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ACS Symposium Series 885, Activation and Functionalization of C-H Bonds, Karen I. Goldberg and Alan S. Goldman, eds. 2004. Chapter 19 C-H Bond Activation with Neutral Platinum Methyl Complexes Carl N. Iverson,1 Charles A. G. Carter1, John D. Scollard,2 Melanie A. Pribisko,1 Kevin D. John1, Brian L. Scott1, R. Tom Baker,1,* John E. Bercaw,2,* and Jay A. Labinger2,* 1 Los Alamos Catalysis Initiative, Chemistry Division, MS J514, Los Alamos National Laboratory, Los Alamos, NM 87545 2 Department of Chemistry, California Institute of Technology, Pasadena CA 91125 The selective metal-catalyzed oxidation of alkanes to alcohols offers immense opportunities for saving energy and reducing waste in the petroleum and chemical manufacturing industries. Robust transition metal complexes with chelating nitrogen ligands show great promise as homogeneous catalysts for this reaction, but a practical system has yet to be identified. In this work a variety of neutral platinum methyl complexes with bidentate anionic N,N- and N,C-donor ligands were prepared. Ligand backbones included one, two, and three-atom bridges between the donor atoms. Redox properties of the new complexes were investigated using competition studies with I2 and evaluating equilibrium constants between divalent methyl complexes and their tetravalent diiodides. The ease of oxidation was amidinate > β-diketiminate > iminopyrrolide. Studies of benzene C-H bond activation using iminopyrrolide platinum complexes were consistent with rate determining benzene association and revealed a novel geometric effect on the rate for iminopyrrolide ligands. © 2004 American Chemical Society 319 320 Introduction Shilov et al. first demonstrated Pt(II)-catalyzed oxidation of alkanes to alcohols and alkyl chlorides some 30 years ago with simple platinum salts(1). In spite of the limitations imposed by the Pt(IV) stoichiometric oxidant and poor catalyst lifetime due to Pt metal precipitation, this mild (120°C) alkane functionalization is notable for its selectivity (1º > 2º > 3º) that is opposite that of radical processes. Much effort has been expended to elucidate the mechanism of this transformation(2-3). The first step is proposed to involve electrophilic activation of an alkane C–H bond by a Pt(II) salt to form a metal alkyl complex. Subsequent oxidation of the Pt(II) alkyl to Pt(IV) is followed by nucleophilic attack at the Pt(IV) alkyl by water or chloride to form the alcohol or alkyl halide and regenerate the starting Pt(II) salt. More recently, Periana et al. reported that solutions of bipyrimidine platinum dichloride in boiling sulfuric acid selectively oxidize methane to methylbisulfate in high one-pass yields(4-5). Subsequent studies showed that nitrogen-ligated, cationic platinum methyl complexes with diamine or diimine ligands undergo facile C-H bond activation reactions in non-coordinating solvents(6,7). While neutral platinum dimethyl complexes with these ligands could also be oxidized with dioxygen in methanol to give Pt(IV) alkyl products(8), cationic Pt methyl complexes derived from methane C-H bond activation proved more difficult to oxidize(9). As a result, platinum methyl complexes with monoanionic nitrogen-based chelates were investigated for their ability to activate C-H bonds and undergo facile oxidation reactions(10). The first example, reported by Goldberg et al., involved a tris(3,5dimethylpyrazolyl)borate (Tp’) platinum complex that oxidatively adds alkane C-H bonds to afford a Pt(IV) methyl alkyl hydride complex stabilized in a sixcoordinate geometry by the tridentate Tp’ ligand(11). More recently, the same group used a bulky β-diketiminate ligand to support the first five-coordinate Pt(IV) methyl compound(12), and further work with Tp ligands by the groups of Templeton and Keinan afforded a rare η2-benzene complex and showed reversible H/D exchange of methyl and hydride positions with alcohol solvent, respectively(13,14). Finally, Peters et al. have developed a bis(phosphinoalkyl)borate ligand that has also shown C–H bond activation capabilities when ligated to platinum(15). In this work a variety of neutral platinum methyl complexes ligated with bidentate anionic N,N- and N,C-donor ligands were prepared. Ligand backbones included one,- two,- and three-atom bridges between the donor atoms. Several of the resulting complexes were then compared for their ability to activate C-H bonds and their ease of oxidation to Pt(IV) methyl products. © 2004 American Chemical Society 321 Preparation and Characterization of Platinum Methyl Complexes with Anionic Bidentate Ligands The ligands 1-5 used in this study were prepared by standard literature procedures(16-20). Reactions of PtMeCl(cod) (6, cod = 1,5-cyclooctadiene) Cy N Ar Ar N N Ar N Li N N Li N Cy N H 1 N S K N N Ar i 2a: R = 4-MeO-Ph Ph2B 3: R = 2,6- Pr2-Ph 2b: R = 4-CF3-Ph 4a: R = 4-MeO-Ph 4b: R = 4-Cl-Ph 5 2c: R = 3,5-(CF3-)2-Ph i 2d: R = 2,6- Pr2-Ph 2e: R = 4-Me-Ph with ligands 1, 5 or deprotonated 2 afforded five-coordinate platinum methyl complexes 7-9 (Eqs. 1-3). Cy N (cod)Pt(Me)Cl + Me Li Pt N N THF N Cy 1 6 Cy Cy 7 Ar Me N (cod)Pt(Me)Cl + N H (2) Pt N N Ar KOtBu 6 (1) THF 2a 8a: Ar = 4-MeO-Ph Me N N (cod)Pt(Me)Cl + Ph2B K THF Pt N N 6 5 © 2004 American Chemical Society 9 N N BPh 2 N N (3) 322 These complexes are all proposed to have a pseudo trigonal bipyramidal geometry with an apical methyl group, based on the similar values of JHPt (73, 78 Hz; Table I) and the molecular structure of 7 as determined by X-ray diffraction (Figure 1). Figure 1. Molecular Structure of PtMe(cod)(CyNCMeNCy), 7 Table I. 1H NMR chemical shifts and Pt-H Coupling Constants (Hz) of Divalent Platinum Methyl Complexes Complex Solvent δ Pt-Me, 2JHPt δ Pt-SMe2, 3JHPt 7 M 0.70, 73 8a C 0.63, 78 9 A 0.76, 73 13 B 1.13, 83 1.73, 52 14a B 1.31, 79 1.52, 51 14b B 1.26, 80 1.43, 51 14c B 1.22, 81 1.45, 52 15a B 0.74, 73 1.73, 57 15b B 0.58, 73 1.68, 58 15d B 0.42, 73 1.71, 58 16 B 1.72, 84 1.58, 29 17 B 1.30, 79 18a B 0.33, 72 1.48, 50 18b M -0.20, 78 2.01, 46 19 B 0.38, 75 2.05, 54 A = acetone-d6, B = C6D6, C = CDCl3, M = CD2Cl2 © 2004 American Chemical Society 323 Four coordinate platinum methyl complexes 13-19 were prepared by a similar salt metathesis route from PtMeCl(SMe2)2 10 or via protonolysis of a methyl group in [PtMe2(µ-SR2)]2 (11, R = Me; 12, R= Et) by the N-H bond of the neutral ligand (Eqs. 4-9). With the exception of the bulky 2,6diisopropylphenyl-substituted example, the unsymmetrical iminopyrrolide ligands 2 gave both stereoisomers from the metathesis route, but a single isomer from the protonation route. Complexes 14 and 15 are labeled cis and trans, respectively, according to the relationship of the anionic methyl and pyrrolide ligands. Reaction of 11 with ligand 3 yielded a methyl complex with an N,Cchelate arising from orthometallation of the β−C-H bond of the thiophene ring(21). The analogous diethylsulfide dimer, 12, also reacts with the neutral iminopyrollides via deprotonation (eq. 8), albeit at a much slower rate, to produce the cis product 17. Characterization of complexes 14 and 15 was facilitated by the magnitude of JHPt for both Pt-Me and SMe2 groups (Table I) that reflected the donor nature of the trans ligand. The two-bond H-Pt coupling constant for the Pt-Me group in 14 (trans to the weaker σ-donor imine) averaged 80 Hz vs. 73 Hz for 15 with Pt-Me trans to pyrrolide. Likewise, the coupling constants for the SMe2 resonances were 58 Hz in 14 vs. 51 Hz for 15. Although comparisons with other complexes were complicated by the different backbone lengths, the coupling constant differences between the symmetrical, three atom bridge βdiketiminate and bis(pyrazolyl)borate ligands were consistent with the higher donor power of the former. Pt(Me)Cl(SMe2)2 Cy Cy N + Li N THF N Cy 10 Pt(Me)Cl(SMe2)2 + THF 2-Li Me 13 Ar N SMe2 Pt N Me 14a: Ar = 4-MeO-Ph 14c: Ar = 4-CF3-Ph Ar N + Me Pt N SMe2 15a: Ar = 4-MeO-Ph 15c: Ar = 4-CF3-Ph i 15e: Ar = 2,6- Pr2-Ph © 2004 American Chemical Society (4) Cy Li N 10 N 1 Ar N SMe2 Pt (5) 324 Ar N Ar N THF [PtMe2(µ-SMe2)]2 + N H 11 14a: Ar = 4-MeO-Ph 14c: Ar = 4-CF3-Ph Ar N + S 11 Ar - CH4 Me S Ar N C6H6 + Li Ar N THF N Ar K 5 © 2004 American Chemical Society Me 18a: Ar = 4-MeO-Ph 18b: Ar = 4-Cl-Ph N N 10 (9) Ar 4 N N SMe2 Pt N + Ph2B Me N Ar Pt(Me)Cl(SMe2)2 (8) 17: Ar = 4-MeO-Ph N 10 SEt2 Pt - CH4 2a + (7) i N H Pt(Me)Cl(SMe2)2 SMe2 Pt 16: Ar = 2,6- Pr2-Ph Ar N 12 N CH2Cl2 3 [PtMe2(µ-SEt2)]2 (6) Me N 2a,c [PtMe2(µ-SMe2)]2 SMe2 Pt THF N N Ph2B SMe2 Pt N N 19 Me (10) 325 Complexes 13, 16 and 17 were also characterized by X-ray diffraction (Figure 2) and the effect of different chelate ligands on geometric parameters of the Pt-Me and Pt-SMe2 moieties was examined. The Pt-C bond distances (Table II) are nearly invariable among the complexes; however, the Pt-S bond in 16 is ~0.1 Å longer than in either 13 or 17. A related structure reported by Templeton has bond lengths similar to those observed in 13 and 17(22). The Pt-C and Pt-S bonds in Tp'PtMe(SMe2) are 2.049 and 2.249Å, respectively. Steric interactions between the 2,6-diisopropylphenyl substituent of the imine and the dimethylsulfide ligand, due to their mutual cis arrangement, is a likely reason for this lengthening. The longer bond is also reflected in the 3JHPt value of 29 Hz for 16 in comparison to ~51 Hz for complexes 14. a) b) c) Figure 2. Molecular Structures of a) PtMe(SMe2)(CyNCMeNCy), 13, b) PtMe(SMe2)[2-(N-[2,6-(i-Pr)2-Ph]imino)thiophenide], 16, and c) PtMe(SEt2)[2-(N-[4-MeO-Ph]imino)pyrollide], 17. © 2004 American Chemical Society 326 Table II. Selected bond lengths (Å) for complexes 13,16 and 17 Complex Pt-Me Pt-SMe2 13 2.048(6) 2.238(2) 16 2.041(6) 2.352(2) 17 2.077(9) 2.249(2) Redox Properties of Neutral Platinum Methyl Complexes In previous work we determined the ease of oxidation of nitrogenligated platinum complexes from competition reactions with I2 and also evaluated equilibrium constants between divalent methyl complexes and their tetravalent diiodides(9). Complexes 13-15 and 18 all reacted cleanly with iodine to afford the trans diiodides 20-23 (Eq. 11, complexes 21 and 22 are produced from 14 and 15, respectively). Complex 19 did not react with iodine at room temperature over several days. Presumably, the steric bulk from the two phenyl rings attached to boron in addition to dimethyl substitution of the pyrazole rings prevents I2 from reacting with the platinum metal center(23). The reduction of electron density in the tetravalent diiodides (vs. the Pt(II) starting compounds) is reflected in the greatly reduced values of JHPt for the SMe2 resonances (Table III)(24). Competition reactions with pairs of complexes indicated that the ease of oxidation of the divalent platinum methyl complexes is 13 > 18 > 15 > 14. N SMe2 Pt N + I2 N I SMe2 Pt N I Me 20-23 C 6D 6 Me 13-15, 18 20: N-N = Cy N N Cy (11) Ar Ar N 21, 22: N-N = (cis, trans) 23: N-N = N N N Ar This ordering was also reflected in estimates of the equilibrium constants between pairs of divalent methyl complexes and the corresponding tetravalent © 2004 American Chemical Society 327 diiodides (Eq. 12). Representative equilibrium constants are given in Table IV. The reaction of 13 with 23, for example, shows the equilibrium greatly favors Table III. 1H NMR chemical shifts and Pt-H Coupling Constants (Hz) of Tetravalent Methylplatinum Diiodides in C6D6 Complex 20 21a 21b 22a 22b 23a δ Pt-Me, 2JHPt 2.65, 70 2.79 ,70 2.74 ,71 2.30, 67 2.14, 67 1.87, 67 δ Pt-SMe2, 3JHPt 2.05, 31 1.72, 32 1.68, 33 2.05, 36 1.99, 38 1.66, 32 the amidinate ligated Pt(IV) complex as 23 converts completely to complex 18b as judged by 1H NMR spectroscopy. In accordance with electronic substitution effects of the aryl group, the more nucleophilic complex 14a (p-OMe substituted) favors oxidation over 14b (p-CF3 substituted). Also, from these equilibrium oxidation studies it would appear that the trans products (15) of the iminopyrrolide ligated complexes are more electron rich than the corresponding cis products (14). N' SMe2 + Pt N' Me N I SMe2 Pt N I Me C 6D 6 I SMe2 Pt + N' I Me N' N SMe2 Pt N Table IV. Equilibrium Constants for Reactions of Pt(II) with Pt(IV) Complexes in C6D6 Equilibrium system [20][18a]/[13][23a] [20][14a]/[13][21a] [23a][14a]/[18a][21a] [21a][14b]/[14a][21b] [21b][15b]/[14b][22b] © 2004 American Chemical Society Q ≥ 13 ≥ 400 3.0 2.8 2.9 Me (12) 328 Electrophilic Activation of Benzene C-H Bonds with Neutral Platinum Methyl Complexes The reactivity of platinum methyl complexes 13-15, 18 and 19 with benzene at 85°C was monitored by 1H NMR spectroscopy (eq. 13). While most of the complexes reacted sluggishly (13, 14a,b, 18b) or not at all (19), iminopyrrolide complexes 15a,b,d activated solvent C-D bonds to form a platinum phenyl product with concomitant formation of CH3D within hours. N' SMe2 Pt N C6D6, 85 oC - CH3D Me N' SMe2 (13) Pt N Ph-d5 13-15, 18, 19 Electronic effects on the rate of benzene C-H bond activation (cf. 15b is ca. four times faster than 15a) were consistent with an electrophilic reaction. More surprising was the ca. 80-fold faster reactions observed for 15 in which the Pt-Me is trans to the pyrrolide nitrogen, vs. isomeric 14 with Pt-Me trans to the imine nitrogen (Scheme 1). We suggest that rate-determining associative displacement of SMe2 by benzene accounts for these differences(25) and the greater electronegativity of the ligand in 14b and 15b would tend to favor associative displacement. Scheme 1 4-CF3-Ph 4-CF3-Ph N N Me Pt SMe2 N t1/2 o C6H6, 85 C - CH3D N Ph Pt SMe2 N 14b:14 days 4-OMe-Ph N N Me SMe2 SMe2 Me N 4-OMe-Ph N t1/2 Pt + 15b: 1 hr Pt 4-CF3-Ph + 15a:4 hr © 2004 American Chemical Society 4-OMe-Ph Pt N SMe2 Me 14a: 3 days o N C6H6, 85 C - CH3D Ph Pt N SMe2 329 An associative mechanism is favored in systems with little steric hindrance and also where the leaving group is trans to a poor donor(26). However, the trigonal bipyramidal intermediate for such displacement in trans complexes 15 would have the better π-acceptor imine ligand equatorial and the better σ-donor pyrrolide axial, the strongly preferred conformation (Scheme 2). Slower displacement in the cis isomer must pass through a higher-energy intermediate and is accompanied by isomerization to the thermodynamically preferred cis isomer of the platinum phenyl product. Scheme 2 Ar Ar N Me Pt L SMe2 N N Me Pt N L trans, 15 favored Ar Ar N Pt N SMe2 Me cis, 14 L N Pt N SMe2 SMe2 L Me disfavored A series of SMe2/SMe2-d6 ligand exchange reactions were performed with complexes 13-15, 18, and 19. The rate of sulfide exchange correlates well with the observed trend in solvent C-H bond activations. For instance, when a 1:1 mixture of 14b and 15b was treated with SMe2-d6 the resonance for coordinated SMe2 in 15b disappeared within one hour whereas the corresponding resonance in 14b required 5 days for exchange to occur. Slow exchange is also observed for complexes 13 and 18a. The borate complex 19 is again the slowest in terms of reactivity, not showing any appreciable exchange at room temperature after several days. In addition, the presence of excess sulfide isomerizes 15b to the more stable thermodynamic complex 14b. Sulfide exchange is also observed between complexes in a crossover experiment involving co-thermolysis of 14e and 17 in C6D6 (Eq. 14); however, the process is very sluggish with respect to the complex/ligand exchange reactions. The mixture requires heating at 85 °C for 48 hours before significant © 2004 American Chemical Society 330 (~ 35%) sulfide exchange is observed. C–D bond activation of the solvent was also observed but it occurs at an even slower rate than the ligand crossover. Me N Pt OMe SMe2 + Me N N Pt N Me SEt2 85 oC N Pt Me OMe N SEt2 + Pt Me N N SMe2 (14) Me In contrast to these sluggish reactions, reaction of 12 with bulky iminopyrrole 2d in perdeuterobenzene at 25°C over several hours afforded two equivalents of methane isotopomers and trans-phenyl complex 24 (Eq. 15). Ca. 10% of trans methyl complex 15e was also formed and methyl groups of the isopropyl substituents were partially deuterated(27). In this reaction we propose Ar Ar N N C 6D 6 1/2 [PtMe2(µ-SMe2)]2 + Pt NH 12 N 2d C 6D 5 SMe2 + 2 CH4-xDx (15) 24d: Ar = 2,6-iPr2Ph + 10% 15d that benzene C-H bond activation must precede chelation since preformed complexes 15 only react with benzene at elevated temperature. The fact that only trans products are observed in Eq. 15 is presumably a consequence of steric interactions between the SMe2 ligand and the ortho-isopropyl substituents. Conclusions Neutral platinum methyl complexes have been shown to be easily oxidized to tetravalent complexes and to activate benzene C-H bonds under mild conditions. In this study, the inherent reactivity of N,N-chelated platinum methyl complexes toward hydrocarbon C-H bonds was masked by the need to substitute strongly coordinating dialkylsulfide ligands. Use of unsymmetrical iminopyrroles and iminopyrrolide ligands shed some light on this process and further work is in progress(28) to identify nitrogen-ligated systems for alkane functionalization outside of strong acid media. © 2004 American Chemical Society 331 Acknowledgements Akzo Nobel, Inc., the US Department of Energy’s Office of Industrial Technologies, the Laboratory-Directed Research and Development program at Los Alamos, and BP are gratefully acknowledged for support and CNI thanks Los Alamos for a Director’s Funded Postdoctoral Fellowship. References 1. 2. 3. 4. 5. 6. 7. Gol’dshleger, N.F.; Es’kova, V.V.; Shilov, A.E.; Shteinman, A.A. Zh. Fiz. Khim. (Engl. Transl.) 1972, 46, 785. a) Kushch, L.A.; Lavrushko, V.V.; Misharin, Y.S.; Moravsky, A.P.; Shilov, A.E. Nouv. J. Chem. 1983, 7, 729-. b) Horvath, I.T.; Cook, R.A.; Millar, J.M.; Kiss, G. Organometallics 1993, 12, 8. c) Hutson, A.C.; Lin, M.R.; Basickes, N.; Sen, A. J. Organomet. Chem. 1995, 504, 69-74. d) Zamashchikov, V.V.; Popov, V.G.; Rudakov, E.S.; Mitchenko, S.A. Dokl. Akad. Nauk SSSR 1993, 333, 34. a) Labinger, J.A.; Herring, A.M.; Lyon, D.K.; Luinstra, G.A.; Bercaw, J.E.; Horvath, I.T.; Eller, K. Organometallics 1993, 12, 895. b) Luinstra, G.A.; Labinger, J.A.; Bercaw, J.E. J. Am. Chem. Soc. 1993, 115, 3004. c) Luinstra, G.A.; Wang, L.; Stahl, S.S.; Labinger, J.A.; Bercaw, J.E. Organometallics 1994, 13, 755. d) Luinstra, G.A.; Wang, L.; Stahl, S.S.; Labinger, J.A.; Bercaw, J.E. J. Organomet. Chem. 1995, 504, 75. e) Stahl, S.S.; Labinger, J.A.; Bercaw, J.E. J. Am. Chem. Soc. 1995, 117, 9371-9372. f) Stahl, S.S.; Labinger, J.A.; Bercaw, J.E. J. Am. Chem. Soc. 1996, 118, 5961-5976. Periana, R.A.; Taube, D.J.; Gamble S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560-564. Periana’s catalytic system of has been examined in detail via theoretical methods. See: a) Kua, J.; Xu, X.; Periana, R.A.; Goddard, III, W.A. Organometallics 2002, 21, 511-525. b) Gilbert, T.M.; Hristov, I.; Ziegler, T. Organometallics 2001, 20, 1183-1189. c) Mylvaganam, K.; Bacskay, G.B.; Hush, N.S. J. Am. Chem. Soc. 2000, 122, 2041-2052. a) Holtcamp, M. H.; Labinger, J.A.; Bercaw, J.E. J. Am. Chem. Soc. 1997, 119, 848-849. b) Johansson, L.; Tilset, M.; Labinger, J.A.; Bercaw, J.E J. Am. Chem. Soc. 2000, 122, 10846-10855. c) Zhong, H.A.; Labinger, J.A.; Bercaw, J.E. J. Am. Chem. Soc. 2002, 124, 1378-1399. a) Johansson, L.; Ryan, O.B.; Tilset, M. J. Am. Chem. Soc. 1999, 121, 1974-1975. b) Johansson, L.; Ryan, O.B.; Romming, C.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 6579-6590. © 2004 American Chemical Society 332 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. a) Rostovtsev, V.V.; Labinger, J.A.; Bercaw, J.E.; Lasseter, T.L.; Goldberg, K.I. Organometallics 1998, 17, 4530-4531. b) Rostovtsev, V.V.; Henling, L.M.; Labinger, J.A.; Bercaw, J.E. Inorg. Chem. 2002, 41, 3608-3619. Scollard, J. D.; Day, M.; Labinger, J.A.; Bercaw, J.E. Helv. Chim. Acta 2001, 84, 3247-3268. a) Baker, R. T.; Watkin, J.; Carter, C. A. G.; Bercaw, J. E.; Labinger, J. A.; Whitwell, G. E., Abstr. I&EC 7, 218th ACS Meeting, New Orleans, LA, August 22, 1999. b) Carter, C. A. G.; Day, M. W.; John, K. D.; Scollard J. D.; Baker, R. T.; Bercaw, J. E.; Labinger, J. A., Abstr. INOR 161 , 223rd ACS Meeting, Orlando, FL, April 8, 2002. Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1997, 119, 10235-10236. Fekl, U.; Kaminsky, W.; Goldberg, K.I. J. Am. Chem. Soc. 2001, 123, 65796590. a) Reinartz, S.; White, P.S.; Brookhart, M.; Templeton, J.L. J. Am. Chem. Soc. 2001, 123, 12724-12725. b) Reinartz, S.; White, P.S.; Brookhart, M.; Templeton, J.L. Organometallics, 2001, 20, 1709-1712. a) Lo, H.C.; Haskel, A.; Kapon, M.; Keinan, E. J. Am. Chem. Soc. 2002, 124, 3226-3228. b) Iron, M.A.; Lo, H.C.; Martin, J.M.L.; Keinan, E. J. Am. Chem. Soc. 2002, 124, 7041-7054. Thomas, J.C.; Peters, J.C. J. Am. Chem. Soc. 2001, 123, 5100-5101. Reviews on amidinate chemistry: a) Edelman, F.T. Coord. Chem. Rev. 1994, 137, 403. b) Barker, J.; Kilner, M. Coord. Chem. Rev. 1994, 133, 219. iminopyrrole ligands: Tanaka, T.; Tamauchi, O. Chem. Pharm. Bull. 1961, 9, 588. iminothiophene ligand: Drisko, R. W.; McKennis, Jr., H. J. Am. Chem. Soc. 1952, 74, 2626-2628. β-diketiminate (nacnac) ligands: Parks, J.E.; Holm, R.H. Inorg. Chem. 1968, 7, 1408-1416. b) McGeachin, S.G. J. Can. Chem. 1968, 46, 19031912. Komorwski, L.; Maringgele, W.; Meller, A.; Niedenzu, K.; Serwatowski, J. Inorg. Chem. 1990, 29, 3845-3849. β-metalation of iminothiophene by dinuclear metal complexes has been reported previously: Wang, D.-L.; Hwang, W.-S.; Liang, L.-C.; Wang, L.-I.; Lee, L.;Chiang, M. Y. Organometallics. 1997, 16, 3109-3113; Imhof, W. J. Organomet. Chem. 1997, 533, 31-43. Reinartz, S. White, P.S.; Brookhart, M.; Templeton, J.L. Organometallics 2000, 19, 3854-3866. Oxidative addition of methyl iodide to a similar square planar Pt(II) complex supported by the less sterically demanding bis(pyrazolyl)ethane ligand has been reported. Byers, P.K.; Canty, A.J.; Honeyman, R.T.; Skelton, B.W.; White, A.H. J. Organomet. Chem. 1992, 433, 223-229. Only a small reduction in 2JHPt values is seen for the Pt-Me group protons of complexes 20-23. These values, however, are typical for methyl groups trans to nitrogen ligands in Pt(IV) complexes. For example, see refs. 11, 12, © 2004 American Chemical Society 333 25. 26. 27. 28. 20, and: a) Canty, A.J.; Honeyman, R.T. J. Organomet. Chem. 1990, 387, 247-263. b) Prokopchuk, E.M.; Puddephatt, R.J. Organometallics 2003, 22, 563-566. c) Hinman, J.G.; Baar, C.R.; Jennings, M.C.; Puddephatt, R.J. Organometallics 2000, 19, 563-570. d) Hill, G.S.; Puddephatt, R.J. J. Am. Chem. Soc. 1996, 118, 8745-8746 A recent paper detailed volume of activation studies with respect to the mechanism of benzene C–H bond activation by related cationic platinummethyl complexes. This report supports a benzene based associative mechanism. Procelewska, J.; Zahl, A.; van Eldik, R.; Shong, H.A.; Labinger, J.A.; Bercaw, J.E. Inorg. Chem. 2002, 41, 2808-2810. a) Frey, U.; Helm, L.; Merbach, A.E.; Romeo, R. J. Am. Chem. Soc. 1989, 111, 8161-8165. b) Pienaar, J.J.; Kotowski, M.; van Eldik, R. Inorg. Chem. 1989, 28, 373-375. c) Cooper, J.; Ziegler, T. Inorg. Chem. 2002, 41, 66146622. Similar exchange has been observed in a related system: Fekl, U.; Goldberg, K. I. J. Am. Chem. Soc. 2002, 124, 6804-6805. Iverson, C. N.; Carter, C. A. G.; Scott, B. L.; Baker, R. T.; Scollard, J. D.; Day, M. W.; Labinger, J. A.; Bercaw, J. E., submitted for publication. © 2004 American Chemical Society