Ring Slippage and Dissociation of Pentamethylcyclopentadienyl Ligand in an (η⁵‑Cp*)Ir Complex with a κ³‑O,C,O Tridentate Calix[4]arene Ligand under Mild Conditions

The first examples of κ³-O,C,O tridentate calix[4]­arene transition-metal complexes [Cp*M­(p-^tBucalix­[4]­arene­(2−)-κ³-O,C,O)] (M = Ir, Rh) that are stable both in the solid state and in solution were synthesized. The reaction of the Ir complex with 2 equiv of xylyl isocyanide afforded the (η¹-Cp*)­Ir­(CNXy)₂ complex in which the Cp* ligand is slipped from an η⁵ to an η¹ fashion, although ring slippage of an η⁵-Cp* ligand is very uncommon in comparison with that of Cp. The reaction of the (η¹-Cp*)Ir complex with additional 1 equiv of isocyanide resulted in the liberation of the Cp* ligand as Cp*H, which was confirmed by ¹H NMR. This study provides the first observation of stepwise dissociation of a Cp* ligand from group 4–10 transition-metals via η¹ coordination. This unique phenomenon seems to be facilitated by flexible coordination of an OH group in the C-metalated calix­[4]­arene ligand.