Structural and mechanistic studies on ion insertion into the molecular box {[CpCo(CN)₃]₄ [Cp*Ru]₄}

The molecular box [CpCo(CN)₃]₄[Cp*Ru] ₄ (Co₄Ru₄) reacts readily with a variety of monocations to form M⊂Co₄Ru₄ ⁺ (M=K⁺, Cs⁺, Rb⁺). Ion competition experiments, monitored by ESI-MS, show that the molecular box binds the smaller K⁺ more rapidly than Cs⁺, but that thermodynamically Co₄Ru₄ prefers the larger ion. The rates of ion-insertion for K⁺ and Cs⁺ into Co₄Ru₄ were found to qualitatively follow second order kinetics with K⁺, 300 M^-1 s^-1 and Cs⁺, 36 M^-1 s^-1. The ratio k_K/k_Cs qualitatively matched the ESI-MS results from ion competition experiments. The rates of ion-insertion into Co₄Ru₄ were found to depend on the counter anions. In particular, RbBF₄ reacted with Co₄Ru₄ more slowly than did RbOTf. The slower rates allowed us to establish second order kinetics. ¹H NMR studies reveal that the Cp signal for Co₄Ru₄ is very sensitive to the presence of entering ions, e.g., Rb⁺, whereas the corresponding Cp signal for Rb⊂Co₄Ru₄⁺ was insensitive to the presence of Rb⁺. The molecular structures of [Co₄Ru₄]·6MeCN, [K⊂Co₄Ru₄]BF₄·7MeCN, [Cs⊂Co₄Ru₄]BF₄·6MeCN and [Tl⊂Co₄Ru₄]BF₄·6MeCN, determined by X-ray diffraction, showed that although the compounds crystallized in the same space group I 23, a correlation exists between the Ru-N/Co-C bond distances and the size of the interstitial ion.