Boosting Effect of Sterically Protected Glucosyl Substituents in Formic Acid Dehydrogenation by Iridium(III) 2-Pyridineamidate Catalysts.

[Cp*Ir(R-pica)Cl] (Cp*=pentamethylcyclopentadienyl anion, pica=2-picolineamidate) complexes bearing carbohydrate substituents on the amide nitrogen atom (R=methyl-β-D-gluco-pyranosid-2-yl, 1; methyl-3,4,6-tri-O-acetyl-β-D-glucopyranosid-2-yl, 2) were tested as catalysts for formic acid dehydrogenation in water. TOF _MAX values over 12000 h ^-1 and 50000 h ^-1 were achieved at 333 K for 1 and 2, respectively, with TON values over 35000 for both catalysts. Comparison with the simpler cyclohexyl-substituted analogue (3) indicated that glucosyl-based complexes are much better performing under the same experimental conditions (TOF _MAX =5144 h ^-1 , TON=5000 at pH 2.5 for 3) owing to a lower tendency to isomerize to the less active k ² -N,O isomer upon protonation. The 5-fold increase in TOF _MAX observed for 2 with respect to 1 is reasonably due to an optimal steric protection by the acetyl substituent, which may prevent unproductive inner-sphere reactivity. These results showcase a powerful strategy for the inhibition of the common deactivation pathways of [Cp*Ir(R-pica)X] catalysts for FA dehydrogenation, paving the way for the development of better performing hydrogen storage systems.
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