Multicomponent isoreticular metal-organic frameworks: Principles, current status and challenges

The Multicomponent Isoreticular Metal-Organic Frameworks (MI-MOFs) are the materials designed either with multiple linkers or mixed-metals as secondary building units (SBUs). These materials are the part of so-called Multinary MOFs by Yaghi and co-workers which are fabricated by fusion of more than two such building units or SBUs and linker in the backbone. The introduction of multiplicity with variant constituents in MOFs conquered extensive growing interest in various developing fields due to their fascinating applications like gas storage and separation, CO2 reduction and energy storage, asymmetric catalysis, photodynamic therapy and cancer treatment, etc. The mixed-linker based multivariate metal–organic frameworks are synthesized employing various systematic strategies which include Linker-Directed Vertex De-symmetrization, Random Mixed-Linker Distribution, covalent pre- and post-synthetic modification techniques, etc. Furthermore, the mixed-metal based multivariate metal–organic frameworks are synthesized by one-pot reticular synthesis, metal-ion metathesis and covalent post-synthetic modification approaches. The systematic incorporation of variance, either functionality or metal, in vacancies generated during removal of SBUs in a highly ordered fashion from crystallographic equivalent sites from single-component MOFs, keeping topology intact, form multicomponent isoreticular-MOFs. All these approaches produce desired multivariate MOFs materials, however, controlling the degree of periodicity with long-range order by arranging the components systematically, is indeed a challenging task. The regular arrangement of multiple components in multivariate MOFs is exceedingly desired for applications like rapid energy transfer, spintronics, sensing, designing of optical logic gates, etc. Of course challenges lie critically with the stability of mixed-component isoreticular MOFs, specifically with mixed-metal type which are highly moisture-sensitive and prone to easy oxidation due to varying oxidation states. The main challenges faced during synthesis of multicomponent porous materials are lack of one’s efficiency over structural control and occurrence of phase separation during mixing of two or more units for synthesis. In this review, we systematically describe all the synthetic methodologies of mixed-component isoreticular metal–organic frameworks, addressing the pore-chemistry along with suitable illustrative applications. We also highlight the probable ways of improving the stabilities of these multi-component MOF materials and identify the possible loopholes which provide extensive opportunities to young researchers to pursue detailed exploration in this field.