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

We have designed a new type of bis(N<sub>2</sub>O<sub>2</sub>) chelate ligand that affords a C-shaped O<sub>6</sub> site on the metalation of the N<sub>2</sub>O<sub>2</sub> sites. UV−vis and <sup>1</sup>H NMR titration clearly showed that the complexation between H<sub>4</sub>L and zinc(II) acetate affords 1:3 complex [LZn<sub>3</sub>]<sup>2+</sup> via a highly cooperative process. Although the O<sub>6</sub>-recognition site of the dinuclear metallohost [LZn<sub>2</sub>] is filled with the additional Zn<sup>2+</sup>, the O<sub>6</sub> site can bind a guest ion with concomitant release of the initially bound Zn<sup>2+</sup>. 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 Ca<sup>2+</sup> > Sr<sup>2+</sup> > Ba<sup>2+</sup> ≫ Mg<sup>2+</sup> 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 [LZn<sub>3</sub>]<sup>2+</sup> is excellent in discriminating charge of the guest ions. The metallohost−guest complexes thus obtained have a helical structure, and the radius <i>d</i> and winding angle θ of the helix depend on the size of the guest. The La<sup>3+</sup> complex has the smallest θ (288°), and the Sc<sup>3+</sup> 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 [LZn<sub>3</sub>]<sup>2+</sup> described here will be utilized for highly selective ion recognition, site-selective synthesis of (3d)<sub>2</sub>(4f) trimetallic complexes, and construction of “tunable” metallohelicenes.