Large Kinetic Isotope Effects for the Protonolysis of Metal–Methyl

ARTICLE pubs.acs.org/Organometallics

Large Kinetic Isotope Effects for the Protonolysis of Metal Methyl Complexes Are Not Reliable Mechanistic Indicators Valerie J. Scott, Jay A. Labinger,* and John E. Bercaw* Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125, United States

bS Supporting Information ABSTRACT: Earlier work on protonolyses of several palladium and platinum methyl complexes (with release of methane) had suggested the possibility that observation of an unusually large kinetic isotope effect, consistent with significant contributions from quantum mechanical tunneling, might be diagnostic of a mechanism involving direct protonation of the metal methyl bond, as opposed to one proceeding via a metal hydride intermediate. By extension of these measurements to a wider set of complexes, we find no support for the proposed correlation.

’ INTRODUCTION The selective functionalization of C H bonds by transitionmetal complexes has been an active area of research for several decades, one which offers the potential for a wide variety of applications. Despite the remarkable advances in this field, significant obstacles to practical processes have yet to be overcome; in particular, reaction rates and selectivities are often insufficient. The step in which C H activation takes place will generally be both rate and selectivity determining; hence, a good deal of effort has been devoted to gaining an understanding of the detailed mechanism(s) of that step.1 Our group and others have focused on electrophilic activation at platinum and palladium centers, which proceeds via the generalized stoichiometry shown in Scheme 1.2 8 In many cases it is quite difficult even to observe the metal alkyl species, let alone determine the mechanism of its formation. Mechanistic studies on the protonolysis of model metal methyl complexes, the microscopic reverse of electrophilic activation (also shown in Scheme 1), have been found to constitute a useful, if indirect, strategy.9 14 Two alternate protonolysis mechanisms are shown in Scheme 2. Pathway A (red) involves direct, concerted protonation at the metal methyl bond, leading directly to a methane σ-adduct, followed by loss of the alkane. In pathway B (blue) protonation takes place at the metal center to generate a (formally oxidized) metal hydride intermediate, which undergoes reductive coupling to generate the σ-adduct and then dissociates alkane. The reverse of those two pathways would correspond to mechanistic alternatives for C H activation (although it should be noted that protonolyses of model compounds are usually irreversible). The observation of platinum(IV) alkyl hydride intermediates in several low-temperature protonolyses has been taken to support r 2011 American Chemical Society

Scheme 1

Scheme 2

the stepwise pathway B.15,16 However, because all steps prior to alkane loss are typically fully reversible under the reaction conditions, observation of a metal hydride species is not conclusive evidence for pathway B: it is conceivable that reductive coupling does not take place, and protonolysis proceeds only by reversion to starting complex followed by pathway A. For Pd-based systems17 20 the concerted pathway has generally been preferred, since PdIV is believed to be less accessible. Indeed, palladium(IV) alkyl hydride intermediates are generally not observable, but again, that does not rule out Received: May 23, 2011 Published: July 18, 2011 4374

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Organometallics their participation. It does not appear possible to rigorously distinguish between the mechanistic alternatives on these grounds. Measurement of the kinetic isotope effect (KIE) has proven to be a highly useful mechanistic tool, which has been applied to many studies of C H activation by organometallic systems.21,22 To date, though, no clear correlation between KIE and mechanism has been established, for either activation or protonolysis.15,16 Recently we reported the observation of unusually large KIE values—around 20 at room temperature—for the protonolysis of several PdII Me species, including (dppe)PdMe2 (dppe = 1,2-bis(diphenylphosphino)ethane), as well as (cod)PtMe2 (cod = 1,5-cyclooctadiene).23 KIEs of that magnitude often signal a substantial quantum-mechanical tunneling component24 26 (although alternate interpretations can be offered27). The KIEs for these protonolyses also exhibited unusual temperature dependence (Arrhenius parameters EaD EaH > 1.2; AH/AD outside the range of 0.5 < AH/AD < 21/2), which is taken as further evidence for tunneling.28 31 We felt that concerted pathway A might be preferred here, not only for the Pd complexes (as discussed above) but also the Pt complex, since the π-acceptor cod ligand should disfavor PtIV. Indeed, no Pt H intermediate could be observed for this complex, even at low temperatures. In contrast, much smaller (or even inverse) KIEs were found in earlier studies of protonolysis for Pt complexes that do lead to observable platinum(IV) alkyl hydride intermediates at low temperatures.12,15,16,32 Comparison of DFT calculations for protonolysis of a PdII complex and (cod)PtMe2 with those for (tmeda)PtII(CH3)Cl (which experimentally exhibited normal KIE values and an observable platinum(IV) alkyl hydride intermediate) supported the proposal that the former pair follow pathway A while the latter goes by pathway B. We took these findings to suggest that KIE values indicating tunneling might be a marker for pathway A,23,33 although with so few examples this suggestion was highly speculative. To further test the proposal, we examined KIEs (including temperature dependences) for protonolysis of several additional metal methyl species, particularly including complexes in their highest accessible formal oxidation state, for which pathway B should not be possible. We report here our findings, which appear to exclude any correlation between a concerted protonolysis pathway and abnormally large KIE values and also call attention to methodological issues in the determination of temperature-dependent KIEs.

’ RESULTS AND DISCUSSION Determination of the Temperature Dependence of KIEs. Initial experiments were aimed at verifying our methodology, by repeating measurements for protonolysis of (cod)PtMe2 by trifluoroacetic acid (TFA). While we were able to reproduce the previously reported23 room-temperature value quite well, agreement at both reduced and elevated temperatures was much less satisfactory: not only was the temperature dependence flatter but also the scatter among duplicate runs was considerably larger than before. In retrospect, perhaps this problem should have been anticipated. KIEs were determined by treating the organometallic complex with a mixture of HX and DX, the ratio of the acidic reagents to one another being chosen so as to give an easily analyzable (by NMR) mixture of CH4 and CH3D. If the KIE is large, thorough mixing is crucial; otherwise, HX can become locally depleted, giving distorted results. However, to achieve

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Table 1. Measured KIEs for the Protonolysis of (cod)PtMe2 as a Function of Temperature and Method temp, °C

a

method

no. of trials

KIE

8

1

7

20.8 ( 1.0

8 0

2 1

4 3

26.7 ( 1.4 20.0 ( 2.1

0

2

5

22.7 ( 0.9

0

3

3

22.9 ( 1.7

0

previousa

25.9 ( 0.3 4

18.0 ( 0.9

23

1

23

previousa

40

1

3

16.4 ( 0.6

40 40

2 previousa

3

15.5 ( 0.5 12.1 ( 0.3

60

previousa

9.2 ( 0.3

80

previousa

6.9 ( 0.3

17.5 ( 0.3

As reported in ref 23.

good mixing and temperature control simultaneously is not so straightforward. We performed a series of experiments, using different methods, to investigate the scope of this problem and (hopefully) to identify the best way to deal with it. For method 1, a mixture of solvent (CD2Cl2) and acid (0.5 and 0.2 mL, respectively) was loaded into a NMR tube with a septum cap. The metal complex was dissolved separately in a small amount of solvent (0.2 mL) and placed in a vial, also equipped with a septum top. The NMR tube was cooled or heated in an appropriate temperature bath. The metal solution was added dropwise by syringe to the NMR tube, which was repeatedly inverted and replaced in the bath between additions. (This method is analogous to that followed in previous measurements.23) For method 2, the acid/solvent mixture was put into a septumcapped GC vial equipped with a stir bar. The vial was immersed in the bath, the metal solution was injected dropwise with constant stirring over a period ranging from 30 s to 2 min (varying the rate of injection had no systematic effect on the observed KIE), and the final solution was transferred by syringe to a septum-capped NMR tube. This method would be expected to provide better mixing and temperature control than method 1, at the potential cost of loss of dissolved methane during transfer; the latter problem proved to be minor enough that good NMR signals could easily be obtained. Method 3 was designed specifically for measurement at 0 °C: samples were prepared as in method 1, and all NMR tubes, vials, and syringes, along with a thermometer, were allowed to equilibrate in a cold room for 1 h before injection as above. Results for the three methods are shown in Table 1, along with the previously reported data for the protonolysis of (cod)PtMe2. It can be seen that (1) the earlier room-temperature result could be reproduced,34 although its estimated uncertainty was probably too optimistic, (2) methods 1 and 2 can give significantly different results, particularly at the lowest temperature studied, and (3) the variation of KIE with temperature is significantly smaller in the present work than in the published data (for (cod)PtMe2). It seems most likely that problems with temperature control are the most important source of uncertainty. The higher uncertainties in KIE values at each temperature are magnified in the determination of Arrhenius parameters, especially AH/AD, which is calculated by extrapolation of the best-fit 4375

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Table 2. Measured KIEs for the Protonolysis of Various Methyl Metal Complexes by TFA at 23 °C complex ZnMe2

KIE 3.1 ( 0.7

tBu2

Cp2ZrMe2 (PONOP)RhMe

10.1 ( 0.7 6.1 ( 0.4

(PONOP)IrMe

10.5 ( 1.1

(dppe)PdMe2 (cod)PtMe2

6.9 ( 0.2 18.0 ( 0.9

line to the intercept. The previous estimates were EaD EaH = 3.2 ( 0.2 and AH/AD = 0.08 ( 0.03.35 The most we can say from our more extended data set is that EaD EaH falls in the range of 0.8 2.6 and AH/AD in the range of 0.1 4.5. These values do not definitively signal tunneling, but they are not inconsistent with it either, and we continue to believe that the extremely large (and completely reproducible) room-temperature KIE results from significant tunneling contributions. KIE Values for Protonolysis of Varied Complexes. A number of further complexes were chosen to test the hypothesis that large KIE values may be characteristic of protonolyses that follow concerted pathway A. First, we selected a main-group compound, ZnMe2, as well as a transition-metal compound in its highest accessible oxidation state, (tBu2Cp)2ZrMe2; for both of these, stepwise pathway B, proceeding via initial protonation at the metal, should not be possible. If the hypothesis were correct, the Zn and Zr complexes would exhibit large KIEs. Next we compared (PONOP)RhMe and (PONOP)IrMe (PONOP = 2,6-bis(di-tert-butylphosphinito)pyridine); Brookhart et al. have found that protonation of the Rh complex at low temperature ( 110 °C) gives an unprecedented observable methane adduct, whereas the Ir analogue gives an observable Ir H species even at room temperature.36 If we assume that the observation (or absence thereof) of a metal hydride is indicative of the reaction pathway being followed, then our hypothesis predicts that the Rh complex would likewise display a large KIE, while the Ir complex would not. We also examined the protonolysis of (dppe)PdMe2 (previously studied using TFE) by TFA. In all cases KIEs were determined by protonolysis with a mixture of TFA and TFA-d, using both methods 1 and 2, over a range of temperatures. Room-temperature values are given in Table 2 (the complete set of data is shown in the Supporting Information); it is obvious that they do not support the hypothesis and that there is no readily discernible correlation between KIE magnitude and mechanism. For the Zn and Zr cases, one has a borderline-high KIE but the other is the lowest of all, the Rh and Ir cases come out opposite to what had been predicted, and the high KIE previously observed for the Pd case disappears on changing the protonating agent from TFE to TFA. Other examples of acid-dependent KIEs have been reported,37 39 although in those cases, where the solvent was either the acid itself39 or THF,37,38 partitioning effects may be operating. Such effects seem less likely to be important here, with CD2Cl2 as the main constituent of the solvent, although they cannot be completely ruled out because of the high concentrations of acid used. Two potentially complicating factors merit comment. First, whereas the earlier study involved dimethyl complexes in which only one of the two methyls underwent protonolysis, both are cleaved for ZnMe2 as well as for (dppe)PdMe2 with TFA. In such cases the measured KIE will represent an average of two

protonolysis events which need not exhibit the same value. However, it is highly unlikely that the observed KIEs, which are fairly small (around 3 and 7, respectively), could be consistent with a very large KIE in one step: that would appear to require a substantially inverse KIE for the other. This factor thus does not appear able to account for the dramatic and unexpected difference in KIE values for Pd on changing from TFE to TFA. Second, protonolysis of (PONOP)IrMe gave a small amount of CD2H2 in addition to the expected isotopologues, indicating that both protonation and reductive coupling are somewhat reversible and loss of methane is relatively slow, leading to some isotopic scrambling. As a consequence, the measured KIE will be somewhat higher than the “true” value for the initial proton-transfer event; hence, the apparent difference between the Rh and Ir analogues may not be significant. The problems with reproducibility away from room temperature, discussed earlier, precluded determination of reliable Arrhenius parameters in most cases: either the scatter was so large that the uncertainties exceeded 100% of the average value (Zn, Pd) or insufficient material was available for multiple determinations (Rh, Ir). For the Zr complex, the values were determined to be EaD EaH = 0.3 ( 0.1 and AH/AD = 5.6 ( 1; the latter suggests tunneling, while the former does not. It does not appear possible to draw any conclusions from the temperature dependence of these KIEs.

’ CONCLUSIONS The absence of any discernible pattern in the room-temperature data (which is quite reproducible) clearly disproves the hypothesis of a correlation between large KIE values (whether attributable to tunneling or not) and protonolysis reaction mechanism. It may be worthwhile to add a general caveat: in light of the complex array of factors that can contribute to the magnitude of KIE in any given case,27 mechanistic interpretations of KIE data—particularly a relatively limited set of data— should be offered with caution. ’ EXPERIMENTAL SECTION General Considerations. (dppe)PdMe 2 , 40 (η 5 -C 5 H 3 -1,3-

(CMe 3 )2 )2 ZrMe 2 , 41 (PONOP)RhMe, and (PONOP)IrMe 36,42 were synthesized following published methods (the last two were kindly provided by Michael Findlater). (cod)PtMe 2 and ZnMe 2 were purchased from Aldrich and used without further purification. CD 2 Cl 2 was purchased from Cambridge Isotope Laboratories; before use it was degassed by three freeze pump thaw cycles and stored over alumina overnight in a N 2 -atmosphere glovebox. For each KIE measurement experiment, the ratio of protio acid to methyl was at least 10:1 to ensure that the ratio of protio to deuterio acid remains approximately the same throughout the course of the reaction. 1 H NMR spectra were recorded on a 600 MHz Varian spectrometer. Kinetic Isotope Effect Measurements. Method 1. In a N2atmosphere glovebox, 0.500 mL of CD2Cl2 was placed in a septumcapped NMR tube along with 0.200 mL of a 10:1 mixture of TFA-d and TFA. The metal complex (5 mg for solids, 0.5 μL for ZnMe2) was dissolved separately in 0.200 mL of CD2Cl2 in a septum-capped vial. Outside of the glovebox, the NMR tube was immersed in a heated oil bath or ice salt bath at the appropriate temperature and allowed to equilibrate for 5 min. The metal complex solution was added to the NMR tube dropwise via syringe with continuous mixing by inversion of the tube. 4376

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Organometallics Method 2. In a N2-atmosphere glovebox, a 1.5 mL vial equipped with a stirbar was charged with 0.500 mL of CD2Cl2 and 0.200 mL of a 10:1 mixture of TFA-d and TFA and capped with a septum. In a separate vial, the metal complex (5 mg for solids, 0.5 μL for ZnMe2) was dissolved in 0.200 mL of CD2Cl2 and capped with a septum. An NMR tube was also capped with a septum under the inert atmosphere. Outside of the glovebox, the vial containing the acid mixture and stirbar was placed into the appropriate temperature bath and allowed to equilibrate for 5 min. The metal solution was then injected via syringe dropwise to the stirring mixture over a period ranging from ∼30 s to 2 min; no systematic effect of addition rate on KIE could be seen. After complete addition, the reaction mixture was transferred to the septum-capped NMR tube using a gastight syringe. Method 3. The experimental setup for this method was identical with that of method 1 prior to removal from the glovebox. The vial and NMR tube were taken from the glovebox and, along with a syringe, allowed to equilibrate in the cold room at 0 °C for 1 h. The metal complex solution was then added to the NMR tube dropwise via syringe, with continuous inversion of the tube as in method 1. NMR Measurements. For each reaction mixture, five single-scan spectra were recorded (with an attenuation time of 5 s), with several minutes between scans. The ratio of integrated intensities for the CH4 and CH3D signals was averaged over the five spectra and multiplied by 10 (because of the 10:1 ratio of TFA-d and TFA used in the experiments) to give the KIE value. Each experiment (at different temperatures and using different methods) was carried out at least in triplicate (except for the Rh and Ir samples, for which there was insufficient material to do so). 1 H NMR Spectral Data for Organometallic Products. In all cases (except ZnMe2) the 1H NMR spectrum of the reaction mixture showed a single organometallic product, consistent with clean protonolysis of one or two methyl groups, while the 19F NMR spectrum showed a single peak, consistent with fast exchange between complex and excess acid. The reaction of (cod)PtMe2 with excess TFA gave (cod)PtMe(TFA): 1H NMR (CD2Cl2) δ 5.50 5.46 (m, 2H), 5.80 5.60 (m, 2H), 2.7 2.2 (m, 8H), 0.78 (s, 3H, 2JPtH = 30 Hz). The reaction of (dppe)PdMe2 with excess TFE gave (dppe)PdMe(TFE): 1 H NMR (CD2Cl2) δ 7.8 7.4 (m, 20H), 2.50 2.10 (m, 4H), 0.54 (br, 3H). The reaction of (dppe)PdMe2 with excess TFA gave (cod)Pt(TFA)2: 1H NMR (CD2Cl2) δ 7.88 7.56 (m, 20H), 2.8 2.55 (m, 4H). The reaction of ZnMe2 with excess TFA (presumably) gave Zn(TFA)2; no 1H NMR signal other than those for the acid, solvent, and methane isotopologues were observed. The reaction of tBu2 Cp2ZrMe2 with excess TFA gave tBu2Cp2ZrMe(TFA): 1H NMR (CD2Cl2) δ 6.7 6.3 (m, 6H), 1.30 (s, 3H), 1.28, (s, 9H), 1.25 (s, 9H). The reaction of (PONOP)IrMe with excess TFA gave (PONOP)Ir(TFA): 1 H NMR (CD2Cl2) δ 8.00 (t, 1H, 3JHH = 12 Hz), 7.11 (d, 2H, 3JHH = 6 Hz), 1.6 1.3 (m, 36H). The reaction of (PONOP)RhMe with excess TFA gave (PONOP)Rh(TFA): 1H NMR (CD2Cl2) δ 7.56 (t, 1H, 3 JHH = 6 Hz), 7.37 (d, 2H, 3JHH = 6 Hz), 1.34 (d, 18H, 3JHH = 6 Hz), 1.26 (d, 18H, 3JHH = 6 Hz).

’ ASSOCIATED CONTENT

bS

Supporting Information. Tables of measured kinetic isotope effects for the protonolyses of metal methyl complexes at different temperatures using different methods, figures giving Arrhenius plots, and equations. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (J.E.B.); [email protected] (J.A.L.).

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’ ACKNOWLEDGMENT This research was funded by BP through the Methane Conversion Cooperative (MC2) program. We thank M. Findlater and M. Brookhart for providing the PONOP complexes and Dr. David Vander Velde for help with NMR experiments. ’ REFERENCES (1) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507–514. (2) Geletii, Y. V.; Shilov, A. E. Kinet. Catal. 1983, 24, 413–416. (3) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2000, 122, 108467–10855. (4) Lin, M. R.; Shen, C. Y.; Garcia-Zayas, E. A.; Sen, A. J. Am. Chem. Soc. 2001, 123, 1000–1001. (5) Luinstra, G. A.; Wang, L.; Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Organomet. Chem. 1995, 504, 75–91. (6) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560–564. (7) Sen, A.; Benvenuto, M. A.; Lin, M. R.; Hutson, A. C.; Basickes, N. J. Am. Chem. Soc. 1994, 116, 998–1003. (8) Shilov, A. E.; Shul’pin, G. B. Usp. Khim. 1987, 56, 754–792. (9) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. Organometallics 1995, 14, 4966–4968. (10) Parmene, J.; Ivanovic-Burmazovic, I.; Tilset, M.; van Eldik, R. Inorg. Chem. 2009, 48, 9092–9103. (11) Romeo, R.; Plutino, M. R.; Elding, L. I. Inorg. Chem. 1997, 36, 5909–5916. (12) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1995, 117, 9371–9372. (13) Wik, B. J.; Ivanovic-Burmazovic, I.; Tilset, M.; van Eldik, R. Inorg. Chem. 2006, 45, 3613–3621. (14) Wik, B. J.; Lersch, M.; Tilset, M. J. Am. Chem. Soc. 2002, 124, 12116–12117. (15) Romeo, R.; D’Amico, G. Organometallics 2006, 25, 3435–3446. (16) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 5961–5976. (17) Ackerman, L. J.; Sadighi, J. P.; Kurtz, D. M.; Labinger, J. A.; Bercaw, J. E. Organometallics 2003, 22, 3884–3890. (18) 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. (19) Kim, Y. J.; Osakada, K.; Sugita, K.; Yamamoto, T.; Yamamoto, A. Organometallics 1988, 7, 2182–2188. (20) Kim, Y. J.; Osakada, K.; Takenaka, A.; Yamamoto, A. J. Am. Chem. Soc. 1990, 112, 1096–1104. (21) Jones, W. D. Acc. Chem. Res. 2003, 36, 140–146.Also see: G omezGallego, M.; Sierra, M. A. Chem. Rev. 2011, DOI: 10.1021/cr100436k. (22) Ryabov, A. D. Chem. Rev. 1990, 90, 403–424. (23) Bercaw, J. E.; Chen, G. S.; Labinger, J. A.; Lin, B. L. J. Am. Chem. Soc. 2008, 130, 17654–17655. (24) Bell, R. P. Chem. Soc. Rev. 1974, 3, 513–544. (25) Bell, R. P. The Proton in Chemistry; Cornell University Press: Ithaca, NY, 1973. (26) Bell, R. P. The Tunnel Effect in Chemistry; Chapman and Hall: London, 1980. (27) Slaughter, L. M.; Wolczanski, P. T.; Klinckman, T. R.; Cundari, T. R. J. Am. Chem. Soc. 2000, 122, 7953–7975. (28) Caldin, E. F. Chem. Rev. 1969, 69, 135–156. (29) Kwart, H. Acc. Chem. Res. 1982, 15, 401–408. (30) Limbach, H. H.; Lopez, J. M.; Kohen, A. Philos. Trans. R. Soc. B 2006, 361, 1399–1415. (31) Melander, L.; Saunders, W. H. Reaction Rates of Isotopic Molecules; Krieger: Malabar, FL, 1987. (32) Crumpton-Bregel, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125, 9442–9456. (33) Bercaw, J. E.; Chen, G. S.; Labinger, J. A.; Lin, B. L. Organometallics 2010, 29, 4354–4359. 4377

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(34) We also reexamined protonolysis of (dppe)PdMe2 by trifluoroethanol, using method 2, and obtained a value (20.0 ( 0.2 at 21 °C) close to that of the previously reported one (21.4 ( 0.7 at 21 °C). (35) The uncertainties reported here take into account the uncertainties in the individual data and are hence somewhat larger than those originally reported, which were based only on scatter in the least-squares line fitting.23 (36) Bernskoetter, W. H.; Hanson, S. K.; Buzak, S. K.; Davis, Z.; White, P. S.; Swartz, R.; Goldberg, K. I.; Brookhart, M. J. Am. Chem. Soc. 2009, 131, 8603–8613. (37) Basallote, M. G.; Duran, J.; Fernandez-Trujillo, M. J.; Manez, M. A. J. Chem. Soc., Dalton Trans. 1998, 2205–2210. (38) Basallote, M. G.; Duran, J.; Fernandez-Trujillo, M. J.; Manez, M. A. J. Organomet. Chem. 2000, 609, 29–35. (39) Bennett, B. L.; Hoerter, J. M.; Houlis, J. F.; Roddick, D. M. Organometallics 2000, 19, 615–621. (40) Degraaf, W.; Boersma, J.; Smeets, W. J. J.; Spek, A. L.; van Koten, G. Organometallics 1989, 8, 2907–2917. (41) King, W. A.; Di Bella, S.; Gulino, A.; Lanza, G.; Fragala, I. L.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1999, 121, 355–366. (42) Bernskoetter, W. H.; Schauer, C. K.; Goldberg, K. I.; Brookhart, M. Science 2009, 326, 553–556.

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