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Acid-To-Base Transmutation

Inorganic chemistry: First stable metal-free borylene adduct could aid catalyst design

by Stu Borman
August 1, 2011 | A version of this story appeared in Volume 89, Issue 31

A basic borylene adduct stabilized by two carbenes (center) forms a conjugate acid when protonated (top) or a radical cation when oxidized (bottom).
A basic borylene adduct stabilized by two carbenes (center) forms conjugate acid (top) when protonated or a racical cation when oxidized (bottom).
A basic borylene adduct stabilized by two carbenes (center) forms a conjugate acid when protonated (top) or a radical cation when oxidized (bottom).

Unlike alchemists’ fruitless efforts to turn base metals into gold, researchers have succeeded in transmuting conventional boron compounds, which are acidic, into stable borylene adducts, which are bases similar to amines. The adducts’ basicity could lead to new catalysts.

Borylenes, monovalent boron compounds with an electron lone pair and two vacant orbitals, have until now only been observed fleetingly, as transient intermediates. Stable species have been created by coordinating them with transition metals. But no stable, basic borylene adducts had been found.

Chemistry professor Guy Bertrand of the University of California, Riverside, and France’s Centre National de la Recherche Scientifique and coworkers have now reacted a carbene with a boron compound to create the first stable nonmetal borylene adduct (Science, DOI: 10.1126/science.1207573). The adduct is a base with a donatable electron lone pair.

Ligands used to create catalytic metal complexes must generally be compounds with electron lone pairs, such as nitrogen or phosphorus bases. Acidic boron compounds don’t qualify, but basic ones might prove useful as ligands for a new class of catalysts. UC Riverside has applied for a patent on such ligands.

Borylenes are electron deficient because they have only four electrons instead of the usual eight in their valence shells. The donation of two carbene electron pairs to boron’s vacant orbitals gives the stable borylene adduct a valence electron configuration identical to that of nitrogen.

Bertrand and coworkers demonstrated the borylene adduct’s basicity by protonating it, forming the conjugate acid, and oxidizing the adduct to form a radical cation similar to those formed by oxidizing amines. Both the borylene adduct and radical cation are stable under argon at room temperature for at least two months, the researchers note.

Boron specialist Holger Braunschweig of the University of Würzburg, in Germany, says the work is “a major breakthrough in organoelement chemistry as it provides unprecedented proof for the existence of a metal-free borylene adduct under ambient conditions.” The development “is truly spectacular” and “clearly an important discovery,” adds Lewis acid specialist François P. Gabbaï of Texas A&M University.



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