Selective Anion Encapsulation by a Metalated Cryptophane with a π-Acidic Interior

Abstract

Metalation of the exterior arene faces of the molecular capsule (±)-cryptophane-E with [Cp*Ru]⁺ moieties results in a π-acidic cavity capable of encapsulating anions. The [CF₃SO₃]^- and [SbF₆]^- salts have been crystallographically characterized and demonstrate the encapsulation of these anions by the metalated cryptophane. ¹H and ¹⁹F NMR spectroscopy establish the binding of anions in NO₂CD₃ solution and reveal the relative affinity of the cavity for different anions (K_X⁻/K_OTf⁻): [BF₄]^- ≈ 0, [PF₆]^- = 1.18, [CF₃SO₃]^- ≡ 1, [SbF₆]^- = 0.30. Variable temperature rate studies reveal the activation barrier for triflate encapsulation to be ΔG^⧧_298K = 18.0(8) kcal·mol^-1 (ΔH^⧧ = 17.5(4) kcal·mol^-1 and ΔS^⧧ = 2(1) cal·mol^-1·K^-1).