Synthesis and properties of iridium bis(phosphinite) pincer complexes (p-XPCP)IrH2, (p-XPCP)Ir(CO), (p-XPCP)Ir(H)(aryl), and {(p-XPCP)Ir}2{μ-N2} and their relevance in alkane transfer dehydrogenation

A series of bis(phosphinite) (p-XPCP)IrH2 pincer complexes {[PCP = eta(3)-5-X-C6H2[OP-(tBu)(2)](2)-1,3], X = MeO (6a), Me (6b), H (6c), F (6d), C6F5 (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-(ArPCP)-P-F)Ir}(2){muN(2)} (11f), which was analyzed by X-ray diffraction. Complexes 6a-f exhibit identical catalytic activity in the transfer dehydrogenation of cyclooetane (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 nu(CO) of (p-YPCP)Ir(CO) complexes (8a-f) as well as on (1)J(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-C6F5PCP)Ir(m-xylyl)(H) (10e) or (p-(ArPCP)-P-F)Ir(m-xylyl)(H) (10f), reductively eliminate m-xylene significantly faster than the more electron-rich complexes, e.g., (p-MeOCP)Ir(m-xylyl)(H) (10a), on the basis of the line widths Deltanu(1/2)(0degreesC) 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.