Large Alkaline-Earths : From Fundamental Organometallic Chemistry to Green Catalysis – GreenLAkE

This project revolves around (i) the organometallic chemistry of large alkalino-earth elements (Ae: calcium, strontium and barium) and (ii) the design of efficient, well-defined catalysts based on such metal centers, to promote highly atom-efficient reactions, namely ring-opening polymerization (ROP) towards biodegradable materials and hydroelementation reactions.

Our first objective is to prepare and study – both from the experimental (NMR, X-ray diffraction crystallography) and theoretical (DFT) point-of-views – the reactivity and structural features of discrete heteroleptic complexes of the general formula [{TpF}M–Nu], where {TpF}- are original fluorinated mono-anionic multidentate tris(indazolyl)borate ligands and Nu– is a nucleophilic group such as alkyl, amido or alkoxide. The metals of focus will be Ca, Sr and Ba. Zn and Mg will be also studied as models since these elements have a closely related chemistry to Aes but yield kinetically more stable complexes. The ancillary ligands {TpF}- provide a convenient and versatile scaffold for the synthesis of a broad range of complexes. According to the needs (both in terms of nature of the metal centre and future catalytic applications), the ligands can be finely tuned through synthetic procedures already developed by the group in Toulouse in order to confer varying degrees of steric protection and electronic density to the metal centers. A special attention will also be given to the nature of the Nu– group, as it is this nucleophile which is directly involved in the targeted catalytic processes to be developed by the Rennes group. Specific [{TpF}M–NR2] amido complexes will be isolated for the study of hydroamination reactions, whereas discrete alkoxy derivatives [{TpF}M–OR] will be developed for ROP, as they mimic the nature of the active species found during the course of the polymerization. Stable [{TpF}M–R] alkyl compounds will be studied essentially for their fundamental interest, as the organometallic chemistry of heavier group 2 metals remains virtually unknown to this date, mostly due to the high reactivity (basicity) of the alkyl group in such species. A special emphasis will also be placed upon the study of homoleptic amido Ae[NEE’]2(S)n and hydrocarbyl AeR2(S)n precursors required for the synthesis of the desired heteroleptic [{TpF}M–Nu] complexes.

Expanding from these fundamental studies, our second objective will be to develop a new class of environmentally benign catalysts capable of performing both polymerization and hydroelementation reactions. Highly active and productive catalysts (metal loading 0.1-1%) will be prepared and the mechanisms of the catalytic reactions will be investigated. Alkoxide complexes [{TpF}M–OR] will be investigated as promoters for the ROP of monomers such as lactide and beta-lactones, with the aim of controlling the polymerization in terms of molecular weights and microstructure. Also, complexes with highly stabilizing {TpF}- ligand cores will be designed to allow the immortal ROP of cyclic esters in the presence of a large excess of an external protic transfer agent. This will open the way to truly catalytic systems in which each metal center will yield hundreds of polymer chains from several thousands of monomer equivalents. The amido- and alkyl- [{TpF}M–Nu] complexes will be tested for the intramolecular (and, when successful, intermolecular) hydro-amination, -silylation, and -phosphination reactions of model unsaturated substrates (alkenes, alkynes).

Owing to the ambitious nature of this proposal, the work program is distributed over a period of 48 months. In addition to the permanent members of staff involved in the proposal, two full-time PhD students will be recruited: one will work with partner 2 in Toulouse (months 1-36), and another one with partner 1 in Rennes (months 13-48). Their starting time will be shifted by one year, in accordance with the scheduled scientific program and respective tasks allocated to the partner teams.