In the Laboratory
1,5 Cyclooctadiene Complexes of Iridium: Synthesis, Characterization, and Reaction with Dihydrogen
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An Experiment for an Integrated Physical/Inorganic Laboratory Course John W. Logan and Mark Wicholas* Department of Chemistry, Western Washington University, Bellingham, WA 98225; *
[email protected] For the past eight years we have taught a two-quarter laboratory sequence with experiments combining inorganic synthesis and physical chemistry measurement. Last year we introduced a new inquiry-based experiment on the chemistry of iridium, specifically, the preparation of [Ir(COD)(Ph3P)2]BF4 from [Ir(COD)Cl]2 (COD is 1,5-cyclooctadiene), the reaction of [Ir(COD)(Ph3P)2]BF4 with dihydrogen, the discovery of the bonding mode of dihydrogen (η2-H2 or dihydride), and the identity of all products formed. The iridium complexes are air-stable in the solid state and relatively easy to work with. The experiment, however, requires a Schlenk line and an attachable valve NMR tube for the reaction of [Ir(COD)(Ph3P)2]BF4 with H2. One of the most important and exciting discoveries in inorganic chemistry in the past 20 years was the synthesis by Kubas and coworkers (1) in 1984 of a tungsten complex containing dihydrogen coordinated in the η2-H2 mode. Since then many hundreds of η 2-H2 complexes have been reported and there now exist spectroscopic techniques that determine how dihydrogen reacts with and binds to a metal (2, 3). The crux of this experiment is the in situ reaction of [Ir(COD)(Ph3P)2]BF4 with H2, done in acetone-d6 solution in a valve NMR tube. When students obtain a 1H NMR spectrum they discover one signal far upfield and realize that it is due to hydrogen directly coordinated to Ir. They subsequently measure T1, the spin–lattice relaxation time, to decide whether H 2 is coordinated to iridium as η 2-H 2 or as a dihydride, and then attempt to identify all products formed in the reaction. Experimental Procedure Students work in pairs and are given 0.10 g of [Ir(COD)Cl]2 as starting material. The first step, carried out in one lab period, is the preparation and isolation of [Ir(COD)(Ph3P)2]BF4. During a second lab period, scheduled individually with each pair of students, the reaction of [Ir(COD)(Ph3P)2]BF4 with H2 is carried out in a RotoTite valve NMR tube (Wilmad Glass) on a double-manifold Schlenk line 30 min prior to the T 1 measurement. Instructions for attaching the NMR tube to the Schlenk line are given in the supplemental documentation.W Each pair of students signs up for three hours of NMR time, and this is more than adequate for the T1 measurement. 1H NMR spectra of other compounds ([Ir(COD)Cl] 2 and [Ir(COD)(Ph3P)2]BF4, for example) are obtained during a third period or by arrangement. NMR spectra were run on a Bruker AC 300 spectrometer.
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Synthesis of [Ir(COD)(Ph3P)2]BF4 To a 10-mL Erlenmeyer flask previously purged with N2, 0.100 g (0.149 mmol) of [Ir(COD)Cl]2, 0.156 g (0.595 mmol) of triphenylphosphine, a magnetic stir bar, and 5 mL of previously boiled, deoxygenated absolute ethanol were added. The flask was stoppered immediately, stirred for a few minutes, and then sonicated for 15 min. The solution turned red and most of the [Ir(COD)Cl]2 dissolved. The solution was quickly filtered and then added to a 10-mL Erlenmeyer flask containing a solution of tetra-n-butylammonium tetrafluoroborate (0.129 g, 0.392 mmol) dissolved in 1.0 mL of ethanol. The volume was reduced by gentle heating on a hot-plate while a stream of N2 was blown over the surface of the solution. When the volume reached approximately 2 mL, the flask was stoppered and cooled in an ice bath for 10 min. The cherry-red, air-stable, crystalline product was filtered, washed with 1 mL of chilled ethanol followed by 1 mL of chilled diethyl ether, and dried in a vacuum desiccator. The typical student yield was 50–60%. Reaction of [Ir(COD)(Ph3P)2]BF4 with H2 A solution of approximately 8 mg of [Ir(COD)(Ph3P)2]BF4 dissolved in 0.5 mL of acetone-d6 was transferred to a 7-in. RotoTite NMR tube and attached to a Schlenk line, which had been flushed with H2 at a reasonable rate for approximately one hour. The acetone solution was then degassed via three freeze–pump–thaw cycles. After the last cycle, H2 was introduced into the head space above the solution and the valve was closed. When the tube was removed from the Schlenk line and shaken, the red solution immediately turned colorless, an indication that reaction with H2 had occurred. 1H
NMR Spectrum and T1 Measurement A routine NMR spectrum of the product, prepared in situ by the procedure described above, was obtained using a 40-ppm sweep width from ᎑30 to +10 ppm. The most salient feature is the triplet (2JP–H = 15.9 Hz) at δ = ᎑27.7 ppm relative to TMS, but other signals in the normal 0–10 ppm region are worthy of comment and are discussed later. Standard Bruker software was used for the inversion–recovery T1 measurement. Sixteen FIDs were collected for each of 10 delay times, τ, using the pulse sequence (π–τ–π/2–Acquire). The relaxation process is expressed by the differential equation dMz/dt = (M∞ – Mz)/T1, where Mz is the z component of the macroscopic magnetization at some time t after the pulse and M∞ is the magnetization at equilibrium. The T1 value for the ᎑27.7 ppm resonance was determined by a nonlinear fit of signal intensity (proportional to Mz ) versus τ using Bruker software.
Journal of Chemical Education • Vol. 78 No. 9 September 2001 • JChemEd.chem.wisc.edu
In the Laboratory
Hazards Because of the highly flammable nature of hydrogen gas, the reaction with H2 should be carried out in a hood. Results The procedure for preparation of [Ir(COD)(Ph3P)2]BF4 is a modification of the method reported by Haines and Singleton (4) for the complexes {Ir(COD)[P(OR)(Ph3P)2]}PF6, R = CH3 and C2H5. The reaction of [Ir(COD)(Ph3P)2]BF4 with H2 in the NMR tube with acetone-d6 produces as its principal product cis,cis,trans-[IrH2(acetone)2(Ph3P)2]BF4, a dihydride complex first reported by Crabtree and coworkers (5). The reaction is as follows: Shift (ppm) Figure 1. 1H NMR spectrum of [Ir(COD)(Ph3P)2]BF4 + H2 in acetone-d6.
[Ir(C8H12)(Ph3P)2]BF4 + 2(CH3)2CO + 2H2 → [IrH2{(CH3)2CO}2(Ph3P)2]BF4 + C8H14 The 1H NMR spectrum of the products from the NMR tube reaction of [Ir(COD)(Ph3P)2]BF4 with H2 for a typical student preparation is shown in Figure 1. Inspection of the NMR spectrum shows a multiplet at 7.5 ppm attributed to Ph3P, three multiplets at approximately 5.6, 2.1, and 1.5 ppm assigned to cyclooctene, H2O and HOD signals at 2.8 ppm, solvent at 2.1 ppm, the upfield triplet at ᎑27.7 ppm, and no resonances due to COD. The spin–lattice relaxation time measurement was carried out at ambient temperature. Fifteen student pairs did the experiment during winter quarter 2000, and the measured T1 values for the upfield triplet signal fell in the rather narrow range of 620–690 ms ± 4–6 ms using Bruker software for the data analysis. A typical stacked plot and the least-squares plot of signal intensity versus delay time, τ, for the upfield triplet resonance are shown in Figures 2 and 3, respectively.
Figure 2. Stacked plot of the ᎑27.7 ppm triplet resonance as a function of delay time, τ.
Figure 3. Nonlinear least squares plot of signal intensity vs delay time for the ᎑27.7 ppm triplet.
Discussion All students received a written handout for the experiment that included a procedure for the synthesis of [Ir(COD)(Ph3P)2]BF4 and its reaction with H2. We did not identify the product formed in the latter reaction, but did state that one molecule of H2 is bound to iridium. It was their task to discover the bonding mode of dihydrogen (η2-H2 or dihydride) and the identity of all products formed. From literature references cited in the lab handout students infer that the triplet at ᎑27.7 ppm is due to coordination of H2 in one or the other of the above two bonding modes. The magnitude of the spin–lattice relaxation time for this signal, 620–690 ms based on student results, allows unequivocal assignment of the resonance to a hydridic proton since η2-H2 coordination is most commonly associated with T1 < 100 ms (3). Complete identification of the products and assignment of all peaks in the 1H NMR spectrum was a more challenging task for the students. In addition to the three review articles (1–3), we assigned two books (6, 7) as references for iridium chemistry and sent students to the university library. Also available in the library is the three-volume Aldrich collection of NMR spectra (8). We encouraged students to obtain 1H NMR spectra as needed of COD, [Ir(COD)Cl]2, Ph3P, [Ir(COD)(Ph3P)2]BF4, acetone-d6, and H2 dissolved in acetoned6. Through literature searching and careful examination of the NMR spectra, students are expected to recognize that a
JChemEd.chem.wisc.edu • Vol. 78 No. 9 September 2001 • Journal of Chemical Education
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In the Laboratory
dihydride is formed, that cyclooctene is the other product, and that two acetone molecules must be coordinated to iridium.1 This experiment has potential for expansion to a semester project if time is available. As a start, there is the intriguing 45-s microwave synthesis of [Ir(COD)Cl]2 from IrCl3⭈xH2O as reported by Baghurst et al. (9). Students could also prepare crystalline [IrH2(acetone)2(Ph3P)2]BF4 (5) and use this for further NMR and infrared spectroscopy investigation. The 1H NMR spectrum in CDCl shows one signal for acetone, 3 conclusive proof of the presence of coordinated acetone, and the 31P NMR spectrum shows the magnetic equivalence of the two phosphine ligands. With infrared spectroscopy students can search the 1700–2200 cm᎑1 region for evidence of H2 coordination. A more elaborate extension could involve the synthesis of [IrH(H2)(bq)(Ph3P)2]BF4 (bq = η2-7,8-benzoquinolinate) from the reaction of [Ir(COD)(Ph3P)2]BF4 with 7,8-benzoquinoline and dihydrogen (10). The Ir(III) benzoquinolate complex contains both η2-H2 and hydride as ligands. Acknowledgments We wish to thank Joseph Morse, Charles Wandler, and Douglas Wick for their assistance with the in-class testing of this experiment. W
Supplemental Material
A handout for students presenting detailed background, experimental procedures, and instructions for the lab report is available in this issue of JCE Online.
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Note 1. Crabtree and coworkers reported that cyclooctane and not cyclooctene is formed when their preparative macroscale procedure is followed (5).
Literature Cited 1. 2. 3. 4. 5.
6.
7.
8.
9. 10.
Kubas, G. J. Acc. Chem. Res. 1988, 21, 120–128. Crabtree, R. H. Acc. Chem. Res. 1990, 23, 95–101. Heinekey, D. M.; Oldham, W. J. Jr. Chem. Rev. 1993, 93, 913. Haines, L. M; Singleton, E. J. Chem. Soc., Dalton Trans. 1972, 1891–1896. Crabtree, R. H.; Hlatky, G. G.; Parnell, C. P.; Segmüller, B. E.; Uriarte, R. J. Inorg. Chem. 1984, 23, 354–358; Crabtree, R. H.; Mellea, M. F.; Mihelcic, M. Inorg. Synth. 1988, 26, 122–126. Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th ed.; Wiley-Interscience: New York, 1999. Comprehensive Organometallic Chemistry II: A Review of the Literature, 1982–1994, Vol. 8; Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Pergamon: Oxford, 1995. Pouchert, C. J.; Behnke, J. The Aldrich Library of 13C and 1H FT-NMR Spectra; Aldrich Chemical Company: Milwaukee, WI, 1993. Baghurst, D. R.; Mingos, D. M. P.; Watson, M. J. J. Organomet. Chem. 1989, 368, C43–C45. Crabtree, R. H.; Lavin, M.; Bonneviot, L. J. Am. Chem. Soc. 1986, 108, 4032–4037.
Journal of Chemical Education • Vol. 78 No. 9 September 2001 • JChemEd.chem.wisc.edu
