Femtosecond Infrared Studies of a Prototypical One-Electron Oxidative-Addition Reaction: Chlorine Atom Abstraction by the Re(CO)5 Radical

One-Electron Oxidative-Addition Reaction: Chlorine Atom Abstraction by the Re(CO)5 Radical Haw Yang,1 Preston T. Snee,1 Kenneth T. Kotz,1 Christine K. Payne,1 Heinz Frei,2 and Charles B. Harris*,1 Department of Chemistry, UniVersity of California Berkeley, California 94720 MS CalVin Laboratory, Ernest Orland Lawrence Berkeley National Laboratory, Berkeley, California 94720 ReceiVed May 20, 1999 ReVised Manuscript ReceiVed July 30, 1999 Oxidative-addition is one class of fundamental reactions of organometallic complexes. A comprehensive understanding of the mechanism requires knowledge of the dynamics of all the intermediates. The extremely fast reaction rates, however, have made it experimentally challenging to elucidate the reaction scheme. Femtosecond infrared (fs-IR) spectroscopy, which is capable of “real-time” observation and characterization of the reactive intermediates down to hundreds of femtosecond, offers the possibility of deducing the elementary steps of a photochemical reaction in room-temperature solutions. Recently, this technique has been successfully used to unravel the reaction dynamics on a time scale faster than that of diffusion in the photoinitiated two-electron oxidative-addition reactions of C-H and Si-H bond activation.3 This communication reports the use of fs-IR spectroscopy to study a prototypical one-electron oxidative-addition. The transition-metal complex under study is the 17-e(CO)5Re radical, generated by photodissociation of the Re-Re bond of Re2(CO)10. The 17-eradical reacts to extract a Cl atom from a chlorinated methane solvent. The reaction may proceed through a strongly solvated 19-eintermediate, through a charge-transfer intermediate, or through a weakly solvated 17-eintermediate as shown in Figure 1.4 This aspect is examined directly for the first time by following the reaction from initiation to completion with 300-fs time resolution. To study the nature of the reaction barrier, the rates are measured along a series of chlorinated methane solutions under ambient conditions. The transition states are studied using density-functional theoretical (DFT) methods. As shown in the static FTIR in Figure 2d, the final product (CO)5ReCl in neat CCl4 solution exhibits two CO stretching peaks at 1982 and 2045 cm-1.5 At shorter time delays (40 ns < 2.5 μs) in Figure 2c,6 there appear five additional bands at 1945, 1985, 1998, 2005, and 2055 cm-1 that are assigned to the equatorially solvated nonacarbonyl species eq-Re2(CO)9(CCl4), in agreement with low-temperature studies.7 On the hundreds of picosecond time scale (Figure 2b), one sees a broad feature centering at around 1990 cm-1, marked by a down-pointing arrow. This spectrum is to be compared with that taken in the chemically inert hexanes solution (Figure 2a), which also exhibits a broad band centering at around 1992 cm-1, assigned to the weakly solvated Re(CO)5 radical in hexanes solution.8 It follows that the broad feature at 1990 cm-1 on the third panel can be attributed to the weakly solvated Re(CO)5 in CCl4. The similar peak positions of the Re(CO)5 band in CCl4 and hexanes solutions suggests that the Re(CO)5/solvent interactions are of similar magnitude in these two solutions. This conclusion is supported by DFT calculations,9 which provide a qualitative estimate of the interaction energies for Re(CO)5/CH4 (ca. -0.2 kcal/mol) and Re(CO)5/CCl4 (ca. -0.6 kcal/mol). Furthermore, the calculated weak interaction energy indicates that the mean thermal energy ∼0.6 kcal/mol at the room temperature is sufficient to disrupt the formation of a stable complex of the form Re(CO)5(solvent). In other words, a dynamic equilibrium is established for Re(CO)5(solvent) h Re(CO)5 + solvent,4c the time scale of which is on the order of collision in liquids (ca. a few picoseconds). This allows the chemically active Re center to undergo recombination reaction with another Re(CO)5 radical to reform the parent Re2(CO)10 molecule. As will be shown later, the aforementioned processes in general occur on a time scale orders of magnitude faster than that of the C-Cl bond activation step, which is in the nanosecond regime. Figure 3 shows the ultrafast kinetics for the parent molecule at 2071 cm-1 in CCl4 (Figure 3a) and hexanes (Figure 3b) (1) University of California at Berkeley. (2) Physical Biosciences Division, MS Calvin Laboratory, LBNL. (3) (a) Lian, T.; Bromberg, S. E.; Yang, H.; Proulx, G.; Bergman, R. G.; Harris, C. B. J. Am. Chem. Soc. 1996, 118 (15), 3769-3770. (b) Yang, H.; Kotz, K. T.; Asplund, M. C.; Harris, C. B. J. Am. Chem. Soc. 1997, 119 (40), 9564-9565. (c) Bromberg, S. E.; Yang, H.; Asplund, M. C.; Lian, T.; McNamara, B. K.; Kotz, K. T.; Yeston, J. S.; Wilkens, M.; Frei, H.; Bergman, R. G.; Harris, C. B. Science 1997, 278, 260-263. (d) Yang, H.; Asplund, M. C.; Kotz, K. T.; Wilkens, M. J.; Frei, H.; Harris, C. B. J. Am. Chem. Soc. 1998, 120 (39), 10154-10165. (e) Asbury, J. B.; Ghosh, H. N.; Yeston, J. S.; Bergman, R. G.; Lian, T. Q. Organometllics 1998, 17 (16), 3417-3419. (4) (a) Stiegman, A. E.; Tyler, D. R. Comments Inorg. Chem. 1986, 5, 215. (b) Baird, M. C. Chem. ReV., 1988, 88, 1217-1227. (c) Tyler, D. R. Acc. Chem. Res. 1991, 24, 325-331 and references therein. (5) Wrighton, M. S.; Ginley, D. S. J. Am. Chem. Soc. 1975, 97 (8), 20652072. (6) Please refer to the Supporting Information for technical details. All uncertainties reported herein represent one standard deviation. (7) Firth, H.; Klotzbücher, W. E.; Poliakoff, M.; Turner, J. J. Inorg. Chem. 1987, 26 (20), 3370-3375. (8) Firth, S.; Hodges, P. M.; Poliakoff, M.; Turner, J. J. Inorg. Chem. 1986, 25 (25), 4608-4610. (9) The results of extensive DFT calculations for the photochemistry of Re2(CO)10 will be described in a separate publication. The interaction energies shown here are gas-phase values. (10) Kim, S. K.; Pedersen, S.; Zewail, A. H. Chem. Phys. Lett. 1995, 233, 500-508. Figure 1.