To reduce the atmospheric accumulation of CO₂, a major cause of global warming, it would be necessary to massively and durably transform CO₂ into fuel or synthetic bricks, a real challenge for chemists. To address this, they can draw inspiration from particularly effective natural metallo-enzymes, such as carbon monoxide dehydrogenase, which reversibly reduces CO₂ to CO. In this context, the Photocatalysis and Biohydrogen team led by Winfried Leibl (SB²SM), in collaboration with ICMMO*, has been developing, for several years, a family of bio-inspired catalysts of the iron porphyrin type, particularly promising for the catalytic electro-reduction of CO₂ (see Joliot highlight of 2020).

In enzymes, according to the work of Arieh Warshel, Nobel laureate in chemistry in 2013, through-space electrostatic interactions play a major role in the activation of substrates, by stabilizing intermediates. What about bio-inspired catalysts? To answer this question, in this study, the researchers designed and synthesized a series of iron porphyrins, mono- and tetra-substituted by cationic imidazolium groups in the second coordination sphere. They also compared the effects of through-space electrostatic interactions to through-structure electronic effects by synthesizing a derivative containing a single imidazolium and six fluorine atoms in the form of electron withdrawing groups (Figure).

The study of the catalytic properties (over-potential and TOF **) of these derivatives has demonstrated the additive effect of electrostatic interactions on the catalytic properties of iron porphyrins: while the over-potential of these catalysts decreases when the number of imidazolium units increases, a gain of six orders of magnitude for the TOFs is observed when switching from the tetra- catalyst to the monosubstituted catalyst. In parallel, the results obtained with the derivative comprising fluorine atoms indicate that the effects of through-space electrostatic interactions surpass the classical through-structure electronic effects.

In conclusion, as part of the quest for durable catalytic systems for CO₂ reduction, this work has improved our understanding of the structure-reactivity relationships of bio-inspired catalysts, necessary for obtaining the combination of chemical functions generating optimized catalytic properties.

This work has been published in ChemSusChem as a "Very Important Paper" and has been the subject of the journal's cover (issue 5/2021).

* ICMMO : Institut de Chimie Moléculaire et des Matériaux d'Orsay.

** The over-potential (in mV) corresponds to the additional electrical potential that must be applied in relation to the thermodynamic potential of the reaction. The TOF (turnover frequency) corresponds to the frequency of renewal of the reaction (in s-1).

Contacts : winfried.leibl@cea.fr ; ally.aukauloo@universite-paris-saclay.fr