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Open AccessArticle

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

1
School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114051, China
2
Faculty of Science and Technology, Technological and Higher Education Institute of Hong Kong (THEi), Hong Kong 999077, China
3
Faculty of Design and Environment, Technological and Higher Education Institute of Hong Kong (THEi), Hong Kong 999077, China
4
Department of Supply Chain and Information Management, The Hang Seng University of Hong Kong, Hong Kong 999077, China
5
Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China
*
Authors to whom correspondence should be addressed.
Catalysts 2020, 10(7), 723; https://doi.org/10.3390/catal10070723
Received: 4 June 2020 / Revised: 25 June 2020 / Accepted: 26 June 2020 / Published: 28 June 2020
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 > (iPr)2P > (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. View Full-Text
Keywords: iridium PNP complexes; cobalt PNP complexes; ammonia borane hydrolysis; density functional theory; hydrogen production iridium PNP complexes; cobalt PNP complexes; ammonia borane hydrolysis; density functional theory; hydrogen production
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MDPI and ACS Style

Li, Y.; Tsang, C.-W.; Chan, E.M.H.; Wong, E.Y.C.; Ho, D.C.K.; Lu, X.-Y.; Liang, C. Sustainable Option for Hydrogen Production: Mechanistic Study of the Interaction between Cobalt Pincer Complexes and Ammonia Borane. Catalysts 2020, 10, 723.

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