Helicenic N-heterocyclic carbenes (helicenic NHCs): synthesis, structure, photophysical properties and catalytic activity – HEL-NHC

The HEL-NHC project involves three complementary research groups: 1) “Phosphorus and Molecular Materials” (PMM); 2) “Nanocatalysis and Molecular Catalysis for Fine Chemistry” (NMCFC) (both are belonging to the “Organometallics: Materials & Catalysis” team of the “Institut des sciences chimiques de Rennes” (ISCR, UMR CNRS 6226, identified as partner P1), and 3) the group “Molecular Design of Transition-Metal Pre-Catalysts” of the “Laboratoire de chimie de coordination du CNRS “ based in Toulouse (LCC, UPR CNRS 8241, identified as partner P2). The main goal of the HEL-NHC project consists in designing the very first helicene-bearing N-Heterocyclic Carbenes (NHCs) and in studying their steric, electronic, chiroptical (optical rotation, electronic and vibrational circular dichroism), and photophysical (non polarized and circularly polarized luminescence) properties, for applications in molecular materials science and in asymmetric catalysis. In this prospect, we propose to prepare a large variety of helicenic NHC ligands along with their transition metal complexes. The anticipated benefits are the following: i) The attachment of helicenic units onto carbenic cycles will generate electronically tunable pi-conjugated helicene-NHC ligands, thanks to the direct interaction between the pi-delocalized systems of both parts. These annelated helicene-NHC systems will be based on a benzimidazolylidene, dipyridocarbene or imidazo[1,5-a]pyridinylidene core. ii) The parallel grafting of a series of different helicenic parts onto the carbenic heterocycle through a sigma N-C bond will provide the target helicene-NHC complexes having diversified architectures, and will allow to investigate the influence of the helicenic core on the properties of the resulting complexes. iii) In addition, more exotic helicene-NHC structures based on an isoquinolinylidene are envisioned in order to study the influence of the nature of the monoaminocarbene on the electronic and reactivity profiles of the assembly. All these helicene-NHCs will be prepared in an enantiopure form. The coordination of the monodentate or bidentate chiral ligands on diverse metallic ions (Au(I), Pt(II), Ir(III), Ru(II) …) will give access to unprecedented metallic helicoidal architectures. These multifunctional molecules will combine the strong chiroptical properties of the helicene with the inherent properties of the metallic centre (coordination geometry, redox activity, electronic and photophysical propertis …). As NHC complexes are already known to display good abilities in luminescence, in switching, or in UV-vis and near IR spectroscopies, molecular materials such as chiroptical switches, chiral emissive materials or two-photons absorbers, are envisaged for applications in molecular electronics, chiral electroluminescent diodes, cryptography or imaging. In parallel, we will take advantage of the strong asymmetry combined with the extended tridimensional structure of the targeted helicene-NHCs to meet several important challenges in asymmetric catalysis, particularly in C-C bond forming reactions from polyene or enyne compounds, which give a direct access to chiral, possibly structurally complex, synthons present in natural and/or pharmaceutical compounds. The helicene-NHC ligands will be associated with diverse transition metals (Cu, Ni, Au) to build efficient and versatile catalytic systems, which will be implemented, for example, in regio-and stereo-selective allylic alkylation and cycloisomerization reactions. The complementary and internationally recognized skills of each collaborator in the various involved domains (chirality, pi-conjugated systems, helicenes, catalysis, NHCs, luminescence, CPL measurements, theoretical calculations on optical properties …) will be the key to success of the HEL-NHC project.