Thermal decomposition modes for four-coordinate ruthenium phosphonium alkylidene olefin metathesis catalysts

Article abstract of DOI:10.1002/chem.200801584

The four-coordinate ruthenium phosphonium alkylidenes 1-Cy and 1-iPr, differing in the substituent on the phosphorus center, were observed to decompose thermally in the presence of 1,1-dichloroethylene to produce [H ₃CPR₃][Cl]. The m

Full text of DOI:10.1002/chem.200801584

FULL PAPER
DOI: 10.1002/chem.200801584
Thermal Decomposition Modes for Four-Coordinate Ruthenium
Phosphonium Alkylidene Olefin Metathesis Catalysts
Erin M. Leitao,[a] Stuart R. Dubberley,[a] Warren E. Piers,*[a] Qiao Wu,[a] and
Robert McDonald[b]
Abstract: The four-coordinate rutheni-
um phosphonium alkylidenes 1-Cy and
1-iPr, differing in the substituent on
the phosphorus center, were observed
to decompose thermally in the pres-
ence of 1,1-dichloroethylene to pro-
duce [H3CPR3][Cl]. The major rutheni-
um-containing product was a trichloro-
bridged ruthenium dimer that incorpo-
rates the elements of the 1,1-dichloro-
ethylene as a dichlorocarbene ligand
and a styrenic vinyl group on the sup-
porting NHC ligand. Spectroscopic, ki-
netic, and deuterium-labeling experi-
ments probed the mechanism of this
process, which involves a rate-limiting
C–H activation of an NHC mesityl
ortho methyl group. These studies pro-
vide insight into intrinsic decomposi-
tion processes of active Grubbs type
olefin metathesis catalysts, pointing the
way to new catalyst design directions.
Keywords: catalysis
· decomposi-
tion · kinetics · olefin metathesis ·
ruthenium
Introduction
Olefin metathesis is a powerful
reaction mediated by homoge-
neous transition-metal-based
catalysts.[1] The most promi-
nent catalyst family, the
Grubbs systems, are based on
well-defined, five-coordinate
ruthenium alkylidenes with the generalized structure shown
in I.[2] While these are a versatile group of catalysts, chal-
lenges remain in catalyst design to produce more active,
longer lived, and more selective catalysts, and work contin-
ues today in catalyst modification aimed at diversifying the
Grubbs catalyst portfolio.[3]
Rational catalyst design to meet defined goals is informed
by a detailed understanding of the intimate mechanism of
the catalytic process and knowledge of the decomposition
modes of catalyst precursors and intermediates. Generally,
mechanistic information leads to ligand modification strat-
egies to influence selectivity, while mapping decomposition
pathways can suggest ways of designing more stable cata-
lysts that exhibit improved longevity.
In the Grubbs systems I, much work has been done on de-
lineating the mechanism by which they operate for olefin
metathesis, including the initiation process[4] and catalyst
structure–activity relationships.[5] Despite the detail avail-
able from these studies, the precise structures of many of
the intermediates present in the catalytic cycle remained the
domain of informed speculation based on model com-
plexes[6] or theory.[7] The discovery of four-coordinate phos-
phonium alkylidene complexes[8] of general structure II in
our laboratories has allowed a more detailed experimental
examination of ruthenacyclobutane intermediates previously
undetected in solution,[9,10] their dynamic behavior,[6c,11] and
the mechanisms by which they interchange.[12] Thus, al-
though work continues in this important area, excellent
progress has been made in terms of our understanding of
the mechanism of catalysis.
[a] E. M. Leitao, Dr. S. R. Dubberley, Prof. W. E. Piers, Dr. Q. Wu
Department of Chemistry, University of Calgary,
2500 University Dr. N.W., Calgary, AB, T2N 1N4 (Canada)
Fax : (+1)403-289-9488
E-mail: wpiers@ucalgary.ca
[b] Dr. R. McDonald
Department of Chemistry, University of Alberta
11227 Saskatchewan Drive, Edmonton, AB, T6G 2G2 (Canada)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801584.
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11565

Comparatively less detail is available regarding decompo-
sition modes of catalyst precursors and intermediates.[13]
What is known is mostly due to the isolation of ruthenium-
containing decomposition products. For example, several
halide-bridged dimers have been isolated and character-
ized,[14] suggesting the lability of the chloride ligands[15] is a
potential source of catalyst death. Alternatively, ruthenium
hydrido carbonyls have been isolated in reactions performed
in the presence of H2O, alco-
um moiety, were prepared as previously described.[8a,11] The
less bulky isopropyl-substituted derivative is best prepared
and stored as its zwitterionic chloride salt 1-iPr-Cl;[19] the
chloride ligand trans to the NHC ligand is readily abstracted
[20]
by BACHTUNGRTNEN(GU C6F5)3 to form 1-iPr partnered with the [ClBACHTUNGTRENNUNG(C6F5)3]
ion. For consistency in the mechanistic studies described
here, both 1-Cy and 1-iPr were generated by this method
(Scheme 1) from the zwitterionic trichlorides.
hols, or using vinyl ether sub-
strates.[16] Other studies sug-
gest that the aryl groups in the
NHC ligands of Grubbs
second-generation
catalysts
can undergo ring expansion in
processes involving the ruthe-
nium alkylidene.[17] In a com-
prehensive study, Grubbs and
co-workers have determined
that
a major decomposition
pathway for generation 1 and
2 catalysts involves the attack
of free phosphine (formed in
the initiation step) on
a
Ru=CH2 group, leading to loss
of [H3CPR3][Cl] and formation
of the carbide-bridged dimer
III.[18] This study clearly shows
that the presence of free phos-
phine not only stunts activity,
it can affect overall catalyst
turnovers due to its role in the
catalyst decomposition pro-
cess.
Scheme 1. Thermal decomposition of ruthenium phosphonium alkylidene olefin metathesis catalysts.
Qualitatively, compounds 1-R are reasonably thermally
While the phosphonium alkylidenes II are quite thermally
robust in the solid state and in solution under ambient con-
ditions, they do decompose in solution at elevated tempera-
tures. As such, they present an opportunity to study the de-
composition modes of four-coordinate ruthenium alkyli-
denes in the absence of free phosphine. The compounds 1-
Cy and 1-iPr are exceptionally active olefin metathesis cata-
lysts by virtue of their rapid initiation kinetics, but they do
suffer from low turnovers due to their moderate thermal in-
stability at low olefin concentrations. Although this likely re-
flects the intrinsic instability of the 14-electron methylidene
species or ruthenacyclobutane resting states present during
catalysis,[9,11,12] an analysis of the decomposition modes of
the catalyst precursors 1-Cy and 1-iPr may provide some in-
sight into the low-energy decomposition pathways available
to catalytic intermediates. Here we describe the pathways
by which these compounds decompose.
stable in solution at room temperature, with half-lives on
the order of hours. However, heating to 708C leads to more
1
rapid decomposition. Monitoring such processes by H and
31P NMR spectroscopy indicates that there are several ruthe-
nium-containing products, but that there is one phosphorus-
containing product formed cleanly, namely the methyl phos-
phonium salts [MePR3]+[Cl]À. These products were easily
identified by their characteristic 31P NMR chemical shifts
(d=34.6 and 45.6 ppm in CD2Cl2 for R=Cy and iPr, respec-
tively), ESI mass spectrometric data (m/z 295 and 175), and
separate syntheses.
In an attempt to identify the ruthenium-containing prod-
uct(s), the compounds were allowed to decompose in the
presence of various trapping agents. The most successful
was 1,1-dichloroethylene, an olefin well known to be a poor
substrate for metathesis.[21] Indeed, this substrate is unreac-
tive towards both 1-Cy and 1-iPr in dichloromethane,[22] but
when allowed to decompose in its presence, compounds 1-R
again produce [MePR3]+[Cl]À cleanly along with a predomi-
nant ruthenium-containing product, in approximately 40–
45% yield based on NMR spectroscopy. X-ray quality crys-
tals of this material were obtained from the crude reaction
mixture and its structure was determined.
Results and Discussion
The phosphonium alkylidene complexes 1-Cy and 1-iPr, dif-
fering only in the steric bulk associated with the phosphoni-
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Decomposition of Ruthenium Phosphonium Alkylidene Olefin Metathesis Catalysts
FULL PAPER
This analysis revealed its structure to be the cationic tri-
chloride bridged dimer shown in Scheme 1, which includes a
dichlorocarbene ligand at each Ru center and a modified
NHC ligand wherein one of the ortho mesityl methyl groups
has been homologated into a vinyl group that coordinates
the Ru center. A thermal ellipsoid diagram of the cationic
portion of the ion pair is shown in Figure 1; here, the crys-
vinyl group are also present, with typical coupling patterns
1
observed in the H NMR spectra. In the 13C NMR spectrum,
a weak resonance at d=265.02 ppm is assignable to the di-
chlorocarbene carbon atom.[23] Finally, ESI mass spectromet-
+
ric analysis of 2 gave a parent ion for C46H52Cl7N4Ru2 (m/z
1111) with an isotope peak pattern that matched the calcu-
lated distribution for this empirical formula.
Since the primary ruthenium-containing product in this
decomposition is dimeric, the reaction order in [Ru] was ex-
amined by following the loss of compounds 1-R using 1H
and/or 31P{1H} NMR spectroscopy. The plots in Figure 2a
Figure 1. Thermal ellipsoid diagram (50%) of the cationic portion of 2;
À
À
the counteranion is [B
G
À
À
2.4201(7), Ru Cl2 2.4982(8), Ru Cl2’ 2.4951(7), Ru C1 1.844(3), Ru C2
À
À
2.065(2), Ru C19 2.255(3), Ru C20 2.209(3), C19 C20 1.388(5). Selected
non-bonding distance [ꢂ]: Ru Ru’ 3.29. Selected bond angles [8]: C1-Ru-
Cl2 169.35(9), C1-Ru-Cl1 93.40(9), C1-Ru-Cl2’ 90.01(9), C1-Ru-C2
90.63(12), C2-Ru-Cl1 91.87(8), C2-Ru-Cl2’172.39(8), C2-Ru-Cl2 98.23(9),
Cl1-Ru-Cl2 80.47(2), Cl1-Ru-Cl2’ 80.44(3), Cl1-Ru-C19 163.51(8), Cl1-
Ru-C20 155.93(9), Ru-Cl1-Ru’ 85.66(3), Ru-Cl2-Ru’ 82.45(2).
tals were obtained from the experiment utilizing 1-Cy part-
Figure 2. a) Exponentially fitted first-order plots for disappearance of 1-
Cy and 1-iPr at 343 K. b) Exponentially fitted first-order plots for disap-
pearance of 1-iPr at 330 K using various initial concentrations of 1-iPr.
nered with the [BACHTUNGTRENNUNG
(C6F5)4]À ion, but 2 is produced in similar
amounts regardless of the anion. Notable features of the
structure are as follows. The two halves of the dimer are re-
lated by a C2 axis that bisects the Ru-Cl1-Ruꢁ angle and
runs through the centroid of the Ru-Cl2-Ru’-Cl2’ ring. The
Ru centers are distorted octahedral in geometry, joined
through the face formed by the three bridging chlorides.
give excellent first-order fits, and show that the phosphoni-
um alkylidene with the less bulky PACTHNUTRGNENUG(iPr)3 group decomposes
with a rate constant about an order of magnitude faster than
1-Cy (all rate constants determined are tabulated in
Table 1). Furthermore, the decomposition of 1-iPr at various
[Ru]0 at 330 K (Figure 2b) also show that the process is
first-order in [Ru]. As shown in Figure 3, Eyring analysis of
the rate constants for the disappearance of 1-iPr at various
temperatures surrenders activation parameters of DH° =
64(2) kJmolÀ1 and DS° =À129(5) JKÀ1 molÀ1.
À
The three Ru Cl distances are slightly different due to the
three different trans ligands. The trapping 1,1-dichloroethyl-
ene reagent has been split into a dichlorocarbene ligand,
À
with a short Ru C1 distance of 1.844(3) ꢂ, whereas the
=CH2 group has been added to an ortho methyl group of an
NHC mesityl substituent, forming a styrenic group that now
coordinates the ruthenium center in an unsymmetrical fash-
ion, the terminal carbon C20 being more closely associated
A plausible mechanism that accounts for these observa-
tions is given in Scheme 2. Since the reaction is first-order in
[Ru], and compounds 1-R do not react with 1,1-dichloro-
ethylene under the conditions of decomposition, the rate-
limiting step is likely an intramolecular process, which pro-
duces a reactive intermediate that is trapped by the olefin to
eventually give 2. We propose that this step involves activa-
with the ruthenium (2.209ACTHNURGTENN(UG 3 ꢂ) than C19 (2.255CAHTUNGTRNE(NUGN 3 ꢂ).
Spectroscopic data for 2 is completely consistent with the
structure as determined by X-ray crystallography. Proton
and 13C NMR analysis of isolated crystals indicates the de-
symmetrization of the NHC ligand as exemplified by the
five separate resonances for mesityl methyl groups observed
in both spectra. Characteristic resonances for a coordinated
À
tion of a C H bond of an ortho methyl group of one of the
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W. E. Piers et al.
Table 1. Rate constants for the decomposition of 1-Cy and 1-iPr at dif-
ferent concentrations and temperatures.
Entry
Compound
[Ru] [m]
Temp [K]
Rate constant[c] [sÀ1
]
1
2
3
4
5
6
7
8
1-Cy
1-Cy
1-Cy
1-Cy
1-Cy
1-Cy
0.069
0.069
0.069
0.069
0.069
0.069
0.069
0.069
0.069
0.042
0.084
0.158
0.084
0.084
0.084
0.084
0.080
0.080
0.080
0.080
324
334
345
354
365[a]
343
343
343
343
330
330
330
313
320
344
355[b]
343
343
343
343
1.5ꢃ10À5
3.6ꢃ10À5
8.6ꢃ10À5
2.0ꢃ10À4
3.9ꢃ10À4
9.3ꢃ10À5
2.1ꢃ10À5
4.8ꢃ10À5
4.9ꢃ10À5
1.1ꢃ10À4
1.2ꢃ10À4
1.3ꢃ10À4
2.6ꢃ10À5
5.8ꢃ10À5
2.8ꢃ10À4
5.9ꢃ10À4
2.4ꢃ10À4
5.1ꢃ10À5
1.4ꢃ10À4
1.6ꢃ10À4
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
9
[D6]-1-Cy
1-iPr
1-iPr
1-iPr
1-iPr
1-iPr
1-iPr
1-iPr
1-iPr
10
11
12
13
14
15
16
17
18
19
20
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[a] Entries 1–5 used in a Eyring plot to give DH° =76(2) kJmolÀ1; DS° =
À104(5) JKÀ1 molÀ1. [b] Entries 11, 13–16 used in a Eyring plot to give
DH° =64(2) kJmolÀ1; DS° =À129(5) JKÀ1 molÀ1. [c] Error in rate con-
stant measurements is Æ0.2ꢃ10À4 or Æ0.2ꢃ10À5 depending on the value.
Scheme 2. Proposed mechanism of decomposition for phosphonium alky-
lidene complexes 1-R.
methyl phosphonium product via what is essentially the re-
À
verse of the initial step, that is, C H bond formation via a
elimination, to give the chelated alkylidene complex V. This
highly reactive species undergoes cross metathesis with the
trapping olefin in a step that institutes both the dichlorocar-
bene unit and the styrenic vinyl group observed in the final
product 2. Intermediate VI thus formed undergoes dimeriza-
tion after loss of chloride; ion exchange yields the observed
products 2 and [H3CPR3][Cl].
None of the proposed intermediates IV–VI are observed
directly, but some precedent and evidence for V is available.
Blechert and co-workers have reported a chelated alkyli-
dene complex (VII, Scheme 2) derived from air oxidation of
NHC-supported Grubbs–Hoveyda type catalysts with N-
phenyl substitution on the NHC donor.[25] While the pro-
posed mechanism of formation of the Blechert compound is
quite different, and it is “completely inactive” as an olefin
metathesis initiator, it serves as an excellent structural
model for the proposed intermediate V.
Evidence for the viability of V is provided by monitoring
the decomposition of 1-iPr as its triflate salt[26] (or 1-iPr
itself) in the absence of any trapping agent at room tempera-
ture. Over 1–2 days at room temperature, a species assigned
as the mixed dimer 3 grows in until it comprises about 20%
of the ruthenium speciation, the remainder being unreacted
1-iPr-X and a small amount (<5%) of other, unidentified
alkylidene-containing species. Compound 3 is formed by the
“self trapping” of V by 1-iPr-X; under the conditions of its
generation, it decomposes further, but cooling enriched sam-
Figure 3. a) Logarithmic first-order plots for the disappearance of 1-iPr
(0.084m) at various temperatures. See Table 1 for first-order rate con-
stants. b) Eyring plot for the decomposition of 1-iPr. See text and Table 1
for activation parameters.
mesityl substituents, possibly via addition across the Ru=C
phosphonium alkylidene bond.[24] The resulting (formally)
RuIV intermediate IV rapidly eliminates the observed
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Decomposition of Ruthenium Phosphonium Alkylidene Olefin Metathesis Catalysts
FULL PAPER
1
ples to À538C allows partial analysis of its H and 13C NMR
spectra.
1
In the alkylidene region of the H NMR spectrum (using
1-iPr-OTf), two new signals in a 1:1 ratio at d=20.23 ppm
(2JH,P =46 Hz) and d=18.57 ppm (singlet) are assignable to
the phosphonium alkylidene and the chelated alkylidene
groups of 3, respectively (see inset, Scheme 3). The magni-
Figure 4. a) Representative intermolecular kinetic isotope effect based on
logarithmic first-order plots for the decomposition of 1-Cy and [D22]-1-
Cy at 343 K (determined by NMR). The ratio of kACHTNUGTRENNUG(1-Cy) to kACHTUGNTRENN(UGN [D22]-1-Cy)
gives the intermolecular KIE of 4.4(4). b) Representative intramolecular
kinetic isotope effect based on the final decomposition mixture of [D11]-
+
1-Cy at 343 K (determined by ESI MS). The ratio of m/z 295 (CH3PCy3
)
Scheme 3. Self-trapping of the proposed intermediate V.
over corrected m/z 297 (CHD2PCy3+); gives an intramolecular KIE of
6.9(4), where the correction is the natural isotopic distribution (gray
bars) of the parent ion (m/z 295) against the experimental spectrum
(black bars).
2
tude of JH,P in the former resonance is indicative of a five-
coordinate ruthenium center; these couplings are about
37 Hz in the four-coordinate phosphonium alkylidenes.
These resonances correlate with 13C signals at d=292.3 ppm
(1JC,H =166Æ4 Hz, c.f. 168 Hz for the starting material phos-
phonium alkylidene) and d=303.3 ppm (1JC,H =156Æ4 Hz),
characteristic of alkylidene carbon atoms.[27] Unfortunately,
all attempts to isolate 3 failed due to its propensity to un-
dergo further decomposition. Furthermore, attempts to
functionalize 3 via reaction with external bases such as PCy3
or pyridine, so as to isolate a derivative, result in the trap-
ping of the starting complexes 1. It should be noted, howev-
er, that precedent exists for the trapping of Grubbs frame-
work 14-electron alkylidene complexes by LMCl2 com-
pounds (M=Ru, Os) to give dichloro-bridged dimers similar
in character to 3.[28]
Table 2. Inter- and intramolecular kinetic isotope effects for the decom-
position of isotopomers of 1-Cy and 1-iPr.
[c]
Type of competition
Compounds
1-Cy/[D22]-1-Cy
[D11]-1-Cy
[D6]-1-Cy
1-iPr/[D22]-1- iPr
[D11]-1- iPr
[D6]-1-iPr
kH/kD (Æ0.4)
intermolecular[a]
intramolecular[b]
intramolecular[b]
intermolecular[a]
intramolecular[b]
intramolecular[b]
G
4.4
6.9
6.8
4.7
6.2
5.9
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
[a] Measured by comparing the rate constants obtained by NMR (en-
tries 6/7 and 17/18 of Table 1). [b] Measured by comparing the intensities
of isotopomers by ESI MS. [c] All decompositions were performed at
343 K; 1-iPr complexes were 0.080m in Ru and 1-Cy complexes were
0.069m in Ru.
Further support for the mechanistic proposals was gar-
nered by selective deuterium-labeling and the determination
of inter- and intramolecular kinetic isotope effects (Figure 4
the intermolecular isotope effects using NMR spectroscopy;
Figure 4a shows a representative kinetic plot. The partially
deuterated compounds were used to determine intramolecu-
lar kH/kD values by quantitatively assessing the ratio of
[H3CPR3]+ and [HD2CPR3]+ isotopomers in the methyl-
phosphonium salt product by using ESI mass spectrometry.
Figure 4b shows a representative mass spectrum; full details
of how these spectra were calibrated and corrected for natu-
ral abundance isotopomers is given in the Supporting Infor-
mation. Consistent with our mechanistic proposal, these are
the two predominant isotopomers observed; less than 2%
À
and Table 2). Since activation of a benzylic C H bond of an
ortho methyl group of an NHC mesityl moiety appears to
be rate-limiting, three deuterated isotopomers of each phos-
phonium alkylidene were prepared in which these methyl
groups were either fully ([D22]-1-R) or partially deuterated.
The partially deuterated were subdivided into [D11]-1-R or
[D6]-1-R isotopomers with label incorporation as shown.
The fully deuterated isotopomers were used to determine
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W. E. Piers et al.
À
ic behavior of the NHC ligand, interaction of a benzylic C
H bond with the ruthenium center or the phosphonium al-
kylidene ligand directly would lead to significant ordering in
the transition state, accounting for the large, negative DS°
observed.[30] The need to bring the C H bond near to the
metal center as a first step is also consistent with the ob-
served faster decay rates for the sterically less bulky 1-iPr
series of compounds.
À
Conclusions
The primary thermal decomposition mode of a family of
highly active phosphonium alkylidene olefin metathesis cat-
À
alysts was found to involve C H activation of one of the N-
of the [H2DCPR3]+ isotopomer was present and likely due
to the less than complete labeling of the starting material,
which was available 98% deuterated. Furthermore, no deu-
terium incorporation into the phosphonium alkylidene posi-
tion was observed during any of these reactions, suggesting
that the C–H activation and [H3CPR3]+ elimination steps
are indeed irreversible as suggested in Scheme 2.
heterocyclic carbene ligandꢁs mesityl groups. This was sup-
ported by characterization of the decomposition product,
and kinetic and labeling mechanistic studies. The implica-
tions of this observation for the further development of li-
gands for this family of catalysts are currently being consid-
ered. For example, N-heterocyclic carbene ligands that are
À
less susceptible to C H activation would be an obvious
As Figure 4 and Table 2 indicate, there is a substantial in-
avenue to pursue.
termolecular isotope effect of kH/kD =4.4(4) for 1-Cy/
[D22]-
Although this decomposition process is eventually detri-
mental to catalyst performance, it is worth noting that initia-
tion rates for these catalysts are much faster than the rate-
limiting step in the decomposition process identified here.
Current studies are focusing on quantifying the initiation
rates and determining the properties of the active propagat-
ing species.
1-Cy and 4.7(4) for 1-iPr/[D22]-1-iPr. These are large effects
AHCTUNGTRENNUNG
considering that the kinetic measurements are made at
343 K. The intramolecular isotope effects are also large and,
surprisingly, larger than the intermolecular kH/kD values by
about 1–3. These differences are reproducible and not due
to the different techniques used to evaluate them; the mea-
sured intramolecular kH/kD for [D11]-1-Cy using ESI MS
methodology (6.9) is in agreement with the value of 6.6(4)
determined by using NMR spectroscopy (see Supporting In-
formation for details). The differences also do not appear to
be due to a conformational equilibrium involving the dy-
namic behavior of the NHC ligand that is influenced by
deuteration. For example, cooling solutions of 1-Cy results
in the freezing out of free rotation about the Ru-NHC
carbon bond, rendering the N-mesityl groups inequivalent
on the NMR timescale. The same behavior is observed upon
cooling samples of [D11]-1-Cy, but there is no measurable
preference for the undeuterated mesityl group to occupy
one environment over the other; the two inequivalent sin-
glets for the mesityl aryl protons are present in a 1:1 ratio.[29]
Furthermore, the intramolecular kH/kD measured for the
symmetrical [D6]-1-R isotopomers, where such an equilibri-
um effect would not be relevant, are identical to the intra-
molecular isotope effects measured in the [D11]-isotopomers.
It is possible that the discrepancy between the intra- and the
intermolecular isotope effects is due to the effect of the vari-
ous isotopic substitution patterns on unseen equilibria be-
tween intermediates IV–VI, but this remains a matter of
speculation.
Experimental Section
General: Argon-filled Innovative Technology System One dry boxes
were used to store air- and moisture-sensitive compounds, and for the
manipulation of air-sensitive materials. Reactions were performed either
on a double manifold vacuum line using standard Schlenk techniques or
under an argon atmosphere in the dry box for small-scale reactions.
Glassware used in anhydrous reactions was stored in a hot oven (1108C)
overnight or flamed-dried prior to immediate use, and then transferred
to the dry box. All non-deuterated starting materials were synthesized ac-
cording to published procedures.[31] The syntheses of the deuterated com-
plexes are given in the Supporting Information. Where applicable, cool-
ing baths consisting of dry ice/acetone (À788C) were used to maintain
low-temperature conditions. 1H, 31P{1H}, 13C{1H}, 19F, 11B NMR experi-
ments were performed on Bruker AMX-300, DMX-300, and DRX-400
spectrometers. Data are given in ppm relative to residual solvent signals
for 1H and 13C. 19F NMR spectra were referenced externally to C6F6 in
C6D6 at d=À163 ppm relative to CFCl3 at d=0.0 ppm. 31P NMR spectra
were referenced to an aqueous external standard of H3PO4 in D2O at d=
0.0 ppm. 11B NMR spectra were referenced to BF3·Et2O in C6D6 at d=
0.0. Temperature calibration for high-temperature NMR experiments was
achieved by monitoring the 1H NMR spectrum of pure ethylene glycol
before and after changing the temperature. For quantitative experiments
16 scans were collected for every spectrum. T1 measurements were per-
formed on the resonances of interest and a delay (d1) of 5*T1 between
single acquisitions was used, taking as a reference the longest relaxation
time (in most cases a d1 of 2 s was sufficient). Electrospray ionization
mass spectrometry (ESI MS) was performed on the Bruker Esquire 3000
(positive mode, 3200 V capillary voltage, 7 psi nebulizer, 5.0 LminÀ1 dry
gas, 3008C dry temperature, 0.10 ms accumulation time, and target mass
The important conclusion is that there is a substantial pri-
mary kinetic isotope effect that confirms that activation of a
À
C H bond in this position is the critical first step in the de-
composition process in these compounds. Given the dynam-
11570
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 11565 – 11572

Decomposition of Ruthenium Phosphonium Alkylidene Olefin Metathesis Catalysts
FULL PAPER
set to 296 or 176 depending on phosphine). Quantitative experiments
were prepared in triplicate and run in triplicate. A test for the capability
of using ESI MS quantitatively for measuring a kinetic isotope effect was
performed using known concentrations of CH3PCy3 and CD3PCy3 (see
Supporting Information). Single-crystal X-ray diffraction was performed
by Robert McDonald on a Bruker Platform/Smart 1000 CCD diffractom-
eter (University of Alberta), and elemental anaylsis were performed on a
Perkin–Elmer Model 2400, Series II, by Johnson Li (University of Calga-
ry).
4F; para CF), À166.1 ppm (m, 8F; meta CF); 11B NMR (128.4 MHz,
C2D2Cl4, 298 K): d=À16.15 ppm (s, B
(C6F5)4À); ESI MS for cationic
ACHTUNGTRENNUNG
dimer C46H52Cl7N4Ru2+: 1111 m/z; elemental analysis calcd (%) for
C70H52BCl7F20N4Ru2·CH2Cl2: C 45.48, H 2.90, N 2.99; found: C 45.88, H
3.04, N 2.92.
+
+
Generation of 3 in solution: Compound 1-iPr-OTf (or 1-iPr) (35 mg,
0.044 mmol) was charged into an NMR tube and CD2Cl2 (0.4 mL) was
added. The tube was left at room temperature overnight (or for two
nights for 1-iPr) and then was measured at the appropriate temperature
in the NMR spectrometer. To prevent over decomposition long measure-
ments were aquired at 220 K. Yield=20% (in solution). 1-iPr-OTf im-
Decomposition kinetics by NMR spectroscopy (for Eyring plot, concen-
tration variation, and intermolecular kinetic isotope effects): Compound
1-R-Cl (25 mg, 0.028 mmol for 1-Cy-Cl and 0.032 mmol for 1-iPr-Cl) was
1
2
portant peaks: H NMR (399.6 MHz, CD2Cl2, 298 K): d=20.23 (d, JH,P
=
46 Hz, 1H; RuCHPiPr3 in 3), 18.57 (s, 1H; RuCHACHTNUTRGNE(UNG NHC) in 3),
charged into a J-Young NMR tube along with BACTHUNRGTNEUNG(C6F5)3 (0.046 mmol for
18.38 ppm (d, 2JH,P =39 Hz, 1H; RuCHPiPr3 in 1-iPr-OTf); 1H NMR
(399.6 MHz, CD2Cl2, 220 K): d=20.09 (d, 2JH,P =45 Hz, 1H; RuCHPiPr3
1-Cy-Cl and 0.054 mmol for 1-iPr-Cl), 1,1-dichloroethane (50 mL),
(CDCl2)2 (0.4 mL), and an internal standard hexamethyldisiloxane
(0.004 mmol). The tube was placed in an acetone/dry ice bath for trans-
port to the NMR spectrometer and then placed in a pre-warmed NMR
probe (708C for intermolecular kinetic isotope effects and a value be-
tween 408C–808C depending on the kinetic run). Proton cycling experi-
ments were used to acquire spectra at various intervals during the de-
composition (depending on the half-life), and the integration of the alky-
lidene proton (d=18 ppm) against an internal standard (d=0.1 ppm) was
used for 1-Cy-Cl. Integration of the methyl doublet of doublets (on the
PiPr3 group) was used for 1-iPr-Cl, where the starting material was divid-
ed into the total integration of the starting material and decomposition
product (methyl signal from the PiPr3 in the phosphonium salt).
2
in 3), 18.75 (d, JH,P =44 Hz, 1H; RuCHPiPr3 in 1-iPr-OTf), 18.43 ppm (s,
1H; RuCHACHTUNGTRNEUNG
(NHC) in 3); 31P{1H} NMR (161.8 MHz, CD2Cl2, 298 K): d=
54.33 (br s, CHPiPr3 in 1-iPr-OTf), 48.57 (s, CHPiPr3 in 3), 45.38 ppm (s,
MePiPr3); 13C NMR (from HMQC (no decoupling), 220 K): d=303.33
ACTHNUTRGNEUNG
(d, 1JC,H =156 Hz, RuCH(NHC) in 3), 292.29 (dd, 1JC,H =166 Hz, RuCH-
PiPr3 in 3), 271.43 ppm (dd, 1JC,H =168 Hz, RuCHPiPr3 in 1-iPr-OTf);
1JC,P couplings were not resolved. 1-iPr important peaks: 1H NMR
(399.6 MHz, CD2Cl2, 298 K): d=20.21 (d, 2JH,P =46 Hz, 1H; RuCHPiPr3
in 3), 18.57 (s, 1H; RuCHACHTNUTGRNEUNG
(NHC) in 3), 17.77 ppm (d, 2JH,P =35 Hz, 1H;
1
RuCHPiPr3 in 1-iPr); H NMR (399.6 MHz, CD2Cl2, 220 K): d=20.08 (d,
2JH,P =45 Hz, 1H; RuCHPiPr3 in 3), 18.43 (s, 1H; RuCH
ACTHUNRTGNE(GNU NHC) in 3),
17.71 ppm (d, 2JH,P =35 Hz, 1H; RuCHPiPr3 in 1-iPr); 31P{1H} NMR
(161.8 MHz, CD2Cl2, 298 K): d=60.95 (s, CHPiPr3 in 1-iPr), 48.40 (s,
CHPiPr3 in 3), 45.24 ppm (s, MePiPr3); 13C NMR (from HMQC (no de-
Decomposition by ESI MS (intramolecular kinetic isotope effects): Com-
pound [D11]-1-R-Cl (25 mg, 0.028 mmol for [D11]-1-Cy-Cl and 0.032 mmol
for [D11]-1-iPr-Cl) was charged into a J-Young NMR tube along with B-
coupling), 220 K): d=303.35 (d, 1JC,H =171 Hz, RuCH
ACHTUNGTERNNU(NG NHC) in 3),
1
ACHTUNGTRENNUNG(C6F5)3 (0.046 mmol for [D11]-1-Cy-Cl and 0.054 mmol for [D11]-1-iPr-Cl),
292.37 (dd, 1JC,H =170 Hz, RuCHPiPr3 in 3), 262.60 ppm (dd, JC,H
178 Hz, RuCHPiPr3 in 1-iPr); JC,P couplings were not resolved.
=
1,1-dichloroethane (50 mL), and (CDCl2)2 (0.4 mL). The tube was heated
by using an oil bath set to 708C on the benchtop. The reaction was moni-
tored by 1H and 31P NMR spectroscopy, and deemed complete when the
starting material 31P NMR peak had completely disappeared, and the
phosphonium salt decomposition product peak had appeared (d=61 to
45 ppm for [D11]-1-iPr; d=55 to 34 ppm for [D11]-1-Cy). Once the reac-
tion was complete, the solution was extracted with water (2.0 mL) to
1
X-ray crystallography: X-ray crystallographic analysis of 2 was performed
on suitable crystals coated in Paratone oil and mounted on a Bruker
PLATFORM diffractometer/SMART 1000 CCD area detector system.
CCDC-697188 (2) contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif
+
remove the CDnH3ÀnPR3 salt mixture (n=0–2, R=Cy, iPr), which was
directly measured in the ESI mass spectrometer. The intensity of the
+
CH3PR3 peak (295 for R=Cy and 175 for R=iPr) was divided by the
corrected intensity (for the natural isotope pattern of the parent ion) of
+
the CHD2PR3 peak (297 for R=Cy and 177 for R=iPr) to provide the
kinetic isotope effect.
Acknowledgements
Synthesis of 2: In a 50 mL reaction vessel, equipped with stir bar, 1-Cy-B-
ACHTUNGTRENNUNG(C6F5)4 (250 mg, 0.17 mmol) was added along with tetrachloroethane
Funding was provided by NSERC of Canada in the form of a Discovery
Grant to W.E.P., and a CGSM scholarship to E.M.L.. Edwin van de
Eide is acknowledged for useful discussions. Dr. Martine Monette
(Bruker Canada) is gratefully acknowledged for performing
(5 mL) and 1,1-dichloroethene (1.25 mL, 16 mmol). The flask was sealed
(with a Kontes tap), and heated to 708C in an oil bath overnight. The
progress of the decomposition was monitored by 31P{1H} NMR spectros-
copy by removal of an aliquot, concentration, and re-dissolving in
CD2Cl2. Once all the starting material was decomposed (only one peak
in the 31P{1H} NMR spectrum at d=34 ppm), the reaction mixture was
cooled and concentrated to dryness. The resulting brown residue was ex-
tracted with dichloromethane and water (to remove the phosphonium
salt), and the organic extracts were combined and concentrated. The new
residue was dissolved in a minimal amount of dichloromethane and lay-
ered with pentane. Red crystals were isolated and washed with pentane.
Yield=11 mg (6.5%, 0.0056 mmol). 1H NMR (399.6 MHz, C2D2Cl4,
298 K; see Supporting Information for the spectrum): d=6.92, 6.89, 6.77,
6.68 (s, 2H each; Mes CH), 5.03 (d, 2JH,H(trans) =14 Hz, 2JH,H(gem) =0 Hz,
2H; vinyl H), 4.66 (d, 2JH,H(cis) =9 Hz, 2JH,H(gem) =0 Hz, 2H; vinyl H), 4.22
(dd, 2JH,H(cis) =9 Hz, 2JH,H(trans) =14 Hz, 2H; vinyl H), 4.60, 4.12, 4.01, 3.78,
(q, 2H each; NHC CH2 backbone), 2.47, 2.30, 2.28, 2.25, 2.22 ppm (s, 6H
each; Mes CH3); 13C{1H} NMR (100.5 MHz, C2D2Cl4, 298 K): d=264.42
(s, CCl2), 198.47 9S, NHC NCN), 139.61, 138.37, 137.69, 135.61, 134.89,
134.65, 134.33 (all s, Mes quaternary C), 134.01, 129.49, 128.97, 126.61 (s,
Mes CH), 84.88, 79.39 (s, vinyl C), 56.45, 49.27 (s, NHC CH2 backbone),
21.32, 20.62, 19.85, 19.23, 19.18 ppm (all s, Mes CH3); 19F NMR
(282.4 MHz, C2D2Cl4, 298 K): d=À132.4 (m, 8F; ortho CF), À162.2 (m,
+
31P{1H} NMR experiments at 242.9 MHz in the analysis of CHnD3ÀnPR3
isotopomers. Materia Inc. provided ruthenium catalysts.
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[22] Monohalogen-substutituted olefins react with compounds
1 and
other ruthenium-based olefin metathesis initiators: See ref. [19] and
M. L. Macnaughtan, M. J. A. Johnson, J. W. Kampf, Organometallics
2007, 26, 780–782.
[23] See reference [1a], page 92. The 13C NMR shift for
a
similar
difluoroACHTUNGERTNcNUNG arbene monomeric complex is d=218.09 ppm: T. M. Trnka,
M. W. Day, R. H. Grubbs, Angew. Chem. 2001, 113, 3549–3552.
À
[24] Other literature precedence for C H activation includes: a) S. H.
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119, 5240–5243; Angew. Chem. Int. Ed. 2007, 46, 5148–5151;
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4944–4945.
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8231; Angew. Chem. Int. Ed. 2007, 46, 8082–8085.
[26] The affect of the counteranion on the rate and mechanism of these
decompositions has not been studied in detail, nor is it well under-
stood. Here, substitution of the borate counteranion for the triflate
renders the self trapping process to generate 3 more rapidly; at this
point, this is simply an empirical observation.
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1
Grubbs catalyst has a JC,H value of 148 Hz: S. Leuthꢅußer, V.
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À
[29] See ref. [8a]. The barrier for rotation about the Ru NHC bond is
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21–22 kcalmolÀ1 are observed in second-generation Grubbsꢁ systems,
see reference [27b].
[11] E. F. van der Eide, P. E. Romero, W. E. Piers, J. Am. Chem. Soc.
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[30] Even a strongly ordered transition state does not fully account for
the magnitude of the entropy of activation. However, we note that
these are ionic compounds and solvent cage reorganization may
contribute significantly to this part of the barrier.
[12] P. E. Romero, W. E. Piers, J. Am. Chem. Soc. 2007, 129, 1698–1704.
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[31] For the syntheses of 1-Cy-Cl, 1-iPr-Cl, and 1-iPr-OTf see ref. [19];
and for the synthesis of 1-Cy-BACHTNUGTRENNUNG
Published online: November 26, 2008
11572
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 11565 – 11572