Water reduction catalysts (WRCs)

Background

Within the larger LightChEC consortium, we are focusing on the proton reduction site of the full water splitting architecture. WRCs are in the center of this research and we are focusing in particular on new ligand frameworks based on (poly)-pyridyl frameworks. These ligands are multidentate and may be cyclic or acyclic. With some of these complexes as shown in the scheme below, we could achieve very high turnover numbers H2/Co and partly high but still far from ideal turn over frequencies or overpotentials and catalytic onset potentials respectively. The WRCs are generally cobalt-based. We functionalize the periphery of the ligands for altering the electronics at the metal center but also for chemosorbing the catalysts on various surfaces. This surface/interface study is done in close collaboration with groups from physics and physical chemistry. Studies are done first in homogeneous solution but in a second step, for approaching coupled systems, also bound to nano- and microparticles. By non-covalent adsorption on reversed-phase fumed silica particle, we could prove this principle in the photocatalytic system.

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Figure 1: WRC and PS non-covalently bound to reversed-phase fumed silica nanoparticles

Current Research

Beside the pyridine periphery in the above-mentioned ligand frameworks, the methylene groups in the cyclic chelators are particularly interesting since they allow the introduction of additional functionalities for interaction with the cobalt center. A number of new architectures have been prepared and their proton reduction activity is being studied. The same time, we are developing novel kind of macrocycles. The macrocycles offer several advantages over the acyclic complexes, namely their improved thermodynamic stability.

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Figure 2. ORTEP presentation of acyclic WRC [CoBr(appy)]+

 
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Figure 3. WRC based on a macrocyclic tetra-pyridyl ligand, doubly functionalized at the bridging methylene groups

Challenges

Rate of H2 formation at the H2 center – catalytic onset potential – lowering the overpotential without loss of stability – photochemistry of hetero-metal complexes addition of functionalities for strong surface binding – photochemistry of hetero-metal complexes –photosensitizers – theoretical understanding

Coworkers

Evelyne Joliat, Stephan Schnidrig, Peter Müller, Nico Weder (PhD students), Dr. Benjamin Probst

Selected publications

  • 3d Element Complexes of Pentadentate Bipyridine-Pyridine-Based Ligand Scaffolds: Structures and Photocatalytic Activities.Bachmann, C., Guttentag, M., Spingler, B. and Alberto, R. (2013): Inorg. Chem. 52, 6055-6061.
  • Photocatalytic proton reduction with ruthenium and cobalt complexes immobilized on fumed reversed-phase silica. Bachmann, C., Probst, B., Oberholzer, M., Fox, T. and Alberto, R. (2016): Chem. Sci. 7, 436-445.
  • Cobalt complexes of tetradentate, bipyridine-based macrocycles: their structures, properties and photocatalytic proton reduction. Joliat, E., Schnidrig, S., Probst, B., Bachmann, C., Spingler, B., Baldridge, K. K., von Rohr, F., Schilling, A. and Alberto, R. (2016): Dalton Trans. 45, 1737-1745.