Unprecedentedly high formic acid dehydrogenation activity on an iridium complex with an N,N'-diimine ligand in water.

Hydrogen production from the dehydrogenation of formic acid (FA) is promising. Most of the current catalysts for FA dehydrogenation are effective only in the presence of bases or additives. We report here newly developed iridium complexes containing conjugated N,N'-diimine ligands for FA dehydrogenation in water without the addition of bases or additives. A turnover frequency (TOF) of 487 500 h(-1) with [Cp*Ir(L1)Cl]Cl (L1=2,2'-bi-2-imidazoline) at 90 °C and a turnover number (TON) of 2 400 000 with in situ prepared catalyst from [IrCp*Cl2 ]2 and 2,2'-bi-1,4,5,6-tetrahydropyrimidine (L2) at 80 °C were obtained, the highest values reported for FA dehydrogenation to date. A mechanistic study reveals that the formation of [Ir-H] intermediate species is the rate-determining step in the catalytic cycle.