Stefano Canossa

Hierarchy of Supramolecular Arrangements and Building Blocks: Inverted Paradigm of Crystal Engineering in the Unprecedented Metal Coordination of Methylene Blue

The aromatic methylene blue cation (MB+) shows unprecedented ligand behavior in the X-ray structures of the trigonal-planar (TP) complexes MBMCl2 (M = CuI, AgI). The two isostructural compounds were exclusively synthesized by grinding together methylene blue chloride and MCl solids. Only in the case of AuCl did the technique lead to a different, yet isoformular, AuI derivative with separated MB+ and AuCl2- counterions and no direct N-Au linkage. While the density functional theory (DFT) molecular modeling failed in reproducing the isolated Cu and Ag complexes, the solid-state program CRYSTAL satisfactorily provided for Cu the correct TP building block associated with a highly compact π stacking of the MB+ ligands. In this respect, the dispersion interactions, evaluated with the DFT functional, provide to the system an extra energy, which likely supports the unprecedented metal coordination of the MB+ cation. The feature seems governed by subtle chemical factors, such as, for instance, the selected metal ion of the coinage triad. Thus, the electronically consistent AuI ion does not form the analogous TP building block because of a looser supramolecular arrangement. In conclusion, while a given crystalline design is generally fixed by the nature of the building block, a peculiarly efficient supramolecular packing may stabilize an otherwise unattainable metal complex.

Hydrogen bonds and π–π inter­actions in two new crystalline phases of methyl­ene blue

Two unprecedented solid phases of methylene blue (MB +), viz. 3,7-bis(dimethylamino)phenothiazin-5-ium chloride dihydrate and 3,7-bis(dimethylamino)phenothiazin-5-ium bisulfite, have been obtained and structurally characterized. The effective absence of hydrogen-bond donors in the second compound has important consequences on the stacking geometry and supramolecular interactions of the MB+ ions, which are analysed by Hirshfeld fingerprint plots.

Flexible porous molecular materials responsive to CO2, CH4 and Xe stimuli

In the search for flexible molecular crystals endowed with porosity, we achieved the fabrication of expandable crystalline prototypal structures, which allow the absorption of gases, without modifying the crystal architecture. The design brings together highly symmetrical tetrahedral elements to construct swellable porous adamantoid frameworks through co-operation of eight surrounding hydrogen bonds mounted on conformationally flexible groups. The flexibility of the porous crystals manifests itself in response to stimuli of selected gases, which promote reversible conformational changes, inducing breathing in the molecular structure. The backbone of the reticular construction is based on the formation of the carboxylic dimers, which project outwards from the tetrahedral molecular core to consolidate the 3D framework. Contact with proper gases such as CO2, Xe and hexane triggers a 56–70% enlargement of the channel cross-section. The accommodation of CO2 and Xe in the channel chambers was revealed by synchrotron-light X-ray diffraction, combined with molecular dynamics and density functional theory (DFT) theoretical calculations. Rare experimental observations of xenon dynamics, in which Xe diffuses along the channels and experiences different chamber orientations in the crystal, were gathered by analysing 129Xe NMR chemical shift anisotropy profiles, which encode the shape and orientation of each visited cavity along the channel. The jump rate and activation energy experienced was uniquely established by exploring Xe atoms in their diffusional path. Nitrogen showed a low affinity to the matrix and was unable to enlarge the pores, thus it was excluded from the restrictive pores of the empty crystal. Given the properties of molecular crystals, it is possible to outline some advantageous aspects, such as simple design, easy self-assembly, solubility, reversible gas uptake and absence of metal ions, and they can thus be considered for eco-friendly gas capture and separation.

Synthesis and structural elucidation of TiO2 aggregates grown inside MOFs

The ordered porosity of Metal Organic Frameworks is commonly acknowledged as the most important feature when their use in the field of host guest materials chemistry is concerned. This is indeed one of their main application, and the possibilities offered by their crystalline nature for structural studies concerning the trapped species make these materials even more versatile and interesting. In this project we aim to exploit the porosity of MOFs to synthesize titania particles inside the pores, in order to obtain MOF-TiO2 composite materials with promising applications in photocatalysis. Structural studies by X-ray diffraction are among the main performed activities, in order to characterize any change in the structure of the original MOF and to gain any useful information about the aggregate grown inside the solid. Preliminary results including the structural characterisation of an titanium oxo-cluster linked by an oxo bridge to Zn atoms of IRMOF-9(1) suggested that Zn based MOFs can be promising starting materials for our purpose. In the cited case, the MOF was not only able to endure the synthetic procedure of the aggregate but also showed different behaviour with temperature changes with respect to the untreated material(1). 1 Yaghi, O. M., et al. (2002). Science. 295, pp 469-472

SCSC metal–organic framework transmetallation: a successful case

Metal-Organic Frameworks (MOFs) are well known porous materials, extensively studied in the last decade and widely used in manifold applications such as gas absorption/storage, catalysis, controlled guest-delivery and many other [1]. A desired MOFs could be ex novo synthesized by direct reaction between metallic precursor and organic bridging ligand(s), usually via solvothermal reaction or, alternatively, it could be obtained via post-synthetic modification from an ad hoc starting framework [2]. Up to the present, the latter method is rarely described in the literature and only few successful examples of metallic node exchange have been reported. A single-crystal-to-single-crystal Zn-to-Co metathesis is here proposed in a post-synthetic metal-organic framework modification performed in mild and reproducible experimental conditions. The modified MOF shows a complete Zn-to-Co transmetallation and results to be isostructural with respect to the starting framework, which only acted as a mould. The new Co-MOF has been widely characterized via SR-SCXRD, SR-XRPD and ESEM analyses: results will be here described in details and compared with the native MOF. In addition, the transmetallated as well as the native MOFs were then exposed to different environmental conditions: they both results to be temperature sensitive going through a similar reversible phase transformation which has been characterized via SCXRD. A deep structure analysis will be also proposed with a description of the hypothesis of the active role of solvent/counter-ion in the transmetallation process. An ex novo approach was also followed to obtain the Co-MOF via direct metal-ligand reaction but none of the synthetic routes proposed has given the same structure obtained via PSM of the native Zn-MOF; on the other hand, a non-porous more stable 1D coordination polymer was obtained. A particular phase transformation of the 1D coordination polymer in which temperature and solvent play a synergistic role is also described. [1] H. Furukawa et al. (2013). Science 341, 6149, 974 [2] M. Lalonde et al. (2013) J. Mater. Chem. A, 1, 5453

Adding flavours to our MOFs

MOFs are highly versatile materials that are made by connecting metal ions with prefixed coordination geometry with rigid ligands acting as spacers, hence affording three-dimensional coordination polymers. The accurate design of the building units allows to design porous MOFs, obtaining cavities of considerable size, which usually accommodate loosely bound solvent molecules. The scope of this work is to find a systematic way to embed small molecular aggregates inside porous crystalline materials, with the multiple aims to explore the structural aspects of nanoconfinement, and of the stabilization of guest molecules inside the cavities of the structure. The feasibility of this approach stems from both the recently developed crystalline-sponge method that describes the structural determination of single molecules trapped inside a microporous framework [1]. The guest that we are here considering for inclusion in MOFs pores are some important compounds for the human health and nutrition which occur as liquids at room temperature, such as eugenol (a major component of clove oil), carvacrol (extracted from the oregano essential oil), thymol (present in the oil of thyme); these are part of a naturally occurring class of compounds known as biocides, with strong antimicrobial attributes. In order to embed these guest in crystalline materials we needed to carefully tailor the MOF cavities; thus we designed a small library of new ligands which could afford flexible MOF architectures. We were able to characterize a series of new materials which are able to release the guests in a controlled way, opening the way to engineer additives for active food packaging. In particular the structure and inclusion properties of PUM168 will be discussed. This new heteroleptic MOF shows triple interpenetration, and nevertheless can accommodate all the guests belonging to the oil family by exhibiting a remarkable flexibility; the selectivity towards mixtures will also be discussed. In the Figure the triple interpenetration of the MOF architecture is shown on the left, and an example of MOF...guest interaction with carvacrol via hydrogen bonds is shown on the right. [1] Inokuma Y., et al., (2013), Nature, 495, 461–466

How to fill MOFs with different flavours

The scope of this work is to find a systematic way to embed small molecular aggregates inside porous crystalline materials, with the multiple aims to explore the structural aspects of nanoconfinement, and of the stabilization of guest molecules inside the cavities of the structure. The feasibility of this approach stems from both the recent report that describes the structural determination of single molecules trapped inside a microporous framework [1] and the various reports showing the inclusion of species such as metal nanoparticles or polyoxometalates into mesoporous MOFs [2]. MOFs are highly versatile materials that are made by connecting metal ions with prefixed coordination geometry with rigid ligands acting as spacers, hence affording three-dimensional coordination polymers. The accurate design of the building units allows to design porous MOFs, obtaining cavities of considerable size, which usually accommodate loosely bound solvent molecules. The guest that we are considering here are some important compounds for the human health and nutrition which occur as liquids at room temperature, some organometallic precursors of inorganic oxides, and organometallic compounds potentially active in catalysis. We initially focused on a collection of already known MOFs, and we determined the interaction mode of some of the guests inside the pores (Figure 1), and the results will be shown. We then moved to the design of new organic linkers in order to better tune the topology and functionality of the MOF network. In particular, we aimed to decorate the inner cavities with hydrogen bond and halogen bond active functional groups, which could serve as anchoring points for the guests. A small library of linkers was synthesized: one group is composed by flexible aminocarboxylic linkers, the second one is characterized by rigid amidic bonds and pyridine as coordinative function. These ligands were used for the synthesis of novel MOFs; due to the nature of the ligands these frameworks resulted quite flexible. Their structure and inclusion properties will be illustrated.

Coordinative properties of the pyridine-2-carbaldehyde thiosemicarbazone ligand towards Ni(II)

Three crystal structures of Ni compounds containing bis(pyridine-2-carbaldehyde thiosemicarbazone) ligand (HL), namely (pyridine-2-carbaldehyde thiosemicarbazonato)(pyridine-2-carbaldeyde thiosemicarbazone) nickel(II) nitrate hydrate [Ni(HL)L][NO3]·(H2O) (1), bis(pyridine-2-carbaldehyde thiosemicarbazone) nickel(II) dinitrate [Ni(HL)2][NO3]2·(2a), and bis(pyridine-2-carbaldehyde thiosemicarbazone) nickel(II) dinitrate dihydrate [Ni(HL)2][NO3]2·2(H2O) (2b) are determined by X ray diffraction methods. Comparative structural studies are carried out.