Cooperative Catalysis in a Crystalline Framework with Templated Acid–Base Sites

Abstract

Nature uses weakly basic residues in conjunction with weakly acidic residues to catalyze challenging heterolytic bond transformations. Here, we show that these cooperative effects can be replicated in a metal−organic framework (MOF) containing bifunctional Brønsted acid−base sites. Using a templating strategy, we show that the colocalization of acid and base sites is key to catalytic activity. Specifically, a thermolabile cross-linker containing tertiary ester and tertiary carbamate linkages is used to tether carboxylic acid and benzylamine pairs in close proximity during framework synthesis. These templated materials are over 4-fold more active aldol condensation catalysts than nontemplated materials containing randomly distributed acid and base sites. Together, this work establishes MOFs as an exciting platform for cooperative acid−base catalysis that couples the advantages of heterogeneous catalysts with the structural precision of enzymes.

Engineering Bimetallic Active Sites in Metal–Organic Frameworks: Challenges and Opportunities

“…To address this challenge, we recently reported a strategy to introduce closely spaced pairs of functional groups within MOF pores, irrespective of functional group loading. ,, We first showed that thermolabile tertiary ester-based cross-linkers can be used to template pairs of carboxylic acids ∼7 Å apart down the pore channels of Mg 2 dotpdc (dotpdc 4– = 4,4″-dioxido-[1,1′:4′,1″-terphenyl]-3,3″-dicarboxylate), a mesoporous framework with one-dimensional hexagonal channels. We later developed tertiary carbamate-based cross-linkers that, upon thermolysis, reveal pairs of templated amines (Figure a) .…”