Synthesis and Structural and Computational Studies of a

Fluorine as a ligand substituent in organometallic chemistry: A second chance and a second research career. Russell P. Hughes. Journal of Fluorine Che...
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Organometallics 2010, 29, 1942–1947 DOI: 10.1021/om1000343

Synthesis and Structural and Computational Studies of a Conformationally Locked (η1-Perfluoroalkylidene)(η2-alkene) Transition Metal Complex: Ir(Cp*)(CFCF3)(C2H4) Jian Yuan,† Russell P. Hughes,*,† James A. Golen,‡ and Arnold L. Rheingold§ †

Department of Chemistry, 6128 Burke Laboratory, Dartmouth College, Hanover, New Hampshire 03755, ‡ University of Massachusetts-Dartmouth, North Dartmouth, Massachusetts 02747, and §Department of Chemistry, University of California, San Diego, California 92093-0358 Received January 12, 2010

Reaction of Cp*Ir(CO)(C2F5)I with N-methylmorpholine-N-oxide (NMO) afforded the iodidebridged dimer complex [Cp*Ir(C2F5)I]2. The dimer reacted reversibly with ethylene to give Cp*Ir(CH2dCH2)(C2F5)I, which dissociated ethylene under reduced pressure. Treatment of Cp*Ir(CH2dCH2)(C2F5)I with AgOTf gave isolable Cp*Ir(CH2dCH2)(C2F5)(OTf). Reduction of this compound with potassium graphite gave the first example of an (ethylene)(perfluoroethylidene) complex, Cp*Ir(CH2dCH2)(CFCF3), which has been characterized crystallographically, spectroscopically, and computationally. Comparisons of a variety of hybrid and gradient-corrected density functionals in predicting geometric parameters, energies, barriers to ligand rotation, and possible alkene metathesis in Cp*Ir(CH2dCH2)(CFCF3) are presented.

Transition metal complexes with alkylidene ligands have been known for decades, and some have been extensively utilized to perform catalytic alkene metathesis.1 Despite the large variety of substrates used for alkene metathesis, no examples of either perfluoroalkene metathesis or metathesis utilizing a perfluoroalkylidene complex have been reported, although a RudCF2 complex is a very slow initiator of hydrocarbon alkene metathesis.2 The key chemical species in alkene metathesis are transition metal-alkylidene complexes as the propagating chain carrier, with a rapid equilibrium between (alkylidene)(alkene) and metallacyclobutane intermediates effecting the metathesis, shown for a hypothetical case as 1 and 2. Successful metathesis requires a facile equilibrium between 1 and 2, which in turn requires that the alkylidene and alkene ligands in 1 can easily achieve the correct cis-parallel conformation required for metallacycle formation. Either the ground-state conformations of these ligands must be correct (as shown for 1) or the barrier to rotation about the MdC and M-alkene bonds must be low. Our recent success in synthesizing fluorinated carbene complexes of iridium by simple reductive conversion of readily available perfluoroalkyl iridium complexes3 to difluorocarbene *Corresponding author. E-mail: [email protected]. (1) (a) Schrock, R. R. Angew. Chem., Int. Ed. 2006, 45, 3748–3759. (b) Grubbs, R. H. Angew. Chem., Int. Ed. 2006, 45, 3760–3765. (c) Grubbs, R. H. Tetrahedron 2004, 60, 7117–7140. (d) Grubbs, R. H. In Handbook of Metathesis; Wiley-VCH: New York, 2003. (e) Schrock, R. R. Chem. Rev. 2002, 102, 145–179. (f) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18–29. (2) Trnka, T. M.; Day, M. W.; Grubbs, R. H. Angew. Chem., Int. Ed. 2001, 40, 3441–3444. (3) (a) Hughes, R. P.; Smith, J. M.; Liable-Sands, L. M.; Concolino, T. E.; Lam, K.-C.; Incarvito, C.; Rheingold, A. L. J. Chem. Soc., Dalton Trans. 2000, 873–879. (b) Hughes, R. P.; Zhang, D.; Ward, A. J.; Zakharov, L. N.; Rheingold, A. L. J. Am. Chem. Soc. 2004, 126, 6169–6178. pubs.acs.org/Organometallics

Published on Web 03/22/2010

complexes 3 and 4, fluoro(perfluoroalkyl)carbene complexes 5-8, and bis(trifluoromethyl)carbene complex 9, using a variety of reducing agents,4 led us to explore extensions to analogues of 1, in the hope that their potential for metathesis reactivity involving fluorinated alkylidene ligands could be evaluated.

Treatment of a yellow methylene chloride solution of Cp*Ir(CO)(CF2CF3)I 10 with N-methylmorpholine-N-oxide (NMO) resulted in the expected oxidative removal of the CO ligand,5 presumably as CO2, and quantitative formation of the air-stable, ruby-red, iodide-bridged dimer 11, whose (4) (a) Hughes, R. P.; Laritchev, R. B.; Yuan, J.; Golen, J. A.; Rucker, A. N.; Rheingold, A. L. J. Am. Chem. Soc. 2005, 127, 15020–15021. (b) Bourgeois, C. J.; Hughes, R. P.; Yuan, J.; DiPasquale, A. G.; Rheingold, A. L. Organometallics 2006, 25, 2908–2910. (c) Yuan, J.; Hughes, R. P.; Rheingold, A. L. Eur. J. Inorg. Chem. 2007, 4723–4725. (5) Hughes, R. P.; Lindner, D. C.; Liable-Sands, L. M.; Rheingold, A. L. Organometallics 2001, 20, 363–366. r 2010 American Chemical Society

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Table 1. Crystallographic Summary for 11 and 14a 11 formula fw space group a, A˚ b, A˚ c, A˚ R, deg β, deg γ, deg V, A˚3 Z cryst color, habit D(calcd), g/cm3 μ(Mo KR(λ = 0.71073)), mm-1 temp, K diffractometer reflns indep R(F) [I > 2σ(I)], %a R(wF2) [I > 2σ(I)], %a

C24H30F10I2Ir2 1146.68 Pbca 12.8514(10) 13.9753(11) 16.3838(13) 90 90 90 2942.6(4) 4 orange, block 2.588 11.211

14a C14H19F4Ir 455.49 Cc 23.7410(12) 8.6351(4) 13.9713(7) 90 92.8130(10) 90 2860.7(2) 8 orange, block 2.115 9.362

208(2)

100(2) Bruker Smart Apex 3497 [R(int) = 0.0447] 5508 [R(int) = 0.0194] 2.17 1.65 5.01 3.60

P P P P a R = ||Fo| - |Fc||/ |Fo|; R(wF2) = { [w(Fo2 - Fc2)2]/ [w(Fo2)2]}1/2.

structure was determined crystallographically. A crystallographic summary is presented in Table 1. An ORTEP plot of 11 is shown in Figure 1; mutual pairs of Cp* ligands and perfluoroalkyl ligands are trans, as might be expected on the basis of steric effects. The symmetry of the complex is also evident in solution, with the perfluoroethyl ligand showing singlet peaks at δ -71.1 ppm (CF2) and -78.8 ppm (CF3).

Exposure of a methylene chloride solution of 11 to ethylene (1 atm) resulted in a rapid color change from red to bright yellow and formation of ethylene compound 12. The 19 F NMR spectrum of 12 revealed that the symmetry of the dimer complex had been destroyed, as evidenced by two sets of peaks (AB quartet) at δ -72.3 and -84.7 (JAB = 272 Hz), corresponding to the now diastereotopic fluorine resonances of the CF2 group. The 1H NMR spectrum showed two peaks for the ethylene ligand at δ 3.09 and 3.59, consistent with rapid propeller rotation of an η2-ethylene ligand bound to a metal stereocenter.6 Unfortunately 12 was not isolable, and exposure to reduced pressure led to rapid ethylene dissociation and re-formation of the dimer complex 11. However, making the iridium a better Lewis acid by treatment of the ethylene complex 12 with AgOTf resulted in smooth conversion to the triflate analogue 13, which could be isolated as a yellow thermally unstable solid. Two-electron reduction of 13 in THF using KC8 afforded an (ethylene)(perfluoroethylidene) complex as an approximately 16:1 mixture of E-isomer 14a and Z-isomer 14b. The major isomer 14a was unambiguously characterized by a single-crystal X-ray diffraction study. There are two independent molecules in the asymmetric unit, yielding two sets of metric parameters. The crystallographic summary appears in Table 1, an ORTEP of one molecule is shown in Figure 2, and selected bond distances and angles are (6) Benn, R. Org. Magn. Reson. 1983, 21, 723–726.

Figure 1. ORTEP diagram of the non-hydrogen atoms of 11 showing the partial atom-labeling scheme. Thermal ellipsoids are shown at the 30% level. Selected bond distances (A˚) and angles (deg): Ct-Ir, 1.839(4); Ir-C11, 2.129(5); Ir-I, 2.7080(3); Ct-Ir-C11, 130.46(16).

Figure 2. ORTEP diagram of 14a showing the partial atomlabeling scheme. Thermal ellipsoids are shown at the 30% level. Hydrogen atoms in the Cp* group are omitted for clarity. Two molecules crystallized in the asymmetric unit. Selected distances and angles appear in Table 2.

provided in Table 2. The average IrdC distance to the perfluoroethylidene carbon C13 [1.853(6) A˚] is not significantly different from the corresponding distance in 5a [1.845(10) A˚] but is significantly shorter than the Ir-C single bond distance in 11 [2.129(5) A˚].4a It is also shorter than the IrdC distance of 1.904(5) A˚ in a hydrocarbon analogue, CpIr(dCPh2)(PiPr3).7 The average distance from Ir to the coordinated carbons of the ethylene ligand [2.216(6) A˚] and the average C-C bond distance of the coordinated ethylene [1.403(11) A˚] are similar to those [2.12(2) and 1.41(3) A˚, (7) Ortmann, D. A.; Webernd€ orfer, B.; Ilg, K.; Laubender, M.; Werner, H. Organometallics 2002, 21, 2369–2381.

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Table 2. Selected Bond Distances and Angles for the Two Independent Molecules of 14a Obtained Crystallographically and Corresponding Values for 14a and 14b Obtained Using Density Functional Theory, Utilizing Various Hybrid and Gradient-Corrected Functionals crystallographic distance (A˚)

compound 14a Mol 1 Mol 2 Average

Ir1-C13

Ir1-C11

Ir1-C12

C11-C12

C13-F1

C13-C14

Ir1-C13-C14

Ir1-C13-F1

C14-C13-F1

C13-Ir-C2H4a

1.866(6) 1.841(6) 1.853(6)

2.127(6) 2.134(7) 2.130(6)

2.115(8) 2.131(5) 2.123(6)

1.398(12) 1.409(10) 1.403(11)

1.388(7) 1.394(7) 1.391(7)

1.497(8) 1.519(8) 1.508(8)

133.1(5) 133.5(5) 133.3(5)

124.2(4) 125.8(4) 125.0(4)

102.7(5) 100.7(4) 101.7(4)

90.86(5) 89.86(5) 90.36(5)

DFT distance (A˚)

compound 14a B3LYP BP86 BPW91 PBE PBE0 M06 14b

DFT angle (deg)

Ir1-C13

Ir1-C11

Ir1-C12

C11-C12

C13-F1

C13-C14

Ir1-C13-C14

Ir1-C13-F1

C14-C13-F1

C13-Ir-C2H4a

1.871 1.878 1.877 1.875 1.860 1.868

2.158 2.150 2.148 2.143 2.127 2.138

2.158 2.152 2.149 2.143 2.126 2.138

1.424 1.437 1.436 1.437 1.425 1.421

1.379 1.395 1.392 1.391 1.364 1.358

1.521 1.525 1.525 1.523 1.517 1.513

132.3 132.5 132.6 132.4 132.0 130.7

124.3 124.2 124.1 124.2 124.5 125.3

103.4 103.4 103.3 103.4 103.5 104.0

90.3 90.4 90.3 90.3 89.8 90.1

DFT distance (A˚)

compound b

crystallographic angle (deg)

Ir1-C13

Ir1-C11

Ir1-C12

C11-C12

DFT angle (deg) C13-F1

C13-C14

Ir1-C13-C14

Ir1-C13-F1

C14-C13-F1

C13-Ir-C2H4a

B3LYP 1.866 2.170 2.170 1.422 1.391 1.523 137.8 119.7 102.5 97.5 BP86 1.868 2.168 2.171 1.434 1.402 1.529 137.1 120.8 102.0 97.2 BPW91 1.872 2.160 2.160 1.434 1.405 1.527 138.0 119.6 102.3 97.3 PBE 1.870 2.155 2.155 1.435 1.404 1.524 137.9 119.6 102.5 97.4 PBE0 1.853 2.144 2.145 1.422 1.372 1.521 136.8 121.1 102.2 97.0 M06 1.865 2.153 2.152 1.419 1.372 1.516 137.5 119.7 102.8 97.0 a Angle including the C2H4 centroid. b Compound numbering for 14b is the same as for 14a (Figure 2) with the positions of F1 and C14(F2F3F4) interchanged.

respectively] in an analogous ethylene complex, CpIr(η2-C2H4)(η2-C6F6),8 and longer than that of free ethylene (1.330 A˚).9 The angle subtended at Ir1 by C13 and the midpoint of the ethylene ligand [90.36(5)°] is almost identical to that between the two CO ligands in Cp*Ir(CO)2 [89.00(5)°],10 indicative of little steric repulsion between the ethylene and perfluoroethylidene ligands. Notably, the alkylidene and alkene ligands in 14a adopt a mutually cis-perpendicular arrangement, precisely orthogonal to the cis-parallel conformation (vide supra) required for correct orbital overlap to afford a metallacyclobutane such as 2.

of both the alkylidene and alkene ligands. Observation of two signals for the ethylene protons in an AA0 XX0 pattern clearly establishes a high barrier to propeller rotation of the ethylene ligand, while a high barrier to rotation about the iridium-carbon double bond is clearly manifested by observation of 1H and 19F resonances for both E- and Z-isomers, 14a and 14b. Unambiguous NMR assignments were obtained using 1H NMR, 1D NOESY, and 19F{1H} HOESY3b experiments; 1D NOESY spectra confirm that the resonance at δ 2.12 ppm represents H11A/H12A and that at δ 3.46 ppm represents H11B/H12B; in the 19F{1H} HOESY spectrum, the CF3 of the major isomer shows a NOE crosspeak with Cp*, while the CF has a NOE cross-peak with H11B/H12B, confirming that the major solution isomer is that (14a) observed in the solid state. These isomers are thermally stable, with no change in isomer ratio observed when the mixture of isomers was heated in refluxing THF. Attempts to induce coupling of the carbene and ethylene ligands, or ligand substitution of ethylene, by treatment with donor ligands CO or CH3CN, or by refluxing in toluene with PMe3, were unsuccessful; no reactions were observed. Density functional theory has become a major tool in understanding structure, bonding, and dynamics in organometallic compounds, although caution dictates that results derived from more than a single functional should be examined where possible.11 Full-molecule calculations were carried out on 14a using a number of popular hybrid (B3LYP,12 PBE0,13 M0614)

NMR spectra of 14a/b illustrate that the solid-state structure is maintained in solution, with conformational rigidity

(11) (a) Harvey, J. N. Annu. Rep. Prog. Chem., Sect. C 2006, 102, 203– 226. (b) Cundari, T. R. Compr. Organomet. Chem. III 2007, 1, 639–669. (12) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 1372–1377. (c) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (d) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623–11627. (13) (a) Perdew, J. P. In Electronic Structure of Solids ’91; Ziesche, P.; Eschrig, H., Eds.; Academie Verlag: Berlin, 1991. (b) Adamo, C.; Cossi, M.; Barone, V. THEOCHEM 1999, 493, 145–157. (14) (a) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157–167. (b) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215-241.

(8) Bell, T. W.; Helliwell, M.; Partridge, M. G.; Perutz, R. N. Organometallics 1992, 11, 1911–1918. (9) (a) Van Nes, G. J. H.; Vos, A. Acta Crystallogr., Sect. B 1979, 35, 2593–2601. (b) Van Nes, G. J. H.; Vos, A. Acta Crystallogr., Sect. B 1978, 34, 1947–1956. (10) Chen, J.; Daniels, L. M.; Angelici, R. J. Acta Crystallogr., Sect. C 1993, 49, 1061–1063.

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Figure 3. Calculated (DFT/LACV3P**þþ) zero-point energy corrected free energy relationships between isomers 14a and 14b and the transition states for ethylene rotation 14c‡, perfluoroethylidene rotation 14d‡, and rotation of both ligands into the conformation 14e‡ required for coplanarity and potential metathesis. Data obtained using different functionals are color coded: B3LYP (black), BP86 (red), BPW91 (magenta), PBE (green), PBE0 (blue), and M06 (light blue). Data in parentheses are computed for benzene-solvated molecules.

and gradient-corrected (BP86,15 BPW91,13a,15b PBE16) functionals, together with the triple-ζ LACV3P**þþ basis set,17 which uses extended core potentials on heavy atoms and a 6-311G**þþ basis for other atoms, as implemented in the Jaguar18 suite of programs. Full details are provided in the Supporting Information, but calculated metric parameters are listed in Table 2, for comparison with distances and angles obtained crystallographically. Clearly all these functionals do very well in predicting distances and angles in 14a. Selected metric parameters calculated for isomer 14b are also presented in Table 1. Isomer 14a is predicted to be more stable than 14b using DFT with all functionals used (Figure 3). The reason is likely steric in nature, with the acute angle between the ethylene and perfluoroethylidene ligands generating steric pressure between the ethylene and the CF3 group in 14b. Calculated metric parameters for 14b bear this out with the carbene ligand pivoting within the Ir-carbene plane. The Ir1-C13-C14 angle opens from ∼132° to ∼138°, the Ir1-C13-F1 angle (15) (a) Perdew, J. P. Phys. Rev. B 1986, 33, 8822–8824. (b) Becke, A. D. Phys. Rev. A: Gen. Phys. 1988, 38, 3098–3100. (16) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865–3868. (17) (a) Dunning, T. H.; Hay, P. J. In Modern Theoretical Chemistry, Vol. 4: Applications of Electronic Structure Theory; Schaefer, H. F., III, Ed.; Plenum: New York, 1977. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299– 310. (d) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284–298. (18) Jaguar, versions 7.0-7.5; Schr€odinger, LLC: New York, 2007-2009.

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contracts from ∼124° to ∼120°, and the C13-Ir-C2H4(centroid) angle enlarges from ∼90° to ∼97°, while the C14-C13-F1 angle, the Ir-C13 distance, and angles within the carbene ligand itself do not change significantly. The ratio of 14a/14b in benzene solution remains unchanged over time, even at reflux; assuming that the ratio of 16:1 corresponds to an equilibrium mixture, a ΔG° of ∼1.6 kcal/ mol is predicted. DFT was also used to compute the free energy differences between 14a and 14b both in the gas phase and for benzene-solvated molecules. The values are presented graphically in Figure 3 and illustrate reasonable correlation between the computed values for ΔG° (average 2.1 kcal/mol with a standard deviation of 1.0) and that calculated (1.6 kcal/mol with a standard deviation of 0.6) from the presumed equilibrium ratio of 16:1 observed in benzene solution; inclusion of a solvent model in the DFT calculations does result in a small change, as expected for a relatively nonpolar molecule, but does result in a closer fit (average 1.5 kcal/mol). Clearly all DFT functionals concur in predicting a small free energy difference favoring 14a over 14b in agreement with experiment. While we have no way to measure experimental data on the barriers to conformational change for the alkylidene and alkene ligands, our NMR observations clearly indicate that rotation of either ligand is slow on the NMR time scale. Results of DFT calculations on these barriers, in the gas phase and in benzene, are also presented in Figure 3. Rotation of the ethylene about the Ir-ethylene bond axis passes through a transition state 14c‡, which lies an average of 26.0 kcal/mol (standard deviation 2.9) above 14a in the gas phase and an average of 22.9 kcal/mol (standard deviation 1.5) in benzene. Likewise, rotation of the alkylidene ligand, a process that interconverts 14a and 14b, passes through transition state 14d‡, lying an average of 21.3 kcal/mol (standard deviation 2.7) in the gas phase and 21.5 kcal/mol (standard deviation 2.1) in benzene. Both these sets of calculations are consistent with rotations too slow to be observed on the NMR time scale, but readily accessible thermally on a chemical time scale. Simultaneous rotation of both the carbene and ethylene ligands through a transition state in which the iridium, the carbene carbon, and the two ethylene carbons are coplanar is energetically quite prohibitive, with 14e‡ lying an average of 43.4 kcal/mol (standard deviation 1.9) higher than 14a in the gas phase; solution values for this were not calculated. The structure of this transition state is interesting in that it contains an η3-C5Me5 ring, as discussed later, but the barrier to its formation does not bode well for accessible metathesis in this system by way of metallacyclobutane intermediates.

Nevertheless the energetics of metathesis were probed computationally. A metallacyclobutane formed directly from 14a would have structure 15 and would cleave either to re-form 14a or to generate the methylidene(fluoroalkene) complex 16. A second metallacyclobutane, 17, is accessible from 16 by coupling the IrdCH2 with the substituted carbon of the alkene. Accordingly the relative free energies of 15, 16, and 17 were calculated using the B3LYP, PBE, PBE0, and M06 functionals using the same basis set described above. Data are presented in Figure 4. All three compounds are minima but lie higher in energy than

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Figure 4. Calculated (DFT/LACV3P**þþ) zero-point energy corrected free energy relationships between 14a alkylidene(alkene) complex 16, metallacyclobutanes 15 and 17, and transition states 18‡ and 19‡. Data obtained using different functionals are color coded: B3LYP (black), PBE (green), PBE0 (blue), and M06 (light blue).

Figure 5. Calculated geometries (B3LYP/LACV3P**þþ) of transition states 14e‡, 18‡, and 19‡ for metallacycle formation. Upper views illustrate the planarity/nonplanarity of the Ir and the three carbon atoms; bottom views show distances (in A˚) in red and angles (in degrees) in green. Cp* rings are omitted for clarity. In 14e‡ the Cp* ring is η3, while in 18‡ and 19‡ the Cp* rings are η5.

14a by average (gas phase) values of 6.6, 7.7, and 1.5 kcal/mol for 15, 16, and 17, respectively; the energies of 14a and 17 are surprisingly close. Transition state 18‡, connecting 14a to 15, and transition state 19‡, connecting 16 to 17, were both located and confirmed to be saddle points by the presence of a single imaginary frequency, which animated to the reaction coordinate for metallacyclobutane formation; their free energies are also included in Figure 4. The conclusion is that while the energies of metallacyclobutanes and alkylidene(alkene) complexes may be quite close in this particular system, the high barriers connecting them clearly preclude facile interconversion. Our synthesized complex 14a is also lowest in energy of all the compounds considered, and any stoichiometric metathesis to give 16 is thermodynamically rather unfavorable.

However these results do suggest that fluorinated 16-electron metallacyclobutanes like 15 and 17 should be isolable and should ring-open under reasonably accessible thermal conditions to give carbene(alkene) compounds. Studies aimed at exploring this option are underway. Finally, there are two possible transition states for metathesis evolving from 14a, as shown in Figures 3 and 4. A comparison of the transition-state geometries (B3LYP) of 14e‡ and 18‡ is presented in Figure 5 along with that of 19‡ with the Cp* rings removed for clarity. Transition state 14e‡ differs from 18‡ in two significant ways, even though both have a single imaginary frequency which animates along the reaction coordinate expected for metallacycle formation; the three carbon atoms in state 14e‡ are coplanar, while those in

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Organometallics, Vol. 29, No. 8, 2010

18‡ are not, and the Cp* ring in 14e‡ is η3, while that in 18‡ is η5. Transition state 19‡ has geometrical features analogous to 18‡. Consequently, even though the metathesis barriers are high in this particular system, there is an approximately 10 kcal/mol advantage to carrying out metathesis via a nonplanar transition state (18‡) than a planar one (14e‡).

Experimental Section All reactions were performed in oven-dried glassware, using standard Schlenk techniques, under an atmosphere of nitrogen, which had been deoxygenated over BASF catalyst and dried over molecular sieves or in an MBraun drybox. Solvents were deoxygenated and dried over activated alumina under nitrogen.19 NMR spectra were recorded on a Varian Unity Plus 300 or 500 FT spectrometer at 21 °C. 1H NMR spectra were reference to the protio impurity in the solvent: C6D6 (7.16 ppm) and CD2Cl2 (5.32 ppm). 19F NMR spectra were referenced to external CFCl3 (0.00 ppm). Coupling constants are reported in hertz (Hz) and are absolute values unless otherwise indicated. X-ray crystallographic analyses were performed at the University of California, San Diego. Elemental analyses were performed by Schwartzkopf (Woodside, NY). Cp*Ir(CO)(C2F5)(I) was prepared as previously reported.3a Potassium graphite was prepared using literature procedures.20 Ethylene (Matheson), CO (GTS), PMe3 (Strem), and AgOTf (Strem) were used as received. N-Methylmorpholine-N-oxide (Lancaster) was dried by azeotropic distillation in toluene. [Cp*Ir(CF2CF3)I]2 (11). To a solution of N-methylmorpholine-N-oxide (NMO, 213 mg, 1.826 mol) in CH2Cl2 (20 mL) was added Cp*Ir(CO)(CF2CF3)I (1.00 g, 1.66 mmol) as a solid. On completion (2-3 h), the yellow solution turned red with a red precipitate. The solvent was removed by vacuum to give a red solid (942 mg, 99%). Recrystallization from CH2Cl2/hexane gave X-ray diffraction quality and analytically pure crystals. Anal. Calcd for C24H20F10I2Ir2 (1146.68): C, 25.14; H, 2.64. Found: C, 25.32; H, 2.65. 1H NMR (500 MHz, C6D6, 21 °C): δ 1.40 (s, 15H, C5Me5). 19F NMR (470 MHz, C6D6, 21 °C): δ -71.1 (s, 2F, CF2), -78.8 (s, 3F, CF3). Cp*Ir(CH2dCH2)(C2F5)(I) (12). [Cp*Ir(C2F5)I]2 (4 mg, 0.0035 mmol) was dissolved in CD2Cl2 (1.0 mL). CH2dCH2 (1 atm) was bubbled into the solution at room temperature for 5 min. After the reddish-orange solution turned yellow, a sample was transferred to an NMR tube. 1H NMR (500 MHz, CD2Cl2, 21 °C): δ 1.83 (s, 15H, C5Me5), 3.09 (br, 2H, CH2dCH2), 3.59 (br, 2H, CH2dCH2). 19F NMR (470 MHz, CD2Cl2, 21 °C): δ -72.3 (br d, 2JFF(AB) = 272 Hz, 1F, CRFA), -77.2 (s, 3F, CF3), -84.7 (br d, 2JFF(AB) = 272 Hz, 1F, CRFB). Evaporation of solvent gave the starting material [Cp*Ir(C2F5)I]2. Cp*Ir(CH2dCH2)(C2F5)(OTf) (13). [Cp*Ir(C2F5)I]2 (30 mg, 0.026 mmol) was dissolved in CH2Cl2 (10 mL). CH2dCH2 (1 atm) was bubbled into the solution at room temperature for 5 min. After the reddish-orange solution turned yellow, AgOTf (15.4 mg, 0.062 mmol) was added. After 1 h, a yellow precipitate was formed and the mixture was filtered. The solvent was removed to give 13 as a yellow solid (31 mg, 96%). The product decomposed to an unknown brown solid after a few hours in the glovebox. 1H NMR (500 MHz, C6D6, 21 °C): δ 1.06 (s, 15H, C5Me5), 3.65 (br, 2H, CH2dCH2), 4.06 (br, 2H, CH2dCH2). 19 F NMR (470 MHz, C6D6, 21 °C): δ -79.8 (s, 3F, OTf), -80.4 (s, 3F, CF3), -86.7 (d, 2JFF(AB) = 274 Hz, 1F, CRFA), -91.6 (d, 2 JFF(AB) = 274 Hz, 1F, CRFB). Cp*Ir(CH2dCH2)(CFCF3) (14). KC8 (67 mg, 0.50 mmol) was suspended in THF (3 mL). The Cp*Ir(C2F5)(CH2dCH2)(OTf) (19) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518–1520. (20) (a) Csuk, R. Nachr. Chem., Tech. Laboratorium 1987, 35, 828– 833. (b) Csuk, R.; Glaenzer, B. I.; Furstner, A. Adv. Organomet. Chem. 1988, 28, 85–137.

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(31 mg, 0.050 mmol) was added as a solution in THF (3 mL). After 2 h, the solvent was removed by vacuum and the residue was extracted with hexane. After filtration the hexane was evaporated by vacuum to give a yellow solid product, Cp*Ir(CH2dCH2)(CFCF3) (18.5 mg, 81%, 14a:14b = 16:1). The ratio did not change upon refluxing in THF for 3 h. Crystals were obtained from hexane. Anal. Calcd for C14H19F4Ir (455.49): C, 36.91; H, 4.20. Found: C, 37.48; H, 4.21. 14a: 1H NMR (500 MHz, C6D6, 21 °C) δ1.56 (s, 15H, C5Me5), 2.12 [m(AA0 XX0 ), 2H, CH2dCH2], 3.46 [m(AA0 XX0 ), 2H, CH2dCH2]. 19F NMR (470 MHz, C6D6, 21 °C): δ -72.9 (d, 3JFF = 11 Hz, 3F, CF3), -8.67 (q, 3JFF = 11 Hz, 1F, CF). 13C{1H} NMR (125 MHz, C6D6, 21 °C): δ 9.0 (q, 1 Hz, 5C, C5Me5), 33.9 (d, 4JCF = 8 Hz, 2C, CH2dCH2), 97.5 (s, 5C, C5Me5), 127.8 (qd, 1JCF = 278 Hz, 2JCF = 50 Hz, 1C, CF3), 197.1 (dq, 1JCF = 349 Hz, 2JCF = 40 Hz, 1C, CF). 14b: 1H NMR (500 MHz, C6D6, 21 °C) δ 1.57 (s, C5Me5), 2.09 [m(AA0 XX0 ), 2H, CH2dCH2], 3.62 [m(AA0 XX0 ), 2H, CH2dCH2]. 19F NMR (470 MHz, C6D6, 21 °C): δ 17.6 (q, 3JFF = 9 Hz, 1F, CF), -71.5 (d, 3JFF = 9 Hz, 3F, CF3). Attempted Reaction of Cp*Ir(CH2dCH2)(CFCF3) with CO. Cp*Ir(CH2dCH2)(CFCF3) (30 mg, 0.066 mmol) was dissolved in CH2Cl2 (6 mL). CO (gas, 1 atm) was added at room temperature. After 4 h, no reaction was observed. Attempted Reaction of Cp*Ir(CH2dCH2)(CFCF3) with PMe3. Cp*Ir(CH2dCH2)(CFCF3) (20 mg, 0.043 mmol) was dissolved in toluene (6 mL). PMe3 (5 μL, 0.05 mmol) was added at -78 °C. No reaction was observed after 30 min stirring at -78 °C. The mixture was warmed to room temperature, and no reaction was observed after 1 h. The mixture was refluxed for 3 h, and no reaction was observed by 19F NMR. Attempted Reaction of Cp*Ir(CH2dCH2)(CFCF3) with MeCN. Cp*Ir(CH2dCH2)(CFCF3) (30 mg, 0.066 mmol) was dissolved in CH2Cl2 (5 mL). MeCN (0.5 mL) was added at room temperature. No reaction was observed after 2 h. The solution was refluxed for 1 h. No reaction was observed. DFT Calculations. Full-molecule calculations were carried out using various hybrid (B3LYP,12 PBE0,13 M0614) and gradient-corrected (BP86,15 BPW91,13a,15b PBE16) functionals, together with the triple-ζ LACV3P**þþ basis set,17 which uses extended core potentials on heavy atoms and a 6-311G**þþ basis for other atoms, as implemented in the Jaguar18 suite of programs. Computations involving solvent used the Poisson-Boltzmann solver21 implemented as part of the Jaguar suite. Transition states were located using the linear synchronous transit (LST) method, refined using the quadratic synchronous transit (QST) method, and confirmed as connecting the two appropriate minima by intrinsic reaction coordinate (IRC) determinations, all as implemented in the Jaguar suite. All computed structures were confirmed as energy minima or as transition states by calculating the vibrational frequencies using second-derivative analytic methods and confirming the absence of imaginary frequencies for minima and the presence of a single imaginary frequency for transition states. Thermodynamic quantities were calculated assuming an ideal gas and are zero point energy corrected.

Acknowledgment. R.P.H is grateful to the U.S. National Science Foundation for generous financial support. Supporting Information Available: Experimental details for all compounds, 1D NOESY and 19F{1H} HOESY spectra for 14, details of all DFT calculations, and crystallographic information files (CIF) for compounds 11 and 14a. This material is available free of charge via the Internet at http://pubs.acs.org. (21) (a) Marten, B.; Kim, K.; Cortis, C.; Friesner, R. A.; Murphy, R. B.; Ringnalda, M. N.; Sitkoff, D.; Honig, B. J. Phys. Chem. 1996, 100, 11775–11788. (b) Tannor, D. J.; Marten, B.; Murphy, R.; Friesner, R. A.; Sitkoff, D.; Nicholls, A.; Honig, B.; Ringnalda, M.; Goddard, W. A., III. J. Am. Chem. Soc. 1994, 116, 11875–11882.