A Case of C−H Activation (Ortho Metalation) Which Is Reversible at 25 °C

Abstract

The reaction of LiC2Ph with Ir(H)2ClL2 (L = PtBu2Ph) gives Ir(H)2(C2Ph)L2, which readily loses H2 to form the ortho-metalated species IrH(η2-C6H4PtBu2)(C2Ph)L. This molecule is unique in showing (sp2)C/H reductive elimination/oxidative addition, which is thermally reversible at 25 °C. Line shape analysis of the 31P{1H} NMR spectra yield ΔH⧧ = 12.3 (±0.4) kcal mol-1 and ΔS⧧ = −2.0 (±1.1) cal deg-1 mol-1 for this process. This implicates the 14-electron species Ir(C2Ph)L2 as highly reactive, yet more easily accessible, than IrClL2 (which ortho metallates essentially irreversibly). The strained four-membered ring in IrH(η2-C6H4PtBu2)(C2Ph)L reacts readily with PhC2H to give IrH(C2Ph)2L2, another unsaturated molecule essentially devoid of π-stabilization. The structure of this molecule, a square pyramid with apical hydride (proton NMR chemical shift −44 ppm), minimizes the Lewis acidity of such a species. Crystal data (−181 °C): a = 9.616(1) Å, b = 23.032(2) Å, c = 8.886(1) Å, α = 95.18(1)°, β = 99.03(1), γ = 98.16(1), with Z = 2 in space group P1̄.