Helical Metallohost−Guest Complexes via Site-Selective Transmetalation of Homotrinuclear Complexes

We have designed a new type of bis(N₂O₂) chelate ligand that affords a C-shaped O₆ site on the metalation of the N₂O₂ sites. UV−vis and ¹H NMR titration clearly showed that the complexation between H₄L and zinc(II) acetate affords 1:3 complex [LZn₃]²⁺ via a highly cooperative process. Although the O₆-recognition site of the dinuclear metallohost [LZn₂] is filled with the additional Zn²⁺, the O₆ site can bind a guest ion with concomitant release of the initially bound Zn²⁺. The novel recognition process “guest exchange” took place quantitatively when rare earth metals were used as a guest. In the case of alkaline earth metals, selectivity of Ca²⁺ > Sr²⁺ > Ba²⁺ ≫ Mg²⁺ was observed. On the other hand, the transmetalation did not take place at all when alkali metals were used for the guest. Accordingly, the trinuclear complex [LZn₃]²⁺ is excellent in discriminating charge of the guest ions. The metallohost−guest complexes thus obtained have a helical structure, and the radius d and winding angle θ of the helix depend on the size of the guest. The La³⁺ complex has the smallest θ (288°), and the Sc³⁺ complex has the largest θ (345°). Because the radius and winding angles of helices are tunable by changing the guest ion, the helical metallohost−guest complexes are regarded as a molecular spring or coil. Consequently, site-specific metal exchange of trinuclear complex [LZn₃]²⁺ described here will be utilized for highly selective ion recognition, site-selective synthesis of (3d)₂(4f) trimetallic complexes, and construction of “tunable” metallohelicenes.