Synthesis and Properties of Iridium Bis(phosphinite) Pincer Complexes (p-XPCP)IrH_2,(p-XPCP)Ir(CO),(p-XPCP)Ir(H)(aryl),and {(p-XPCP)Ir}_2{mu-N_2} and Their Relevance in Alkane Transfer Dehydrogenation

摘要

A series of bis(phosphinite) (p-XPCP)IrH_2 pincer complexes {[PCP=eta~3-5-X-C_6H_2[OP-(tBu)_2]_2-l,3],X=MeO (6a),Me (6b),H (6c),F (6d),C_6F_5 (6e),and Ar~F [=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-Ar~FPCP)Ir}_2{mu-N_2} (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 v_(co) of (p-XPCP)Ir(CO) complexes (8a-f) as well as on ~1J_(HD) coupling constants of monodeuterated complexes (p-XPCP)IrHD (6a-f-d_1).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-C_6F_5PCP)Ir(m-xylyl)(H) (10e) or (p-Ar~FPCP)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 DELTAv_(1/2)(0degC) 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.