Catalytic Properties of Molecular Systems Combining Porphyrins and N-Heterocyclic Carbene-Metal Complexes – COmPaCt

In the context of increasing energy demands and environmental problems, using photocatalysis and electrocatalysis to obtain valuable chemicals or fuels is receiving increasing attention and will probably become a ubiquitous challenge in the future. Porphyrins and N-heterocyclic carbenes (NHC) are two important classes of ligands in the field of catalysis, because they are able to form strong bonds with transition metals ions in low and high oxidation states. The proposal “COmPaCt” aims at combining these two kinds of ligands within unimolecular systems and to study their catalytic, photocatalytic and electrocatalytic properties. Different NHC-metal complexes will be synthesized to cover a broad range of possible reactions. The opportunity to observe cooperative effects between porphyrins and NHC-metal complexes will notably be investigated according to their binding modes. In a first time, we will investigate the modulation of the electronic and catalytic properties of NHC-metal complexes by tuning the electronic properties of the neighbouring porphyrins in different ways: coordination of metal ions in porphyrin cores, protonation of free base porphyrins, oxidation/reduction of (metallo)porphyrins, pi-stacking interactions with other pi-conjugated molecules. This study will allow for the identification of molecular systems offering null, weak or strong interactions between porphyrins and peripheral NHC-metal complexes. In a second time, these compounds will be used for applications in photocatalysis. Several systems combining photocatalysis and transition metal catalysis were reported in the literature, but none of them uses simultaneously porphyrins and NHC-metal complexes. Here, porphyrins will be used as visible-light harvesting units able to perform photoinduced energy and/or electron transfer reactions with the peripheral NHC-metal complexes. Different light-driven chemical reactions will be investigated like C-C and C-N cross-coupling reactions or small molecules activation reactions (CO2 reduction, proton reduction, H2O activation reactions), with the aim to see if synergistic effects can be observed when combining porphyrins and NHC-metal complexes within all-in-one unimolecular systems. These synergistic effects could yield photocatalytic systems with enhanced efficiency or having new or unexpected reactivity compared to the corresponding bimolecular systems. Advanced photophysical studies using different steady-state and time-resolved techniques will be performed to understand the mechanisms at work. Finally, the versatility of the chemistry of porphyrins and NHC ligands allowed us to obtain cofacial porphyrin dimers assembled from NHC-metal bonds. As a part of this project, we are seeking to explore the possibility of using these new molecular objects for applications in electrocatalysis. Cofacial cobalt- and iron-porphyrin dimers will be synthesized and their electrocatalytic properties towards O2 and CO2 reduction reactions will be investigated. The nature of the NHC ligands and the assembling metal ions will modulate the porphyrin-porphyrin distance, a very important parameter to ensure that both face-to-face metalloporphyrins can act in a cooperative way.