Organometallics 2004, 23, 4349-4356
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Articles Reactions of the Transient Species W(CO)5(Cyclohexane) with Thiophene and Tetrahydrothiophene Studied by Time-Resolved Infrared Absorption Spectroscopy Richard H. Schultz† Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Received June 7, 2004
The transient species W(CO)5(CyH) (CyH ) cyclohexane) is prepared by laser photolysis of a CyH solution of W(CO)6. Time-resolved infrared spectroscopy is used to probe the ligand substitution reactions of W(CO)5(CyH) with thiophene and tetrahydrothiophene (THT). From the temperature dependence of the second-order rate constants derived from the kinetic study, we obtain for reaction with thiophene ∆Hq ) 5.7 ( 0.1 kcal mol-1 and ∆Sq ) -11.2 ( 1.1 eu, while for reaction with THT, ∆Hq ) 2.6 ( 0.1 kcal mol-1 and ∆Sq ) -15.5 ( 0.8 eu. As expected, THT, the stronger electron donor, is the more reactive ligand of the two. THT is much more reactive than would be expected based on its strength as an electron donor, however. We show that the relative reactivities of O-, N-, and S-containing five-membered heterocycles toward ligand substitution at W(CO)5(CyH) can be accounted for by a model that takes into account the attacking ligand’s polarizability in addition to its electron-donating ability. Introduction Concerns about negative environmental effects arising from emissions of sulfur-containing species during combustion of fossil fuels have led to a significant effort toward development of efficient hydrodesulfurization (HDS) catalysts to reduce and remove sulfur from the fuel prior to combustion. The systems being developed as commercial HDS catalysts tend to be based on transition metals.1 The development of practical HDS catalysts has thus seen a parallel research effort aimed at understanding at a more fundamental level the principles that guide the reactivity of transition-metal complexes with sulfur-containing organic species. In particular, many research groups have been working toward the goal of finding transition-metal systems that can perform homogeneous HDS under mild conditions in solution. Most of the systems thus far studied have used thiophene and its derivatives as models for the more complicated (and chemically heterogeneous) environment found in actual fossil fuel processing. In the course of the development of this chemistry, numerous † E-mail:
[email protected]. Fax: +972-3-535-1250. (1) Recent reviews include, inter alia: (a) Breysse, M.; DjegaMariadassou, G.; Pessayre, S.; Geantet, C.; Vrinat, M.; Perot, G.; Lemaire, M. Catal. Today 2003, 84, 129. (b) Oyama, S. T. J. Catal. 2003, 216, 343. (c) Song, C.; Ma, X. Appl. Catal. B 2003, 41, 207. (d) Furimsky, E. Appl. Catal. A 2003, 240, 1. (e) Chianelli, R. R.; Berhault, G.; Raybaud, P.; Kasztelan, S.; Hafner, J.; Toulhoat, H. Appl. Catal. A 2002, 227, 83. (f) Brorson, M.; King, J. D.; Kiriakidou, K,; Prestopino, F.; Nordlander, E. In Metal Clusters in Chemistry; Braunstein, P., Oro, L. A., Raithby, P. R., Eds.; Wiley-VCH: Weinheim, Germany, 1999; Vol. 2, p 741. (g) Rodriguez, J. A.; Hrbek, J. Acc. Chem. Res. 1999, 32, 719. (h) Whitehurst, D. D.; Isoda, T.; Mochida, I. Adv. Catal. 1998, 42, 345.
modes of reactivity have been found. Complexes have been developed in which a metal atom can insert into a C-S2-6 or C-H5a,i,7 bond of thiophene or one of its derivatives. In addition, numerous complexes are now known that can, under the proper conditions, reduce or desulfurize thiophenes in solution.6b,8-11 In addition to these studies of the reactivity of transition-metal complexes toward thiophene and related compounds, considerable research effort has gone into the more basic problem of the behavior of thiophene (2) (a) Chen, J.; Angelici, R. A. Organometallics 1990, 9, 894. (b) Chen, J.; Daniels, L. M.; Angelici, R. A. J. Am. Chem. Soc. 1991, 113, 2544. (c) Reynolds, M. A.; Guzei, I. A.; Angelici, R. A. Chem. Commun. 2000, 513. (3) (a) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Moneti, S.; Herrera, V.; Sa´nchez-Delgado, R. A. J. Am. Chem. Soc. 1994, 116, 4370. (b) Bianchini, C.; Jime´nez, M. V.; Meli, A.; Vizza, F. Organometallics 1995, 14, 3196. (c) Bianchini, C.; Herrera, V.; Jime´nez, M. V.; Laschi, F.; Meli, A.; Sa´nchez-Delgado, R.; Vizza, F.; Zanello, P. Organometallics 1995, 14, 4390. (d) Bianchini, C.; Jime´nez, M. V.; Meli, A.; Moneti, S.; Vizza, F. Inorg. Chim. Acta 1998, 272, 55. (4) Smith, V. C. M.; Aplin, R. T.; Brown, J. M.; Hursthouse, M. B.; Karalulov, A. I.; Abdul Malik, K. M.; Cooley, N. A. J. Am. Chem. Soc. 1994, 116, 5180. (5) (a) Jones, W. D.; Dong, L. J. Am. Chem. Soc. 1991, 113, 559. (b) Dong, L.; Duckett, S. B.; Ohman, K. F.; Jones, W. D. J. Am. Chem. Soc. 1992, 114, 151. (c) Jones, W. D.; Chin, R. M. J. Am. Chem. Soc. 1992, 114, 9851. (d) Jones, W. D.; Chin, R. M. Organometallics 1992, 11, 2698. (e) Jones, W. D.; Chin, R. M. J. Organomet. Chem. 1994, 472, 311. (f) Myers, A. W.; Jones, W. D. Organometallics 1996, 15, 2905. (g) Jones, W. D.; Chin, R. M.; Hoaglin, C. L. Organometallics 1999, 18, 1786. (h) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1999, 121, 7606. (i) Chantson, J.; Gorls, H.; Lots, S. J. Organomet. Chem. 2003, 687, 39. (6) (a) Yu, K.; Li, H.; Watson, E. J.; Virkaitis, K. L.; Carpenter, G. B.; Sweigart, D. A. Organometallics 2001, 20, 3550. (b) Yu, K.; Li, H.; Watson, E. J.; Virkaitis, K. L.; D’Acchioli, J. S.; Carpenter, G. B.; Sweigart, D. A.; Czech, P. T.; Overly, K. R.; Coughlin, F. Organometallics 2002, 21, 1262.
10.1021/om049588z CCC: $27.50 © 2004 American Chemical Society Publication on Web 08/14/2004
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and its derivatives as ligands independent of any subsequent reactions they may undergo at transitionmetal centers. These ligands have been found to exhibit a rich and varied chemistry.12,13 For example, η1, η2, η4, and η5 binding modes have all been observed for thiophene,12 and ligands such as benzothiophene (BT) or dibenzothiophene (DBT) can bind η1 through the S atom, η6 through a benzene ring, or η2 through two adjacent C atoms in a benzene or thiophene ring.14 In fact, in room-temperature solution, the complexes CpRe(CO)2(BT) and Cp*Re(CO)2(BT) are actually equilibrium mixtures of the η1(S)-BT and 2,3-η2-BT linkage isomers.15 In our laboratory, we have been concerned primarily with the kinetics of the interaction between a coordinatively unsaturated transition-metal intermediate and an incoming ligand L to form a more stable complex and how the properties of the ligand and of the intermediate affect the reactivity. We have performed, inter alia, a series of studies of the reactions of the cyclohexanesolvated intermediate W(CO)5(CyH) (CyH ) a molecule of the cyclohexane solvent) with five-membered heterocyclic molecules containing oxygen16-18 or nitrogen.19 We observed that in these systems, as the incoming ligand becomes more electron-donating, ∆Hq (and ∆Gq at temperatures around room temperature) for the sub(7) (a) Bianchini, C.; Jime´nez, M. V.; Meli, A.; Moneti, S.; Vizza, F. J. Organomet. Chem. 1995, 504, 27. (b) Stafford, P. R.; Rauchfuss, T. B.; Wilson, S. R. Inorg. Chem. 1995, 34, 5220. (c) Bianchini, C.; Casares, J. A.; Osman, R.; Pattison, D. I.; Peruzzini, M.; Perutz, R. N.; Zanobini, F. Organometallics 1997, 16, 4611. (d) Paneque, M.; Poveda, M. L.; Salazar, V.; Taboada, S.; Carmona, E.; Gutie´rrez-Puebla, E.; Monge, A.; Ruiz, C. Organometallics 1999, 18, 139. (8) (a) Eisch, J. J.; Im, K. R. J. Organomet. Chem. 1977, 139, C51. (b) Angelici, R. J. Acc. Chem. Res. 1988, 21, 387. (9) (a) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Frediani, P.; Herrera, V.; Sa´nchez-Delgado, R. A. J. Am. Chem. Soc. 1993, 115, 2731. (b) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Herrera, V.; Sa´nchez-Delgado, R. A. Organometallics 1994, 13, 721. (c) Bianchini, C.; Frediani, P.; Herrera, V.; Jime´nez, M. V.; Meli, A.; Rinco´n, L.; Sa´nchez-Delgado, R.; Vizza, F. J. Am. Chem. Soc. 1995, 117, 4333. (d) Bianchini, C.; Jime´nez, M. V.; Meli, A.; Moneti, S.; Vizza, F.; Herrera, V.; Sa´nchez-Delgado, R. A. Organometallics 1995, 14, 2342. (e) Bianchini, C.; Fabbri, D.; Gladiali, S.; Meli, A.; Pohl, W.; Vizza, F. Organometallics 1996, 15, 4604. (f) Herrera, V.; Fuentes, A.; Rosales, M.; Sa´nchez-Delgado, R.; Bianchini, C.; Meli, A.; Vizza, F. Organometallics 1997, 16, 2465. (g) Bianchini, C.; Meli, A.; Moneti, S.; Oberhauser, W.; Vizza, F.; Herrera, V.; Fuentes, A.; Sa´nchez-Delgado, R. A. J. Am. Chem. Soc. 1999, 121, 7071. (10) (a) Rosini, G. P.; Jones, W. D. J. Am. Chem. Soc. 1992, 114, 10767. (b) Jones, W. D.; Chin, R. M. J. Am. Chem. Soc. 1994, 116, 198. (c) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1997, 119, 10855. (d) Vicic, D. A.; Jones, W. D. Organometallics 1998, 17, 3411. (11) (a) Frediani, P.; Salvini, A.; Finocchiaro, S. J. Organomet. Chem. 1999, 584, 265. (b) Janak, K. E.; Tanski, J. M.; Churchill, D. G.; Parkin, G. J. Am. Chem. Soc. 2002, 124, 4182. (c) Borowski, A. F.; SaboEtienne, S.; Donnadieu, B.; Chaudret, B. Organometallics 2003, 22, 4803. (d) Che´rioux, F.; Therrien, B.; Su¨ss-Fink, G. Chem. Commun. 2004, 204. (12) (a) Rauchfuss, T. B. Prog. Inorg. Chem. 1991, 39, 259. (b) Harris, S. Organometallics 1994, 13, 2628. (c) Harris, S. Polyhedron 1997, 16, 3219. (d) Angelici, R. J. Organometallics 2001, 20, 1259. (13) (a) Sanger, M. J.; Angelici, R. J. Organometallics 1994, 13, 1821. (b) Mills, P.; Korlann, S.; Bussell, M. E.; Reynolds, M. A.; Ovichinnikov, M. V.; Angelici, R. J.; Stinner, C.; Weber, T.; Prins, R. J. Phys. Chem. A 2001, 105, 4418. (14) (a) Choi, M.-G.; Robertson, M. J.; Angelici, R. J. Inorg. Chem. 1991, 113, 4005. (b) Rao, K. M.; Day, C. L.; Jacobson, R. A.; Angelici, R. J. Inorg. Chem. 1991, 30, 5046. (15) Choi, M.-G.; Angelici, R. J. Organometallics 1992, 11, 33283334. (16) Paur-Afshari, R.; Lin, J.; Schultz, R. H. Organometallics 2000, 19, 1682. (17) Lugovskoy, A.; Paur-Afshari, R.; Schultz, R. H. J. Phys. Chem. A 2000, 104, 10587. (18) Krishnan, R.; Schultz, R. H. Organometallics 2001, 20, 3314. (19) Lugovskoy, A.; Shagal, A.; Lugovskoy, S.; Huppert, I.; Schultz, R. H. Organometallics 2003, 22, 2273.
Schultz
stitution decreases, unless specific steric factors intervene.18 In this report, we extend these studies to include reactions of two S-containing ligands, thiophene and tetrahydrothiophene (THT). Our goal was to see if the trends we observed for reaction partners containing first-row heterocyclic atoms would still occur with ligands containing a second-row heteroatom, and whether the model we developed in our previous studies can be extended to include the second-row analogues of furan and THF. Experimental Section The spectrometer on which these experiments were performed has been described in detail previously,16 so only a brief description will be given here. A CyH solution containing (0.51) × 10-3 mol L-1 W(CO)6 and an at least 10-fold excess of the reaction partner L is prepared. The solution flows through a CaF2 IR cell (0.5 mm path length, held to within (0.3 °C of the nominal reaction temperature), where reaction is initiated by the output of a pulsed XeCl excimer laser (308 nm, ∼20 ns/pulse, typically 2-5 Hz and 60-80 mJ/pulse). To ensure that each photolysis pulse irradiates fresh solution, the solution flows continuously through the cell. The UV flash leads to loss of a CO ligand and formation, within the photolysis pulse,20 of the solvated W(CO)5(CyH) intermediate. The consequent reaction of this intermediate is then monitored by following the time evolution of the solution’s IR absorption. To determine the C-O stretching frequencies of the species present during the course of the reaction, we use a step-scan FTIR (S2FTIR) spectrometer, which enables collection of the time-resolved IR spectrum with sub-microsecond time resolution. In the particular case of the experiments discussed here, the S2FTIR was run at 4 cm-1 spectral resolution and with time steps of 200 ns. The temperature-dependent reaction kinetics of W(CO)5(CyH) are monitored by using a continuously tunable CW Pb-salt diode laser as the IR source. This source is tuned to a specific frequency corresponding to a C-O stretching absorption of W(CO)5(CyH) or of W(CO)5L, and the time dependence of the IR signal impinging upon an InSb detector (ca. 50 ns rise time) following the UV pulse is recorded. The raw IR signal is then converted to the change in absorbance ∆A, and pseudo-first-order rate constants kobs are determined by a linear fit to ln|∆A∞ - ∆A0|. The values of kobs reported here were determined from the exponential decay of the 1954 cm-1 W(CO)5(CyH) absorbance. In the case of reaction with thiophene, because of overlap of the peak of the W(CO)5(CyH) absorbance with a product C-O stretch, the best results were obtained when the laser was tuned ∼2 cm-1 to the blue of the W(CO)5(CyH) absorbance peak. In some cases, the time dependence of the rise of the absorbance corresponding to W(CO)5L was measured as well. The value of kobs for growth of the product was found to be identical to that for the decay of the W(CO)5(CyH) absorbance to within experimental error. The values of kobs reported in Tables S-1 and S-2 represent averages of at least two independent measurements made at each temperature and concentration. CyH was obtained in HPLC grade and distilled from Na/ benzophenone. The concentration of benzophenone in the distilled solvent was confirmed by UV-vis spectroscopy to be 99% purity (confirmed by NMR spectroscopy) and used without further purification except for storage over molecular sieves to remove any residual H2O.
Results W(CO)5(CyH) + Thiophene. S2FTIR results for the reaction of W(CO)5(CyH) are shown in Figure 1. The intermediate decays cleanly to a single product with two strong C-O stretches at 1950 and 1933 cm-1 and a much weaker one at 2080 cm-1. We identify this product as W(CO)5(thiophene), produced in reaction 1:
W(CO)5(CyH) + thiophene f W(CO)5(thiophene) + CyH (1) To our knowledge, the sole report of this species that has so far appeared in the literature is an IR study from 1965,22 in which the C-O stretching frequencies for W(CO)5(thiophene) at 20 °C in n-hexane were reported to be 2085, 1953, and 1937 cm-1, quite similar to the frequencies reported in the present work (Table 1). The authors of the 1965 study also reported that at -180 °C in a 1:4 isopentane/methylcyclohexane mixture the C-O stretches of W(CO)5(thiophene) appear at 2092, 1952, and 1921 cm-1, but that upon warming to about -110 °C, the peaks shift to those of the room-temperature species. They proposed that the two spectra reflected different structures and hypothesized that at low temperature they had trapped W(CO)5(η5-thiophene), which converted to the η1 structure as the temperature was raised. In our experiment, we found no evidence for the existence at room temperature of more than one product appearing on a microsecond time scale. Efforts to synthesize a sufficiently large quantity of W(CO)5(21) Spies, G. H.; Angelici, R. J. Organometallics 1987, 6, 1897. This procedure demonstrates that while the label of the commercially supplied sample indicates that thiophene has a “stench”, the observed foul odor actually arises from the alkanethiols present as
