Iron Complex-Catalyzed Ammonia-Borane Dehydrogenation. A Potential Route toward B-N-Containing Polymer Motifs Using Earth-Abundant Metal Catalysts

Ammonia-borane (NH3BH3, AB) has garnered interest as a hydrogen storage material due to its high weight percent hydrogen content and ease of H-2 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(PCy3)[N(SiMe3)(2)](2) (1) and four-coordinate Fe(DEPE)[N(SiMe3)(2)](2) (2) yield a mixture of (NH2BH2) and (NHBH)(n) products with up to 1.7 equiv of H-2 released per AB but cannot be recycled (DEPE = 1,2-bis(diethylphosphino)ethane). In contrast, Fe supported by a bidentate P-N ligand (4) can be used in a second cycle to afford a similar product mixture. Intriguingly, the symmetric analogue of 4 (Fe(N-N)(P-P), 3), only generates (NH2BH2) 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).