Reversible Interconversion between 2,5‐Dimethylpyrazine and 2,5‐Dimethylpiperazine by Iridium‐Catalyzed Hydrogenation/Dehydrogenation for Efficient Hydrogen Storage

Abstract

A new hydrogen storage system based on the hydrogenation and dehydrogenation of nitrogen heterocyclic compounds, employing a single iridium catalyst, has been developed. Efficient hydrogen storage using relatively small amounts of solvent compared with previous systems was achieved by this new system. Reversible transformations between 2,5-dimethylpyrazine and 2,5-dimethylpiperazine, accompanied by the uptake and release of three equivalents of hydrogen, could be repeated almost quantitatively at least four times without any loss of efficiency. Furthermore, hydrogen storage under solvent-free conditions was also accomplished.

Development and Application of New Iridium Catalysts for Efficient Dehydrogenative Reactions of Organic Molecules

“…Thereafter, we succeeded in developing a hydrogen-storage system based on the reversible conversion between 2,5dimethylpiperazine and 2,5-dimethylpyrazine using catalyst 6 (eq 10). 22 In this system, in addition to achieving hydrogenation at a comparatively low pressure (15 atm), the main feature was that no solvent was required (or only a very small amount of solvent was used). Reversible and repetitive reactions accompanied by the uptake and release of three equivalents of hydrogen, could be repeated almost quantitatively at least four times without any loss of efficiency.…”

Effect of a Substituent in Cyclopentadienyl Ligand on Iridium-Catalyzed Acceptorless Dehydrogenation of Alcohols and 2-Methyl-1,2,3,4-tetrahydroquinoline

“…On the basis of our previous studies, in which Cp* iridium complexes exhibited excellent catalytic activity [36][37][38][39][40][41][42][43][44][45][46][47], we attempted to modify one methyl group in the Cp* ligand to hydrogen, ethyl, isopropyl, and t-butyl groups in order to improve the catalytic properties (Scheme 2). A series of cyclopentadienyl-ligated iridium dichloride dimers 1a-1e were synthesized by the reaction of iridium trichloride with parent cyclopentadiene derivatives [62][63][64].…”

“…We have previously reported that the dehydrogenation of a cyclic amine, which leads to aromatized N-heterocycles, is also catalyzed by the same iridium complex used for the dehydrogenation of alcoholic substrates [45][46][47]. Hence, we also examined the catalytic activity of a series of iridium complexes (3) in the dehydrogenation of 2-methyl-1,2,3,4-tetrahydroquinoline (10) as a model substrate (Table 4).…”

“…Our research group has consistently studied the catalytic activity of pentamethylcyclopentadienyl (Cp*) iridium complexes for the hydrogen transfer process of alcoholic substrates [31][32][33][34][35]. By combining hydroxypyridine or dihydroxybipyridine derivatives as non-innocent ligands, the Cp* iridium complex shows an extremely high catalytic activity in acceptorless dehydrogenation reactions of alcoholic substrates and N-heterocyclic compounds (Scheme 1c) [36][37][38][39][40][41][42][43][44][45][46][47]. Theoretical studies suggest that the spectator Cp* ligand contributes to the stabilization of catalytically active species and to the milder electron-population change on the iridium center during the reaction, which decreases the overall reaction barrier [48].…”

Reversible interconversion between methanol-diamine and diamide for hydrogen storage based on manganese catalyzed (de)hydrogenation

“…To lower the endothermicity, LOHCs systems based on nitrogen-containing organic hydrides 24 , e.g., N-heterocycles, were developed. These have high H 2 capacities, in the range 5.3-7.3 wt% [31][32][33][34][35][36][37] . These systems can be promoted by various homogeneous and heterogeneous catalysts [38][39][40] under relatively mild conditions ( Fig.…”