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Formic acid is an attractive means to reversibly store dihydrogen. In this context, d(6) pianostool complexes rank among the most-effective formate dehydrogenase catalysts. With biologically generated formic acid in mind, we evaluated the performances of iridium-based pianostool complexes bearing a cooperative ligand, which are known to catalyze formate decomposition. Interestingly, the phenylpyrazole-derived catalyst [Cp*Ir(phenpz)(OH2)](+) (7, Cp* = pentamethylcyclopentadienyl, phenpz = 1-phenylpyrazole) compares favourably with the very best systems [Cp*Ir(phenpzCO(2)H)H2O](+) [8, phenpzCO(2)H = 4-(pyrazol-1-yl)benzoic acid] and [Cp*Ir(imim)H2O](2+) [11, imim = 2,2'-bis(4,5-dimethylimidazole)]. These catalysts display remarkable air tolerance, recyclability and activity under dilute aqueous conditions.
