Multi-redox porous crystalline hybrid chalcogenides – THIOMOFS

Our strategy relies on the use of sulfur-based, 1,2-dithiolene type ligands. With their fully delocalized frontier orbitals, three accessible redox states and ability to form stable complexes with both 3d and 4d metal ions, they can be considered as ideal non innocent ligands (meaning that at least one frontier orbital of the derived complex involves both the cation and the p system of the ligand). This leads to unique electronic properties and complex electrochemical behaviors in solution, but such properties have been scarcely exploited in extended solids yet. Synthesis, structure resolution and physico-chemical studies of new coordination networks will then be the core of the project. It relies on the combination of (i) innovative synthetic strategies, (ii) the thorough determination of the crystal structures by a combination of advanced diffraction methods and spectroscopies, (ii) the in-depth investigation of the electronic properties by a combination of experimental and computational tools. Finally, the performance of these solids as electrode materials for ion-batteries and hydrogen evolution reaction (HER) will be evaluated, anticipating that their characteristics allow overcoming some limitations of O-based MOFs and inorganic sulfides for such applications.

The project effectively started Fall 2020. The work was initially focused on the synthesis of molecular precursors of various chemical nature and geometry. Their reactivity with metal precursors presenting a single accessible coordination site was evaluated, in order to (i) determine the most efficient proligand deprotection pathways (ii) identify the parameters controlling the reaction kinetics, and (iii) produce model molecular complexes, which will be used as references in spectroscopic studies. This study allowed us to conclude that (i) the presence of a conjugated core separating the dithiolene groups is needed for the complete deprotection of the proligand, and that (ii) the position of the dithiolene groups on this core strongly impacts the electronic communication between the metal centers. These results serve as a guide in the synthetic choices for the preparation of MOFs. First tests have identified two proligand-metal cation couples that seem to lead to polymeric compounds. However, these solids are amorphous, which makes their structural characterization difficult for the moment.

In the next year, our future work will focus on:
- the intensive exploration of the reactivity of the above-mentioned molecular building blocks, in order to produce polymeric coordination compounds. Improvement of the crystallinity will be the main target.
- the characterization of these polymeric compounds by different spectroscopic and diffraction techniques, which, coupled with numerical simulation, should allow producing first structural models.
Later on, we will extend our activities to the study of electronic properties and electrochemical and electrocatalytic activities, studies coupled if necessary to in situ characterizations in order to identify the redox centers involved in the processes.

- The potential of MOFs in the field of electrochemical energy storage. T. Devic, Metal-Organic Frameworks in Biomedical and Environmental Field, Ed. P. Horcajada and S. Rojas Macias, Springer Nature 2021, 111-154.

While zeolites or Metal-Organic Frameworks (MOFs) are suitable in numerous fields involving adsorption-desorption processes, their poor electronic conductivity, associated to the nature of the constitutive bonds involving the very electronegative oxygen, is a noticeable drawback in every application involving electron transfer. This project thus aims at bridging the gap between such insulating, porous materials and dense inorganic conductors, by focusing on the preparation and in-depth characterization of new crystalline hybrid organic-inorganic chalcogenides, with the final aim at combining in the designed solids porosity, electronic conductivity and multi-redox (cationic/inorganic and anionic/organic) activity. Our strategy relies on the use of sulfur-based, 1,2-dithiolene type ligands. With their fully delocalized frontier orbitals, three accessible redox states and ability to form stable complexes with both 3d and 4d metal ions, they can be considered as ideal non innocent ligands (meaning that at least one frontier orbital of the derived complex involves both the cation and the p system of the ligand). This leads to unique electronic properties and complex electrochemical behaviors in solution, but such properties have been scarcely exploited in extended solids yet. Synthesis, structure resolution and physico-chemical studies of new coordination networks will then be the core of the project. It relies on the combination of (i) innovative synthetic strategies, (ii) the thorough determination of the crystal structures by a combination of advanced diffraction methods and spectroscopies, (ii) the in-depth investigation of the electronic properties by a combination of experimental and computational tools. Finally, the performance of these solids as electrode materials for ion-batteries and hydrogen evolution reaction (HER) will be evaluated, anticipating that their characteristics allow overcoming some limitations of O-based MOFs and inorganic sulfides for such applications.