Dehydrogenation of amine-boranes

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Dehydrogenation of amine-boranes or dehydrocoupling of amine-boranes is a chemical process wherein dihydrogen is released from amine-boranes. This process was once of some interest for hydrogen storage. [1]

Contents

Substrates

Ammonia-borane, the parent amine-borane, is a molecule with the formula H3B−NH3. The hydrogen content for double dehydrogenation of ammonia-borane is nearly 12% ((4/33.7)x100):

H3B−NH3 → 2 H2 + (HBNH)

The amine boranes have the formula H3B−NH2R where R = alkyl. Their hydrogen content is necessarily lower, but the dehydrogenated product is more processable. [1]

Catalysis

Many metal complexes catalyze the dehydrogenation of amine-borane (AB). Catalysis in the absence of metals has also been observed. [1]

Pathways

The dehydrogenation of AB would in principle afford (H2BNH2)n and (HBNH)n. The monomers (n = 1) are highly unstable with respect to oligomerization. DAB primary revised.png DAB secondary path.png

Metal carbonyl catalysts

Group 6 metal carbonyls upon photolytic activation catalyze dehydrogenation of AB. [2] 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. [2] [CpFe(CO)2]2 is also an effective precatalyst, requiring 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. [1]

Rhodium and iridium catalysts

The first catalysts for the dehydrogenation of ABs were derived from reduction of Rh(I) complexes to form the active colloidal heterogeneous catalyst. [1] Homogeneous catalysts are of the type RhL2, RhClL3, and Rh(H)2L2 where L = Triisopropylphosphine|P(iPr)3]], P(iBu)3, and PH(cyclohexyl)2. [1]

Related iridium-based catalysts are less active for dehydrogenation of non sterically hindered amine-boranes but more active for sterically hindered substrates. [1] Dehydrocoupling of primary diborazanes NH2R—BH2—NHR—BH3 is catalyzed by Brookhart's catalyst via conversion to the metal-bound species MeNH—BH2 and subsequent polymerization/oligomerization. This same reaction occurs in the absence of the iridium metal, upon heating of the reaction mixture. [1] 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. [3] When catalyzing ammonia-borane dehydrogenation, the catalyst acts homogeneously at a 0.5 mol% catalyst loading. [3] Rather than the typical high temperatures needed for this dehydrogenation, the reaction proceeds at room temperature, with high conversion. [3]

Metallocenes

Group 4 metallocenes also catalyze dehydrogenation of ABs. Activity is affected by metal (Ti > Zr > Hf) and inhibited by bulk. Unlike 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. [4] 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. [4]

Potential applications

Hydrogen storage

Dehydrogenation of amine-boranes is thermodynamically favourable, making the process attractive for hydrogen storage systems. Ammonia borane has attracted particular interest due to its high weight percent of hydrogen (19.6%). [5] [6] Dehydrogenation occurs in three steps, creating polyamino-boranes and borazines as insoluble side products. [5] [6] The dehydrogenation reactions are irreversible, which limits the utility of this process for hydrogen storage.

Hydrogen transfer

Amine-borane dehydrogenation can be coupled with hydride transfer to unsaturated functional groups, usually olefins in an anti-Markovnikov fashion. [7] [8] Hydroboration of the olefin and release of H2 from the amine-borane occur in parallel reactions, reducing the percent of olefin reduced. [7]

References

  1. 1 2 3 4 5 6 7 8 9 Staubitz, Anne; Robertson, Alasdair P. M.; Manners, Ian (2010). "Ammonia-Borane and Related Compounds as Dihydrogen Sources". Chemical Reviews. 110 (7): 4079–4124. doi:10.1021/cr100088b. PMID   20672860.
  2. 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
  3. 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
  4. 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
  5. 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
  6. 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
  7. 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
  8. ↑ 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
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