Dehydrogenation of amine-boranes

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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.


DAB primary revised DAB secondary path


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[edit]

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[edit]

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[edit]

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]


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]


Hydrogen Storage[edit]

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[edit]

Hydrogen Transfer[edit]

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]


  1. ^ a b c d 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. ^ a b c 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
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