NMR-Enhanced Crystallography Aids Open Metal–Organic Framework Discovery Using Solvent-Free Accelerated Aging

We demonstrate the combined use of NMR-enhanced crystallography and solvent-free synthesis by accelerated aging (AA), for the discovery and structural characterization of a novel cadmium-based open metal–organic framework (MOF) belonging to the class of zeolitic imidazolate frameworks (ZIFs). Although solid-state NMR spectroscopy has been used to assist in structural characterization of crystalline solids by powder X-ray diffraction (PXRD), typically through quantification of the contents of the asymmetric unit, this work highlights how it can take a more active role in guiding structure determination, by elucidating the coordination environment of the metal node in a novel MOFs. Exploration of AA reactions of cadmium oxide (CdO) and 2-methylimidazole (HMeIm) enabled the synthesis of not only the previously reported yqt1-topology framework but also a new material (1) exhibiting a Cd/MeIm ratio of 1:3, contrasting the 1:2 ratio expected for a ZIF. Structural characterization of 1 was enabled by using 111Cd solid-state nuclear magnetic resonance (SSNMR) to provide information on the coordination environment of the cadmium node. Specifically, 111Cd SSNMR experiments were conducted on a series of model compounds to correlate the cadmium coordination environment to the observed isotropic chemical shift, δiso(111Cd), followed by multinuclear SSNMR experiments on 1 to determine the nature of the metal coordination environment and the number of distinct chemical sites. This information was used in refinement of the molecular level structure from the available PXRD data, a technique termed NMR-enhanced crystallography, revealing that 1 is an open diamondoid (dia) topology Cd(MeIm)2 framework based on Cd2+ ions tetrahedrally coordinated with MeIm– ligands and additional HMeIm guest molecules within the framework pores. Although AA was initially devised as a clean, mild route for making MOFs, these results provide a proof-of-principle of how, by combining it with SSNMR spectroscopy as a means to overcome limitations of PXRD structure determination, it can be used to screen for new solid phases in the absence of solvents, high temperatures, or mechanical impact that are inherent to other thermally-, solution-, or mechanochemically-based techniques

Halogen bonding with carbon:directional assembly of non-derivatised aromatic carbon systems into robust supramolecular ladder architectures †

Carbon, although the central element in organic chemistry, has been traditionally neglected as a target for directional supramolecular interactions. The design of supramolecular structures involving carbon-rich molecules, such as arene hydrocarbons, has been limited almost exclusively to non-directional π-stacking, or derivatisation with heteroatoms to introduce molecular assembly recognition sites. As a result, the predictable assembly of non-derivatised, carbon-only π-systems using directional non-covalent interactions remains an unsolved fundamental challenge of solid-state supramolecular chemistry. Here, we propose and validate a different paradigm for the reliable assembly of carbon-only aromatic systems into predictable supramolecular architectures: not through non-directional π-stacking, but via specific and directional halogen bonding. We present a systematic experimental, theoretical and database study of halogen bonds to carbon-only π-systems (C-I⋯πC bonds), focusing on the synthesis and structural analysis of cocrystals with diversely-sized and -shaped non-derivatised arenes, from one-ring (benzene) to 15-ring (dicoronylene) polycyclic atomatic hydrocarbons (PAHs), and fullerene C60, along with theoretical calculations and a systematic analysis of the Cambridge Structural Database. This study establishes C-I⋯πC bonds as directional interactions to arrange planar and curved carbon-only aromatic systems into predictable supramolecular motifs. In &gt;90% of herein presented structures, the C-I⋯πC bonds to PAHs lead to a general ladder motif, in which the arenes act as the rungs and halogen bond donors as the rails, establishing a unique example of a supramolecular synthon based on carbon-only molecules. Besides fundamental importance in the solid-state and supramolecular chemistry of arenes, this synthon enables access to materials with exciting properties based on simple, non-derivatised aromatic systems, as seen from large red and blue shifts in solid-state luminescence and room-temperature phosphorescence upon cocrystallisation.</p

Halogen Bonding to the Azulene Pi-System: Cocrystal Design of Pleochroism

We describe the rational development of a design to create pleochroic molecular solids, by using C-I...pi halogen bonds to pre-organise different types of chromophores in a cocrystal. Using as a blueprint the structure of a non-dichroic cocrystal of naphthalene, we show how the systematic introduction of chromophores able to act as halogen bond donors or acceptors enables the construction of optically interesting dichroic or pleochroic solids, demonstrating the reliability of C-I...pi halogen bonds in designing new properties.<br /

Three-in-one: dye-volatile cocrystals exhibiting intensity-dependent photo-chromic, photo-mechanical and photo-carving response

Cocrystallization of a cis-azobenzene dye with volatile cocrystal former molecules, such as pyrazine and dioxane, leads to materials that exhibit at least three different light intensity-dependent responses upon irradiation with low-energy visible light. Specifically, the halogen bond-driven assembly of cis-(p-iodoperfluorophenyl)azobenzene with volatile halogen bond acceptors produces cocrystals whose light-induced behaviour varies significantly depending on the intensity of the light applied. Low-intensity (<1 mW∙cm-2) light irradiation leads to a colour change due to low levels of cis-trans isomerization. Irradiation at higher intensities (150 mW∙mm-2) produces photo-mechanical bending, caused by more extensive azo dye isomerization. At still higher irradiation intensities (2.25 W∙mm-2) the cocrystals undergo photo-carving, i.e. they are readily shaped, punctured, and cut with micrometer precision using laser light. This work demonstrates how the recently reported photo-carving behaviour can be combined with different types of photo-responses, providing a design for multi-responsive materials that can respond to different levels of irradiation with optical colour change, photo-mechanical bending, or photo-carving, as laser power is increased

Low-Power Laser Micro-Shaping of Dye-Volatile Cocrystals: The Gentle Cutting Edge of Photoresponsive Materials

Cocrystallisation of a fluorinated azobenzene with volatile cocrystal components dioxane or pyrazine yields halogen-bonded cocrystals that can be cut, carved or engraved with low-powered visible laser light. This process, termed cocrystal laser micro-shaping (CLMS), is enabled by cocrystallisation of a visible light dye with a volatile component, giving rise to materials that can be selectively disassembled with micrometer precision using gentle, non-burning irradiation in a commercial confocal microscope setup. The ability to shape and even machine cocrystals in 3D using laser powers between 0.5 and 20 mW, which are 2-4 orders of magnitude lower compared to laser powers used for machining metals, ceramics or polymers, is rationalized by CLMS targeting the disruption of weak supramolecular interactions between cocrystal components, rather than the breaking of covalent bonds in polymers or disruption of ionic structures required for conventional laser beam or focused ion beam machining processes, mainly by high-power laser heating.<br /

Halogen Bonding to Carbon: a Directional Interaction for the Reliable Design of Supramolecular Architectures Based on Non-derivatized Aromatic Carbon Systems

Carbon, although the central element in organic chemistry has been traditionally neglected as a target for directional supramolecular interactions. The design of supramolecular structures involving carbon-rich molecules, such as arenes, has almost exclusively been limited to π-stacking of aromatic systems, or derivatization with heteroatoms as sites for molecular recognition. Here, we demonstrate that C-I···Cπ halogen bonds to carbon-based π-systems can be reliably used as direction-al interactions for the creation of extended structures based on planar, as well as curved aromatic systems, without any need for derivatization or π-stacking. Specifically, we describe the first systematic study of a series of cocrystals containing non-derivatized carbon-only aromatic systems of different sizes and shapes, including polycyclic aromatic hydrocarbons (PAHs) and fullerene C60, which are held together by directional halogen bonds to aromatic carbon atoms. In a large majority (~90%) of structures, the C-I···Cπ halogen bonding with PAHs leads to a supramolecular ladder-like motif, in which the PAHs act as the rungs and halogen bond donors as rails, demonstrating this motif as the first example of a supramolecular synthon based on carbon. These results, supported by novel cocrystal structures, theoretical calculations, and a systematic analysis of the Cambridge Structural Database, offer a new, previously overlooked paradigm for the assembly of carbon-only aromatic systems, not based on π-stacking, but via specific, directional halogen bonding. This new ability to use a car-bon-based supramolecular synthon to direct the assembly aromatic systems provides an exciting opportunity to create materials with new and modified properties based on non-derivatized aromatic systems, as seen from large red and blue shifts in solid-state luminescence for cocrystals of pyrene, coronene and perylene, as well as the appearance of room-temperature phosphorescence upon cocrystal formation

Halogen bonding with carbon: directional assembly of non-derivatised aromatic carbon systems into robust supramolecular ladder architectures

Carbon, although the central element in organic chemistry, has been traditionally neglected as a target for directional supramolecular interactions. The design of supramolecular structures involving carbon-rich molecules, such as arene hydrocarbons, has been limited almost exclusively to non-directional π-stacking, or derivatisation with heteroatoms to introduce molecular assembly recognition sites. As a result, the predictable assembly of non-derivatised, carbon-only π-systems using directional non-covalent interactions remains an unsolved fundamental challenge of solid-state supramolecular chemistry. Here, we propose and validate a different paradigm for the reliable assembly of carbon-only aromatic systems into predictable supramolecular architectures: not through non-directional π-stacking, but via specific and directional halogen bonding. We present a systematic experimental, theoretical and database study of halogen bonds to carbon-only π-systems (C-I⋯πC bonds), focusing on the synthesis and structural analysis of cocrystals with diversely-sized and -shaped non-derivatised arenes, from one-ring (benzene) to 15-ring (dicoronylene) polycyclic atomatic hydrocarbons (PAHs), and fullerene C60, along with theoretical calculations and a systematic analysis of the Cambridge Structural Database. This study establishes C-I⋯πC bonds as directional interactions to arrange planar and curved carbon-only aromatic systems into predictable supramolecular motifs. In &gt;90% of herein presented structures, the C-I⋯πC bonds to PAHs lead to a general ladder motif, in which the arenes act as the rungs and halogen bond donors as the rails, establishing a unique example of a supramolecular synthon based on carbon-only molecules. Besides fundamental importance in the solid-state and supramolecular chemistry of arenes, this synthon enables access to materials with exciting properties based on simple, non-derivatised aromatic systems, as seen from large red and blue shifts in solid-state luminescence and room-temperature phosphorescence upon cocrystallisation.</p