Amine-rich Nickel(II)-Xerogel as a Highly Active Bifunctional Metallo-organo Catalyst for Aqueous Knoevenagel Condensation and Solvent-free CO2 Cycloaddition

Metallogels formed from supramolecular interactions of low-molecular-weight gelators (LMWGs) combine the qualities of heterogeneous catalysts and offer the advantages of multifunctionality owing to the facile installation of desired task-specific moieties on the surface and along the channels of the gels. We discuss the applications of a triazole-based Ni(II) gel-derived xerogel (NiXero) having a high density of Ni(II)-nodes and appended primary amines as a recyclable heterogeneous catalyst for Knoevenagel condensation of aldehyde and malononitrile in water and the solvent-free cycloaddition of CO2 to form a series of cyclic carbonates with near-quantitative conversion of the respective epoxides, with low catalyst loading (0.59 mol %), high catalyst stability, and recyclability. The structural advantages of NiXero, due to the concurrent presence of bifunctional Lewis acid-base sites on the channels, open Ni(II) nodes, Ntriazole, pendant -NH2 and its chemical stability, are conducive to the cooperative heterogeneous catalytic activity under mild conditions. This work emphasizes the effective amalgamation of metals with purpose-built ligand systems for the construction of metallogels and their utility as heterogeneous catalysts for desired organic transformations.

Hierarchical porous metal–organic gels and derived materials: from fundamentals to potential applications

This review summarizes recent progress in the development and applications of metal–organic gels (MOGs) and their hybrids and derivatives dividing them into subclasses and discussing their synthesis, design and structure–property relationship.

Supramolecular Strategies for the Recycling of Homogeneous Catalysts

Supramolecular approaches are increasingly used in the development of homogeneous catalysts and they also provide interesting new tools for the recycling of metal-based catalysts. Various non-covalent interactions have been utilized for the immobilization homogeneous catalysts on soluble and insoluble support. By non-covalent anchoring the supported catalysts obtained can be recovered via (nano-) filtration or such catalytic materials can be used in continuous flow reactors. Specific benefits from the reversibility of catalyst immobilization by non-covalent interactions include the possibility to re-functionalize the support material and the use as "boomerang" type catalyst systems in which the catalyst is captured after a homogeneous reaction. In addition, new reactor design with implemented recycling strategies becomes possible, such as a reverse-flow adsorption reactor (RFA) that combines a homogeneous reactor with selective catalyst adsorption/desorpion. Next to these non-covalent immobilization strategies, supramolecular chemistry can also be used to generate the support, for example by generation of self-assembled gels with catalytic function. Although the stability is a challenging issue, some self-assembled gel materials have been successfully utilized as reusable heterogeneous catalysts. In addition, catalytically active coordination cages, which are frequently used to achieve specific activity or selectivity, can be bound to support by ionic interactions or can be prepared in structured solid materials. These new heterogenized cage materials also have been used successfully as recyclable catalysts.

Tuning Gel State Properties of Supramolecular Gels by Functional Group Modification

The factors affecting the self-assembly process in low molecular weight gelators (LMWGs) were investigated by tuning the gelation properties of a well-known gelator N-(4-pyridyl)isonicotinamide (4PINA). The N-H∙∙∙N interactions responsible for gel formation in 4PINA were disrupted by altering the functional groups of 4PINA, which was achieved by modifying pyridyl moieties of the gelator to pyridyl N-oxides. We synthesized two mono-N-oxides (INO and PNO) and a di-N-oxide (diNO) and the gelation studies revealed selective gelation of diNO in water, but the two mono-N-oxides formed crystals. The mechanical strength and thermal stabilities of the gelators were evaluated by rheology and transition temperature (Tgel) experiments, respectively, and the analysis of the gel strength indicated that diNO formed weak gels compared to 4PINA. The SEM image of diNO xerogels showed fibrous microcrystalline networks compared to the efficient fibrous morphology in 4PINA. Single-crystal X-ray analysis of diNO gelator revealed that a hydrogen-bonded dimer interacts with adjacent dimers via C-H∙∙∙O interactions. The non-gelator with similar dimers interacted via C-H∙∙∙N interaction, which indicates the importance of specific non-bonding interactions in the formation of the gel network. The solvated forms of mono-N-oxides support the fact that these compounds prefer crystalline state rather than gelation due to the increased hydrophilic interactions. The reduced gelation ability (minimum gel concentration (MGC)) and thermal strength of diNO may be attributed to the weak intermolecular C-H∙∙∙O interaction compared to the strong and unidirectional N-H∙∙∙N interactions in 4PINA.

Beyond Covalent Crosslinks: Applications of Supramolecular Gels

Traditionally, gels have been defined by their covalently cross-linked polymer networks. Supramolecular gels challenge this framework by relying on non-covalent interactions for self-organization into hierarchical structures. This class of materials offers a variety of novel and exciting potential applications. This review draws together recent advances in supramolecular gels with an emphasis on their proposed uses as optoelectronic, energy, biomedical, and biological materials. Additional special topics reviewed include environmental remediation, participation in synthesis procedures, and other industrial uses. The examples presented here demonstrate unique benefits of supramolecular gels, including tunability, processability, and self-healing capability, enabling a new approach to solve engineering challenges.

A supramolecular Cd(ii)-metallogel: an efficient semiconductive electronic device

A supramolecular Cd(ii)-metallogel with oxalic acid in DMF medium has been achieved, and the semi-conductive characteristics such as the Schottky barrier diode nature of the metallogel were explored.

Crystal habit modification of Cu(ii) isonicotinate–N-oxide complexes using gel phase crystallisation

We report the crystallisation of three forms of the copper(ii) isonicotinate–N-oxide complex and their phase interconversion via solvent-mediated crystal-to-crystal transformation and the selective crystallisation of one form via gel phase crystallisation.

A carboxylic acid functionalized benzimidazole-based supramolecular gel with multi-stimuli responsive properties

We fabricated an organogel C11-OG and a novel metallogel Pb-MG, both of which exhibit strong blue AIEE in gel states.

A new small molecule gelator and 3D framework ligator of lead(ii)

A new allene dicarboxylate ligand reacts with hydrated lead(ii) acetate in DMF to yield either a crystalline 3D framework or a metallogel dependent on the reaction temperature.

A highly selective fluorescent probe for cadmium ions in aqueous solution and living cells

An organic salt based on double 1,3,4-oxadiazole derivatives as fluorophores and BAPTA as a receptor has been designed for detection of Cd(2+). The fluorescent probe exhibits high selectivity for Cd(2+) and a low detection limit of 20 nM in aqueous solution, making it useful for Cd(2+) imaging in living MCF-7 cells.