Catalytic Generation of Hydrogen with Titanium Citrate and a Macrocyclic Cobalt Complex

Hydrogen evolution from acidic aqueous solutions of TiIIIcitrate is strongly catalyzed by Co(dmgBF2)2. The reaction generates an intermediate with maximum absorbance at 770 nm. The slow disappearance of this intermediate takes place simultaneously with the generation of H2 in a process that was most efficient at pH 1.6 (turnover number 53). The loss of the catalytic activity is caused by the loss of the macrocyclic ligand and formation of Coaq2+. Control experiments implicate CoIII as the most likely oxidation state responsible for catalyst destruction, and thus provide indirect evidence for the involvement of CoIII in the catalytic cycle. Taken together, the data suggest that hydrogen generation takes place at least in part by the H+/HCoIII(dmgBF2)2 route. Incitrate‐containing solutions at 7 ≤ pH ≤ 8, the protonation of CoI(dmgBF2)2– to yield HCoIII(dmgBF2)2 has a rate constant kH = 1.4 × 106 M–1 s–1. This reaction is about ten times slower in the absence of citrate. The complex CoII(dmgBF2)2 catalyzes hydrogen evolution from acidic aqueous solutions of TiIII citrate. The main H2‐generating pathway appears to involve attack by H+ or protonated citrate at a hydridocobalt(III) intermediate.