Redox-switched complexation/decomplexation of K⁺ and Cs ⁺ by molecular cyanometalate boxes

The reaction of [N(PPh₃)₂][CpCo(CN)₃] and [Cb*Co(NCMe)₃]PF₆ (Cb* = C₄Me ₄) in the presence of K⁺ afforded {K⊂[CpCo(CN) ₃]₄[Cb*Co]₄}PF₆, [KCo ₈]PF₆. IR, NMR, ESI-MS indicate that [KCo ₈]PF₆ is a high-symmetry molecular box containing a potassium ion at its interior. The analogous heterometallic cage {K⊂[Cp*Rh(CN)₃]₄[Cb*Co] ₄}PF₆ ([KRh₄Co₄]PF₆) was prepared similarly via the condensation of K[Cp*Rh(CN)₃] and [Cb*Co(NCMe)₃]PF₆. Crystallographic analysis confirmed the structure of [KCo₈]PF₆. The cyanide ligands are ordered, implying that no Co-CN bonds are broken upon cage formation and ion complexation. Eight Co-CN-Co edges of the box bow inward toward the encapsulated K⁺, and the remaining four μ-CN ligands bow outward. MeCN solutions of [KCo₈]⁺ and [KRh₄Co ₄]⁺ were found to undergo ion exchange with Cs⁺ to give [CsCo₈]⁺ and [CsRh₄Co ₄]⁺, both in quantitative yields. Labeling experiments involving [(MeC₅H₄)Co(CN)₃]^- demonstrated that Cs⁺-for-K⁺ ion exchange is accompanied by significant fragmentation. Ion exchange of NH₄⁺ with [KCo₈]⁺ proceeds to completion in THF solution, but in MeCN solution, the exclusive products were [Cb*Co(NCMe) ₃]PF₆ and the poorly soluble salt NH₄CpCo(CN) ₃. The lability of the NH₄⁺-containing cage was also indicated by the rapid exchange of the acidic protons in [NH ₄Co₈]⁺. Oxidation of [MCo₈] ⁺ with 4 equiv of FcPF₆ produced paramagnetic (S = 4/2) [Co₈]⁴⁺, releasing Cs⁺ or K⁺. The oxidation-induced dissociation of M⁺ from the cages is chemically reversed by treatment of [Co₈]⁴⁺ and CsOTf with 4 equiv of Cp₂Co. Cation recognition by [Co₈] and [Rh ₄Co₄] cages was investigated. Electrochemical measurements indicated that E_1/2(Cs⁺) - E_1/2(K⁺) ∼ 0.08 V for [MCo₈]⁺.