Postsynthetic Modification: An Enabling Technology for the Advancement of Metal-Organic Frameworks

Abstract

Metal-organic frameworks (MOFs) are a class of porous materials with immense chemical tunability derived from their organic and inorganic building blocks. Presynthetic approaches have been used to construct tailor-made MOFs, but with a rather restricted functional group scope limited by the typical MOF solvothermal synthesis conditions. Postsynthetic modification (PSM) of MOFs has matured into an alternative strategy to broaden the functional group scope of MOFs. PSM has many incarnations, but two main avenues include (1) covalent PSM, in which the organic linkers of the MOF are modified with a reagent resulting in new functional groups, and (2) coordinative PSM, where organic molecules containing metal ligating groups are introduced onto the inorganic secondary building units (SBUs) of the MOF. These methods have evolved from simple efforts to modifying MOFs to demonstrate proof-of-concept, to becoming key synthetic tools for advancing MOFs for a range of emerging applications, including selective gas sorption, catalysis, and drug delivery. Moreover, both covalent and coordinative PSM have been used to create hierarchal MOFs, MOF-based porous liquids, and other unusual MOF materials. This Outlook highlights recent reports that have extended the scope of PSM in MOFs, some seminal reports that have contributed to the advancement of PSM in MOFs, and our view on future directions of the field.

Figure 1

Illustrative schematic for the synthesis of UiO-66. A Zr⁴⁺-metal source is combined with a H₂bdc under solvothermal conditions to form a three-dimensional extended framework (UiO-66). For simplicity, the cartoon of a small subunit (bottom right) of the MOF lattice will be used throughout the manuscript to generally depict an extended MOF crystallite and is not only used to represent the UiO-66 framework. Green polyhedra represent Zr⁴⁺ ions, red spheres O atoms, and silver sticks C bonds.

Figure 2

Illustrative schematic of MOFs and PSM. Depiction of covalent PSM using reactive groups on the MOF linker (blue spheres representing amine groups) for modification by an organic reagent (blue acid chloride reagent). Depiction of coordinative PSM using a coordinating organic molecule (red acid reagent) to bind to the MOF SBU.

Figure 3

Top: illustrative schematic depicting the PSP synthesis of MOF–nylon materials. First, amine functional groups on the MOF are covalently bonded to diacyl chloride molecules through covalent PSM. Subsequently, through interfacial PSP, diamine is introduced resulting in polyamide chains growing and linking MOF particles. Bottom: illustrative schematic depicting the synthesis of MOF–PTU hybrid materials. First, a PSM step in which the −NCS groups on the MOF (red semicircles) undergo covalent PSM with an excess of amine-functionalized polymer (blue). Subsequently, through the introduction of a diisothiocyanate molecule (red), chains of polymer are grown resulting in a monolithic MOF–PTU hybrid material.

Figure 4

Illustrative schematic of the design of hierarchal MOF–polymer hybrid materials formed through covalent PSM linkages. First, using a Zr⁴⁺-based MOF for nucleation, Cu-based MOF ligands and metal salts are added forming a hetero-MOF structure. Similarly, the Zr⁴⁺-Cu²⁺-MOF structure is used as a nucleation site for the growth of a Zn²⁺-based MOF. Then, the Zn-MOF linkers are polymerized through covalent PSM. Subsequently, the polymerized MOF assembly is treated with acid resulting in deterioration of the Zn²⁺- and Cu²⁺-based MOFs resulting in a hierarchal MOF–polymer hybrid gel.

Figure 5

Illustrative schematic for the design of MOF porous liquids. First, coordinative PSM is used to tether a polymer to the MOF surface that contains polymer initiator sites. Subsequently, polymer chains are grafted from the MOF surface resulting in a porous liquid.

Figure 6

Illustrative schematic for the synthesis of phospholipid MOFs. A phospholipid coordinative molecule is first linked at the MOF SBU. Subsequently, the favorable interactions at the MOF result in the formation of colloidally stable MOFs as a function of the hydrophobic lipid assembly.

Figure 7

Scheme for the design of DNA-functionalized MOFs through coordinative PSM. Single-stranded DNA with phosphate coordinating groups was tethered to the MOF via the SBUs. Subsequently, with the addition of AuNPs surface coated with a complementary DNA strand, MOF particles are assembled around the AuNP as a function of the double helix formation.