Synthesis of a novel Iridium-CCC Pincer Catalyst?

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Synthesis of a novel Iridium-CCC Pincer Catalyst?

• Justin Chong
• University of Washington
• Heinekey Research Group

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Catalytic uses of Pincer Complexes

• Transfer dehydrogenation
• COA tert-butylethylene ? COE
  tert-butylethane
• Heck olefination
• ArX ethylene ? phenylethene HX
• (X I, Br, Cl)

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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 ? NH2BH2n n H2
• Studies have shown a Nickel-based catalyst
  utilizing Enders N-heterocyclic carbene (NHC)
  dehydrogenating a 25 wt solution of AB at 60C,
  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
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NHCs vs. Phosphine-based Pincer Ligands

• N-heterocyclic carbenes are superior s-donor
  ligands compared to many phosphine ligands. In
  addition, carbenes are also poor p-acceptor
  ligands.
• Carbenes stabilized by resonance -- donation of
  electrons from two nitrogen atoms to the carbene
  carbon through pp-pp interactions. With
  heterocyclic carbenes, stabilization also gained
  from aromaticity.
• NHCs 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 190-200C, 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
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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)
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1H NMR Spectrum of 1-(mesityl)imidazole (500
MHz, CDCl3)
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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.

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1H NMR of Pincer Ligand Precursor (500 MHz,
CD2Cl2)

• dgsdg

(R Mesityl)
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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
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1H NMR of Silver Metalated Pincer Complex (500
MHz, CD2Cl2)
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Transmetalation with Ir(COD)Cl2 (maybe)

• Ag-CCC pincer ligand (NHCs 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.

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1H NMR Spectrum of Mystery Solution (500 MHz,
CD2Cl2)
CD2Cl2
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Promising Evidence for Successful Transmetalation
From Hahn, E. F. Jahnke, M. C. Pape, T.
Organometallics, 2007, 26, 150-154
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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)
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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.

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Conclusions and Future Work

• React other substituted imidazoles
  1-(cyclohexyl)imidazole and (2,6-diisopropyl)phen
  ylimidazole with Ir(COD)Cl2. 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)Cl2 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 NHCs.

(2) Peris, E. Tejeda, J. Royo, B. et. al.
Organometallics, 2008, 27, 1305-1309
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Cheers!(Thanks for listening!)