Carbene Curiosities

Two advances in the chemistry of N-heterocyclic carbenes (NHCs) demonstrate the range of reactivity for these unusual molecules. One study reports a new use for NHCs as organometallic catalysts, and another details the isolation of so-called abnormal NHCs at room temperature. Both discoveries, published in Science, offer new avenues for chemical synthesis.

Carbenes have a reputation as highly reactive species, due to their divalent carbon possessing only six electrons. Over the years, chemists have tamed these molecular beasts for use as catalysts and ligands through clever use of electronic and steric stabilization. NHCs—in which the carbene carbon exists within a heterocycle and is flanked by two nitrogen atoms—are one such example.

Through careful selection of NHC moiety, Caltech’s Robert H. Grubbs and Vincent Lavallo used the molecules to catalyze unusual organometallic transformations (Science 2009, 326, 559). Although NHCs have been employed as metal ligands and organocatalysts in the past, their use as catalysts in organometallic reactions is rare.

In one stoichiometric reaction, Grubbs and Lavallo used an NHC with mesityl group substituents to convert bis(cyclooctatetraene)iron into a tetra­metallic species with two diiron regions. When they replaced the NHC’s mesityl units with bulky diisopropylphenyl groups, the reaction gave rise to a triiron cyclooctatetraene cluster in a catalytic process.

“This rare case of a catalytic organometallic transformation challenges the paradigm that metal-carbene bond making and breaking are one-way processes,” writes University College Dublin’s Martin Albrecht in a commentary that accompanies the papers. “Such bond making between iron centers may help to construct electronically active systems,” such as molecular wires, he adds.

In a discovery that gives chemists access to new NHC-based ligands, a team led by the University of California, Riverside’s Guy Bertrand synthesized a metal-free abnormal NHC that’s stable at room temperature (Science 2009, 326, 556). Abnormal NHCs have a carbene carbon that’s adjacent to only one nitrogen atom.

Bertrand’s team created this unusual molecule by putting phenyl groups at two of the three carbons and adding bulky diisopropylphenyl substituents to the nitrogens. Through judicious choice of a base, the group created an abnormal NHC that is stable both as a solid and in solution. They also showed that the compound forms adducts with gold and carbon dioxide.

“Normal carbenes rapidly became a key tool for organometallic chemistry and organic synthesis once they were available as stable free ligands some 20 years ago,” Albrecht comments. “Given the unique impact of abnormal carbenes on the reactivity of transition metals, the accessibility of free abnormal carbenes may become another cornerstone in this field, and it will be exciting to witness developments in these directions.”