Characterization of an unanticipated indium-sulfur metallocycle complex.

We have produced a novel indium-based metallocycle complex (In-MeSH), which we initially observed as an unanticipated side-product in metal-organic framework (MOF) syntheses. The serendipitously synthesized metallocycle forms via the acid-catalysed decomposition of dimethyl sulfoxide (DMSO) during solvothermal reactions in the presence of indium nitrate, dimethylformamide and nitric acid. A search through the Cambridge Structural Database revealed isostructural zinc, ruthenium and palladium metallocycle complexes formed by other routes. The ruthenium analogue is catalytically active and the In-MeSH structure similarly displays accessible open metal sites around the outside of the ring. Furthermore, this study also gives access to the relatively uncommon oxidation state of In(II), the targeted synthesis of which can be challenging. In(II) complexes have been reported as having potentially important applications in areas such as catalytic water splitting.

Tuning the Stereoselectivity of an Intramolecular Aldol Reaction by Precisely Modifying a Metal-Organic Framework Catalyst.

We report the catalysis of an enantioselective, intramolecular aldol reaction accelerated by an organocatalyst embedded in a series of multicomponent metal-organic frameworks. By precisely programming the pore microenvironment around the site of catalysis, we show how important features of an intramolecular aldol reaction can be tuned, such as the substrate consumption, enantioselectivity, and degree of dehydration of the products. This tunability arises from non-covalent interactions between the reaction participants and modulator groups that occupy positions in the framework remote from the catalytic site. Further, the catalytic moiety can be switched form one framework linker to another. Deliberately building up microenvironments that can influence the outcome of reaction processes in this way is not possible in conventional homogenous catalysts but is reminiscent of enzymes.

Photoinduced Electron Transfer in Multicomponent Truxene-Quinoxaline Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) can respond to light in a number of interesting ways. Photochromism is observed when a structural change to the framework is induced by the absorption of light, which results in a color change. In this work, we show that introducing quinoxaline ligands to MUF-7 and MUF-77 (MUF = Massey University Framework) produces photochromic MOFs that change color from yellow to red upon the absorption of 405 nm light. This photochromism is observed only when the quinoxaline units are incorporated into the framework and not for the standalone ligands in the solid state. Electron paramagnetic resonance (EPR) spectroscopy shows that organic radicals form upon irradiation of the MOFs. The EPR signal intensities and longevity depend on the precise structural details of the ligand and framework. The photogenerated radicals are stable for long periods in the dark but can be switched back to the diamagnetic state by exposure to visible light. Single-crystal X-ray diffraction analysis reveals bond length changes upon irradiation that are consistent with electron transfer. The multicomponent nature of these frameworks allows the photochromism to emerge by allowing through-space electron transfer, precisely positioning the framework building blocks, and tolerating functional group modifications to the ligands.

Probing Nonuniform Adsorption in Multicomponent Metal-Organic Frameworks via Segmental Dynamics by Solid-State Nuclear Magnetic Resonance.

The guest adsorption phenomena in multicomponent metal-organic frameworks (MOFs) are intricate due to their structural complexities. In this work, we studied two members of the isostructural series of MUF-77 frameworks that consist of long or short alkyl groups. The adsorption of methanol, N,N-dimethylaniline (DMA) and acridine orange (AO) in two structures of MUF-77 has been investigated. 2H solid-state nuclear magnetic resonance (SSNMR) and two-dimensional 1H-13C NMR spectroscopy were used to probe the dynamics of various compartments of MUF-77. Through the analyses of dynamic behavior by SSNMR and molecular dynamics simulations, we elucidate the spatial distribution of guest molecules are nonuniform around different chemical components, in different pore structures, and across different parts of MOF lattice. In addition, we reveal that the framework flexibility of MUF-77 with short alkyl groups is reduced upon guest adsorption yet the framework flexibility of MUF-77 with long alkyl groups increases upon loading with methanol.

Tritopic Triazatruxene Ligands for Multicomponent Metal-Organic Frameworks.

Multicomponent metal-organic frameworks (MOFs) are built up from multiple ligands that are geometrically distinct. These ligands occupy specific positions in the MOF lattice. Installing different functionalities at precise locations in the framework is an important step in making MOFs for specific applications. This can be achieved by designing functionalized ligands for multicomponent MOFs. Here, we report a simple synthetic procedure for a new tritopic triazatruxene based tricarboxylic acid, H3 tat. We show that this ligand can be symmetrically derivatized with various substituents on its nitrogen centres. We report a new isoreticular series of well-ordered quaternary MOFs based on these new triazatruxene ligands together with two linear carboxylate ligands and Zn4 O clusters. These MOFs are isostructural to the previously reported MUF-77 series and show similar high surface areas and large pore volumes. Furthermore, H-bonding between the NH sites of the incorporated triazatruxene ligands and guest molecules is employed to modify their fluorescence behavior.

Systematic Tuning of the Luminescence Output of Multicomponent Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) exhibit a broad range of luminescence characteristics due to the vast array of metal ions and organic linkers available as building blocks. Systematic control over the emissive output of MOFs is highly sought after. Methods for tuning emission profiles are emerging based largely on luminescent metal ions and the encapsulation of emissive guests. Herein, we show how the functionalization of the organic linkers of a series of multicomponent MUF-77 (MUF = Massey University Framework) materials can methodically tune their spectral output. This was quantified by chromaticity diagrams. White-light emission was obtained by combining the photophysical characteristics of the three distinct organic fluorophores present in these materials. Our results also show that both (i) energy transfer interactions between the organic components and (ii) noncovalent interactions with guests can also be harnessed to tune the emission. These results establish multicomponent metal-organic frameworks as fluorescent materials with unique spectral characteristics.