Département de chimie moléculaire

The authors recently reported an efficient mol. homogeneous photocatalytic system for H (H2) prodn. in H2O combining [RhIII(dmbpy)2Cl2]+ (dmbpy = 4,4'-dimethyl-2,2'-bipyridine) as a H2 evolving catalyst, [RuII(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 Rh intermediates and mechanistic pathways for H2 prodn. with this system were studied at DFT/B3LYP level of theory and the most probable reaction pathways are proposed. The calcns. 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 [RuII(bpy)2(bpy ̇-)]+ form, which is capable, in turn, of reducing the RhIII catalyst to the distorted square planar [RhI(dmbpy)2]+ species. This two-electron redn. by [RuII(bpy)2(bpy ̇-)]+ is sequential and occurs according to an ECEC mechanism which involves the release of one chloride after each 1-electron redn. step of the Rh catalyst. The mechanism of disproportionation of the intermediate RhII species, much less thermodynamically favored, cannot be barely ruled out since it could also be favored from a kinetic point of view. The RhI catalyst reacts with H3O+ to generate the hexacoordinated hydride [RhIII(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 aq. soln. Besides, the calcns. showed that H2 is preferentially released through an heterolytic mechanism by reaction of the RhIII(H) hydride and H3O+; the homolytic pathway, involving the reaction of two RhIII(H) hydrides, being clearly less favored. In parallel to this mechanism, the redn. of the RhIII(H) hydride into the pentacoordinated species [RhII(H)(dmbpy)2]+ by [RuII(bpy)2(bpy ̇-)]+ is also possible, according to the potentials of the resp. species detd. exptl. and this is confirmed by the calcns. From this RhII(H) species, the heterolytic and homolytic pathways are both thermodynamically favorable to produce H2 confirming that RhII(H) is as reactive as RhIII(H) towards the prodn. of H2. [on SciFinder(R)]