CE07 - Chimie moléculaire

Mixed (Anti)Aromaticity, Unorthodox Interactions and Topologies – MixAr

Mixed (Anti)Aromaticity, Unorthodox Interactions and Topologies

The MixAr project aims at expanding the knowledge concerning antiaromaticity. Our approach explores the mixing of (anti)aromaticity, unorthodox interactions and topologies following 3 main axes: the study of “mixed (anti)aromaticity” with antiaromatic species forced to “interact” with each-others or with aromatic ones. Probe “unorthodox interactions” of molecules with antiaromatic 4n p-system to target antiaromatic catalysis. Access to “[4n+2] Möbius antiaromaticity”, of fundamental interest.

MixAr objectives

A first objective of MixAr is to evaluate the behavior of antiaromatic compounds forced to «interact« with other antiaromatic species or mixed with aromatic compounds (mixed [anti]aromaticity) in order to better understand the delocalization of antiaromaticity (through space and bonds). The conversion to «3D aromaticity« and the propagation of antiaromaticity are, for example, phenomena that we wish to observe and control. They would strongly stimulate the design of information storage and transfer devices.<br />A second objective of MixAr is to evaluate the ability of a 4n-electron p-antiaromatic system to interact with other compounds (unorthodox interactions), as this would open new opportunities for molecular recognition or even catalysis. Indeed, influencing the outcome of a reaction according to the (anti)aromatic character of a catalyst is a challenge that has never been attempted before.<br />A third objective of the MixAr project is the synthesis and study of [4n+2] p-electron antiaromatic molecules in the Möbius configuration, a difficult task of great fundamental interest limited to rare cases, as most of the antiaromatic compounds reported to date adopt a planar conformation.

Our teams have pioneered the synthesis of hybrid hexaphyrin-cyclodextrin structures that together form a new class of highly versatile compounds. Indeed, this unique design based on triply linked units affords chirality, a tightly associated confined space and a global conformational restriction of the hexaphyrin, stabilizing an antiaromatic state. In this proposal, we wish to maintain chiral communications and an associated cavity, but we also wish to get more flexibility in the hybrid structures to be able to switch between antiaromatic and aromatic character either by redox processes or conformational adaptation (Möbius twist) triggered by external stimuli. Very recently, we have also developed hybrid structures based on doubly linked units, starting from a readily available A,D-modified cyclodextrin platform. These new hybrids show the best compromise in term of accessibility, conformation and aromaticity control, chirality transfer phenomena, coordination behaviour, and redox stability. These multi-functional molecules thus afford a cutting-edge toolkit to address the challenging issues of the current MixAr project. We thus propose to explore in parallel two complementary approaches, depending on the environment experienced by an antiaromatic ring :
(i) a first one focusing on through space interaction (3D arrangement). This part will be developed in Paris (scientific leader: M. Ménand) and will concern dimeric architectures allowing to bring hexaphyrins in close proximity (“face-to-face” design). This approach will address the concerns identified as “mixed (anti)aromaticity” and “unorthodox interactions”;
(ii) a second one exploring through bonds communications (in line arrangement). This part will be developed in Rennes (scientific leader: S. Le Gac) and will target meso-meso ethynyl linked hexaphyrins. This approach will address the concerns identified as “mixed (anti)aromaticity”.
For both approaches, a third concern identified as “[4n+2] Möbius antiaromaticity” will be explored in different arrangement.

Access to the sandwich dimer by [2+4] macrocyclization did not work and led to novel HCD structures (bent and figure-8 structures) whose potential is currently under investigation. We revised our strategy and used a more direct [2+2] macrocyclization, but ran into the instability of the species. Therefore, we are currently developing more stable counterparts. The current synthesis work is still within the time frame announced in the project (36 months). We hope to obtain the dimer very soon and continue the project according to the initial plan.
In parallel, a pathway to C6F5/alkynyl disubstituted tripyrrans has been validated on a small scale. We are working on the scale up to achieve the corresponding HCD-bisalkynyl structures, key compounds towards mixed aromaticity. An M2 trainee will be recruited soon to further our efforts.
Finally, the double-bridged HCDs have been synthesized on a large scale, and the Ni(II) and Pd(II) metalations have been successfully performed. Studies of the redox properties by cyclic voltammetry, spectroelectrochemistry (abs. UV-vis and circular dichroism), chemical oxidation (NMR spectroscopy) and by theoretical calculation (DFT) are promising. A chiroptical switch process under redox control has been validated (unprecedented) and the isolation and characterization of structures with antiaromatic Möbius character is in progress.

We are currently developing stable hexaphyrins homologues that we will soon metalate and conjugate to the cyclodextrins. As soon as the dimer structures are obtained, we will start the spectroscopic studies of the species alone and in interaction with other molecules.
As soon as we obtain C6F5/alkynyl disubstituted tripyrrans in sufficient quantity, we will carry out the corresponding HCD-bisalkynyl structures, key compounds towards a mixed aromaticity. An M2 trainee will be recruited soon to further our efforts.
Finally, for the double-bridged Ni(II) and Pd(II) HCDs complexes, we will complete the ongoing studies to determine the (anti)aromaticity of the systems in their different redox states.

F. Robert, B. Boitrel, M. Ménand, S. Le Gac, J. Porphyrins Phthalocyanines 2021, 25, 1022–1032.

Nature builds complex architectures and organizes them into sophisticated dynamic networks thanks to an intensive use of a limited set of weak and reversible interactions. Unorthodox ones recently joined the toolbox and changed the paradigm of device conception. Besides, whereas aromatic p-type interactions are ubiquitous, those originating from antiaromatic p-systems remain to be discovered. MixAr general aim is to lay down the foundations of conceptually new molecular systems taking advantage of antiaromaticity. Tuneable (anti)aromatic porphyrinoids- cyclodextrins edifices will be prepared and their recognition properties, combined to aromaticity reversal emerging from conformational (Möbius twisting) or stacking events (three-dimensional aromaticity), will be studied. While face-to-face species will be devoted to antiaromatic p-type interactions, oligomeric hybrids will target multiple (anti)aromaticity switching through iterative twisting events.

Project coordination

Mickaël Ménand (Institut Parisien de Chimie Moléculaire)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

ISCR INSTITUT DES SCIENCES CHIMIQUES DE RENNES
IPCM Institut Parisien de Chimie Moléculaire

Help of the ANR 405,093 euros
Beginning and duration of the scientific project: January 2021 - 48 Months

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