Attached to MOFs, frustrated Lewis pair catalysts become recyclable

Frustrated Lewis pairs (FLPs) stepped into the synthetic limelight just over a decade ago. Composed of a Lewis acid and a Lewis base whose blocked inclination to bond grants the molecules great powers, the main-group molecules were hailed for catalyzing hydrogenation reactions previously accomplished only with transition metals. The use of FLPs has been limited, however, by their lack of recyclability and their sensitivity to air and moisture.

Now, researchers led by Shengqian Ma at the University of South Florida have developed stable FLPs. Anchored inside a metal-organic framework (MOF), they efficiently catalyze hydrogenation and imine reduction reactions (Chem 2018, DOI: 10.1016/j.chempr.2018.08.018). With simple filtration, the catalyst can be recycled in the latter reaction at least seven times without loss of activity.

To construct the MOF-FLP pair, the team docked an amine Lewis base at the MOF’s open metal sites and then attached a boron Lewis acid (shown). The team then compared the heterogeneous MOF-FLP’s ability to catalyze reactions with that of a homogeneous catalyst. While the researchers observed comparable yields for the hydrogenation reaction, Ma says the MOF-FLP showed “interesting” steric and size selectivity in the imine reduction. The MOF-FLP matched the homogeneous catalyst’s high yield unless the substrate contained a buried imine, suggesting that the MOF environment restricts access to certain substrates. Substrates beyond a certain size were not reduced, likely because they can’t squeeze through the MOF’s pores, the authors say.

“The beauty of MOFs is that they’re very tunable,” Ma says. Future MOF designs could easily introduce chirality or superhydrophobicity to expand the FLP catalysts’ reaction scope and durability, he says.

The University of Toronto’s Douglas W. Stephan, whose group pioneered the use of FLPs, calls the work a “remarkably clever and facile strategy.” The readily handled MOF-LPs avoid the air and moisture sensitivity of most homogeneous FLP catalysts, and their recyclability could also lower costs, Stephan says. The study, he adds, “foreshadows a vast potential for an array of new stable, recyclable, and selective FLP catalysts embedded in MOFs.”