Sterically Encumbered Iridium Bis(N-heterocyclic ... - ACS Publications

Apr 23, 2009 - Multiple C-H Activation Processes and Isomeric Normal/Abnormal. Carbene Complexes. Christina Y. Tang, William Smith, Dragoslav Vidovic,...
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Organometallics 2009, 28, 3059–3066

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Sterically Encumbered Iridium Bis(N-heterocyclic carbene) Systems: Multiple C-H Activation Processes and Isomeric Normal/Abnormal Carbene Complexes Christina Y. Tang, William Smith, Dragoslav Vidovic, Amber L. Thompson, Adrian B. Chaplin, and Simon Aldridge* Inorganic Chemistry, UniVersity of Oxford, South Parks Road, Oxford, U.K., OX1 3QR ReceiVed January 6, 2009

The reaction of [Ir(coe)2Cl]2 (coe ) cyclooctene) with the N-heterocyclic carbene N,N′-bis(2,6diisopropylphenyl)imidazol-2-ylidene (IPr) in tetrahydrofuran under anaerobic conditions leads to the formation of two main products, the planar four-coordinate Ir(I) complex Ir(IPr)(IPr′′)Cl (1) formed by dehydrogenation of one of the IPr isopropyl substituents (to give the mixed NHC/alkene donor IPr′′) and the trigonal bipyramidal Ir(III) system Ir(IPr)(aIPr)(H)2Cl (6), which features both “normal” and “abnormal” C-bound isomers of the NHC ligand. Formation of 1 presumably proceeds via initial C-H activation at iridium(I); moreover, subsequent reactivity for 1 initiated by chloride abstraction suggests that C-H oxidative addition chemistry is facile for the methyl C-H bonds of the carbene isopropyl substituent. Thus, the square pyramidal Ir(III) alkene alkyl hydride [Ir(IPr′)(IPr”)H]+[BArf4]- (2) is formed on reaction with Na[BArf4] in fluorobenzene. In contrast to the Ir(I) system 1, the alignment of the alkene ligand in solid 2 is such that it lies coplanar with the IrC4 basal plane. Quantum chemical investigations imply that the energetic difference between this alkene orientation and an alternative perpendicular conformation is small (ca. 5 kcal mol-1), with steric factors (notably at the alkene 2-position) being important. Hydrogenation of 1 proceeds via an intermediate identified as Ir(IPr)(IPr′′)(H)2Cl to give the Ir(III) dihydride Ir(IPr)2(H)2Cl (4), the structure of which can be compared with those of the tautomeric isomer 6 and the mixed IPr/IMes carbene complex Ir(IPr)(IMes)(H)2Cl [IMes ) N,N′-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]. Crystallographically determined bond lengths for the Ir-C linkages trans to the respective carbenes imply relatively similar σ-donor properties for the IPr, aIPr, and IMes ligands. Introduction The increasingly widespread use of N-heterocyclic carbenes (NHCs) in the synthesis of catalytically relevant transition metal complexes1 and in the stabilization of otherwise highly electrophilic systems reflects, at least in part, the strong σ-donor capacity of this ligand class.2 In addition, the high steric demands of ortho-disubstituted aryl substituents suggest that ligands such as N,N′-bis(2,6-diisopropylphenyl)imidazol-2ylidene and N,N′-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IPr and IMes) might find application in the stabilization of metal centers with low coordination numbers and/or formal electron * Corresponding author. Tel: +44 (0)1865 285201. Fax: +44 (0)1865 272690. E-mail: [email protected]. Web-page: http: //users.ox.ac.uk/∼quee1989. (1) For reviews of NHC chemistry, see, for example: (a) Arduengo, A. J. Acc. Chem. Res. 1999, 32, 913. (b) Bourissou, D.; Guerret, O.; Gabbaı¨, F. P.; Bertrand, G. Chem. ReV. 2000, 100, 39. (c) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (d) Peris, E.; Crabtree, R. H. Coord. Chem. ReV. 2004, 248, 2239. (e) Scott, N. M.; Nolan, S. P. Eur. J. Inorg. Chem. 2005, 1815. (f) Hahn, F. E. Angew. Chem., Int. Ed. 2006, 45, 1348. In addition, a series of reviews on NHC chemistry were published in a recent issue of Coord. Chem. ReV.: Coord. Chem. ReV. 2007, 251, issues 5-6. (2) For articles dealing with issues of electronic structure and donor properties for NHCs see, for example: (a) Boehme, C.; Frenking, G. Organometallics 1998, 17, 5801. (b) Huang, J.; Schanz, H.-J.; Stevens, E. D.; Nolan, S. P. Organometallics 1999, 18, 5375. (c) Weskamp, T.; Kohl, F. J.; Hieringer, W.; Gleich, D.; Herrmann, W. A. Angew. Chem., Int. Ed. 1999, 38, 2416. (d) Trnka, T. N.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18. (e) Tafipolsky, M.; Scherer, W.; o¨fele, K.; Artus, G.; Pedersen, B.; Herrmann, W. A.; McGrady, G. S. J. Am. Chem. Soc. 2002, 124, 5865. (f) Lee, M.-T.; Hu, C.-H. Organometallics 2004, 23, 976. (g) Nemcsok, D.; Wichmann, K.; Frenking, G. Organometallics 2004, 23, 3640.

counts (e.g., of the types L2MX, [L2MX2]+, and [L2ML′]+; M ) group 9 metal).3 Despite this, previous reports suggest that the use of the IPr ligand in conjunction with conventional rhodium/iridium cyclooctadiene (cod) precursors leads to the introduction of only one NHC donor (presumably, at least in part, due to steric factors).4-6 Furthermore, NHCs bearing pendant methyl or phenyl substituents are often prone to facile (3) For examples of formally 14-electron bis(phosphine) and bis(NHC) complexes of group 9 metals see: (a) Cooper, A. C.; Clot, E.; Huffman, J. C.; Streib, W. E.; Maseras, F.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1999, 121, 97. (b) Scott, N. M.; Pons, V.; Stevens, E. D.; Heinekey, D. M.; Nolan, S. P. Angew. Chem., Int. Ed. 2005, 44, 2512. (c) Scott, N. M.; Dorta, R.; Stevens, E. D.; Correa, A.; Cavallo, L.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 3516. (d) Lavallo, V.; Canac, Y.; DeHope, A.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2005, 44, 7236. (e) Douglas, T. M.; Chaplin, A. B.; Weller, A. S. Organometallics 2008, 27, 2918. (4) (a) Yu, X.-Y.; Patrick, B. O.; James, B. R. Organometallics 2006, 25, 2359. (b) Yu, X.-Y.; Patrick, B. O.; James, B. R. Organometallics 2006, 25, 4870. (5) (a) Hillier, A. C.; Lee, H. M.; Stevens, E. D.; Nolan, S. P. Organometallics 2001, 20, 4246. (b) Kelly, R. A.; Clavier, H.; Giudice, S.; Scott, N. M.; Stevens, E. D.; Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.; Nolan, S. P. Organometallics 2008, 27, 202. (6) Displacement of ancillary carbonyl ligands has been shown to lead to the synthesis of Rh(IPr)2L(X) system; see, for example: Praetorius, J. M.; Kotyk, M. W.; Webb, J. D.; Wang, R.; Crudden, C. M. Organometallics 2007, 26, 1057. (7) For examples of group 9 NHC complexes featuring C-H activated alkyl substituents see refs 3b,c together with: (a) Huang, J.; Stevens, E. D.; Nolan, S. P. Organometallics 2000, 19, 1194. (b) Hanasaka, F.; Tanabe, Y.; Fujita, K.; Yamaguchi, R. Organometallics 2006, 25, 826. (c) Cordoba´n, R.; Sanau´, M.; Peris, E. Organometallics 2006, 25, 4002.

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C-H activation processes.3b,c,7,8 Thus, Nolan and co-workers report that the reaction of [Rh(coe)2Cl]2 (coe ) cyclooctene) with IMes leads to the formation of the rhodium(III) complex Rh(IMes)(IMes′)(H)Cl, via oxidative addition of one of the ortho-methyl C-H bonds (to give the chelating NHC/alkyl donor IMes′);8 similarly, 14-electron rhodium and iridium systems containing two metalated tBu substituents can be isolated from related reactions with ItBu [N,N′-bis(tert-butyl)imidazol-2-ylidene].3b,c In the former case, the reversibility of the C-H activation process means that in its reactivity toward CO this system can act as a “masked” source of the RhL2Cl fragment.8 Pendant substituents possessing β-hydrogen atoms offer an additional ligand modification pathway in the presence of unsaturated metal centers. Thus, the formal dehydrogenation of alkyl substituents (e.g., isopropyl, cyclohexyl, cyclopentyl), pendant to phosphine or β-ketiminate (Nacnac) type donors, has been reported by a number of groups9-12 and has been shown, for example, to be a relevant side reaction in systems targeting the functionalization of external alkanes via dehydrogenation chemistry.10a,b An alternative type of C-H activation pathway implicit in the organometallic chemistry of NHC ligands, and which has emerged since 2001 as a mode of coordination relevant not only to transition metal systems but also to main group and f-block

(8) For examples of group 9 NHC complexes featuring C-H activated phenyl and related substituents see: (a) Hitchcock, P. B.; Lappert, M. F.; Terreros, P. J. Organomet. Chem. 1982, 239, C26. (b) Danopoulos, A. A.; Winston, S.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 2002, 3090. (c) Sajoto, S.; Djurovich, P. I.; Tanayo, A.; Yousufuddin, M.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 7992. (d) Nanchen, S.; Pfalz, A. Chem.sEur. J. 2006, 12, 4550. (e) Cordoba´n, R.; Sanau´, M.; Peris, E. J. Am. Chem. Soc. 2006, 128, 3974. (f) Cordoba´n, R.; Lillo, V.; Mata, J. A.; Fernandez, E.; Peris, E. Organometallics 2007, 26, 4350. (g) Chien, C.-H.; Fujita, S.; Yamoto, S.; Hara, T.; Yamagata, T.; Watanabe, M.; Mashima, K. Dalton Trans. 2008, 916. (h) Chang, C.-F.; Cheng, Y.-M.; Chi, Y.; Chiu, Y.-C.; Lin, C.-C.; Lee, G.-H.; Chou, P.-T.; Chen, C.-C.; Chang, C.-H.; Wu, C.-C. Angew. Chem., Int. Ed. Engl. 2008, 47, 4542. (9) For examples of the dehydrogenation of alkyl phosphines see, for example: (a) Bennett, M. A.; Clark, P. W.; Robertson, G. B.; Whimp, P. O. J. Chem. Soc., Chem. Commun. 1972, 1011. (b) Hietkamp, S.; Stufkens, D. J.; Vrieze, K. J. Organomet. Chem. 1978, 152, 347. (c) Amoroso, D.; Yap, G. P. A.; Fogg, D. E. Can. J. Chem. 2001, 79, 958. (d) Six, S.; Gabor, B.; Go¨rls, H.; Mynott, R.; Philipps, P.; Leitner, W. Organometallics 1999, 18, 3316. (e) Borowski, A. F.; Sabo-Etienne, S.; Christ, M. L.; Donnadieu, B.; Chaudret, B. Organometallics 1996, 15, 1427. (f) Deblon, S.; Liesum, L.; Harmer, J.; Scho¨nberg, H.; Schweiger, A.; Gru¨tzmacher, H. Chem.sEur. J. 2002, 8, 601. (g) Glaser, P. B.; Tilley, T. D. Organometallics 2004, 23, 5799. (h) Baya, M.; Buil, M. A.; Esteruelas, M. A.; Onate, E. Organometallics 2004, 23, 1416. (i) Grellier, M.; Vendier, L.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2005, 46, 2613. (j) Piras, E.; L¨; ang, F.; Ru¨egger, H.; Stein, D.; Wo¨rle, M.; Gru¨tzmacher, H. Chem.sEur. J. 2006, 12, 5849. (k) Douglas, T. M.; Le Noˆtre, J.; Brayshaw, S. K.; Frost, C. G.; Weller, A. S. Chem. Commun. 2006, 3408. (l) Douglas, T. M.; Brayshaw, S. K.; Dallanegra, R.; Kociok-Ko¨hn, G.; Macgregor, S. A.; Moxham, G. L.; Weller, A. S.; Wondimagegn, T.; Vadivelu, P. Chem.sEur. J. 2008, 14, 1004. (10) For examples of the dehydrogenation of alkyl substituents pendant to Nacnac and related ligand types see:(a) Fekl, U.; Goldberg, K. I. J. Am. Chem. Soc. 2002, 124, 6804. (b) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125, 15286. (c) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Organometallics 2005, 24, 6250. (d) Kloek, S. M.; Goldberg, K. I. J. Am. Chem. Soc. 2007, 129, 3460. (e) Giri, R.; Maugel, N.; Foxman, B. M.; Yu, J.-Q. Organometallics 2008, 27, 1667. (11) For examples of the dehydrogenation of alkyl substituents pendant to NHC ligands see: (a) Prinz, M.; Grosche, M.; Herdtweck, E.; Herrmann, W. A. Organometallics 2000, 19, 1692. (b) Dible, B. R.; Sigman, M. S.; Arif, A. M. Inorg. Chem. 2005, 44, 3774. (12) For examples of the dehydrogenation of alkyl substituents pendant to other classes of donor ligands see, for example: (a) Yu, J. S.; Fanwick, P. E.; Rothwell, I. P. J. Am. Chem. Soc. 1990, 112, 8171. (b) Ma, Y.; Bergman, R. G. Organometallics 1994, 13, 2548. (c) Holtcampl, M. W.; Henling, L. M.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. Inorg. Chim. Acta 1998, 270, 467.

Tang et al. Chart 1. “Abnormal” Binding of the NHC Ligand Class via C(4)

derivatives,13 involves metalation at the C(4) position of the NHC backbone.14 The so-called “abnormal” carbene complexes that result (Chart 1) have been the subject of a number of structural and computational studies, with a view to comparing their properties with the more widespread C(2)-bound tautomers and assessing the potential effects (e.g., of differential σ-donor capabilities) on the catalytic properties of NHC complexes.15 As part of a recently instituted program to examine the application of novel low-coordinate group 9 metal complexes in E-H bond activation processes (E ) B, C, N), we set out to examine the reactivity of [Ir(coe)2Cl]2 toward the sterically demanding IPr ligand. In doing so, we were surprised to find that previous reports of the reactivity of this system were confined to studies carried out in the presence of protic solvents and/or air.16 By contrast, under anaerobic and aprotic conditions a wide range of C-H activation chemistry is uncovered, leading to the isolation of dehydrogenated ligand frameworks and to isomeric NHC/abnormal NHC tautomers, which provide a useful basis for further comparison of ligand properties.

Experimental Section (i) General Considerations. All manipulations were carried out under a nitrogen or argon atmosphere using standard Schlenk line or drybox techniques. Solvents (thf, pentane, benzene) were dried using a commercial Braun SPS system and stored over molecular sieves before use; fluorobenzene was distilled from CaH2. d6Benzene and d2-dichloromethane (both Goss) were degassed and dried over the appropriate drying agent (potassium or molecular sieves) prior to use. [Ir(coe)2Cl]2, IPr, IMes, Na[BArf4] [Arf ) C6H3(CF3)2-3,5], ammonia borane (AB), and iPr2NH · BH3 were prepared by literature methods.17 Selected spectroscopic data for Ir(IPr)2(H)2Cl (4) and Ir(IPr)2(O2)Cl (5) have been reported previously by Sames;16 additional crystallographic data for these compounds are reported herein. In addition, structural data for the IMes-containing complexes Ir(IPr)(IMes)(H)2Cl and Ir(IMes)(IMes′)(H)Cl and for the salt [(IPr)H]+[Ir(coe)2Cl2]- are included (for comparative purposes) in the Supporting Information. NMR spectra were measured on a Varian Mercury VX-300 or Bruker AVII 500 FT-NMR spectrometer. Residual signals of solvent (13) Grundemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.; Crabtree, R. H. Chem. Commun. 2001, 2274. (14) For a recent review of abnormal NHCs see, for example: (a) Arnold, P. L.; Pearson, S. Coord. Chem. ReV. 2007, 251, 596. For an example of a metal complex containing a third (N-bound) mode of coordination of an NHC see, for example: (b) Burling, S.; Mahon, M. F.; Powell, R. E.; Whittlesey, M. K.; Williams, J. M. J. J. Am. Chem. Soc. 2006, 128, 13702. (15) For a recent quantum chemical exploration of the donor capabilities of various N-heterocyclic carbene isomers see, for example: Tonner, R.; Heydenrych, G.; Frenking, G. Chem. Asian J. 2007, 2, 1555. (16) DeBoef, B.; Pastine, S. J.; Sames, D. J. Am. Chem. Soc. 2004, 126, 6556. (17) (a) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15, 18. (b) Jafarpour, L.; Stevens, E. D.; Nolan, S. P. J. Organomet. Chem. 2000, 606, 49. (c) Arduengo, A. J., III.; Rasika Dias, H. V.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1992, 114, 5530. (d) Reger, D. L.; Wright, T. D.; Little, C. A.; Lamba, J. J. S.; Smith, M. D. Inorg. Chem. 2001, 40, 3810. (e) Shore, S. G.; Parry, R. J. Am. Chem. Soc. 1955, 77, 6084. (f) No¨th, H.; Beyer, H. Chem. Ber. 1960, 93, 928.

Iridium Bis(N-heterocyclic carbene) Systems were used for reference for 1H and 13C NMR spectroscopy; 11B, 19 F, and 31P NMR spectra were referenced with respect to Et2O · BF3, CFCl3, and 85% aqueous H3PO4, respectively. Infrared spectra were measured for each compound either pressed into a disk with excess dry KBr or as a solution in the appropriate solvent, on a Nicolet 560 FTIR spectrometer. Mass spectra were measured by the EPSRC National Mass Spectrometry Service Centre, Swansea University; perfluorotributylamine was used as the standard for high-resolution EI mass spectra. Elemental microanalyses were carried out at London Metropolitan University. Characterization of new compounds is typically based on multinuclear NMR, IR, and mass spectrometry data (including accurate mass measurement), supplemented by elemental microanalysis. In all cases the purity of the bulk material was established by multinuclear NMR to be >95%. In addition, single-crystal X-ray diffraction studies were carried out for compounds 1, 2, 4 · OEt2, 5, 6 · C6H6, Ir(IPr)(IMes)(H)2Cl, Ir(IMes)(IMes′)(H)Cl, and [(IPr)H]+[Ir(coe)2Cl2]-. Abbreviations: s ) singlet, d ) doublet, sep ) septet, m ) multiplet. (ii) Crystallographic and Computational Methods. Crystallographic data were collected at low temperature using an Oxford Cryosystems N2 open-flow cooling device on a Nonius KappaCCD diffractometer.18a Data collection and cell refinement were carried out using DENZO and COLLECT.18b Structure solution was carried out using either SIR9218c or SHELXS-97 [for Ir(IMes)(IMes′)(H)Cl],18d and the structures were refined by full-matrix least-squares using the CRYSTALS suite.18e In general, all nonhydrogen atoms were refined with anisotropic displacement parameters; hydrogen atoms were initially found in the difference map and refined with soft constraints before inclusion in the model with the parameters riding on those of the parent atom. Exceptions to this occurred in cases of disorder, for example as seen in the CF3 groups in 2, where a partially isotropic model was used. In the cases of 1, 2, and Ir(IMes)(IMes′)(H)Cl, examination of the difference map indicated the presence of diffuse electron density. Efforts made to model it were unsuccessful, so SQUEEZE18f within PLATON18g was used to provide the discrete Fourier transform of the void region, which was treated as contributions to the A and B parts of the calculated structure factors within CRYSTALS. In the case of 6, the solvent of crystallization was clearly benzene, but examination of the three-dimensional electron density plot made it (18) (a) Cosier, J.; Glazer, A. M. J. Appl. Crystallogr. 1986, 19, 105. (b) Otwinowski, Z.; Minor, W. Methods in Enzymology; Carter, C. W., Sweet, R. M., Eds.; Academic Press: New York, 1996; Vol. 276, p 307; Collect: Nonius B.V., Delft, The Netherlands, 1997-1002. (c) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (d) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112. (e) CRYSTALS: Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, J.; Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487. (f) Sortav: Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33. Spek, A. J. Appl. Crystallogr. 2003, 36, 7. (g) PLATON, A Multipurpose Crystallographic Tool; Spek, A. L. Utrecht, The Netherlands, 1998; (h) Schro¨der, L.; Watkin, D. J.; Cousson, A.; Cooper, R. I.; Paulus, W. J. Appl. Crystallogr. 2004, 37, 545. (19) (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, ReVision D.01; Gaussian, Inc.: Wallingford, CT, 2004. (b) Dapprich, S.; Koma´romi, I.; Byun, K. S.; Morokuma, K.; Frisch, M. J. J. Mol. Struct. (THEOCHEM) 1999, 462, 1. (c) Vreven, T.; Morokuma, K. J. Comput. Chem. 2000, 21, 1419.

Organometallics, Vol. 28, No. 10, 2009 3061 Scheme 1. Syntheses of Mixed NHC/Alkene Complex 1 by Ligand Dehydrogenation; Subsequent C-H Oxidative Addition to Give Iridium(III) Alkene Alkyl Hydride Complex 2a

a Key reagents and conditions: (i) IPr (3.98 equiv.), thf, 20°C, 12 h, isolated yield 40% (6 also isolated in 7% yield); (ii) Na[BArf4] (1.0 equiv.), fluorobenzene, 20°C, 12 h, 26% isolated yield.

Scheme 2. Model Complexes 2a and 2b Used As Part of Quantum Chemical Investigations of Alkene Ligand Orientation

clear that the benzene was rotating around the axis perpendicular to the carbon/hydrogen plane. Conventional modeling of the disorder was unsatisfactory, so the benzene was modeled using a pair of concentric annuluses18h representing the disordered carbon and hydrogen atoms. This left two residual peaks approximately three angstroms apart, which were modeled as dichloromethane with a site occupancy factor of 5%. A figure showing this is included with the Supporting Information, together with all the crystallographic data and special details of the refinement in CIF format. Crystallographic data (excluding structure factors) for all the structures included here have been deposited with the Cambridge Crystallographic Data Centre (CCDC 715009-715016) and have been included in the Supporting Information. Copies of the data can be obtained free of charge from the Cambridge Crystallographic Data Centre at www. ccdc.cam.ac.uk/data_request/cif. All geometry optimizations and frequency calculations were carried out using the Gaussian03 suite of programs.19a The geometries of 2 and 2b were optimized and verified by harmonic analysis using the ONIOM approach;19b,c the aryl groups on the carbene ligands, with the exception of coordinated atoms and substituents directly bound to these atoms (H or Me), constituted the low level (see Scheme 2). The low layer was modeled using HF, and the LanL2MB basis set was used for all atoms; LanL2MB pseudopotentials were used for iridium. B3LYP was used for the high layer, with the 6-31G(d,p) basis set used for all atoms except for iridium, for which the LanL2DZ basis set and pseudopotentials were used. Energies were calculated by single-point calculation on the full optimized geometry at the B3LYP level using the high layer model chemistry and were not zero-point corrected. The geometry of 2a was optimized and verified by harmonic analysis also using this model chemistry. A pruned grid consisting of 75 radial shells and 302 angular points was used for all calculations. Energies and geometries are compiled in the Supporting Information.

3062 Organometallics, Vol. 28, No. 10, 2009 (iii) Syntheses. Ir(IPr)(IPr′′)Cl (1) and Ir(IPr)(aIPr)(H)2Cl (6). To a solution of [Ir(coe)2Cl]2 (2.0 g, 2.4 mmol) in thf (100 cm3) was added, over 15 min, 3.7 g (9.54 mmol) of solid IPr. The reaction mixture was stirred for a further 12 h, during which time it changed color from bright orange to dark brown. Volatiles were then removed in vacuo, leaving an oily red-orange solid; addition of pentane (100 cm3) afforded a dark brown solution and an insoluble orange solid. Filtration, concentration of the filtrate, and cooling to -30 °C for 12 h led to the formation of single red crystals of 1 suitable for X-ray diffraction. Isolated yield: 0.96 g (40%). The product insoluble in pentane was recrystallized from benzene as small orange crystals of 6 (as the benzene solvate) suitable for X-ray diffraction. Isolated yield: 0.16 g (7%). Data for 1: 1H NMR (300 MHz, C6D6): δ 0.67 (d, J ) 6.4 Hz, 3H, iPr Me), 0.87 (d, J ) 7.0 Hz, 3H, iPr Me), 0.91 (d, J ) 6.7 Hz, 3H, iPr Me), 0.95 (d, J ) 6.8 Hz, 3H, iPr Me), 0.96 (d, J ) 6.8 Hz, 3H, iPr Me), 0.98 (d, J ) 6.7 Hz, 3H, iPr Me), 0.99 (d, J ) 7.0 Hz, 3H, iPr Me), 1.08 (d, J ) 6.7 Hz, 3H, iPr Me), 1.11 (d, J ) 7.0 Hz, 3H, iPr Me), 1.19 (d, J ) 6.7 Hz, 3H, iPr Me), 1.20 (d, J ) 6.7 Hz, 3H, iPr Me), 1.36 (d, J ) 1.2 Hz, 1H, alkene CH), 1.39 (d, J ) 6.5 Hz, 3H, iPr Me), 1.41 (d, J ) 6.4 Hz, 3H, iPr Me), 1.43 (d, J ) 6.4 Hz, 3H, iPr Me), 1.49 (s, 3H, alkene Me), 1.74 (d, J ) 1.2 Hz, 1H, alkene CH), 2.54 (sep, J ) 6.9 Hz, 1H, iPr CH), 3.13 (sep, J ) 6.5 Hz, 1H, iPr CH), 3.27 (sep, J ) 6.5 Hz, 2H, iPr CH), 3.57 (sep, J ) 6.9 Hz, 2H, iPr CH), 3.72 (sep, J ) 6.9 Hz, 1H, iPr CH), 6.25 (d, J ) 2.1 Hz, 1H, NCH), 6.41 (d, J ) 2.1 Hz, 1H, NCH), 6.53 (d, J ) 2.1 Hz, 1H, NCH), 6.56 (d, J ) 2.1 Hz, 1H, NCH), 6.99-7.33 (m, 12H, aromatic CH). 13C NMR (75 MHz, C6D6): δ 13.9, 21.4, 21.5, 21.9, 22.3, 22.5, 23.0, 23.3, 23.7, 24.2, 24.6, 24.8, 25.3, 25.9 (2 signals), 26.1 (CH3 of IPr and IPr′′), 26.5, 28.3, 28.4, 28.5, 28.7, 29.3, 31.5, 34.0 (CH of IPr and IPr′′; alkene CH2 and alkene quaternary of IPr′′), 118.4, 122.5, 122.7, 123.0, 123.2, 123.3, 123.7, 124.5, 124.8, 125.3, 125.5, 126.0, 128.5, 128.8 (aromatic CH and carbene backbone CH of IPr and IPr′′), 135.7, 137.5, 137.6, 138.7, 140.3, 143.9, 144.1, 145.0, 145.7, 147.8, 148.1, 148.2 (aromatic quaternary of IPr and IPr′′), 178.3, 187.3 (carbene quaternary of IPr and IPr′′). EI-MS: m/z 1002 (2%, correct isotope profile for M+), 386 (100%, IPr′′); accurate mass (calcd for M+, 191Ir, 35Cl isotopomer) 1000.4889, (measd) 1000.4892. Elemental microanalysis: (calcd for 1, C54H70ClIrN4) C 64.67, H 7.04, N 5.59, (measd) C 64.41; H, 6.92; N, 5.23. Data for 6: 1H NMR (300 MHz, C6D6): δ -33.53 (s, 2H, IrH), 0.86 (d, J ) 7.2 Hz, 6H, iPr Me of aIPr), 0.91 (d, J ) 6.6 Hz, 6H, i Pr Me of aIPr), 1.04 (d, J ) 6.6 Hz, 6H, iPr Me of aIPr), 1.17 (d, J ) 6.9 Hz, 6H, iPr Me of aIPr), 1.18 (d, J ) 7.5 Hz, 12H, iPr Me of IPr), 1.45 (d, J ) 7.2 Hz, 12H iPr Me of IPr), 2.42 (sep, J ) 6.6 Hz, 2H, iPr CH of aIPr), 2.70 (sep, J ) 6.6 Hz, 2H, iPr CH of aIPr), 3.43 (sep, J ) 6.9 Hz, 4H, iPr CH of IPr), 6.63 (s, 2H, NCH of IPr), 7.10 (s, 1H imidazolium CH of aIPr), 7.13 (s, 1H, NCHCN of aIPr), 6.80-7.33 (m, 12H, aromatic CH of IPr and aIPr). 13C NMR (75 MHz, C6D6): δ 23.3, 23.7 (2 overlapping signals), 24.2, 24.8, 25.5 (CH3 of IPr and aIPr), 28.0, 28.2, 28.4 (CH of IPr and aIPr), 123.1 (carbene backbone CH of aIPr), 123.4 (carbene backbone CH of IPr), 121.1, 122.3, 128.7, 128.8, 128.9, 129.8, 131.1, 132.1, 136.9, 137.9, 145.4, 146.5 (aromatic CH and aromatic quaternary of IPr and aIPr), 145.9 (CH of imidazolium), 166.8 (carbene quaternary of aIPr), 187.5 (carbene quaternary of IPr). EI-MS: m/z 1006 (weak, correct isotope profile for M+). [Ir(IPr′)(IPr′′)H]+[BArf4]- (2). To a solution/suspension of Na[BArf4] (0.088 g, 0.10 mmol) in fluorobenzene (10 cm3) was added at -30 °C a solution of 1 (0.100 g, 0.10 mmol) also in fluorobenzene (50 cm3). The reaction mixture was stirred for 12 h, during which time the color changed from red to yelloworange. Filtration, layering with hexanes, and storage at -30 °C for one week led to the isolation of 2 as large yellow crystals suitable for X-ray diffraction. Isolated yield: 0.47 g (26%). 1H NMR (300 MHz, CD2Cl2): δ -45.50 (s, 1H, IrH),

Tang et al. 0.24 (d, J ) 7.0 Hz, 3H, iPr Me), 0.40 (d, J ) 6.7 Hz, 3H, iPr Me), 0.76 (d, J ) 6.7 Hz, 3H, iPr Me), 0.86 (d, J ) 6.7 Hz, 3H, i Pr Me), 0.90 (d, J ) 6.7 Hz, 3H, iPr Me), 1.11 (d, J ) 6.7 Hz, 3H, iPr Me), 1.20 (d, J ) 6.7 Hz, 3H, iPr Me), 1.22 (d, J ) 6.6 Hz, 3H, iPr Me), 1.28 (d, J ) 6.5 Hz, 3H, iPr Me), 1.30 (d, J ) 6.7 Hz, 3H, iPr Me), 1.41 (d, J ) 6.7 Hz, 3H, iPr Me), 1.42 (d, J ) 6.7 Hz, 3H, iPr Me), 1.45 (d, J ) 7.0 Hz, 3H, iPr Me), 1.50 (s, 3H, alkene Me), 2.08-2.24 (m, 2H, IrCH2), 2.28 (sep, J ) 7.0 Hz, 1H, iPr CH), 2.41 (overlapping m, 3H, iPr CH), 2.66 (sep, J ) 6.6 Hz, 1H, iPr CH), 3.04 (sep, J ) 7.0 Hz, 1H, iPr CH), 3.27 (sep, J ) 7.0 Hz, 1H, iPr CH), 2.98 (s, 1H, alkene CH), 3.28 (s, 1H, alkene CH), 7.11 (overlapping m, 2H, NCH of IPr′′), 7.32 (d, J ) 2.0 Hz, 1H, NCH of IPr′), 7.42 (d, J ) 2.0 Hz, 1H, NCH of IPr′), 6.55-7.49 (overlapping m, 12H, aromatic CH), 7.56 (s, 4H, para-CH of [BArf4]-), 7.72 (s, 8H, ortho-CH of [BArf4]-). 13C NMR (126 MHz, C6D6): δ 22.3, 22.9 (2 overlapping signals), 23.4, 23.6, 23.8 (2 overlapping signals), 23.9, 24.2, 25.1, 25.4 (2 overlapping signals), 26.0, 26.2, 26.3, 27.6, 27.9, 28.7, 28.8 (2 overlapping signals), 28.9, 34.3 (CH, CH2, and CH3 of IPr′ and IPr′′), 65.9, 96.1 (alkene), 121.4, 123.3, 123.4, 123.9, 124.1, 124.8 (2 overlapping signals), 125.3, 126.7, 126.9, 127.5, 129.6, 130.5, 130.9, 133.3, 134.2, 135.8, 136.8, 137.6, 140.0, 144.0, 144.9, 145.1, 147.1 (aromatic CH and aromatic quaternary of IPr′ and IPr′′), 168.8, 172.4 (carbene quaternary of of IPr′ and IPr′′), N-bound aryl quaternary signals not observed; [BArf4]- signals: 118.1 (para-CH), 125.3 (q, 1JCF ) 275 Hz, CF3), 129.7 (q, 2JCF ) 33 Hz, meta-C), 135.5 (orthoCH), 162.8 (q, 1JCB ) 48 Hz, boron-bound ipso-C). 11B NMR (96 MHz, CD2Cl2): δ -1.8. 19F NMR (283 MHz, CD2Cl2): δ -62.9. Elemental microanalysis: (calcd for 2, C86H82BF24IrN4) C 56.43, H 4.52, N 3.06; (measd) C 56.45, H 4.49, N 2.93. NMR Monitoring of the Reaction of 1 with H2. A solution of 1 (0.020 g, 0.02 mmol) in C6D6 (ca. 1 cm3) in a J. Young NMR tube was freeze-pump-thaw degassed and exposed to H2 (ca. 4 atm) before being sealed and sonicated over a period of 24 h. The progress of the reaction was monitored by periodic acquisition of 1 H NMR spectra, revealing the presence of an intermediate 3, prior to the formation of the ultimate product Ir(IPr)2(H)2Cl (4). The identity of 4 was inferred from a comparison of the spectroscopic data with those reported by Sames et al.16 and confirmed by an X-ray diffraction study (single crystals of the diethyl ether solvate obtained from diethyl ether in ca. 60% yield, 1 g scale). Data for 3 (acquired at 1 h reaction time): 1H NMR (300 MHz, C6D6): δ -25.69 (s, br, 1H, IrH), -24.97 (s, br, 1H, IrH), 1.01 (d, J ) 1.2 Hz, 1H, alkene CH), 1.14 (d, J ) 7.2 Hz, 3H, iPr Me of IPr′′), 1.17 (d, J ) 7.7 Hz, 12H, iPr Me of IPr), 1.20 (d, J ) 6.5 Hz, 12H, i Pr Me of IPr), 1.27 (d, J ) 7.2 Hz, 6H, iPr Me of IPr′′), 1.38 (d, J ) 6.5 Hz, 6H, iPr Me of IPr′′), 1.50 (d, J ) 6.5 Hz, 3H, iPr Me of IPr′′), 1.64 (d, J ) 1.2 Hz, 1H, alkene CH), 2.06 (s, 3H, alkene Me), 2.93 (sep, J ) 6.6 Hz, 1H, iPr CH of IPr′′), 3.17 (sep, J ) 6.3 Hz, 4H, iPr CH of IPr), 3.39 (sep, J ) 6.9 Hz, 2H, iPr CH of IPr′′), 6.53 (d, J ) 1.3 Hz, 1H, NCH of IPr′′), 6.57 (s, 2H, NCH of IPr), 6.76 (d, J ) 1.3 Hz, 1H, NCH of IPr′′), 7.11-7.45 (overlapping m, 12H, aromatic CH). Interconversion of 4 and 5. Exposure of a benzene solution of 4 (0.5 g, 0.5 mmol) to air over a period of 24 h led to a color change from bright yellow to deep purple. After the removal of volatiles in vacuo, the resulting deep purple powder was recrystallized from cyclohexane (by slow evaporation) to give single purple crystals of 5 suitable for X-ray diffraction.16 Isolated yield: 0.36 g, 70%. Exposure of solutions of 5 in diethyl ether to ammonia borane (or MeH2N · BH3, Me2HN · BH3, iPr2HN · BH3, or LiBH4) or to hydrogen (at 4 atm pressure) in benzene led to the regeneration of 4, as judged by 1H NMR monitoring.

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Figure 1. (a) Crystallographically determined molecular structure of Ir(IPr)(IPr′′)Cl (1); (b) crystallographically determined molecular structure of the cationic component of [Ir(IPr′)(IPr′′)(H)]+[BArf4]- (2); and (c) computationally determined structure of the alternative (perpendicular alkene) conformer of 2 (see Table 1) showing sterically relevant interactions adjacent to the alkene donor. For crystal structures: counterion (in 2) and the majority of hydrogen atoms omitted for clarity; thermal displacement ellipsoids drawn at the 50% probability level. Key bond lengths (Å) and angles (deg): (for 1) Ir(1)-Cl(2) 2.329(1), Ir(1)-C(3) 2.027(2), Ir(1)-C(66) 2.074(3), Ir(1)-C(10) 2.178(3), Ir(1)C(11) 2.086(4), C(10)-C(11) 1.425(5), C(3)-Ir(1)-C(66) 176.4(1); (for 2) Ir(1)-C(2) 2.237(6), Ir(1)-C(3) 2.303(6), Ir(1)-C(8) 2.041(6), Ir(1)-C(31) 2.053(6), Ir(1)-C(55) 2.117(7), Ir(1)-H(1) 1.51, C(2)-C(3) 1.385(9), C(8)-Ir(1)-C(31) 167.1(2).

Results and Discussion Reaction of [Ir(coe)2Cl]2 with excess IPr in tetrahydrofuran followed by recrystallization from pentane leads to the isolation of Ir(IPr)(IPr′′)Cl (1) as a red crystalline material in ca. 40% isolated yield (Scheme 1). A second, pentane-insoluble product is isolated from the reaction mixture in low yield and subsequently shown to be the mixed NHC/abnormal NHC complex Ir(IPr)(aIPr)(H)2Cl, 6 (vide infra). The ability of the IPr donor to displace both (monofunctional) alkene ligands in this system contrasts markedly with the behavior of related precursors featuring rhodium and/or chelating cyclooctadiene co-ligands.4-6 The composition of 1 (C54H70ClIrN4) is suggested by mass spectrometry and by elemental microanalysis. Moreover, the activation of an isopropyl substituent is implied by the presence in the 1H NMR spectrum of (i) three distinct isopropyl CH resonances in the ratio 4:2:1 and (ii) a singlet methyl resonance (of relative intensity 3) at δH 1.46 ppm. Additionally, the lack of any discernible Ir-H resonance (in the range δH 0 to -50 ppm) is consistent with a different mode of reactivity from the simple methyl group C-H oxidative addition chemistry observed for the corresponding reaction of [Ir(coe)2Cl]2 with IMes.20 These spectroscopic inferences were confirmed crystallographically (Figure 1). In common with classical d8 transition metal alkene complexes,21 the Ir(I) center in 1 is planar four-coordinate with Ir(1) lying only 0.064 Å above the least-squares plane defined by the centroid of the coordinated alkene ligand, Cl(2), C(3), and C(66). The alkene ligand adopts an approximately perpendicular alignment with respect to the coordination plane [C(3)Ir(1)-alkene centroid-C(11) torsion angle ) 79.4°] and features a C-C distance [1.425(5) Å] consistent with previous reports of Ir(I)-ligated geminal-disubstituted alkenes [e.g., 1.430(11) Å (20) Reaction of [Ir(coe)2Cl]2 with IMes in thf generates Ir(IMes)(IMes′)(H)Cl (see Supporting Information) by activation of one of the mesityl orthomethyl substituents in a manner similar to related rhodium chemistry reported by Nolan and co-workers.7a (21) See, for example: Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Sausalito, CA, 1987.

Table 1. Calculated Energies for Different Orientations of the Alkene Ligand in Complex 2 and Related Model Systems

compounda 2 2a 2b

R1

H H i Pr

R2

∆E /kJ mol-1b

torsion angle (parallel alignment)c

H Me Me

-4.9 +2.6 +0.5 -3.5 -5.8

171.9 (174.4) 175.7 172.8 179.8 178.3

torsion angle (perpendicular alignment)c 69.0 89.3 82.4 75.9 71.8

a Compound 2 features the “full” substituent set, i.e., as realized experimentally; model compounds 2a and 2b are as defined in Scheme 2. b ∆E is the energy difference between the local minima corresponding to approximately parallel and approximately perpendicular arrangements of the alkene C-C vector and the basal plane of the iridium coordination sphere, i.e., E(parallel) - E(perpendicular). c Calculated torsion angle (CNHC-Ir-alkene centroid-Calkene) for the local minimum energy geometry; relevant crystallographically determined data are given in parentheses.

for (1,4,5-η-2,4-dimethylpentadienyl)Ir(PPh3)2(CO)].22 The marked asymmetry in the accompanying Ir-C distances [2.086(4) and 2.178(3) Å] presumably reflects the geometric constraints imposed by the ligand tether, as well as the steric implications of the alkene substitution pattern. A similar asymmetry, reflecting a shorter bond to the less substituted carbon atom [2.126(2) vs 2.163(2) Å], has been reported by Chirik and co-workers for a related iridium(I) system featuring a dehydrogenated isopropyl substituent pendant to a Nacnac donor.10c,23 While C-H activation processes at the peripheral substituents of NHC ligands have previously been observed at group 9 centers,7,8 and the formal dehydrogenation of alkyl groups attached to phosphine and Nacnac ligands has recently been described,9-12 the analogous dehydrogenation of NHC substituents is relatively rare.11 Goldberg has proposed a combination of C-H oxidative addition, β-hydride elimination, and reductive (22) Bleeke, J. R.; Boorsma, D.; Chiang, M. Y.; Clayton, T. W., Jr.; Haile, T.; Beatty, A. M.; Xie, Y.-F. Organometallics 1991, 10, 2391. (23) For other examples of iridium NHC complexes featuring (preconstructed) pendant alkene donors see, for example: (a) Hahn, F. E.; Hiltgrewe, C.; Pape, T.; Martin, M.; Sola, E.; Oro, L. A. Organometallics 2005, 24, 2203. (b) Cordoba´n, R.; Sanau´, M.; Peris, E. Organometallics 2007, 26, 3492. (c) Zanardi, A.; Peris, E.; Mata, J. A. New J. Chem. 2008, 32, 120.

3064 Organometallics, Vol. 28, No. 10, 2009

Tang et al.

Scheme 3. Reactivity of Dehydrogenated System 1a

a Key reagents and conditions: (i) H (4 atm), C D , 20 °C, ca. 1 h, quantitative by 1H NMR; (ii) H (4 atm), C D , 20 °C, 24 h, quantitative; (iii) 2 6 6 2 6 6 exposure to air, benzene, 20 °C, 24 h, 70%; (iv) H3N · BH3, MeH2N · BH3, Me2HN · BH3, iPr2HN · BH3, or LiBH4, diethyl ether, 20 °C,1 h, quantitative 1 by H NMR.

elimination pathways to account for the dehydrogenation of Nacnac isopropyl substituents,10a,b and subsequent reactivity observed for 1 (vide infra) suggests that C-H oxidative addition chemistry is facile for the methyl C-H bonds of the isopropyl substituent. Reaction of 1 with Na[BArf4] in fluorobenzene generates the cationic species [Ir(IPr′)(IPr′′)(H)]+ as the [BArf4]- salt (2), in which chloride ion abstraction has been accompanied by oxidative addition of one of the methyl C-H bonds of the previously unactivated IPr ligand (Scheme 1). 2 has been characterized by standard spectroscopic and analytical techniques, with the presence of a high-field resonance at δH -46.6 ppm being diagnostic not only of a C-H activation process but also of a hydride ligand trans to a vacant coordination site.3c The structure of 2 determined by single-crystal X-ray diffraction (Figure 1b) reveals a five-coordinate (approximately square pyramidal) Ir(III) cation featuring trans carbene donors, trans alkyl/η2-alkene substituents, and a hydride ligand (located in the difference map but refined using a riding model). 2 represents a rare example of a transition metal complex containing alkene, alkyl, and hydride ligands;24 in the absence of a hydrogen acceptor (cf. cyclooctene in the formation of 1) 2 does not appear to undergo further substituent activation processes. Moreover, variable-temperature and spin saturation transfer experiments in solution imply that there is no exchange on the NMR time scale between the iridium-bound hydride and the β-hydrogens of the alkyl ligand; that is, a degenerate fluxional process involving alkene insertion into the Ir-H bond and β-hydride elimination from the alkyl substituent is not facile. The square pyramidal geometry at the iridium center in 2 is entirely consistent with quantum chemical predictions for d6 ML5 systems, being largely dictated by the strong σ-donor and nonexistent π-bonding characteristics of the apical hydride ligand.25 In contrast to 1, the orientation of the alkene ligand in 2 is effectively parallel to the basal plane [C(8)-Ir(1)-alkene centroid-C(2) torsion angle ) 174.4°], a finding difficult to rationalize on the basis of simple electronic arguments, given the formal degeneracy of the metal dxy, dxz, and dyz orbitals in the (C4V) ML5 σ-only molecular orbital picture.26 Quantum chemical calculations carried out on the cationic component of 2 using the methods detailed above reveal a fully optimized geometry consistent with that derived from crystallographic measurements (see Table 1); the local minimum corresponding to the alternative perpendicular alkene ligand orientation is 4.9 kcal mol-1 higher in energy. While the orientational preferences of alkene ligands in planar four-coordinate d8 systems (such as 1) have been well documented,21,26 the coordination geometry

in the square pyramidal Ir(III) system 2 is more unusual.27 In order to ascertain whether such a geometry is enforced by steric (rather than predominantly electronic) factors or by the constraints of the alkene ligand “tether”, a further series of calculations were carried out on the model systems 2a and 2b (Scheme 2). Calculations carried out on sterically unencumbered and non-tethered model system [Ir(IH)2(Me)(C2H4)H]+ (2a; IH ) imidazol-2-ylidene) are consistent with a relatively small energetic difference (2.6 kcal mol-1) between local minima corresponding to essentially the parallel (torsion angle ) 175.7°) and perpendicular (89.3°) alignments of the alkene ligand. However, in contrast to the conformation observed in the solidstate structure of 2, the perpendicular ligand alignment is favored, thus implying that the steric bulk and/or tether constraints present in the experimentally realized system are likely to be important conformational influences. Calculations carried out on a further series of model systems (2b; Scheme 2) in which both the alkyl and alkene ligands are tethered to the ancillary NHC ligands imply that steric factors at the alkene R2 position are important in determining the lowest energy conformation. Thus, model systems featuring a 2,2 alkene disubstitution pattern feature a parallel ground-state alignment of the alkene ligand analogous to that found crystallographically for 2 (Table 1), while the corresponding monosubstituted system (i.e., with R2 ) H) shows a marginal (0.5 kcal mol-1) preference for the perpendicular ligand orientation. Figure 1c provides some indication of the possible origins of this steric effect for R2 ) Me. The local minimum geometry corresponding to the (unfavored) approximately perpendicular alkene ligand orientation brings with it a contact between the hydrogens of the R2 methyl substituent and a proximal isopropyl CH, which falls well within (24) For previous reports of structurally characterized iridium(III) complexes containing hydride, alkyl, and alkene ligands see: (a) Scherer, O. J.; Florchinger, M.; Gobel, K.; Kaub, J.; Sheldrick, W. S. Chem. Ber. 1988, 121, 1265. (b) Sjovall, S.; Svensson, P. H.; Andersson, C. Organometallics 1999, 18, 5412. (c) Padilla-Martine´z, I. I.; Poveda, M. L.; Carmona, E.; Monge, M. A.; Ruiz-Valero, C. Organometallics 2002, 21, 93. (d) Kataoka, Y.; Shizuma, K.; Imanishi, M.; Yamagata, T.; Tani, K. J. Organomet. Chem. 2004, 689, 3. Related systems have been identified spectroscopically from the photolysis of iridium bis(alkene) complexes: (e) Rodrı´guez, P.; Dı´az-Requejo, M. M.; Belderrain, T. R.; Trofimenko, S.; Nicasio, M. C.; Pe´rez, P. Organometallics 2004, 23, 2162. (25) See, for example: (a) Rachidi, I. E.; Eisenstein, O.; Jean, Y. New J. Chem. 1990, 14, 671. (b) Riehl, J. F.; Jean, Y.; Eisenstein, O.; Pelissier, M. Organometallics 1992, 11, 729. (26) See, for example: Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry; Wiley-Interscience: Chichester, 1985. (27) For studies of related square pyramidal 16-electron Ru(II) alkene hydrides see: (a) Gusev, D. G.; Lough, A. J. Organometallics 2002, 21, 2106. (b) Ge´rard, H.; Eisenstein, O. Dalton Trans. 2003, 839. (c) Kuznetsov, V. F.; Abdur-Rashid, K.; Lough, A. J.; Gusev, D. G. J. Am. Chem. Soc. 2006, 128, 14388.

Iridium Bis(N-heterocyclic carbene) Systems

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Figure 2. Molecular structures of (a) Ir(IPr)2(H)2Cl · OEt2 (4 · OEt2), (b) Ir(IPr)2(O2)Cl (5), and (c) Ir(IPr)(aIPr)(H)2Cl · C6H6 (6 · C6H6). Solvent molecules (for 4 · OEt2 and 6 · C6H6) and hydrogen atoms except those attached to iridium (for 4 · OEt2 and 6 · C6H6) omitted for clarity; thermal displacement ellipsoids set at the 50% probability level. Key bond lengths (Å), and angles (deg): (for 4 · OEt2) Ir(1)-Cl(2) 2.351(2), Ir(1)-C(4) 2.044(6), Ir(1)-C(33) 2.012(5), C(4)-Ir(1)-C(33) 178.3(2); (for 5) Ir(1)-O(2) 2.003(4), Ir(1)-O(3) 1.991(5), Ir(1)-C(4) 2.055(6), Ir(1)-C(33) 2.053(7), Ir(1)-Cl(62) 2.278(2), O(2)-O(3) 1.361(7), C(4)-Ir(1)-C(33) 177.4(2), O(2)-Ir(1)-O(3) 39.8(2); (for 6 · C6H6) Ir(1)-C(3) 2.032(5), Ir(1)-C(32) 2.051(3), Ir(1)-Cl(2) 2.413(1), C(32)-N(33) 1.417(5), N(33)-C(46) 1.341(5), C(46)-N(47) 1.316(5), N(47)-C(48) 1.393(4), C(32)-C(48) 1.377(5), C(3)- Ir(1)-C(32) 177.8(2).

the sum of their van der Waals radii. The closest approach (2.106 Å) is markedly shorter than any H · · · H contacts found in the corresponding “parallel” structure and thus provides a rationale for its lower energy. Hydrogenation of 1 under 4 atm of pressure ultimately proceeds to give the iridium(III) bis(hydride) complex Ir(IPr)2(H)2Cl, 4 (Scheme 3), which was originally reported by Sames and co-workers to be the major product isolated from the reaction of [Ir(coe)2Cl]2 with IPr in cyclohexane.16 In our hands, and using thf as the reaction medium, only very small quantities of 4 are isolated from the direct reaction of [Ir(coe)2Cl]2 with IPr. 1H NMR monitoring of the hydrogenation of 1 over a period of 24 h is consistent with the buildup of measurable concentrations of an intermediate at short reaction times (ca. 1 h). Thus, the reaction at this point is characterized by the appearance of two broad hydride resonances at δH -24.97 and -25.69 and by the retention (albeit shifted) of signals for the -C(Me)dCH2 fragment [at δH 1.36, 1.74 (both doublets, one hydrogen, 2JHH ) 1.2 Hz) and δH 2.06 (singlet, three hydrogens)]. Crabtree and co-workers have established for related cationic L2Ir(I) systems that hydrogenation of coordinated alkenes typically proceeds via initial formation of an alkene bis(hydride) complex,28 and Brookhart has identified a related iridium pincer complex containing alkene and cis hydride ligands by low-temperature NMR spectroscopy.29 Moreover, in related chemistry, Budzelaar has shown that the hydrogenation of the “masked” alkene complex LIr(H)(cyclooctenyl) {L ) [(2,6-Me2C6H3)NC(Me)]2CH} generates the iridium(III) alkene bis(hydride) complex LIr(H)2(coe) in virtually quantitative yield, with the mutually cis hydride ligands giving rise to signals at δH -22.60 and -22.74 ppm.30 On this basis we propose that the intermediate observed in the hydrogenation of 1 is the dihydride complex Ir(IPr)(IPr′′)(H)2Cl, 3, which over a period of ca. 24 h then takes up a second equivalent of dihydrogen to yield the bis(IPr) complex 4. (28) Crabtree, R. H.; Demou, P. C.; Eden, D.; Mihelcic, J. M.; Parnell, C. A.; Quirk, J. M.; Morris, G. E. J. Am. Chem. Soc. 1982, 104, 6994. (29) Go¨ttker-Schnetmann, I.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 9330. (30) Budzelaar, P. H. M.; Moonen, N. N. P.; de Gelder, R.; Smits, J. M. M.; Gal, A. W. Eur. J. Inorg. Chem. 2000, 753.

Chart 2. Mixed NHC/Abnormal NHC Complex Ir(IPr)(aIPr)(H)2Cl, 6, an Isomer of Ir(IPr)2(H)2Cl, 4

Single crystals of 4 are accessible as the diethyl ether monosolvate, and the molecular structure so obtained (Figure 2) can be compared to that of the isomeric mixed NHC/abnormal NHC complex Ir(IPr)(aIPr)(H)2Cl, 6 (Chart 2.and Figure 2), which is obtained as a minor byproduct in the synthesis of 1 after recrystallization from benzene.31-34 Both compounds feature a trans disposition of the carbene ligands [∠C-Ir-C (31) For other examples of transition metal complexes that contain different modes of NHC ligand coordination within the same molecule see, for example: (a) Danopoulos, A. A.; Tsoureas, N.; Wright, J. A.; Light, M. A. Organometallics 2004, 23, 166. [Fe]. (b) Caddick, S.; Cloke, F. G. N.; Hitchcock, P. B.; Lewis, A. K. de K. Angew. Chem., Int. Ed. 2004, 43, 5824. [Ni]. (c) Lebel, H.; Janes, M. K.; Charette, A. B.; Nolan, S. P. J. Am. Chem. Soc. 2004, 126, 5046. [Pd]. (d) Bacciu, D.; Cavell, K. J.; Fallis, I. A.; Ooi, L.-L. Angew. Chem., Int. Ed. 2005, 44, 5282. [Pt]. (e) Arnold, P. L.; Liddle, S. T. Organometallics 2005, 25, 1485. [Y, K]. (f) Viciano, M.; Feliz, M.; Corbera´n, R.; Mata, J. A.; Clot, E.; Peris, E. Organometallics 2007, 26, 5304. [Ir]. (g) Appelhans, L. N.; Incarvito, C. D.; Crabtree, R. H. J. Organomet. Chem. 2008, 693, 2761. [Ir]. (32) For other examples of iridium complexes containing abnormally bound NHC ligands see refs 13 and 31f and (a) Grundemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.; Crabtree, R. H. J. Am. Chem. Soc. 2002, 124, 10473. (b) Chianese, A. R.; Kovacevic, A.; Zeglis, B. M.; Faller, J. W.; Crabtree, R. H. Organometallics 2004, 23, 2461. (c) Stylianedes, N.; Danopoulos, A. A.; Tsoureas, N. J. Organomet. Chem. 2005, 690, 5948. (33) The C-N and C-C bond lengths within the “abnormal” carbene heterocycle in 6 agree well with previous reports of this type of ligand.32 (34) The direct reaction of [Ir(coe)2Cl]2 with [(IPr)H]+Cl- in thf simply leads to coordination of the chloride ion and the generation of [(IPr)H]+[Ir(coe)2Cl2]- (see Supporting Information).

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) 178.3(2)° and 177.8(2)° for 4 · OEt2 and 6, respectively] and thereby offer a means of comparing the relative trans influences of the ‘normal’ and abnormal’ forms of the IPr ligand. Although recent quantum chemical studies imply that the abnormal mode of coordination of the carbene ligand typically brings about stronger σ-donor behavior,15,32 the Ir-C distance trans to the abnormal carbene ligand in 6 {d[Ir(1)-C(3)] ) 2.032(3) Å} actually lies between the two distinct Ir-C distances measured for 4 [2.044(6) and 2.012(5) Å]. The distance measured for 6 is also statistically identical to that determined for the Ir-C distance trans to the IMes ligand in Ir(IPr)(IMes)(H)2Cl [2.026(4) Å; see Supporting Information]. Within compound 6 itself, the Ir-C bond length associated with the abnormal carbene ligand [Ir(1)-C(32) ) 2.051(3) Å] is marginally longer than the “normal” linkage Ir(1)-C(3) [2.032(3) Å], consistent with related structural studies reported by Crabtree and by Peris and Clot.31f,32a Steric factors have previously been shown to be of importance in the synthesis of abnormal NHC complexes,14a,32a and although the adoption of this coordination mode (as in 6) implies reduced steric loading at the iridium center, the isolation of both 1 and 6 from the same reaction mixture implies that this is unlikely to be the sole discriminating factor. Exposure of bis(hydride) complex 4 to air leads to the formation of a deep violet compound which can be obtained as single crystals suitable for X-ray diffraction by recrystallization from cyclohexane (Scheme 3 and Figure 2) and thus shown to be Ir(IPr)2(O2)Cl (5).16 The crystallographically determined O-O distance [1.361(7) Å] and IR-measured ν(O-O) stretching frequency (863 cm-1) are consistent with an iridium(III) peroxide formulation.35 Moreover, exposure of 5 to dihydrogen (at 4 atm. pressure) or to excess ammonia borane leads to quantitative recovery of dihydride 4, thus offering 5 as an easily (35) For a related recent example of O2 binding by L2IrX systems see: Williams, D. B.; Kaminsky, W.; Mayer, J. M.; Goldberg, K. I. Chem. Commun. 2008, 4195. (36) The stability of 5 in air and its reaction with ammonia borane (AB) to give 4 suggest its potential use as a precatalyst in the dehydrogenation of AB and related amine boranes.37 5 can indeed be shown to be competent for the catalytic dehydrogenation of the model system iPr2NH · BH3 (to give monomeric iPr2NBH2 and H2) at 50 °C and 2-5 mol % loading, although the reaction is typically very slow to reach completion (half-life ca. 100 h). (37) For related examples of amine borane dehydrogenation by IrL2X(H)2 systems see, for example: (a) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 12048. (b) Hebden, T. J.; Denney, M. C.; Pons, V.; Piccoli, P. M. B.; Koetzle, T. F.; Schultz, A. J.; Kaminsky, W.; Goldberg, K. I.; Heinekey, D. M. J. Am. Chem. Soc. 2008, 130, 10812. (c) Staubitz, A.; Soto, A. P.; Manners, I. Angew. Chem., Int. Ed. 2008, 47, 6212.

Tang et al.

handled air-stable source of Ir(IPr)2Cl-containing reagents, e.g., for use in hydrogenation/dehydrogenation chemistry.36

Conclusions Reaction of [Ir(coe)2Cl]2 with the N-heterocyclic carbene IPr has been shown to proceed via a number of C-H activation processes, leading to the isolation of the Ir(III) mixed normal/ abnormal NHC complex Ir(IPr)(aIPr)(H)2Cl (6) together with an Ir(I) system, Ir(IPr)(IPr′′)Cl (1), containing a chelating NHC/ alkene donor (IPr”) which results from the dehydrogenation of a pendant isopropyl substituent. While the corresponding reactivity toward IMes proceeds via simple methyl C-H oxidative addition, the IPr isopropyl substituents offer competing sites for activation. Subsequent reactivity for 1 initiated by chloride ion abstraction suggests that C-H oxidative addition chemistry is facile for the methyl C-H bonds of the carbene isopropyl substituents. Thus, the Ir(III) alkene alkyl hydride [Ir(IPr′)(IPr′′)H]+[BArf4]- (2) featuring an additional 1-metalated isopropyl substituent is isolated from the reaction of 1 with Na[BArf4] in the absence of a hydrogen acceptor. Structurally, square pyramidal 2 is shown to feature an alkene ligand which is essentially coplanar with the basal coordination plane, an orientation which is shown by quantum chemical calculations to be influenced by the steric properties of the alkene substituents. Hydrogenation of 1 leads ultimately to the formation of Ir(IPr)2(H)2Cl (4), the solid-state structure of which [along with those of Ir(IPr)(aIPr)(H)2Cl and Ir(IPr)(IMes)(H)2Cl] offers a basis for comparison of the trans influences of the IPr, aIPr and IMes ligands. The observation of similar Ir-C bond lengths trans to each of these ligands, in otherwise identical Ir(III) coordination environments, is consistent with relatively similar σ-donor capabilities.

Acknowledgment. The EPSRC for grant EP/F01600X/1 including funding for a postdoctoral position for C.Y.T. We also thank the EPSRC National Mass Spectrometry Service Centre, Swansea University. Supporting Information Available: Crystallographic data for 1, 2, 4 · OEt2, 5, 6 · C6H6; synthetic, spectroscopic, and crystallographic data for Ir(IPr)(IMes)(H)2Cl, Ir(IMes)(IMes’)(H)Cl and [(IPr)H]+[Ir(coe)2Cl2]-; all crystallographic data in CIF format; diagram of the disordered benzene/CH2Cl2 solvate molecules for 6; details of optimized geometries for calculated molecular structures.This material is available free of charge via the Internet at http://pubs.acs.org. OM9000082