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

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.

Well‐Defined Ruthenium Complex for Acceptorless Alcohol Dehydrogenation in Aqueous Medium

“…Notwithstanding in their potential applications, not many catalyst systems have been reported which can carry out such reactions in aqueous medium. Reported catalyst systems that can carry out acceptorless dehydrogenation of alcohols to the corresponding aldehyde or ketone in water are Cp*‐Ir catalyst systems in neutral medium by Fujita,, Fukuzumi and their co‐workers, and a bimetallic Rh−Rh catalyst under basic condition by Wang and co‐workers . Though ruthenium in water is known to produce aldehyde as intermediate during the oxidation of primary alcohol to the corresponding carboxylic acid, there is no report available for the acceptorless dehydrogenation of alcohols to aldehydes or ketones as final products in aqueous medium .…”

A Computational Study on Iridium‐Catalyzed Production of Acetic Acid from Ethanol and Water Solution

“…In 2012, Fukuzumi and co‐workers reported a [C,N] cyclometalated Ir complex, which could catalyze aliphatic alcohols dehydrogenation at room temperature in a basic aqueous solution . Huang and co‐workers performed a detailed computational study on this catalytic system, and found that the inner sphere mechanism is more feasible than the outer‐sphere mechanism …”

Pentamethylcyclopentadienyl Iridium(III) Metallacycles Applied to Homogeneous Catalysis for Fine Chemical Synthesis

“…In a basic aqueous solution, aquo complex 13 was initially shown to be equilibrated with complex 14 and hydroxo complex 15 . Interestingly, complex 15 catalysed the dehydrogenation of alcohols and led to iridium hydrido species 16 , which could subsequently reduce NAD + molecules with a turnover frequency (TOF) up to 18 h −1 and regenerate complex 15 …”