New “NCOP” Pincer Ligands Supporting Iridium Catalysts for the Catalytic Activation of Alkanes

The development of new catalysts for the activation and functionalization of C-H bonds remains a key area of research. Pincers are among the most widely used supporting ligands for catalysts in this field, and particularly for iridium-catalyzed dehydrogenation. All such pincer ligands used for this purpose to date have included two phosphine groups trans to each other when coordinated (e.g PCP, POCOP, POCP). It is considered that the two moieties are necessary to furnish thermal stability and steric and electronic features needed to promote C-H bond activation. Here, we report that newly prepared POCN pincers (bearing only one phosphine group) effectively support the synthesis of thermally stable Ir(I) and Ir(III) complexes. Surprisingly, these complexes serve as good catalysts for the transfer-dehydrogenation of linear and cyclic alkanes. Complexes (^iPrNCOP^tBu)Ir(C₂H₄), (^iPrNCOP^iPr)Ir(C₂H₄), (^iPrNCOP^tBu)IrHCl (plus base) and (^iPrNCOP^iPr)IrHCl (plus base), where (^iPrNCOP^tBu) and (^iPrNCOP^iPr) are the monoanionic phosphino-amino pincer ligands 1,3-C₆H₃(CH₂NⁱPr₂)(OP^tBu₂) and 1,3-C₆H₃(CH₂NⁱPr₂)(OPⁱPr₂) respectively. were obtained in good yields and characterized by spectroscopic methods including multinuclear NMR spectroscopy and X-ray single crystal analyses. We will discuss their reactivity and compare with the corresponding PCP pincer complexes, with respect to both the differences engendered by a coordinated amino group vs. a phosphino group, as well as the possibility of hemilability resulting from dissociation of the amino group.