New Water-Soluble Iridium(I)–N-Heterocyclic Carbene–Tertiary Phosphine Mixed-Ligand Complexes as Catalysts of Hydrogenation and Redox Isomerization

Seven
new [Ir­(NHC)­(η4-cod)­(L)] complexes (4–9, 11) with NHC = bmim,
emim, L = Cl–, H2O, or the water-soluble
phosphines mtppms-Na, mtppts-Na3, and pta were synthesized and characterized. Na2[Ir­(bmim)­(η4-cod)­(mtppts)] (6) and [Ir­(bmim)­(η4-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)­(η4-cod)], [IrCl­(η4-cod)­(emim)], or [IrCl­(η4-cod)­(IMes)] and mtppts-Na3 or
pta. In the presence of HCOONa, [IrCl­(η4-cod)­(bmim)]
+ mtppts-Na3 showed a TOF of 150 h–1in 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 H2. Furthermore, Na2[Ir­(bmim)­(η4-cod)­(mtppts)] (6) was also
shown to catalytically decompose aqueous formate to H2 and
bicarbonate.