Amine-boranes are a class of covalent compounds based on the ammonia borane parent structure. These compounds were once of some interest for hydrogen storage, as they can undergo dehydrogenation (dehydrocoupling) to release dihydrogen.
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 13% ((4/30.9)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.
Catalysis Many metal complexes catalyze the dehydrogenation of amine-borane (AB). Catalysis in the absence of metals has also been observed.
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.
Metal carbonyl catalysts Group 6 metal carbonyls upon photolytic activation catalyze dehydrogenation of AB. 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. [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.
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. Homogeneous catalysts are of the type RhL2, RhClL3, and Rh(H)2L2 where L = P(iPr)3, P(iBu)3, and PH(cyclohexyl)2. Related iridium-based catalysts are less active for dehydrogenation of non sterically hindered amine-boranes but more active for sterically hindered substrates. 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. 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. When catalyzing ammonia-borane dehydrogenation, the catalyst acts homogeneously at a 0.5 mol% catalyst loading. Rather than the typical high temperatures needed for this dehydrogenation, the reaction proceeds at room temperature, with high conversion.
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. 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. 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.
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%). Dehydrogenation occurs in three steps, creating polyamino-boranes and borazines as insoluble side products. 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. Hydroboration of the olefin and release of H2 from the amine-borane occur in parallel reactions, reducing the percent of olefin reduced.
References
External links Media related to Amine boranes at Wikimedia Commons