Cobalt(III) Metallacryptates and Their Guest Cation-Exchange in Solution Monitored by ⁵⁹Co NMR

One-pot reactions of the catechol-scaffolding aroylbis(N,N-diethylthiourea) H₂L^cat with mixtures of CoCl₂ and MCl (M⁺ = Cs⁺, Rb⁺, K⁺, Tl⁺, or NH₄⁺) or with a Co(NO₃)₂/TlNO₃ mixture lead to the self-assembly of a series of cationic Co(III) metallacryptates of the general formula [M ⊂ {Co₂(L^cat)₃}]⁺ (M⁺ = Cs⁺, Rb⁺, K⁺, Tl⁺, or NH₄⁺). Crystalline PF₆^- salts were obtained after workup with (n-Bu₄N)(PF₆), and the single-crystal structures of all five metallacryptates have been determined. Depending on the nature of the guest cations, the directional interactions between guest cations and the metallacryptand {Co₂(L^cat)₃} are either weak coordination contacts or hydrogen bonds. The bonding mode and the size of the guest ions slightly influence the molecular skeleton of the host molecule. These small structural variations also exist in solution and could be detected by means of ⁵⁹Co NMR spectroscopy, which is shown to be a unique tool for an easy characterization of such compounds. ⁵⁹Co NMR chemical shifts are extraordinarily sensitive to the guest cation in the metallacryptates, and time-arrayed ⁵⁹Co NMR experiments show that cation-exchange processes in biphasic organic/aqueous systems can be studied in detail. This leads to insights into the relative rates of cation exchange, as well as the relative conditional distribution coefficients of such Co(III) metallacryptates between the aqueous and organic phases. Thus, the extent and the relative rate of the NH₄⁺ ion exchange in [NH₄ ⊂ {Co₂(L^cat)₃}](PF₆) by Cs⁺ and K⁺ ions across the organic/aqueous phase boundary at room temperature have been studied by in situ ⁵⁹Co NMR experiments. Preliminary ⁵⁹Co NMR experiments show that the K⁺ ion in [K ⊂ {Co₂(L^cat)₃}](PF₆) can be removed by its competitive complexation with the highly potassium-selective [2.2.2]cryptand, to give a transient ⁵⁹Co NMR signal of the relatively unstable "empty" {Co₂(L^cat)₃} complex, which slowly decomposes in solution.