Iron Complex-Catalyzed
Ammonia–Borane Dehydrogenation.
A Potential Route toward B–N-Containing Polymer Motifs Using
Earth-Abundant Metal Catalysts
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Abstract
Ammonia–borane (NH<sub>3</sub>BH<sub>3</sub>,
AB) has garnered
interest as a hydrogen storage material due to its high weight percent
hydrogen content and ease of H<sub>2</sub> release relative to metal
hydrides. As a consequence of dehydrogenation, B–N-containing
oligomeric/polymeric materials are formed. The ability to control
this process and dictate the identity of the generated polymer opens
up the possibility of the targeted synthesis of new materials. While
precious metals have been used in this regard, the ability to construct
such materials using earth-abundant metals such as Fe presents a more
economical approach. Four Fe complexes containing amido and phosphine
supporting ligands were synthesized, and their reactivity with AB
was examined. Three-coordinate Fe(PCy<sub>3</sub>)[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub> (<b>1</b>) and four-coordinate Fe(DEPE)[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub> (<b>2</b>) yield a mixture
of (NH<sub>2</sub>BH<sub>2</sub>)<sub><i>n</i></sub> and
(NHBH)<sub><i>n</i></sub> products with up to 1.7 equiv
of H<sub>2</sub> released per
AB but cannot be recycled (DEPE = 1,2-bis(diethylphosphino)ethane).
In contrast, Fe supported by a bidentate P–N ligand (<b>4</b>) can be used in a second cycle to afford a similar product
mixture. Intriguingly, the symmetric analogue of <b>4</b> (Fe(N–N)(P–P), <b>3</b>), only generates (NH<sub>2</sub>BH<sub>2</sub>)<sub><i>n</i></sub> and does so in minutes at room temperature. This
marked difference in reactivity may be the result of the chemistry
of Fe(II) vs Fe(0)
