CHEMISTRY: Bimetallic Bases

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Science  13 May 2005:
Vol. 308, Issue 5724, pp. 929a
DOI: 10.1126/science.308.5724.929a

The typical notion of a chemical base is a reactive molecular anion bound to a charge-balancing metal cation. Although the influence of anion structure on reactivity has long been appreciated, the metals are often regarded as interchangeable bystanders. Recent work suggests that the cations can play a strong role, however, particularly when two of them cooperate.

Andrikopoulos et al. prepared a hybrid base of butyl sodium and magnesium bis(tetramethylpiperidide). The butyl anion in the complex strips a proton from the meta-carbon in toluene, three carbons away from the more acidic methyl group. The origins of this unexpected reactivity were clarified by determining a crystal structure and by performing density functional calculations. The Mg cation stabilizes the deprotonated meta-carbon, whereas the Na cation interacts with electrons above the phenyl ring.

Similarly, García et al.treated aminophenylphosphine (a phenyl ring bearing adjacent NH2 and PH2 groups) successively with butyl lithium and bis(dimethylamido)tin. In the resulting product, which is not produced by either base alone, all four protons have been stripped from the N and P centers. Crystallography revealed a complex structure in which four of the amido phosphide molecules are stabilized by a network of six Sn and four Li cations. — JSY

Angew. Chem. Int. Ed. 10.1002/anie.200500379; 10.1002/anie.200500340 (2005).

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