This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Communication pubs.acs.org/Organometallics
An Air-Stable Dimeric Ru−S Complex with an NHC as Ancillary Ligand for Cooperative Si−H Bond Activation Susanne Baḧ r, Antoine Simonneau,† Elisabeth Irran, and Martin Oestreich* Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany S Supporting Information *
ABSTRACT: The preparation of a coordinatively unsaturated NHC ruthenium complex with a tethered 2,6-dimesitylphenyl thiolate is described. Unlike related mononuclear phosphine complexes, the NHC complex forms an air-stable dimer with bridging sulfur ligands in both the solid state and solution. The dinuclear complex is a precatalyst and activates Si−H bonds upon heat- or donor-assisted dissociation into the catalytically active monomer. Its usefulness is illustrated with representative dehydrogenative Si−X coupling and hydrosilylation reactions.
T
couplings,11,12 chemoselective hydrosilylations,13,14 and hydrodefluorination.15 However, none of these complexes are air stable. We report here the preparation and unexpected formation of the dimeric tethered ruthenium complex 4 with an NHC as an ancillary ligand (Figure 1, bottom right). Complex 4 is air stable while retaining catalytic activity in Si−H bond activation similar to 3. In earlier work, we had found that the phosphine ligand in 3 had significant influence not only on yields and selectivities12,14 but also on the nature of intermediates.15 This ancillary ligand effect prompted us to explore the ability of other donors to stabilize the ruthenium center and possibly alter its properties. Hence, replacement of the phosphine by an N-heterocyclic carbene (NHC) ligand seemed to be a good starting point. 1,3,4,5-Tetramethylimidazol-2-ylidene (5; Figure 1, bottom right) was chosen as a proxy due to its low steric demand.16 Following the established procedure for the preparation of 3,9c the free carbene 5 was added to the coordinatively unsaturated ruthenium complex 6 in toluene to afford 7 in 89% isolated yield (6 → 7, Scheme 1). Complex 7 was fully characterized by NMR spectroscopy and X-ray diffraction (see the Supporting Information). Chloride abstraction from 7 with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate in CH2Cl2 resulted in a color change from red to blue (7 → 8); this is, in analogy with the phosphine series, usually a good indication of the generation of the cationic complexes. However, monomeric 8 was elusive, and another color change from blue to brown occurred as the reaction progressed. After evaporation of the volatiles, a brown solid formed, and suitable crystals for X-ray crystallography were obtained by slow evaporation from CH2Cl2 (Figure 2). This revealed that monomeric 8 had dimerized to quantitatively yield 4 (8 → 4, Scheme 1).17 The molecular structure of 4 shows two sulfurbridged ruthenium centers with the NHC ligands in an almost
he activation of dihydrogen at the dinuclear, sulfurbridged [FeNi] center of hydrogenases1 inspired investigations into cooperative (reversible) H−H bond cleavage at metal−sulfur bonds.2 Notable advancements were made with dinuclear sulfide complexes ([Rh2],3 [Ir2],4 [IrW],5 [IrMo],5 and [Ru2]6), but the number of examples remains limited. The conceptually related Si−H bond activation at metal−sulfur sites of monomeric complexes was achieved with compounds 1 and 2 reported by Bergman7 and Stradiotto,8 respectively (Figure 1,
Figure 1. Metal−sulfur complexes for Si−H bond activation. ArF = 3,5-bis(trifluoromethyl)phenyl.
left). Ohki and Tatsumi introduced iridium and rhodium (not shown) as well as tethered ruthenium thiolate complexes 3 for dihydrogen activation (Figure 1, top right),9 and we later accomplished in collaboration with them the heterolytic splitting of hydrosilanes with 3.10 Complexes 3 are broadly applicable catalysts, enabling unusual dehydrogenative Si−X © 2016 American Chemical Society
Received: February 10, 2016 Published: March 24, 2016 925
DOI: 10.1021/acs.organomet.6b00110 Organometallics 2016, 35, 925−928
Communication
Organometallics Scheme 1. Preparation of the Air-Stable Dimer
Scheme 2. Dehydrogenative Si−X Coupling Reactions
a
Performed with 1.0 equiv of EtMe2SiH; see ref 12a. bPerformed with 1.0 equiv of Me2PhSiH; see ref 11a. cSee ref 11c.
We began with dehydrogenative Si−X coupling reactions (Scheme 2). Treatment of enolizable 9 with EtMe2SiH in the presence of catalyst 3a would usually afford silyl enol ether 10 chemoselectively within minutes.12a However, both 4 and 8 required hours to reach full conversion, and 10 was formed along with silyl ether 11; 8 was nevertheless markedly more reactive than 4 (Scheme 2a). The situation was similar in the Friedel−Crafts-type silylation of indole (12 → 13, Scheme 2b). Several hours at 80 °C were necessary with 4 and 8 as catalysts, while a few hours at room temperature were enough with 3a.11a We note here that, in contrast to the above transformation at room temperature, thermal activation of 4 and heating of 8 was vital for the catalysis to proceed. An intramolecular version of this reaction is also catalyzed by 4 and 8, but yields are lower than with 3d as catalyst11c (14 → 15, Scheme 2c). To complete our panel of test reactions, we had a look at hydrosilylations (Scheme 3). We exposed Et3SiH to catalytic amounts of 4 and 8, respectively, under CO2 pressure at 80 °C (Scheme 3a). As expected, the reaction was faster with 8, and full consumption of the hydrosilane was observed after 3 h, whereas 4 required 24 h under an otherwise identical setup. The in situ generated NHC complex 8 was as reactive as the phosphine complex 3a.14 These three hydrosilylations stopped selectively at the formaldehyde oxidation level, that is, bis(silyl)acetal 17, but 4 and 8 did not fully convert the intermediate silyl formate 16 into 17. We also examined the chemo- and regioselective hydrosilylation of pyridine with 4 and 8 as catalysts (19 → 20, Scheme 3b). Both 4 and 8 performed equally well in terms of selectivity, and isolated yields of the 1,4-dihydropyridine compared well with that
Figure 2. Molecular structure of 4 with thermal ellipsoids at the 50% probability level. Counteranions and hydrogen atoms are omitted for clarity.
coplanar arrangement, most likely due to π−π interactions.18 The Ru−S bonds range from 2.416 to 2.449 Å, and these are substantially longer than that of the phosphine analogue 3a (2.212 Å).9c The Ru−Ru distance of 3.673 Å excludes metal− metal bonding.19 Complex 4 remains dimeric in solution, as evidenced by an 1 H NMR spectrum with 10 chemically inequivalent methyl groups. This was further confirmed by an NOE experiment where transfer of spin polarization was observed between protons positioned on the two different tethering thiolate ligands (see the Supporting Information for assignment and a NOESY spectrum). These measurements are in accordance with the solid-state structure. We next tested the ability of dimer 4 to promote the activation of Si−H bonds in several representative catalyses (experiments i). Moreover, we prepared monomer 8 in situ from 7 and NaBArF4 and used it as the catalyst in the same set of reactions (experiments ii). The results obtained with 4 and 8 were compared with those achieved with the optimal phosphine complex 3 (experiments iii). The data are summarized in Schemes 2 and 3. 926
DOI: 10.1021/acs.organomet.6b00110 Organometallics 2016, 35, 925−928
Organometallics
■
Scheme 3. Hydrosilylation Reactions
Communication
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.O.:
[email protected]. Present Address †
Laboratoire de Chimie de Coordination (LCC), CNRS, 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France. Notes
The authors declare no competing financial interest.
■ a
ACKNOWLEDGMENTS S.B. thanks the Studienstiftung des deutschen Volkes for a predoctoral fellowship (2015−2017), and A.S. gratefully acknowledges the Alexander von Humboldt Foundation for a postdoctoral fellowship (2014−2015). M.O. is indebted to the Einstein Foundation (Berlin) for an endowed professorship. We thank Sebastian Kemper (TU Berlin) for expert advice with the NMR measurements.
See ref 14. bSee ref 13.
■
obtained with 3a.13 No overreduction or formation of the 1,2dihydropyridine 21 was detected. The general observation from the above sets of reactions is that dimer 4 is far less reactive than in situ generated monomer 8. This strongly indicates that robust 4 must break into 8 with a vacant coordination site prior to the Si−H bond activation event. 1H NMR spectroscopic measurements confirmed that 4 and hydrosilanes do not react with each other at room temperature. The reversible dissociation of 4 into 8 is achieved thermally (see Scheme 2b), as indicated by VT 1H NMR experiments (see the Supporting Information). If the substrate is sufficiently Lewis basic, it will assist catalyst dissociation at ambient temperature, thereby allowing room-temperature reactions (see Schemes 2a and 3b). For example, 1H NMR experiments show that pyridine converts 4 partially into the adduct 8·py (see the Supporting Information). NHC complex 8 is less reactive than the phosphine complexes 3, but we cannot draw any conclusions from this, as ligand effects are potentially antagonistic in the various steps of the catalytic cycles.15 We reported here the air-stable Ru−S complex 4, formed by dimerization of the coordinatively unsaturated NHC ruthenium complex 8. Such aggregation was not observed for the related phosphine ruthenium complexes 3.9c,10 The dicationic dimer 4 serves as a precatalyst for Si−H bond activation by thermal or donor-assisted dissociation into the catalytically active cationic monomer 8. We demonstrated the practicality of 4 and also in situ generated 8 in several dehydrogenative Si−X coupling and hydrosilylation reactions. While dinuclear 4 is less reactive than mononuclear 3, its robustness is an advantage in handling. We are not aware of other air-stable metal−sulfur complexes active in cooperative Si−H bond activation.
■
REFERENCES
(1) (a) Vignais, P. M.; Billoud, B. Chem. Rev. 2007, 107, 4206−4272. (b) Fontecilla-Camps, J. C.; Volbeda, A.; Cavazza, C.; Nicolet, Y. Chem. Rev. 2007, 107, 4273−4303. (2) For a recent review of metal−ligand cooperativity, see: Khusnutdinova, J. R.; Milstein, D. Angew. Chem., Int. Ed. 2015, 54, 12236−12273. (3) Bianchini, C.; Mealli, C.; Meli, A.; Sabat, M. Inorg. Chem. 1986, 25, 4617−4618. (4) Linck, R. C.; Pafford, R. J.; Rauchfuss, T. B. J. Am. Chem. Soc. 2001, 123, 8856−8857. (5) Kato, H.; Seino, H.; Mizobe, Y.; Hidai, M. J. Chem. Soc., Dalton Trans. 2002, 1494−1499. (6) Sellmann, D.; Prakash, R.; Heinemann, F. W.; Moll, M.; Klimowicz, M. Angew. Chem., Int. Ed. 2004, 43, 1877−1880. (7) (a) Sweeney, Z. K.; Polse, J. L.; Andersen, R. A.; Bergman, R. G.; Kubinec, M. G. J. Am. Chem. Soc. 1997, 119, 4543−4544. (b) Sweeney, Z. K.; Polse, J. L.; Bergman, R. G.; Andersen, R. A. Organometallics 1999, 18, 5502−5510. (8) Hesp, K. D.; McDonald, R.; Ferguson, M. J.; Stradiotto, M. J. Am. Chem. Soc. 2008, 130, 16394−16406. (9) For iridium and rhodium complexes, see: (a) Ohki, Y.; Sakamoto, M.; Tatsumi, K. J. Am. Chem. Soc. 2008, 130, 11610−11611. (b) Sakamoto, M.; Ohki, Y.; Kehr, G.; Erker, G.; Tatsumi, K. J. Organomet. Chem. 2009, 694, 2820−2824. For the preparation of tethered ruthenium complexes, see: (c) Ohki, Y.; Takikawa, Y.; Sadohara, H.; Kesenheimer, C.; Engendahl, B.; Kapatina, E.; Tatsumi, K. Chem. - Asian J. 2008, 3, 1625−1635. (10) Stahl, T.; Hrobárik, P.; Königs, C. D. F.; Ohki, Y.; Tatsumi, K.; Kemper, S.; Kaupp, M.; Klare, H. F. T.; Oestreich, M. Chem. Sci. 2015, 6, 4324−4334. (11) (a) Klare, H. F. T.; Oestreich, M.; Ito, J.-i.; Nishiyama, H.; Ohki, Y.; Tatsumi, K. J. Am. Chem. Soc. 2011, 133, 3312−3315. (b) Königs, C. D. F.; Müller, M. F.; Aiguabella, N.; Klare, H. F. T.; Oestreich, M. Chem. Commun. 2013, 49, 1506−1508. (c) Omann, L.; Oestreich, M. Angew. Chem., Int. Ed. 2015, 54, 10276−10279. (12) (a) Königs, C. D. F.; Klare, H. F. T.; Oestreich, M. Org. Lett. 2012, 14, 2842−2845. (b) Hermeke, J.; Klare, H. F. T.; Oestreich, M. Chem. - Eur. J. 2014, 20, 9250−9254. (13) Königs, C. D. F.; Klare, H. F. T.; Oestreich, M. Angew. Chem., Int. Ed. 2013, 52, 10076−10079. (14) Metsänen, T. T.; Oestreich, M. Organometallics 2015, 34, 543− 546. (15) Stahl, T.; Klare, H. F. T.; Oestreich, M. J. Am. Chem. Soc. 2013, 135, 1248−1251.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00110. Synthetic procedures and NMR spectra of the compounds synthesized in this paper, as well as analytical data for the unknown compounds (PDF) Crystallographic data (CIF) 927
DOI: 10.1021/acs.organomet.6b00110 Organometallics 2016, 35, 925−928
Communication
Organometallics (16) (a) Kuhn, N.; Kratz, T. Synthesis 1993, 561−562. (b) Ansell, M. B.; Roberts, D. E.; Cloke, F. G. N.; Navarro, O.; Spencer, J. Angew. Chem., Int. Ed. 2015, 54, 5578−5582. (17) Ohki and Tatsumi had reported on the formation of a dimeric complex similar to 4 with acetophenone (9) instead of 5 as ligand.9c This particular complex was isolated as decomposed material and turned out to be inactive in the hydrogenation of 9. (18) Intramolecular π stacking in organometallic compounds is rare but known for aromatic ligands: (a) Magistrato, A.; Pregosin, P. S.; Albinati, A.; Rothlisberger, U. Organometallics 2001, 20, 4178−4184 and references cited therein. Distances for offset or slipped stacking range from 3.3 to 3.8 Å; see: (b) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 21, 3885−3896. The distance of the centroids of the NHC ligands is 3.6 Å. To the best of our knowledge, π−π interactions of NHC ligands in organometallic compounds have not been reported so far. (19) Reported Ru−Ru single bonds range from 2.6 to 2.9 Å; see: Cameron, B. R.; Bridger, G. J.; Maresca, K. P.; Zubieta, J. Inorg. Chem. 2000, 39, 3928−3930 and references cited therein.
928
DOI: 10.1021/acs.organomet.6b00110 Organometallics 2016, 35, 925−928
