Reports: UR1050635-UR10: Synthesis of Crystalline Porous Materials with Functional Open Metal Sites

Xianhui Bu, PhD, California State University (Long Beach)

In the PI’s earlier work reported to the PRF in 2012, the PI’s group developed a strategy for encapsulating both single metal ions and di- or trimeric clusters into metal-organic framework materials. With this approach, a trifunctional ligand, 1,3,5-benzenetricarboxylate (BTC) in particular, uses two -COO groups to form the 3D framework while resorting to the third -COO group (called the hook) for metal capture. The BTC method is fundamentally different from the metalloligand method, because it is the cooperative action of multiple ligands (instead of multiple donor atoms from a single ligand) with 2, 3, or 4 hooks (one hook per ligand) that results in the capture of metal ions/clusters. Still, both the BTC method and the metalloligand method rely on specific features of framework organic ligands for metal capture (i.e., they have more functional groups than needed for the framework formation). As such, the strategies based on BTC and metalloligands are unusable for many common ligands (e.g., bifunctional ligands used in the synthesis of well-known MOFs such as MOF-5, MIL-88, and MIL-101) that are devoid of sparefunctional group after the framework formation.

In the past year, the PI’s group developed a new strategy (called the extended hook method) that makes it possible to construct MOFs with hooks from bifunctional ligands and thereby removes the intrinsic limitations of either the BTC or metallologand method, in terms of the need for polyfunctional ligands. The essence of the PI’s new method is to have hooks coming from the inorganic nodes, in contrast with those located on framework crosslinking ligands such as BTC or porphyrin. As a result, this method for the hook incorporation is in principle independent of the framework organic ligands. Clearly, it is possible to devise various ways for the anchoring of hooks onto inorganic nodes in the framework wall, considering the diversity of inorganic nodes. In this work, the PI’s group took advantage of a commonly observed feature in MOFs, open metal sites, to add an auxiliary ligand (called the extended hook here, because it is much longer than the –COO hook on BTC) whose pyridal end is anchored to the open metal sites on the framework, while its carboxyl end serves as the lengthened hook to capture metal ions or clusters at the centers of channels. This extended hook method can be tailored to different MOFs by matching the length of the hook with the radius of cages or channels in MOFs.

The research supported by this PRF grant made a significant impact on the PI's project development and on the career of students involved. With this PRF support, the PI extended the application of urothermal synthesis to the heterometallic system and expanded urothermal synthesis method to include co-solvents with urea derivatives as either a major or a minor co-solvent. One graduate student, Andrew Clough finished his MS thesis (only MS degree is offered on campus) through participating in this PRF-supported project and has joined the PhD program at University of Southern California. An undergraduate student, Addis Fuhr, was accepted into the PhD program at UCLA, in part due to his successful participation in this PRF-supported project.