Highly Active Catalysts for the Transfer Dehydrogenation of Alkanes: Synthesis and Application of Novel 7-6-7 Ring-Based Pincer Iridium Complexes

A series of Ir-PCP pincer precatalysts [(7-6-7-(PCP)-P-R)Ir(H)(Cl)] and [(7-6-7-(PCP)-P-Ar)Ir(H)(Cl)(MeCN)] bearing a novel 7-6-7 fused-ring skeleton have been synthesized based upon the postulate that the catalytic species would have durability due to their rather rigid structure and high activity owing to the low but sufficient flexibility of their backbones, which are not completely fixed. tBu gave rise to the active 14 electron (14e) species [(7-6-7-(PCP)-P-iPr)Ir] and [(7-6-7-(PCP)-P-Ph)Ir], which can trap hydrogen and were spectroscopically characterized as the tetrahydride complexes. Both [(7-6-7-(PCP)-P-iPr)Ir] and [(7-6-7-(PCP)-P-Ph)Ir] were found to be highly effective in the transfer dehydrogenation of cyclooctane with tert-butylethylene as the hydrogen acceptor, the initial reaction rate at high temperature (230 degrees C) being higher for [(7-6-7-(PCP)-P-iPr)Ir] than [(7-6-7-(PCP)-P-Ph)Ir], and the turnover number ( (PCP)-P-Ph)Ir] complex is the catalyst of choice for this reaction. Structural analysis and computational studies support the importance of the low flexibility of the ligand core.