Hydrogen storage and evolution catalysed by metal hydride complexes

Abstract

The storage and evolution of hydrogen are catalysed by appropriate metal hydride complexes. Hydrogenation of carbon dioxide by hydrogen is catalysed by a [C,N] cyclometalated organoiridium complex, [Ir^III(Cp*)(4-(1H-pyrazol-1-yl-κN²)benzoic acid-κC³)(OH₂)]₂SO₄ [Ir–OH₂]₂SO₄, under atmospheric pressure of H₂ and CO₂ in weakly basic water (pH 7.5) at room temperature. The reverse reaction, i.e., hydrogen evolution from formate, is also catalysed by [Ir–OH₂]⁺ in acidic water (pH 2.8) at room temperature. Thus, interconversion between hydrogen and formic acid in water at ambient temperature and pressure has been achieved by using [Ir–OH₂]⁺ as an efficient catalyst in both directions depending on pH. The Ir complex [Ir–OH₂]⁺ also catalyses regioselective hydrogenation of the oxidised form of β-nicotinamide adenine dinucleotide (NAD⁺) to produce the 1,4-reduced form (NADH) under atmospheric pressure of H₂ at room temperature in weakly basic water. In weakly acidic water, the complex [Ir–OH₂]⁺ also catalyses the reverse reaction, i.e., hydrogen evolution from NADH to produce NAD⁺ at room temperature. Thus, interconversion between NADH (and H⁺) and NAD⁺ (and H₂) has also been achieved by using [Ir–OH₂]⁺ as an efficient catalyst and by changing pH. The iridium hydride complex formed by the reduction of [Ir–OH₂]⁺ by H₂ and NADH is responsible for the hydrogen evolution. Photoirradiation (λ > 330 nm) of an aqueous solution of the Ir–hydride complex produced by the reduction of [Ir–OH₂]⁺ with alcohols resulted in the quantitative conversion to a unique [C,C] cyclometalated Ir–hydride complex, which can catalyse hydrogen evolution from alcohols in a basic aqueous solution (pH 11.9). The catalytic mechanisms of the hydrogen storage and evolution are discussed by focusing on the reactivity of Ir–hydride complexes.