Since the beginning of the project, we have been working on making molecules with two central atoms, and shown their ability to activate the dinitrogen molecule in order to make it react under very mild conditions. We have demonstrated some mixed metal and main group as well as hetero-bimetallic combinations are indeed able to activate the dinitrogen molecule. We have attached importance to ease of production of these molecules, as well as their complete description in terms of spectroscopic methods, in particular by nuclear magnetic resonance and x-ray diffraction.
Underpinning our research is the concept of cooperativity. By the combination of an element rich in electron (a "donor") and another one poor (an "acceptor"), we could eveidence that the interference of the two elements with dinitrogen led to significantly enhanced activation thereof, through a "push-pull" or "donor-acceptor" mechanism. This under-explored way for N2 activation has been the object of 2 reviews written by us (Simonneau et al., Chemistry — A European Journal, 2018 and Coffinet et al., Encyclopedia of Inorganic and Bioinorganic Chemistry, 2020). Moreover, we have shown those systems to be competent for the building of bonds between dinitrogen and elements such as silicon and boron. Additionnally, our research on cooperative activation of N2 have led us to explore uncommon acceptor elements. We thus described the first N2 activation donor-acceptor system incorporating gold and demonstrated the importance of the positive charge on the acceptor (Specklin et al., Inorganic Chemistry 2021), as well as the influence of a ditopic acceptor (i.e. that carries two electron-poor sites), which was never achieved before. In this last case, the activation of dinitrogen went beyond expectations, since the small molecule was transformed into a dianion (doubly negatively charged species).
A final aspect of reactivity we explored with those systems was their reaction with hydrogen. We have found that our donor-acceptor N2 activation systems were able to activate dihydrogen, thus gathering all the ingredients to make ammonia within the same molecule, under ambient temperature and pressure. Some reactions were attempted with the aim to promote the formation of bonds between dinitrogen and hydrogen, but we were not successful. Instead, we show that it remains possible to build bonds between dinitrogen and silicon or boron. This study also led to the discovery of metal-hydride species with high hydrogen loadings (Queyriaux et al., European Journal of Inorganic Chemistry). In the frame of future projects we will explore deeper their porperties.
Another aspect of the OrFuNCo project is to study the possibility of creating bonds between dinitrogen and carbon by using "organometallic" reagents (reactive organic species). We prepared all-metal donor-acceptor N2 activation systems (Le Dé et al., Journal of Organometallic Chemistry 2023)
with the aim of using them as platform to promote dinitrogen carbon-bond formation. We finally manage to embed organometallic species in those systems, but the reactivity studies were difficult to rationalize — we could reproducibly evidence the scission of the strong N-N tripple bond. From this perspective, we were also interested in the reactivity of dinitrogen with organolithium reagents (reactive species bearing a lithium-carbon bond) that was reported in the literature because it was somehow comparable to what we anted to achieve in this project. Reproduction of those results led us to rule such a possibility, but the team gained a thorough expertise in mechanistic investigations along this study (Bouammali et al., European Journal of Inorganic Chemistry and Le Dé et al. Angewandte Chemie International Edition 2023).
