Synthesis and Characterization of PBP Pincer Iridium Complexes and Their Application in Alkane Transfer Dehydrogenation

This work reports on the synthesis of several new complexes of Ir supported by a diarylboryl/bis­(phosphine) PBP pincer ligand. The previously reported complexes (PBP)­Ir­(Ph)­(Cl) (1) and (PBP)­Ir­(H)­(Cl) (2) were converted to the new complexes (PBP)­IrH4 (3) and (PBP)­Ir­(Ph)­(H) (4). Complexes 3 and 4 serve similarly as precatalysts for transfer dehydrogenation of cyclooctane. The turnover numbers achieved were relatively modest but were increased (to 220 at 200 °C) when 1-hexene was used as a sacrificial hydrogen acceptor vs tert-butylethylene. The dicarbonyl complex (PBP)­Ir­(CO)2 (6) was also synthesized, by the reaction of CO with either 3 or 4. Intermediates (PBPhP)­Ir­(H)­(CO)2 (5) and (PBP)­IrH2(CO) (7) were observed in these reactions. Complex 7 could be obtained in pure form by comproportionation of 3 and 6. Solid-state structures of 3 and 6 were determined by X-ray crystallography.