The focus of our lab is supramolecular catalysis. A long-standing goal of the field is the construction of catalysts that more accurately mimic enzyme active sites, i.e catalytic sites that are confined, highly functionalized, and flexible. To this end, our group has introduced metal-organic framework (MOF) materials as scaffolds on which we can deliberately organize complex chemical functionality within confined, 3-dimensional space. Our methodology entails the assembly of modifiable organic linkers into sufficiently porous, independently modifiable MOF materials, and their sequential elaboration with additional functionality.
In our NSF-funded research we have developed a strategy to uniformly multifunctionalize MOFs post-synthesis (Figure 1, Link). We have recently extended this work with the use of orthogonal functionalization chemistry which allows us to generate uniformly multifunctional MOFs simultaneously in one pot (Link).
Figure 1. A: The self-assembly of modifiable organic linkers with zinc cations to generate a bifunctionalizable metal-organic framework (MOF) material, KSU-1. B: Schematic representation of the sequential, independent functionalization of KSU-1 to yield a uniformly multifunctional MOF material.
With these multifunctional MOFs we have also demonstrated confinement effects that result in emergent properties that are reminiscent to those observed in enzymes. We observed a reversal of reactivity of amine-functionalized and hydroxyl-functionalized organic linkers when both are immobilized together in MOF pores (Figure 2, Link).
Figure 2. When immobilized in MOFs individually, amine-functionalized and hydroxyl-functionalized linkers react as expected with isocyanates: the amines react faster than the hydroxyls despite being immobilized in a MOF with smaller pores. When the two linkers are immobilized together, the reactivity reverses for less electrophilic isocyanates, with the hydroxls reacting to a larger extent than the amines.
Our uniform multifunctionalization strategy depends on our ability to construct mixed-linker frameworks that have large enough channels to accommodate additional functionality. In this context, we have provided evidence to support the theory that the use of organic linkers that have hydrogen-bonding substituents can prevent interpenetration of frameworks. We synthesized a non-catenated, mixed-ligand MOF that, to our knowledge, has the largest pore volume and lowest density of its topology (Scheme 1, Link). We have subsequently synthesized several other non-catenated mixed-linker frameworks where at least one of the organic linkers bears a functional group capable of H-bonding interactions.
Scheme 1. The self-assembly of modifiable organic linkers that have H-bonding substituents to generate a large-pore, low-density, independently functionalizable MOF material, KSU-100.