A computational mechanistic investigation of hydrogen production in water using the [Rh^III(dmbpy)₂Cl₂]⁺/[Ru^II(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.

Hydroxyl-Decorated Diiron Complex as a [FeFe]-Hydrogenase Active Site Model Complex: Light-Driven Photocatalytic Activity and Heterogenization on Ethylene-Bridged Periodic Mesoporous Organosilica

“…The Xe lamp provided a higher light intensity illuminated on the active area of Once the experimental conditions of the photocatalytic reaction were optimized for homogeneous catalysis, the light source was replaced by a Xenon lamp with a higher irradiation intensity to evaluate the effect of this parameter on the photocatalytic performance. The Xe lamp provided a higher light intensity illuminated on the active area of the sample of 90 mW/cm 2 and an increased range of wavelength (λ > 420 nm) to cover most of the maximum absorption of the photosensitizer [Ru(bpy) 3 ] 2+ [56]. As can be seen in Figure 11, the FeFeOH complex reached the maximum TON of 70 after 6 h, which is amongst the highest reported catalytic performances of diiron catalysts under similar experimental conditions, using [Ru(bpy) 3 ] 2+ as the photosensitizer and ascorbate as the sacrificial electron donor (Table S1).…”

Probing the Diphosphine Ligand’s Impact within Heteroleptic, Visible-Light-Absorbing Cu(I) Photosensitizers for Solar Fuels Production

“…Our Cu­(I)/Rh­(III) PS/CAT photosystem is proposed to function via reduction quenching of the [Cu­(Xantphos)­(biq)] +, * ES by intermolecular electron transfer from a sacrificial electron donor (e.g., N,N′ -dimethylaniline, DMA) to produce [Cu­(Xantphos)­(biq – )], which possesses sufficient reducing potential ( E 1/2 (biq 0/– ) = −1.57 V) to undergo GS electron transfer to cis -[Rh­(Me 2 bpy) 2 ­Cl 2 ] + CAT ( E p c (Rh III/II/I ) = −1.46 V). Following photoinduced multielectron reduction to produce Rh­(I) CAT (either by 2Rh­(II) disproportionation − or sequential electron transfer steps , ) and subsequent protonation, catalytic amounts of H 2 were observed which increased linearly up to 1 h . In an effort to extend light absorption further into the visible, the Cu­(I) PS was modified to include electron-withdrawing substituents on the biq diimine ligand, [Cu­(Xantphos)­(dmebiq)] + (dmebiq = 2,2′-biquinoline-4,4′-dimethyl ester), which produced substoichiometric levels of H 2 under the same photolysis conditions (λ irr = 447.5 nm).…”

Tetra- and Heptametallic Ru(II),Rh(III) Supramolecular Hydrogen Production Photocatalysts

“…[56][57][58][59][60] Comparatively few multi-chromophore supramolecular photocatalysts have been tested for hydrogen production despite the improved light absorption per catalytic center. These include Ru,Rh,Ru complexes that function efficiently in fully aqueous media, [61][62]26 multinuclear Ru,Pt complexes, 63 and a metal organic cage with eight Ru chromophores and six Pd catalysts. 64 The Brewer group has investigated supramolecular photocatalysts coupling multiple Ru(II) chromophores to Rh(III) or Pt(II) catalytic centers in numerous arrangements including…”