Sustainable Option for Hydrogen Production: Mechanistic Study of the Interaction between Cobalt Pincer Complexes and Ammonia Borane

Abstract

<jats:p>The mechanism of the solvolysis/hydrolysis of ammonia borane by iridium (Ir), cobalt (Co), iron (Fe) and ruthenium (Ru) complexes with various PNP ligands has been revisited using density functional theory (DFT). The approach of ammonia borane (NH3BH3) to the metal center has been tested on three different possible mechanisms, namely, the stepwise, concerted and proton transfer mechanism. It was found that the theoretical analyses correlate with the experimental results very well, with the activities of the iridium complexes with different PNP ligands following the order: (tBu)2P &gt; (iPr)2P &gt; (Ph)2P through the concerted mechanism. The reaction barriers of the rate-determining steps for the dehydrogenation of ammonia borane catalyzed by the active species [(tBu)2PNP-IrH] (Complex I-8), are found to be 19.3 kcal/mol (stepwise), 15.2 kcal/mol (concerted) and 26.8 kcal/mol (proton transfer), respectively. Thus, the concerted mechanism is the more kinetically favorable pathway. It is interesting to find that stable (tBu)2PNP Co-H2O and (tBu)2PNP Co-NH3 chelation products exist, which could stabilize the active I-8 species during the hydrolysis reaction cycle. The use of more sterically hindered and electron-donating PNP ligands such as (adamantyl)2P- provides similar activity as the t-butyl analogue. This research provides insights into the design of efficient cobalt catalysts instead of using precious and noble metal, which could benefit the development of a more sustainable hydrogen economy.</jats:p>

Computational Catalysis—Trends and Outlook

“…Of particular recent interest have been metal pincer complexes, which have the potential to provide highly active and selective catalytic sites, and they may provide an economical alternative to traditional precious metal catalysts (Pt, Ru, etc.). These pincer complexes may play a future role in hydrogen production and storage [4] and the hydrogenation of alkenes or carbonyl sites [5,6]. In terms of a hydrogen economy, the development of economic fuel cell designs still presents a challenging bottleneck, and the catalyst cost has been a key focus for many years.…”