Investigations into the mechanisms of catalytic aliphatic dehydrogenation by transbisphosphine iridium complexes

Abstract

transbisphosphine iridium complexes represent a class of homogeneous catalysts with high potential for alkane to olefin transformation applications. The chemical industry is almost exclusively olefin-based and catalysts are responsible for the manufacture of most of the products. Consequently, the design and synthesis of a catalyst that can selectively produce the desired olefins from the enormous stock of alkanes available from petroleum and natural gas, is highly desirable. An understanding of the fine mechanistic processes occurring during the catalytic cycle is invaluable to the successful synthesis of a practical system. Therefore, we investigated the mechanisms of catalytic aliphatic dehydrogenations by transbisphosphine iridium complexes. A deuterium labeling study was undertaken to probe agostic interactions in the dehydrogenation reaction pathways of four related transbisphosphine iridium complexes in order to understand what role intramolecular interactions play in the catalyst's performance. The labeling studies included a rate study to compare the catalysts' agostic activities. These studies were followed with reaction of the complexes in neat neohexene to further elucidate the mechanisms. The results of these investigations reveal how agostic interactions stabilize significant intermediates and even stimulate important reaction steps of the catalytic cycle. The higher activity of the pincer type catalysts was shown to be attributable to their agostic access facility endowed by their molecular structure.