Helical Metallohost-Guest Complexes via Site-Selective Transmetalation of Homotrinuclear Complexes Shigehisa Akine, Takanori Taniguchi, and Tatsuya Nabeshima* Contribution from the Department of Chemistry, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan Received July 12, 2006; E-mail: nabesima@chem.tsukuba.ac.jp Abstract: 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 1 H 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 Zn 2+ , the O6 site can bind a guest ion with concomitant release of the initially bound Zn 2+ . 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 2+ > Sr 2+ > Ba 2+ . Mg 2+ 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 3] 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 La 3+ complex has the smallest θ (288°), and the Sc 3+ 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 3] 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. Introduction Recently, metallohosts have attracted much interest because they would have various functions of the parent metal com- plexes. Photochemical 1 and redox 2 properties arising from hybridization between the organic and metal complex moieties are successfully applied to guest sensing or detection. Revers- ibility of coordination bonds between a metal and ligands is also an important factor to construct supramolecular metallo- hosts. 3 In particular, conversion of an acyclic molecule to the corresponding cyclic metallohost is effective to control guest recognition (Scheme 1A). 3 We have investigated allosteric regulation of guest recognition by pseudomacrocycles obtained by the complexation of bis- or tris(bipyridine) ligands with tran- sition metals. 4 The unique strategy is also applied to sophisti- cated functions such as transduction of molecular information. 5 Salen-type N 2 O 2 chelates are also attractive candidates for a metal-binding site to be incorporated into metallohosts. 6 Salen ligands coordinate to various kinds of transition and typical metals in a tetradentate fashion to give stable complexes, some of which are used as catalysts for organic reactions, 7 models of reaction centers of metalloenzymes, 8 nonlinear optical materials, 9 (1) For reviews, see: (a) Fabbrizzi, L.; Licchelli, M.; Rabaioli, G.; Taglietti, A. 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Two Strategies for Controllable Guest Binding System Utilizing Metal Coordination Published on Web 11/14/2006 10.1021/ja0646702 CCC: $33.50 © 2006 American Chemical Society J. AM. CHEM. SOC. 2006, 128, 15765-15774 9 15765