D Exchange Reactions

Jun 10, 2010 - E-mail: [email protected]. ... OTf = trifluoromethanesulfonato), were prepared and assessed by GC/MS in catalytic H/D exchange reactions ...
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Organometallics 2010, 29, 2857–2867 DOI: 10.1021/om100018x

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Effect of Ancillary Ligands and Solvents on H/D Exchange Reactions Catalyzed by Cp*Ir Complexes Yuee Feng, Bi Jiang, Paul A. Boyle, and Elon A. Ison* Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204 Received January 6, 2010

A series of complexes of the form Cp*Ir(NHC)(X)n and [Cp*Ir(NHC)(L)2][OTf]2, where NHC = 1,3,4,5-tetramethylimidazol-2-ylidene (n = 2, X = Cl- (1-Cl), NO3- (1-NO3), -OC(O)CF3 (=TFA, 1-TFA); n = 1, X = SO42- (1-SO4); L = H2O (1-H2O), CH3CN (1-CH3CN), OTf = trifluoromethanesulfonato), were prepared. X-ray crystal structures of 1-OH2, 1-SO4, and 1-NO3 and the dimeric complex [(Cp*Ir(NHC)Cl)2][OTf]2 (2) were obtained. In solution, the complex 1-TFA was found to exist in equilibrium with [Cp*Ir(NHC)(OH2)2][OCOCF3]2 (1-aqua-TFA), where the aqua ligands are strongly hydrogen bound to the -OCOCF3 counterion. A van’t Hoff plot from -10 to 30 °C yielded values for the reaction enthalpy and entropy of ΔH° = -7.6 ( 0.7 kcal/mol and ΔS° = -30.6 ( 2.4 eu, respectively. These data are consistent with the observation that at higher temperatures the complex 1-TFA is favored. An X-ray crystal structure of 1-aqua-TFA was obtained. Catalytic H/D exchange reactions between benzene and various deuterium sources (CD3OD, CF3COOD, CD3COCD3, and D2O) were performed and assessed by GC-MS. The best deuterium sources for this reaction were found to be CD3OD or CD3OD/D2O (1:1) mixtures. The highest turnover numbers (TONs) were observed for the H/D exchange reactions catalyzed by Cp*Ir(NHC) complexes with labile ligands. These results suggest that the dissociation of the ancillary ligand to form an unsaturated 16-electron intermediate is an important step prior to C-H activation in the catalytic cycle, which is consistent with the Shilov electrophilic C-H activation mechanism. In contrast, the most effective deuterium source for H/D exchange with the aqua complex [Cp*Ir(OH)3][OTf2] (3) was the acidic solvent CF3COOD. Thus, the σ-donating NHC ligand serves to attenuate the electrophilicity of the metal center so that milder reaction conditions are required for the C-H activation reactions.

Introduction Tremendous research effort has been devoted to the catalytic activation and functionalization of C-H bonds *To whom correspondence should be addressed. E-mail: eaison@ ncsu.edu. Tel: (919) 513-4376. Fax: (919) 515-8909. (1) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. (2) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154. (3) Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989, 22, 91. (4) Crabtree, R. H. Chem. Rev. 1985, 85, 245. (5) Crabtree, R. H. Chem. Rev. 1995, 95, 987. (6) Goldberg, K. I.; Goldman, A. S. Activation and Functionalization of C-H Bonds; Oxford University Press: Washington, DC, 2004. (7) Crabtree, R. H. Dalton Trans. 2001, 2437. (8) Periana, R. A.; Bhalla, G.; William J. Tenn, I.; Young, K. J. H.; Liu, X. Y.; Mironov, O.; Jones, C.; Ziatdinov, V. R. J. Mol. Catal. A: Chem. 2004, 220, 7. (9) Labinger, J. A. J. Mol. Catal. A: Chem. 2004, 220, 27. (10) Tenaglia, A.; Heumann, A. Angew. Chem., Int. Ed. 1998, 38, 2180. (11) Dyker, G. Handbook of C-H Transformations; Wiley-VCH: Weinheim, Germany, 2005. (12) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102, 1731. (13) Dyker, G. Angew. Chem., Int. Ed. 1999, 38, 1698. (14) Feng, Y.; Lail, M.; Barakat, K. A.; Cundari, T. R.; Gunnoe, T. B.; Petersen, J. L. J. Am. Chem. Soc. 2005, 127, 14174. (15) Tenn, W. J.; Young, K. J. H.; Bhalla, G.; Oxggard, J.; Goddard, W. A.; Periana, R. A. J. Am. Chem. Soc. 2005, 127, 14172. r 2010 American Chemical Society

mediated by transition-metal complexes.1-19 The most efficient catalysts reported thus far contain an electrophilic metal center, such as PdII, PtII, HgII, and AuI/II, and require strong acidic conditions because of the accumulation of H2O and CH3OH during the catalytic cycle.20,21 In order to avoid Lewis base inhibition and harsh reaction conditions, it is desirable to design transition-metal complexes with less electrophilic metal centers.22-30 (16) Feng, Y.; Lail, M.; Foley, N. A.; Gunnoe, T. B.; Barakat, K. A.; Cundari, T. R.; Petersen, J. L. J. Am. Chem. Soc. 2006, 128, 7982. (17) Meier, S. K.; Young, K. J. H.; Ess, D. H.; Tenn, W. J.; Oxgaard, J.; Goddard, W. A.; Periana, R. A. Organometallics 2009, 28, 5293. (18) Young, K. J. H.; Oxgaard, J.; Ess, D. H.; Meier, S. K.; Stewart, T.; Goddard, W. A.; Periana, R. A. Chem. Commun. 2009, 3270. (19) Ahlquist, M.; Periana, R. A.; Goddard, W. A. Chem. Commun. 2009, 2373. (20) Goldshleger, N. F.; Tyabin, M. B.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. 1969, 43, 2174. (21) Shilov, A. E.; Shteinman, A. A. Coord. Chem. Rev. 1977, 24, 97. (22) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1982, 104, 352. (23) Buchanan, J. M.; Stryker, J. M.; Bergman, R. G. J. Am. Chem. Soc. 1986, 108, 1537. (24) Arndtsen, B. A. B.; R., G. Science 1995, 270, 1970. (25) Tellers, D. M.; Yung, C. M.; Arndtsen, B. A.; Adamson, D. R.; Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 1400. (26) Klei, S. R.; Golden, J. T.; Tilley, T. D.; Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 2092. Published on Web 06/10/2010

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Iridium complexes that contain the Cp*Ir(phosphine) fragment have been shown to efficiently catalyze H/D exchange between various organic substrates and deuterium sources, including D2O.31-35 The use of the basic phosphine ligands in these catalysts seems to be essential in the C-H activation step for these catalysts.24,36,37 However, because phosphine ligands are easily oxidized, their further utilization for oxidative C-H bond functionalization is limited. N-heterocyclic carbene (NHC) ligands, on the other hand, are suitable alternatives to phosphines as ligands. NHCs, like phosphines, are strong σ-donor ligands, but they are not as easily oxidized. The development of Ir complexes with NHC ligands as potential catalysts for the activation and functionalization of C-H bonds, therefore, is highly desirable. Cp*Ir(NHC) complexes (NHC = 1-n-butyl-4,5-dichloroimidazol-2-ylidene, 1,3-di-n-butyl-4,5-dichloroimidazol-2ylidene, 1-benzyl-3-methylimidazol-2-ylidene, 1,3-bis-n-butylimidazol-2-ylidene) have been recently utilized for catalytic H/D exchange between methanol-d4 and organic substrates.38-42 However, these studies were limited to the activation of C-H bonds with bond dissociation energies lower than that of benzene. In addition, no information on the mechanism of C-H bond activation was provided for this class of compounds. In this article, a series of complexes of the form Cp*Ir(NHC)(X)n and [Cp*Ir(NHC)(L)2][OTf]2 (NHC = 1,3,4,5tetramethylimidazol-2-ylidene; X = Cl-, NO3-, -OC(O)CF3 (=TFA), n = 2; X = SO42-, n = 1; L = H2O, CH3CN; OTf = trifluoromethanesulfonato) have been synthesized and utilized for catalytic H/D exchange reactions between benzene and various deuterium sources. The reactivity of these complexes in catalytic H/D exchange reactions was assessed by GC/MS using a quantitative assay recently reported by Sanford et al.43 The influence of the ancillary ligands (Cl-, SO42-, NO3-, TFA, H2O, and CH3CN), and deuterium sources (CD3OD, CF3COOD, CD3COCD3, and D2O) on the catalytic reactivity in H/D exchange reactions were probed using this method. In addition, the influence of the NHC (27) Klei, S. R.; Tilley, T. D.; Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 1816. (28) Fujita, K.; Nakaguma, H.; Hamada, T.; Yamaguchi, R. J. Am. Chem. Soc. 2003, 125, 12368. (29) Iverson, C. N.; Smith, M. R., III. J. Am. Chem. Soc. 1999, 121, 7696. (30) Diversi, P.; Iacoponi, S.; Ingrosso, G.; Laschi, F.; Lucherini, A.; Pinzino, C.; Uccello-Barretta, G.; Zanello, P. Organometallics 1995, 14, 3275. (31) Skaddan, M. B.; Yung, C. M.; Bergman, R. G. Org. Lett. 2004, 6, 11. (32) Klei, S. R.; Golden, J. T.; Burger, P.; Bergman, R. G. J. Mol. Catal. A: Chem. 2002, 189, 79. (33) Klei, S. R.; Tilley, T. D.; Bergman, R. G. Organometallics 2002, 21, 4905. (34) Golden, J. T.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc. 2001, 123, 5837. (35) Yung, C. M.; Skaddan, M. B.; Bergman, R. G. J. Am. Chem. Soc. 2004, 126, 13033. (36) Ellames, G. J. G.; J., S.; Herbert, J. M.; Kerr, W. J.; McNeill, A. H. J. Labelled Compd. Radiopharm. 2004, 47, 1. (37) Bergman, R. G. Science 1984, 223, 902. (38) Corberan, R. S.; M. Peris, E. J. Am. Chem. Soc. 2006, 128, 3974. (39) Tanabe, Y.; Hanasaka, F.; Fujita, K.; Yamaguchi, R. Organometallics 2007, 26, 4618. (40) Prinz, M.; Grosche, M.; Herdtweck, E.; Herrmann, W. A. Organometallics 2000, 19, 1692. (41) Hanasaka, F.; Tanabe, Y.; Fujita, K.; Yamaguchi, R. Organometallics 2006, 25, 826. (42) Corberan, R.; Sanau, M.; Peris, E. Organometallics 2006, 25, 4002. (43) Hickman, A. J.; Villalobos, J. M.; Sanford, M. S. Organometallics 2009, 28, 5316.

Feng et al. Scheme 1

Scheme 2

Scheme 3

ligand itself was examined by comparing the reactivity of 1-OH2 with that of [Cp*Ir(OH2)3][(OTf)2] (3) in catalytic H/D exchange reactions between benzene and CD3OD and CF3COOD. From these investigations, valuable insights into the mechanism of C-H activation by Cp*Ir(NHC) complexes were attained.

Results and Discussion Syntheses of Iridium Complexes with N-Heterocyclic Carbene Ligands. We began our investigations with the complexes Cp*Ir(NHC)Cl2 (1-Cl) and [Cp*Ir(NHC)(NCMe)2][OTf]2 (1-NCMe) (NHC = 1,3,4,5-tetramethylimidazol-2ylidene, OTf = trifluoromethanesulfonato), which have been previously utilized in catalytic Oppenauer-type oxidations of alcohols and C-H bond activation reactions.38-42,44 In addition, derivatives of 1-Cl, Cp*Ir(NHC)(SO4) (1-SO4), Cp*Ir(NHC)(NO3)2 (1-NO3), and Cp*Ir(NHC)(TFA)2 (1-TFA; TFA = trifluoroacetate), where the anionic ligand Cl- has been replaced with SO42-, NO3-, or TFA, and a derivative of 1-NCMe, [Cp*Ir(NHC)(OH2)2][OTf]2 (1-OH2), where the neutral MeCN ligand is replaced with H2O, were also synthesized so that systematic comparisons of the reactivity of these complexes in catalytic H/D exchange reactions could be made as the ancillary ligands were varied. As depicted in Schemes 1- 4, the complexes 1-OH2, 1-SO4, 1-NO3, and 1-TFA were readily obtained from 1-Cl by metathesis with the corresponding Ag salts. Synthesis and Characterization of 1-OH2. The reaction of 1-Cl with 2 equiv of AgOTf in methylene chloride results in the formation of a yellow solid, 1-OH2 (Scheme 1), which is soluble in water and polar solvents such as CH2Cl2 but insoluble in nonpolar solvents such as Et2O, hexanes, and benzene. This complex is also stable indefinitely in solution (44) Hanasaka, F.; Fujita, K.; Yamaguchi, R. Organometallics 2005, 24, 3422.

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Scheme 4

and the solid state. In the 1H NMR spectrum (CD2Cl2) of this complex, resonances occur at 3.59 (6H, N-Me), 2.25 (6H, C-Me) and 1.63 (15H, Cp*) ppm. In addition, several absorption bands were observed in the IR spectrum (KBr) in the OTf region (1377, 1263, 1166, 1046, 1031 cm-1), indicative of both ionic and covalently bound triflate anions. This is in contrast to a recent report for the complex Cp*Ir(NHC)(OTf)2, where the IR spectrum (CH2Cl2) revealed one absorption band at 1309 cm-1 and no absorption bands in the region indicative of ionic triflate (1235-1288 cm-1).44 Cp*Ir(NHC)(OTf)2 was synthesized under an inert atmosphere, in contrast to the case for 1-OH2, which was not protected from air and moisture. The presence of an absorption band (1377 cm-1) in the region for a covalently bound triflate anion can be explained by H bonding of the -OTf anion to the coordinated water ligands (vide infra). X-ray Crystal Structure of 1-OH2. Crystals suitable for X-ray diffraction were obtained by slow diffusion of pentane into a concentrated CH2Cl2 solution of the complex at room temperature. The unit cell consisted of two chemically equivalent but crystallographically independent Ir dications, four triflate anions, and two water molecules. The structure of the cations (Figure 1) can be regarded as three-legged piano stools, with the Ir-Ccarbene bonds (2.075(6) A˚) comparable to those in known Cp*Ir(NHC) complexes.44 The Ir-O bond lengths were 2.155(4) and 2.191(6) A˚ and are comparable to an Ir-O bond length (2.156 (8) A˚) for the complex Cp*Ir(H2O)(Cl2), reported by Lu and co-workers.45 The IR data (KBr) described above for 1-OH2 in the region is suggestive of a coordinated -OTf ligand (1377 cm-1). However, a closer examination of the crystal structure reveals the existence of strong H bonding between the -OTf anions and the coordinated water ligands (see the Supporting Information). These data are also consistent with the absence of signals in the 1H NMR spectrum for the coordinated water ligands at room temperature. Rapid exchange of the protons from the aqua ligand and free water and -OTf anions will be expected to broaden the signal from these groups. The exchange of protons will be slower at lower temperatures, which is consistent with the observation of a signal at 7.95 ppm that integrates to four protons (broad at -20 °C and sharp at -78 °C) (see the Supporting Information). Synthesis and Characterization of 1-SO4. The complex Cp*Ir(NHC)(SO4) (1-SO4) was prepared by stirring Cp*Ir(NHC)Cl2 (1-Cl) and Ag2SO4 in water overnight (Scheme 2). 1-SO4 was obtained as an orange solid that is soluble in polar solvents such as H2O, CH3OH, and CH2Cl2 but is insoluble in nonpolar solvents such as benzene and Et2O. The complex is stable indefinitely, both in solution and in the solid state, and does not need protection from air and moisture. Resonances characteristic of η2 coordination of the SO42anion46 are observed in the IR spectrum (KBr) at 1120, 1140, (45) Ma, L.; Zhao, W. J.; Lu, K. Acta Crystallogr. 2006, E62, 2524. (46) Barraclough, C. G.; Tobe, M. L. J. Chem. Soc. 1961, 1993.

Figure 1. Thermal ellipsoid plot of the cation [Cp*Ir(NHC)(OH2)2]2þ (1-OH2). Thermal ellipsoids are given at the 50% probability level. Hydrogen atoms and the -OTf anions have been omitted for clarity. Selected bond distances (A˚) and angles (deg): Ir2-C11B = 2.075(6), Ir2-O1B = 2.155(4), Ir2-O2B = 2.191(6); C11B-Ir2-O2B = 81.5(2), C11B-Ir2-O1B = 87.9(2), O1B-Ir2-O2B = 81.5(2).

and 1163 cm-1. In the 1H NMR spectrum of this complex, resonances occur at 3.58 (s, 6H, NMe), 2.19 (s, 6H, CMe), and 1.67 (s, 15H, Cp*) ppm. X-ray Crystal Structure of 1-SO4. The crystal structure for 1-SO4 confirmed our assignment of η2 binding of the sulfate ligand (Figure 2). The Ir-O bond lengths were 2.1397(13) and 2.1574(13) A˚, respectively, which are comparable to Ir-O bonds for other reported Ir-SO4 complexes. For example, Atwood et al. have reported Ir-O bond lengths of 2.065(6) A˚ for Ir-SO4 bonds of the Ir(OMe)(CO)(PPh3)2(SO4) complex47 and Ir-O bond lengths of 2.119(4) A˚ for the Ir-SO4 bonds of the Ir(CO)(Me)(SO4)(P(p-tolyl)3)2 complex.48 In addition, the sulfate ligand S-O single (1.5250(14) and 1.5312(14) A˚) and double bonds (1.4431(16) and 1.4439(15) A˚) are comparable to those of reported complexes.47-49 Synthesis and Characterization of 1-NO3. Treatment of 1-Cl with 2 equiv of AgNO3 provided a new Cp*Ir complex, Cp*Ir(NHC)(η1-NO3)2 (1-NO3) (Scheme 3), which was identified by 1H NMR, 13C NMR, and elemental analysis. In the IR spectrum (KBr) for 1-NO3, bands characteristic of unidentate η1 binding of the nitrate ligands are observed at 1489 and 1385 cm-1. In comparison, Amouri et al. have reported nitrato bands at 1550 and 1273 cm-1 for the chelating bidentate nitrate ligand and bands at 1493 and 1384 cm-1 for the unidentate nitrate ligand for the complex Cp*Ir(η2-NO3)(η1-NO3).50 X-ray Crystal Structure of 1-NO3. Crystals suitable for X-ray diffraction were obtained by slow diffusion of pentane into a concentrated CH2Cl2 solution of the complex at room temperature (Figure 3). The X-ray structure confirmed the unidentate nature of the nitrate ligands. The Ir-O bond lengths of 1-NO3 are 2.136(2) and 2.142(2) A˚, respectively, which are comparable to the Ir-O bond length observed in Cp*Ir(NO3)2 (2.120(9) A˚).51 (47) Fettinger, J. C.; Churchill, M. R.; Bernard, K. A.; Atwood, J. D. J. Organomet. Chem. 1988, 340, 377. (48) Randall, S. L.; Miller, C. A.; See, R. F.; Churchill, M. R.; Janik, T. S.; Lake, C. H.; Atwood, J. D. Organometallics 1994, 13, 5088. (49) Ciriano, M. A.; Lopez, J. A.; Oro, L. A.; Perez-Torrente, J. J.; Lanfranchi, M.; Tiripicchio, A.; Camellini, M. T. Organometallics 1995, 14, 4764. (50) Amouri, H.; Guyard-Duhayon, C.; Vaissermann, J. Inorg. Chem. 2002, 41, 1397.

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Figure 2. X-ray single-crystal structure of Cp*Ir(NHC)(SO4) 3 CH2Cl2 (1-SO4). Thermal ellipsoids are given at the 50% probability level. Hydrogen atoms and CH2Cl2 are omitted for clarity. Selected bond distances (A˚) and angles (deg): Ir1-C11 = 2.045(2), Ir1-O2 = 2.140(1), Ir1-O1 = 2.157(1), S1-O1 = 1.525(1), S1-O2 = 1.531(1), S1-O3 = 1.443(2), S1-O4 = 1.444(2); C11-Ir1-O2 = 86.07(6), C11-Ir1-O1 = 85.10(6), O2-Ir1-O1 = 66.29(5), O3-S1-O4 = 113.41(10), O1-S1O2 = 100.49(8).

Figure 3. X-ray single-crystal structure of Cp*Ir(NHC)(η1-NO3)2 (1-NO3). Thermal ellipsoids are given at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (A˚) and angles (deg): Ir1-C11 = 2.050(2), Ir1O1 = 2.136(2), Ir1-O4 = 2.142(2); C11-Ir1-O1 = 85.28(7), C11-Ir1-O4 = 80.63(7), O1-Ir1-O4 = 76.00(6), O3-N1O2 = 123.29(18), O3-N1-O1 = 120.25(18), O2-N1-O1 = 116.45(18).

Synthesis and Characterization of 1-TFA. 1-TFA was prepared by stirring 1-Cl and 2 equiv of Ag(OCOCF3)2 in CH2Cl2 at room temperature overnight (Scheme 4). The formation of 1-TFA was confirmed by 1H NMR, 13C NMR, and elemental analysis. In the 1H NMR (CD2Cl2) spectrum at 30 °C peaks were observed at 3.54, 2.17, and 1.60 ppm, which correspond to the carbene N-Me, carbene C-Me, and Cp* methyl peaks, respectively. However, at 20 °C, four more resonances due to the complex 1-aqua-TFA were observed (Scheme 5). Signals for the aqua ligand were observed at 9.46 ppm, while resonances corresponding to the carbene C-Me and N-Me were observed at 3.49 and 2.11 ppm, respectively; the Cp* ligand was observed at 1.55 ppm (51) Ogo, S.; Nakai, H.; Watanabe, Y. J. Am. Chem. Soc. 2002, 124, 597.

Feng et al.

Figure 4. X-ray single-crystal structure of [Cp*Ir(NHC)(OH2)2][OCOCF3]2 (1-aqua-TFA). Thermal ellipsoids are given at the 50% probability level. Hydrogen atoms other than the hydrogens of aqua ligands have been omitted for clarity. Selected bond distances (A˚) and angles (deg): Ir1-C11 = 2.045(5), Ir1-O1 = 2.153(3), Ir1-O2 = 2.146(3); C11-Ir1-O1 = 86.24(15), C11Ir1-O2 = 88.32(15), O1-Ir1-O2 = 80.20(12). Scheme 5

(see the Supporting Information). As suggested in Scheme 5, the complex 1-aqua-TFA contains two aqua ligands that are strongly hydrogen bound to the TFA counterions. The complex 1-TFA exists in equilibrium with 1-aqua-TFA. The temperature dependence of the equilibrium rate constant (Keq) was investigated from -10 to 30 °C. From the van’t Hoff equation, both the entropy and the enthalpy of the reaction were found to be negative (ΔS° = -30.6 ( 2.4 eu and ΔH° = -7.6 ( 0.7 kcal/mol). These data suggest that at 30 °C and above, the reaction in Scheme 5 is entropically driven and complex 1-TFA is favored. However, the reaction becomes driven by enthalpy at approximately 20 °C, where both complexes 1-TFA and 1-aqua-TFA are observed. In the solid-state IR (KBr) spectrum of 1-TFA at room temperature an absorbance (νCdO) is observed at 1681 cm-1. This is consistent with other Cp*Ir(TFA) complexes (νC-O 1685 cm-1 for Cp*Ir(OH2)(TFA)2 and νC-O 1685 cm-1 for [Cp*Ir(TFA)(μ-Cl)]2).52 X-ray Crystal Structure of 1-aqua-TFA. Crystals of 1-aqua-TFA suitable for X-ray diffraction were obtained by slow diffusion of pentane into a concentrated CH2Cl2 solution of the complex at room temperature (Figure 4). Only the 1-aqua-TFA complex was observed. The Ir-O bond lengths (2.153(3) and 2.146(3) A˚) of 1-aqua-TFA are comparable to the Ir-O bond lengths (2.158(5) and 2.156(4) A˚) of 1-OH2. The distances between the hydrogen of the aqua ligands and oxygen of the TFA (H1A- - -O3 = 1.77 (4) A˚, H1B- - -O6 = 1.76(4) A˚, H2A- - -O4 = 1.82(4) A˚, H2B- - -O5 = 1.79(4) A˚) reveal the existence of very strong H bonding between the TFA anions and the coordinated water ligands. (52) Ison, E. A.; Jiang, B. Unpublished work, 2009.

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Scheme 6

Table 1. Ratio of 2-dim to 2-mon in Various Solvents As Observed by 1H NMR Spectroscopy

a

solvent

2-dim:2-mona

CF3COOD CD2Cl2 CD3OD CD3OD þ D2O CD3CN pyridine-d5

4:1 3:1 2:1 2:1 0:1 0:1

The ratios of 2-dim to 2-mon were monitored by 1H NMR spectroscopy.

Synthesis and Characterization of 2. Stirring 1-Cl with 1 equiv of AgOTf in CH2Cl2 overnight at room temperature resulted in the formation of complex 2, which exists as a mixture of the iridium dimer [(Cp*Ir(NHC)Cl)2][OTf]2 (2-dim) and the iridium monomer [Cp*Ir(NHC)(Cl)(solvent)][OTf] (2-mon) (Scheme 6). In poorly coordinating solvents (CF3COOD, CD2Cl2, CD3OD, mixtures of CD3OD and D2O), resonances were observed in the 1H NMR spectrum for both 2-dim and 2-mon with the ratio of dimer to monomer dependent on the solvent (Table 1). However, in coordinating solvents (CD3CN and pyridine-d5), complex 2 was completely converted to monomer. Removing CD3CN and dissolving the complex in CD2Cl2 results in a mixture of 2-dim, 2-mon-CD3CN, and 2-mon-CD2Cl2 (Supporting Information). This observation provided experimental evidence that the exact composition of complex 2 depends on the donating ability of the solvents. In a mixture of CD3OD and D2O at room temperature, complex 2 existed as a mixture of 2-dim and 2-mon; a VT NMR experiment was conducted to determine the composition of complex 2 at higher temperatures, since the H/D exchange reactivity studies were conducted at 150 °C (see below). At temperatures up to 70 °C, there were no changes in the dimer to monomer ratio (1:1) of complex 2 monitored by 1H NMR. X-ray Crystallography of 2-dim. Crystals of complex 2-dim suitable for X-ray crystallography were obtained by slow diffusion of pentane into a CH2Cl2 solution of 2 (Figure 5). The Ir-Cl bond lengths are 2.444(1) and 2.446(1) A˚, which are comparable to the Ir-Cl bond lengths 2.456(3) and 2.449(3) A˚ of the dimeric complex (Cp*IrCl2)2 reported by Churchill and co-workers.53 Catalytic H/D Exchange. A standard assay for Pt-catalyzed H/D exchange between C6H6 and various deuterium sources was recently reported.43 We employed this assay under the same reaction conditions for catalytic H/D exchange reactions between C6H6 and various deuterium sources using the Cp*Ir(NHC) complexes 1-Cl, 1-NCMe, 1-OH2, 1-SO4, 1-NO3, 1-TFA, and 2. This method allowed us to rapidly compare the ability of these catalysts to perform C-H activation reactions by comparing turnover numbers (53) Churchill, M. R.; Julis, S. A. Inorg. Chem. 1977, 16, 1488.

Figure 5. X-ray single-crystal structure of [(Cp*Ir(NHC)Cl)2][OTf]2 (2-dim). Thermal ellipsoids are given at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (A˚) and angles (deg): Ir1-C11 = 2.053(2), Ir1Cl1a = 2.444(1), Ir1a-Cl1 = 2.446(1), Ir1a-Cl1a = 2.444(0); C11-Ir1-Cl1a = 89.05(5), Cl1a-Ir1-Cl1 = 79.97(2).

(TONs) after a defined period of time (24 h). We were also able to systematically examine the effect of ligand electronics on chemical reactivity using this method. All the complexes examined catalyzed H/D exchange between C6H6 and CD3OD (Table 2). Complexes 1-Cl, 1OH2, 1-SO4, 1-NO3, and 1-TFA exhibited modest reactivity toward catalytic H/D exchange reactions (TONs between 15 and 62); however, the complex 1-NCMe exhibited better reactivity under the same reaction conditions (TON 157 ( 33). This difference in reactivity can be attributed to the ability of the ancillary ligand to dissociate from the metal center. Acetonitrile, NCMe, is more labile than Cl-, OH2, SO42-, NO3-, or TFA; thus, the NCMe ligand readily dissociates to form a 16-electron iridium metal fragment with an open coordination site. These results imply that the dissociation of a ligand to form an open coordination site is likely an important step prior to C-H bond activation for the catalytic H/D exchange reaction between benzene and CD3OD. The TON for H/D exchange between C6H6 and CD3OD observed for complex 2 was 100 ( 28. Even though this TON was higher than those for reactions performed with complexes 1-Cl, 1-OH2, 1-SO4, 1-NO3, and 1-TFA, complex 2 exists as a mixture of the dimer, 2-dim, and the monomer, 2-mon, in solution at room temperature. Despite repeated attempts, we have not been able to obtain either 2-dim or 2-mon in pure isolated form and therefore are not in a position to assess the reactivity of 2-mon, 2-dim, or a mixture of the two as H/D exchange catalysts; as a result, we could

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Table 2. Catalytic H/D Exchange between C6H6 and Various Deuterium Solvents Catalyzed by Various Cp*Ir(NHC) Complexesa

TON entry

catalyst

CD3OD

CF3COOD

CD3COCD3

D2O

1 2 3 4 5 6 7

Cp*Ir(NHC)Cl2 (1-Cl) [Cp*Ir(NHC)(NCMe)2][OTf]2 (1-NCMe) [Cp*Ir(NHC)(OH2)2][OTf]2 (1-OH2) Cp*Ir(NHC)(SO4) (1-SO4) Cp*Ir(NHC)(NO3)2 (1-NO3) Cp*Ir(NHC)(TFA)2 (1-TFA) [(Cp*Ir(NHC)Cl)2][OTf]2 and Cp*Ir(NHC)(Cl)(OTf) (2)

62 ( 11 156 ( 33 57 ( 21 50 ( 2 16 ( 1 35 ( 1 100 ( 28

8 7 8 1 8 9 6

0 0 0 0 3 3 2

0 2 3 3 2 3 0

Conditions: Cp*Ir(NHC) catalyst (2 mol %, 0.025 mmol) and benzene (111 μL, 1.25 mmol) in deuterium solvent (25 mmol); solvents CD3OD, CF3COOD, CD3COCD3, D2O; 150 °C for 24 h. Values are reported as TON, with 2 mol % catalyst load and maximum TON = 300. a

not directly compare its reactivity with that of the other Cp*Ir(NHC) complexes. Dependence of Catalytic H/D Exchange Reactivity on the Source of Deuterium. Deuterium sources other than CD3OD were screened for H/D exchange with C6H6. Poor TONs (