5-C5H7)Mn(CO)2(PMe2P),
apparently by an associative mechanism [17]. An interme-
diate, (g3-C5H7)Mn(CO)3(PMe2Ph), can be isolated at
room temperature although the kinetics of the reaction
have not been reported. Reaction of (g5-pentadienyl)-
Re(CO)3 with PEt3 or PMe3 in refluxing diethylether yields
g1 complexes, (g1-C5H7)Re(CO)3(PR3)2 (R@Et, Me) [18].
Furthermore, in reactions of 1 with excess PMe2Ph it is
possible to isolate (g3-C5H7)Ru(PMe2Ph)3Cl (9) [6]. Never-
theless, we believe that the zero-order dependence of the
reaction rate on PMe2Ph concentration argues for a disso-
ciative pathway for the reaction between 1 and PMe2Ph.
The larger C5H7 ligand also prevents successful substitu-
tion by large, electron-rich phosphines like tri(cyclohexyl)-
phosphine. The complete conversion of 1 to 5 at room
temperature is observed under pseudo-first order condi-
tions. The observation of (g5-C5H7)Ru(PMe2Ph)2Cl (6)
only after complete conversion of 1 to 5 suggests that the
rate of substitution in 5 is slower than in 1.
Ph P
3
NCMe
H
3
Ru
H
H
2
4
PPh
3
H
5, exo
H
H
5, endo
1,endo
H
1,exo
7
The 13C NMR spectrum of 7 is similar to that reported
for 1 [6]. Solutions of 7 in acetonitrile-d3 are stable toward
further substitution of PPh3 by solvent. The spectra of 7
are similar to low temperature spectra of 1 and 5, where
a solution structure is unsymmetrical. In 1, one PPh3
ligand rather than the Cl ligand lies under the open edge
of the pentadienyl ligand [6b]. It is unclear which ligand
occupies this position under the open edge in 5, but both
1 and 5 are fluxional under the conditions of the kinetic
experiments [6b].
The insolubility of 7 in non-polar solvents such as ben-
zene prevents a direct comparison of the activity of 7 as a
catalyst in the dimerization of alkynes. Phenylacetylene
does not react at ambient temperature in CD3CN and
CD2Cl2 in the presence of ꢁ5 mol% 7. At 80 ꢀC in
CD3CN, 26 h are required to effect a 48% conversion of
phenylacetylene to a 2.5:1 ratio of Z- to E-1,4-diphenyl-
1-buten-3-yne. A similar rate (43% conversion), but a
slightly different ratio of Z- to E-1,4-diphenyl-1-buten-3-
yne (3.3:1, respectively), is observed in CD2Cl2 at reflux
after 26 h. In refluxing THF, 7 is also active as a catalyst
for the dimerization of phenylacetylene yielding a 47%
isolated yield of a 4:1 mixture of Z- to E-1,4-diphenyl-
1-buten-3-yne. The absence of dark residues that adhere
to an alumina column suggests that oligomerization of
phenylacetylene is suppressed.
4. Discussion
4.1. Kinetics of phosphine substitution
The substitution of PPh3 by smaller tertiary phosphines
in (g5-pentadienyl)Ru(PPh3)2Cl (1) is faster than in the iso-
electronic CpRu(PPh3)2Cl (2), or (g5-indenyl)Ru(PPh3)2Cl
(3). The enthalpy of activation (DHꢀ = 16.1 0.4 kcal
molꢀ1) for the reaction of 1 with PMe2Ph is much less than
4.2. Catalytic activity
for the reaction of 2 with PMePh2 (DHꢀ = 29 kcal molꢀ1
)
Having demonstrated the increased rate of phosphine
substitution in 1 relative to 2 and 3, we can begin to com-
pare the catalytic activity of these three complexes in the
dimerization of terminal alkynes. The two most interesting
results of the investigations of the catalytic activity of 1 are:
or 3 with PMe2Ph (DHꢀ = 26 kcal molꢀ1). The entropy of
activation for the reaction of 1 with PMe2Ph (DSꢀ =
ꢀ16 1 cal Kꢀ1 molꢀ1) differs from the reaction of 2 and
3 with the same tertiary phosphine (DSꢀ = 17 and
11 cal Kꢀ1 molꢀ1, respectively). The activation parameters
for substitution reactions of 2 and 3 are consistent with a
dissociative mechanism. The observed negative entropy of
activation in the reaction of 1 suggests the possibility of
an associative process, but does not allow us to draw defin-
itive conclusions in a polar solvent like THF [16].
(1) the significant increase in rate of dimerization of phe-
nylacetylene when 1 is used as a catalyst compared to
structurally related ruthenium(II) complexes 2–4 and
(2) the extensive oligomerization of PhCCH in the pres-
ence of 1 compared to 2–4.

1724
M. Daniels, R.U. Kirss / Journal of Organometallic Chemistry 692 (2007) 1716–1725
Neither 2, 3, nor 4 are reported to be active as catalysts
of phenylacetylene containing 5 mol% 7. Heating the mix-
tures to 80 ꢀC for 26 h led to between 48% and 47% conver-
sion to phenylacetylene dimers. The effect of substitution of
CH3CN for Cl in the coordination sphere of 1 on catalytic
activity is not surprising as similar effects are evident in
reactions of other ruthenium compounds. For example,
significant conversion of phenylacetylene to 1,4-diphenyl-
but-1-en-3-ynes is observed in the presence of 3 (80%) while
68% conversion is found with (g5-C9H7)Ru(PPh3)2CCPh
and only 52% conversion is reported when (g5-
C9H7)Ru(PPh3)2H is used as the catalyst. The relative
amounts of dimeric and oligomeric products is also highly
dependant on the ligands in these complexes.
Significant amounts (>50%) of oligomeric and poly-
meric products are only observed in reactions of phenyl-
acetylene catalyzed by 1. Diphenylbutenynes are the
major products from reactions of phenylacetylene in the
presence of 2–4 and 7. Nevertheless, the formation of phe-
nylacetylene oligomers in the presence of Ru(II) complexes
has precedent in the literature. Even greater amounts of
oligomeric and polymeric products are observed in the
reactions of PhCCH catalyzed by (g5-C9H7)Ru(PPh3)2-
CCPh (70%) and (g5-C9H7)Ru(COD)Cl (80%). Lesser
amounts are seen in the presence of (g5-C9H7)Ru-
(PPh3)2H [15]. The polymerization of terminal alkynes is
at ambient temperature while 1 converts phenylacetylene to
a mixture of dimers, oligomers, and polymers after 24 h at
ambient temperature. In the presence of 5 mol% of 5, the
conversion of phenylacetylene is complete in 90 min at
80 ꢀC. In contrast, the conversions of PhCCH after 20–
24 h at 80 ꢀC in the presence of 5 mol% 2 and 5 mol 3
are 23% and 80%, respectively. Complete conversion of
phenylacetylene to dimers in the presence of TpRu(PPh3)2-
Cl (4) is reported at 110 ꢀC [9]. The lability of the PPh3
ligand in 1 may be responsible for the rate enhancement
in the catalytic reactions of phenylacetylene. The rate of
PPh3 substitution for a series of isoelectronic ruthenium(II)
complexes has now been shown to be in the order:
(g5-C5H7)Ru(PPh3)2Cl > (g5-C9H7)Ru(PPh3)2Cl > CpRu-
(PPh3)2Cl. The rate of phosphine substitution in TpRu-
(PPh3)2Cl has not been reported. The accepted mechanism
for the coupling of terminal alkynes (Fig. 2) starts with the
dissociation of a phosphine ligand. Faster dissociation of
PPh3 could account for a faster dimerization reaction.
One expects (g5-C5H7)Ru(PPh3)2(CH3CN)+ (7) to show
a different activity in the oligomerization of alkynes given
the presence of three potentially labile ligands (two PPh3
and the coordinated acetonitrile). No reaction is observed
at ambient temperature in acetonitrile or THF solutions
Tp
Ru
Tp
Ru
L
Cl
L
C
L
Cl
C
Ph
L = PPh3
H
- HCl
Ph
Tp
Ph
Ru
L
Tp
Ru
Tp
Ru
PhCCH
Ph
Ph
L
L
Ph
Ph
Ph
Tp
Ru
L
•
Ph
Ph
Fig. 2. Proposed mechanism for the dimerization of terminal alkynes in the presence of 4 [9].

M. Daniels, R.U. Kirss / Journal of Organometallic Chemistry 692 (2007) 1716–1725
1725
also observed in the presence of TpRu(COD)Cl [19]. The
COD ligand in both (g5-indenyl)Ru(COD)Cl and TpRu-
(COD)Cl is very labile and may be responsible for opening
two coordination sites at ruthenium upon dissociation and
polymerization. These data suggest that the lability of the
phosphine ligands in 1 may be responsible for the forma-
tion of oligomeric products. The kinetic data for the con-
version of 1 to 5 suggest that reaction of 5 with PMe2Ph
(forming 6) is slower than the conversion of 1 to 5. Using
5 as the catalyst in the dimerization of phenylacetylene,
the observation of a slower reaction rate and less oligomer-
ization correlate with lower rates of phosphine substitution
in 5 compared to 1.
ligand contributes to the increased rate of substitution of
PPh3 ligands in 1 compared to 3. The greater lability of
the PPh3 ligands in 1 is reflected in the increased catalytic
activity of 1 in the dimerization of terminal alkynes. While
an increased rate is often welcome in catalytic process, the
reduction in selectivity (dimer vs. polymer) makes 1 less
attractive than 3, 4 and a number of other transition metal
catalysts in the dimerization of alkynes. Further work is in
progress on exploring the scope of reactions catalyzed by 1
where rapid ligand dissociation and a strong donor/accep-
tor pentadienyl ligand may lead to improved yields or dif-
ferent selectivity relative to 3, 4, or other ruthenium(II)
compounds.
Further insight into the mechanism of the (g5-
C5H7)Ru(PPh3)2Cl catalyzed dimerization of phenylacety-
lene is limited. The observation of ‘‘free’’ PPh3 in reactions
catalyzed by (g5-C5H7)Ru(PPh3)2Cl is consistent with the
first step in the proposed mechanism in Fig. 2 (the forma-
tion of a vinylidene complex). The formation of a vinyli-
dene compound, TpRuCl(PPh3){@C@CPh(H)} (31P: d
34 ppm) is reported to be immediate upon addition of
References
[1] R.D. Ernst, Comments Inorg. Chem. 21 (1999) 285.
[2] J.W. Freeman, N.C. Hallinan, A.M. Arif, R.W. Gedridge, R.D.
Ernst, F.F. Basolo, J. Am. Chem. Soc. 113 (1991) 6509.
[3] G.L. Miessler, D.A. Tarr, Inorganic Chemistry, second ed., Prentice-
Hall, New York, 1999.
[4] The abbreviation Tp will be used for the tris(pyrazolylborate) ligand.
Cp refers to the g5-cyclopentadienyl group, C5H5.
[5] (a) B.M. Trost, F.D. Toste, A.B. Pinkerton, Chem. Rev. 101 (2001)
2067;
excess phenylacetylene to
a
benzene-d6 solution of
TpRuCl(PPh3)2. The former can be isolated from the reac-
tion mixture [9]. Vinylidene complexes such as CpRu(PPh3)
{@C@CR(H)}+ and (g5-indenyl)Ru(PPh3){@C@CR(H)}+
are well characterized with non-coordinating anions such
as BFꢀ4 [20]. Evidence for the formation of vinylidene deriv-
atives of 1 remains elusive. The only ruthenium-containing
product from reactions between phenylacetylene and 1
lacks 1H resonances for the pentadienyl group and 13C res-
onances characteristic of ruthenium vinylidene complexes
(ꢁ350 ppm) [20]. The sole 31P resonance at 25 ppm in
C6D6 is nearly identical to the unidentified and catalytically
inactive material isolated from reactions between phenyl-
acetylene and (g5-indenyl)Ru(PPh3)2H or (g5-C9H7)Ru-
(PPh3)2CCPh (31P resonance at 25 ppm in toluene-d8) [15].
Loss of HCl generates a 16-electron alkynyl complex as
the catalytically active species shown in Fig. 2. While such
intermediates are not observed in catalytic dimerization of
PhCCH by 2–4, some 70% of the available phenylacetylene
was converted to oligomers at 110 ꢀC in the presence of
(g5-C9H7)Ru(PPh3)2CCPh [15]. It is possible that similar
compounds are formed in reactions of 1 with phenylacety-
lene leading to oligomerization. Further study on the
mechanistic aspects of 1 catalyzed alkyne dimerization is
clearly needed.
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5. Conclusion
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The pentadienyl (C5H7) ligand has a significant effect on
the catalytic activity of ruthenium(II) complexes. The
donor properties of the pentadienyl ligand may account
for the increased rate of substitution of PPh3 ligands in 1
compared to 2 while the steric bulk of the pentadienyl