Long–Short-Arm Acridine Ru-Pincer Catalysts for Reversible Hydrogen Storage Based on Ethylene Glycol

Liquid organic hydrogen carriers (LOHCs) offer an attractive strategy for efficient hydrogen storage and release, thereby facilitating the effective use of hydrogen as a carbon-neutral energy carrier. The advancement of LOHC technology is highly dependent on the innovation of the catalysts. Herein, based on a strategy combining rigidity and flexibility in a single molecular catalyst, a novel class of PNP-pincer ligands, called long-short-arm acridine ligands, and their Ru complexes have been developed and successfully used in the LOHC system based on ethylene glycol (EG). In comparison to previously reported catalytic systems, which suffered from low conversions or insufficient H2 release due to the dual challenge of catalyst stability and catalytic activity in the acceptorless dehydrogenative coupling of EG, this new catalytic system overcomes these challenges and achieves high conversion (up to >99%) with high H2 yield (up to 96%), achieving a hydrogen storage capacity of 6.2 wt %. Mechanistic and computational studies reveal that the special coordination mode, one 5-membered metallacycle and one 6-membered metallacycle, is essential for the high reactivity and late-stage dehydrogenative coupling. Moreover, both dehydrogenation and hydrogenation can be achieved under solvent- and additive-free conditions, highlighting the robustness and application potential of this new catalytic system. This advances the promising liquid-to-liquid paired LOHC systems based on inexpensive, widely accessible, and biobased EG toward practical application.