Anionic Calixarene Complexes of Copper(I) and Silver(I) as Cation Receptors

A tetraphosphonitocalixresorcinarene ligand (PhCH₂CH₂CHC₆H₂O ₂PPh)₄, bowl, forms tetracopper(I) and tetrasilver(I) complexes [(bowI)M₄(μ-Cl)₄]. 1, M = Cu or Ag, and the chloride inclusion compounds [(bowl)Cu₄(μ-Cl)₄(μ₃-Cl)]^-, 2a, and [(bowl)Ag₄(μ-Cl)₄(μ₄-Cl)]^-, 2b. The anionic complexes 2a and 2b are now shown to act as hosts for the selective binding of metal cations. This metal cation inclusion is effected by chelation by some or all of the four μ-Cl groups present in 2. Isolated complexes have the formula [2·M] (3, M = Na, K, Rb, Cs) for M⁺ ions and either [2·MCl] (4, M = Zn, Cd) or [2₂·M] (5, M = Hg, Pb) for M⁺ ions. The complexes [(2a· Cs)₂], 3d. and [2b₂·Hg], 5a. have been characterized by X-ray structure determinations [3d·solvate; triclinic, P1, a = 16.5142(1) Å, b = 25.8269(2) Å, c = 26.3615(4) Å, α = 61.785(1)°, β= 80.403(1)°, γ = 72.533(1)°, Z = 2; 5a·solvate; monoclinic, P2₁/n, a = 13.4174(2) Å, b = 43.2897(7) Å, c = 15.7462(3) Å, β= 98.734°, Z = 2]. In both 3d and 5a, the metal ions are sandwiched between two units of the corresponding anionic complex 2. In the 2:2 complex 3d, each Cs⁺ ion is coordinated by four Cl atoms of one unit of 2a and by two Cl atoms and a phenyl ring of the second unit of 2a. In the 2:1 complex 5a, the Hg²⁺ ion is coordinated by one Cl atom of one unit of 2b and by two Cl atoms (with a weak bond to a third Cl atom) of the second unit of 2b. The metal cation complexation also influences the binding of the occluded chloride anion in 3d. Both the guest cation and the guest anion can migrate easily between donor and acceptor sites, respectively, such that the host-guest complexes appear to have 4-fold symmetry as characterized by ¹H or ³¹P NMR. Overall, the formation of 3d and 5a from 1 may be considered to occur by salt inclusion.