This review highlights the current progress in the chemistry of MOFs and their dye adsorption mechanisms.

A 2-D copper(II)-based coordination polymer, [Cu(SCN)(hmp)]_n CP (1), was crystallized via the slow evaporation method after the reaction of CuSO₄·5H₂O, 2-pyridinemethanol (hmpH), sodium thiocyanate and sodium hydroxide in water for functional applications. CP (1) was characterized via elemental analysis, FTIR, PXRD, magnetic, EPR, crystallographic and TGA studies. The crystal structure and EPR data confirmed a square pyramidal geometry around the Cu(II) ions. The topological analysis revealed that CP 1 has a {6^{^}3} point symbol with a [6.6.6] extended point symbol and 3-c net, uninodal net having hcb and Shubnikov hexagonal plane net/(6,3) type of uncommon topology. The magnetic studies suggested the strong antiferromagnetic nature of CP (1). The fluorescence sensing property of CP (1) was investigated with different nitro aromatic compounds and hazardous metal ions. CP (1) demonstrated high selectivity and sensitivity towards nitrobenzene, even in the presence of other competitive nitro aromatics. In addition, CP (1) showed excellent selectivity and sensitivity toward Fe³⁺ over other metal ions. The possible detection mechanisms were proposed employing UV-visible and fluorescence spectroscopy and DFT calculations. CP (1) also showed excellent recyclability towards both analytes, and its initial intensity was almost regained after several washings. Moreover, CP (1) acted as an excellent adsorbent material for natural dyes with different charges and sizes, i.e., methylene blue (MB), methyl orange (MO) and Rhodamine-B. Furthermore, CP (1) was utilized repeatedly for the effective adsorption of MB from wastewater without significant loss in its adsorption capacity. Hence, the present CP (1) was designed to relate coordination chemistry with various functional applications of interest.

In this report, metal organic frameworks (MOFs) are designed and tuned for structural variations in order to induce metal capture which in turn directs dye adsorption properties. The three MOFs, Cu-MOF-2COOH, Ni-MOF-COOH and Cd-MOF, are synthesized by employing 1,3,5-benzenetricarboxylic acid (H₃-BTC) as the main ligand and 4,4′-dipyridyl (bipy) as the spacer. The MOFs have been characterized using various spectral techniques and single crystal X-Ray studies. A topological analysis using TOPOS Pro reveals that the MOFs possess varying topologies i.e.hcb, hxl, sql and 2C1. Cu-MOF-2COOH and Ni-MOF-COOH contain two and three uncoordinated carboxylic acid groups, respectively, and in Cd-MOF, all three –COOH groups are utilized in bonding. The dye adsorption properties of the MOFs with free carboxylate group(s) were checked and we found that both MOFs are unable to adsorb any of the dyes significantly. The free carboxylate group(s) in the MOFs inspire us to elaborate their metal capturing properties. In different solvents we checked the metal capturing properties of Cu-MOF-2COOH and Ni-MOF-COOH with different metal salts. Surprisingly, both MOFs show better metal capturing properties towards the hard and highly polarizing Fe³⁺ ion in aqueous medium. Theoretical studies show that the free carboxylate(s) are involved in binding with metals. The post synthetically modified materials (Fe@Cu-MOF-2COOH and Fe@Ni-MOF-COOH) were further checked for their dye adsorption properties and both the doped MOFs show better adsorption properties towards the MB and MO. Furthermore, three kinetic models were employed to understand the reaction mechanism of adsorption and the pseudo second order kinetic model fits the best in both cases. The uncoordinated carboxylate groups in the channels act as post synthetic modification sites for metal capture and the post synthetically modified material thus formed attracts organic dyes following the HSAB concept. The strong interaction existing between the hard Fe³⁺ ion and hard donors of the dyes is responsible for the enhanced adsorption.

Herein, two metal organic frameworks (MOFs), namely, [{Cu₂(μ-OH)(μ-OAc)(H₂O)(OAc)(bpy)₂}NO₃·4H₂O]_n (CuMOF-1) and [{Co(bpy)(ClO₄)₂(H₂O)₂}·bpy]_n (CoMOF-2), where bpy = 4,4′-bipyridine, were synthesized, characterized and explored for their material importance in magnetism and dye adsorption. Under solvothermal conditions, two different metal salts (nitrate and perchlorate) were reacted with a simple bench chemical, i.e., a dipyridine derivative, namely, bpy to afford two MOFs having different yet interesting topologies and adsorption properties. The structure of CuMOF-1 was built from the repeating building blocks of the dinuclear cationic Cu(II) units composed of two Cu(II) ions, two bpy ligands, two acetate ions (OAc), one hydroxide ion (OH⁻) and a water molecule. Interestingly, two different Cu atoms existed in the structure with two different coordination geometries, i.e., octahedral and trigonal bipyramidal. The sql topology of the resulting 4-c uninodal net with point symbol {4⁴·6²} was obtained for CuMOF-1. The cluster representation of CuMOF-1 further gave hxl topology with 6-c uninodal nets (point symbol: {3⁶·4⁶·5³}). CoMOF-2 was built from repeating mononuclear Co(II) units, where each Co atom coordinated with two bpy ligands, two perchlorate ions and two water molecules, resulting in octahedral geometry around the Co(II) ions. The surprising nature of the structure of CoMOF-2 was that one bpy ligand was coordinated with cobalt, while the second bpy ligand was present in the lattice in between layers, thus consolidating the structure by strong hydrogen bonding with the coordinated water molecules of the neighboring units. The resulting underlying net in CoMOF-2 was 2C1 with sql topology. The magnetic data indicated the presence of strong antiferromagnetic interactions in MOFs (J = −98.21 cm⁻¹ in CuMOF-1 and θ = −8.8 K in CoMOF-2). The present MOFs were exploited for the adsorption of two dyes, namely, methylene blue (MB) and methyl orange (MO), which are organic pollutants. The adsorption studies confirmed that CoMOF-2 is a more selective adsorbent for MB. The kinetic data disclosed the pseudo-second-order mechanism of adsorption in both cases. The cation/anion–π interaction between the dyes and π-electron-rich bpy was responsible for the adsorption behaviour of MOFs. Moreover, the presence of free bpy in the lattice of CoMOF-2 gave rise to the strongest interactions, making CoMOF-2 the best adsorbent.