Hydrogen Evolution from Aliphatic Alcohols and 1,4-Selective Hydrogenation of NAD+ Catalyzed by a [C,N] and a [C,C] Cyclometalated Organoiridium Complex at Room Temperature in Water

A [C,N] cyclometalated Ir complex, [Ir-III(Cp*)(4-(1H-pyrazol-1-yl-kN(2))benzoic acid-kC(3))(H2O)](2)SO4 [1](2)center dot SO4, was reduced by aliphatic alcohols to produce the corresponding hydride complex [Ir-III(Cp*)(4-(1H-pyrazol-1-yl-kN(2))-benzoate-kC(3))H](-) 4 at room temperature in a basic aqueous solution (pH 13.6). Formation of the hydride complex 4 was confirmed by H-1 and C-13 NMR, ESI MS, and UV vis spectra. The [C,N] cyclometalated Ir-hydride complex 4 reacts with proton to generate a stoichiometric amount of hydrogen when the pH was decreased to pH 0.8 by the addition of diluted sulfuric acid. Photoirradiation (lambda > 330 nm) of an aqueous solution of the [C,N] cyclometalated Ir-hydride complex 4 resulted in the quantitative conversion to a unique [C,C] cyclometalated Ir-hydride complex 5 with no byproduct. The complex 5 catalyzed hydrogen evolution from ethanol in a basic aqueous solution (pH 11.9) under ambient conditions. The 1,4-selective catalytic hydrogenation of beta-nicotinamide adenine dinucleotide (NAD(+)) by ethanol was also made possible by the complex 1 to produce 1,4-dihydro-beta-nicotinamide adenine dinucleotide (1,4-NADH) at room temperature. The overall catalytic mechanism of hydrogenation of NAD(+), accompanied by the oxidation of ethanol, was revealed on the basis of the kinetic analysis and detection of the reaction intermediates.