Light-driven hydrogen evolution a novel pincer/no pincer mechanism including a possible concerted proton electron transfer

The road to efficient molecular catalysts for artificial photosynthesis requires a great deal of basic research to address the challenges of anthropogenic global warming. An unexpected result of such research is a novel pincer/no pincer mechanism discovered for the DFT-calculated catalytic cycle of...

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Veröffentlicht in:Energy advances 2024-01, Vol.3 (1), p.149-162
Hauptverfasser: Viertl-Strasser, Wolfgang, Pann, Johann, Pehn, Richard, Roithmeyer, Helena, Kopacka, Holger, Hofer, Thomas S, Thompson, Mark E, Mencke, Austin, Brüggeller, Peter
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Zusammenfassung:The road to efficient molecular catalysts for artificial photosynthesis requires a great deal of basic research to address the challenges of anthropogenic global warming. An unexpected result of such research is a novel pincer/no pincer mechanism discovered for the DFT-calculated catalytic cycle of PNP-C2 complexes. Structural parameters from theoretical modeling are supported by single crystal X-ray data for the corresponding complexes. Using the examples of [M( ii )Cl(PNP-C2-R)] + (M = Pd 2+ , Ni 2+ ; R = CH 3 , H; PNP-C2 = N , N -bis{(di(2-methoxy-phenyl)phosphanyl)ethyl}- N -alkylamine) this pincer/no pincer mechanism is supported by differential pulse voltammetric, cyclic voltammetric, UV-Vis spectroscopic (combined with voltammetry) and 31 P nuclear magnetic resonance spectroscopic studies. The efficiency of hydrogen evolution during artificial photosynthesis depends significantly on the ratio of photosensitiser and water reduction catalysts. At first sight, the results from irradiation experiments (gas chromatography and mass spectrometry) seem obvious, but show a considerable impact of secondary and tertiary amines within the pincer structure. The best result obtained for the palladium pincer complex was with a secondary amine as a proton relay, delivering a turnover number of 2237 after only 2.5 hours. In contrast the analogous water reduction catalyst with a methylated (tertiary) amine instead of a secondary one gives only 972 turnovers under the same experimental conditions, over the same time. For all photocatalytic measurements the amount of hydrogen is approximately 2.5-times higher when a secondary amine is used as the proton relay. The influences of these different amines on the activity of water reduction catalysts during irradiation experiments in H 2 O or D 2 O lead to evidence of a kinetic isotope effect, supporting a possible concerted proton electron transfer. DFT calculations of reaction enthalpies, activation energies and intrinsic reaction coordinates of the catalytic cycle support the high efficiency of the presented catalysts. In particular, results from secondary amines suggest new candidates as examples of the rarely known concerted proton electron transfer. New water reduction catalysts containing secondary amines in the backbones show turnover numbers for photochemical hydrogen evolution up to 2237.
ISSN:2753-1457
DOI:10.1039/d3ya00354j