Conference Agenda

Rational and creative design of organic and metal building blocks has successfully enabled the genesis of a variety of coordination polymers or metal-organic frameworks (MOFs) that are of fundamental scientific importance as well as provide a myriad of practical applications including gas storage and separation, catalysis, and sensing. One of the most attractive features in MOFs is flexibility because they show distinctive properties that cannot be achieved with rigid MOFs and other porous inorganic materials. In this talk, we will present strategies that exploit flexible MOFs for effectively separating hydrogen isotopes through the dynamic pore change of flexible MOFs. Especially, a unique isotope-responsive breathing transition of the flexible MOF was studied, which selectively recognize and respond to only D₂ molecules through a secondary breathing transition, monitored by in situ neutron diffraction experiments.

Biomacromolecules, such as enzymes, are ubiquitous in nature and essential for maintaining basic life activities. Apart from the fundamental biological functions, biomacromolecules are also of great values in industrial applications, especially in food and pharmaceutical production. However, their industrial applications are often handicapped by low operational stability, poor robustness, difficult recovery and reuse. Incorporation of biomolecules within protective exteriors has been proved to be an effective method to promote their stabilities and applications. As new classes of crystalline solid-state materials, porous frameworks materials (such as covalent-organic frameworks, COFs and metal-organic frameworks, MOFs) feature high surface area, tunable pore size, high stability, and easily tailored functionality, which entitle them as ideal supports for encapsulation of biomolecules to form novel composite materials for various applications. Moreover, the formed composites can combine the properties of both constitutes, where crystalline frameworks materials and biomolecules are indeed mutually beneficial. Our researches mainly focus on their biocatalysis, separation and medicinal applications. This novel crystalline platform composed of biomolecules-incorporation and framework materials exhibited various functionality and superior poteintials in biocatalysis, bioseparation, and biopharmaceutical formulations.

Supramolecular chemistry places focus on the weak non-covalent interactions between molecules in the solution or solid phases. These “soft” interactions are reversible and allow one to build materials which are responsive to their chemical or physical environment, changing form and properties under the influence of heat, light, pressure or chemical probes.

Dynamic materials, capable of responding to their environment, require flexibility which may be achieved using interactions such as hydrogen- or halogen-bonding or through the use of suitable metals and ligands in coordination compounds. Frameworks may be made up of relatively strongly bound entities such as those that make up metal-organic frameworks (MOFs) or may be more loosely bound such as host-guest systems where the host molecules crystallise as independent entities but leave spaces which can accommodate guest molecules. The process of guest exchange within porous solids can be used in a range of applications, such as selective absorption or separation of gases and heterogeneous catalysis. Among the more interesting examples of dynamic processes in frameworks are those which result in thermochromic and/or mechanochromic effects. Materials of this type are particularly of interest if they are able to revert to their original state on application of another external perturbation signal. Rational design of such systems remains a challenge however, and is thus an exciting area for application of crystal engineering principles.

In this presentation, examples from recent work in our laboratory will be presented, including MOFs and 3D hydrogen bonded frameworks constructed from the same flexible ditopic ligands. The influence of halogen versus hydrogen bonding on a molecular host-guest system will also be described. Frameworks exhibit thermochromic and mechanochromic properties, depending on the application of external stimuli such as heat, grinding or exposure to solvent vapours. Further examples will include the selective inclusion of halogenated volatile organic compounds in a porous metal-organic framework (Fig. 1).

The use of mechanical bonds for the synthesis of catenanes is a challenging process because of the many factors controlling the interpenetration process.[1,2] We report the kinetic control in the presence of various aromatic solvents of a poly-[n]-catenane (1). The polymeric structure is composed of interlocked M₁₂L₈ icosahedral nanometric cages with internal voids of ca. 2500 ų.[3] Using the symmetric exotridentate tris-pyridyl benzene (TPB) ligand and ZnCl₂ with appropriate templating solvent molecules due to the good ligand aromatic interactions are used, the metal-organic nanocages can be synthesized very fast, homogeneously, and in large amounts as microcrystals (Figure 1). Synchrotron single-crystal X-ray data (100 K) allowed the resolution of nitrobenzene guest molecules at the internal walls of the M₁₂L₈ cages, while in the centre of the nanocages the solvent is disordered and not observable by X-ray diffraction data. The guest release occurs in two steps with the disordered nitrobenzene released in the first step (lower temperatures) because of the lack of strong cage-guest interactions. Solid-state quantum mechanics provided a rationalization of the results, in particular, solid-state approaches, showed theoretical evidence of the kinetic nature in the formation of the polycatenation of the M₁₂L₈ nanocages by the analysis of the packing energy considering monomeric and dimeric cages.

Figure 1. Synthesis of the M₁₂L₈ interlocked nanocages forming the poly-[n]-catenane 1 under aromatic control.

[1] J. F. Stoddart (2009). Chem. Soc. Rev. 38, 1802-1820.

[2] Frank, M., Johnstone, M. D. & Clever, G. (2016). Chem.- Eur. J. 22, 14104-14125.

[3] Torresi, S., Famulari, A. & Martí-Rujas, J. (2020). J. Am. Chem. Soc. 142, 9537-9543.

The synthesis and conception of coordination polymers and supramolecular networks based on gold(I) complexes used as metallo-ligands (especially dicyanoaurate) is an established procedure to obtain materials with exciting properties: phosphorescence, non-linear optical behaviour, vapochromism and non-classical response to temperature and pressure [1-2]. However, the appearance of these solid-state properties is often connected to the manifestation of aurophilic interaction. The Au(I)‧‧‧Au(I) interaction, an attraction between closed shell d¹⁰ metal centres, is a relativistic effect that has a strength comparable to that of classical hydrogen bond [3]. Therefore, the study of new functional materials based on gold(I) properties must encourage the formation of these contacts in the crystal environment. We prepared, by a judicious choice of chelating ligands and balance in coordination equilibria [4], a series of 12 new coordination polymers or supramolecular networks based on dicyanoaurate anion and copper complexes presenting aurophilic interactions. The choice of copper as metal centre to connect to [Au(CN)₂]^- makes the synthesis particularly predictable due to the Jahn-Teller effect in the case of Cu(II), and the appearance of Cu(I) compounds due to redox effect of specific ligands will be commented. These compounds have been tested for vapochromism, and their behaviour in presence of ammonia has been interpreted with Raman, Ir and Uv-Vis absorption spectroscopy. On the same time, the response to temperature (T= 100-420 K) and pressure (P= 0.1-1.5 GPa) of {Cu(bipy)₂[Au(CN)₂]}[Au(CN)₂] (bipy =2,2’-bipyridine), a prototypical bimetallic aurophilic supramolecular network, has been investigated. Both the dependence of structural and reticular parameters to thermal and compression stimuli has been studied, and a phase transition at 1.2 GPa has been revealed. Moreover, we investigated the possibility to modulate the structural behaviour with the cocrystallization with other d¹⁰ metal tectons, and we demonstrate the possibility to obtain inclusion compounds with the presence of Hg(CN)₂ with a 3D weakly interacting framework still presenting Au‧‧‧Au contacts.

The supramolecular compound catena-{[Co(amp)₃][Cr(C₂O₄)₃]·6H₂O}(I) was synthetized as reported earlier [1,2]. To get insight into the structural modifications of its architecture within the water sorption processes (see Fig.1 (a)), in situ powder X-Ray Diffraction (PXRD) measurements were performed on a Bruker D8 Advance Diffractometer in a Bragg−Brentano geometry, using Cu(Kα₁) radiation. The humidity was controlled by exposing the sample to a nitrogen flow heated at 40°C and having humidity rates ranging from 0 to 90% relative humidity (r.H.) and then from 90 to 0% r.H. The PXRD carpet plot diagram ((see Fig.1 (b)) and the refinements of the PXRD patterns coupled with the single crystal diffraction results were used. During the adsorption and desorption processes, only two phases are involved, that of the dehydrated phase ([Co(amp)₃][Cr(C₂O₄)₃] (I’), P 2₁/n, a= 12.0542, b=16.0920, c = 13.8841, β = 99.8013) and the hydrated phase (I, P 2₁/n; a= 13.2330, b=18.2611, c = 14.1396, β = 100.5016). For the adsorption process, during the first step (from 0 to 30% r.H) corresponding to an adsorption of ∼ 1 mol H₂O / mol of I’, only phase I’ is involved and the volume of its unit cell does not change significantly. During the second step (from 30 to 35% r.H.) corresponding to an abrupt adsorption of ∼5.6 mol H₂O / mol of I’, both phases are involved with different percentages (deduced from Rietveld refinements) progressing to the complete conversion of I’ to I. During the third phase where the quantity of water adsorbed shows a plateau (from 35 to 90 % r.H.), only phase I is present and the volume of its unit cell does not change significantly with the humidity. For the desorption process, the same observations apply. During the first step (from 90 to 20 % r.H.) only I is present and its volume decreases just slightly. During the deep desorption process (from 20 to 14 % r.H.), both phases are involved with different percentages and during the last step (from 14 to 0% r.H) at the contrary to the adsorption process, both phases are still present while the sorption isotherm in this region looks like a type-I isotherm in the IUPAC classification [4]. These results suggest a quick capillary condensation followed by a pore filling process that produces a type-V isotherm profile [4], in relation with the first order structural transition followed by insignificant changes of the unit cell volume. The adsorption and desorption branches in the S- shaped isotherms of H₂O-vapor for this compound occur at the values of relative humidity at which these phase transitions start. The conversion of I to I’ and vis-versa is followed by the cleavage and formation of the hydrogen bonds in the architectures of these materials.