Regioselective hydrogenation of the oxidized form of β-nicotinamide adenine dinucleotide (NAD(+)) to the reduced form (NADH) with hydrogen (H(2)) has successfully been achieved in the presence of a catalytic amount of a [C,N] cyclometalated organoiridium complex [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))benzoic acid-κC(3))(H(2)O)](2) SO(4) [1](2)·SO(4) under an atmospheric pressure of H(2) at room temperature in weakly basic water. The structure of the corresponding benzoate complex Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))-benzoate-κC(3))(H(2)O) 2 has been revealed by X-ray single-crystal structure analysis. The corresponding iridium hydride complex formed under an atmospheric pressure of H(2) undergoes the 1,4-selective hydrogenation of NAD(+) to form 1,4-NADH. On the other hand, in weakly acidic water the complex 1 was found to catalyze the hydrogen evolution from NADH to produce NAD(+) without photoirradiation at room temperature. NAD(+) exhibited an inhibitory behavior in both catalytic hydrogenation of NAD(+) with H(2) and H(2) evolution from NADH due to the binding of NAD(+) to the catalyst. The overall catalytic mechanism of interconversion between NADH and NAD(+) accompanied by generation and consumption of H(2) was revealed on the basis of the kinetic analysis and detection of the catalytic intermediates.

No abstract

A [C,N] cyclometalated Ir complex, [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))benzoic acid-κC(3))(H(2)O)](2)SO(4) [1](2)·SO(4), was reduced by aliphatic alcohols to produce the corresponding hydride complex [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))-benzoate-κC(3))H](-)4 at room temperature in a basic aqueous solution (pH 13.6). Formation of the hydride complex 4 was confirmed by (1)H and (13)C 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 (λ > 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 β-nicotinamide adenine dinucleotide (NAD(+)) by ethanol was also made possible by the complex 1 to produce 1,4-dihydro-β-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.