Platinum(II) Complexes Containing Quaternized Nitrogen Ligands

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Organometallics 2010, 29, 257–262 DOI: 10.1021/om900889k

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Platinum(II) Complexes Containing Quaternized Nitrogen Ligands: Synthesis, Stability, and Evaluation as Catalysts for Methane and Benzene H/D Exchange Janette M. Villalobos, Amanda J. Hickman, and Melanie S. Sanford* Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109 Received October 12, 2009

This paper describes an efficient synthesis of the cationic platinum complex [(N-CH3-bpym)PtCl2]þ (N-CH3-bpym=N-methylbipyrimidinium) and evaluation of its catalytic activity in H/D exchange reactions of CH4 (with D2SO4) and benzene (with CF3CO2D). With both substrates [(N-CH3-bpym) PtCl2]þ shows C-H activation reactivity comparable to that of its neutral analogue (bpym)PtCl2 (bpym=bipyrimidine). The origin of this similar reactivity is proposed to be the in situ formation of the same active catalyst in both cases.

The development of direct, efficient, and selective catalysts for the oxygenation of hydrocarbons continues to be a great challenge in organometallic chemistry.1 Advances in this field could significantly impact the synthesis of fuels, commodity chemicals, and pharmaceuticals. In 1972, Shilov demonstrated the use of Pt salts to catalyze the oxidation of CH4 to CH3OH in water under mild conditions,2 and this seminal discovery sparked tremendous research effort in the area of Pt-catalyzed C-H bond oxidation.3 The best Pt-based CH4 oxidation process to date involves the conversion of methane to methyl bisulfate in fuming H2SO4 with (bpym)PtCl2 (1; bpym=2,2-bipyrimidine) as a precatalyst (Scheme 1).4,5 The cationic complex [(N-H-bpym)Pt(X)2]þ (2; X=Cl, OSO3H)6 is believed to form in situ under the highly acidic reaction conditions and has been proposed as the active *To whom correspondence should be addressed. E-mail: mssanfor@ umich.edu. (1) For select pertinent reviews, see: (a) Shilov, A. E.; Shul’pin, G. B. Chem. Rev. 1997, 97, 2879. (b) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. (c) Dick, A. R.; Sanford, M. S. Tetrahedron 2006, 62, 2439. (d) Godula, K.; Sames, D. Science 2006, 312, 67. (e) Bergman, R. G. Nature 2007, 446, 391. (f) Hermans, I.; Spier, E. S.; Neuenschwander, U.; Turra, N.; Baiker, A. Top. Catal. 2009, 52, 1162. (2) Gol’dshleger, N. F.; Eskova, V. V.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. 1972, 46, 1353. (3) For select pertinent reviews, see: (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. 1998, 37, 2181. (b) Lersch, M.; Tilset, M. Chem. Rev. 2005, 105, 2471. (c) Fekl, U.; Goldberg, K. I. Adv. Inorg. Chem. 2003, 54, 259. (d) Conley, B. L.; Tenn, W. J.; Young, K. J. H.; Ganesh, S. K.; Meier, S. K.; Ziatdinov, V. R.; Mironov, O.; Oxgaard, J.; Gonzales, J.; Goddard, W. A.; Periana, R. A. J. Mol. Catal. A 2006, 251, 8. (4) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560. (5) For second-generation versions of analogous catalyst systems, see: (a) Cheng, J.; Li, Z.; Haught, M.; Tang, Y. Chem. Commun. 2006, 4617. (b) Palkovits, R.; Antonietti, M.; Kuhn, P.; Thomas, A.; Sch€uth, F. Angew. Chem., Int. Ed. 2009, 48, 6909. (6) (a) Kua, J.; Xu, X.; Periana, R. A.; Goddard, W. A. Organometallics 2002, 21, 511. (b) Xu, X.; Kua, J.; Periana, R. A.; Goddard, W. A. Organometallics 2003, 22, 2057. (c) Ahlquist, M.; Periana, R. A.; Goddard, W. A. Chem. Commun. 2009, 2373. (7) Paul, A.; Musgrave, C. B. Organometallics 2007, 26, 793. r 2009 American Chemical Society

Scheme 1. Oxidation of CH4 to CH3OSO2H Catalyzed by 1

catalyst (Scheme 1). Computational studies6,7 suggest that this protonation is critical for limiting oxidative catalyst degradation and increasing the electrophilicity of the Pt center (and thereby its reactivity with CH4). However, these effects have proven difficult to study experimentally, due to the challenges of working in fuming H2SO4 and the multiple accessible protonation states and X-type ligands in this medium. As such, the exact nature of the active catalyst remains the subject of some debate.8 Additionally, the limited experimental understanding of the effects of protonation on reactivity has hindered further catalyst development, which is required to render this methane oxidation process commercially viable. In an effort to address these challenges, we aimed to replace bpym with the cationic quaternized nitrogen-containing ligand N-methyl-2,20 -bipyrimidininum (N-CH3-bpymþ). Quaternary (8) (a) Gilbert, T. M.; Hristov, I.; Ziegler, T. Organometallics 2001, 20, 1183. (b) Hristov, I. H.; Ziegler, T. Organometallics 2003, 22, 1668. (9) Black, M. L. J. Phys. Chem. 1955, 59, 670. (10) For examples of other metal complexes containing ligands with quaternized nitrogen substituents, see: (a) Johnson, C. R.; Shepherd, R. E. Inorg. Chem. 1983, 22, 2439. (b) Wishart, J. F.; Bino, A.; Taube, H. Inorg. Chem. 1986, 25, 3318. (c) Kaim, W.; Matheis, W. Chem. Ber. 1990, 123, 1323. (d) Matheis, W.; Kaim, W. J. Chem. Soc., Faraday Trans. 1990, 86, 3337. (e) Matheis, W.; Poppe, J.; Kaim, W.; Zaliz, S. J. Chem. Soc., Perkin Trans. 2 1994, 1923. (f) Waldh€or, E.; Kaim, W.; Olabe, J. A.; Slep, L. D.; Fiedler, J. Inorg. Chem. 1997, 36, 2969. (g) Coe, B. J.; Chamberlain, M. C.; Essex-Lopresti, J. P.; Gaines, S.; Jeffery, J. C.; Houbrechts, S.; Persoons, A. Inorg. Chem. 1997, 36, 3284. (h) Fujihara, T.; Wada, T.; Tanaka, K. Inorg. Chim. Acta 2004, 357, 1205. Published on Web 12/15/2009

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Scheme 2. Methane Oxidation with Proposed Catalyst A12

Figure 1. pKa of N-methylisonicotinic acid.

Figure 3. X-ray structure of the complex 4-PF6 (PF6 counterion omitted for clarity).

Figure 2. PtII complexes compared in this study.

nitrogen substituents are highly electron withdrawing; for example, the pKa of N-methylisonicotinic acid (Figure 1) is 1.72, which corresponds to a Hammett σpara value of 2.47 for the N-CH3 group.9,10 We anticipated that the cationic PtII NCH3-bpymþ complex 3 (Figure 2) would possess electronic properties similar to those of 2 but would be accessible in the absence of strong acids such as fuming H2SO44,6 or HF/ SbF6.11 This paper describes studies on the synthesis, characterization, stability, and C-H activation reactivity of complex 3 and its analogues.

Results and Discussion Our initial investigations focused on reproducing the literature synthesis of [(N-CH3 -bpym)PtCl2 ][HSO4 ] (3), 12 a reported intermediate in the preparation of [(N-CH 3bpym)PtCl 2 ][H 4PV 2 Mo10 O 40 ] (A) (Scheme 2). Complex A exhibits modest activity for the aerobic oxidation of CH4 to a mixture of CH3OH, HCHO, and CH3 CHO under mild conditions (50 °C, 30 bar of CH4, and 2 bar of O 2 in acidic H 2O) (Scheme 2). The reaction conditions and the product distribution differ substantially from CH4 oxidation reactions with precatalyst 1, and this unusual reactivity was originally attributed to the presence of the polyoxometalate counterion. Surprisingly, possible contributions from the alkylation of the bipyrimidine ring were not discussed; furthermore, the catalytic activity of the proposed precursor 3 was not evaluated. The literature procedure for the synthesis of 3 involved methylation of (bpym)PtCl2 (1) with (CH3)2SO4 in dimethyl sulfoxide (DMSO) (eq 1).12 In our hands, this reaction provided a compound with identical 1H and 13C NMR spectra to those in the literature report. However, a variety (11) Seidel, S.; Seppelt, K. Inorg. Chem. 2003, 42, 3846. (12) Bar-Nahum, I.; Khenkin, A. M.; Neumann, R. J. Am. Chem. Soc. 2004, 126, 10236.

of additional experiments implicated [(bpym)Pt(DMSO)Cl][CH3OSO3] (4-CH3OSO3) as the structure of this product. First, elemental analysis was consistent with 4-CH3OSO3, but not with 3. Second, electrospray mass spectrometry showed an [Mþ] peak at m/z 466, consistent with [(bpym)Pt(DMSO)Cl]þ and not [(N-CH3-bpym)PtCl2]þ ([Mþ] at m/z 438). Third, no NOE enhancement of any of the bpym protons was observed upon irradiation of the CH3 resonance at 3.36 ppm (originally assigned as the N-methyl group of 3).12 In structure 3, this group should be in close proximity to H4. Finally, reaction of the product with NaPF6 led to disappearance of the 1H NMR signal at 3.36 ppm, consistent with ion exchange to form the hexafluorophosphate salt [(bpym)Pt(DMSO)Cl]PF6 (4-PF6) (eq 2). The structure of 4-PF6 was confirmed by elemental analysis and X-ray crystallography (Figure 3).

An alternate route to 3 was next explored via metalation of N-methylbipyrimidinium [(N-CH3-bpym)]BF4. However, treatment of (DMSO)2PtCl213 with [(N-CH3-bpym)]BF4 in (13) Price, J. H.; Williamson, A. N; Schramm, R. F.; Wayland, B. B. Inorg. Chem. 1972, 11, 1280.

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Figure 4. X-ray structure of complex 6.

methanol at 60 °C did not afford the desired complex. Instead, metalation proceeded with concomitant addition of CH3OH into the ligand to provide the neutral complex (2-(4-methoxy-3-methyl-4,5-dihydropyrimidin-2-yl)pyrimidine)PtCl2 (5) in 86% yield (eq 3). The 1H and 13C NMR spectra of 5 show signals consistent with loss of aromaticity in one ring of the bpym; for example, upfield 1H NMR resonances at 5.49 and 5.69 ppm are observed for H4 and H5, respectively. Crystals of 5 were obtained by diffusion of Et2O into a solution of 5 in wet CH3CN. This led to substitution of the OCH3 for an OH group to form complex 6 (eq 4), which was characterized by X-ray crystallography (Figure 4). Figure 5. Turnover numbers (TON’s) for H/D exchange between C6H6 and TFA-d catalyzed by 1 and 3. Conditions: PtII catalyst (2 mol %, 5 μmol), benzene (23.2 μL, 0.26 mmol), AgOAc (1 equiv, 0.85 mg, 5 μmol; 2 equiv, 1.7 mg, 10 μmol) in TFA-d (0.5 mL, 6.5 mmol, 25 equiv relative to benzene) at 75 or 150 °C for 2 or 24 h. Reported TON’s are the average of at least two trials.

We hypothesized that 3 could be accessed from 5 via protonation of the CH3O group followed by loss of CH3OH. Gratifyingly, dissolving complex 5 in trifluoroacetic acid-d (TFA-d) led to instantaneous formation of a new inorganic species with concomitant release of 1 equiv of CH3OD (which was converted in situ to CF3CO2CH3) (eq 5). The 1 H and 13C NMR resonances associated with the new complex (3) are shifted significantly downfield relative to those of 5. For example, while the ligand proton resonances for 5 appear between 9.84 and 5.49 ppm, those in the product appear between 10.6 and 8.0 ppm. Similarly, in the 13C NMR spectrum of 3, six aromatic resonances are observed between 162.7 and 158.6 ppm, while the starting material 5 shows six peaks between 161.9 and 123.9 ppm. These data are all fully consistent with aromatization of the N-CH3-bpymþ ligand to form 3.

Having generated the desired complex in trifluoroacetic acid, we next examined the C-H activation reactivity of 1 and 3 (generated in situ from 5) in TFA-d. In this solvent, the (14) Keq for protonation of coordinated bpym in TFA should be greater than 2 orders of magnitude lower than in H2SO4. The pKa of H2SO4 is -3, while the pKa of trifluoroacetic acid is -0.3 in H2O.

protonation of 1 to afford cationic complex 2 should be significantly less favorable than in more strongly acidic media typically employed in methane oxidation.14-16 Our group has recently developed a procedure to directly compare Pt catalysts for H/D exchange between C6H6 and TFA-d.17,18 Thus, precatalysts 1 and 3 were evaluated under our standard conditions (2 mol % [Pt], 4 mol % AgOAc, 0.5 M C6H6 in TFA-d at 75 and 150 °C) after 2 and 24 h. The extent of H/D exchange was assayed by GCMS,18 and the data are summarized in Figure 5. Interestingly, 1 and 3 provided very similar TON’s under all conditions examined. In both cases, very little H/D exchange activity was observed at 75 °C, even after 24 h (TON values after 24 h are 19 and 15, respectively, for 1 and 3). Both showed higher activity at 150 °C, with similar TON values (202 and 187 after 24 h) and turnover frequencies (TOF = 0.8 and 0.3 min-1 after 2 h) at this temperature. To gain insights into the similar reactivities of 1 and 3 in C6H6/TFA-d H/D exchange, we probed the structure of the active Pt catalyst in both systems. We have previously demonstrated that complex 1 reacts rapidly with AgOAc in TFA-d to form (bpym)Pt(TFA)2 (8; TFA = O2CCF3).18 (15) Ernst, S.; Kaim, W. J. Am. Chem. Soc. 1986, 108, 3578. (16) Negligible H/D exchange was observed between CH4 and TFA-d with precatalyst 1 or 3. (17) (a) Hodges, R. J.; Webster, D. E.; Wells, P. B. J. Chem. Soc., Chem. Commun. 1971, 462. (b) Hodges, R. J.; Webster, D. E.; Wells, P. B. J. Chem. Soc. A 1971, 3230. (c) Hodges, R. J.; Webster, D. E.; Wells, P. B. J. Chem. Soc., Dalton Trans. 1972, 2571. (d) Gerdes, G.; Chen, P. Organometallics 2004, 23, 3031. (e) Ziatdinov, V. R.; Oxgaard, J.; Mironov, O. A.; Young, K. J. H.; Goddard, W. A.; Periana, R. A. J. Am. Chem. Soc. 2006, 128, 7404. (f) Kloek, S. M.; Goldberg, K. I. J. Am. Chem. Soc. 2007, 129, 3460. (18) Hickman, A. J.; Villalobos, J. M.; Sanford, M. S. Organometallics 2009, 28, 5316.

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Similarly, dissolving 5 in TFA-d at room temperature in the presence of 2 equiv of AgOAc led to the formation of a single detectable organometallic product, along with release of 1 equiv of CF3CO2CH3 and 2 equiv of AcOD. On the basis of the stoichiometry of organic byproducts as well as the observed 1H and 13C NMR resonances (which show an unsymmetrical species in which the N-methyl group is intact),18 this new complex is proposed to be [(N-CH3-bpym)Pt(TFA)2](TFA) (7) (eq 6).

Heating 7 at 150 °C for 15 min in TFA-d in a flame-sealed NMR tube resulted in complete disappearance of its diagnostic 1H NMR resonances and concomitant formation of symmetrical (bpym)Pt(TFA)2 (8) along with 1 equiv of CF3CO2CH3 (eq 7).18 This result indicates that nucleophilic demethylation of the N-CH3-bpymþ ligand is very fast under the H/D exchange conditions. Although demethylation was slower at 75 °C, significant (∼30%) decomposition of 7 to 8 was observed after 24 h even at this temperature.19 In sum, these data indicate that 7 readily decomposes to 8 at elevated temperatures in TFA-d. On this basis, we hypothesize that the similar H/D exchange reactivity of 1 and 3 is likely due to the generation of the same active catalyst, 8, under the reaction conditions.

Figure 6. Turnover numbers for H/D exchange between CH4 and D2SO4 catalyzed by 1, 3, and 4. Conditions: PtII catalyst (1 mol %, 20 μmol), CH4 (∼13 bar, 2.0 mmol, 1 equiv), D2SO4 (1 mL, 20 mmol, 10 equiv) at 200 °C for 24 h. Reported TON’s are the average of two trials.

bpym under these conditions. Investigation of the active catalyst in D2SO4 by 1H NMR spectroscopy is consistent with rapid demethylation of 1 (see the Supporting Information). Therefore, it is likely that both 1 and 3 are converted to the N-protonated species 2 in situ; however, we were unable to definitively characterize the active species in sulfuric acid.4,6,7 We next examined the catalytic activity of 1, 3 (generated in situ from 5),20 and 4 in the H/D exchange between CH4 and D2SO4, which serves as a direct assay for methane C-H activation. The three catalysts were compared under a standard set of conditions (1 mol % of [Pt], 2 mmol of CH4, 1 mL of D2SO4, 200 °C, 24 h).4,21 The headspace gases were analyzed by GCMS, and the distribution of CDnH4-n isotopologues was determined using a published worksheet (see the Supporting Information).22 As summarized in Figure 6, turnover numbers (TON’s)23 of 193, 208, and 151 were obtained with 1, 3, and 4, respectively.24 The similarity between the TON’s for 1 and 3 suggests that the N-methylated ligand does not offer a significant advantage over simple

(19) Complex 8 was also the sole inorganic product observed when 1 was heated to 150 °C for 15 min in TFA-d in the presence of 2 equiv of AgOAc. (20) Complex 3 is expected to be generated in situ by protonation of the OCH3 group by D2SO4 followed by an SN1 reaction to aromatize the N-CH3-bpymþ ligand. (21) Young, K. J. H.; Meier, S. K.; Gonzales, J. M.; Oxgaard, J.; Goddard, W. A.; Periana, R. A. Organometallics 2006, 25, 4734. (22) Notably, under these conditions, all three of the catalytic reaction solutions remained homogeneous, and no Pt black or other precipitate was observed after 24 h. (23) Turnover numbers were calculated as mol of D incorporated per mol of catalyst minus the background D incorporation in the absence of catalyst. (24) The moderately lower reactivity of 4 versus that of 1 suggests that the DMSO may inhibit H/D exchange.

Conclusions In conclusion, we have developed a new, efficient synthetic route to [(N-CH3-bpym)PtCl2]þ (3). Complex 3 catalyzes H/D exchange between D2SO4 and CH4 as well as between CF3CO2D and C6H6 with turnover numbers similar to those for (bpym)Pt(Cl)2 (1). In both cases, the similar reactivities are proposed to result from in situ demethylation of the N-CH3-bpymþ precatalyst. These results indicate that more stable ligands are needed to definitively study the influence of quaternized nitrogen substituents on Pt C-H activation catalysts. The preparation of such ligands and their application in arene and alkane H/D exchange and oxidation reactions is currently underway in our laboratory and will be reported in due course.

Experimental Section General Procedures. 1H and 13C NMR spectra were recorded on Varian Inova 500 or 400 MHz NMR spectrometers with the residual solvent peak (CDCl3: 1H 7.27 ppm, 13C 77.23 ppm) as the internal reference unless otherwise noted. Chemical shifts are reported in parts per million (ppm) (δ). Multiplicities are reported as follows: br (broad resonance), s (singlet), t (triplet), q (quartet), d (doublet), m (multiplet), app (apparent). Coupling constants (J) are reported in Hz. Infrared (IR) spectroscopy was performed on a Perkin-Elmer FTIR. Peaks are reported in

Article cm-1. Elemental analyses were performed by Atlantic Microlab, Inc., Norcross, GA. Materials and Methods. Dichloromethane (CH2Cl2), dimethyl sulfoxide (DMSO), and methanol (CH3OH) were purchased from Aldrich and used as received unless otherwise noted. Diethyl ether (Et2O), ethyl acetate (EtOAc), and acetonitrile (MeCN) were obtained from EM Science and used as purchased. Ethanol (EtOH), 200 proof, was obtained from Deacon Laboratories, Inc., and used as received. Celite was purchased from EM Science. Silver acetate (AgOAc), Me2SO4, NaPF6, bipyrimidine (bpym), and [Me3O]BF4 were purchased from Aldrich. (DMSO)2PtCl2 was prepared according to a literature procedure.25 D2SO4 and CF3CO2D (TFA-d) were purchased from Cambridge Isotopes Lab in 10 g ampules and were stored in Schlenk tubes under N2. Ultrahigh-purity methane gas was obtain from Airgas, Inc. Stock solutions of AgOAc were prepared using volumetric glassware, and all liquid reagents were dispensed by difference using gastight Hamilton syringes. Benzene H/D exchange was carried out using a literature procedure.18 (bpym)PtCl2 (1). Complex 1 was prepared according to a literature procedure.12 The spectroscopic data for this complex matched those reported in the literature. Anal. Calcd for C8H6Cl2N4Pt: C, 22.65; H, 1.43; N, 13.21. Found: C, 22.83; H, 1.32; N, 13.16. [(bpym)Pt(DMSO)Cl][CH3OSO3] (4-CH3OSO3). Under N2, (bpym)PtCl2 (1; 344 mg, 0.81 mmol, 1.0 equiv) was dissolved in dry DMSO (60 mL, dried over 4 A˚ molecular sieves). Me2SO4 (0.65 mL, 7.1 mmol, 8.75 equiv) was added dropwise at room temperature over 5 min. The resulting mixture was heated at 60 °C for 15 h. The solvent volume was reduced to 10 mL by vacuum distillation. Ethanol (15 mL) was then added to precipitate the product. The resulting light yellow solid was collected, washed with ethanol (2  10 mL) and Et2O (1  10 mL), and then dried under vacuum. The product was obtained as a pale yellow solid (361 mg, 77% yield). 1H NMR (500 MHz, DMSO-d6): δ 9.88 (dd, J=5.8, 1.4 Hz, 1H), 9.59 (dd, J=5.8, 1.7 Hz, 1H), 9.47 (m, 2H), 8.20 (app t, J=5.1 Hz, 1H), 8.16 (app t, J=5.3 Hz, 1H), 3.36 (s, 3H), 2.54 (s, 6H). 13C NMR (125 MHz, DMSO-d6): δ 162.5, 161.3, 160.8, 160.6, 156.3, 155.3, 124.6, 124.5, 52.8, 40.4. IR (KBr pellet, cm-1) 1584 (m), 1410 (m), 1246 (s), 1226 (s), 1128 (m). HRMS (EI): m/z calcd for [C10H12ClN4OPtS]þ 466.0068, found 466.0073. Anal. Calcd for C11H15ClN4O5PtS2: C, 22.86; H, 2.62; N, 9.69. Found: C, 22.64; H, 2.52; N, 9.49. [(bpym)Pt(DMSO)Cl][PF6] (4-PF6). [(bpym)Pt(DMSO)Cl][CH3OSO3] (4; 361 mg, 0.62 mmol, 1.0 equiv) was dissolved in deionized water (10 mL) to give a homogeneous solution. NaPF6 (341 mg, 2.0 mmol, 3.0 equiv) in deionized water (5 mL) was added. A white precipitate formed and was immediately collected via filtration and dried under vacuum. The product was obtained as a white solid (309 mg, 81% yield). 1H NMR (500 MHz, DMSO-d6): δ 9.89 (dd, J=5.8, 1.4 Hz, 1H), 9.59 (dd, J=5.8, 1.7 Hz, 1H), 9.49-9.46 (multiple peaks, 2H), 8.22-8.14 (multiple peaks, 2H), 2.54 (s, 6H). 1H NMR (400 MHz, acetone-d6): δ 10.15 (dd w/Pt satellites, 3JPt-H=39.7 Hz, J=6.1, 2.3 Hz, 1H), 9.79 (dd, w/Pt satellites, 3JPt-H =39.0 Hz, J=6.1, 1.5 Hz, 1H), 9.55 (m, 2H), 8.29 (dd, J=5.8, 4.8 Hz, 1H), 8.26 (dd, J=6.1, 4.6 Hz, 1H), 3.87 (s w/Pt satellites, 3JPt-H=20.6 Hz, 6H). 19F NMR (282 MHz, DMSO-d6): δ -70.17 (d, 1JP-F= 711.4 Hz). 19F NMR (377 MHz, acetone-d6): δ -72.53 (d, 1 JP-F =707.0 Hz). 13C NMR (100 MHz, acetone-d6): δ 162.8, 161.7, 161.1, 161.0, 156.9, 155.7, 124.8, 124.7, 45.0. IR (KBr pellet, cm-1): 1585 (m), 1416 (m), 1133 (m), 839 (s). HRMS (EI): m/z calcd for [C10H12ClN4OPtS]þ 466.0068, found 466.0063. Anal. Calcd for C10H12ClF6N4OPPtS: C, 19.63; H, 1.98; N, 9.16. Found: C, 19.67; H, 1.89; N, 9.13.

(25) Price, J. H.; Williamson, A. N.; Schramm, R. F.; Wayland, B. B. Inorg. Chem. 1972, 11, 1280.

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[N-Me(bpym)]BF4. Under N2, bipyrimidine (bpym) (100 mg, 0.63 mmol, 1.0 equiv) was dissolved in CH2Cl2 (13 mL, dried over 4 A˚ molecular sieves). The mixture was cooled to -5 °C in an ice bath. [Me3O]BF4 (103 mg, 0.69 mmol, 1.1 equiv) was then added as a solid, and the resulting mixture was stirred for 2 h at -5 °C. A white precipitate slowly formed that was subsequently filtered. The crude product was washed with CH2Cl2 (2  5 mL) and Et2O (3  5 mL) to remove unreacted bipyrimidine. The resulting mixture of mono- and dimethylated bipyrimidine was purified by recrystallization from MeOH/Et2O at -35 °C to give the product as a white powder (78 mg, 48% yield). This product was unstable; therefore, it was stored at -15 °C and used for subsequent reactions within 12 h. 1H NMR (500 MHz, CD3CN): δ 9.54 (d, J=4.9 Hz, 1H), 9.13 (d, J=4.9 Hz, 2H), 9.09 (d, J = 6.6 Hz, 1H), 8.26 (app t, J = 5.6 Hz, 1H), 7.78 (app t, J=4.9 Hz, 1H), 4.39 (s, 3H). HRMS (EI): m/z calcd for C9H9N4 173.0827, found 173.0819. (2-(4-methoxy-3-methyl-4,5-dihydropyrimidin-2-yl)pyrimidine)PtCl2 (5). Under an inert atmosphere, (DMSO)2PtCl2 (74.2 mg, 0.22 mmol, 1.0 equiv) was dissolved in CH3OH (27 mL). This mixture was heated to 60 °C, and then [N-Me(bpym)]BF4 (58 mg, 0.22 mmol, 1.0 equiv) was added. The reaction was refluxed overnight and then cooled to room temperature, resulting in the precipitation of a pale yellow solid. The precipitate was collected, washed with Et2O (3  5 mL), and dried under vacuum to afford the product as a yellow solid (77 mg, 75% yield). 1H NMR (500 MHz, CD3CN): δ 9.84 (dd, J=5.7, 1.9 Hz, 1H), 9.09 (dd, J=4.5, 1.9 Hz, 1H), 7.79 (d, J=7.6 Hz, 1H), 7.68 (app t, J=4.5 Hz, 1H), 5.69 (dd, J=7.3, 5.0 Hz, 1H), 5.49 (d, J= 5.0 Hz, 1H), 3.89 (s, 3H), 3.22 (s, 3H). 13C NMR (100 MHz, CD3CN): δ 161.9, 157.2, 155.3, 155.2, 133.4, 123.9, 106.7, 85.8, 51.6, 42.0. IR (KBr pellet, cm-1): 1562 (s). Anal. Calcd for C10H12Cl2N4OPt: C, 25.54; H, 2.57; N, 11.92. Found: C, 25.69; H, 2.51; N, 11.82. Procedure for CH4 H/D Exchange. All glassware and stir bars were treated with aqua regia, washed with copious amounts of water and acetone, and dried before each use. Figure S9 (Supporting Information) shows the glassware and setup used for these reactions. A 7 mL glass Schlenk tube with 1.6 mm walls capable of sustaining pressures up to 200 psi equipped with a resealable Teflon stopcock and side arm 14/20 female adaptor was attached to a 50 mL bulb equipped with a screwcap septum, a 14/20 male adaptor, and a gas/vacuum inlet. The Schlenk tube was charged with catalyst (0.02 mmol, 0.01 equiv, 1 mol %) in D2SO4 (1 mL, 20 mmol, 10 equiv). The entire system was then degassed by three freeze-pump-thaw cycles. The 50 mL bulb was backfilled with ultrapure methane (2.0 mmol, 1.0 equiv) three times to 1 atm at room temperature. The Schlenk tube was submerged in liquid N2 and then opened to the methane-filled bulb. The methane condensed into the reaction vessel for 10 min. After the Schlenk tube was resealed and warmed to room temperature, it was submerged up to the stopcock channel in a 200 °C oil bath for 18 h. At the end of the reaction, the reaction vessel was cooled to room temperature. The gas was re-expanded in the same 50 mL bulb and analyzed by GC-MS using a GSCarbonPLOT column obtained from Agilent Technologies. The percent deuterium incorporation was defined as the percent of C-H bonds converted to C-D bonds. Turnover numbers (TON’s) are calculated as mol of D incorporated per mol of catalyst. Reported values have been corrected for the background reaction. The reported TON is the average of at least two trials. The error associated with deconvolution is 5%.

Acknowledgment. We thank the National Science Foundation for support through the Center for Enabling New Technologies through Catalysis (CENTC). In addition, A.J.H. thanks the NSF for a graduate fellowship. We

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also thank Dr. Jeff Kampf for crystallography support, Brannon Gary for repeating several CH4 H/D exchange experiments, and Professors Karen Goldberg, Bill Jones, Mike Heinekey, and Elon Ison for valuable discussions.

Villalobos et al. Supporting Information Available: Text and figures giving additional experimental details and characterization data and CIF files giving crystal data for 4-PF6 and 6. This material is available free of charge via the Internet at http://pubs. acs.org.