Communication Cite This: Organometallics XXXX, XXX, XXX-XXX
pubs.acs.org/Organometallics
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 ringclosing metathesis (RCM) and ring-opening metathesis polymerization (ROMP) reactions.
T
complexes supported by a bent allene ligand, and these examples are metathesis inactive.9 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 (H 2 IMes)(py)2(Cl)2RuCHPh11 (Mes = 2,4,6-Me3Ph) with carbodicarbene 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 H2IPrsubstituted derivative (H2IPr)(CDC)(Cl)2RuCHPh (3, IPr = 2,6-iPr2Ph) was prepared by an analogous method.12
he reactivity of olefin metathesis catalysts is critically influenced by the properties of their supporting ligands.1 In particular, ligand selection for metathesis catalysts of the type (L)2(X)2RuCHR 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 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 [RuCHR] © XXXX American Chemical Society
Received: August 10, 2017
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DOI: 10.1021/acs.organomet.7b00615 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
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)2RuCHPh 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).
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)2RuCHPh, (PCy3)(py)2(Cl)2RuCHPh, and (PCy3)(Cl)2RuCH-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
Scheme 3. Living ROMP of DEE by Catalyst 3a
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). 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 monitored by 1H NMR spectroscopy in benzene-d6 at 40 °C. RCM
a
Molecular weight and dispersity determined by SEC light scattering detector; conversion measured by 1H NMR spectroscopy.
The corresponding poly(norbornene) product displayed a wellcontrolled 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)2RuCHPh are used to prepare monodisperse poly(norbornenes), as the phosphine-substituted analogue (H2IMes)(PCy3)(Cl)2RuCHPh generates polymers 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).
Scheme 2. RCM of Diethyl Diallylmalonate with 2 and 3, Monitored by 1H NMR Spectroscopy
B
DOI: 10.1021/acs.organomet.7b00615 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics 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.
Scheme 4. Reaction of Complexes 2 and 3 with 2Isopropoxy-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⧧298 K = 23.5 ± 0.1 kcal/mol) is, within error, identical with the value measured for the analogous PCy3 complex (H2IMes)(PCy3)(Cl)2RuCHPh (ΔG⧧298 K = 23.0 ± 0.4 kcal/mol),3a in spite of the fact that CDCs are often considered significantly stronger donors than phosphine or NHC ligands.7 Additional experiments determined the relative affinity of the four-coordinate (NHC)(Cl)2RuCHPh intermediate to reassociate 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
rate =
k1k 2[Ru][EVE] k −1[CDC] + k 2[EVE]
rate = k1[Ru]
free and protonated CDC (1 and 1-H+, respectively).18 To clarify whether CDC dissociation occurs prior to decomposition, complexes 2 and 3 were treated with 5 equiv of PCy3. After 12 h at 25 °C, ∼50% conversion to the analogous PCy3supported complexes of the type (NHC)(PCy3)(Cl)2Ru CHPh was observed by 1H 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).
1 kobs
(1)
(when k 2[EVE] ≫ k −1[CDC])
(2)
k −1[CDC] 1 + k1k 2[EVE] k1
=
(3)
1 H 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
Scheme 5. Proposed Dissociative Mechanism for Initiation of Catalysts 2 and 3 in the Presence of Ethyl Vinyl Ether (EVE)
complex
T (°C)
k−1/k2
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 Table 1. 1H NMR Initiation Kinetics for Complexes 2 and 3 complex
k1(40 °C) (10−3 s−1)
ΔH⧧ (kcal/mol)
ΔS⧧ (eu)
ΔG⧧298 K (kcal/mol)
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 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
(4) (a) Hong, S. H.; Wenzel, A. G.; Salguero, T. T.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2007, 129, 7961−7968. (b) Samojłowicz, C.; Bieniek, M.; Grela, K. Chem. Rev. 2009, 109, 3708−3742. (c) Marx, V. M.; Sullivan, A. H.; Melaimi, M.; Virgil, S. C.; Keitz, B. K.; Weinberger, D. S.; Bertrand, G.; Grubbs, R. H. Angew. Chem., Int. Ed. 2015, 54, 1919−1923. (5) (a) Schuster, M.; Blechert, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 2036−2056. (b) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4490−4527. (c) Hoveyda, A. H. J. Org. Chem. 2014, 79, 4763−4792. (6) (a) Sutthasupa, S.; Shiotsuki, M.; Sanda, F. Polym. J. 2010, 42, 905−915. (b) Martinez, H.; Ren, N.; Matta, M. E.; Hillmyer, M. A. Polym. Chem. 2014, 5, 3507−3532. (c) Sinclair, F.; Alkattan, M.; Prunet, J.; Shaver, M. P. Polym. Chem. 2017, 8, 3385−3398. (7) (a) Tonner, R.; Frenking, G. Angew. Chem., Int. Ed. 2007, 46, 8695−8698. (b) Dyker, C. A.; Lavallo, V.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 3206−3209. (c) Fürstner, A.; Alcarazo, M.; Goddard, R.; Lehmann, C. W. Angew. Chem., Int. Ed. 2008, 47, 3210−3214. (d) Alcarazo, M.; Lehmann, C. W.; Anoop, A.; Thiel, W.; Fürstner, A. Nat. Chem. 2009, 1, 295−301. (e) Klein, S.; Tonner, R.; Frenking, G. Chem. - Eur. J. 2010, 16, 10160−10170. (f) Soriano, E.; Fernández, I. Chem. Soc. Rev. 2014, 43, 3041−3105. (8) (a) Goldfogel, M. J.; Roberts, C. C.; Meek, S. J. J. Am. Chem. Soc. 2014, 136, 6227−6230. (b) Pranckevicius, C.; Fan, L.; Stephan, D. W. J. Am. Chem. Soc. 2015, 137, 5582−5589. (c) Roberts, C. C.; Matías, D. M.; Goldfogel, M. J.; Meek, S. J. J. Am. Chem. Soc. 2015, 137, 6488−6491. (d) Hsu, Y.-C.; Shen, J.-S.; Lin, B.-C.; Chen, W.-C.; Chan, Y.-T.; Ching, W.-M.; Yap, G. P. A.; Hsu, C.-P.; Ong, T.-G. Angew. Chem., Int. Ed. 2015, 54, 2420−2424. (e) Goldfogel, M. J.; Meek, S. J. Chem. Sci. 2016, 7, 4079−4084. (f) Goldfogel, M. J.; Roberts, C. C.; Manan, R. S.; Meek, S. J. Org. Lett. 2017, 19, 90−93. (9) DeHope, A.; Donnadieu, B.; Bertrand, G. J. Organomet. Chem. 2011, 696, 2899−2903. (10) (a) Trnka, T. M.; Morgan, J. P.; Sanford, M. S.; Wilhelm, T. E.; Scholl, M.; Choi, T.-L.; Ding, S.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 2546−2558. (b) Zhang, W.; Bai, C.; Lu, X.; He, R. J. Organomet. Chem. 2007, 692, 3563−3567. (c) Ledoux, N.; Allaert, B.; Linden, A.; Van Der Voort, P.; Verpoort, F. Organometallics 2007, 26, 1052−1056. (d) Vorfalt, T.; Leuthäβer, S.; Plenio, H. Angew. Chem., Int. Ed. 2009, 48, 5191−5194. (e) Sashuk, V.; Peeck, L. H.; Plenio, H. Chem. - Eur. J. 2010, 16, 3983−3993. (11) Love, J. A.; Morgan, J. P.; Trnka, T. M.; Grubbs, R. H. Angew. Chem., Int. Ed. 2002, 41, 4035−4037. (12) Leitao, E. M.; Piers, W. E.; Parvez, M. Can. J. Chem. 2013, 91, 935−942. (13) Ritter, T.; Hejl, A.; Wenzel, A. G.; Funk, T. W.; Grubbs, R. H. Organometallics 2006, 25, 5740−5745. (14) Courchay, F. C.; Sworen, J. C.; Wagener, K. B. Macromolecules 2003, 36, 8231−8239. (15) Lin, T.-P.; Chang, A. B.; Chen, H.-Y.; Liberman-Martin, A. L.; Bates, C. M.; Voegtle, M. J.; Bauer, C. A.; Grubbs, R. H. J. Am. Chem. Soc. 2017, 139, 3896−3903. (16) Bielawski, C. W.; Grubbs, R. H. Prog. Polym. Sci. 2007, 32, 1−29. (17) Choi, T.-L.; Grubbs, R. H. Angew. Chem., Int. Ed. 2003, 42, 1743−1746. (18) (a) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 8168−8179. (b) Blum, A. P.; Ritter, T.; Grubbs, R. H. Organometallics 2007, 26, 2122−2124. (19) Atwood, J. D. Inorganic and Organometallic Reaction Mechanisms; VCH: New York, 1997; p 13. (20) Jordan, R. B. Reaction Mechanisms of Inorganic and Organometallic Systems; Oxford University Press: New York, 1998; pp 35−45. (21) A computational study predicted that allene ligands could be leaving groups from ruthenium olefin metathesis catalysts: Kuriakose, N.; Vanka, K. J. Comput. Chem. 2015, 36, 795−804.
complexes catalyze RCM and ROMP reactions. The H2IPr− CDC complex 3 demonstrates living ROMP characteristics, allowing for the synthesis of a poly(norbornene) product with a controlled molecular weight. Mechanistic studies indicate that the CDC ligand of the NHC−CDC ruthenium complexes is labile, with faster CDC dissociation observed for the bulkier H2IPr-supported complex 3. The lability of the CDC ligand was unexpected by us, given precedent indicating that CDC ligands are remarkably strong donors.7 To our knowledge, there are no prior experimental reports discussing the lability of CDC ligands.21 These findings reiterate that thermodynamic metrics, such as carbonyl stretching frequencies for (L)Rh(CO)2Cl complexes, do not necessarily translate across different coordination environments or to kinetic metrics such as ligand lability.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00615. Experimental information, characterization data, and Xray crystallographic details (PDF) Accession Codes
CCDC 1568028−1568029 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for R.H.G.:
[email protected]. ORCID
Allegra L. Liberman-Martin: 0000-0002-8447-905X Robert H. Grubbs: 0000-0002-0057-7817 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS 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 determination. Drs. William J. Wolf and Tzu-Pin Lin are thanked for helpful discussions and for synthesizing (H 2 IPr) (py)2(Cl)2RuCHPh and DEE, respectively. Materia, Inc. is thanked for generous donations of metathesis catalysts.
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REFERENCES
(1) Nelson, D. J.; Manzini, S.; Urbina-Blanco, C. A.; Nolan, S. P. Chem. Commun. 2014, 50, 10355−10375. (2) Diver, S. T.; Griffiths, J. R. Factors Affecting Initiation Rates. In Handbook of Metathesis; Wenzel, A. G., Grubbs, R. H., Eds.; Wiley: New York, 2014; Vol. 1, Catalyst Development and Mechanism. (3) (a) Sanford, M. S.; Ulman, M.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 749−750. (b) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 6543−6554. D
DOI: 10.1021/acs.organomet.7b00615 Organometallics XXXX, XXX, XXX−XXX
