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 Kekulé'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 also exhibit aromatic properties. These heterocyclic benzenoid compounds, in which a CH group is formally replaced by an isoelectronic heteroatom, include pyridine, phosphabenzene, pyrylium, and thiopyrylium. In contrast, virtually nothing 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-bonds.
We have succeeded in synthesizing metallabenzenes using an approach that employs pentadienide reagents as the source of the ring carbon atoms. As shown in the scheme below, treatment of (Cl)Ir(PEt3)3 with potassium 2,4-dimethylpentadienide produces the 1-metallacyclohexa-2,4-diene complex 1, via iridium-mediated activation of a C-H bond on the terminus of the 1-pentadienyl ligand in intermediate A. Compound 1 is then "dehydrogenated" in two steps: a) removal of the hydride ligand with methyltriflate and b) deprotonation of the alpha-ring carbon with base. The red crystalline product, "iridabenzene" 2, is obtained in high yield.
The solid state structure of 2 has been determined by single crystal X-ray diffraction. As expected, the six-membered ring is nearly planar and bonding within the ring is delocalized. The C-C bond lengths are similar to those found in benzene itself and the Ir-C distances are intermediate between normal single and double bonds. In the 1H NMR, the ring protons appear at low-field chemical shift values, as expected for an aromatic molecule capable of sustaining an induced ring current.
As shown in the scheme at right, iridabenzene 2 exhibits a rich and varied reaction chemistry. While some of these reactions mimic those of conventional organic arenes, others differ sharply due to the powerful influence of the transition metal center. From these studies, the guidelines for what constitutes "aromatic behavior" in metallacycles are beginning to emerge.