Dalton Transactions PAPER Cite this: Dalton Trans., 2020, 49, 14690 Received 22nd August 2020, Accepted 21st September 2020 DOI: 10.1039/d0dt02949a rsc.li/dalton Metal organic frameworks decorated with free carboxylic acid groups: topology, metal capture and dye adsorption properties† M. Naqi Ahamad, a M. Shahnawaz Khan, a M. Shahid * a and Musheer Ahmad b 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 (H 3 -BTC) as the main ligand and 4,4’-dipyridyl (bipy) as the spacer. The MOFs have been character- ized 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 captur- ing 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 Fe 3+ 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 Fe 3+ ion and hard donors of the dyes is responsible for the enhanced adsorption. 1. Introduction Coordination polymers and metal–organic frameworks (MOFs) have gained much attention from chemists owing to their interesting properties and applications in diverse areas like gas storage, sensing, separation, catalysis, luminescence and magnetism. 1–3 MOFs are, in general, designed by linking mul- tidentate bridging ligands with transition metals. 4 It is impor- tant to select a particular ligand (spacer) and metal ion (node) to achieve MOFs with desired applications. In this regard, MOFs consisting of aromatic multicarboxylate ligands such as 1,3-/1,4-benzene dicarboxylate, 1,3,5-benzenetricarboxylate, and 1,2,3-benzenetricarboxylate are considered to be promis- ing functional linkers due to their rigid nature, interesting topologies and high thermal stability. 5–7 Several reports throw light on the state of the art design and functional applications of MOFs. 8,9 To date, a number of rationally designed one dimensional (1D) chains, two-dimensional (2D) sheets and three-dimensional (3D) networks have come into existence by the combination of suitable metal ions and organic ligands. Although the chemistry of MOFs has been explored over decades, a better understanding regarding metal–ligand inter- action that can result in intriguing topological features is still a challenge. Particularly, nickel–organic frameworks, 10 copper– organic frameworks 11 and cadmium–organic frameworks 12 constructed from benzene-1,3,5-tricarboxylate (H 3 -BTC) using solvothermal reactions are important from the material point of view. 1,3,5-Benzenetricarboxylic acid (trimesic acid) is a † Electronic supplementary information (ESI) available: Crystal and spectral data. CCDC 1945412–1945414. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0dt02949a a Functional Inorganic Materials Lab (FIML), Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India. E-mail: shahid81chem@gmail.com b Department of Applied Chemistry (ZHCET), Aligarh Muslim University, Aligarh 202002, India 14690 | Dalton Trans. , 2020, 49, 14690–14705 This journal is © The Royal Society of Chemistry 2020 Published on 22 September 2020. Downloaded by Aligarh Muslim University on 10/30/2020 3:22:31 AM. View Article Online View Journal | View Issue