Ruthenium Olefin Metathesis Catalysts Featuring a Labile Carbodicarbene Ligand

Article abstract of DOI:10.1021/acs.organomet.7b00615

Ruthenium benzylidene complexes containing a carbodicarbene (CDC) ligand are reported. Mechanistic studies indicate that the CDC ligand can dissociate under relatively mild conditions to afford active olefin metathesis catalysts. These catalysts were found to be effective at ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP) reactions.

Full text of DOI:10.1021/acs.organomet.7b00615

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX-XXX
Ruthenium Olefin Metathesis Catalysts Featuring a Labile
Carbodicarbene Ligand
Allegra L. Liberman-Martin and Robert H. Grubbs*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125,
United States
S
* Supporting Information
ABSTRACT: Ruthenium benzylidene complexes containing a
carbodicarbene (CDC) ligand are reported. Mechanistic
studies indicate that the CDC ligand can dissociate under
relatively mild conditions to afford active olefin metathesis
catalysts. These catalysts were found to be effective at ring-
closing metathesis (RCM) and ring-opening metathesis
polymerization (ROMP) reactions.
he reactivity of olefin metathesis catalysts is critically
influenced by the properties of their supporting ligands.1
complexes supported by a bent allene ligand, and these
examples are metathesis inactive.9
T
In particular, ligand selection for metathesis catalysts of the type
(L)2(X)2RuCHR is known to affect the initiation rate,2
affinity for olefin binding,3 and catalyst longevity.4 The ability
to fine-tune catalyst activity and selectivity through ligand
design has enabled olefin metathesis to become ubiquitous in
the fields of organic synthesis5 and materials science.6
Herein, we report the synthesis of mixed NHC−CDC
ruthenium benzylidene complexes. The CDC ligand in these
complexes is unexpectedly labile relative to the NHC ligand
and readily dissociates to generate active metathesis catalysts. In
contrast, bis(NHC) ruthenium catalysts are typically slow to
initiate via NHC dissociation, and these catalysts often display
only modest activity in olefin metathesis reactions at relatively
high temperatures.10 The performance of CDC-supported
precatalysts is evaluated in ring-closing metathesis (RCM) and
ring-opening metathesis polymerization (ROMP) reactions.
Treatment of the bis(pyridine) complex (H2IMes)-
(py)2(Cl)2RuCHPh11 (Mes = 2,4,6-Me3Ph) with carbodi-
carbene 1 in benzene led to an immediate color change from
green to orange, and conversion to the H2IMes−CDC complex
2 was complete within 3 h at 25 °C (Scheme 1). The H2IPr-
substituted derivative (H2IPr)(CDC)(Cl)2RuCHPh (3, IPr
= 2,6-iPr2Ph) was prepared by an analogous method.12
We became interested in pursuing the use of carbodicarbene7
(CDC) ligands to support ruthenium metathesis catalysts due
to their unusual donor properties. These compounds can be
described on a continuum among (i) bent allene, (ii) double
ylide, and (iii) divalent C(0) resonance forms (Figure 1). The
Scheme 1. Synthesis of Carbodicarbene Complexes 2 and 3
Figure 1. Resonance structures of carbodicarbene 1.
very strong donor ability of CDC ligands has been shown by
the exceptionally low average carbonyl stretching frequency
observed for a (CDC)Rh(CO)2Cl complex (CDC = 1, νCO
2014 cm−1), in comparison to N-heterocyclic carbene (NHC)
analogues (νCO 2036−2058 cm−1).7b Despite the unique ability
of CDC ligands to potentially serve as neutral four-electron
donors, relatively few reports have described the catalytic
reactivity of CDC-ligated metal complexes.8 To our knowledge,
there is only one previous report describing two [RuCHR]
Received: August 10, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00615
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Complexes 2 and 3 were isolated by precipitation from THF/
Et2O solutions at −30 °C. Attempts to incorporate the CDC
ligand 1 into PCy3-supported ruthenium complexes, including
(PCy3)2(Cl)2RuCHPh, (PCy3)(py)2(Cl)2RuCHPh, and
(PCy3)(Cl)2RuCH-o-OiPr-Ph, were unsuccessful, leading to
the formation of the protonated CDC 1-H+ (see the
Supporting Information for details).
Crystals suitable for X-ray diffraction were grown by vapor
diffusion of pentane into a saturated benzene solution of 2 at 25
°C. The X-ray structure of 2 indicates a slightly distorted square
pyramidal ruthenium center, with the benzylidene ligand
occupying the apical position (Figure 2). The NHC and
of diethyl diallylmalonate with 1 mol % of 2 or 3 proceeded to
95% conversion within 35 or 10 min, respectively. For
comparison, the PCy3-supported analogues (NHC)(PCy3)-
(Cl)2RuCHPh are faster RCM catalysts and reach 95%
conversion within 25 min (NHC = H2IMes) or 1 min (NHC =
H2IPr) at 30 °C.13 Faster RCM by H2IPr-supported complexes
relative to H2IMes analogues was attributed to both faster
initiation and propagation rates for the bulkier H2IPr
catalyst.13,14 Despite the high conversions for diethyl
diallylmalonate ring closure obtained using catalysts 2 and 3,
the activity of both catalysts decreases during the course of the
reaction, as evidenced by curvature in logarithmic plots of the
kinetics data (see the Supporting Information), which suggests
catalyst decomposition.
The ring-opening metathesis polymerization (ROMP)
activity of complexes 2 and 3 was also investigated. ROMP
of racemic endo,exo-norbornenyl diethyl diester (DEE; 0.05 M
in CH2Cl2, 125 equiv)15 mediated by catalyst 3 proceeded to
complete (>98%) conversion over 2 h at 25 °C (Scheme 3).
a
Scheme 3. Living ROMP of DEE by Catalyst 3
Figure 2. X-ray crystal structure of complex 2. Displacement ellipsoids
are drawn at 50% probability, and hydrogen atoms have been omitted
for clarity. Color scheme: Ru, blue; Cl, green; C, black; N, light blue.
CDC ligands are positioned in a trans arrangement (H2IMes−
Ru−CDC = 168.14(8)°). In contrast, a related ruthenium
complex of a cyclic bent allene ligand features a cis arrangement
of the bent allene and phosphine ligands at ruthenium, a
geometry that is associated with poor metathesis activity.9 The
Ru−CDC bond distance of 2 (2.2069(18) Å) is significantly
longer than the Ru−H2IMes bond length (2.0798(18) Å). The
X-ray structure of 3 shows similar bond lengths and angles (see
the Supporting Information).
a
Molecular weight and dispersity determined by SEC light scattering
1
detector; conversion measured by H NMR spectroscopy.
The catalytic activities of complexes 2 and 3 were evaluated
in the benchmark ring-closing metathesis (RCM) of diethyl
diallylmalonate (Scheme 2).13 Reaction progress was moni-
The corresponding poly(norbornene) product displayed a well-
controlled molecular weight (observed Mn = 32.0 kDa;
theoretical Mn = 29.9 kDa) and low dispersity (Đ = Mw/Mn
= 1.03). ROMP of DEE by 3 exhibits living characteristics,16 as
demonstrated by a linear increase in the product molecular
weight with DEE conversion, and nearly constant dispersity
over the course of the polymerization. Controlled chain
extension occurs upon addition of a second 100 equiv of
DEE (observed Mn = 57.6 kDa; theoretical Mn = 53.8 kDa; Đ =
1.07), which further suggests that there is not significant
catalyst decomposition under these ROMP conditions.
Typically, fast-initiating bis(pyridine) complexes of the type
(H2IMes)(py)2(Cl)2RuCHPh are used to prepare mono-
disperse poly(norbornenes), as the phosphine-substituted
analogue (H2IMes)(PCy3)(Cl)2RuCHPh generates poly-
mers with uncontrolled molecular weights and broad
dispersities.17 Using catalyst 2, ROMP of DEE proceeded to
completion within 20 min; however, the molecular weight was
not as well controlled (observed Mn = 37.3 kDa; theoretical Mn
= 29.9 kDa) and the dispersity was higher (Đ = 1.14).
1
tored by H NMR spectroscopy in benzene-d6 at 40 °C. RCM
Scheme 2. RCM of Diethyl Diallylmalonate with 2 and 3,
1
Monitored by H NMR Spectroscopy
B
DOI: 10.1021/acs.organomet.7b00615
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Organometallics
Communication
On the basis of the catalytic activity of 2 and 3, it is likely that
L-type ligand dissociation occurs from these precatalysts to
generate 14-electron catalytic intermediates.3 To assess which
L-type ligand dissociates (NHC or CDC), complexes 2 and 3
were treated with 3 equiv of 2-isopropoxy-β-methylstyrene and
heated to 40 °C. Exclusive formation of the previously reported
NHC-supported o-isopropxybenzylidene chelate complexes 4
and 5 was observed (Scheme 4), along with formation of both
CHPh affords the rate expression in eq 1. Under saturation
conditions (k2[EVE] ≫ k−1[CDC]), this rate expression
simplifies to the form in eq 2, and CDC dissociation becomes
rate determining.
k1k2[Ru][EVE]
rate =
k−1[CDC] + k2[EVE]
(1)
rate = k1[Ru] (when k2[EVE] ≫ k−1[CDC])
(2)
Scheme 4. Reaction of Complexes 2 and 3 with 2-
Isopropoxy-β-methylstyrene, Indicating CDC Ligand
Dissociation
Rate constants for the disappearance of complexes 2 and 3
were measured under saturation conditions at 40 °C in
benzene-d6 and are presented in Table 1. The CDC ligand
dissociation rate (kobs = k1) is over 1 order of magnitude faster
for H2IPr complex 3 in comparison to the H2IMes analogue 2,
likely due to steric effects.13,14 Activation parameters for CDC
dissociation from 2 and 3 were determined from the
temperature dependence of k1 (Table 1). The positive sign of
the activation entropies (ΔS⧧) is consistent with a dissociative
ligand exchange pathway.19 Notably, the free energy of
activation for complex 2 (ΔG⧧
= 23.5 0.1 kcal/mol) is,
298 K
within error, identical with the value measured for the
analogous PCy3 complex (H2IMes)(PCy3)(Cl)2RuCHPh
(ΔG⧧
= 23.0 0.4 kcal/mol),3a in spite of the fact that
CDCs are often considered significantly stronger donors than
298 K
phosphine or NHC ligands.7
free and protonated CDC (1 and 1-H+, respectively).18 To
clarify whether CDC dissociation occurs prior to decom-
position, complexes 2 and 3 were treated with 5 equiv of PCy3.
After 12 h at 25 °C, ∼50% conversion to the analogous PCy3-
supported complexes of the type (NHC)(PCy3)(Cl)2Ru
Additional experiments determined the relative affinity of the
four-coordinate (NHC)(Cl)2RuCHPh intermediate to re-
associate CDC or bind olefin. Under pseudo-first-order
conditions in EVE, and assuming that EVE coordination is
irreversible, eq 1 can be rewritten to a form that describes 1/
kobs as a function of [CDC]/[EVE] (eq 3).3,20
1
CHPh was observed by H and 31P NMR spectroscopy, along
with exclusive formation of free CDC 1. Taken together, these
experiments indicate that dissociation of the CDC ligand from
2 and 3 can occur and is more favorable than NHC
dissociation.
To study the initiation mechanisms for catalysts 2 and 3,
reactions with ethyl vinyl ether (EVE) were performed under
pseudo-first-order conditions and were monitored by 1H NMR
spectroscopy.3 The disappearance of complex 2 or 3 proceeded
with clean first-order kinetics and was independent of ethyl
vinyl ether concentration above 0.3 M. This behavior is
consistent with a two-step mechanism involving CDC
dissociation followed by olefin coordination (Scheme 5).
k−1[CDC]
k1k2[EVE]
1
kobs
1
k1
=
+
(3)
1H NMR kinetics experiments were performed to measure
rate constants for reactions of ruthenium complexes with ethyl
vinyl ether in the presence of added CDC.3 From these studies,
the k−1/k2 ratios for complexes 2 and 3 were determined, which
reflect both CDC rebinding (k−1) and olefin binding (k2) steps
(Table 2). Both of these rate constants (k−1 and k2) may be
Table 2. Values of the k−1/k2 Ratio for Complexes 2 and 3
complex
T (°C)
k−1/k2
Scheme 5. Proposed Dissociative Mechanism for Initiation
of Catalysts 2 and 3 in the Presence of Ethyl Vinyl Ether
(EVE)
2
3
60
40
0.93
4.9
influenced by NHC structure and jointly contribute to the
overall k−1/k2 ratios for complexes 2 and 3. The 5-fold greater
k−1/k2 value observed for catalyst 3 relative to 2 is consistent
with results from catalytic RCM and ROMP studies showing
slower substrate conversion utilizing catalyst 3, as preferential
CDC rebinding will inhibit catalytic turnover.
In summary, we have synthesized two ruthenium benzylidene
complexes containing N-heterocyclic carbene (NHC) and
carbodicarbene (CDC) ligands in a trans arrangement. These
Applying the steady-state approximation to the concentration
of four-coordinate ruthenium intermediate (NHC)(Cl)2Ru
1
Table 1. H NMR Initiation Kinetics for Complexes 2 and 3
complex
k1(40 °C) (10−3 s−1
)
ΔH⧧ (kcal/mol)
ΔS⧧ (eu)
ΔG⧧
(kcal/mol)
298 K
2
3
0.40 0.01
9.5 0.1
29.8 1.3
25.2 1.0
20.9 3.9
12.4 3.3
23.5 0.1
21.5 0.1
C
DOI: 10.1021/acs.organomet.7b00615
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Organometallics
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Experimental information, characterization data, and X-
ray crystallographic details (PDF)
Accession Codes
CCDC 1568028−1568029 contain the supplementary crystal-
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charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
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AUTHOR INFORMATION
Corresponding Author
*E-mail for R.H.G.: rhg@caltech.edu.
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Plenio, H. Chem. - Eur. J. 2010, 16, 3983−3993.
Allegra L. Liberman-Martin: 0000-0002-8447-905X
Robert H. Grubbs: 0000-0002-0057-7817
Notes
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Chem., Int. Ed. 2002, 41, 4035−4037.
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935−942.
The authors declare no competing financial interest.
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Organometallics 2006, 25, 5740−5745.
ACKNOWLEDGMENTS
■
(14) Courchay, F. C.; Sworen, J. C.; Wagener, K. B. Macromolecules
2003, 36, 8231−8239.
This work was supported by the National Science Foundation
(CHE-1212767). A.L.L.-M. is grateful to the Resnick
Sustainability Institute at Caltech for fellowship support. Dr.
Michael K. Takase is acknowledged for X-ray crystallographic
analysis, and Dr. Mona Shahgholi and Naseem Torian are
acknowledged for mass spectrometry assistance. Dr. David
VanderVelde is thanked for aid in NMR structural determi-
nation. Drs. William J. Wolf and Tzu-Pin Lin are thanked for
helpful discussions and for synthesizing (H2IPr)
(py)2(Cl)2RuCHPh and DEE, respectively. Materia, Inc. is
thanked for generous donations of metathesis catalysts.
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Bates, C. M.; Voegtle, M. J.; Bauer, C. A.; Grubbs, R. H. J. Am. Chem.
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DOI: 10.1021/acs.organomet.7b00615
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