Versatile κ²,η²-<i>C</i> Ligands Assembled at Iridium via [2+2] Oxidative Cyclization

The cationic Ir(I) complex [Ir(1,2,5,6-η-C8H12)(NCMe)(PMe3)]BF4 has been observed to react with ester-substituted internal alkynes via [2+2] oxidative cyclization to form iridacyclopentene derivatives: [Ir(1-κ-5,6-η-C8H12-2-Z-{2′-κ-C(CO2R)C(CO2R)})(NCMe)2(PMe3)]BF4 (R = Me, tBu). These compounds contain fac-tridentate ligands that coordinate the Ir(III) centers κ2, through sp3 and sp2 carbons, and η2 through the remaining CC bond of the former cyclooctadienes. The derivative with R = Me has been subjected to different substitution reactions of its labile acetonitriles, including sequential reactions against monodentate ligands such as CO and PMe3, bidentate ligands such as 2,2′-bipy, and anions such as phenylacetylide, hydride, acetylacetonate, and cyclopentadienyl. The use of these reagents, individually or in combination, has led to compounds containing the new C-donor ligand in various coordination modes, including κ3; κ2,η3; κ2,η2; κ2; and κ,η2 (or its κ3 extreme version). The transformations observed for this ligand include a C−H bond activation reaction at the coordination sphere of a formally Ir(V) complex effected by an external base as weak as chloride.