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

Abstract

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.

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

“…[5a] Among the two cobaloximes, 1 showed superior catalytic activity than 2 with both reductants, especially with Eu II -EGTA. [19,21] Our results are the first clear demonstration that the BF 2 -annulated cobaloxime 2 is capable of acting as ac ompetent H 2 -evolving catalysta tn eutral pH. [19] However, this is in line with previous reports that stronga cids are required for catalysis with 2 in CH 3 CN or DMF [10a, 20] and that an acidic pH is required in aqueous solutions.…”

Mechanistic aspects of photocatalytic activity of metalloporphyrin–titanium mixtures in microemulsions

“…ZnTPP at the interface may accept electrons from the complex or be activated directly by irradiation. The ZnTPP activation may occur similarly to the activation of a macrocycle cobalt complex caused by titanium citrate in aqueous solution as previously reported, 38 where the cobalt complex is reduced to its lowest oxidation state, +1, from where the reaction starts. In the system with ZnTPP, the light promotes electrons to its excited state (LUMO) and the titanium complex exists as titanium (III) citrate; therefore, a redox reaction starts: oxidation to titanium (IV) citrate with the HOMO of the porphyrin and reduction of any organic compound with the electron in the LUMO of the porphyrin.…”

Splitting Water with Cobalt

“…[86] A Co-H stretching band is observed at 2240 cm À1 in the IR spectrum; this band is observed at 1680 cm À1 for the deuteride analogue. The reported 1 H NMR chemical shift of the hydride ligand is d = 6.0 ppm, which is a surprising value for a formal hydride ligand but not unprecedented among transition metal hydrides [87][88][89] [90] Other hydride derivatives have been prepared or characterized in the course of pulse radiolysis experiments. These include studies with cobalt [14]diene-N 4 macrocyclic cobalt complexes, including 1 [91,92] and aqueous CoSO 4 /2,2'-bipyridine mixtures, generating [Co(bipy) 2 …”