Synthesis and Properties of Iridium Bis(phosphinite) Pincer Complexes (<i>p</i>-XPCP)IrH₂, (<i>p</i>-XPCP)Ir(CO), (<i>p</i>-XPCP)Ir(H)(aryl), and {(<i>p</i>-XPCP)Ir}₂{μ-N₂} and Their Relevance in Alkane Transfer Dehydrogenation

Abstract

A series of bis(phosphinite) (p-XPCP)IrH2 pincer complexes {[PCP = η3-5-X-C6H2[OP(tBu)2]2-1,3], X = MeO (6a), Me (6b), H (6c), F (6d), C6F5 (6e), and ArF [=3,5-bis(trifluoromethyl)phenyl] (6f)} have been synthesized by dehydrochlorination of (p-XPCP)IrHCl precursor complexes 4a−f with NaOtBu in the presence of hydrogen. Dehydrochlorination of 4f in the presence of nitrogen yields {(p-ArFPCP)Ir}2{μ-N2} (11f), which was analyzed by X-ray diffraction. Complexes 6a−f exhibit identical catalytic activity in the transfer dehydrogenation of cyclooctane (COA) with tert-butylethylene (TBE) when compared to mixtures of precatalysts 4a−f and NaOtBu. The electronic properties of the fragments (p-XPCP)Ir (Aa−f) are discussed on the basis of the νCO of (p-XPCP)Ir(CO) complexes (8a−f) as well as on 1JHD coupling constants of monodeuterated complexes (p-XPCP)IrHD (6a−f-d1). Reaction of 4a−f with NaOtBu in arene solvents generates (p-XPCP)Ir(aryl)(H) complexes (9 and 10), which undergo rapid arene exchange on the NMR time scale. Exchange rates are zero-order in free arene, implying a dissociative exchange mechanism. More electron-deficient complexes, e.g., (p-C6F5PCP)Ir(m-xylyl)(H) (10e) or (p-ArFPCP)Ir(m-xylyl)(H) (10f), reductively eliminate m-xylene significantly faster than the more electron-rich complexes, e.g., (p-MeOPCP)Ir(m-xylyl)(H) (10a), on the basis of the line widths Δν1/2(0 °C) of the hydridic NMR resonances of (p-XPCP)Ir(m-xylyl)(H) complexes 10a−f. The same correlation with substituent effects applies to the catalytic activity (initial turnover frequencies) of complexes 6a−f in the transfer dehydrogenation of COA with TBE.