Synthesis of a novel Iridium-CCC Pincer Catalyst?

1. Synthesis of a novel Iridium-CCC Pincer Catalyst? Justin Chong University of Washington Heinekey Research Group

2. Catalytic uses of Pincer Complexes • Transfer dehydrogenation: COA + tert-butylethylene → COE + tert-butylethane • Heck olefination: ArX + ethylene → phenylethene + HX (X = I, Br, Cl)

3. Hydrogen Fuel Economy • We need a clean-burning alternative to fossil fuels. Hydrogen fuels cells are a very popular alternative. • Ammonia-Borane (NH3BH3) has both low molecular weight and a high chemical weight percent hydrogen (30.9 g/mol, 19.6 wt % H2). Useful for storage. • n NH3BH3 → [Catalyst 1] → [NH2BH2]n + n H2 • Studies have shown a Nickel-based catalyst utilizing Enders’ N-heterocyclic carbene (NHC) dehydrogenating a 25 wt % solution of AB at 60°C, completely consuming AB in 150 minutes.1 • Nearly 20 times faster than an uncatalyzed reaction! Catalyst 1 Enders’ NHC (1) Baker, T.; Keaton R.; Blacquiere, J. JACS, 2007, 129, 1844-1845

4. NHC’s vs. Phosphine-based Pincer Ligands • N-heterocyclic carbenes are superior σ-donor ligands compared to many phosphine ligands. In addition, carbenes are also poor π-acceptor ligands. • Carbenes stabilized by resonance -- donation of electrons from two nitrogen atoms to the carbene carbon through pπ-pπ interactions. With heterocyclic carbenes, stabilization also gained from aromaticity. • NHC’s are thermodynamically stable. Phosphine-based pincer ligands can undergo P-C bond scission and can decompose quickly at high temperatures. • Transfer dehydrogenation often uses temperatures of 190-200°C, and unwanted phopshine-borane side products can form during AB dehydrogenation reactions. • Carbenes are generally often cheaper than their phosphine counterparts. Hollis, K. T.; Rubio, R. J.; et. al. Journal of Organometallic Chemistry, 2005, 690, 5353-5364 Hahn, E. F.; Jahnke, M. C.; Pape, T. Organometallics, 2007, 26, 150-154

5. Synthesis of Substituted Imidazoles • Substituted imidazoles were used for generating the NHC part of the CCC pincer ligand. • In general, the imidazoles were synthesized in the following manner prior to work-up: • All imidazoles characterized by 1H NMR spectroscopy. (40% solution) (R = cyclohexyl, mesityl [(2,4,6-trimethyl)benzene], (2,6-diisopropyl)phenyl)

6. 1H NMR Spectrum of 1-(mesityl)imidazole (500 MHz, CDCl3)

7. Synthesis of the Bis-Imidazolium Ligand Precursor • 2,6-bis(bromomethyl)-1-bromobenzene (200 mg, .583 mmol) and substituted imidazole (2.2 equiv, 1.3 mmol) in MeCN, refluxing for 16 hours. • Off-white/white solid powder in moderate yields (25-48%). • Characterized by 1H NMR spectroscopy.

8. 1H NMR of Pincer Ligand Precursor (500 MHz, CD2Cl2) dgsdg (R= Mesityl)

9. Metalation of the CCC NHC Pincer Ligand • Silver bromide acts as a base to deprotonate the carbenic hydrogen atoms. • Silver(I)-carbene complexes are very useful for transmetalation. Light sensitive compounds. • Reaction done in J. Young tube. Nearly all silver oxide consumed in matter of minutes. Very stable in solution and under vacuum. No decomposition observed over 2-3 weeks. (R = Mesityl) Lin I. J. B.; Wang, H. M. J. Organometallics, 1998, 17, 972-975

10. 1H NMR of Silver Metalated Pincer Complex (500 MHz, CD2Cl2)

11. Transmetalation with [Ir(COD)Cl]2 (maybe) • Ag-CCC pincer ligand (NHC’s = 1-(mesityl)imidazole) was used for this experiment. Solvent removed under vacuum and Ir-dimer added. The reactants were then dissolved by back-transferring CD2Cl2. • Solution immediately turns bright yellow. White precipitate quickly forms at bottom of J.Y. tube and supernatant is now orange-yellow. • Proton NMR of solution was taken in DCM. Difficult to interpret.

12. 1H NMR Spectrum of Mystery Solution (500 MHz, CD2Cl2) CD2Cl2

13. Promising Evidence for Successful Transmetalation From: Hahn, E. F.; Jahnke, M. C.; Pape, T. Organometallics, 2007, 26, 150-154

14. 1H NMR Spectrum of Mystery Solution (500 MHz, CD2Cl2) Ortho-methyl substituents on the mesityl ring show doublet splitting! -- (Danopoulos, A. et. al. The Royal Society of Chemistry, Dalton Trans., 2003, 1009-1015)

15. Isolating the Iridium CCC Pincer Catalyst • White precipitate turns black when exposed to light. Good evidence for transmetalation. • 13C NMR is an excellent diagnostic tool for metal-carbene bonding as well as carbon-halide bond activation. • May attempt to add more equivalents of Ir-dimer. • Try and potentially isolate the catalyst as a solid.

16. Conclusions and Future Work • React other substituted imidazoles [1-(cyclohexyl)imidazole and (2,6-diisopropyl)phenylimidazole] with [Ir(COD)Cl]2. Compare with the reactivity of 1-(mesityl)imidazole. • Attempt deprotonation of the bis-imidazolium ligand precursor using Cs2CO3 in acetonitrile to pull off acidic protons and form the bis-imidazol-2-ylidene pincer ligand. This method has been used to oxidatively add [Ir(COD)Cl]2 to a carbene and Cp* moiety.2 • Once the desired product is isolated, attempt to add hydrogen to achieve a useful dehydrogenation catalyst. • Use various subsititued imidazoles (benzimidazole, di-substituted and tri-substituted imidazoles) to observe electronic and steric influences of substituted NHC’s. (2) Peris, E.; Tejeda, J.; Royo, B.; et. al. Organometallics, 2008, 27, 1305-1309

17. Cheers!(Thanks for listening!)