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Organometallics 2010, 29, 1522–1525 DOI: 10.1021/om901039u
Computational and Experimental Studies of Methyl Group Exchange between Palladium(II) Centers Matthew S. Remy,† Thomas R. Cundari,‡ and Melanie S. Sanford*,† †
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, and ‡Department of Chemistry, Center for Advanced Scientific Computing and Modeling (CASCaM),University of North Texas, Denton, Texas 76203 Received December 2, 2009
Summary: The disproportionation of 2 equiv of (L)2Pd(CH3)(X) to form 1 equiv of (L)2Pd(CH3)2 and 1 equiv of (L)2Pd(X)2 could serve as a key step in the catalytic oxidative oligomerization of methane. The thermodynamics associated with this transformation have been evaluated as a function of the supporting ligands L and X using DFT calculations. With these calculations as a guide, we demonstrate the first experimental example of disproportionation of a PdII monomethyl complex to generate a PdII dimethyl species using (tBubpy)Pd(CH3)(CH2COCH3) (tBu-bpy = 4,40 -di-tert-butyl2,20 -bipyridine) as the starting material. The development of catalysts for the conversion of methane to more valuable chemical feedstocks is an important goal of both homogeneous and heterogeneous catalysis.1 Currently, such transformations are achieved via an indirect two-step sequence involving steam reforming of methane to generate syngas (CO þ H2), followed by reductive homologation to form a mixture of methane, higher hydrocarbons, and oxygenates (the Fischer-Tropsch synthesis).2 An attractive alternative to this energy-intensive, two-step process would be the direct conversion of methane to either methanol (eq 1) or to higher alkanes (eq 2). cat: ½M
H3 C-H þ 1=2 O2 s H3 C-OH cat: ½M
2 H3 C-H þ 1=2 O2 s H3 C-CH3 þ H2 O
ð1Þ
ð2Þ
A variety of single-site catalysts have been developed for the synthesis of oxygenates such as methanol,3 methyl bisulfate,4 *To whom correspondence should be addressed. E-mail: mssanfor@ umich.edu. (1) Derouane, E. G., Parmon, V., Lemos, F., Ribeiro, F. R., Eds. Sustainable Strategies for the Upgrading of Natural Gas: Fundamentals, Challenges, and Opportunities; Springer: Dordrecht, The Netherlands, 2005. (2) (a) Khodakov, A. Y.; Chu, W.; Fongarland, P. Chem. Rev. 2007, 107, 1692. (b) Dry, M. E. Catal. Today 2002, 71, 227. (3) For examples, see: (a) Bar-Nahum, I.; Khenkin, A. M.; Neumann, R. J. Am. Chem. Soc. 2004, 126, 10236. (b) Jones, C. J.; Taube, D.; Ziatdinov, V. R.; Periana, R. A.; Nielsen, R. J.; Oxgaard, J.; Goddard, W. A., III Angew. Chem., Int. Ed. 2004, 43, 4626. (c) Shul'pin, G. B.; Nizova, G. V.; Kozlov, Y. N.; Gonzalez Cuervo, L.; S€uss-Fink, G. Adv. Synth. Catal. 2004, 346, 317. (d) Gol'dshleger, N. F.; Es'kova, V. V.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. 1972, 46, 1358. (4) For examples, see: (a) Michalkiewicz, B.; Kosowski, P. Catal. Commun. 2007, 8, 1939 and references therein. (b) Mukhopadhyay, S.; Zerella, M.; Bell, A. T. Adv. Synth. Catal. 2005, 347, 1203 and references therein. (c) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560. (d) Periana, R. A.; Taube, D. J.; Evitt, E. R.; L€ offler, D. G.; Wentrcek, P. R.; Voss, G.; Masuda, T. Science 1993, 259, 340. pubs.acs.org/Organometallics
Published on Web 03/05/2010
and methyl trifluoroacetate5 from methane. In contrast, there are no reported examples of homogeneous catalysts for direct conversion of methane to higher alkanes.6-8 Each of these transformations is likely to involve the C-H activation of methane to form a M-CH3 complex. However, the unique challenge for direct methane-to-higher-alkane chemistry is to convert this M-CH3 species into an intermediate from which C-C coupling can occur. One potential approach to this type of reaction would be a catalytic cycle similar to that proposed for the Pd-catalyzed oxidative dimerization of arenes.9,10 As shown in Figure 1, this transformation is believed to proceed via C-H activation of benzene to form the Pd-Ph complex A (step 1), disproportionation of 2 equiv of A to generate the Pd(Ph)2 complex B (step 2), C-C bond-forming reductive elimination from B to release a biaryl product (step 3), and oxidation of Pd(0) with O2 to regenerate the C-H activation catalyst (step 4). Substitution of C6H6 for CH4 in an otherwise analogous catalytic cycle could provide a route to the desired transformation. However, while steps 1, 3, and 4 of this cycle have precedent for both C6H69-14 and CH4,4d,5,8,15,16 the key disproportionation event (step 2) has not, to our knowledge, been reported for (5) For examples, see: (a) Muehlhofer, M.; Strassner, T.; Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1745. (b) Gretz, E.; Oliver, T. F.; Sen, A. J. Am. Chem. Soc. 1987, 109, 8109. (6) (a) Derk, A. R.; Funke, H. H.; Falconer, J. L. Ind. Eng. Chem. Res. 2008, 47, 6568. (b) Olah, G. A. Acc. Chem. Res. 1987, 20, 422 and references therein. (7) For examples of heterogeneous catalysts for direct methane-tohigher alkane conversion see: (a) Soulivong, D.; Norsic, S.; Taoufik, M.; Coperet, C.; Thivolle-Cazat, J.; Chakka, S.; Basset, J. M. J. Am. Chem. Soc. 2008, 130, 5044. (b) Maitra, A. M. Appl. Catal. A 1993, 104, 11. (c) Wolf, E. E., Ed. Methane Conversion by Oxidative Processes; Van Nostrand Reinhold: New York, 1992. (8) For examples of the conversion of CH4 to CH3CO2H, see: (a) Periana, R. A.; Mironov, O.; Taube, D.; Bhalla, G.; Jones, C. J. Science 2003, 301, 814. (b) Jia, C. G.; Kitamura, T.; Fujiwara, Y. Acc. Chem. Res. 2001, 34, 633. (9) For recent examples of Pd-mediated Ar-H/Ar-H coupling, see: (a) Rong, Y.; Li, R.; Lu, W. Organometallics 2007, 26, 4376. (b) Li, R.; Jiang, L.; Lu, W. Organometallics 2006, 25, 5973. (c) Ackerman, L. J.; Sadighi, J. P.; Kurtz, D. M.; Labinger, J. A.; Bercaw, J. E. Organometallics 2003, 22, 3884. (d) Burton, H. A.; Kozhevnikov, I. V. J. Mol. Catal. A 2002, 185, 285. (e) Iretskii, A. V.; Sherman, S. C.; White, M. G.; Kenvin, J. C.; Schiraldi, D. A. J. Catal. 2000, 193, 49. (10) For reviews on Pd-catalyzed Ar-H/Ar-H coupling, see: (a) Li, B. J.; Yang, S. D.; Shi, Z. J. Synlett 2008, 949. (b) Kakiuchi, F.; Kochi, T. Synthesis 2008, 19, 3013. (c) Alberico, D.; Scott, M. E.; Lautens, M. Chem. Rev. 2007, 107, 174. (d) Catellani, M.; Motti, E.; Della Ca', N.; Ferraccioli, R. Eur. J. Org. Chem. 2007, 4153. (e) Stahl, S. S. Angew. Chem., Int. Ed. 2004, 43, 3400 and references therein. (11) For Pd-catalyzed benzene H/D exchange: Lee, J. H.; Yoo, K. S.; Park, C. P.; Olsen, J. M.; Sakaguchi, S.; Prakash, G. K. S.; Mathew, T.; Jung, K. W. Adv. Synth. Catal. 2009, 351, 563. r 2010 American Chemical Society
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at PtII (with R = Ph).24 However, as discussed above, the disproportionation of analogous PdII or PtII methyl species does not appear to be known. As such, we initiated DFT calculations (B3LYP/CEP-31G(d))25,26 to assess the thermodynamics associated with methyl disproportionation at square-planar PdII complexes as a function of the ligands R, L, and X.
Figure 1. Catalytic cycle for Pd-catalyzed oxidative coupling of benzene (R = Ph) or methane (R = CH3).
palladium or platinum methyl complexes.17,18 This communication reports our computational and experimental success in identifying ancillary ligands that facilitate methyl disproportionation at palladium(II) centers. The catalytic cycle in Figure 1 requires the disproportionation of 2 equiv of (L)2Pd(R)(X) to generate 1 equiv of (L)2Pd(R)2 and 1 equiv of (L)2Pd(X)2 (eq 3). The microscopic reverse of this reaction (comproportionation between (L)2Pd(R)2 and (L)2Pd(X)2) has precedent at PdII centers with R = Ph,19 alkynyl.20 Analogous comproportionation reactions are also well-known at PtII centers with R = Ph,21 alkynyl,22 and CH3.23 The corresponding disproportionation reactions are significantly less common but have been directly observed at both PdII (with R = Ph,12 alkynyl20) and (12) For examples of Pd-Ar disproportionation, see: (a) Grushin, V. V.; Marshall, W. J. J. Am. Chem. Soc. 2009, 131, 918. (b) Suzaki, Y.; Osakada, K. Organometallics 2003, 22, 2193. (c) Yagyu, T.; Hamada, M.; Osakada, K.; Yamamota, T. Organometallics 2001, 20, 1087. (13) For a review on Ar-Ar reductive elimination from PdII, see: Hartwig, J. F. Inorg. Chem. 2007, 46, 1936. (14) For reviews on the oxidation of Pd0 to PdII with O2, see: (a) Schultz, M. J.; Sigman, M. S. Tetrahedron 2006, 62, 8227. (b) Reference 10e and references therein. (c) Stoltz, B. M. Chem. Lett. 2004, 33, 362. (d) Sheldon, R. A.; Arends, I. W. C. E.; Brink, G.-J. T.; Dijksman, A. Acc. Chem. Res. 2002, 35, 774. (15) For an example of Pd-catalyzed functionalization of methane see: Nakata, K.; Yamaoka, Y.; Miyata, T.; Taniguchi, Y.; Takaki, K.; Fujiwara, Y. J. Organomet. Chem. 1994, 473, 329. (16) For an example of ethane reductive elimination from PdMe2(L)2: Gillie, A.; Stille, J. K. J. Am. Chem. Soc. 1980, 102, 4933. (17) For an example of CH3 disproportionation at a NiII center, see: Yamamoto, T.; Kohara, T.; Yamamota, A. Bull. Chem. Soc. Jpn. 1981, 54, 2010. (18) For examples of oxidatively inducted CH3 disproportionation at Pd and Pt: (a) Lanci, M. P.; Remy, M. S.; Kaminsky, W.; Mayer, J. M.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 15618. (b) Moret, M.-E.; Chen, P. J. Am. Chem. Soc. 2009, 131, 5675. (c) Johansson, L.; Ryan, O. B.; Roe mming, C.; Tilset, M. Organometallics 1998, 17, 3957. (d) Canty, A. J.; Jin, H.; Roberts, A. S.; Skelton, B. W.; White, A. H. Organometallics 1996, 15, 5713. (e) Ling, S. S. M.; Payne, N. C.; Puddephatt, R. J. Organometallics 1985, 4, 1546. (19) (a) Suzaki, Y.; Yagya, T.; Osakada, K. J. Organomet. Chem. 2007, 692, 326. (b) Casado, A. L.; Casares, J. A.; Espinet, P. Organometallics 1997, 16, 5730. (20) Osakada, K.; Hamada, M.; Yamamota, T. Organometallics 2000, 19, 458. (21) (a) Suzaki, Y.; Osakada, K. Bull. Chem. Soc. Jpn. 2004, 77, 139. (b) Peters, R. G.; White, S.; Roddick, D. M. Organometallics 1998, 17, 4493. (c) Eaborn, C.; Odell, K. J.; Pidcock, A. J. Chem. Soc., Dalton Trans. 1978, 357. (22) (a) Osakada, K.; Yamamoto, T. Coord. Chem. Rev. 2000, 198, 379. (b) Cross, R. J.; Gemmill, J. J. Chem. Soc., Dalton Trans. 1984, 205. (23) (a) Scott, J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643. (b) Puddephatt, R. J.; Thompson, P. J. J. Chem. Soc., Dalton Trans. 1975, 1810. (c) Puddephatt, R. J.; Thompson, P. J. J. Chem. Soc., Dalton Trans. 1977, 1219.
We first examined the relative thermodynamics of methyl, phenyl, and alkynyl group disproportionation at (MeDAB)PdII(R)(Cl) (MeDAB = N,N0 -dimethyl-1,4-diazabutene). This diimine was selected because Pt and Pd complexes containing related ligands are known to promote both stoichiometric27 and catalytic27,28 C-H activation reactions. The singlet ground-state structures for (MeDAB)Pd(R)(Cl), (MeDAB)Pd(R)2, and (MeDAB)Pd(Cl)2 were optimized for each R ligand in the gas phase at 298 K and 1 atm. Solvent corrections in acetone were performed as single-point energy calculations on the optimized gas-phase structures using the integral equation formalism of the polarizable continuum model (IEFPCM).25 ΔGdisprop was then calculated on the basis of these optimized ground-state energies. As summarized in Table 1, the disproportionation reactions with R = Ph, CH3 are thermodynamically uphill at 298 K, while when R = alkynyl the disproportionation is thermoneutral. Significantly, ΔG for methyl disproportionation is >3 kcal/mol further uphill than the analogous phenyl reaction (Table 1, entries 1 and 2). Notably, for all R groups, the acetone solvent correction had minimal effect (7 kcal/mol with all of the ligands examined. Nonetheless, L did have a significant influence on the value of ΔGdisprop, with strongly σ-donating ligands such as 1,3-dimethylimidazol-2-ylidene (Me2Im), PH3, C5H5N, and NH3 (entries 1-5) providing the most favorable equilibria for this transformation. Gratifyingly, chelating carbenes,5a diamines,31 diimines,27 bipyridine derivatives,4 and phosphines32 are all known to support group 10 metal complexes capable of alkane activation. In contrast, weaker σ-donors such as MeCN, SH2, and OH2 (entries 6, 8, and 10) as well as strong π-acceptors such as PF3, CO, and ethylene (entries 7, 9, and 11) provided much larger values of ΔGdisprop in this system. To more systematically probe the electronic influence of the L-type ligand on ΔGdisprop, we computationally examined a series of pyridine ligands containing diverse para substituents. As summarized in Table S8, more electron rich pyridines generally provided smaller values of ΔGdisprop. A Hammett plot of the data versus σ showed a modest correlation (R2 = 0.69), with a F value of -1.57 (Figure 2). However,
it is important to note that even with X = N(CH3)2, ΔGdisprop remained relatively large (nearly 10 kcal/mol). The influence of the X-type ligands on ΔGdisprop was also examined by DFT, using MeDAB as a chelating L-L ligand.33 Again, ground-state structures were optimized in the gas phase for (MeDAB)Pd(CH3)(X), (MeDAB)Pd(CH3)2, and (MeDAB)Pd(X)2, and ΔGdisprop was determined on the basis of these data.29 As summarized in Table 3, ΔGdisprop showed a reasonable correlation with the basicity of X. For example, with weakly basic X ligands such as iodide, bromide, chloride, and trifluoroacetate, ΔGdisprop was g11.7 kcal/mol (entries 1-4). However, ΔGdisprop decreased by close to 8 kcal/mol upon moving to alkoxide and phenoxide ligands (entries 6 and 7). Furthermore, when X was changed to a C-bound enolate,34 ΔGdisprop was only 1.2 kcal/mol (entry 8). This value corresponds to a Keq value of 0.14, indicating that the disproportionation should be readily detectable by 1H NMR spectroscopy. To further study the effect of the X-type ligand on ΔGdisprop, DFT calculations were carried out on the complexes (MeDAB)Pd(CH3)(OAr), where OAr is a series of para-substituted phenoxides. An excellent correlation was observed between ΔGdisprop and the σ value of the para
(30) Tethering the ligands had a relatively small impact on ΔG; for example, ΔGdisprop changed from 10.6 kcal/mol for (py)2Pd(CH3)(I) to 11.1 kcal/mol for (bpy)Pd(CH3)(I). (31) Holtcamp, M. W.; Henling, L. M.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. Inorg. Chim. Acta 1998, 270, 467. (32) Hackett, M.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 1449.
(33) Similar trends in ΔGdisprop as a function of X-type ligand were observed for the carbene-ligated complexes (Me2Im)2Pd(CH3)X. See the Supporting Information for details. (34) The ground-state structure containing an O-bound enolate was significantly higher in energy (þ15.8 kcal/mol) than the corresponding molecule with a C-bound enolate. The same trend holds true for Cbound versus O-bound enolates in (tBu-bpy)Pd(CH2COCH3)2.
Figure 2. Hammett plot for variation of the para substituent on the pyridine ligand.
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Table 3. Effect of X-Type Ligands on ΔGdisprop
entry
X
pKaa
ΔGdisprop (kcal/mol)
1 2 3 4 5 6 7 8 9
I Br Cl CF3CO2 CH3CO2 C6H5O CH3O CH3C(O)CH2 NH2
-10 -9 -8 -0.3 4.8 10.0 15.5 26.5 38
11.8 12.1 11.7 12.4 8.1 4.4 2.0 1.2 0.1
a
pKa of the conjugate acid of X.
substituent, with F = -3.21 (R = 0.94) (Table S9 and Figure 3). This indicates that disproportionation is strongly favored with more electron rich alkoxide ligands. All of the observed trends in ΔGdisprop as a function of X can be rationalized on the basis that as this ligand becomes more and more similar to CH3- (pKa = 48, ΔGdisprop = 0 (by definition)), the free energy for disproportionation approaches zero. The DFT calculations above suggested that the disproportionation equilibrium should be detectable with X = CH2COCH3, and we sought to experimentally confirm this by studying the PdII methyl enolate complex (tBu-bpy)Pd(CH3)(CH2COCH3) (4) (eq 5). An analytically pure sample of 4 was prepared by the reaction of (tBu-bpy)Pd(CH3)(I) with 1.5 equiv of Ag2O in acetone at room temperature.24,35 2
When complex 4 was dissolved in THF-d8 at 25 °C in a sealed NMR tube in ambient (room) light, disproportionation was observed to generate a 36:1:1.2 ratio of 4:1:(tBubpy)Pd(CH2COCH3)2 (5) after 24 h, as determined by 1H NMR spectroscopy.36,37 The observed ratio 4:5 after 24 h would correspond to ΔG = 4.2 kcal/mol; however, competing decomposition of 1 over the course of the disproportionation means that this can only serve as an estimate of the thermodynamics associated with this reaction (eq 5). For comparison, DFT calculations predict ΔG = 2.8 kcal/mol (35) The synthesis of 4 was carried out in the absence of light. (36) No disproportionation was observed in the absence of light, even at elevated temperatures (3 h at 50 °C). Studies of the origin of this effect are ongoing. (37) Disproportionation of 4 was observed in a variety of solvents, including THF, acetone, and benzene. However, competing decomposition of 1 under the reaction conditions prohibited quantitatively establishing the equilibrium constant in any solvent. Further details are summarized in the Supporting Information.
Figure 3. Hammett plot for variation of the para substituent on the phenoxide ligand.
for disproportionation of the model complex (4,40 -dimethyl2,20 -dipyridyl)Pd(CH3)(CH2COCH3) in THF, which is in reasonable agreement with our estimate (eq 5). In summary, we have conducted DFT calculations to gain insights into the influence of supporting ligands on the disproportionation chemistry of cis-(L)2Pd(CH3)(X). In general, these calculations predict that strongly electron donating L and X ligands will afford the largest equilibrium populations of the disproportionation products. On the basis of these calculations, we have identified the first experimental example of methyl disproportionation at PdII, with (tBubpy)Pd(CH3)(CH2COCH3) (4) as the starting material. These studies offer valuable insights into the influence of ligands on the thermodynamics of methyl disproportionation. However, it is important to note that if the dimethyl species is kinetically accessible, only small equilibrium concentrations of (L)2Pd(CH3)2 should be required to achieve catalytic turnover via the mechanism in Figure 1. This is exemplified by the oxidative coupling of benzene catalyzed by (bpy)Pd(OAc)2.9e,38 This transformation proceeds efficiently, despite an unfavorable calculated ΔGdisprop of 1.7 kcal/mol for (bpy)Pd(Ph)(OAc). In this case, the disproportionation equilibrium is believed to be rapid and reversible and a subsequent irreversible C-C bond-forming step drives the reaction in the forward direction. As such, our ongoing work seeks to assess the kinetics and mechanism of disproportionation via a combination of theory and experiment. In addition, we are utilizing the present results in the design of novel catalysts for the oxidative dimerization of methane.
Acknowledgment. We thank the NSF Center for Enabling New Technologies through Catalysis (CENTC) for support of this research. We also thank the Center for Advanced Scientific Computing and Modeling (CASCaM) for use of their computing and training resources, which are supported in part by the NSF (No. CHE-0701247). Finally, we acknowledge Professor Jim Mayer and Dr. Michael Lanci for valuable discussions throughout these studies. Supporting Information Available: Text, figures, and tables giving experimental details regarding the synthesis and characterization of inorganic compounds as well as computational details and Cartesian coordinates of all species used to obtain thermodynamic data. This material is available free of charge via the Internet at http://pubs.acs.org. (38) Shiotani, A.; Yoshikiyo, M.; Itatani, H. J. Mol. Catal. 1983, 18, 23.
