A series of Cu(II), Zn(II), Cd(II) and Zr(IV) MOFs with acetylene and triazole functionalized linkers

The last two decades have yielded remarkable progress in the development of reticular porous materials including metal organic frameworks (MOFs). However, it is still significant challenge to precisely control the secondary coordination sites or functionalities within the framework. This complexity arises due to the multitude of parameters influencing the synthesis process. Precise control of secondary sites within MOFs can be achieved through accurate linker design, overcoming limitations imposed by diverse reaction conditions. By employing modular approach in the linker design, we prepared four new di and tetra carboxylate-based linkers where acetylene and triazole functionalities were incorporated in the side arms of linkers through sequential Sonogashira, Suzuki and “click” reactions. Under similar reaction conditions, acetylenic tagged non-symmetrical linear ditopic linker, H2-1, gave 4-fold interpenetrated Cu(II) 3D framework while acetylenic tagged symmetrical linear ditopic linker, H2-2, produced 2D framework. This indicates the importance of subtle change in linker core towards the targeted assemblies. An isostructural to UiO-68, an acetylene functionalised Zirconium MOF was also prepared using H2-1 where the acetylene group was found disordered due to linker different orientational arrangements and rotation within framework. With linear triazole functionalized ditopic linker, H2-3, two isostructural 3D Cu(II) and Cd(II) 3D frameworks were prepared where triazoles of one ligand were found axially coordinated with paddlewheel units and the triazoles of the second ligand were partially hydrogen bonded. On the other hand, two isostructural 3D Zn(II) and Cd(II) MOFs were built from reaction with a tetratopic linker, H2-4, where all triazoles sites were found axially coordinated with the paddlewheel SBU. A triazole functionalized 12- connecting zirconium MOF was prepared by linker H2-3, which ester hydrolyse to linker H2-4 during MOF synthesis. All the triazole sites in this Zr MOF were found available as secondary sites. This shows with subtle changes in the linker, not only we can control the topology, interpenetration and pore size of MOFs but also availability of secondary coordination sites within framework.