Metal–organic frameworks (MOFs) based on mixed linker systems: structural diversities towards functional materials

Abstract

Coordination polymers are well known organic–inorganic hybrids which can be of different dimensionalities; 1D, 2D and 3D. Based on the geometry of the inorganic metal ions and coordination mode of the organic linkers/clusters the polymers extend to different dimensions and adopt versatile topologies. Frameworks can be built with one linker or more than one linker (mixed linkers) with assembly of different metal ions. Use of a single linker is a well known and accepted methodology to generate high surface area frameworks such as isoreticular metal–organic frameworks (IRMOFs), zeolitic imidazolate frameworks (ZIFs), HKUST and MILs (Material Lavoisier Laboratory). In this context, MOFs composed of mixed linkers provide greater flexibility in terms of surface area, modifiable pore size and chemical environment. In general, one anionic linker and one neutral linker connect with the metal ion/cluster to generate a mixed linker framework. Depending on the binding mode of the anionic linker, the framework extends in different dimensions; a V-shaped anionic linker would not grow in two dimensions, rather a 1D chain will form. The neutral linkers mostly serve as pillars and further increase the dimensionality. Depending on the length of the linkers, porosity can be achieved and systematic control is possible. The extent of entanglement in a 3D framework can also be tuned by altering the neutral or anionic linker. Moreover the linkers can be functionalized extensively to meet the aimed applications such as gas separation, catalysis, magnetism and molecular sensing. Such modulation over functionality and porosity is not possible with a single linker system. In this highlight we aimed to discuss mixed linkers based framework structures, their versatile topologies and tunable porous properties.