A Tale of Two Lattices

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Science  05 Jan 2007:
Vol. 315, Issue 5808, pp. 19
DOI: 10.1126/science.315.5808.19c

One promising aspect of metal organic framework (MOF) solids is the ease with which chiral components can be incorporated into the structural lattice. By linking metal centers with a network of chiral bridging ligands, researchers can prepare porous crystals with the potential to serve as robust asymmetric catalysts. However, assembling an extended MOF with specific steric and electronic properties remains highly challenging.

Wu and Lin highlight this challenge in presenting two MOF structures composed largely of the same building blocks, but exhibiting strikingly different lattice geometries and consequent properties. The first MOF was crystallized from a solution of cadmium nitrate and a chiral binaphthol derivative, appended with pyridines to bridge two metal centers. The hydroxyl groups in the lattice remained free to bind Ti(IV) centers, which in turn catalyzed ethylation of aromatic aldehydes with high yields and enantioselectivities. When a second MOF was prepared from the same precursors, but with the nitrate counterions replaced by perchlorate, a very different lattice structure emerged, which failed to catalyze the reaction. The authors suggest that steric crowding near the hydroxyls in this second structure inhibited effective binding of the titanium ions. — JSY

Angew. Chem. Int. Ed. 45, 10.1002/anie.200602099(2006).

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