Femtosecond infrared studies of a prototypical one-electron oxidative- addition reaction: Chlorine atom abstraction by the Re(CO)s radical [5]

Full text of DOI:10.1021/ja991682x

J. Am. Chem. Soc. 1999, 121, 9227-9228
Femtosecond Infrared Studies of a Prototypical
9227
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
Figure 1.
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 photochemi-
cal reaction in room-temperature solutions. Recently, this tech-
nique 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 spec-
troscopy 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-e- radical reacts to extract a Cl atom from a
chlorinated methane solvent. The reaction may proceed through
a strongly solvated 19-e- intermediate, through a charge-transfer
intermediate, or through a weakly solvated 17-e- intermediate
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
Figure 2. Transient difference spectra in the CO stretching region for
Re2(CO)10 in neat CCl4 and hexanes. In panel c, peaks due to Re2(CO)9-
(CCl4) are marked with asterisks.
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.
(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), 2065-
2072.
(6) Please refer to the Supporting Information for technical details. All
uncertainties reported herein represent one standard deviation.
(7) Firth, H.; Klotzbu¨cher, 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.
Figure 3 shows the ultrafast kinetics for the parent molecule
(10) Kim, S. K.; Pedersen, S.; Zewail, A. H. Chem. Phys. Lett. 1995, 233,
500-508.
at 2071 cm-1 in CCl4 (Figure 3a) and hexanes (Figure 3b)
10.1021/ja991682x CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/15/1999

9228 J. Am. Chem. Soc., Vol. 121, No. 39, 1999
Communications to the Editor
Figure 3. Representative parent bleach kinetics (open circles) of Re2-
(CO)10 in (a) neat CCl4 and (b) hexanes solution. Fits to a diffusion model
to account for geminate recombination are shown as solid lines. Except
for the macroscopic viscosity, identical molecular parameters are used
for all the solvents studied (hexanes, and CHnCl4-n, n ) 0, 1, 2).
Figure 4. DFT structures for the final product and transition-state
structures. Geometrical parameters are in angstroms for bond lengths and
degree for bond angles. Also shown are the imaginary vibrational
frequencies associated with each transition state.
simultaneous dissociation of the C-Cl bond and formation of
the Re-Cl bond. The structural variation along the series of
chlorinated methanes suggests that the transition state becomes
more product-like as the number of hydrogen n in CHnCl4-n
increases. For example, the Re‚‚‚Cl distance in (CO)5Re‚‚‚Cl‚‚‚CHn-
Cl3-n decreases monotonically from 2.95 (n ) 0) to 2.84 (n ) 1)
to 2.76 (n ) 2), and to 2.52 Å for the final the product (CO)5Re-
Cl. The calculated energy barriers ∆E0q in CCl4, CHCl3, and CH2-
Cl2 are respectively 0.86, 3.0, and 8.5 kcal/mol, where the solvent
effects are treated self-consistently in the reaction-field ap-
proximation. The current level of theory, however, does not permit
an immediate comparison of the experimental energy barriers to
the calculated values. Nonetheless, comparison of the experimental
trend and the qualitative trend from the theoretical calculations
along the series of chloromethane provides greater insight in
understanding the reactivity. The observed trends in the barrier
and the transition-state structure are in agreement with Ham-
mond’s postulate,14 which associates a later transition state with
a higher energy barrier.
solutions. The parent kinetics in CCl4 recovers on two time scales
of ∼50 and ∼500 ps if fitted to two exponentials. Considering
that the photoinitiation step is most likely a direct dissociation
(∼100 fs)10 and the fact that no other transient intermediate is
observed, the most likely origin for the biphasic recovery is
geminate recombination of the Re(CO)5 pairs. To account for such
dynamics, a diffusion model is adapted to reproduce the parent
bleach kinetics in all the solvents studied.11 According to the
model, the fast 50 ps component is attributed to a convolution of
vibrational relaxation and the probability of reactive collision of
the Re(CO)5 pair within the solvent cage to form the parent Re2-
(CO)10. On a longer time scale, the mean separation of the two
Re(CO)5 fragments increases as a result of diffusive motion, thus
diminishing the probability of recombination.
In the nanosecond time regime, the Re(CO)5 radicals that have
not undergone geminate recombination (>50%, indicated by the
parent kinetics shown in Figure 3, or from the asymptotes of the
diffusion model) may activate the C-Cl bond to form the final
product (CO)5ReCl in chlorinated methane solutions. By monitor-
ing the product kinetics at 2045 cm-1, the time scales for
activation are found 1.33 ( 0.06 ns for CCl4,12 128 ( 40 ns for
CHCl3, and 270 ( 60 ns for CH2Cl2. These time scales correspond
to mean free-energy barriers ∆Gq of 5.35 ( 0.03, 8.03 ( 0.20,
and 8.48 ( 0.13 kcal/mol, respectively.13 The fact that no other
intermediates were detected prior to the product formation and
that no appreciable CO stretch frequency shift for Re(CO)5 was
observed suggest that the reaction involves only the rate-limiting
Cl-atom transfer step without the 19-e- nor the charge-transfer
intermediates.
In summary, femtosecond IR spectroscopy that allows for direct
assessment of the roles of the transient intermediates during a
reaction continues to demonstrate its ability to elucidate otherwise
intricate chemical reaction mechanisms. The present work pro-
vides experimental and theoretical evidence that one-electron
oxidative-addition of a Cl atom (from chlorinated methanes) to
the Re(CO)5 radical does not involve an appreciable 19-e-
intermediate. Nor do the results support the idea of a charge-
transfer intermediate [(CO)5Re+
+
-Cl-R] for this reaction.15
For if this is the case, the positively charged (CO)5Re+ will exhibit
a substantial νCO shift, which was not observed. With the above
evidence and the fact that no other intermediates were observed,
it is suggested that the reaction be considered as involving only
the C-Cl bond activation process. More studies are needed to
arrive at a model that is capable of predicting, even qualitatively,
the reaction rates from macroscopic parameters such as the
strength or electron affinity of the carbon-halogen bond.16
To better understand the reactivity, the transition states for the
series of reactions were studied using DFT methods. As illustrated
in Figure 4, the results show that each transition state can be
characterized by a single imaginary frequency that involves
(11) (a) Naqvi, K. R.; Mork, K. J.; Waldenstrøm, S. J. Phys. Chem. 1980,
84 (11), 1315-1319. (b) Bultmann, T.; Ernsting, N. P. J. Phys. Chem. 1996,
100 (50), 19417-19424. (c) Scott, T. W.; Liu, S. N. J. Phys. Chem. 1989,
93, 1393. (d) Churchill, M. R.; Amoh, K. N.; Wasserman, H. J. J. Inorg.
Chem. 1981, 20 (5), 1609-1611.
Acknowledgment. This work was supported by a grant from the
National Science Foundation. We also acknowledge equipment used under
the Office of Basic Energy Science, Chemical Science Division, U.S.
Department of Energy contract DE-AC03-76SF00098. We are grateful
to E. J. Heilweil of NIST and D. Triman of Talktronics for helpful sugges-
tions with the infrared focal plane detector. We also thank C. B. Moore for
use of an FTIR spectrometer and A. Rizvi for assistance with experiments.
Supporting Information Available: Details of the experiments, the
DFT calculations, and the diffusional model for geminate recombination
(PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
(12) This time scale corresponds to a reaction rate of (7.3 ( 0.4) × 107
M-1 s-1, and is in good agreement with the literature value of (1.8-9.1) ×
107 M-1 s-1, calculated from a set of Arrhenius parameters by: Meckstroth
W. K.; Reed, D. T.; Wojcicki, A. Inorg. Chim. Acta 1985, 105, 147-151.
(13) The energy barriers are calculated using 1/τ ≈ kTST ) (kBT/h) exp-
(-∆Gq/RT), T ) 298 K.
(14) Hammond, G. S. J. Am. Chem. Soc. 1954, 77, 334-338.
(15) There is, however, only a limited extent of charge-transfer character
in the calculated transition states (Table 1 of the Supporting Information).
(16) (a) Herrick, R. S.; Herrinton, T. R.; Walker, H. W.; Brown, T. L.
Organometallics 1985, 4, 42-45. (b) Lee, K.-W.; Brown, T. L. J. Am. Chem.
Soc. 1987, 109, 3269-3275.
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