Protonolysis of Platinum(II) and Palladium(II) Methyl Complexes: A

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Organometallics 2010, 29, 4354–4359 DOI: 10.1021/om100655w

Protonolysis of Platinum(II) and Palladium(II) Methyl Complexes: A Combined Experimental and Theoretical Investigation John E. Bercaw,* George S. Chen, Jay A. Labinger,* and Bo-Lin Lin Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125 Received July 6, 2010

The protonolysis of platinum(II) and palladium(II) methyl complexes has been investigated by both experiment and computation. Previously the protonolysis of (COD)PtII(CH3)2 by CF3COOY or (dppe)PdII(CH3)2 by CF3CY2OY (Y=H, D) was found to be accompanied by abnormally large and highly temperature-dependent kinetic isotope effects (KIEs), suggesting the involvement of tunneling. Here we find normal KIEs and no evidence of tunneling for protonolysis of (tmeda)PtII(CH3)Cl by CF3COOY (Y = H, D). Density functional theory (DFT) calculations indicate that protonation at the metal center followed by reductive coupling to the methane σ adduct (stepwise pathway) is favored for Pt complexes with good electron donor ligands, whereas direct protonation of the M-CH3 bond to generate the methane σ adduct (concerted pathway) is favored for Pt with electron-withdrawing ligands as well as for Pd. We suggest that KIE behavior consistent with tunneling may be an experimental indicator of the concerted pathway.

Introduction The selective activation and functionalization of saturated alkane C-H bonds has far-reaching practical implications.1 Over the past several decades, numerous examples of partial alkane oxidations catalyzed by homogeneous transition metal (especially platinum and palladium) complexes, often under remarkably mild conditions and with high selectivity, have been reported in the literature.2 While a great deal of progress has been made, the development of practical catalysts to transform alkanes to value-added products remains an ongoing *To whom correspondence should be addressed. E-mail: jal@ caltech.edu; [email protected]. (1) (a) Goldshlegger, N. F.; Tyabin, M. B.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. 1969, 43, 2174–2175. (b) Goldshlegger, N. F.; Eskova, V. V.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. 1972, 46, 1353–1354. (c) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154–162. (d) Crabtree, R. H. Chem. Rev. 1995, 95, 987–1007. (e) Shilov, A. E.; Shul'pin, G. B. Chem. Rev. 1997, 97, 2879– 2932. (f ) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. 1998, 37, 2180–2192. (g) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507–514. (h) Dyker, G., Ed. Handbook of C-H Transformations; WileyVCH: Weinheim, Germany, 2005, and references therein. (2) (a) Kao, L. C.; Hutson, A. C.; Sen, A. J. Am. Chem. Soc. 1991, 113, 700–701. (b) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560–564. (c) Sen, A. Acc. Chem. Res. 1998, 31, 550–557. (d) Lin, M.; Shen, C.; Garcia-Zayas, E. A.; Sen, A. J. Am. Chem. Soc. 2001, 123, 1000–1001. (e) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc. Chem. Res. 2001, 34, 633–639. (f ) Periana, R. A.; Mironov, O.; Taube, D.; Bhalla, G.; Jones, C. J. Science 2003, 301, 814–818. (g) Lersch, M.; Tilset, M. Chem. Rev. 2005, 105, 2471–2526. (h) An, Z.; Pan, X.; Liu, X.; Han, X.; Bao, X. J. Am. Chem. Soc. 2006, 128, 16028–16029. (3) (a) Luinstra, G. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1993, 115, 3004–3005. (b) Labinger, J. A.; Herring, A. M.; Lyon, D. K.; Luinstra, G. A.; Bercaw, J. E.; Horvath, I. T.; Eller, K. Organometallics 1993, 12, 895–905. (c) Luinstra, G. A.; Wang, L.; Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Organometallics 1994, 13, 755–756. (d) Hutson, A. C.; Lin, M.; Basickes, N.; Sen, A. J. Organomet. Chem. 1995, 504, 69–74. pubs.acs.org/Organometallics

Published on Web 09/10/2010

challenge. As part of the search for further improvements, efforts have been devoted to understanding the mechanisms of alkane C-H activation by platinum complexes.3 Significant mechanistic insights have been gained through investigations of the microscopic reverse of the C-H activation step, namely, protonolysis of alkylplatinum(II) model systems (Scheme 1).4 Two alternative mechanisms for protonolysis have been proposed: direct, concerted electrophilic attack at the metalcarbon bond (red pathway) leading to the methane σ adduct, and a stepwise route consisting of protonation at the metal to generate a metal hydride followed by reductive coupling to the σ adduct (blue pathway).5 The subsequent loss of methane is usually believed to proceed via associative displacement;6 that conclusion has been supported by the observation of statistical isotopic scrambling in deuterolysis, for cases when that step is rate determining.7 (4) (a) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. Organometallics 1995, 14, 4966–4968. (b) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1995, 117, 9371–9372. (c) Romeo, R.; Plutino, M. R.; Elding, L. I. Inorg. Chem. 1997, 36, 5909–5916. (d) Wik, B. J.; Lersch, M.; Tilset, M. J. Am. Chem. Soc. 2002, 124, 12116–12117. (e) Wik, B. J.; IvanovicBurmazovic, I.; Tilset, M.; Eldik, R. V. Inorg. Chem. 2006, 45, 3613–3621. (f ) Parmene, J.; Ivanovic-Burmazovic, I.; Tilset, M.; Eldik, R. V. Inorg. Chem. 2009, 48, 9092–9103. (5) (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 5961–5976. (b) Romeo, R.; D'Amico, G. Organometallics 2006, 25, 3435–3446. (6) (a) Johansson, L.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 739– 740. (b) Johansson, L.; Ryan, O. B.; Rømming, C.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 6579–6590. (c) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2002, 124, 1378–1399. (d) Procelewska, J.; Zahl, A.; van Eldik, R.; Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. Inorg. Chem. 2002, 41, 2808–2810. (7) (a) Owen, J. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2006, 128, 2005–2016. (b) Chen, G. S.; Labinger, J. A.; Bercaw, J. E. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 6915–6920. r 2010 American Chemical Society

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Scheme 1

Scheme 2

The stepwise pathway has been strongly implicated by observation of Pt(IV)-H intermediates in the protonolysis of several alkylplatinum(II) model systems.5 While the mechanism of alkane C-H activation by Pd(II) and the corresponding microscopic reverse process8 have received much less attention than Pt(II), formation of alkylpalladium(II) intermediates via the concerted pathway has been preferred, because Pd(IV)-H complexes are thought to be less accessible than their Pt(IV)-H counterparts.9 In general, though, distinguishing between these two pathways is highly problematic. Failure to observe a Pt(IV)-H intermediate does not necessarily rule out its involvement: it can be difficult to detect, depending on factors such as solvent, proton sources, and supporting ligands. It is also possible that the concerted pathway could be operating even when a Pt(IV)-H can be observed, if it is formed reversibly and does not undergo reductive coupling rapidly. Measurements of kinetic isotope effect (KIE) have also been used to investigate the mechanism of protonolysis,10 but no conclusive correlation between mechanism and KIE values is available.5 Romeo has noted that inverse KIEs (kH/kD < 1) can be observed for the protonolysis of alkylplatinum(II) (8) (a) Kim, Y. J.; Osakada, K.; Sugita, K.; Yamamoto, T.; Yamamoto, A. Organometallics 1988, 7, 2182–2188. (b) Kim, Y. J.; Osakada, K.; Takenaka, A.; Yamamoto, A. J. Am. Chem. Soc. 1990, 112, 1096–1104. (c) Kapteijn, G. M.; Dervisi, A.; Grove, D. M.; Kooijman, H.; Lakin, M. T.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 1995, 117, 10939–10949. (d) Ackerman, L. J.; Sadighi, J. P.; Kurtz, D. M.; Labinger, J. A.; Bercaw, J. E. Organometallics 2003, 22, 3884–3890. (9) For example, our recent computational investigations on benzene C-H bond activation by a monomethylpalladium(II) β-diketiminate indicate that the activation barrier for the stepwise oxidative addition/ reductive elimination pathway is ∼10 kcal/mol higher than that for the σ-bond metathesis pathway: Lin, B. L.; Bhattacharyya, K. X.; Labinger, J. A.; Bercaw, J. E. Organometallics 2009, 28, 4400–4405. (10) (a) Ryabov, A. D. Chem. Rev. 1990, 90, 403–424. (b) Jones, W. D. Acc. Chem. Res. 2003, 36, 140-146, and references therein.

complexes that proceed by the stepwise mechanism, but only if a step subsequent to protonation is rate-determining. He further proposed that if the actual protonation of the complex is rate-limiting, it may well not be possible to distinguish between the alternatives: “the similarity of the energy profiles [for the two pathways] suggests that, under these circumstances, any discussion of the site of proton attack risks becoming semantic in nature”.5b Although rigorous distinction between these pathways may well be elusive, in our opinion that does not reduce the problem to one of semantics; rather it highlights the need to develop new mechanistic tools. Recently we reported abnormally large KIEs (greater than 10 at 298 K) in the protonolysis of several dimethylpalladium(II) complexes, 1a-d, and (COD)PtII(CH3)2 (2, COD = 1,5cyclooctadiene) (Scheme 2) and invoked tunneling to account for the magnitude and temperature dependence of the KIEs.11 While the possibility of the stepwise, oxidative protonolysis route could not be firmly excluded for these systems, we favored the concerted mechanism for a number of reasons: Pd(IV)-H intermediates are not easily accessible; the electron-deficient COD ligand will disfavor formation of Pt(IV)-H; no Pt(IV)-H is detected by 1H NMR in the protonolysis of 2 at low temperatures; no scrambling of H/D between methyl/methane positions is observed in any of the systems studied, nor are measurable amounts of methane isotopologues with more than a single deuterium produced. We suggested the possibility that such unusually high KIEs, consistent with tunneling, might be a signature of operation of a concerted mechanism. Here we report computational studies on two of the systems exhibiting high KIEs, as well as a combined experimental and (11) Bercaw, J. E.; Chen, G. S.; Labinger, J. A.; Lin, B. L. J. Am. Chem. Soc. 2008, 130, 17654–17655.

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computational investigation of the protonolysis of (tmeda)PtII(CH3)Cl (3, tmeda=N,N,N0 ,N0 -tetramethylethylenediamine), in which the likely involvement of the stepwise mechanism has been supported by the observation of the Pt(IV) hydride 4 at low temperatures (Scheme 3).5a Our findings support the proposal that KIE observations indicating tunneling may be correlated with a preference for the concerted protonation pathway.

Results and Discussion Computational Studies on Protonolysis of (COD)PtII(CH3)2 by Trifluoroacetic Acid (TFA) and (dmpe)PdII(CH3)2 by Trifluoroethanol (TFE) in DCE. Density functional theory (DFT) was used to examine the systems that exhibited tunneling. Of the Pd complexes examined experimentally,11 protonolysis of 1a-c was complicated by competing reductive elimination of ethane, and the computational cost required to study dppe complex 1d would have been extreme, so calculations were performed on the bis(dimethylphosphino)ethane (dmpe) analogue 9. The calculated reaction energy profiles for the protonolyses of 2 by TFA and 9 by TFE in DCE are shown in Table 1 (calculated enthalpy and entropy values for each species are provided in the Supporting Information (SI)). Transition states (TS1a and TS1b) were located for the proton transfer step in the concerted mechanism of both systems; some interaction between the proton and its Scheme 3

Bercaw et al.

counteranion (as shown in Table 1) remains at the transition states. In the stepwise pathway for the protonolysis of 2, a Pt(IV)-H intermediate 8 and the transition state (TS2a) for the reductive coupling from 8 to the methane σ adduct 6 were both located, but no transition state could be located for the protonation of 2 to 8; nor did the calculations reveal any stabilization resulting from proton-counterion interaction along that pathway. The energy of TS2a (which is significantly higher than that of TS1a) can be taken as a lower limit for the overall activation barrier of the stepwise mechanism in the protonolysis of 2; hence the calculated barrier for the concerted mechanism (ΔGq=26.9 kcal/mol) is lower than that for the stepwise mechanism (ΔGq =32.5 kcal/mol). In contrast to 2, no Pd(IV)-H intermediate could be located for the stepwise pathway in the protonolysis of 9 by TFE. This result suggests that the stepwise pathway is even less favorable relative to the concerted mechanism, most likely due to both the instability of Pd(IV)-H and the unfavorable combination of a hard Pd(IV) center and two soft phosphorus donors. The transition states for the loss of methane from 6 and 10 were not located, but we can estimate upper limits for the activation barriers of this step by adding the overall enthalpy of methane dissociation (ΔH=5.8 kcal/mol for 6 to 7 and 5.9 kcal/mol for 10 to 11; we use enthalpy values because little or none of the favorable entropic changes will probably contribute to the transition-state free energies) to the free energies of the methane complexes, giving 19.3 and 22.1 kcal/mol, respectively, lower than the ΔGq values calculated for protonation of the M(II)-C bond. (As noted earlier, an associative mechanism for methane displacement is likely.6) The experimental findings, that CH3D was the only deuterated methane isotopologue observed and that no deuterium incorporation into the resultant Pt(II)-CH3 or Pd(II)-CH3 of the product was detected,11 suggested that protonation is

Table 1. Calculated Energy (kcal/mol) Profiles for the Protonolysis of 2 and 9 at 298 K

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Table 2. Experimental11 and Calculated (without Tunneling Corrections) KIEs for the Protonolysis/Deuterolysis of 2 by TFA at Various Temperatures

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Table 3. Experimental and Calculated (without Tunneling Corrections) KIEs for the Protonolysis/Deuterolysis of Complex 3 by Trifluoroacetic Acid in DCE-d4 at Various Temperatures

temperature, K

(kH/kD)expt

(kH/kD)calc

temperature, K

(kH/kD)expt

(kH/kD)calc

273 294 313 333 353

25.9 ( 0.3 17.5 ( 0.3 12.1 ( 0.3 9.2 ( 0.3 6.9 ( 0.3 AH/AD = 0.075 ( 0.007 EaD - EaH = 3.2 ( 0.1 kcal/mol

5.9 5.2 4.8 4.4 4.0 AH/AD = 1.1 EaD - EaH = 0.9 kcal/mol

273 294 313 333

5.1 ( 0.5 4.4 ( 0.5 4.1 ( 0.5 3.9 ( 0.5 AH/AD = 1.1 ( 0.1 EaD - EaH = 0.8 ( 0.1 kcal/mol

4.2 3.8 3.5 3.3 AH/AD = 1.1 EaD - EaH = 0.7 kcal/mol

Scheme 4

the rate-determining step for the protonolysis of 2 by TFA and 9 by TFE, respectively, followed by rapid loss of methane, and the computational result is thus consistent with that conclusion. KIEs calculated (see SI for details) without tunneling corrections for the protonolysis of 2 by TFA acid in DCE are much smaller than the experimental values (Table 2), especially at low temperatures. As the temperature increases, the hydrogen tunneling contribution to the KIE is expected to decrease; thus the calculated and experimental KIEs gradually approach each other. On the basis of these calculations, it appears that hydrogen tunneling is most likely the reason behind the abnormally large experimental KIEs as well as their temperature dependence behavior for the protonolysis of 2. Protonolysis of (tmeda)PtII(CH3)Cl by Trifluoroacetic Acidd0 and -d1 in 1,2-Dichloroethane-d4. We have previously examined the protonolysis of (tmeda)PtII(CH3)Cl (3, tmeda=N,N, N0 ,N0 -tetramethylethylenediamine) in dichloromethane.5a With HCl a six-coordinate Pt(IV)-H intermediate (stable below about -60 °C) could be observed by NMR, while use of triflic acid, which contains only a very weakly coordinating anion, led directly to methane even at -80 °C. No multiply deuterated toluene was detected when the analogous benzyl complex was reacted with a mixture of HCl and DCl, indicating that the rapid isotope scrambling often observed in the protonolysis of dimethylplatinum(II) species is absent here, thus allowing us to measure the KIE of protonolysis/ deuterolysis under competitive conditions across a wide range of temperatures. Addition of a 2:1 mixture of CF3COOD/CF3COOH to a solution of 3 in 1,2-dichloroethane-d4 (DCE-d4) at room temperature (Scheme 4) gave (tmeda)PtII(OOCCF3)Cl (12) and CH3D/CH4 in the ratio of 0.45:1 (by 1H NMR), with no (12) It should be noted that values for Arrhenius parameters outside the ranges 0.5 < AH/AD < 21/2 (calculations on model systems suggest 0.7-1.2 is a more realistic range) and (EaD - EaH) < 1.2 kcal/mol (zeropoint energy difference of O-H stretching of the proton source) are generally taken to demonstrate the presence of tunneling.13 However, the involvement of proton tunneling cannot always be ruled out for systems with normal KIEs and Arrhenius parameters that fall within semiclassical limits.14 For example, temperature-independent KIEs have been observed for enzymatic systems that have been shown to have significant proton tunneling.15 A recent DFT study also suggests that proton tunneling may play a major role in the reductive elimination of methane from (PPh3)2PtII(CH3)(H) even though the observed KIE is 3.3 at 248 K in toluene.16

Figure 1. Plot of ln(kH/kD) vs 1/T for the protonolysis/ deuterolysis of complex 3 by trifluoroacetic acid in DCE.

other isotopologues of methane observed. The calculated KIE is 4.4. The KIEs measured for the protonolysis of 3 are much less temperature dependent (Table 3) than those for 1 and 2 (see Table 2). A linear correlation between ln(kH/kD) and 1/T was obtained, with both AH/AD (1.1 ( 0.1) and EaD - EaH (0.8 ( 0.1 kcal/mol) falling within semiclassical limits (Figure 1), suggesting that tunneling does not play a significant role.12 DFT calculations on the protonolysis of 3 by TFA gave reaction profiles (Table 4) similar to those obtained for 2 (see above), but the replacement of COD by tmeda lowers both the activation enthalpy (from 32.8 to 29.2 kcal/mol) and activation free energy (from 32.5 to 29.0 kcal/mol) for the stepwise mechanism, while increasing those values (from 18.8 to 20.7 and from 26.9 to 30.1 kcal/mol, respectively) for the concerted mechanism. Moreover, the formation of 15 from 3 is calculated to be more favorable than that of 8 from 2 by 3.4 kcal/mol, probably attributable to tmeda being a better electron donor and poorer acceptor than COD. Although the five-coordinate Pt(IV)-H intermediate 15 is calculated to be unstable relative to 3 þ TFA, the addition of a sixth ligand lowers its energy: (tmeda)PtIV(CH3)(H)Cl2, 4, which was experimentally observed at low temperatures, is calculated to be more stable than 3 þ HCl (Scheme 5). Again, (13) (a) Bell, R. P. Chem. Soc. Rev. 1974, 4, 513–544. (b) Bell, R. P. The Proton in Chemistry; Cornell University Press: Ithaca, NY, 1973. (c) Bell, R. P. The Tunnel Effect in Chemistry; Chapman and Hall: London, 1980. (d) Caldin, E. F. Chem. Rev. 1969, 69, 135–156. (e) Kwart, H. Acc. Chem. Res. 1982, 15, 401–408. (f ) Melander, L.; Saunders, W. H. Reaction Rates of Isotopic Molecules; Krieger: Malabar, FL, 1987. (g) Limbach, H.; Lopez, J. M.; Kohen, A. Phil. Trans. R. Soc. B 2006, 361, 1399–1415. (h) Schneider, M. E.; Stern, M. J. J. Am. Chem. Soc. 1972, 95, 1517–1522.

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Table 4. Calculated Energy (kcal/mol) Profiles for the Protonolysis of 3 at 298 K

Scheme 5

Conclusions In summary, our computational results favor a concerted mechanism with direct protonation at the M-C bond over a stepwise mechanism in which protonation at M is followed by reductive coupling, for the protonolysis of two systems that exhibit abnormally large KIEs, attributed to tunneling. For a platinum(II) complex with a better electron-donating ligand, the stepwise mechanism becomes more favorable computationally, and no tunneling effects are observed in the experimentally measured KIEs. We suggest that the observation of tunneling-related high KIEs may be a useful indicator of systems that follow the concerted mechanism, but calculations that include tunneling would be needed to reach a more confident conclusion on this point. We would also like to have considerably more information on the detailed mechanism of protonolysis (as well as the microscopic reverse, C-H bond activation) and the relation to KIE values; some of that work is currently underway in our laboratories.

the estimated barrier to loss of methane is lower than that for protonation, consistent with the absence of isotopic scrambling in the protonolysis/deuterolysis. The overall activation free energy is calculated to be slightly (∼1 kcal/mol) lower for the stepwise pathway than for the concerted pathway for 3. This small difference is not sufficient, by itself, to conclude that the stepwise mechanism is favored, since the uncertainties in the calculated parameters must be considerably larger. In particular, activation entropies in proton transfer reactions are difficult to calculate reliably, since they can be profoundly influenced by solvent effects,17 which current computational methods are still not sophisticated enough to model accurately. Indeed, the experimentally measured activation parameters for the protonolysis of 3 by triflic acid (ΔHq =19.4 ( 1.5 kcal/mol, ΔSq = 14 ( 5 eu)5a differ considerably from the calculated values (ΔHq = 29.2 kcal/mol, ΔSq = 1 eu). However, the relative calculated values appear much more significant: on going from 2 to 3, the stepwise path becomes more favorable compared to the concerted path by about 7 kcal/mol. This comparison involves calculations on closely related systems, so that any major systematic errors are likely to cancel out; coupled with the experimental observation of a Pt(IV)-H intermediate for the latter but not the former, it provides strong support for the argument that the two systems follow different mechanisms. Additionally, calculated KIEs for the stepwise mechanism agree well with the experimental values (Table 3), further suggesting that proton tunneling does not play an important role in the protonolysis of 3.

General Information. All air- and/or moisture-sensitive compounds were manipulated by using standard high-vacuum line, Schlenk, or cannula techniques, or in a glovebox under a nitrogen atmosphere. TFA-d1 and DCE-d4 were purchased from Cambridge Isotope Laboratories and stored under nitrogen in the glovebox. TFA-d0 was purchased from Sigma-Aldrich and stored in the glovebox. Compound 3 was prepared according to literature procedures.18 All NMR tubes were dried overnight in a 180 °C oven. Protonolysis studies were performed in a screw-cap NMR tube with a PTFE/silicone septum. The error introduced by residual protons from the glass surface of the NMR tubes was found to be negligible by 1H NMR spectroscopy with an internal standard (within 1H NMR error). All NMR spectra were recorded at room temperature on a Varian Mercury 300 spectrometer.

(14) Stern, M. J.; Weston, R. E. J. Chem. Phys. 1974, 60, 2808–2814. (15) Klinman, J. P.; Sharma, S. C. J. Am. Chem. Soc. 2008, 130, 17632–17633. (16) Datta, A.; Hrovat, D. A.; Borden, W. T. J. Am. Chem. Soc. 2008, 130, 2726–2727.

(17) (a) Evans, A. G.; Hamann, S. D. Trans. Faraday Soc. 1951, 47, 25–49. (b) Bunting, J. W.; Stefanidis, D. J. Am. Chem. Soc. 1990, 112, 779– 786. (18) Luinstra, G. A.; Wang, L.; Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Organomet. Chem. 1995, 504, 75–91.

Experimental Section

Article Protonolysis of 3. Stock solutions of 2:1 TFA-d1/TFA-d0 and 3 in DCE-d4 were prepared in the glovebox in glass vials with PTFE/silicone septa. The vials with the stock solutions were then placed in an ice or oil bath with three screw-cap NMR tubes sealed with PTFE/silicone septa at a constant temperature (273, 294, 313, 333 K). After the solutions were allowed to equilibrate for a few minutes, the TFA-d1/TFA-d0 mixture (0.2 mL) and 3/ DCE-d4 solution (0.7 mL) were slowly transferred into the NMR tubes by syringe. After quickly shaking the tubes outside the bath to homogenize the mixtures, the NMR tubes were inserted back into the bath for 10 min before NMR analysis. The average CH3D/CH4 ratios of three runs at each temperature were used. Computational Methods. All computations were performed using the Gaussian03 software package unless otherwise stated.19 All species were treated as singlets. Geometries were optimized by density functional theory method (BP86) with a hybrid basis set (Pd and Pt: LANL2TZ(f),20,21 consisting of Wadt and Hay relativistic effective core potentials (RECPs),20a valence triple-ζ contraction functions, and an f-orbital polarization (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian03, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2004. (20) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (b) Ehlers, A. W.; Bohme, M.; Dapprich, S.; Gobbi, A.; Hollwarth, A.; Jonas, V.; Kohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111–114. (c) Roy, L. E.; Hay, P. J.; Martin, R. L. J. Chem. Theory Comput. 2008, 4, 1029–1031.

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function; all other elements: 6-31þG(d), an all-electron basis set developed by Pople et al.22). Similar methods have been widely used in computational investigations of C-H bond activation by transition metal complexes.23 A harmonic oscillator model was used for vibration frequency analysis of the optimized structures. All frequencies of the minima are positive, while transition states have only one negative frequency. The vibration mode of the negative frequency in the transition state was confirmed to be the one that corresponds to the reaction coordinate. Gas phase enthalpies and entropies (pressure=1 atm, 298.15 K) of all species were obtained via frequency calculations with appropriate isotopic contents at various temperatures. No scaling factor was used for the calculated frequencies. An implicit solvation model, the CPCM polarizable continuum model, was then employed for the calculation of solvation energies. The sum of the gas phase enthalpies and the solvation energies was used directly as the enthalpies in DCE. Finally, the gas phase entropies were converted to corresponding entropies (1 M in DCE) according to an empirical method developed by Wertz.24 Details of the latter calculation, along with all calculated structural and energetic parameters, are given in the SI.

Acknowledgment. This work was generously supported by BP through the MC2 program. An NSF Graduate Research Fellowship to G.S.C. is gratefully acknowledged. Supporting Information Available: Detailed calculational results are available free of charge via the Internet at http:// pubs.acs.org. (21) (a) Feller, D. J. Comput. Chem. 1996, 17, 1571–1586. (b) Schuchardt, K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.; Li, J.; Windus, T. L. J. Chem. Inf. Model. 2007, 47, 1045–1052. (22) (a) Hariharan, P. C.; Pople, J. A. Chem. Phys. Lett. 1972, 16, 217–219. (b) Franel, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654–3665. (23) Balcells, D.; Clot, E.; Eisenstein, O. Chem. Rev. 2010, 102, 749– 823. (24) Wertz, D. H. J. Am. Chem. Soc. 1980, 102, 5316–5322.