Organometallics 2010, 29, 1061–1063 DOI: 10.1021/om9010774
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On the Nature of the Formation of [(η5-Ind)Ru(CO)2H]: Evidence of a Water-Gas Shift Reaction Mechanism via Triethylamine Reduction Robert C. Badger, Jason S. D’Acchioli,* Tracey A. Oudenhoven and Brennan J. Walder Department of Chemistry, The University of Wisconsin;Stevens Point, 2001 Fourth Avenue, Stevens Point, Wisconsin 54481 Received December 16, 2009 Summary: The formation of [(η5-Ind)Ru(CO)2H] from [(η5-Ind)Ru(CO)3]þ was investigated using both 1H and 2H NMR spectroscopy and GC-mass spectrometry. Both NMR and GC-MS data support a water-gas shift reaction mechanism as the cause for formation of [(η5-Ind)Ru(CO)2H]. The chemical and electrochemical properties of 1þ (Figure 1) were the focus of a prior investigation.1 The chemical and electrochemical reduction of 1þ ultimately leads to 3. The pathway leading to formation of compound 3, however, depends on the reaction conditions. It was found that reduction of 1þ with the organic base triethylamine leads to formation of 2 prior to the formation of 3.1 Reduction of 1þ with cobaltocene under the same conditions, however, immediately formed 3. The existence of both 2 and 3 was verified by both 1H NMR and IR spectroscopy.1 Understanding the formation of 2 is especially important for the organometallic community. Species of the form [(η5aromatic)Ru(L)2X] (η5-aromatic = Cp, Ind; L = variety of phosphines, carbonyl ligands; X = halides) are known to be active in the transformation of small organic molecules.2 While some [(η5-aromatic)Ru(L)2X] complexes have known hydride intermediates, it is helpful to know the true source of the hydride and to determine whether these intermediates compete with catalytic transformations. There are a number of probable mechanisms for the formation of transition-metal hydride complexes. Indeed, hydride formation could proceed via H abstraction from a solvent,3 adventitious water in a water-gas shift (WGS) mechanism,4 or another molecule of 1þ. The mechanism of formation of 2 was studied utilizing a variety of synthetic and spectroscopic methods; they are described below. The most conspicuous spectroscopic handle for 2 is the hydride resonance in the 1H NMR spectra. The hydride chemical shift is -13.25 ppm in both d2-methylene chloride
Figure 1. Ruthenium complexes investigated.
(1) Badger, R. C.; D’Acchioli, J. S.; Gamoke, B. C.; Kim, S. B.; Oudenhoven, T. A.; Sweigart, D. A.; Tanke, R. S. Organometallics 2009, 28, 418–424. (2) See, for example: (a) Fung, W.; Huang, X.; Man, M.; Ng, S.; Hung, M. J. Am. Chem. Soc. 2003, 125, 11539–11544. (b) Ghosh, P.; Fagan, P. J.; Marshall, W. J.; Hauptman, E.; Bullock, R. M. Inorg. Chem. 2009, 48, 6490–6500. (c) Bartoszewicz, A.; Livendahl, M.; Martín-Matute, B. Chem. Eur. J. 2008, 14, 10547–10550. (d) Martín-Matute, B.; Åberg, J. B.; Edin, M.; B€ackvall, J.-E. Chem. Eur. J. 2007, 13, 6063–6072. (e) Martin-Matute, B.; Edin, M.; Bogar, K.; Kaynak, F. J. Am. Chem. Soc. 2005, 127, 8817–8825. (3) Zhang, J.; Grills, D.; Huang, K.; Fujita, E.; Bullock, R. J. Am. Chem. Soc. 2005, 127, 15684–15685. (4) For an excellent recent review of WGS reactions with transitionmetal carbonyl complexes, see: Esswein, A.; Nocera, D. Chem. Rev. 2007, 107, 4022–4047.
and d-chloroform. We originally believed the hydride could not be formed in the presence of adventitious water. Chemical reduction of 1þ with cobaltocene in a water-saturated methylene chloride solution did not lead to formation of 2.1 We subsequently believed that the hydride source for formation of 2 was from Et3N, since 2 only formed in the presence of Et3N, and that the most probable route for hydride formation was R-hydrogen abstraction from Et3N (Scheme 1). The resulting iminium cation would be resonance stabilized and would act as a counterion to 1þ’s tetrafluoroborate anion. In order to prove hydrogen abstraction from Et3N, a 2H NMR labeling study was performed. The compound d15Et3N was utilized in the reduction of 1þ. The 2H NMR spectrum of the reaction mixture, however, did not show a deuteride resonance. A deuteride resonance, however, would have been quite small relative to the d15-Et3N peaks and could very well have been obscured by baseline noise (see the Supporting Information, Figure S1). It was quite surprising, though, that a 1H NMR spectrum of the same solution confirmed the presence of hydride species 2 (Supporting Information, Figure S2). It became obvious at this point in our investigations that the hydride source was not from an indenyl ring proton (from 1þ), or from the residual protio impurity in the NMR solvents. The key to the mystery lay in 2’s formation in the presence of the basic Et3N. It is well-known that transition-metal carbonyl complexes can, in basic media, catalyze the so-called water-gas shift (WGS) reaction.4 Three criteria would be necessary to prove a water-gas shift reaction as a viable mechanistic candidate for the formation of 2 (Scheme 2). First, there would need to be trace amounts of water present during Et3N reduction.
r 2010 American Chemical Society
Published on Web 02/01/2010
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Scheme 1. Potential Reaction If an r-Hydrogen Abstraction Mechanism Is Followed
Scheme 2. Potential Reaction iIf a Water-Gas Shift Reaction Mechanism Is Followed
Second, the formation of 2 should occur in the presence of bases other than Et3N. Finally, the formation of CO2(g) should be observed. The aforementioned points are subsequently addressed (vide infra). Trace Water in the NMR Solvents. Fresh bottles of NMR solvent were opened under a stream of dry nitrogen. It was observed that compound 2 formed via Et3N reduction of 1þ in d2-methylene chloride, d-chloroform, and d6-acetone. However, when the NMR solvents were dried over freshly activated 3 A˚ molecular sieves, Et3N-induced formation of 2 was suppressed. Indeed, an experiment was performed where 1þ was reacted with Et3N in dry d2-methylene chloride (Supporting Information, Figure S3); no hydride peak was observed in the 1H NMR. When a drop of water was immediately added to the NMR tube, however, the hydride resonance appeared in the 1H spectra. The next most logical experiment was to try Et3N reduction of 1þ in the presence of D2O. The 2H NMR spectra, however, did not show any indication of a deuteride peak. Again, it is conceivable that such a small peak was lost in baseline noise. Formation of 2 in the Presence of Other Bases. The WGS reaction for transition-metal carbonyl compounds proceeds in the presence of bases.3 Since compound 2 formed in the presence of Et3N, two other bases were tested. The weak base NH4OH was observed to initiate the formation of 2 in CHCl3 (see the Supporting Information, Figure S4). The weak base diethylamine also initiated the formation of 2 in d6-acetone (see the Supporting Information, Figure S5). Formation of CO2 Gas: Detection via GC-MS. The most conclusive proof for a WGS reaction mechanism would be detection of CO2(g) from the reaction mixture. There is literature precedence for using mass spectrometry in detection of the WGS reaction, often coupled with other spectroscopic techniques such as IR spectroscopy.5 Reactions of 1þ with Et3N were carried out in septa-capped LPV NMR tubes in an Ar(g)-filled glovebag (see the Supporting Information for details). All reactions were carried out in protioacetone that had been freeze-pump-thawed three times and subsequently back-filled with Ar(g). Two drops of argon-degassed (5) See, for example: (a) Goguet, A.; Meunier, F. C.; Tibiletti, D.; Breen, J. P.; Burch, R. J. Phys. Chem. B. 2004, 108, 20240–20246. (b) Kalamaras, C. M.; Olympiou, G. G.; Efstathiou, A. M. Catal. Today 2008, 138, 228–234. (c) Meunier, F. C.; Tibiletti, D.; Goguet, A.; Shekhtman, S.; Hardacre, C.; Burch, R. Catal. Today 2007, 126, 143–147. (d) Olympiou, G. G.; Kalamaras, C. M.; Zeinalipour-Yazdi, C. D.; Efstathiou, A. M. Catal. Today 2007, 127, 304–318.
water were added to the reaction mixtures. An Ar(g) (molar mass 39.95 g/mol) atmosphere was chosen so as not to interfere with gases liberated during the reaction, since a N2(g) (molar mass 28.01 g/mol) m/z peak would mask a CO (g) (molar mass 28 g/mol) m/z peak. Gases from the septa-capped LPV NMR tubes were collected with a gastight syringe prior to addition of Et3N, upon addition of Et3N, and 30 min after addition of Et3N. No peak at m/z 44 was observed prior to Et3N addition. After Et3N addition, however, the growth of a peak at m/z 44 was observed (Supporting Information, Figure S6), indicating the formation of CO2(g). A second reaction using cobaltocene as a reducing agent was carried out under the aforementioned conditions. Gases from the septa-capped LPV NMR tubes were again collected with a gastight syringe prior to addition of cobaltocene, upon addition of cobaltocene, and 30 min after addition of cobaltocene (Supporting Information, Figure S7). While no peak was observed at m/z 44, a peak at m/z 28 grew in after addition of cobaltocene. The growth of the m/z 28 peak is expected, since dimerization of 1þ to 3 liberates CO(g). It has been shown that compound 1þ is involved in a WGS reaction when reduced with the weak organic base Et3N in the presence of water. We initially believed Et3N was involved in an R-hydrogen transfer to 1þ. Reduction of 1þ with d15-Et3N, however, did not show evidence of a deuteride in the 2H NMR spectrum. GC-mass spectrometry on gas samples taken from above the reactions of 1þ with Et3N and cobaltocene showed growth of peaks at m/z 44 for the former and m/z 28 for the latter. The peak at m/z 44; resulting from Et3N reduction;is consistent with liberation of CO2(g) and supports a WGS reaction mechanism. The peak at m/z 28;resulting from cobaltocene reduction;is consistent with liberation of CO(g), which would be expected when 1þ dimerizes to 3.
Acknowledgment. J.S.D. thanks Dr. Aaron Wilson for fruitful discussions regarding the WGS reaction. J.S.D. also thanks Professor Dwight Sweigart at Brown University for fruitful discussions on hydride abstraction. B.J.W. and T.A.O. thank Professor Laura Cole for help in acquiring GC-mass spectral data. J.S.D., B.J.W., and T.A.O. thank Professors William F. Wacholtz and Brant Kredowski from UW-Oshkosh for help acquiring NMR spectra and Professor Nate Bowling from UWSP for stimulating discussions. J.S.D. thanks Jim Tuszka for assistance tuning the
Communication
NMR. J.S.D. thanks the UWSP College of Letters and Science (CoLS) for startup funds and UWSP for a UPDC grant. T.A.O. and B.J.W. thank the CoLS for stipends under the Undergraduate Education Initiative. Note Added after ASAP Publication. In the version of this paper published on the web on Feb 1, 2010, the wrong molar
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masses of N2 and CO were given. In the version of the paper that appears as of Feb 10, 2010, the masses are correct. Supporting Information Available: Text and figures giving all H and 2H NMR spectra, GC-MS spectra, and experimental conditions. This material is available free of charge via the Internet at http://pubs.acs.org. 1
