Amido Complexes of Iridium with a PNP Pincer Ligand: Reactivity toward Alkynes and Hydroamination Catalysis

The pincer ligand HN(CH 2 CH 2 PPh 2 ) 2 ( 1; PN H P) reacted with [{Ir(μ-X)(cod)} 2 ] (X = Cl, OMe), affording complexes [ fac -(PN H P)Ir(cod)]Cl ( 2 ) and [ fac -(PNP)Ir(cod)] ( 3 ), respectively. The X-ray molecular structure of 2 showed that the PNP ligand coordinates in a facial fashion, with the N atom in an axial site and both P atoms coordinated in the equatorial plane. Compound 1 is able to protonate the hydroxo bridges in the complex [{Ir(μ-OH)(coe) 2 } 2 ] forming the new amido complex [ mer -(PNP)Ir(coe)] ( 4 ). Complex 4 is an extremely air sensitive compound, as confirmed by the isolation of the oxo complex [ mer -(PNP)Ir(η 2 -O 2 )] ( 8 ) from its interaction with air. Protonation of 4 with HBF 4 afforded the corresponding amino complex [ mer -(PN H P)Ir(coe)]BF 4 ( 5 ), whose molecular structure enlightened by X-ray crystallography confirmed the PNP ligand to be coordinated in a meridional fashion. The coe ligand in 4 is tightly bonded to iridium; however, under an atmosphere of ethylene at 60 °C or with acrylonitrile at 70 °C complex 4 exchanges the olefin, affording compounds [ mer -(PNP)Ir(η 2 -C 2 H 4 )] ( 6 ) and [ mer -(PNP)Ir(η 2 -C 2 H 3 CN)] ( 7 ), respectively. Interaction of 4 with alkynes depends on the nature of the substrate; therefore, methyl phenylpropiolate reacted with 4, affording the adduct [ mer -(PNP)Ir(η 2 -PhCCC(O)OMe)] ( 9 ), while the parent acetylene undergoes a double C–H activation, affording the Ir(III) complex [ fac -(PN H P)IrH(C≡CH) 2 ] ( 10 ). A DFT theoretical analysis of this transformation supports a metal–ligand cooperation mechanism. The reaction starts by deprotonation of an alkyne moiety by the PNP ligand followed by oxidative addition of the C–H bond to the metal of a second alkyne molecule. Additionally, we have tested complex 4 as a catalyst for the addition of gaseous ammonia to activated unsaturated substrates. A DFT theoretical analysis disclosed the operative mechanism on these organic transformations, which starts with a nucleophilic attack of ammonia to the bound alkyne, hydrogen migration to the metal, and reductive elimination steps.