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Organometallics 2009, 28, 4646–4648 DOI: 10.1021/om900657f
A Monomeric Perfluoroalkyl Iridium(III) Amido Complex with an IrdN Double Bond and Its Reactions To Activate sp3 Carbon-Hydrogen Bonds at Room Temperature Jian Yuan,† Russell P. Hughes,*,† and Arnold L. Rheingold‡ †
Department of Chemistry, 6128 Burke Laboratory, Dartmouth College, Hanover, New Hampshire 03755, and ‡Department of Chemistry, University of California, San Diego, California 92093-0358 Received July 24, 2009
Summary: A simple iridium amido complex has been prepared and shown to contain an unambiguous IrdN double bond, using X-ray crystallography and DFT calculations. The compound exhibits remarkable reactivity by activating the allylic C-H bonds of terminal alkenes at room temperature, liberating tBuNH2 and forming η3-allylic complexes of iridium. Late-transition-metal complexes with MdNR2 doubly bonded terminal amido ligands are rare.1 For d6 metal centers, the d orbitals required for pπ/dπ bonding are nominally filled, giving rise to pπ-dπ repulsion, although orbital descriptions involving partial π-bonding2,3 and other models that do not involve a π-bond have been put forth.4 Here we report chemistry in which a perfluoroethyl group serves as a nonreactive ligand during inner-sphere construction of an unusually simple IrdN double-bonded amido complex and the remarkable reactivity of this compound in the room-temperature activation of sp3 C-H bonds. As observed previously for Rh analogues,5 the red dimeric complex 1a can be prepared from monomeric IrCp*(CF2CF3)(CO)(I)6 by oxidative removal of CO using N-methylmorpholine N-oxide. Addition of tBuNH2 to a CH2Cl2 solution of 1a resulted in a rapid color change from red to bright yellow, with formation of the amine complex 2a. The 19F NMR spectrum of 2a showed broad AB quartet peaks (δ -75.4 and -87.7 (JAB = 261 Hz)), for the diastereotopic CF2 fluorines adjacent to the Ir stereocenter. Exposure of solutions of 2a to vacuum resulted in amine dissociation to give 1a. However, a CH2Cl2 solution of 2a reacted with AgBF4 in the presence of excess tBuNH2 to afford the amido complex 3 (92%) as orange-red crystals, along with tBuNHþ 3 BF4 . Reaction of 3 with HCl afforded t 2b, from which BuNH2 was lost under vacuum to give 1b. The chloro(amine) complex 2b could be crystallized from a solution containing tBuNH2 and was crystallographically *To whom correspondence should be addressed. E-mail: rph@ Dartmouth.edu. (1) Nugent, W. A.; Mayer, J. M., Metal-Ligand Multiple Bonds; Wiley: New York, 1988. (2) Riehl, J.-F.; Jean, Y.; Eisenstein, O.; Pelissier, M. Organometallics 1992, 11, 729–737. (3) Lunder, D. M.; Lobkovsky, E. B.; Streib, W. E.; Caulton, K. G. J. Am. Chem. Soc. 1991, 113, 1837–1838. (4) Holland, P. L.; Andersen, R. A.; Bergman, R. G. Comments Inorg. Chem. 1999, 21, 115–129. (5) Hughes, R. P.; Lindner, D. C.; Liable-Sands, L. M.; Rheingold, A. L. Organometallics 2001, 20, 363–366. (6) Hughes, R. P.; Smith, J. M.; Liable-Sands, L. M.; Concolino, T. E.; Lam, K.-C.; Incarvito, C.; Rheingold, A. L. Dalton Trans. 2000, 873–879. pubs.acs.org/Organometallics
Published on Web 07/30/2009
characterized; details are provided as Supporting Information.
The amido complex 3 was characterized crystallographically; an ORTEP drawing is shown in Figure 1. The Ir-N distance (1.912(4) A˚) is longer than the Ir-N triple bond in Cp*Ir(NtBu) (1.712(7) A˚)7,8 but is significantly shorter than either the single bond in 2b (2.199(4) A˚) or IrdN bonds in the few other five-coordinate 16e iridium amido complexes.9-11 The Ir1-N1-C13 angle of 134.4(3)° is also consistent with an sp2 N atom. DFT calculations on 3 reproduce the crystallographically determined metric parameters well, and the calculated Kohn-Sham orbitals clearly illustrate π-bonding between Ir and N, as shown in Figure 2.12 HOMO-3 and HOMO-5 involve iridium-N π-bonding using different d orbitals, each of which is shared with bonding contributions between Ir and Cp*: the HOMO is Ir-N π* but is bonding with respect to Ir and Cp*; the LUMO is both Ir-N and IrCp* antibonding in character. NBO13 calculations on the DFT optimized structure also support a strong π interaction between N and Ir. The Ir-N σ bond involves Ir(sd1.4) and N(sp1.8) hybrids, while the Ir-N π bond involves an essentially pure N(pπ)-Ir(dπ) interaction; both components are polarized toward N (σ 74%; π 67%). The distance between H1A and F1 (2.08 A˚) is rather long for a possible H-bonding interaction, and NBO analysis reveals insignificant delocalization of the F1 lone pair into the N-H σ* orbital. (7) Glueck, D. S.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc. 1989, 111, 2719–2721. (8) Glueck, D. S.; Wu, J.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc. 1991, 113, 2041–2054. (9) Kanzelberger, M.; Zhang, X.; Emge, T. J.; Goldman, A. S.; Zhao, J.; Incarvito, C.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 13644– 13645. (10) Zhao, J.; Goldman, A. S.; Hartwig, J. F. Science 2005, 307, 1080– 1082. (11) Stykes, A. C.; White, P.; Brookhart, M. Organometallics 2006, 25, 1664–1675. (12) Details of the DFT calculations appear in the Supporting Information. (13) Glendening, E. D.; Badenhoop, J. K.; Reed, A. K.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0; Theoretical Chemistry Institute, University of Wisconsin, Madison, WI, 2001. r 2009 American Chemical Society
Communication
Figure 1. ORTEP diagram of 3 showing the partial atomlabeling scheme. Thermal ellipsoids are shown at the 30% level. Hydrogen atoms in the Cp* and tBu groups are omitted for clarity.
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Figure 3. ORTEP diagram of 5 showing the partial atomlabeling scheme. Thermal ellipsoids are shown at the 30% level. Hydrogen atoms in the Cp* and tBu groups are omitted for clarity.
Figure 4. ORTEP diagram of 6 showing the partial atomlabeling scheme. Thermal ellipsoids are shown at the 30% level. Hydrogen atoms in the Cp* group are omitted for clarity.
Figure 2. Kohn-Sham orbitals of 3 involved in Ir-N π-bonding.
The 1H NMR spectrum of 3 showed a typical broad lowfield peak at δ 11.4 ppm for the N-H proton;14 1D NOESY spectroscopy confirmed the Z isomer as the solution structure with the Cp* cis to tBu. The effective symmetry plane in 3 was manifested by singlet 19F NMR peaks for the CF3 and CF2 groups. Using the monomer IrCp*(C2F5)(CO)I and dimer 1a as internal references, diffusion-ordered NMR spectroscopy (DOSY) confirmed a monomeric structure for 3 in solution.15,16 The basicity of singly bonded terminal amido complexes of iridium is well established, and their ability to deprotonate other amines, phenol, and (p-nitrophenyl)acetonitrile has been demonstrated.17 Not unexpectedly, the amido complex (14) Legzdins, P.; Rettig, S. J.; Ross, K. J. Organometallics 1993, 12, 2103–2110. (15) Li, D.; Keresztes, I.; Hopson, R.; Williard, P. G. Acc. Chem. Res. 2009, 42, 270–280. (16) Li, D.; Kagan, G.; Hopson, R.; Williard, P. G. J. Am. Chem. Soc. 2009, 131, 5627–5634. (17) Rais, D.; Bergman, R. G. Chem. Eur. J. 2004, 10, 3970–3978.
3 rapidly deprotonates propionic acid to give the chelating carboxylate complex 4, with liberation of tBuNH2. However, it also reacts at room temperature with acetonitrile to give 5 (97%), which was crystallographically characterized; an ORTEP drawing is shown in Figure 3. The free cyanoNacNac ligand is known from reaction of acetonitrile with strong bases18 and has been made in the coordination sphere of cobalt (electrochemically)19 or ruthenium at 180 °C;20 ours represents an exceptionally mild synthesis.
A more interesting C-H activation occurs on exposure of a hexane solution of 3 to propene (1 atm) at room temperature, resulting in a color change from red to off-white, with liberation of tBuNH2 and formation of the exo-η3-allyl complex 6 (90%). Compound 6 was characterized crystallographically; the asymmetric unit contained two independent molecules, and an ORTEP drawing of one of them is (18) Nishijo, J.; Nishi, N. Eur. J. Inorg. Chem. 2006, 3022–3027. (19) Duran, M. L.; Garcia-Vazquez, J. A.; G omez, C.; Sousa-Pedrares, A.; Romero, J.; Sousa, A. Eur. J. Inorg. Chem. 2002, 2348–2354. (20) Foley, N. A.; Gunnoe, T. B.; Cundari, T. R.; Boyle, P. D.; Petersen, J. L. Angew. Chem., Int. Ed. 2008, 47, 726–730.
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shown in Figure 4. As expected, the average C-C bond lengths in the allyl ligand are the same (C11-C12=1.427(9) A˚ and C12-C13=1.425(9) A˚), as are the average distances from Ir to the terminal allylic carbons (Ir1-C11=2.169(6) A˚ and Ir1-C13=2.179(6) A˚), with a slightly shorter distance from Ir to the internal allylic carbon (Ir1-C12=2.107(6) A˚). The 1H NMR spectrum of 6 showed the expected AMM0 XX0 pattern for the allylic protons,21 and 1D NOESY spectra confirmed the exo configuration of the allyl ligand. DFT calculations confirm that the exo isomer is thermodynamically more stable than the endo isomer (in the gas phase ΔG° = 3.1 kcal/mol); therefore, it is not possible to discern whether selective formation of 6 is the result of kinetic or thermodynamic control. Likewise, 3 reacts with 1-butene to afford initially a 2:1 mixture of exo,syn- and exo,anti-methylallyl complexes 7 and 8. Over time this ratio slowly changed to 9:1, a phenomenon observed in hydrido methylallyl analogues.21
Yuan et al.
less hindered terminal alkene can bind, but not an internal one. While NMR monitoring showed no evidence of intermediates involving propene binding, low-temperature exposure of 3 to ethylene results in binding, as evidenced by diastereotopic CF2 resonances, but warming to room temperature resulted in ethylene dissociation without any further chemistry, indicating that the observed chemistry may be limited to allylic C-H bonds in terminal alkenes. Since free tBuNH2 is a sufficiently good base to deprotonate coordinated tBuNH2 during formation of 3, the basicity of the amido group in 3 is clearly incompatible with simple deprotonation of free CH3CN or CH3CHdCH2 under these conditions. A bound substrate seems required, and either intramolecular deprotonation or oxidative addition of the allylic C-H bond to give an Ir(V) intermediate followed by reductive elimination of tBuNH2 may afford routes to 6-8. Studies of the scope of these C-H bond activation reactions and experimental and computational studies of mechanistic options are ongoing.
Acknowledgment. R.P.H. is grateful to the U.S. National Science Foundation for generous financial support.
No reaction was observed with cis- or trans-2-butene, suggesting that alkene binding precedes C-H activation; a (21) McGhee, W. D.; Bergman, R. G. J. Am. Chem. Soc. 1988, 110, 4246–4262.
Supporting Information Available: Text, tables, and figures giving experimental details for all compounds and details of DFT and NBO calculations and CIF files giving crystallographic data for compounds 2b and 3-6. This material is available free of charge via the Internet at http://pubs.acs. org.
