Forging Ligands

By coupling a carborane and an amidinate—two ligands that coordinate metals well on their own—a team of German chemists has created a new class of bifunctional ligands that can chelate metals while leaving an NH group available for further derivatization. These combined abilities of carboranylamidinates could prove useful for an array of applications, from catalysis to making polymer films.

Carboranes are carbon-boron icosahedral cluster compounds that feature extensively in the synthesis of polymers, ceramics, catalysts, and radiopharmaceuticals. Amidinate ligands, RC(NR')₂ ^–, are widely used to construct main-group, transition-metal, and f-element complexes, often as alternatives to cyclopentadienyl ligands. They make possible a large degree of variability through use of different substituents at carbon or nitrogen.

Peter Dröse, Cristian G. Hrib, and Frank T. Edelmann of Otto von Guericke University, in Magdeburg, Germany, decided to see what would happen if the two ligands were joined (J. Am. Chem. Soc., DOI: 10.1021/ja108051u). The team treated C₂B₁₀H₁₂ carborane with butyllithium followed by diisopropylcarbodiimide, a reaction that created an isolable lithium complex. Hydrolysis of this complex yielded the free carboranylamidine.

A crystal structure of the lithium complex led to a surprise: The carborane’s unsubstituted CH is preferentially deprotonated over the amidinate’s NH, Edelmann notes. The result is bidentate C,N coordination of lithium by both the carborane and amidinate instead of the expected N,N coordination only by the amidinate. The NH remains uncoordinated and is available for further chemistry, Edelmann says. Initial experiments show that the carboranylamidinate coordinates chromium and tin to form monomeric or dimeric C,N complexes.

“The simplicity of the one-pot synthesis is quite neat, and the unexpected outcome of the ligand’s reactivity is remarkable,” says M. Frederick Hawthorne of the University of Missouri, Columbia, a leading expert in carborane chemistry. Because the ligand binds chromium and tin, “it should bind many metals in the periodic table, leading to opportunities to make functional materials and catalysts,” he says.

Chemical & Engineering News

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