Understanding C-C agostic interactions – a joint experimental / theoretical challenge – THEOEXP07

1- Scientific background and objectives. The activation of strong and inert bonds such as CH and CC bonds is a current scientific, economical and environmental challenge. The catalytic activation of CH bonds has made significant progress but CC bond activation is even more challenging due in part to kinetic (steric) problems such as accessibility to the metal. CH activation reactions are very often assisted by secondary interactions between the metal and the substrate. Such interactions are at the heart of enzymatic activity or supramolecular chemistry. In organometallic catalysis, CH agostic interactions (3 centre- 2 electron bond) play a similar role. They assist the migratory insertion of an alkene into an M-C bond (olefin polymerisation by soluble catalysts) or they stabilise the catalyst resting sate for example. They also model metal mediated CH bond activation. The understanding of their nature (still under debate) by experimental and theoretical means has allowed a detailed knowledge of catalyst activity thereby leading to an improvement of their properties. We have recently described a C-C agostic system among the five examples known to date. We propose herein a multidisciplinary project aimed at understanding these C-C agostic interactions with the joint, synergistic efforts of leading groups in synthetic organometallic chemistry, in experimental determination of electron density by X-ray diffraction and in theoretical chemistry. Consequences in terms of reactivity will be briefly considered. 2- Description of project, methodology. Following our detailed studies of CH agostic interactions, we have recently characterized a rare C-C agostic interaction in a cyclopropyl niobium complex supported by an hydrotris(pyrazoly)borate TpMe2 ligand [TpMe2NbCl(c-C3H5)(MeCCMe)]. In the crystal, one of the CC bonds of the C3 ring, that oriented towards Nb, is significantly elongated. QM/MM calculations indicate that the preferred interaction with the empty Nb d orbital is due to the destabilization of the C-C bonds due to poor overlap between C-C orbitals in the C3 ring. There is no CH agostic interaction. To date, we have no spectroscopic signature of the C-C agostic interaction. In this project, we will synthesize a whole family of cyclopropyl complexes based on systematic variations of the electronic properties of isosteric Tp' ligands. A relationship between the electrophilicity of the Nb complexes and the strength of the C-C agostic interaction will be sought. By NMR in solution, and despite the low abundance of 13C, we will analyze (in addition to chemical shifts), the 1JCC coupling constants in the C3 ring, looking for a characteristic decrease as a signature. Systematic determination of X-ray structures on single crystals will be carried out with the same goal. These spectroscopic and structural data, which give an indirect view of the interaction, will be probed thanks to DFT calculations. In order to get an intimate view of the interaction, experimental electron density measurements will be carried out. It is now possible to experimentally determine electron density (ED) with high accuracy with X-ray diffraction even when heavy elements are present. Significant results have already been obtained in the context of CH agostic interactions. High quality crystals of all possible new complexes will be studied, either on conventional X-ray diffractometers or on synchrotron if necessary, and the topology of the ED will be analyzed thanks to methods like AIM. The experimental topology will then be compared to that calculated through quantum calculations. It is thus crucial to gather leading groups in their fields (Etienne, organometallic chemistry and agostic interactions; Lecomte and Lugan, experimental determination of ED using X-ray crystallography; McGrady and Maseras, quantum calculations) for this unified project. Fruitful collaborations already exist between some partners or collaborators, and this testifies to strong commitments and interests in the project. 3- Expected results. Our multidisciplinary approach combining synthetic chemists, crystallographers and computational chemists will yield an accurate view at the most intimate level of what appears to be a new type of bonding situation: an interaction between a metal and C-C bond. This is a major scientific challenge. This project will also be a first step towards the stœchiometric and catalytic activation of strong and inert C-C bonds.