A Systematic Comparative Study of Hydrogen-Evolving Molecular Catalysts in Aqueous Solutions.

We describe here a systematic, reliable, and fast screening method that allows the comparison of H2-forming catalysts that work under aqueous conditions with two readily prepared chemical reductants and two commonly used photosensitizers. This method uses a Clark-type microsensor for H2 detection and complements previous methods based on rotating disk electrode measurements. The efficiencies of a series of H2 -producing catalysts based on Co, Ni, Fe, and Pt were investigated in aqueous solutions under thermal conditions with europium(II) reductants and under photochemical conditions in the presence of two different photosensitizers {[Ru(bipy)3]Cl2(bipy=2,2-bipyridine) and eosin-Y} and sacrificial electron donors (ascorbate and triethanolamine, respectively). The majority of catalysts tested were active only under specific conditions. However, our results also demonstrate the impressive versatility of a group of Co catalysts, which were able to produce H2 under different reducing conditions and at various pH values. In particular, a cobaloxime, [Co(dmgH)2(H2O)2] (dmgH2 =dimethylglyoxime), and a cobalt tetraazamacrocyclic complex, {Co(CR)Cl2}(+) [CR=2,12-dimethyl-3,7,11,17-tetraazabicylo(11.3.1)heptadeca-1(17),2,11,13,15-pentaene], displayed excellent catalytic rates under the studied conditions, and the best rates were observed under thermal conditions.