Dehydrogenation of amine-boranes

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.

Substrates

Ammonia-borane, the parent amine-borane, is a molecule with the formula. The hydrogen content for double dehydrogenation of ammonia-borane is nearly 12% :
The amine boranes have the formula 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. Catalysis in the absence of metals has also been observed.

Pathways

The dehydrogenation of AB would in principle afford _n and _n. The monomers 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. Cyclopentadienyliron dicarbonyl dimer|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 complexes to form the active colloidal heterogeneous catalyst. Homogeneous catalysts are of the type RhL₂, RhClL3, and Rh₂L₂ where L = P(ⁱPr)₃, P₃, and PH₂.
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 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 and inhibited by bulk. Unlike other catalytic processes, the reaction proceeds via a linear aminoborane 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 ⁺ 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. 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.