Photochemical hydrogen production and cobaloximes: the influence of the cobalt axial N-ligand on the system stability

Abstract

We report on the first systematic study of cobaloxime-based hydrogen photoproduction in mixed pH 7 aqueous/acetonitrile solutions and demonstrate that H2 evolution can be tuned through electronic modifications of the axial cobalt ligand or through introduction of TiO2 nanoparticles. The photocatalytic systems consist of various cobaloxime catalysts [Co(dmgH)2(L)Cl] (L = nitrogen-based axial ligands) and a water soluble porphyrin photosensitizer. They were assayed in the presence of triethanolamine as a sacrificial electron donor. Optimal turnover numbers related to the photosensitizer are obtained with electron-rich axial ligands such as imidazole derivatives (1131 TONs with N-methyl imidazole). Lower stabilities are observed with various pyridine axial ligands (443 TONs for para-methylpyridine), especially for those containing electron-acceptor substituents. Interestingly, when L is para-carboxylatopyridine the activity of the system is increased from 40 to 223 TONs in the presence of TiO2 nanoparticles.

Towards Noble‐Metal‐Free Dyads: Ground and Excited State Tuning by a Cobalt Dimethylglyoxime Motif Connected to an Iron N‐Heterocyclic Carbene Photosensitizer

“…Unfortunately, the stretching vibration of Co‐N pyridine cannot be analyzed since it is overlapped by the signals of Fe‐N vibrations. Nevertheless, this suggests a decreased basicity of the pyridine ring of the iron fragment 1 in comparison to isolated pyridine . On the other hand, vibrations of 1 are not significantly influenced, either by methylation in complex 2 or by the coordination of the cobalt center.…”

“…Realizing photocatalytic proton reduction as partial reaction of the complete sun‐driven water splitting process is crucial to allow sustainable production of hydrogen. So far, numerous model systems based on ruthenium, or iridium photosensitizers in combination with catalysts containing platinum,, cobalt, or iron have been employed. A definite disadvantage of such systems is the use of noble metals in the active photosensitizing and catalyst components, associated to unfavorable economic and ecological reasons.…”

“…To the best of our knowledge, 3 is the first example of a bimetallic complex connecting a NHC‐ligand based iron(II) complex with a cobalt(III) dmgH motif. The former are promising candidates for iron‐based photosensitizers, the latter are known to be effective catalysts in photocatalytic water reduction processes , . Thorough characterization by means of absorption spectroscopy, Valence‐to‐Core X‐ray emission spectroscopy (VtC‐XES), High Resolution Fluorescence Detected X‐ray Absorption Near Edge Structure (HERFD‐XANES), cyclic voltammetry (CV) and transient optical absorption spectroscopy insights into the ground and excited state properties are given in order to pave the way to future photocatalytic applications.…”

A Smorgasbord of 17 Cobalt Complexes Active for Photocatalytic Hydrogen Evolution

“…[19] At high concentrations,t he cobalt catalystd rives the photocatalytic turnover for hydrogen generation. [20] In water, TOFs and TONs are reported with respectt oc obaltc atalyst, that is, TOF cat. and TON cat.…”

Tuning the Electron Storage Potential of a Charge‐Photoaccumulating RuII Complex by a DFT‐Guided Approach

“…Most of these transformations involvet he transfer of multiple electrons, and significant progress has been made in developing molecular catalysts able to perform such kinetically arduous multi-proton multi-electronp rocesses. [5][6][7][8] Widely used photosensitizers (PS) for solarf uelp roduction include Ru II [9][10][11] and Ir III [12,13] polypyridyl coordination complexes, porphyrins, [14][15][16] and organic push-pull dyes. [17][18][19] While these chromophoresf eature broad absorption bands with relatively high extinction coefficients in the visible region, they fundamentally generate one photoexcited electron at at ime.…”