A supramolecular gel results from the immobilization of solvent molecules on a 3D network of gelator molecules stabilized by various supramolecular interactions that include hydrogen bonding, π-π stacking, van der Waals interactions, and halogen bonding. In a metallogel, a metal is a part of the gel network as a coordinated metal ion (in a discrete coordination complex), as a cross-linking metal node with a multitopic ligand (in coordination polymer), and as metal nanoparticles adhered to the gel network. Although the field is relatively new, research into metallogels has experienced a considerable upsurge owing to its fundamental importance in supramolecular chemistry and various potential applications. This focus review aims to provide an insight into the development of designing metallogelators. Because of the limited scope, discussions are confined to examples pertaining to metallogelators derived from discrete coordination complexes, organometallic gelators, and coordination polymers. This review is expected to enlighten readers on the current development of designing metallogelators of the abovementioned class of molecules.

Five new coordination polymers (CPs) namely, [{Zn(μ-HO)(5N-IPA)(2,2'-bpe)}] (1), [{Zn(μ-HO)(5N-IPA)(1,10-phen)}] (2), [{Zn(5N-IPA)(1,2-bpe)}] (3), [{Zn(5N-IPA)(1,2-bpey)}] (4), and [{Zn(HO)(5N-IPA)(4,4'-tme)}(HO)] (5) (5N-HIPA = 5-azidoisophthalic acid, 2,2'-bpe= 2,2'-bipyridine, 1,10-phen = 1,10-phenanthroline, 1,2-bpe = 1,2-bis(4-pyridyl)ethane, 1,2-bpey = 1,2-bis(4-pyridyl)ethylene, 4,4'-tme = 4,4'-trimethylenedipyridine), have been synthesized based on a mixed ligand approach adopting a solvothermal technique. Depending upon the intrinsic structural flexibility of the bis-pyridyl coligands, interesting structural topologies have also been observed in the resulting CPs: Sra SrAl2 type topology for 3 and a 3-fold interpenetrated dmp topology for 4. A green hand grinding technique has been implemented to reduce the particle size of the CPs to generate nanoscale CPs (NCPs). SEM studies of NCPs reveal the formation of square and spherical particles for NCP 1 and 2, respectively, and nano rod for NCP 3, 4, and 5. Remarkably, when scaled down to nano range all the NCPs retain their crystalline nature. The cytotoxic activity of the NCPs (1-5) has been studied using human colorectal carcinoma cells (HCT 116). Significant cell death is observed for NCP 2, which is further corroborated by cell growth inhibition study. The observed cell death is likely to be due to mitochondrial-assisted apoptosis as is evident from immunofluorescence study.

This work demonstrates the solvothermal synthesis of two new lanthanide-based MOFs, namely [{Dy(2N 3 -TPA) 2 (H 2 O)(CH 3 OH)}] ∞ (1) and [{Dy 4, derived from two carboxylate ligands, namely 2-azidoterephthalate (2N 3 -TPA) and 5-azidoterephthalate (5N 3 -IPA), respectively, with immobilized azide functionalities oriented toward the pore surfaces. Singlee-crystal X-ray diffraction (SXRD) analyses reveal that 1 exhibits an infinite 1D network while 2 displays a 2D sheet structure with helical metal carboxylate chains. Both compounds are found to exhibit excellent thermal and aqueous phase stabilities. A simple and convenient route has been adopted to reduce the particle size of the assynthesized compounds to prepare nanoscale metal−organic frameworks (NMOFs). Unprecedentedly, nanospherical architectures are observed for both cases on SEM and TEM analyses. These NMOFs have been successfully exploited as fluorescent probes for the detection of various nitro analytes such as nitrotoluene (NT), nitrobenzene (NB), 2,6-dinitrotoluene (DNT), 4-nitrobenzoic acid (4-NBA), picric acid (PA), etc. As suggested by the values of quenching constants obtained from Stern−Volmer plots, the sensitivity of both NMOFs toward PA sensing is exceptionally high. Furthermore, time-resolved fluorescence studies and theoretical calculations have been performed to speculate the excited state interaction of NMOFs with PA specifically. Moreover, the selectivity toward PA sensing in the presence of other structurally similar nitro analytes and excellent reusability of the employed sensory platforms manifest great potentials of these materials for real-life applications.

Solvothermally synthesized CoII based MOFs have been exploited as visible light induced dye degradation catalyst having real life applicability where introduction of H2O2 enhances the efficiency markedly.

A newly synthesized bis-pyridyl ligand having a diphenyl ether backbone (LP6) displayed the ability to form crystalline coordination polymers (CP1-CP6) which were fully characterized by single crystal X-ray diffraction. Most of the resulting polymers were lattice-occluded crystalline solids-a structural characteristic reminiscent to gels. The reactants of the coordination polymers produced metallogels in DMSO/water confirming the validity of the design principles with which the coordination polymers were synthesized. Some of the metallogels displayed material properties like in situ synthesis of Ag nanoparticles and stimuli-responsive gel-sol transition including sensing hazardous gases like ammonia and hydrogen sulfide.