Binding of Mono- and Dianions within Silver Thiazolylurea Tweezers and Capsules

A silver thiazolylurea complex, [Ag(TUTh)₂]⁺, has been used as a host species for geometrically differently shaped mono- and dianions: trigonal planar (NO₃^-), tetrahedral (SO₄^2-), and octahedral (SiF₆^2-). In the presence of nitrate a 1:1 near-planar tweezer host-guest species is formed, with poor binding in solution despite excellent geometric complementarity being found between the host and the anion in the solid state. In the presence of either SO₄^2- or SiF₆^2- a 2:1 host-guest species is formed, whereby the guest is held in a capsulelike arrangement stabilized by an array of eight NH hydrogen-bond donors, as confirmed by X-ray crystallographic studies. Solution studies in DMSO-d₆ support the host-guest stoichiometry seen in the solid state. The binding constant between SO₄^2- and [Ag(TUTh)₂]⁺ was calculated to be K₂₁ = 2511 M^-2 and was shown to be the dominant species in solution, in excellent agreement with the solid-state studies. However, upon the addition of SiF₆^2- ions different speciation is observed: H₂·G (capsule), H·G (tweezer), and H·G₂ during the course of the study.