Dehydrogenation of formic acid using half-sandwich iridium complexes with bidentate nitrogen ligands has high potential for hydrogen production, because hydrogen is formed in water without organic additive under mild reaction conditions with no CO contamination. Formic acid can be produced by hydrogenation of CO2, and therefore formic acid as hydrogen carrier leads to CO2 utilization. A series of complexes containing various diazole (imidazole and pyrazole) moieties as ligands were comprehensively evaluated as catalysts for the dehydrogenation of formic acid in water. Complexes with bidiazole without any substituent outperformed the complexes with dihydroxylbipyridine reported previously. Introduction of a methyl group as an electron-donating substituent into the diazole moiety enhanced the catalytic activity. The catalyst properties for practical use were also investigated. Simple biimidazole complexes retained their activity for at least one week at 70 degrees C in a concentrated formic acid solution. Moreover, the complex with 4,4'-biimidazole mixed with FA in a closed vessel produced high-pressure gas without significant decrease of catalytic activity, in comparison with the same reaction under atmospheric pressure. The diazole moiety on the ligand enables effective hydrogen production and is a promising basic ligand structure.