Since joining the chemistry faculty in 1981, Profesor Bleeke has established a nationally and internationally recognized research program in the area of transition metal organometallic chemistry. He is particularly well known for his research with metallabenzenes and other aromatic metallacycles.
Ever since Kekule’s intuitive idea on the structure of benzene, “aromaticity” has been one of the most fascinating and provocative research topics in chemistry. While benzene is the archetypical aromatic compound, it is now well known that heterocyclic analogues of benzene containing N, O, P or S also exhibit aromatic properties. In contrast very little is known about metallacyclic benzenoid compounds, i.e., benzene analogues in which a CH group has been formally replaced by a transition metal and its associated ligands. Such “metallabenzenes” represent a fundamentally new class of aromatic compounds in which metal d orbitals participate fully with carbon p orbitals in the formation of ring pi-bonds. We have succeeded in synthesizing metallabenzenes using an approach that employs pentadienyl reagents as the source of ring carbon atoms and C-H bond activation in the key ring-forming step. Using this strategy, we have synthesized red crystalline “iridabenzene” (see figure 1) in high yield.
X-ray crystallography of 1 has confirmed the presence of a fully delocalized (and almost planar) metallacyclic ring, while the 1H NMR spectrum exhibits downfield chemical shifts, consistent with the presence of an aromatic ring current. Iridabenzene 1 exhibits a rich and varied reaction chemistry. Some of these reactions are typical of conventional organic arenes while others differ sharply due to the powerful influence of the transition metal center.
More recently, we have begun to study the synthesis of heteroatom-containing analogues of 1, species such as irdidafuran, iridapyrylium, iridathiophene, iridathiabenzene, and iridapyrrole. Our goal is to generate a family of closely-related molecules, enabling us to assess the effects of ring size and heteroatom incorporation on stability, structure, spectroscopy, and reactivity.