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

The reaction of N(PPh(3))(2)]CpCo(CN)(3)] and Cb*Co(NCMe)(3)]PF(6) (Cb* = C(4)Me(4)) in the presence of K(+) afforded {K subsetCpCo(CN)(3)](4)Cb*Co](4)}PF(6), KCo(8)]PF(6). IR, NMR, ESI-MS indicate that KCo(8)]PF(6) is a high-symmetry molecular box containing a potassium ion at its interior. The analogous heterometallic cage {K subsetCp*Rh(CN)(3)](4)Cb*Co](4)}PF(6) (KRh(4)Co(4)]PF(6)) was prepared similarly via the condensation of KCp*Rh(CN)(3)] and Cb*Co(NCMe)(3)]PF(6). Crystallographic analysis confirmed the structure of KCo(8)]PF(6). 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 mu-CN ligands bow outward. MeCN solutions of KCo(8)](+) and KRh(4)Co(4)](+) were found to undergo ion exchange with Cs(+) to give CsCo(8)](+) and CsRh(4)Co(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 NH(4+) with KCo(8)](+) proceeds to completion in THF solution, but in MeCN solution, the exclusive products were Cb*Co(NCMe)(3)]PF(6) and the poorly soluble salt NH(4)CpCo(CN)(3). The lability of the NH(4+)-containing cage was also indicated by the rapid exchange of the acidic protons in NH(4)Co(8)](+). Oxidation of MCo(8)](+) with 4 equiv of FcPF(6) produced paramagnetic (S = 4/2) Co(8)](4+), releasing Cs(+) or K(+). The oxidation-induced dissociation of M(+) from the cages is chemically reversed by treatment of Co(8)](4+) and CsOTf with 4 equiv of Cp(2)Co. Cation recognition by Co(8)] and Rh(4)Co(4)] cages was investigated. Electrochemical measurements indicated that E(1/2)(Cs(+))--E(1/2)(K(+)) approximately 0.08 V for MCo(8)](+).