A computational mechanistic investigation of hydrogen production in water using the [RhIII(dmbpy)2Cl2]+/[RuII(bpy)3]2+/ascorbic acid photocatalytic system

Abstract

We recently reported an efficient molecular homogeneous photocatalytic system for hydrogen (H₂) production in water combining [Rh^III(dmbpy)₂Cl₂]⁺ (dmbpy = 4,4’-dimethyl-2,2’-bipyridine) as a H₂ evolving catalyst, [Ru^II(bpy)₃]²⁺ (bpy = 2,2’-bipyridine) as a photosensitizer and ascorbic acid as a sacrificial electron donor (Chem. – Eur. J., 2013, 19, 781). Herein, the possible rhodium intermediates and mechanistic pathways for H₂ production with this system were investigated at DFT/B3LYP level of theory and the most probable reaction pathways were proposed. The calculations confirmed that the initial step of the mechanism is a reductive quenching of the excited state of the Ru photosensitizer by ascorbate, affording the reduced [Ru^II(bpy)₂(bpy˙⁻)]⁺ form, which is capable, in turn, of reducing the Rh^III catalyst to the distorted square planar [Rh^I(dmbpy)₂]⁺ species. This two-electron reduction by [Ru^II(bpy)₂(bpy˙⁻)]⁺ is sequential and occurs according to an ECEC mechanism which involves the release of one chloride after each one-electron reduction step of the Rh catalyst. The mechanism of disproportionation of the intermediate Rh^II species, much less thermodynamically favoured, cannot be barely ruled out since it could also be favoured from a kinetic point of view. The Rh^I catalyst reacts with H₃O⁺ to generate the hexa-coordinated hydride [Rh^III(H)(dmbpy)₂(X)]ⁿ⁺ (X = Cl⁻ or H₂O), as the key intermediate for H₂ release. The DFT study also revealed that the real source of protons for the hydride formation as well as the subsequent step of H₂ evolution is H₃O⁺ rather than ascorbic acid, even if the latter does govern the pH of the aqueous solution. Besides, the calculations have shown that H₂ is preferentially released through an heterolytic mechanism by reaction of the Rh^III(H) hydride and H₃O⁺; the homolytic pathway, involving the reaction of two Rh^III(H) hydrides, being clearly less favoured. In parallel to this mechanism, the reduction of the Rh^III(H) hydride into the penta-coordinated species [Rh^II(H)(dmbpy)₂]⁺ by [Ru^II(bpy)₂(bpy˙⁻)]⁺ is also possible, according to the potentials of the respective species determined experimentally and this is confirmed by the calculations. From this Rh^II(H) species, the heterolytic and homolytic pathways are both thermodynamically favourable to produce H₂ confirming that Rh^II(H) is as reactive as Rh^III(H) towards the production of H₂.