Molecular hydrogen, obtained by water electrolysis or photocatalytic water splitting, can be used to store energy obtained from intermittent sources such as wind and solar power. The storage and safe transportation of H2 , however, is an open and central question in such a hydrogen economy. Easy-to-synthesize, water-soluble iridium-N-heterocyclic carbene-phosphine (Ir(I) -NHC-phosphine) catalysts show unprecedented high catalytic activity in dehydrogenation of aqueous sodium formate. Fast reversible generation and storage of hydrogen can be achieved with these catalysts by a simple decrease or increase in the hydrogen pressure, respectively.

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The new water-soluble cis-mer-[IrH2Cl(mtppms)3] (mtppms = monosulfonated triphenylphosphine) was employed as a catalyst for selective decomposition of formic acid to H2 + CO2 in aqueous solution at T = 30-100 °C. The easily synthesized compound showed high catalytic activity (TOF up to 298 000 h(-1)) and could be reused several times with no loss of activity (total TON = 67 650). A sharp maximum in the reaction rate was observed at pH = 3.75; its coincidence with the pKa of formic acid shows that both H(+) or HCOOH and HCOO(-) play important roles in the reaction mechanism.

Seven new [Ir(NHC)(η 4 -cod)(L)] complexes (4−9, 11) with NHC = bmim, emim, L = Cl − , H 2 O, or the water-soluble phosphines mtppms-Na, mtppts-Na 3 , and pta were synthesized and characterized. Na 2 [Ir(bmim)(η 4 -cod)(mtppts)] ( 6) and [Ir(bmim)(η 4 -cod)-(pta)]Cl (7) actively catalyzed hydrogenation of alkenoic and oxo acids in aqueous solution. These catalysts were also found to be active in the hydrogenation of highly substituted CC bonds (such as those in methylmaleic and methylfumaric acids). The stability of 6 was significantly increased by the addition of formate or oxalate. Under hydrogen, active catalysts were formed in situ from [IrCl(bmim)(η 4 -cod)], [IrCl(η 4 -cod)(emim)], or [IrCl(η 4 -cod)-(IMes)] and mtppts-Na 3 or pta. In the presence of HCOONa, [IrCl(η 4 -cod)(bmim)] + mtppts-Na 3 showed a TOF of 150 h −1 in the hydrogenation of itaconic acid in water at 60 °C, which is the highest value determined to date for a water-soluble Ir(I) hydrogenation catalyst in an aqueous system. Both 6 and 7 selectively isomerized alk-1-en-3-ols to the corresponding methyl ketones with no need for an external reducing agent such as H 2 . Furthermore, Na 2 [Ir(bmim)(η 4cod)(mtppts)] (6) was also shown to catalytically decompose aqueous formate to H 2 and bicarbonate.