Ring expansion of a platinacyclopropane to a platinacyclopentane by double insertion of isocyanides into Pt-C bonds

A platinum isocyanide complex bearing a η²-TCNE ligand, (η²-TCNE)Pt(η-CNC₆H₃-2,6-Me ₂)2 (1), was synthesized, and its crystallographic study revealed the coordination mode of the η²-TCNE ligand to be a typical metallacyclopropane extreme, in which back-donation from metal to TCNE contributes primarily to the stabilization of 1. Treatment of 1 with 2 equiv of CNC₆H₃-2,6-Me₂ resulted in ring expansion of the platinacyclopropane by insertion of CNC₆H₃-2,6-Me ₂ into each Pt-C bond of the metallacycle, leading to formation of a new complex having a platinacyclopentane structure, (TCNE)Pt(η-CNC ₆H₃-2,6-Me₂)4 (4). Although the molecular structures of palladium and nickel homologues of 1 were close to the corresponding metallacyclopropane, no metallacyclopentane complex was obtained under similar conditions. Studies on variable-temperature NMR spectroscopy and spin-saturation transfer measurement of 4 showed exchange of the imidoyl CNC₆H₃-2,6-Me₂ group and the coordinated CNC₆H₃-2,6-Me₂ ligand in solution, suggesting the ring expansion to be reversible.