Organometallics 2009, 28, 1575–1578
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Notes Serendipitous Discovery of a Simple Compound with an Unsupported Ir-Ir Bond Hui Huang,† Arnold L. Rheingold,‡ and Russell P. Hughes*,† Departments of Chemistry, 6128 Burke Laboratory, Dartmouth College, HanoVer, New Hampshire 03755, and UniVersity of California, San Diego, California 92093-0358 ReceiVed NoVember 21, 2008 Summary: The long-known compound Ir(acac)(CO)2 (acac ) 2,4-pentanedionato) undergoes a formal oxidation reaction when treated with AgOTf (Tf ) CF3SO2) to giVe the dinuclear complex [Ir(acac)(OTf)(CO)2]2, containing an unsupported Ir-Ir bond. The compound has been characterized by a singlecrystal X-ray diffraction study, and DFT and NBO calculations haVe been employed to probe the nature of the Ir-Ir bond. Reduction of the complex regenerates Ir(acac)(CO)2. There are very few examples of formal Ir(II)-Ir(II) compounds containing unsupported Ir-Ir bonds. The first example, Ir2(Tcbiim)2(CO)4(CH3CN)2 (H2Tcbiim ) tetracyanobiimidazole), was obtained during electrolysis of [Ir(CO)2Tcbiim]+; reaction with P(OEt)3 afforded the crystallographically characterized analogue 1.1 Oxidation of IrCp*(CO)2 with the oneelectron oxidant [Ph3C][BF4] in nitromethane afforded the ionic dimer 2, while oxidation using [NO][BF4] in CH2Cl2 yielded 3.2 A similar neutral dimer, 4, was prepared by consecutive treatment of IrCp*(CO)(H)2 with acid and base.3,4 Very recently, compound 5 was prepared and crystallographically characterized in an unusual reaction in which CH2Cl2 was the oxidant.5 Oxidation of Ir(I) precursors has also been used to prepare compounds with chains of unsupported Ir-Ir bonds in the socalled “iridium blues”.6,7 The 16-electron iridium complex 6 is a commercially available catalyst for anti-Markovnikov arylation of olefins and olefin isomerization8 and has been broadly employed to prepare other organometallic complexes through ligand substitution9,10 * To whom correspondence should be addressed. E-mail: rph@ dartmouth.edu. † Dartmouth College. ‡ University of California, San Diego. (1) Rasmussen, P. G.; Anderson, J. E.; Bailey, O. H.; Tamres, M.; Bayo´n, J. C. J. Am. Chem. Soc. 1985, 107, 279–281. (2) Einstein, F. W. B.; Jones, R. H.; Zhang, X.; Yan, X.; Nagelkerke, R.; Sutton, D. J. Chem. Soc., Chem. Commun. 1989, 1424–1426. (3) Heinekey, D. M.; Fine, D. A.; Harper, T. G. P.; Michel, S. T. Can. J. Chem. 1995, 73, 1116–1125. (4) Heinekey, D. M.; Fine, D. A.; Barnhart, D. Organometallics 1997, 16, 2530–2538. (5) Patra, S. K.; Rahaman, S. M. W.; Majumdar, M.; Sinha, A.; Bera, J. K. Chem. Commun. 2008, 2511–2513. (6) Tejel, C.; Ciriano, M. A.; Oro, L. A. Chem. Eur. J. 1999, 5, 1131– 1135. (7) Tejel, C.; Ciriano, M. A.; Villarroya, B. E.; Gelpi, R.; Lopez, J. A.; Lahoz, F. J.; Oro, L. A. Angew. Chem., Int. Ed. 2001, 40, 4084–4086. (8) Matsumoto, T.; Yoshida, H. Catal. Lett. 2001, 72, 107–109. (9) Buron, C.; Stelzig, L.; Guerret, O.; Gornitzka, H.; Romanenko, V.; Bertrand, G. J. Organomet. Chem. 2002, 664, 70–76.
or cluster formation.11,12 Recently we discovered that oxidative addition of CF3I to 6 afforded the octahedral Ir(III) compound 7, which could be converted to the triflate analogue 8 by treatment with AgO3SCF3; a variety of other perfluoroalkyl analogues of 7 and 8 could also be prepared.13 Failure to adequately purify 7 led serendipitously to reaction of AgO3SCF3 with samples of 7 that were contaminated with unreacted 1, affording an additional new compound, which was isolated and crystallographically characterized as 9. Retrospective analysis of this observation led to the reaction of pure 1 with 1 equiv of AgO3SCF3 to give a black precipitate of Ag metal and exclusive formation of 9. Treatment of 9 with 2 equiv of a reducing agent, such as KC8, afforded clean regeneration of 6. (10) Miranda-Soto, V.; Pe´rez-Torrente, J. J.; Oro, L. A.; Lahoz, F. J.; Martı´n, M. L.; Parra-Hake, M.; Grotjahn, D. B. Organometallics 2006, 25, 4374–4390. (11) Camerano, J. A.; Casado, M. A.; Ciriano, M. A.; Lahoz, F. J.; Oro, L. A. Organometallics 2005, 24, 5147–5156.
10.1021/om801110x CCC: $40.75 2009 American Chemical Society Publication on Web 02/02/2009
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An ORTEP representation of 9 with selected bond lengths and angles is provided in Figure 1, and a summary of the crystallographic methods is presented in Table 1. Compound 9 is another rare example of an unbridged Ir-Ir bond, with each Ir formally in the Ir(II) oxidation state. The Ir-Ir bond length of 9 (2.6622(3) Å) is significantly shorter than those in 1 (2.826(2) Å),1 2 (2.8393(12) Å), 3 (2.8266(6) Å),2 4 (2.724(1) Å),3,4 and 5 (2.7121(8) Å).5 The Ir-C(O) bond lengths in 9 (1.894(5), 1.893(5), 1.907(5), 1.891(5) Å) are slightly longer than those in the Ir(I) precursor 6 (1.856(10), 1.883(14) Å)13 and slightly shorter than those in the formal Ir(III) complex 8 (1.916(5), 1.917(5) Å);13 these features are consistent with iridium in 9 being in a higher oxidation state, with less back-bonding from IrfCO(π*). The Ir-O(acac) bond lengths in 9 (2.031(3), 2.031(3), 2.025(3), 2.025(3) Å) are slightly shorter than those in 1 (2.053(6), 2.078(8) Å) and slightly longer than those in 8 (2.013(3), 2.014(3) Å). The Ir-O(triflate) distances in 1 (2.205(3), 2.186(3) Å) are significantly different from each other, with the lower value identical with that in 8 (2.184(3) Å).13 Each iridium has an approximately planar coordination geometry of its two CO ligands and the O atoms of the acac, but each acac ligand is significantly distorted from planarity with its associated iridium atom, distorting toward the apical triflate ligand; the angle between least-squares planes C8-C9-Ir2-O7-O8 and O7C10-C11-C12-O8 is 14.3° and that between C1-C2-Ir1O3-O4 and O3-C3-C4-C5-O4 is 11.7°. Moreover, the angles between the triflate O atoms and the Ir-Ir bond differ considerably from 180°, with O(12)-Ir(2)-Ir(1) being 168.99(9)° and O(9)-Ir(1)-Ir(2) being 170.65(9)°, in each case the atoms being displaced toward the acac ligand. This distorted bond angle in 9 is quite similar to that found in compound 8 (C-Ir-O ) 171.95(16)°), but the folding of the acac ring in 9 is significantly greater than the corresponding distortion in 8 (2.4°) or in the square-planar starting material 6 (1.2°).13 In order to obtain some insight into the electronic structure of compound 9 and possibly address the reasons for the distortions described above, density functional theory was employed. The gas-phase structure of 9 was optimized without any symmetry constraints using the B3LYP hybrid functional and the LACV3P**++ basis set, as implemented in the Jaguar14 suite of programs; the optimized structure was established as a minimum by carrying out a vibrational calculation and establishing the absence of imaginary frequencies. In addition, an NBO analysis15 was performed on the optimized structure. The DFT structure of 9 reproduced reasonably well the crystallographic one in terms of the distance and angular metrics, (12) Li, F.; Gates, B. C. J. Phys. Chem. B 2004, 108, 11259–11264. (13) Huang, H.; Hurubeanu, N. R.; Bourgeois, C. J.; Cheah, S.-M.; Yuan, J.; Rheingold, A. L.; Hughes, R. P. Can. J. Chem. 2009, 87, 151–160. (14) Jaguar, version 7.0; Schro¨dinger LLCm New York, 2007. (15) 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.
Notes
Figure 1. ORTEP diagram and partial atom labeling scheme for 9 with ellipsoids drawn at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ir1-Ir2, 2.6622(3); Ir1-C1, 1.894(5); Ir1-C2, 1.893(5); Ir1-O3, 2.031(3); Ir1-O4, 2.031(3); Ir1-O9, 2.205(3); Ir2-C8, 1.907(5); Ir2-C9, 1.891(5); Ir2-O7, 2.025(3); Ir2-O8, 2.025(3); Ir2-O12, 2.186(3); C1-Ir1-C2, 92.7(2); O3-Ir1-O4, 91.39(13); O9-Ir1-Ir2, 170.65(9);C8-Ir2-C9,91.3(2);O7-Ir2-O8,91.07(13);O12-Ir2-Ir1, 168.99(9). Table 1. Crystallographic Summary for 9 formula fw space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z cryst color, habit D(calcd), g/cm3 µ(Mo KR), mm-1 (λ ) 0.710 73 Å) temp, K diffractometer no. of indep rflns R(F) (I > 2σ(I)), %a R(wF2) (I > 2σ(I)), %a a
C16H14F6Ir2O14S2 992.79 P1j 8.5155(9) 11.6573(12) 14.0377(15) 79.2200(10) 89.9160(10) 71.3540(10) 1294.5(2) 2 orange, block 2.547 10.540 296(2) Bruker Smart Apex 4782 (R(int) ) 0.0303) 2.41 6.23
R ) ∑||Fo|| - ||Fc||/∑|Fo|; R(wF2) ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2.
including the folding distortion of the acac rings; values of 18.2 and 17.4° were computed for the fold angles, as described above. Full details are available in the Supporting Information. The highest occupied and lowest unoccupied Kohn-Sham orbitals of 9 are depicted in Figure 2 and are clearly the σ and σ* orbitals of the TfO-Ir-Ir-OTf interaction. The distortion from linearity in the TfO-Ir-Ir-OTf angles presumably helps reduce antibonding between the TfO p-orbital components and
Notes
Organometallics, Vol. 28, No. 5, 2009 1577
Figure 2. The highest occupied and lowest unoccupied Kohn-Sham orbitals of 9 (B3LYP/LACV3P**++). Table 2. CO Stretching Frequencies, NPA Charges on Ir and CO Ligands, Ir-CO Back-Bonding Interaction Energies (∆Ebb, and NBO Hybridizations 6 -1
νCO (cm ; in CH2Cl2) ∆Ebb(IrfC-O (π*)) (kcal/mol) NPA charge on Ir NPA charge on CO (C) NPA charge on CO (O) Ir σ-hybrid to CO CO σ-hybrid to Ir Ir σ-hybrid to CF3 Ir σ-hybrid to Ir
2072, 1994 96.5 +0.26 +0.57 -0.44 sd1.22 sp0.62
8
9
2168, 2125 2128, 2095 61.5 63.3 +0.67 +0.62 +0.65 +0.64 -0.36 -0.38 sd1.80 sd1.81 sp0.63 sp0.62 sd2.50 sd2.57
the in-phase combination of the dz2 orbitals on each Ir, while the folding of the acac rings helps diminish the antibonding between the acac π-component and the same Ir dz2 combination. Not unexpectedly, the lowest unoccupied orbital is Ir-Ir σ*antibonding, explaining the loss of the metal-metal bond on reduction. Despite the presence of four CO ligands in 9, only two IR stretching frequencies are observed, as shown in Table 2, indicating that the two Ir centers are not coupled. These frequencies are slightly lower than those in 8 but significantly higher than those in the Ir(I) starting material 6, consistent with a formal oxidation at the metal centers, with correspondingly less back-bonding from IrfC-O(π*). Natural bond orbital (NBO)16,17 and natural population analysis (NPA)18 methods are powerful tools for probing electronic structure in main-group and transition-metal compounds,19 and the use of second-order perturbative NBO analysis for estimating metal-ligand backbonding interaction energies (∆Ebb), obtained from the offdiagonal Fock matrix element expressed in the NBO basis, is now well established.20-22 Values of ∆Ebb reflect stabilization (16) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211– 7218. (17) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899–926. (18) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735–746. (19) Weinhold, F.; Landis, C. R., Valency and Bonding: A Natural Bond Orbital Donor-Acceptor PerspectiVe; Cambridge University Press: Cambridge, U.K., 2005. (20) Leyssens, T.; Peeters, D.; Orpen, A. G.; Harvey, J. N. Organometallics 2007, 26, 2637–2645. (21) Leyssens, T.; Peeters, D. J. Org. Chem. 2008, 73, 2725–2730.
Figure 3. Reference Lewis structures used in the NBO analyses of 6, 8, and 9.
energies resulting from delocalization of the formal “lone pairs” on Ir (nonbonding 5d orbitals in the NBO analysis) into the π* antibonding orbitals of the CO ligands. The NBO approach to transition-metal complexes19 requires that a square-planar d8 complex such as 6 be regarded as a bent two-coordinate ML2 complex, with approximate sd hybridization for the IrL σ-bonds, four lone pairs in d orbitals on Ir, and additional donor-acceptor interactions between the IrL2 fragment and the other two ligands to give three-center-four-electron bonds. Similarly, the NBO treatment of an octahedral d6 compound requires a facial threecoordinate IrL3 fragment, with approximate sd2 hybridization for the Ir-L σ-bonds, three lone pairs in Ir d orbitals, and additional donor-acceptor interactions between IrL3 and the three additional ligands to give three-center-four-electron bonds. Clearly, in order to make any comparisons between compounds 6, 8, and 9, appropriate structures must be chosen as the reference ML2 (for 6) and ML3 (for 8 and 9). For compound 6 the $CHOOSE keyword in the NBO program was used to specify a reference Lewis structure with single bonds from Ir to each CO, resulting in Ir(CO)2 and acac fragments, for 8 single bonds were specified from Ir to each CO and to CF3, resulting in the Ir(CO)2(CF3), OTf, and acac fragments, and for 9 the Ir2(CO)4 fragment was specified, with OTf and acac; the Ir reference structures are shown in Figure 3. Values of ∆Ebb for compounds 6, 8, and 9 are presented in Table 2, along with the natural charges from the natural population analysis on iridium and the CO ligand atoms and atom hybridizations arising from the NBO treatment.15 It is noteworthy that all these values indicate that the Ir atoms in 9 have natural charge and back-bonding properties almost identical with those in 8 and quite different from those in 6. (22) Leyssens, T.; Peeters, D.; Orpen, A. G.; Harvey, J. N. New. J. Chem. 2005, 29, 1424–1430.
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Notes
Experimental Section
symmetry higher than triclinic was observed. The centrosymmetric alternative was assumed from initial E statistics and later confirmed by the results of refinement. The structure was solved using direct methods, completed by subsequent difference Fourier syntheses, and refined by full-matrix least-squares procedures. DIFABS absorption corrections were applied. All non-hydrogen atoms were refined with anisotropic displacement coefficients, and hydrogen atoms were treated as idealized contributions. All software and sources of the scattering factors are contained in the SHELXTL (5.10) program library. Computational Studies. Structures were optimized using the hybrid B3LYP functional26,27 and the triple-ζ LACV3P**++ basis set,28-31 which uses extended core potentials on heavy atoms and a 6-311G**++ basis for other atoms, as implemented by Jaguar, version 7.0.14 Optimized geometries are provided in the Supporting Information. Natural population analyses (NPA)16-19 were carried out at the B3LYP level of theory using the NBO program,15 also implemented as part of the Jaguar suite. As part of the NBO program, second-order perturbation analysis was used to estimate metal-ligand back-bonding energies (∆Ebb), with the overall value obtained by summing all second-order perturbation E(2) terms between the relevant metal d-orbital donor lone pair and the appropriate π* C-O acceptor orbitals.
General Considerations. Reactions were performed in ovendried glassware using standard Schlenk techniques under an atmosphere of nitrogen, which was deoxygenated over BASF catalyst and dried over Aquasorb, or in a Braun drybox. Methylene chloride, hexanes, diethyl ether, tetrahydrofuran, and toluene were dried over an alumina column under nitrogen.23 NMR spectra were recorded on a Varian Unity Plus 300 spectrometer. IR spectra were recorded on a Perkin-Elmer FTIR 1600 series spectrophotometer. Elemental analyses were performed by Schwartzkopf (Woodside, NY). AgOTf (Synquest) was commercially available. Ir(acac)(CO)2 and C8K were prepared by literature procedures.24,25 [Ir(CO)2(acac)(OTf)]2 (9). A yellow CH2Cl2 (10 mL) solution of Ir(acac)(CO)2 (174 mg, 0.500 mmol) was added to a CH2Cl2 (10 mL) suspension of AgOTf (129 mg, 0.500 mmol). The color of the suspension turned to black quickly, and that of the solution turned to orange. After 30 min, 19F NMR and IR spectra showed the reaction had finished. The solution was filtered and the solvent was removed to give a yellow solid (150 mg, 60%), which was crystallized from CH2Cl2/hexane to give yellow crystals suitable for X-ray diffraction. Anal. Calcd for C16H14F6Ir2O14S2: C, 19.36; H, 1.42. Found: C, 19.54; H, 1.60. 1H NMR (CDCl3, 300 MHz, 21 °C): δ 5.93 (H, s, CH), 2.28 (6H, s, 2CH3). 19F NMR (CDCl3, 282 MHz, 21 °C): δ -77.1 (3F, s, OTf). IR (CH2Cl2, cm-1): 2128 (s), 2095 (vs). IR (THF, cm-1): 2107 (s), 2072 (s). Reduction of 9. To a yellow CH2Cl2 solution (10 mL) of [Ir(acac)(CO)2(OTf)]2 (120 mg, 0.121 mmol) was added KC8 (81 mg, 0.60 mmol) at room temperature. The solution turned colorless quickly, and a large amount of yellow solid precipitated. The solution was filtered, and the yellow residue was extracted with CH2Cl2 (20 mL). The filtrate and the extract were combined, and the solvent was removed to give a golden yellow solid (70 mg, 83%). NMR and IR spectra showed it to be Ir(acac)(CO)2.24 Crystallographic Structural Determination. Crystal, data collection, and refinement parameters are collected in Table 1. No (23) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518–1520. (24) Bonati, F.; Ugo, R. J. Organomet. Chem. 1968, 11, 341–352. (25) Csuk, R.; Fuerstner, A.; Weidmann, H. J. Chem. Soc., Chem. Commun. 1986, 775.
Acknowledgment. R.P.H. is grateful to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for generous financial support. Supporting Information Available: A CIF file giving full crystallographic data for 9 and text and tables giving the optimized DFT structure of 9. This material is available free of charge via the Internet at http://pubs.acs.org. OM801110X (26) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (27) Becke, A. D. J. Chem. Phys. 1993, 98, 1372–1377. (28) Dunning, T. H.; Hay, P. J., In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1977; Vol. 4, Applications of Electronic Structure Theory. (29) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283. (30) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (31) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284–298.
