Photocatalytic Production of Hydrogen by Disproportionation of One‐Electron‐Reduced Rhodium and Iridium–Ruthenium Complexes in Water

Abstract

One at a time: A rhodium aqua complex and a heterodinuclear iridium–ruthenium complex (see picture) act as effective catalysts for the photocatalytic two‐electron reduction of protons. Hydrogen is produced with the aid of a photosensitizer and an electron donor by disproportionation of the corresponding one‐electron‐reduced metal complexes.

Product Selectivity in Homogeneous Artificial Photosynthesis Using [(bpy)Rh(Cp*)X]n+-Based Catalysts

“…Special attention will be given to slightly basic conditions, since the TONs for NAD + reduction have been observed to be higher by a factor of 100 in contrast to the TONs for hydrogen production using a heterobimetallic photocatalyst [99]. Based on other literature reports, this finding could be anticipated, since for the photocatalytic formation of hydrogen, an optimal pH value of 3.6 has been found [37]. Although under electrochemical conditions the lowering of the pH value was reported to increase the rate of hydrogen generation [34], for the formation of NADH, a maximum of catalytic activity was observed at neutral pH conditions [56].…”

“…A negligible formation of hydrogen was observed at pH values higher than five and lower than two, which indicates on the one hand, that a sufficient thermodynamic driving force for the proton hydride reaction needs to be generated, which can be achieved by a high proton concentration, and that on the other hand ascorbate rather than ascorbic acid acts as the electron donor [37]. Additionally, molecular hydrogen was also evolved by thermally produced [(bpy)Rh(Cp*)H] + in the presence of photoactivated platinum nanoparticles [71].…”

“…However, only turnover numbers (TONs) below 10 were found for the Rh catalyst and the Ru chromophore. [33][34][35][36][37][38], reaction (b) [39,40], reaction (c) [26,29,[41][42][43][44], reaction (d) [45][46][47][48][49][50][51][52][53][54][55], and reaction (e) [48,[56][57][58][59].…”

“…Access to the reactive [(NN)Rh I (Cp*)] intermediate is accomplished via a two electron reduction at a rather low cathodic voltage (ranging from −700 to −900 mV against Ag/AgCl depending on the N,N-chelating ligand) [51]. After the addition of one electron, the primarily obtained coordinatively unstable Rh II species disproportionates extremely rapidly, leading to the desired Rh I compound (for the majority of the applied complexes [64]), as this process is merely connected with the easy loss of a weakly bound monodentate ligand [33,37,65]. In addition, a crystal structure of the reduced d 8 intermediate [(bpy)Rh I (Cp*)] revealed that the bpy and the Cp* plane are lying perpendicular to each other, as depicted in Figure 4, different to the piano-stool like structure of the d 6 species [(bpy)Rh III (Cp*)X] n+ [66].…”

“…Steckhan and coworkers Identically to the photocatalytic generation of hydrogen, the catalytically active rhodium complex is most likely formed via the fast disproportionation of the intermediate Rh II species [33,37]. Since the underlying mechanism for the generation of the protonated active catalyst is similar with respect to the electrochemical and the photochemical approach, the same reactivity trends for different N,N-chelating ligands are assumed to be true.…”