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Supramolecular allosteric catalyst Supramolecular coordination chemistry has been used to prepare a catalyst that functions like an allosteric enzyme. Graduate student Nathan C. Gianneschi and chemistry professor Chad A. Mirkin at Northwestern University and coworkers synthesized a macrocyclic assembly containing two rhodium(I) and two chromium(III) centers (the Cr centers are in the colored moieties shown) [J. Am. Chem. Soc., 125, 10508 (2003)]. "The rhodium portion of our complex allows us to control structure via well-defined coordination chemistry, and the chromium sites effect structure-dependent catalysis," Mirkin tells C&EN. The Rh(I) centers are weakly linked to ligands that can be selectively displaced by more strongly binding anionic or neutral ligands. Mirkin's group showed that the cavities of the tetrametallic complex can be switched between "closed" (top) and "opened" (bottom) states by reversible binding of Cl– ions and CO at the Rh(I) centers. In the closed state, the complex catalyzes the ring opening of cyclohexene oxide by trimethylsilyl azide to yield 1-azido-2-(trimethylsiloxy)cyclohexane with 68% enantiomeric excess. In the opened state, the reaction rate is doubled. This "weak-link" approach is a relatively straightforward way to build supramolecular structures inspired by biological systems, according to Mirkin. |
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