Acetonitrile Coupling at an Electron-Rich Iridium Center Supported by a PCsp2P Pincer Ligand

Aryl amido iridium complexes supported by a tridentate phosphanyl-carbene-phosphanyl pincer ligand [(PCsp2P)Ir-N(H)Ar, Ar = C6H5, 1a; Ar = 2,6-iPr(2)C(6)H(3), 1b] react with acetonitrile to afford the dimeric complex 2, in which two (PCsp2P)Ir fragments are bridged by a diiminato ligand derived from two molecules of CH3CN. Metrical parameters obtained from X-ray structural determination of 2 confirm that the bridging ligand is a diiminato species rather than an enediamido moiety, which indicates that the reductive coupling is mediated by one electron per iridium center. Experiments suggest that the reaction proceeds by heterolytic cleavage of the Ir-N(H)Ar bond followed by one-electron reduction of the resulting Ir-I-NCCH3 cation by the amido anion. The resulting anilino radical rapidly abstracts a hydrogen atom from the solvent. The reductive coupling of acetonitrile at a late transition metal center is unusual and in this instance occurs as a result of the highly sigma-donating, electron-rich nature of the PCsp2P ligand.