DOI: 10.1002/chem.201404131
Communication
& Carbene Complexes
Methandiide as a Non-Innocent Ligand in Carbene Complexes: From the Electronic Structure to Bond Activation Reactions and Cooperative Catalysis Julia Becker, Tanja Modl, and Viktoria H. Gessner*[a] Abstract: The synthesis of a ruthenium carbene complex based on a sulfonyl-substituted methandiide and its application in bond activation reactions and cooperative catalysis is reported. In the complex, the metal–carbon interaction can be tuned between a Ru C single bond with additional electrostatic interactions and a Ru=C double bond, thus allowing the control of the stability and reactivity of the complex. Hence, activation of polar and non-polar bonds (O H, H H) as well as dehydrogenation reactions become possible. In these reactions the carbene acts as a non-innocent ligand supporting the bond activation as nucleophilic center in the 1,2-addition across the metal– carbon double bond. This metal–ligand cooperativity can be applied in the catalytic transfer hydrogenation for the reduction of ketones. This concept opens new ways for the application of carbene complexes in catalysis.
Figure 1. Complexes with non-innocent ligands used in bond activation reactions.
Non-innocent ligands have received great attention over the past few years due to their capability to support transformations, which are not possible at a single metal center.[1] Prominent examples are redox-active ligands that allow multielectron transformations with metals such as Fe, Co, or Cu, which usually tend to undergo only 1 e oxidation state changes (e.g. CoI, CoII, CoIII).[2] This has become particularly interesting in the context of substituting precious metals in catalytic transformations, where typically 2 e oxidation state changes are required.[3] Indeed, even redox-neutral ligands have been applied as non-innocent ligands. Particularly interesting examples are ligands supporting proton transfer reactions by moderating the loss or gain of protons at the metal center. In these processes, the ligand itself exhibits two stable tautomeric forms, usually an anionic and a neutral one.[4] A remarkable example is the ruthenium pincer complex A reported by the Milstein group (see Figure 1).[5] This complex was found to be active in
[a] Dipl.-Chem. J. Becker, T. Modl, Dr. V. H. Gessner Institut fr Anorganische Chemie, Julius-Maximilians-Universitt Wrzburg Am Hubland, 97074 Wrzburg (Germany) E-mail: [email protected] Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/forschungsgruppen/dr_v_gessner/ Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404131. Chem. Eur. J. 2014, 20, 11295 – 11299
multiple bond activation reactions such as of H H, N H, or O H bonds, including the splitting of water. Other examples for complexes with non-innocent ligands used in bond activation reactions are the iron complex B reported by Knçlker et al.[6] and the nickel carbene complex C recently reported by Piers and co-workers.[7] Our group has become particularly interested in the methandiide ligand family, which was shown to give access to a novel class of carbene complexes.[8, 9] The metal–carbon interaction in these complexes was found to cover a range of bonding situations, starting from a kind of masked methandiide with mainly electrostatic M2 + ···C2 interactions, to alkyl complexes with a carbanionic ligand, M + C , and complexes with a real M=C double bond.[10] This electronic flexibility suggests a possible application of these complexes in bond activation reactions by means of metal–ligand cooperation, in which the ligand exhibits a dianionic and an anionic tautomeric form. Based thereon, we focused on the following questions: i) Can the electronic situation of the M=C bond be tuned to allow the activation of polar as well as non-polar bonds, such as that of dihydrogen? ii) Can the bond activation reactions be conducted in a reversible manner, enabling the re-formation of the carbene complex from the activation product under liberation of the activated species? iii) Can the non-innocent behavior of the carbene ligand be used in cooperative catalysis? Herein we show that these applications of carbene complexes are possible. Based on the sulfonyl-substituted methandiide ligand 1, tuning of the metal–carbon interaction enabled small molecule activation and catalytic transfer hydrogenation by means of metal–ligand cooperation.[11] Previously, we reported on the reaction of methandiide 1 with [(PPh3)3RuCl2]. The obtained carbene complex 2-int turned out to be unstable even at low reaction temperature resulting in a cyclometallation to 2 via C H bond activation of the sulfonyl bound phenyl group.[12, 13] The origin of the instability was found to be the facile decoordination of the sulfonyl
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Communication moiety as well as the highly nucleophilic carbon center and the weak p-interaction in the carbene complex. We assumed that to increase the charge transfer from the methandiide to the metal and to strengthen the metal–carbon interaction a less electron-rich metal center would be beneficial. Thus, we chose [{(p-cymene)RuCl2}2] with a stronger p-accepting ligand (Scheme 1).[14] Salt metathesis by treatment of methandiide
Scheme 1. Preparation of Ru complexes 2 and 3 from methandiide 1.
1 with [{(p-cymene)RuCl2}2] instantly resulted in the reaction mixture turning purple and the formation of a single new species characterized by a signal at dP = 66.6 ppm (1: dP = 26.5 ppm) in the 31P{1H} NMR spectrum (see the Supporting Information for Experimental Details). No protonation of the central carbon atom was observed. Work-up delivered carbene complex 3 as purple solid in 86 % yield. Contrary to the phosphine complex 2-int the cymene complex turned out to be stable at room temperature and less reactive towards protonabstraction reactions. The carbene carbon atom resonates as a doublet at dC = 140.0 ppm (1JCP = 30.2 Hz), which is upfieldshifted compared to that in other ruthenium carbene complexes (220–320 ppm), but considerably down-field shifted compared to typical alkyl complexes and many carbene complexes derived from dilithio methandiides.[10] The nature of the carbene complex was unambiguously confirmed by single-crystal X-ray diffraction (XRD) analysis (Figure 2). In the crystal, the methandiide acts as a bi-dentate ligand via a C,S-coordination mode. The Ru C1 distance is 1.965(2) , which is shorter than the Ru C bond found in alkyl complex 2 (Ru C 2.191(6) ). The central carbon atom C1 features a planar coordination environment (sum of angles: 358.9(1)8) as expected for a carbene complex. Interestingly, both the C1 P and C1 S2 distances in 3 are considerably longer than the bond lengths found in the methandiide 1 and other carbene complexes of this ligand with little p-contribution of the M C bond.[12] This can be attributed to reduced electrostatic interactions within the P-C-S framework and thus to an efficient charge transfer from the methandiide carbon to the metal center. The different stabilities and bonding situation of 2-Int and 3 were investigated by DFT studies using the B3LYP functional in combination with the 6-311 + + g(d,p) and LANL2TZ(f) basis sets (for computational details see the Supporting Information). The calculated natural atomic charges, the Wiberg bond Chem. Eur. J. 2014, 20, 11295 – 11299
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Figure 2. Molecular structure of carbene complex 3. Displacement ellipsoids drawn at the 50 % probability level. Selected bond lengths [] and angles [8]: Ru–C1 1.965(2), Ru–S1 2.4441(5), S1–P 2.0312(7), S2–O2 1.454(1), S2–O1 1.460(1), S2–C1 1.717(2), S2–C14 1.784(2), P–C1 1.739(2), P–C8 1.815(2), P–C2 1.815(2); C1-Ru-S1 79.79(6), S2-C1-P 122.8(1), S2-C1-Ru 133.9(1), P-C1-Ru 102.27(9).
indices (WBI) and natural bond orbital (NBO) analyses provide a clear picture of the differences between both compounds (Figure 3). Thus, the phosphine complex 2-int is best described by an ylidic structure with a lone pair remaining at the central carbon atom. This is reflected by a high negative charge of qC = 1.28 at the carbon atom, the low WBI of the Ru C bond of only 0.63 and the missing p-interaction in the NBO analysis. In contrast, the M C bond in 3 features both, a s- and a p-interaction, which are only slightly polarized towards the carbon end. The carbenic carbon atom still carries a negative charge of qC = 0.98. The WBI of 1.06 is smaller than that of a real double bond [for example, WBI(ethene) = 2.04] or of an undisturbed aromatic system [for example, WBI(benzene) = 1.44], however comparable with other Schrock-type complexes.[10, 15] Overall, the different co-ligands in 2-int and 3 result in a transition from a bipolar ylidic structure to a doubly bonded carbene complex. Yet, the charge distribution still accounts for a considerable nucleophilic character of 3, which should be beneficial for bond activation reactions across the Ru=C bond. As shown in Figure 4 this expectation is confirmed by a series of O H activation reactions supported by metal–ligand coop-
Figure 3. Calculated charges, WBI of the Ru C bond, and NBO analysis of the complexes 2-int and 3.
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Figure 4. E H bond activation reaction of carbene complex 3 via metal–ligand cooperation. Displacement ellipsoids drawn at the 50 % probability level. Selected bond lengths [] and angles [8]: 4 a: Ru–C1 2.201(3), Ru–O3 2.098(3), S2–C1 1.739(4), P–C1 1.788(4), S1–P 1.9997(13), S2–O1 1.444(3), S2–O2 1.445(3); S2-C1-P 118.5(2), P-C1-Ru 94.45(15), O3-Ru-C1 76.65(12). 5: Ru–C1 2.185(4), C1–S2 1.768(5), C1–P 1.787(4), S2–O1 1.445(3), S2–O2 1.449(3), S1–P 1.9971(16); S2-C1-P 117.1(3), P-C1-Ru 93.33(19).
eration. Treatment of a solution of 3 in toluene with one equivalent of an aromatic alcohol ROH (R = Ph, p-C6H4OMe, pC6H4CF3) instantly resulted in a color change from purple to red and the formation of the 1,2-addition products 4 via O H bond cleavage (see the Supporting Information). For R = Ph, this is evidenced by the appearance of a resonance at dH = 4.21 ppm in the 1H NMR spectrum for the proton at the PCS bridge and a new 31P{1H} NMR signal at dP = 74.2 ppm (in C6D6). No simple coordination complex of the alcohol to the metal is observed, which often limits a direct functionalization of E H bonds by transition-metal complexes. The O H activation was accomplished by using electron-rich and electron-poor arenes (4 b and 4 c). Depending on the nature of the alcohol, an equilibrium between the 1,2-addition to 4 and the re-formation of the carbene complex 3 was observed at room temperature. As such, quantitative formation of the addition product was only found for the electron-poor p-CF3 functionalized alcohol delivering 4 c in 89 % isolated yield. On the contrary, phenol and pmethoxyphenol exhibit an equilibrium between 3 and the alcoholato complex 4. This reversibility of the O H activation and its temperature dependency is confirmed by variable-temperature NMR studies of 4 b (see the Supporting Information for spectra). Almost complete transformation to the addition product 4 b is observed at 80 8C, whereas an approximate 10:3 mixture between 4 b and 3 is found at room temperature and a 10:5.5 mixture at 35 8C. The O H activation of the less electron-rich phenol shows an analogous equilibrium with 95 % conversion to the O H activation product at room temperature. Such equilibration processes are unprecedented in the chemistry of methandiide ligands in carbene complexes. Addition reactions (e.g. of alkynes or aldehydes) have so far been irreversible due to the high nucleophilicity of the carbenic carbon atom.[16] This underlines the efficient charge transfer from the ligand to the ruthenium center and the stability of carbene complex 3. Chem. Eur. J. 2014, 20, 11295 – 11299
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The phenolato complex 4 a was additionally characterized by XRD analysis, which confirmed the 1,2-addition across the former R=C double bond. In the molecular structure of 4 a (Figure 4) the Ru C bond lengthens from 1.965(2) in the carbene complex 3 to 2.201(3) , which is consistent with a change from a Ru C bond with double bond character to a single bond. The phenolato ligand and the proton at the PC-S bridge are attached on the same side of the former Ru=C bond, suggesting that a cis-type addition reaction might be operative. Both groups are still in close contact with each other (e.g. small C1-Ru-O3 angle of 76.7(1)8), indicating a facile elimination of the activated alcohol as observed in the experiment. Preliminary DFT studies (for computational details, see the Supporting Information) on the equilibrium between the O H activation and the carbene re-formation showed a rather low reaction barrier for the 1,2-addition of only 5.7 kcal mol 1 relative to the carbene complex 3. Thereby, the phenolato complex was found to be only 14.5 kcal mol 1 favored over the carbene species, thus confirming the equilibrium between both complexes.[17] Besides the activation of the O H bonds also the activation of the non-polar H H bond in dihydrogen could be realized. Exposure of a solution of 3 in toluene to 1 atm of H2 resulted in a color change from purple to yellow and the conversion of 3 to the hydrido complex 5. The hydrido complex could be isolated in 72 % yield as a yellow solid. Complex 5 is characterized by a singlet at dH = 6.62 ppm in the 1H NMR spectrum for the terminal hydrido ligand and at dH = 3.77 ppm for the proton at the PCS bridge. The phosphorus resonates at dP = 51.2 ppm. In the molecular structure (Figure 4), the Ru C bond elongates from 1.965(2) in the carbene complex 3 to 2.185(4) . Contrary to the O H activation, no equilibrium was observed for the H H activation at room temperature. This is confirmed by calculations that show a higher barrier of 27.0 kcal mol 1 for the cis-addition of H2 across the metal–
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Communication carbon bond and a preference of 5 over 3 by 12.0 kcal mol 1. Interestingly, the formation of the hydrido complex 5 was also observed by the dehydrogenation of isopropanol or formic acid under formation of acetone and carbon dioxide, respectively.[18] Treatment of carbene complex 3 with one equivalent of iPrOH in toluene or pure formic acid selectively gave way to 5 without the formation of any by-products. Thereby, the dehydrogenation was found to be faster with formic acid, instantly resulting in the formation of 5. The observed dehydrogenation and the moderate energetic preference of hydrido complex 5 over carbene 3 suggested that a catalytic cycle including both species should be possible. Transfer hydrogenation with isopropanol as hydrogen source seemed to be a good starting point to evaluate the potential of complex 3 as catalyst making use of the non-innocent behavior of the carbene ligand.[19] Preliminary studies confirmed this hypothesis by means of the reduction of acetophenone as test reaction (Table 1, Supporting Information). In a general protocol iPrOH was used as solvent in combination
Table 1. Cooperative catalytic transfer hydrogenation for the reduction of acetophenone using 3 as catalyst.
Entry
Time [h]
T [8C]
Cat. [mol %]
Base
Base [mol %]
Yield [%][a]
1 2 3 4 5 6 7 8 9 10 11
24 24 24 24 24 24 24 48 48 24 24
75 75 75 75 75 75 75 75 90[b] 75 75
0.34 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.34 0.10
– KHMDS KHMDS KOtBu KOtBu KOtBu KOtBu KOtBu KOtBu KOtBu KOtBu
– 1.8 3.5 1.6 6.2 12 19 6.2 6.2 19 19
12 16 18 24 36 43 55 62 82 51 44
with varying catalyst loadings and potassium salts as base additive. Without additional base only 12 % conversion to the alcohol could be achieved with 0.34 mol % catalyst within 24 h at 75 8C (entry 1). Depending on the applied base, yields of up to 55 % were obtained at the same reaction temperature. Best results were observed with KOtBu, which is commonly applied in transfer hydrogenation reactions as a base additive. Longer reaction times and higher temperatures of 90 8C (entries 8 and 9) finally led to good conversions (> 80 %) of the ketone to the alcohol with only 6 mol % KOtBu and 0.53 mol % catalyst. Reduction of the catalyst loading to 0.1 mol % resulted in slightly lower yields (entries 10 and 11). Overall, this transfer hydrogenation using 3 as catalyst is—to the best of our knowledge— the first example of a cooperative catalysis making use of the non-innocent behavior of a carbene ligand. In the case of carbene complex 3 further optimizations of the ligand framework have to be made to improve the catalytic activity. www.chemeurj.org
Acknowledgements V.H.G. thanks the DFG for an Emmy Noether grant and the Fonds der Chemischen Industrie as well as the Otto-Rçhm-Gedchtnisstiftung for financial support. We also thank Rockwood Lithium GmbH for the supply of chemicals. Keywords: bond activation · carbene complexes · dianions · non-innocent ligands · transition metal catalysis
[a] Determined by 1H NMR spectroscopy. [b] Solvent: iPrOH/toluene = 2:1.
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In conclusion, we presented the synthesis of a ruthenium– carbene complex based on a dilithio methandiide and its application in bond activation reactions as well as cooperative catalysis. Depending on the co-ligands at the metal center, the charge transfer from the dianionic ligand to the metal can be tuned thus allowing for a controlled manipulation of the metal–carbon interaction and hence of the complex stability and reactivity. This gives way to the application of the carbene complex in the activation of the O H bond in aromatic alcohols and the non-polar H H bond in dihydrogen. Additionally, the carbene complex was found to be applicable in the dehydrogenation of formic acid and isopropanol which could be used in catalytic transfer hydrogenation for the reduction of acetophenone. All activation and dehydrogenation reactions were found to proceed—supported by metal–ligand cooperation—via a 1,2-addition across the Ru=C double bond. This demonstrates the non-innocent behavior of the methandiide ligands and opens new pathways for the application of these carbene complexes in homogenous catalysis.
[1] For reviews, see: a) V. K. K. Praneeth, M. R. Ringenberg, T. R. Ward, Angew. Chem. 2012, 124, 10374; Angew. Chem. Int. Ed. 2012, 51, 10228; b) O. R. Luca, R. H. Crabtree, Chem. Soc. Rev. 2013, 42, 1440 – 1459; c) H. Grtzmacher, Angew. Chem. 2008, 120, 1838; Angew. Chem. Int. Ed. 2008, 47, 1814; d) J. I. van der Vlugt, J. N. H. Reek, Angew. Chem. 2009, 121, 8990; Angew. Chem. Int. Ed. 2009, 48, 8832; e) S. Schneider, J. Meiners, B. Askevold, Eur. J. Inorg. Chem. 2012, 412 – 429; f) V. Lyaskovskyy, B. de Bruin, ACS Catal. 2012, 2, 270 – 279. [2] a) C. A. Lippert, K. I. Hardcastle, J. D. Soper, Inorg. Chem. 2011, 50, 9864; b) P. J. Chirik, K. Wieghardt, Science 2010, 327, 794. [3] For examples, see: a) W. I. Dzik, J. N. H. Reek, B. de Bruin, Chem. Eur. J. 2008, 14, 7594; b) B. de Bruin, D. G. H. Hetterscheid, Eur. J. Inorg. Chem. 2007, 211; c) P. Chaudhuri, M. Hess, J. Mller, K. Hildenbrand, E. Bill, T. Weyhermller, K. Wieghardt, J. Am. Chem. Soc. 1999, 121, 9599; d) J. Scott, S. Gambarotta, I. Korobkov, Q. Knijnenburg, B. de Bruin, P. H. M. Budzelaar, J. Am. Chem. Soc. 2005, 127, 17204; e) K. T. Sylvester, P. J. Chirik, J. Am. Chem. Soc. 2009, 131, 8772; f) S. K. Russell, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2011, 133, 8858; g) S. C. Bart, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2004, 126, 13794 – 13807; h) M. W. Bouwkamp, A. C. Bowman, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2006, 128, 13340 – 13341. [4] For examples, see: a) P. Maire, T. Bttner, F. Breher, P. Le Floch, H. Grtzmacher, Angew. Chem. 2005, 117, 6477; Angew. Chem. Int. Ed. 2005, 44, 6318; b) R. Noyori, T. Ohkuma, Angew. Chem. 2001, 113, 40; Angew. Chem. Int. Ed. 2001, 40, 40; c) R. Noyori, M. Yamakawa, S. Hashiguchi, J. Org. Chem. 2001, 66, 7931; d) M. Kß, A. Friedrich, M. Drees, S. Schneider, Angew. Chem. 2009, 121, 922; Angew. Chem. Int. Ed. 2009, 48, 905; e) B. Askevold, J. Torres Nieto, S. Tussupbayev, M. Diefenbach, E. Herdtweck, M. C. Holthausen, S. Schneider, Nature Chem. 2011, 3, 532. [5] a) S. W. Kohl, L. Weiner, L. Schwartsburd, L. Konstantinovski, L. J. W. Shimon, Y. Ben-David, M. A. Iron, D. Milstein, Science 2009, 324, 74 – 77; b) R. Langer, Y. Diskin-Posner, G. Leitus, L. J. W. Shimon, Y. Ben-David, D.
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[6]
[7] [8]
[9]
Milstein, Angew. Chem. 2011, 123, 10122 – 10126; Angew. Chem. Int. Ed. 2011, 50, 9948 – 9952; c) R. Langer, G. Leitus, Y. Ben-David, D. Milstein, Angew. Chem. 2011, 123, 2168 – 2172; Angew. Chem. Int. Ed. 2011, 50, 2120 – 2124; d) C. Gunanathan, D. Milstein, Acc. Chem. Res. 2011, 44, 588 – 602. a) H. J. Knçlker, E. Baum, H. Goesmann, R. Klauss, Angew. Chem. 1999, 111, 2196 – 2199; Angew. Chem. Int. Ed. 1999, 38, 2064 – 2066; b) C. P. Casey, H. Guan, J. Am. Chem. Soc. 2007, 129, 5816 – 5817; c) P. J. Turrell, J. A. Wright, J. N. T. Peck, V. S. Oganesyan, C. J. Pickett, Angew. Chem. 2010, 122, 7670 – 7673; Angew. Chem. Int. Ed. 2010, 49, 7508 – 7511; d) D. F. Chen, R. Scopelliti, X. L. Hu, Angew. Chem. 2011, 123, 5789 – 5791; Angew. Chem. Int. Ed. 2011, 50, 5671 – 5673. D. V. Gutsulyak, W. E. Piers, J. Borau-Garcia, M. Parvez, J. Am. Chem. Soc. 2013, 135, 11776 – 11779. For reviews, see: a) N. D. Jones, R. G. Cavell, J. Organomet. Chem. 2005, 690, 5485 – 5496; b) T. Cantat, N. Mzailles, A. Auffrant, P. Le Floch, Dalton Trans. 2008, 1957 – 1972; c) S. T. Liddle, D. P. Mills, A. Wooles, J. Organomet. Chem. 2010, 695, 29; d) S. Harder, Coord. Chem. Rev. 2011, 255, 1252 – 1267; e) T. K. Panda, P. W. Roesky, Chem. Soc. Rev. 2009, 38, 2782 – 2804; f) S. T. Liddle, D. P. Mills, A. J. Wooles, Chem. Soc. Rev. 2011, 40, 2164 – 2176; g) G. R. Giesbrecht, J. C. Gordon, Dalton Trans. 2004, 2387 – 2393; h) O. T. Summerscales, J. C. Gordon, RSC Adv. 2013, 3, 6682 – 6692. For selected examples, see: a) O. J. Cooper, D. P. Mills, J. McMaster, F. Moro, E. S. Davies, W. Lewis, A. J. Blake, S. T. Liddle, Angew. Chem. 2011, 123, 2431 – 2434; Angew. Chem. Int. Ed. 2011, 50, 2383 – 2386; b) R. G. Cavell, R. P. K. Babu, A. Kasani, R. McDonald, J. Am. Chem. Soc. 1999, 121, 5805 – 5806; c) R. P. Kamalesh Babu, R. McDonald, R. G. Cavell, Chem. Commun. 2000, 481 – 482; d) G. Lin, N. D. Jones, R. A. Gossage, R. McDonald, R. G. Cavell, Angew. Chem. 2003, 115, 4188 – 4191; Angew. Chem. Int. Ed. 2003, 42, 4054 – 4057; e) T. Cantat, T. Arliguie, A. Nol, P. Thury, M. Ephritikhine, P. Le Floch, N. Mzailles, J. Am. Chem. Soc. 2009, 131, 963 – 972; f) D. P. Mills, O. J. Cooper, F. Tuna, E. J. L. McInnes, E. S. Davies, J. McMaster, F. Moro, W. Lewis, A. J. Blake, S. T. Liddle, J. Am. Chem. Soc. 2012, 134, 10047 – 10054; g) T. Cantat, N. Mzailles, L. Ricard, Y. Jean, P. Le Floch, Angew. Chem. 2004, 116, 6542 – 6545; Angew. Chem.
Chem. Eur. J. 2014, 20, 11295 – 11299
www.chemeurj.org
[10] [11]
[12] [13]
[14]
[15] [16] [17] [18]
[19]
Int. Ed. 2004, 43, 6382 – 6385; h) D. P. Mills, L. Soutar, W. Lewis, A. J. Blake, S. T. Liddle, J. Am. Chem. Soc. 2010, 132, 14379 – 14381. V. H. Gessner, F. Meier, D. Uhrich, M. Kaupp, Chem. Eur. J. 2013, 19, 16729 – 16739. a) P. Schrçter, V. H. Gessner, Chem. Eur. J. 2012, 18, 11223 – 11227; b) J. Becker, V. H. Gessner, Dalton Trans. 2014, 43, 4320 – 4325; c) V. H. Gessner, Acta Crystallogr. Sect. E 2012, 68, o1045. J. Becker, V. H. Gessner, Organometallics 2014, 33, 1310 – 1317. An analogous reaction has been reported for dppm-based ligands at elevated temperatures: a) T. Cantat, M. Demange, N. Mzailles, L. Ricard, Y. Jean, P. Le Floch, Organometallics 2005, 24, 4838 – 4841; b) H. Heuclin, X. F. Le Goff, N. Mzailles, Chem. Eur. J. 2012, 18, 16136 – 16144. a) Y. Smurnyy, C. Bibal, M. Pink, K. G. Caulton, Organometallics 2005, 24, 3849 – 3855; b) V. Cadierno, J. Dez, J. Garca- lvarez, J. Gimeno, Organometallics 2005, 24, 2801 – 2810. S. F. Vyboishchikov, G. Frenking, Chem. Eur. J. 1998, 4, 1428 – 1438. R. G. Cavell, R. P. K. Babu, K. Aparna, J. Organomet. Chem. 2001, 617 – 618, 158 – 169. E. Nicolas, B. Martin-Vaca, N. Mzailles, D. Bourissou, L. Maron, Eur. J. Inorg. Chem. 2013, 4068 – 4076. For reviews on the catalytic dehydrogenation of alcohols, see: a) C. Gunanathan, D. Milstein, Science 2013, 341, 249 – 260; b) A. Friedrich, S. Schneider, ChemCatChem 2009, 1, 72 – 73; c) G. E. Dobereiner, R. H. Crabtree, Chem. Rev. 2010, 110, 681 – 703. For examples of catalytic transfer hydrogenation, see: a) H. V zquezVilla, S. Reber, M. A. Ariger, E. M. Carreira, Angew. Chem. 2011, 123, 9141 – 9143; b) S.-F. Hsu, B. Plietker, Chem. Eur. J. 2014, 20, 4242 – 4245; c) T. Ohkuma, N. Utsumi, K. Tsutsumi, K. Murata, C. Sandoval, R. Noyori, J. Am. Chem. Soc. 2006, 128, 8724 – 8725; d) H. Brunner, W. Leitner, Angew. Chem. 1988, 100, 1231; Angew. Chem. Int. Ed. Engl. 1988, 27, 1180; e) S. Hashiguchi, A. Fuji, J. Takehara, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 1995, 117, 7562; f) J. R. Miecznikowski, R. H. Crabtree, Organometallics 2004, 23, 629.
Received: June 26, 2014 Published online on July 22, 2014
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& Carbene Complexes
Methandiide as a Non-Innocent Ligand in Carbene Complexes: From the Electronic Structure to Bond Activation Reactions and Cooperative Catalysis Julia Becker, Tanja Modl, and Viktoria H. Gessner*[a] Abstract: The synthesis of a ruthenium carbene complex based on a sulfonyl-substituted methandiide and its application in bond activation reactions and cooperative catalysis is reported. In the complex, the metal–carbon interaction can be tuned between a Ru C single bond with additional electrostatic interactions and a Ru=C double bond, thus allowing the control of the stability and reactivity of the complex. Hence, activation of polar and non-polar bonds (O H, H H) as well as dehydrogenation reactions become possible. In these reactions the carbene acts as a non-innocent ligand supporting the bond activation as nucleophilic center in the 1,2-addition across the metal– carbon double bond. This metal–ligand cooperativity can be applied in the catalytic transfer hydrogenation for the reduction of ketones. This concept opens new ways for the application of carbene complexes in catalysis.
Figure 1. Complexes with non-innocent ligands used in bond activation reactions.
Non-innocent ligands have received great attention over the past few years due to their capability to support transformations, which are not possible at a single metal center.[1] Prominent examples are redox-active ligands that allow multielectron transformations with metals such as Fe, Co, or Cu, which usually tend to undergo only 1 e oxidation state changes (e.g. CoI, CoII, CoIII).[2] This has become particularly interesting in the context of substituting precious metals in catalytic transformations, where typically 2 e oxidation state changes are required.[3] Indeed, even redox-neutral ligands have been applied as non-innocent ligands. Particularly interesting examples are ligands supporting proton transfer reactions by moderating the loss or gain of protons at the metal center. In these processes, the ligand itself exhibits two stable tautomeric forms, usually an anionic and a neutral one.[4] A remarkable example is the ruthenium pincer complex A reported by the Milstein group (see Figure 1).[5] This complex was found to be active in
[a] Dipl.-Chem. J. Becker, T. Modl, Dr. V. H. Gessner Institut fr Anorganische Chemie, Julius-Maximilians-Universitt Wrzburg Am Hubland, 97074 Wrzburg (Germany) E-mail: [email protected] Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/forschungsgruppen/dr_v_gessner/ Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404131. Chem. Eur. J. 2014, 20, 11295 – 11299
multiple bond activation reactions such as of H H, N H, or O H bonds, including the splitting of water. Other examples for complexes with non-innocent ligands used in bond activation reactions are the iron complex B reported by Knçlker et al.[6] and the nickel carbene complex C recently reported by Piers and co-workers.[7] Our group has become particularly interested in the methandiide ligand family, which was shown to give access to a novel class of carbene complexes.[8, 9] The metal–carbon interaction in these complexes was found to cover a range of bonding situations, starting from a kind of masked methandiide with mainly electrostatic M2 + ···C2 interactions, to alkyl complexes with a carbanionic ligand, M + C , and complexes with a real M=C double bond.[10] This electronic flexibility suggests a possible application of these complexes in bond activation reactions by means of metal–ligand cooperation, in which the ligand exhibits a dianionic and an anionic tautomeric form. Based thereon, we focused on the following questions: i) Can the electronic situation of the M=C bond be tuned to allow the activation of polar as well as non-polar bonds, such as that of dihydrogen? ii) Can the bond activation reactions be conducted in a reversible manner, enabling the re-formation of the carbene complex from the activation product under liberation of the activated species? iii) Can the non-innocent behavior of the carbene ligand be used in cooperative catalysis? Herein we show that these applications of carbene complexes are possible. Based on the sulfonyl-substituted methandiide ligand 1, tuning of the metal–carbon interaction enabled small molecule activation and catalytic transfer hydrogenation by means of metal–ligand cooperation.[11] Previously, we reported on the reaction of methandiide 1 with [(PPh3)3RuCl2]. The obtained carbene complex 2-int turned out to be unstable even at low reaction temperature resulting in a cyclometallation to 2 via C H bond activation of the sulfonyl bound phenyl group.[12, 13] The origin of the instability was found to be the facile decoordination of the sulfonyl
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Communication moiety as well as the highly nucleophilic carbon center and the weak p-interaction in the carbene complex. We assumed that to increase the charge transfer from the methandiide to the metal and to strengthen the metal–carbon interaction a less electron-rich metal center would be beneficial. Thus, we chose [{(p-cymene)RuCl2}2] with a stronger p-accepting ligand (Scheme 1).[14] Salt metathesis by treatment of methandiide
Scheme 1. Preparation of Ru complexes 2 and 3 from methandiide 1.
1 with [{(p-cymene)RuCl2}2] instantly resulted in the reaction mixture turning purple and the formation of a single new species characterized by a signal at dP = 66.6 ppm (1: dP = 26.5 ppm) in the 31P{1H} NMR spectrum (see the Supporting Information for Experimental Details). No protonation of the central carbon atom was observed. Work-up delivered carbene complex 3 as purple solid in 86 % yield. Contrary to the phosphine complex 2-int the cymene complex turned out to be stable at room temperature and less reactive towards protonabstraction reactions. The carbene carbon atom resonates as a doublet at dC = 140.0 ppm (1JCP = 30.2 Hz), which is upfieldshifted compared to that in other ruthenium carbene complexes (220–320 ppm), but considerably down-field shifted compared to typical alkyl complexes and many carbene complexes derived from dilithio methandiides.[10] The nature of the carbene complex was unambiguously confirmed by single-crystal X-ray diffraction (XRD) analysis (Figure 2). In the crystal, the methandiide acts as a bi-dentate ligand via a C,S-coordination mode. The Ru C1 distance is 1.965(2) , which is shorter than the Ru C bond found in alkyl complex 2 (Ru C 2.191(6) ). The central carbon atom C1 features a planar coordination environment (sum of angles: 358.9(1)8) as expected for a carbene complex. Interestingly, both the C1 P and C1 S2 distances in 3 are considerably longer than the bond lengths found in the methandiide 1 and other carbene complexes of this ligand with little p-contribution of the M C bond.[12] This can be attributed to reduced electrostatic interactions within the P-C-S framework and thus to an efficient charge transfer from the methandiide carbon to the metal center. The different stabilities and bonding situation of 2-Int and 3 were investigated by DFT studies using the B3LYP functional in combination with the 6-311 + + g(d,p) and LANL2TZ(f) basis sets (for computational details see the Supporting Information). The calculated natural atomic charges, the Wiberg bond Chem. Eur. J. 2014, 20, 11295 – 11299
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Figure 2. Molecular structure of carbene complex 3. Displacement ellipsoids drawn at the 50 % probability level. Selected bond lengths [] and angles [8]: Ru–C1 1.965(2), Ru–S1 2.4441(5), S1–P 2.0312(7), S2–O2 1.454(1), S2–O1 1.460(1), S2–C1 1.717(2), S2–C14 1.784(2), P–C1 1.739(2), P–C8 1.815(2), P–C2 1.815(2); C1-Ru-S1 79.79(6), S2-C1-P 122.8(1), S2-C1-Ru 133.9(1), P-C1-Ru 102.27(9).
indices (WBI) and natural bond orbital (NBO) analyses provide a clear picture of the differences between both compounds (Figure 3). Thus, the phosphine complex 2-int is best described by an ylidic structure with a lone pair remaining at the central carbon atom. This is reflected by a high negative charge of qC = 1.28 at the carbon atom, the low WBI of the Ru C bond of only 0.63 and the missing p-interaction in the NBO analysis. In contrast, the M C bond in 3 features both, a s- and a p-interaction, which are only slightly polarized towards the carbon end. The carbenic carbon atom still carries a negative charge of qC = 0.98. The WBI of 1.06 is smaller than that of a real double bond [for example, WBI(ethene) = 2.04] or of an undisturbed aromatic system [for example, WBI(benzene) = 1.44], however comparable with other Schrock-type complexes.[10, 15] Overall, the different co-ligands in 2-int and 3 result in a transition from a bipolar ylidic structure to a doubly bonded carbene complex. Yet, the charge distribution still accounts for a considerable nucleophilic character of 3, which should be beneficial for bond activation reactions across the Ru=C bond. As shown in Figure 4 this expectation is confirmed by a series of O H activation reactions supported by metal–ligand coop-
Figure 3. Calculated charges, WBI of the Ru C bond, and NBO analysis of the complexes 2-int and 3.
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Figure 4. E H bond activation reaction of carbene complex 3 via metal–ligand cooperation. Displacement ellipsoids drawn at the 50 % probability level. Selected bond lengths [] and angles [8]: 4 a: Ru–C1 2.201(3), Ru–O3 2.098(3), S2–C1 1.739(4), P–C1 1.788(4), S1–P 1.9997(13), S2–O1 1.444(3), S2–O2 1.445(3); S2-C1-P 118.5(2), P-C1-Ru 94.45(15), O3-Ru-C1 76.65(12). 5: Ru–C1 2.185(4), C1–S2 1.768(5), C1–P 1.787(4), S2–O1 1.445(3), S2–O2 1.449(3), S1–P 1.9971(16); S2-C1-P 117.1(3), P-C1-Ru 93.33(19).
eration. Treatment of a solution of 3 in toluene with one equivalent of an aromatic alcohol ROH (R = Ph, p-C6H4OMe, pC6H4CF3) instantly resulted in a color change from purple to red and the formation of the 1,2-addition products 4 via O H bond cleavage (see the Supporting Information). For R = Ph, this is evidenced by the appearance of a resonance at dH = 4.21 ppm in the 1H NMR spectrum for the proton at the PCS bridge and a new 31P{1H} NMR signal at dP = 74.2 ppm (in C6D6). No simple coordination complex of the alcohol to the metal is observed, which often limits a direct functionalization of E H bonds by transition-metal complexes. The O H activation was accomplished by using electron-rich and electron-poor arenes (4 b and 4 c). Depending on the nature of the alcohol, an equilibrium between the 1,2-addition to 4 and the re-formation of the carbene complex 3 was observed at room temperature. As such, quantitative formation of the addition product was only found for the electron-poor p-CF3 functionalized alcohol delivering 4 c in 89 % isolated yield. On the contrary, phenol and pmethoxyphenol exhibit an equilibrium between 3 and the alcoholato complex 4. This reversibility of the O H activation and its temperature dependency is confirmed by variable-temperature NMR studies of 4 b (see the Supporting Information for spectra). Almost complete transformation to the addition product 4 b is observed at 80 8C, whereas an approximate 10:3 mixture between 4 b and 3 is found at room temperature and a 10:5.5 mixture at 35 8C. The O H activation of the less electron-rich phenol shows an analogous equilibrium with 95 % conversion to the O H activation product at room temperature. Such equilibration processes are unprecedented in the chemistry of methandiide ligands in carbene complexes. Addition reactions (e.g. of alkynes or aldehydes) have so far been irreversible due to the high nucleophilicity of the carbenic carbon atom.[16] This underlines the efficient charge transfer from the ligand to the ruthenium center and the stability of carbene complex 3. Chem. Eur. J. 2014, 20, 11295 – 11299
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The phenolato complex 4 a was additionally characterized by XRD analysis, which confirmed the 1,2-addition across the former R=C double bond. In the molecular structure of 4 a (Figure 4) the Ru C bond lengthens from 1.965(2) in the carbene complex 3 to 2.201(3) , which is consistent with a change from a Ru C bond with double bond character to a single bond. The phenolato ligand and the proton at the PC-S bridge are attached on the same side of the former Ru=C bond, suggesting that a cis-type addition reaction might be operative. Both groups are still in close contact with each other (e.g. small C1-Ru-O3 angle of 76.7(1)8), indicating a facile elimination of the activated alcohol as observed in the experiment. Preliminary DFT studies (for computational details, see the Supporting Information) on the equilibrium between the O H activation and the carbene re-formation showed a rather low reaction barrier for the 1,2-addition of only 5.7 kcal mol 1 relative to the carbene complex 3. Thereby, the phenolato complex was found to be only 14.5 kcal mol 1 favored over the carbene species, thus confirming the equilibrium between both complexes.[17] Besides the activation of the O H bonds also the activation of the non-polar H H bond in dihydrogen could be realized. Exposure of a solution of 3 in toluene to 1 atm of H2 resulted in a color change from purple to yellow and the conversion of 3 to the hydrido complex 5. The hydrido complex could be isolated in 72 % yield as a yellow solid. Complex 5 is characterized by a singlet at dH = 6.62 ppm in the 1H NMR spectrum for the terminal hydrido ligand and at dH = 3.77 ppm for the proton at the PCS bridge. The phosphorus resonates at dP = 51.2 ppm. In the molecular structure (Figure 4), the Ru C bond elongates from 1.965(2) in the carbene complex 3 to 2.185(4) . Contrary to the O H activation, no equilibrium was observed for the H H activation at room temperature. This is confirmed by calculations that show a higher barrier of 27.0 kcal mol 1 for the cis-addition of H2 across the metal–
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Communication carbon bond and a preference of 5 over 3 by 12.0 kcal mol 1. Interestingly, the formation of the hydrido complex 5 was also observed by the dehydrogenation of isopropanol or formic acid under formation of acetone and carbon dioxide, respectively.[18] Treatment of carbene complex 3 with one equivalent of iPrOH in toluene or pure formic acid selectively gave way to 5 without the formation of any by-products. Thereby, the dehydrogenation was found to be faster with formic acid, instantly resulting in the formation of 5. The observed dehydrogenation and the moderate energetic preference of hydrido complex 5 over carbene 3 suggested that a catalytic cycle including both species should be possible. Transfer hydrogenation with isopropanol as hydrogen source seemed to be a good starting point to evaluate the potential of complex 3 as catalyst making use of the non-innocent behavior of the carbene ligand.[19] Preliminary studies confirmed this hypothesis by means of the reduction of acetophenone as test reaction (Table 1, Supporting Information). In a general protocol iPrOH was used as solvent in combination
Table 1. Cooperative catalytic transfer hydrogenation for the reduction of acetophenone using 3 as catalyst.
Entry
Time [h]
T [8C]
Cat. [mol %]
Base
Base [mol %]
Yield [%][a]
1 2 3 4 5 6 7 8 9 10 11
24 24 24 24 24 24 24 48 48 24 24
75 75 75 75 75 75 75 75 90[b] 75 75
0.34 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.34 0.10
– KHMDS KHMDS KOtBu KOtBu KOtBu KOtBu KOtBu KOtBu KOtBu KOtBu
– 1.8 3.5 1.6 6.2 12 19 6.2 6.2 19 19
12 16 18 24 36 43 55 62 82 51 44
with varying catalyst loadings and potassium salts as base additive. Without additional base only 12 % conversion to the alcohol could be achieved with 0.34 mol % catalyst within 24 h at 75 8C (entry 1). Depending on the applied base, yields of up to 55 % were obtained at the same reaction temperature. Best results were observed with KOtBu, which is commonly applied in transfer hydrogenation reactions as a base additive. Longer reaction times and higher temperatures of 90 8C (entries 8 and 9) finally led to good conversions (> 80 %) of the ketone to the alcohol with only 6 mol % KOtBu and 0.53 mol % catalyst. Reduction of the catalyst loading to 0.1 mol % resulted in slightly lower yields (entries 10 and 11). Overall, this transfer hydrogenation using 3 as catalyst is—to the best of our knowledge— the first example of a cooperative catalysis making use of the non-innocent behavior of a carbene ligand. In the case of carbene complex 3 further optimizations of the ligand framework have to be made to improve the catalytic activity. www.chemeurj.org
Acknowledgements V.H.G. thanks the DFG for an Emmy Noether grant and the Fonds der Chemischen Industrie as well as the Otto-Rçhm-Gedchtnisstiftung for financial support. We also thank Rockwood Lithium GmbH for the supply of chemicals. Keywords: bond activation · carbene complexes · dianions · non-innocent ligands · transition metal catalysis
[a] Determined by 1H NMR spectroscopy. [b] Solvent: iPrOH/toluene = 2:1.
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In conclusion, we presented the synthesis of a ruthenium– carbene complex based on a dilithio methandiide and its application in bond activation reactions as well as cooperative catalysis. Depending on the co-ligands at the metal center, the charge transfer from the dianionic ligand to the metal can be tuned thus allowing for a controlled manipulation of the metal–carbon interaction and hence of the complex stability and reactivity. This gives way to the application of the carbene complex in the activation of the O H bond in aromatic alcohols and the non-polar H H bond in dihydrogen. Additionally, the carbene complex was found to be applicable in the dehydrogenation of formic acid and isopropanol which could be used in catalytic transfer hydrogenation for the reduction of acetophenone. All activation and dehydrogenation reactions were found to proceed—supported by metal–ligand cooperation—via a 1,2-addition across the Ru=C double bond. This demonstrates the non-innocent behavior of the methandiide ligands and opens new pathways for the application of these carbene complexes in homogenous catalysis.
[1] For reviews, see: a) V. K. K. Praneeth, M. R. Ringenberg, T. R. Ward, Angew. Chem. 2012, 124, 10374; Angew. Chem. Int. Ed. 2012, 51, 10228; b) O. R. Luca, R. H. Crabtree, Chem. Soc. Rev. 2013, 42, 1440 – 1459; c) H. Grtzmacher, Angew. Chem. 2008, 120, 1838; Angew. Chem. Int. Ed. 2008, 47, 1814; d) J. I. van der Vlugt, J. N. H. Reek, Angew. Chem. 2009, 121, 8990; Angew. Chem. Int. Ed. 2009, 48, 8832; e) S. Schneider, J. Meiners, B. Askevold, Eur. J. Inorg. Chem. 2012, 412 – 429; f) V. Lyaskovskyy, B. de Bruin, ACS Catal. 2012, 2, 270 – 279. [2] a) C. A. Lippert, K. I. Hardcastle, J. D. Soper, Inorg. Chem. 2011, 50, 9864; b) P. J. Chirik, K. Wieghardt, Science 2010, 327, 794. [3] For examples, see: a) W. I. Dzik, J. N. H. Reek, B. de Bruin, Chem. Eur. J. 2008, 14, 7594; b) B. de Bruin, D. G. H. Hetterscheid, Eur. J. Inorg. Chem. 2007, 211; c) P. Chaudhuri, M. Hess, J. Mller, K. Hildenbrand, E. Bill, T. Weyhermller, K. Wieghardt, J. Am. Chem. Soc. 1999, 121, 9599; d) J. Scott, S. Gambarotta, I. Korobkov, Q. Knijnenburg, B. de Bruin, P. H. M. Budzelaar, J. Am. Chem. Soc. 2005, 127, 17204; e) K. T. Sylvester, P. J. Chirik, J. Am. Chem. Soc. 2009, 131, 8772; f) S. K. Russell, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2011, 133, 8858; g) S. C. Bart, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2004, 126, 13794 – 13807; h) M. W. Bouwkamp, A. C. Bowman, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2006, 128, 13340 – 13341. [4] For examples, see: a) P. Maire, T. Bttner, F. Breher, P. Le Floch, H. Grtzmacher, Angew. Chem. 2005, 117, 6477; Angew. Chem. Int. Ed. 2005, 44, 6318; b) R. Noyori, T. Ohkuma, Angew. Chem. 2001, 113, 40; Angew. Chem. Int. Ed. 2001, 40, 40; c) R. Noyori, M. Yamakawa, S. Hashiguchi, J. Org. Chem. 2001, 66, 7931; d) M. Kß, A. Friedrich, M. Drees, S. Schneider, Angew. Chem. 2009, 121, 922; Angew. Chem. Int. Ed. 2009, 48, 905; e) B. Askevold, J. Torres Nieto, S. Tussupbayev, M. Diefenbach, E. Herdtweck, M. C. Holthausen, S. Schneider, Nature Chem. 2011, 3, 532. [5] a) S. W. Kohl, L. Weiner, L. Schwartsburd, L. Konstantinovski, L. J. W. Shimon, Y. Ben-David, M. A. Iron, D. Milstein, Science 2009, 324, 74 – 77; b) R. Langer, Y. Diskin-Posner, G. Leitus, L. J. W. Shimon, Y. Ben-David, D.
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[6]
[7] [8]
[9]
Milstein, Angew. Chem. 2011, 123, 10122 – 10126; Angew. Chem. Int. Ed. 2011, 50, 9948 – 9952; c) R. Langer, G. Leitus, Y. Ben-David, D. Milstein, Angew. Chem. 2011, 123, 2168 – 2172; Angew. Chem. Int. Ed. 2011, 50, 2120 – 2124; d) C. Gunanathan, D. Milstein, Acc. Chem. Res. 2011, 44, 588 – 602. a) H. J. Knçlker, E. Baum, H. Goesmann, R. Klauss, Angew. Chem. 1999, 111, 2196 – 2199; Angew. Chem. Int. Ed. 1999, 38, 2064 – 2066; b) C. P. Casey, H. Guan, J. Am. Chem. Soc. 2007, 129, 5816 – 5817; c) P. J. Turrell, J. A. Wright, J. N. T. Peck, V. S. Oganesyan, C. J. Pickett, Angew. Chem. 2010, 122, 7670 – 7673; Angew. Chem. Int. Ed. 2010, 49, 7508 – 7511; d) D. F. Chen, R. Scopelliti, X. L. Hu, Angew. Chem. 2011, 123, 5789 – 5791; Angew. Chem. Int. Ed. 2011, 50, 5671 – 5673. D. V. Gutsulyak, W. E. Piers, J. Borau-Garcia, M. Parvez, J. Am. Chem. Soc. 2013, 135, 11776 – 11779. For reviews, see: a) N. D. Jones, R. G. Cavell, J. Organomet. Chem. 2005, 690, 5485 – 5496; b) T. Cantat, N. Mzailles, A. Auffrant, P. Le Floch, Dalton Trans. 2008, 1957 – 1972; c) S. T. Liddle, D. P. Mills, A. Wooles, J. Organomet. Chem. 2010, 695, 29; d) S. Harder, Coord. Chem. Rev. 2011, 255, 1252 – 1267; e) T. K. Panda, P. W. Roesky, Chem. Soc. Rev. 2009, 38, 2782 – 2804; f) S. T. Liddle, D. P. Mills, A. J. Wooles, Chem. Soc. Rev. 2011, 40, 2164 – 2176; g) G. R. Giesbrecht, J. C. Gordon, Dalton Trans. 2004, 2387 – 2393; h) O. T. Summerscales, J. C. Gordon, RSC Adv. 2013, 3, 6682 – 6692. For selected examples, see: a) O. J. Cooper, D. P. Mills, J. McMaster, F. Moro, E. S. Davies, W. Lewis, A. J. Blake, S. T. Liddle, Angew. Chem. 2011, 123, 2431 – 2434; Angew. Chem. Int. Ed. 2011, 50, 2383 – 2386; b) R. G. Cavell, R. P. K. Babu, A. Kasani, R. McDonald, J. Am. Chem. Soc. 1999, 121, 5805 – 5806; c) R. P. Kamalesh Babu, R. McDonald, R. G. Cavell, Chem. Commun. 2000, 481 – 482; d) G. Lin, N. D. Jones, R. A. Gossage, R. McDonald, R. G. Cavell, Angew. Chem. 2003, 115, 4188 – 4191; Angew. Chem. Int. Ed. 2003, 42, 4054 – 4057; e) T. Cantat, T. Arliguie, A. Nol, P. Thury, M. Ephritikhine, P. Le Floch, N. Mzailles, J. Am. Chem. Soc. 2009, 131, 963 – 972; f) D. P. Mills, O. J. Cooper, F. Tuna, E. J. L. McInnes, E. S. Davies, J. McMaster, F. Moro, W. Lewis, A. J. Blake, S. T. Liddle, J. Am. Chem. Soc. 2012, 134, 10047 – 10054; g) T. Cantat, N. Mzailles, L. Ricard, Y. Jean, P. Le Floch, Angew. Chem. 2004, 116, 6542 – 6545; Angew. Chem.
Chem. Eur. J. 2014, 20, 11295 – 11299
www.chemeurj.org
[10] [11]
[12] [13]
[14]
[15] [16] [17] [18]
[19]
Int. Ed. 2004, 43, 6382 – 6385; h) D. P. Mills, L. Soutar, W. Lewis, A. J. Blake, S. T. Liddle, J. Am. Chem. Soc. 2010, 132, 14379 – 14381. V. H. Gessner, F. Meier, D. Uhrich, M. Kaupp, Chem. Eur. J. 2013, 19, 16729 – 16739. a) P. Schrçter, V. H. Gessner, Chem. Eur. J. 2012, 18, 11223 – 11227; b) J. Becker, V. H. Gessner, Dalton Trans. 2014, 43, 4320 – 4325; c) V. H. Gessner, Acta Crystallogr. Sect. E 2012, 68, o1045. J. Becker, V. H. Gessner, Organometallics 2014, 33, 1310 – 1317. An analogous reaction has been reported for dppm-based ligands at elevated temperatures: a) T. Cantat, M. Demange, N. Mzailles, L. Ricard, Y. Jean, P. Le Floch, Organometallics 2005, 24, 4838 – 4841; b) H. Heuclin, X. F. Le Goff, N. Mzailles, Chem. Eur. J. 2012, 18, 16136 – 16144. a) Y. Smurnyy, C. Bibal, M. Pink, K. G. Caulton, Organometallics 2005, 24, 3849 – 3855; b) V. Cadierno, J. Dez, J. Garca- lvarez, J. Gimeno, Organometallics 2005, 24, 2801 – 2810. S. F. Vyboishchikov, G. Frenking, Chem. Eur. J. 1998, 4, 1428 – 1438. R. G. Cavell, R. P. K. Babu, K. Aparna, J. Organomet. Chem. 2001, 617 – 618, 158 – 169. E. Nicolas, B. Martin-Vaca, N. Mzailles, D. Bourissou, L. Maron, Eur. J. Inorg. Chem. 2013, 4068 – 4076. For reviews on the catalytic dehydrogenation of alcohols, see: a) C. Gunanathan, D. Milstein, Science 2013, 341, 249 – 260; b) A. Friedrich, S. Schneider, ChemCatChem 2009, 1, 72 – 73; c) G. E. Dobereiner, R. H. Crabtree, Chem. Rev. 2010, 110, 681 – 703. For examples of catalytic transfer hydrogenation, see: a) H. V zquezVilla, S. Reber, M. A. Ariger, E. M. Carreira, Angew. Chem. 2011, 123, 9141 – 9143; b) S.-F. Hsu, B. Plietker, Chem. Eur. J. 2014, 20, 4242 – 4245; c) T. Ohkuma, N. Utsumi, K. Tsutsumi, K. Murata, C. Sandoval, R. Noyori, J. Am. Chem. Soc. 2006, 128, 8724 – 8725; d) H. Brunner, W. Leitner, Angew. Chem. 1988, 100, 1231; Angew. Chem. Int. Ed. Engl. 1988, 27, 1180; e) S. Hashiguchi, A. Fuji, J. Takehara, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 1995, 117, 7562; f) J. R. Miecznikowski, R. H. Crabtree, Organometallics 2004, 23, 629.
Received: June 26, 2014 Published online on July 22, 2014
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