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)IrH 2 pincer complexes {[PCP = η 3 -5-X-C 6 H 2 [OP( t Bu) 2 ] 2 -1,3], X = MeO ( 6a ), Me ( 6b ), H ( 6c ), F ( 6d ), C 6 F 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 NaO t Bu in the presence of hydrogen. Dehydrochlorination of 4f in the presence of nitrogen yields {( p -Ar F PCP)Ir} 2 {μ-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 NaO t Bu. The electronic properties of the fragments ( p -XPCP)Ir ( Aa − f ) are discussed on the basis of the ν CO of ( p -XPCP)Ir(CO) complexes ( 8a − f ) as well as on 1 J HD coupling constants of monodeuterated complexes ( p -XPCP)Ir HD ( 6a − f - d 1 ). Reaction of 4a − f with NaO t Bu 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 6 F 5 PCP)Ir( m -xylyl)(H) ( 10e ) or ( p -Ar F PCP)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 Δν 1/ 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.