Switchable molecular cages as catalysts for CO2 valorisation – SWITCHABLE CAGES

The construction of these complex molecular systems relies on the assembly of several molecular sub-components able to act either as substrate-binding sites or as metal chelators. DABCO has been efficiently used as a template to pre-organize the two different metalloporphyrin precursors in a coordination dimer to favor the intramolecular cyclisation reaction. We chose the Copper-catalyzed Alkyne Azide Cycloaddition to link covalently the metalloporphyrins, since, besides its efficiency, it provides triazole ligands that will be used as allosteric sites to control guest release or to regulate the catalytic activity of the cages.

The coordination of ditopic guests inside the metalloporphyrinic covalent cages will be explored, thanks to UV-visible and NMR spectroscopies. Photophysical measurements could also be performed. Then, the effect of the addition of metal ions to the metalloporphyrinic covalent cages will be monitored to check their ability to change the cage conformation and its affinity for the guests.

We will test the ability of the cages to catalyse transformations of carbon dioxide into valuable products, using gas chromatography, IR and NMR spectroscopies. These tests have to be carried out both with the open and closed forms of the cages, and with the corresponding precursors. If the open cage is catalytically active, and the closed counterpart inactive, the objective of preparing a switchable catalyst based on a chemical stimuli-responsive cage will be fully achieved.

A DABCO-templated Copper-Catalysed Alkyne-Azide Cycloaddition reaction afforded in good yields two flexible covalent cages 1 and 1bis, endowed with orthogonal recognition sites, 2 metalloporphyrins and 8 peripheral triazole ligands.

Coordination of 4 silver(I) ions to the peripheral triazoles induced in solution a large conformational change, locking the porphyrins in a face-to-face disposition. Ag(I) can be removed by precipitation of AgCl and the initial flattened conformation of 1 and 1bis were restored. Thus, Ag(I) acts as a chemical stimulus able to fix the cavity size and the porphyrins disposition.

These cages are also able to reversibly accommodate DABCO in a so-called induced-fit mechanism.

Besides, a coordination cage [Ag4(14)2]4+ has been quantitatively formed by self-assembly of silver(I) ions and zinc porphyrins with four appended peripheral triazolyl-pyridine ligands. The overall stability constant determined by UV-visible titration in a dichloromethane/methanol 9/1 solution is large, K = 5.0 × 10 26 M-5.

The encapsulation properties of the obtained cages have to be explored, both in the absence and presence of metal ions such as Ag(I). If noticeable differences of affinity are observed, this would prove that the cages can release guests upon a chemical stimulus.

Catalytic activity of the cages in their open conformation has also to be assessed. If the closed conformation of the cages is inactive, the cages could then be considered as switchable catalysts.

Besides, if the catalysed reaction involves carbon dioxide, it could be a way to valorise this cheap, abundant and renewable carbon source.

Articles

A1. L. Kocher, S. Durot, V. Heitz Chem. Commun., 2015, 51, 13181-13184, « Control of the cavity size of flexible covalent cages by silver coordination to the peripheral binding sites »

A2. P. Ballester, M. Claudel, S. Durot, L. Kocher, L. Schoepff, V. Heitz Chem. Eur. J. 2015, 21, 15339-15348, « A porphyrin coordination cage assembled from four silver(I) triazolyl-pyridine complexes »

International conferences

CI1. S. Durot, L. Kocher, V. Heitz, « Silver(I)-Assembled Porphyrin Coordination Cages : towards stimuli-responsive cages with a size-controlled cavity », 10th International Symposium on Macrocyclic and Supramolecular Chemistry, Strasbourg (France), 28 juin-2 juillet 2015

CI2. V. Heitz, S. Durot, L. Kocher, J. Taesch, « Synthesis and properties of large and flexible porphyrin cages incorporating orthogonal functional groups », 6th EuCheMS Conference on Nitrogen Ligands, Beaune, 13-17 septembre 2015

CI3. L. Kocher, S. Durot, V. Heitz, Synthesis and coordination properties of a large and flexible cage », Student Winter Workshop Strasbourg/Hokkaido, Strasbourg (France), 14-15 mars 2016

CI4. S. Durot, « Stimuli-responsive cages with a size-controlled cavity », International Conference on Coordination Chemistry (42th ICCC), Brest (France), 3-8 juillet 2016

National conferences

CF1. L. Kocher, S. Durot, V. Heitz, « Cage moléculaire à taille de cavité contrôlée », GECOM CONCOORD 2015, Lyon (France), 26-29 mai 2015

CF2. L. Kocher, S. Durot, V. Heitz, « Synthèse d’une cage covalente flexible et contrôle de la taille de la cavité par coordination », Journées Scientifiques de l’Institut de Chimie de Strasbourg, Strasbourg (France), 29-30 octobre 2015

CF3. L. Schoepff, S. Durot, V. Heitz, « Synthèse et propriétés de coordination d’une cage covalente commutable », GECOM CONCOORD 2016, Obernai (France), 16-20 mai 2016

Developing nanoreceptors or nanoreactors, based on three-dimensional cage compounds in which the activity is controlled by an external stimulus and able to perform a defined function, is still a synthetic challenge which relies on the capacity of chemists to design and assemble multifunctional systems. Controlling the release of a guest from a molecular container or of a product formed within a nanoreactor is particularly important for controlled guest delivery or catalysis. Furthermore, switchable catalysis is a booming area, offering new promising perspectives to tailor chemical reactivity. Stimulus-responsive control of catalysis by artificial molecular devices, able to cut activity on and off or to switch products distribution, is highly desirable for better regulation of a wide variety of chemical reactions. Such a property of smart molecular systems is reminiscent of allosteric modulation of catalysis in some natural enzymes.

The aim of this project is to build artificial molecular machines based on dynamic cages incorporating peripheral regulation sites to trigger, under the action of an external stimulus, a reversible and large modulation of the cavity size, in order to perform controlled guest delivery and switchable catalysis. To date, only few artificial systems have reached the goal of converting mechanical motions into useful functions, and synthesis of such smart molecular architectures is never trivial and remains a stimulating challenge. An efficient synthesis requires a judicious choice of the precursors, permitting a limited number of steps, high yield reactions for each step, and ingenious methods to covalently assemble the different components. For the construction of the desired complex molecular systems of this project, we will take advantage of a templated multi-component strategy and of click chemistry for the efficient covalent cage closure step.

The flexible covalent cages will incorporate orthogonal active and regulation sites: metalloporphyrins for substrate binding within the cavity and ligands on the periphery to modulate the cage activity. The cavity of the flexible cages will shrink or expand in response to different chemical stimuli interacting with the peripheral ligands. We will explore the capacity of these controlled-dynamic cages to trigger the release of guests and to perform catalysis. The cages of this project will gather two metalloporphyrins in a face-to-face disposition and in dynamic structures that allow to bring them closer or to move them away reversibly, by coordination or removal of metal ions to peripheral ligands. These unique systems have indeed an appealing potential as on/off switchable catalysts that will be exploited. The two porphyrins metallated with Al(III) will be the catalytic sites. Closing the cage will hide one face of each porphyrin and prevent the porphyrin reactivity whereas opening the cage will trigger the catalytic activity. One reaction of interest is the catalytic carbon dioxide activation since carbon dioxide, an air pollutant and industrial waste, is also considered as a renewable carbon source. This project will thus give access to new artificial molecular machines able to perform specific tasks, like controlled guest release and switchable catalysis.

This fundamental research project, which is strongly bio-inspired from various viewpoints, has strong connections with the themes « molecular machines », « molecular cages », « supramolecular chemistry », « switchable receptor » and « switchable catalysis ».