Efficient catalytic interconversion between NADH and NAD ⁺ accompanied by generation and consumption of hydrogen with a water-soluble iridium complex at ambient pressure and temperature

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 ₂O)] 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 ₂O) 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.