Dehydrogenation of amine-boranes

Dehydrogenation of amine-boranes or dehydrocoupling of amine-boranes is a chemical process in main group and organometallic chemistry wherein dihydrogen is released by the coupling of two or more amine-borane adducts. This process has gained significant interest in the past few decades due to the potential of using amine-boranes for hydrogen storage.

Mechanism

Catalysts

Many types of catalysts have been found to catalyze amine-borane dehydrogenation, however, with apparent substrate specificity.[1] Although almost every dehydrogenation reaction of amine-boranes has been examined with transition metal catalysts, recent evidence points toward the possibility of certain dehydrogenations occurring in the absence of any metal.[2][3]

Metal-Carbonyl Catalysts

Group 6 homoleptic metal carbonyls activate dehydrogenation of both primary and secondary amine-borane adducts with photolytic activation.[4] Secondary amine-boranes dehydrogenate to form cyclic dimers, or monomeric aminoboranes in the case of more bulky groups on the amine. Similarly, primary amine-boranes dehydrogenate through a two step intramolecular process to give aminoborane polymers, which further dehydrogenate to form borazines.[4][5] The iron-carbonyl catalyst [CpFe(CO)2]2 also mediates the dehydrogenation process via photolytic activation. The two step process is proposed to occur first by dehydrogenation of the amine-borane coordinated to the metal, followed by cyclodimerization in an off-metal step.[5]

Rhodium Catalysts

The first catalysts known to effect amine-borane dehydrogenation were Rh(I) complexes, where the rhodium was reduced in situ to Rh(0) to form the active colloidal heterogeneous catalyst.[6][7] As in the case with the metal carbonyl catalysts, bulky secondary amine-boranes form monomeric aminoboranes due to steric bulk preventing dimerization.[7] For RhL2 and Rh(H)2L2 type catalysts, the active species is a homogeneous catalyst, with the phosphine ligands interacting directly with the dehydrocoupling process.[8] Notably, changing the phosphine ligands from PiPr3 to PiBu3 significantly increases the turnover rate of the catalyst.[8] Unlike other Rh(I) catalysts, the rhodium analogue of Wilkinson's catalyst RhCl(PHCy2)3 (Cy=cyclohexyl) behaves like the RhL2 and Rh(H)2L2 catalysts as a homogeneous species.[9]

Iridium Catalysts

In comparison to RhCl(PHCy2)3, the iridium analogue has reduced catalytic activity on the dehydrogenation of non sterically hindered amine-boranes, and increased activity on more sterically hindered substrates.[9] Dehydrocoupling of primary diborazanes NH2R—BH2—NHR—BH3 can be catalyzed by Brookhart's catalyst via conversion to the metal-bound species MeNH—BH2 and subsequent polymerization/oligomerization.[2] This same reaction has been found to occur in the absence of the iridium metal, upon heating of the reaction mixture.[2] Dehydrogenation of ammonia-borane with Brookhart's catalyst results in quantitative formation of the cyclic pentamer [NH2BH2]5 rather than the typically seen cyclic dimers from other amine-borane dehydrogenations.[10] When catalyzing ammonia-borane dehydrogenation, the catalyst acts homogeneously at a 0.5 mol% catalyst loading.[10] Rather than the typical high temperatures needed for this dehydrogenation, the reaction proceeds cleanly at room temperature, with complete substrate conversion in 14min.[10]

Metallocenes

Group 4 complexes are catalytically active for amine-borane dehydrogenation, with decreasing activity going down in the group.[1] Activity is also reduced by substituting the cyclopentadienyl ligands with bulky and electron-donating groups.[1] Unlike in other catalytic processes, the reaction proceeds via a linear aminoborane [NR2BH2]2, which then cyclodimerizes through a dehydrocoupling process on the metal.[1] Most of the zirconocene complexes contain the zirconium in the +4 oxidation state, and the systems are not very active catalysts for amine-borane dehydrogenation.[11] In contrast to these systems, the cationic zirconocene complex [Cp2ZrOC6H4P(tBu)2]+ effectively catalyzes the reaction, with the most notable example being the dehydrogenation of dimethylamineborane in 10min at room temperature.[11]

Applications

Hydrogen Storage

Main article: Hydrogen Storage

Dehydrogenation of amine-boranes is thermodynamically favourable, making the process attractive for hydrogen storage systems. Ammonia borane is the prototypical amine-borane being investigated for hydrogen storage, due to its high weight percent of hydrogen (19.6%).[12][13] Dehydrogenation occurs in three steps, creating polyamino-boranes and borazines as insoluble side products.[12] Recent findings have enabled solubilization of the dehydrogenated products, while still maintaining a decent quantitative release of dihydrogen.[13]

Boron-Nitride Ceramics


Hydrogen Transfer

Amine-borane dehydrogenation is often coupled with hydride transfer to unsaturated functional groups, usually olefins in an anti-Markovnikov fashion.[14] Through coordination of the amine-borane to a transition metal catalyst, the B-H bond is activated enough to release H− under mild reaction conditions.[15] Hydroboration of the olefin and release of H2 from the amine-borane occur in parallel reactions, reducing the percent of olefin reduced.[14]

References

  1. 1 2 3 4 Sloan, M.E.; Staubitz, A.; Clark, T.J.; Russell, C.A.; Lloyd-Jones, G.C.; Manners, I. "Homogeneous Catalytic Dehydrocoupling/Dehydrogenation of Amine-Borane Adducts by Early Transition Metal, Group 4 Metallocene Complexes" J. Amer. Chem. Soc. 2010, 132, 3831-3841. doi:10.1021/ja909535a
  2. 1 2 3 Robertson, A.P.M.; Leitao, E.M.; Manners, I. "Catalytic Redistribution and Polymerization of Diborazanes: Unexpected Observation of Metal-Free Hydrogen Transfer between Aminoboranes and Amine-Boranes" J. Amer. Chem. Soc. 2011, 133, 19322-19325. doi:10.1021/ja208752w
  3. ↑ Helten, H.; Robertson, A.P.M.; Staubitz, A.; Vance, J.R.; Haddow, M.F.; Manners, I. "'Spontaneous' Ambient Temperature Dehydrocoupling of Aromatic Amine-Boranes" Chem. Eur. J. 2012, 18, 4665-4680. doi:10.1002/chem.201103241
  4. 1 2 Kawano, Y.; Uruichi, M.; Shimoi, M.; Taki, S.; Kawaguchi, T.; Kakizawa, T.; Ogino, H. "Dehydrocoupling Reactions of Borane-Secondary and -Primary Amine Adducts Catalyzed by Group-6 Carbonyl Complexes: Formation of Aminoboranes and Borazines" J. Amer. Chem. Soc. 2009, 131, 14946-14957. doi:10.1021/ja904918u
  5. 1 2 Vance, J.R.; Robertson, A.P.M.; Lee, K.; Manners, I. "Photoactivated, Iron-Catalyzed Dehydrocoupling of Amine Borane Adducts: Formation of Boron-Nitrogen Oligomers and Polymers" Chem. Eur. J. 2011, 17, 4099-4103. doi:10.1002/chem.201003397
  6. ↑ Jaska, C.; Temple, K.; Lough, A.J.; Manners, I. "Rhodium-catalyzed formation of boron-nitrogen bonds: a mild route to cyclic aminoboranes and borazines" Chem. Commun. 2001, 962-963. doi:10.1039/b102361f
  7. 1 2 Jaska, C.A.; Temple, K.; Lough, A.J.; Manners, I. "Transition Metal-Catalyzed Formation of Boron-Nitrogen Bonds: Catalytic Dehydrocoupling of Amine-Borane Adducts to Form Aminoboranes and Borazines" J. Amer. Chem. Soc. 2003, 125, 9424-9434. doi:10.1021/ja030160I
  8. 1 2 Chaplin, A.B.; Weller, A.S. "Amine- and Dimeric Amino-Borane Complexes of the {Rh(PiPr3)2}+ Fragment and their Relevance to the Transition-Metal-Mediated Dehydrocoupling of Amine-Boranes" Inorg. Chem. 2010, 49, 1111-1121. doi:10.1021/ic9020542
  9. 1 2 Sloan, M.E.; Clark, T.J.; Manners, I. "Homogeneous Catalytic Dehydrogenation/Dehydrocoupling of Amine-Borane Adducts by the Rh(I) Wilkinson's Complex Analogue RhCl(PHCy2)3 (Cy = cyclohexyl)" Inorg. Chem. 2009, 48, 2429-2435. doi:10.1021/ic801752k
  10. 1 2 3 Denney, M.C.; Pons, V.; Hebden, T.J.; Heinekey, D.M.; Goldberg, K.I. "Efficient Catalysis of Ammonia Borane Dehydrogenation" J. Amer. Chem. Soc. 2006, 128, 12048-12049. doi:10.1021/ja062419g
  11. 1 2 Chapman, A.M.; Haddow, M.F.; Wass, D.F. "Frustrated Lewis Pairs beyond the Main Group: Cationic Zirconocene-Phosphinoaryloxide Complexes and Their Application in Catalytic Dehydrogenation of Amine Boranes" J. Amer. Chem. Soc. 2011, 133, 8826-8829. doi:10.1021/ja201989c
  12. 1 2 Frueh, S.; Kellett, R.; Mallery, C.; Molter, T.; Willis, W.S.; King'ondu, C.; Suib, S.L. "Pyrolytic Decomposition of Ammonia Borane to Boron Nitride" Inorg. Chem. 2011, 50, 783-792. doi:10.1021/ic101020k
  13. 1 2 Mal, S.S.; Stephens, F.H.; Baker, R.T. "Transition metal catalyzed dehydrogenation of fuel blends." Chem. Commun. 2011,47,2922-2924. doi:10.1039/c0cc03585h
  14. 1 2 Sewell, L.J.; Chaplin, A.B.; Weller, A.S. "Hydroboration of an alkene by amine-boranes catalyzed by a [Rh(PR3)2]+ fragment. Mechanistic insight and tandem hydroboration/dehydrogenation" Dalton Trans. 2011, 40, 7499-7501. doi:10.1039/C1DT10819K
  15. ↑ Couturier, M.; Andresen, B.M.; Tucker, J.L.; Dubé, P.; Brenek, S.J.; Negri, J.T. "The use of borane-amine adducts as versatile palladium-catalyzed hydrogen-transfer reagents in methanol" Chem. Commun. 2001, 42, 2763-2766. doi:10.1016/S0040-4039(01)00300-8
This article is issued from Wikipedia - version of the 10/20/2015. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.