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40329-B3
Investigating the Strength and Reactivity of the Metal-eta-2- Arene Bond: A Laser Flash Photolysis Study Employing Infrared Detection
Since the start of the funding period in June 2004, five students have worked on the proposed research project which has resulted in two publications and a presentation at a National Meeting of the American Chemical Society. During the last year of the project, we continued our investigation into the factors that influence the reactivity of metal-η2-arene complexes. The results of these studies were presented at the Chicago meeting of the American Chemical Society in March 2007 by my students Amy Grunbeck and Laura Bahorich.
Earlier we found that in the case of the (η6-C6H6)Cr(CO)2(η2-C6H6) complexes, the primary bonding interaction between the metal center and the η2 coordinated arene was σ donation of electron density from the arene to the metal. Having investigated the electronic component of this interaction, we were interested in studying the steric influence of the coordinated arene upon the reactivity of the Cr-η2-arene bond. Thus experiments were conducted in which the steric bulk of the η2-arene was systematically increased by adding a CH3 group. Thus, the relative rates at which η2 coordinated benzene, toluene, m-xylene, and mesitylene were displaced from (η6-C6H6)Cr(CO)2(η2-arene) was studied. These complexes were generated by photolysis of the (η6-C6H5CF3)Cr(CO)3 complex by 355 nm light in the presence of the appropriate arene. The rate of displacement of the η2-arene by pyridine was then measured using the technique of laser flash photolysis.
The results suggested that the displacement rate was not affected significantly as the steric bulk of the η2-arene increased from benzene to m-xylene. However, upon addition of the third methyl group (mesitylene) the second order rate constant increased by almost a factor of 8. These results shed some light on the interplay between the electronic and steric factors in determining the overall reactivity of the Cr-η2-arene bond. As discussed earlier, our previous work has demonstrated that the primary component of the (η6-C6H6)Cr(CO)2-(η2-arene) bond is the donation of σ electron density from the arene to the metal. Thus, systematic addition of CH3 groups should result in a progressively more electron rich arene and therefore lead to a progressively less reactive Cr-(η2-arene) bond. However, these groups also add to steric crowding which makes it more difficult for the arene to approach the metal at an optimum bonding distance. Our results indicate that from benzene to m-xylene, the increasing steric hindrance counteracts the effect of increasing electron density such that the displacement rate does not change significantly. However, steric effects overwhelm electronic considerations upon addition of a third methyl group, resulting in a marked increase in the rate of displacement of mesitylene compared to the other arenes from the Cr center. Further studies with different metal centers are planned to fully understand the interplay between steric and electronic considerations in determining stability of these types of complexes.
