Insights into the decomposition of olefin metathesis precatalysts

Article abstract of DOI:10.1002/anie.201403770

The decomposition of a series of benzylidene, methylidene, and 3-phenylindenylidene complexes has been probed in alcohol solution in the presence of base. Tricyclohexylphosphane-containing precatalysts are shown to yield [RuCl(H)(H₂)(PCy₃)₂] in isopropyl alcohol solutions, while 3-phenylindenylidene complexes lead to η⁵-(3- phenyl)indenyl products. The potential-energy surfaces for the formation of the latter species have been probed using density functional theory studies.

Full text of DOI:10.1002/anie.201403770

Angewandte
Chemie
DOI: 10.1002/anie.201403770
Reaction Mechanisms
Insights into the Decomposition of Olefin Metathesis Precatalysts**
Simone Manzini, Albert Poater, David J. Nelson, Luigi Cavallo, Alexandra M. Z. Slawin, and
Steven P. Nolan*
Abstract: The decomposition of a series of benzylidene,
methylidene, and 3-phenylindenylidene complexes has been
probed in alcohol solution in the presence of base. Tricyclo-
hexylphosphane-containing precatalysts are shown to yield
[RuCl(H)(H2)(PCy3)2] in isopropyl alcohol solutions, while 3-
phenylindenylidene complexes lead to h5-(3-phenyl)indenyl
products. The potential-energy surfaces for the formation of
the latter species have been probed using density functional
theory studies.
to explore the stability of metathesis precatalysts towards
various reagents and conditions. Notably, when the Grubbs-
type precatalysts G1 and G2 are heated in the presence of
primary alcohols, the hydridocarbonyl complexes 1 and 2,
respectively, are formed (Scheme 1).[2] These complexes have
been implicated in deleterious isomerization processes during
metathesis reactions.[3] Recently, our efforts have been
S
tability and activity are two key requirements for robust
and efficient catalysts. While these two properties are often
inversely proportional, a thorough understanding of both is
important. To develop catalysts with increased performance it
is crucial to understand possible decomposition pathways, not
only during catalysis but also during catalyst synthesis and
handling. In addition, the characterization and investigation
of decomposition products can lead to new understanding in
organometallic chemistry.
Olefin metathesis has been the key step in many elegant
syntheses of complex molecules,[1] and therefore (pre-)cata-
lysts are routinely exposed to a plethora of functional groups.
Previous work by numerous researchers has been conducted
[*] S. Manzini, Dr. D. J. Nelson, Prof. Dr. A. M. Z. Slawin,
Prof. Dr. S. P. Nolan
EaStCHEM School of Chemistry, University of St Andrews
North Haugh, St Andrews, Fife, KY16 9ST (UK)
E-mail: snolan@st-andrews.ac.uk
Scheme 1. Decomposition of G1, G2, M10, and M11 in an alcohol
solution. SIMes=1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-
ylidene.
Dr. A. Poater
Institut de Quꢀmica Computacional i Catꢁlisi and Departament de
Quꢀmica, Universitat de Girona, 17071 Girona (Spain)
and
Catalan Institute for Water Research (ICRA), Scientific and Tech-
nological Park of the University of Girona, H2O Building
Emili Grahit 101, 17003 Girona (Spain)
focused on understanding the stability of metathesis com-
plexes, especially the versatile synthon [RuCl2((3-
phenyl)indenylidene)(PPh3)2] (M10), which is used for the
synthesis of first-, second-, and third-generation metathesis
precatalysts.[4] When M10 was heated in alcohol solution, the
new species [RuCl(h5-(3-phenyl)indenyl)(PPh3)2] (3) was
formed, wherein the indenylidene ligand coordinated in an
h5-fashion (Scheme 1).[5] This complex, and its cationic
derivative,[6] have since been shown to be highly active
in a number of transformations.[5–7] More recently, we
have shown that this indenylidene-to-h5-indenyl rear-
rangement is not limited to M10, but that it also occurs for
the related complex [RuCl2(3-phenylindenylidene)(iBu-
Phoban)2] (M11), thus yielding the hydride species [Ru(H)-
(h5-(3-phenyl)indenyl)(iBu-Phoban)2] (4; Scheme 1; iBu-
Phoban = 9-isobutyl-9-phosphabicyclo[3.3.1]nonane).[8]
Prof. Dr. L. Cavallo
KAUST Catalyst Center, Physical Sciences and Engineering Division
King Abdullah University of Science and Technology
Thuwal, 23955-6900 (Saudi Arabia)
[**] Research leading to these results has received funding from the EU
Seventh Framework Programme (FP7/2007-2013) under grant
agreement n8CP-FP 211468-2 EUMET. S.P.N. thanks the ERC for an
Advanced Investigator Award “FUNCAT” and KAUST for support.
S.P.N. is a Royal Society Wolfson Research Merit Award holder. L.C.
thanks the HPC team of Enea for using the ENEA-GRID and the
HPC facilities CRESCO in Portici (Italy) for access to remarkable
computational resources. A.P. thanks the Spanish MINECO for
a Ramꢂn y Cajal contract (RYC-2009-05226) and European Com-
mission for a Career Integration Grant (CIG09-GA-2011-293900).
We thank Umicore for gifts of materials and ligands.
Notably, in the latter case, a much cleaner reaction
occurred when a secondary alcohol was used. Seeking to have
a more comprehensive understanding of the decomposition of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403770.
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ruthenium indenylidene complexes, the stability of different
first-generation species was evaluated in primary and secon-
dary alcohols in the presence of a base. The two key questions
were 1) whether indenylidene complexes behave differently
from their benzylidene congeners and 2) whether the behav-
ior of a complex might be predicted based on its chemical
structure. To this end, a series of complexes were systemati-
cally explored in alcohol solutions in the presence of triethyl-
=
amine. The methylidene [RuCl2( CH2)(PCy)2] (5; see
Scheme 4) was also studied, as it is commonly implicated as
a fragile species in metathesis reactions. This compound was
prepared using a modified version of the protocol from Fogg
and co-workers,[9] where 1,7-octadiene was used in place of
ethene, as the former undergoes rapid and complete meta-
thesis[10] to yield ethene in situ (see the Supporting Informa-
tion for details).
Scheme 3. Decomposition of indenylidene complexes in isopropyl
alcohol.
The products of each of the reactions in primary alcohols
were investigated first (Scheme 2). As disclosed previously,[8]
M1 reacts in the same manner as G1, thus forming the
hydridocarbonyl complex 1. M2 decomposed slowly to yield
low quantities of 2; this slow reaction is most likely
to 1 by methanolysis,[13] so 7 is a plausible potential
intermediate in the primary-alcohol-mediated decomposition
of metathesis catalysts.
To understand how the alkylidene might affect decom-
position, G1 and 5 (the catalyst resting state) were heated
with triethylamine in isopropyl alcohol (Scheme 4). In both
cases, 7 was obtained, thus suggesting that this pathway is
general to bis(tricyclohexylphosphane) complexes. Notably,
methylidene, benzylidene, and (3-phenyl)indenylidene com-
plexes bear different degrees of alkylidene substitution, yet
yield the same product.
Clearly, the nature of the phosphine ligand appears to play
a role in determining the reactivity. Triphenylphosphine and
tricyclohexylphosphine are quite different in terms of both
Scheme 2. Decomposition of indenylidene complexes in alcohol solu-
tion.
Scheme 4. Decomposition of G1 and 4 in isopropyl alcohol.
a consequence of its low initiation rate.[11] M20 features
a more labile phosphane, and decomposed more quickly to
the corresponding hydridocarbonyl species 6. The decompo-
sition of M11 yielded 4.
steric and electronic properties (cone angles of 1458and 1708,
and TEPs of 2068.9 cmÀ1 and 2056.4 cmÀ1, respectively), when
studied on model [Ni(CO)3(PR3)] complexes.[14] However, in
The same set of complexes was exposed to isopropyl
alcohol, to investigate the reactivity differences in this solvent
(Scheme 3). Only traces of hydridocarbonyl complexes were
observed in these reactions, presumably because of either the
need to cleave two carbon–carbon bonds to form a carbonyl
ligand, or the presence of traces of primary alcohols in the
solvent. Despite the previously reported lack of reactivity of
G1 in secondary alcohols,[2b] M1 yielded [RuCl(H)(H2)-
(PCy3)2] (7). The analytically-pure product was isolated in
76% yield after work-up and the NMR data were consistent
with previous reports.[12] The compound 7 is typically
a decomposition product from hydrogenolysis reactions,[12a]
but this is the first example where this complex is obtained
without the direct use of dihydrogen, thus suggesting that
alcoholysis and hydrogenolysis may proceed by similar path-
ways. Fogg and co-workers has shown that 7 can be advanced
M10 and M1 they show similar steric bulk, having average
[15]
percent buried volumes (% Vbur
)
of 26.5% (M10; Ru-P =
2.39 ꢀ) and 27.6% (M1; Ru-P = 2.42 ꢀ).[16] Single crystals of
M11 were grown from CH2Cl2/pentanes and analyzed by
X-ray crystal diffraction (Figure 1). The % Vbur was found to
be 27.4%, which is similar to PCy3. The electronic properties
of iBu-Phoban were measured from IR spectroscopy of
[IrCl(CO)2(iBu-Phoban)][17] (see the Supporting Informa-
tion). The average of the carbonyl stretching frequencies
was 2029 cmÀ1, which compares to 2028 cmÀ1 for PCy3 and
2044 cmÀ1 for PPh3. Insofar as can be measured using these
common metrics, iBu-Phoban possesses similar steric and
electronic properties to PCy3.
We therefore turned to theoretical studies (at the
M06L\TZP\\BP86\SVP level of theory) to explore the impor-
tant issue of how structure influences the decomposition
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formaldehyde (G). The barrier FG° (pictured in Figure 3) is
the highest identified for each complex. Rearrangement of G
furnishes the chloride complex H, which corresponds to 3 in
the case of PPh3.
Interestingly, much can be gained from the inspection of
the free-energy differences between A and H, and from the
FG° barrier. For M10, where 3 is obtained, H is favored by
15 kcalmolÀ1. For M1 and M11, H is unfavorable by 10.8 and
5.5 kcalmolÀ1, respectively. Notably, H is not recovered from
the reaction of M11, but the hydride species is recovered.
Prolonged heating of 3 in alcohol in the presence of a base is
known to yield the hydride complex 8. At this point, the
remarkable difference in stability of M10, M1, and M11 is
clearly related to the height of the FG° barrier and to the
stability of H relative to A. Considering that in H the two
phosphine ligands are placed cis to each other, whereas in the
A they are trans, we wondered if the height of the FG° barrier
and the stability of H and A could be related to increased
steric clashes between the phosphine ligands as they move
from trans to cis, since the FG° barrier is lower and H is more
stable than A for the smaller PPh3 relative to the bulkier PCy3
(Figure 3). To this end, we evaluated the energy difference
between A and H using the smaller PMe3. To our delight, in
this case we calculated the FG° barrier to be only 20.2 kcal
molÀ1, and that H is more stable than A by 26.5 kcalmolÀ1.
This result highlights which factor is key to the observed
decomposition, and provides a way to predict the stability of
new complexes, with respect to this decomposition pathway.
To complete the study, we modeled the reaction of the
chloride complexes H to yield the hydride species M
(Figure 4; normalized to H plus NEt3 and methanol). The
relative energies of H and M for M1, show that M lies
12.7 kcalmolÀ1 above H, which is already 10.8 kcalmolÀ1
above M1 (i.e. 23.5 kcalmolÀ1 in total).
Figure 1. X-ray crystal structure of M11. Thermal ellipsoids are shown
at 50% probability, and hydrogen atoms are excluded for clarity.
pathway.[19] The full pathway for M10 to 3 (PPh3) has been
modeled previously.[5] This pathway was therefore modeled
for M1 (PCy3) and M11 (iBu-Phoban; Figure 2). The poten-
tial-energy surfaces (PESs) for the rearrangement of preca-
talysts M10, M1, and M11, generically indicated as A in the
following calculations, to yield complexes of the form [RuCl-
(h5-(3-phenyl)indenyl)(PR3)2], generically indicated as H in
the following calculations, are presented in Figure 2 (energies
are normalized to A plus NEt3 and methanol). Before starting
the discussion we remark that in the case of the PPh3 complex,
the final product H corresponds to the experimentally
observed product 3.
Endergonic dissociation of the phosphine ligand from A
yields the 16-electron species B, which can undergo reaction
with methanol (C) and subsequent base-promoted loss of HCl
(D, E). Recoordination of (F) is followed by elimination of
Figure 2. Potential-energy surfaces for the rearrangement of ruthenium indenylidene complexes to yield h5-complexes. Energies are free energies
in solvent, in kcalmolÀ1
.
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Experimental Section
Typical decomposition experiment: In the glovebox (under an Ar
atmosphere), the ruthenium complex (typically ca. 100 mg) was
weighed into a screw-cap vial. Dry, oxygen-free alcohol (2 mL) was
added, followed by a stirrer bar, and the vial was closed, sealed with
tape, and removed from the glovebox. At the bench, triethylamine
(ca. 0.5 mL) was added by syringe, and the vial was heated at reflux
temperature overnight. The vial was returned to the glovebox and the
reaction mixture was worked up. Full experimental details and
characterization data can be found in the Supporting Information.
DFT calculations: All calculations were performed with the
Gaussian09 package at the BP86 GGA level using the SDD ECP on
Figure 3. DFT-derived transition state FG for M1 (selected distances in
ꢃ, cyclohexyl moieties are omitted for the sake of clarity).
Figure 4. Potential-energy surfaces for the onward reaction of the chloride complexes H to yield hydride species M. Energies are free energies, in
kcalmolÀ1
.
Ru and the SVP basis set on all main group atoms. The reported
energies have been obtained by single-point calculations at the M06 L
MGGA and BP86 level of theory with the TZVP basis set on main
group atoms and an additional diffuse function on Cl and O. Solvent
effects, MeOH, were included with the PCM model. Full computa-
tional details and xyz coordinates, absolute energies, and three-
dimensional view of all computed species can be found in the
Supporting Information.
The reaction of 3 to form 8 is considerably uphill, even
though the formation of 3 from M10 is energetically very
favorable. The difference in energy between M10 and 8 is
18.3 kcalmolÀ1. For M11, while the formation of H is slightly
uphill, the subsequent reaction to form the hydride species 4 is
slightly favored (by 0.4 kcalmolÀ1). In addition, the barriers
on the chloride to hydride pathway are all much smaller than
the largest barrier on the indenylidene to chloride pathway in
Figure 2 (FG° is 35.5 kcalmolÀ1 higher in energy than F). We
therefore believe that these DFT calculations are in full
agreement with the observed behavior of the indenylidene
species. These DFT calculations may well hold predictive
value for future studies, to assess the decomposition behavior
of new generations of ruthenium indenylidene metathesis
precatalysts.
Received: March 27, 2014
Revised: April 25, 2014
Published online: && &&, &&&&
Keywords: density functional calculations · hydride ligands ·
.
metathesis · reaction mechanisms · ruthenium
In conclusion, we have probed the decomposition of first-
and second-generation metathesis catalysts in the presence of
alcohols, thus revealing the formation of [RuCl(H)(H2)-
(PCy3)2] (7) in the case of PCy3-containing complexes. While
simple measures of the steric and electronic properties of the
phosphine ligands were unable to rationalize the different
behavior of the complexes examined, DFT calculations to
probe the effect of structure on this rearrangement reaction
revealed that the primary decomposition mechanism is in fact
predictable using theoretical methods. We believe this
information will be of significant utility and interest to
researchers preparing new generations of metathesis preca-
talysts, and to chemists applying alkene metathesis trans-
formations in their laboratories.
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Communications
Reaction Mechanisms
S. Manzini, A. Poater, D. J. Nelson,
L. Cavallo, A. M. Z. Slawin,
S. P. Nolan*
&&&& — &&&&
Insights into the Decomposition of Olefin
Metathesis Precatalysts
Ligands make a difference: The decom-
position of olefin metathesis precatalysts
in isopropyl alcohol leads to a series of
differing complexes. Bis(tricyclohexyl-
phosphane)-bearing complexes decom-
pose to yield [RuCl(H)(H2)(PCy3)2], while
bis(triphenylphosphane) and bis(iBu-
Phoban)-bearing indenylidene complexes
yield h5-(3-phenyl)indenyl complexes. The
(3-phenyl)indenylidene to h5-indenyl
rearrangement for three complexes has
been probed using DFT calculations.
6
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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