Isoreticular Expansion of Metal-Organic Frameworks with Multiple Functionalities and Controlled Pore Sizes

Metal-Organic Frameworks (MOFs) are made by linking organic and inorganic molecular building blocks into extended structures through strong bonds. With a judicious choice of inorganic joints and various functional groups available in organic links, a large number of MOFs have been synthesized in the past decade. Along with the fast expansion of the family of MOFs, important applications emerge including hydrogen storage and carbon dioxide capture, both of which address the most pressing societal demand for clean and sustainable energy resources.;Although numerous MOFs are now known and they have found widespread applications, the introduction of more than one kind of building block into their crystal structures remains challenging. One of the main objectives of this study is to demonstrate the successful incorporating of multiple functional groups into MOFs. Here, a new strategy has been developed to achieve the synthesis of a series of eighteen multivariate MOFs (MTV-MOFs) containing up to eight distinct functional groups, while their parent topologies were fully preserved. The backbone of these MTV-MOFs was found to be ordered, while the orientation, number, relative position and ratio of the functionalities along the backbone could be controlled by virtue of the unchanged length of the link and its unaltered connectivity. This strategy allows us to endow the pores of these MOFs with a new level of complexity which far exceeds any held by that of the original mono-functional MOFs—an aspect that makes it possible to fine-tune the pore environment of a porous crystal with favorable implications. Indeed, one member of these MTV-MOFs has already shown an 87% improvement of the hydrogen uptake while another member demonstrated a 400% increase in CO2 selectivity comparing to their mono-functional counterparts.;Another goal of this study has been to maximize MOF porosity and pore size. There were three major obstacles against expanding the pore size of porous crystals: (a) long organic links usually exhibit low solubility, making them inaccessible for MOF synthesis; (b) the formation of interpenetrated structures will block the pores; and (c) frameworks with large pores usually collapse upon guest removal. In this study, strategies were designed for the first time to overcome all three limitations and thus allow us to expand the pores of MOFs into a new size regime (> 50 Å). In particular, organic links containing one to eleven phenylene groups were used to produce a series of isoreticular MOFs (IRMOF-74) with progressively increased pore aperture up to nearly 100 Å. It is remarkable that all nine members of this series have non-interpenetrating structures, high thermal stability, and permanent porosity. Seven of them break pore aperture record, a key parameter in porosity because it controls the size of molecules that might be captured into the pore. Furthermore, several macromolecules, including natural proteins such as myoglobin and green fluorescent protein, were chosen to demonstrate the accessibility of large pores in some of these MOFs.