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

The pincer ligand HN­(CH₂CH₂PPh₂)₂ (<b>1</b>; PN<i>H</i>P) reacted with [{Ir­(μ-X)­(cod)}₂] (X = Cl, OMe), affording complexes [<i>fac</i>-(PN<i>H</i>P)­Ir­(cod)]Cl (<b>2</b>) and [<i>fac</i>-(PNP)­Ir­(cod)] (<b>3</b>), respectively. The X-ray molecular structure of <b>2</b> 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 <b>1</b> is able to protonate the hydroxo bridges in the complex [{Ir­(μ-OH)­(coe)₂}₂] forming the new amido complex [<i>mer</i>-(PNP)­Ir­(coe)] (<b>4</b>). Complex <b>4</b> is an extremely air sensitive compound, as confirmed by the isolation of the oxo complex [<i>mer</i>-(PNP)­Ir­(η²-O₂)] (<b>8</b>) from its interaction with air. Protonation of <b>4</b> with HBF₄ afforded the corresponding amino complex [<i>mer</i>-(PN<i>H</i>P)­Ir­(coe)]­BF₄ (<b>5</b>), whose molecular structure enlightened by X-ray crystallography confirmed the PNP ligand to be coordinated in a meridional fashion. The coe ligand in <b>4</b> is tightly bonded to iridium; however, under an atmosphere of ethylene at 60 °C or with acrylonitrile at 70 °C complex <b>4</b> exchanges the olefin, affording compounds [<i>mer</i>-(PNP)­Ir­(η²-C₂H₄)] (<b>6</b>) and [<i>mer</i>-(PNP)­Ir­(η²-C₂H₃CN)] (<b>7</b>), respectively. Interaction of <b>4</b> with alkynes depends on the nature of the substrate; therefore, methyl phenylpropiolate reacted with <b>4</b>, affording the adduct [<i>mer</i>-(PNP)­Ir­(η²-PhCCC­(O)­OMe)] (<b>9</b>), while the parent acetylene undergoes a double C–H activation, affording the Ir­(III) complex [<i>fac</i>-(PN<i>H</i>P)­IrH­(CCH)₂] (<b>10</b>). 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 <b>4</b> 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