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 is of due to the potential of using amine-boranes for hydrogen storage.
Many metal complexes catalyze the dehydrogenation of amine-borane (AB). Catalysis in the absence of metals has also been observed.[1]
The dehydrogenation of AB would in principle afford (H2BNH2)n and (HBNH)n. The monomers (n = 1) are highly unstable with respect to oligomerization.
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]
The first catalysts for the dehydrogenation of ABs were derived from reduction of Rh(I) complexes to form the active colloidal heterogeneous catalyst.[1] As in the case with the metal carbonyl catalysts, bulky secondary amine-boranes form monomeric aminoboranes. For RhL2- and Rh(H)2L2-derived catalysts, the active species is a homogeneous catalyst, with the phosphine ligands interacting directly with the dehydrocoupling process.[3] Changing the phosphine ligands from PiPr3 to PiBu3 significantly increases the turnover rate of the catalyst.[3] 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.[1]
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.[1] 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. This same reaction has been found to occur 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.[4] When catalyzing ammonia-borane dehydrogenation, the catalyst acts homogeneously at a 0.5 mol% catalyst loading.[4] Rather than the typical high temperatures needed for this dehydrogenation, the reaction proceeds cleanly at room temperature, with complete substrate conversion in 14min.[4]
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.[5] 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.[5]
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%).[6][7] Dehydrogenation occurs in three steps, creating polyamino-boranes and borazines as insoluble side products.[6][7] The dehydrogenation reactions are irreversible, which limits the utility of this process for hydrogen storage.
Amine-borane dehydrogenation can be coupled with hydride transfer to unsaturated functional groups, usually olefins in an anti-Markovnikov fashion.[8][9] Hydroboration of the olefin and release of H2 from the amine-borane occur in parallel reactions, reducing the percent of olefin reduced.[8]