Engineering of iridium complexes for the efficient hydrogen evolution of formic acid without additives

Formic acid (FA), as a promising hydrogen storage carrier, plays a key role in the implementation of a hydrogen economy with its efficient hydrogen evolution, especially for additive-free systems. To this end, we construct a series of tetrahydroquinoxaline-dioximino iridium complexes (Ir/Ln) that meet the above criteria. After optimization of the ligand structure, Ir/L8 (L8 = 6,7-dimethyl-2,3-dioxo-1,2,3,4-tetrahydroquinoxaline) exhibits an initial TOF of 80,000 h(-1) at 60 degrees C (337,500 h(-1) at 80 degrees C), outperforming the previous reported counterparts for additive-free systems. Besides, a possible dehydrogenation mechanism, i.e. the synergistic auxiliary effect of beta-NH units, water and HCOOH molecules favored Ir/L8-driven FA decomposition, is adequately elucidated by experimental and theoretical studies, filling the gap in the existing mechanism system. In brief, the ingenious design and systematic research lay the foundation for the efficient release of hydrogen from FA, promising the overall implementation of "green hydrogen economy". (C) 2022 Elsevier Inc. All rights reserved.