AmericanChemicalSociety.com

The original goal of this project was to develop extended, bowl-shaped catalysts for organic reactions that would direct selectivity and specifity based on the global size and shape of substrate molecules. In Years 1 and 2 of the grant period, we developed a synthesis of dendritic, bowl-shaped N-heterocyclic carbene ligands, which were applied in the substrate-selective hydrosilylation of ketone substrates. We found that the largest, 2nd-generation dendritic carbene ligand promoted specificity for smaller ketone substrates over larger ones. This work was published in 2009. Although this proof-of-concept project was moderately successful, efforts at developing improved ligands were hindered by increasingly long and low-yielding syntheses. Because the project described below has, in our judgment, a greater potential for making a significant impact in homogeneous catalysis, we have decided to focus our efforts in Year 3 here instead.

During Year 2, we initiated a project involving the synthesis and metalation of a series of rigid, aryl-substituted CCC-pincer ligands, with the goal of discovering useful applications in the catalysis of C-H functionalization reactions. Three new iridium complexes have been synthesized and structurally characterized, with the general formula Ir(CCC)(MeCN)HCl. These complexes are active catalysts for arene C-H borylation and alkane dehydrogenation, two reactions of great potential importance for the functionalization of lower-cost hydrocarbons extracted from petroleum. We have recently (May 2010) published on the ligand synthesis, metalation, and initial catalytic trials. We are currently examining the stoichiometric reactivity of our CCC-Ir complexes, with the goals of isolating more reactive precatalysts for alkane dehydrogenation, and identifying potential catalytic intermediates for mechanistic study. We are also designing the synthesis of a second generation of CCC-pincer ligands, based on the results of the first study, which indicated that extremely bulky side groups favor increased catalyst stability and higher turnover numbers.