Exploring Promising Catalysts for Chemical Hydrogen Storage in Ammonia Borane: A Density Functional Theory Study

Deciphering the Origin of Regioselectivity in Ru(II)-Catalyzed C-H Annulation of N-Chlorobenzamides with 1,3-Diynes

Understanding the reaction mechanism and origin of regioselectivity in transition metal-catalyzed C-H activation/annulation reactions with 1,3-diynes has remained an intriguing challenge. In this article, to establish the mechanism and decipher the origin of regioselectivity, we report a detailed computational density functional theory-based mechanistic investigation on the recently developed Ru(II)-catalyzed [4 + 2] annulation of N-chlorobenzamides with 1,3-diynes for the synthesis of 3-alkynylated isoquinolone derivatives. Our calculations reveal a redox-neutral pathway for the annulation reaction. The stepwise analysis of the reaction channels indicates the migratory insertion step and the concerted reductive elimination/oxidative addition of the Ru(p-cymene) moiety to form the N-C bond leading to the 3-alkynylated product to be the selectivity- and rate-determining steps, respectively. Finally, the distortion/interaction analysis using the activation-strain model suggests the steric effect as the determining factor for the observed regioselectivity for the formation of the 3-alkynylated product. Overall, the computationally obtained key insights into the catalytic mechanism and the origin of regioselectivity in the C-H activation/annulation reaction can be used as a guide to rationally design and develop novel transformation strategies for heterocycle synthesis.

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

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.

Hydrogen Production from Ammonia Borane over PtNi Alloy Nanoparticles Immobilized on Graphite Carbon Nitride

Graphite carbon nitride (g-C3N4) supported PtNi alloy nanoparticles (NPs) were fabricated via a facile and simple impregnation and chemical reduction method and explored their catalytic performance towards hydrogen evolution from ammonia borane (AB) hydrolysis dehydrogenation. Interestingly, the resultant Pt0.5Ni0.5/g-C3N4 catalyst affords superior performance, including 100% conversion, 100% H2 selectivity, yielding the extraordinary initial total turnover frequency (TOF) of 250.8 molH2 min−1 (molPt)−1 for hydrogen evolution from AB at 10 °C, a relatively low activation energy of 38.09 kJ mol−1, and a remarkable reusability (at least 10 times), which surpass most of the noble metal heterogeneous catalysts. This notably improved activity is attributed to the charge interaction between PtNi NPs and g-C3N4 support. Especially, the nitrogen-containing functional groups on g-C3N4, serving as the anchoring sites for PtNi NPs, may be beneficial for becoming a uniform distribution and decreasing the particle size for the NPs. Our work not only provides a cost-effective route for constructing high-performance catalysts towards the hydrogen evolution of AB but also prompts the utilization of g-C3N4 in energy fields.

Palladium Supported on Carbon Nanotubes as a High-Performance Catalyst for the Dehydrogenation of Dodecahydro-N-ethylcarbazole

Hydrogen storage in the form of liquid organic hydrides, especially N-ethylcarbazole, has been regarded as a promising technology for substituting traditional fossil fuels owing to its unique merits such as high volumetric, gravimetric hydrogen capacity and safe transportation. However, unsatisfactory dehydrogenation has impeded the widespread application of N-ethylcarbazole as ideal hydrogen storage materials in hydrogen energy. Therefore, designing catalysts with outstanding performance is of importance to address this problem. In the present work, for the first time, we have synthesized Pd nanoparticles immobilized on carbon nanotubes (Pd/CNTs) with different palladium loading through an alcohol reduction technique. A series of characterization technologies, such as X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometer (ICP-AES), X-ray photoelectron spectroscopy (XPS) and transmission electron spectroscopy (TEM) were adopted to systematically explore the structure, composition, surface properties and morphology of the catalysts. The results reveal that the Pd NPs with a mean diameter of 2.6 ± 0.6 nm could be dispersed uniformly on the surface of CNTs. Furthermore, Pd/CNTs with different Pd contents were applied in the hydrogen release of dodecahydro-N-ethylcarbazole. Among all of the catalysts tested, 3.0 wt% Pd/CNTs exhibited excellent catalytic performance with the conversion of 99.6% producing 5.8 wt% hydrogen at 533 K, low activation energy of 43.8 ± 0.2 kJ/mol and a high recycling stability (>96.4% conversion at 5th reuse).