Although salen and its analogues are versatile chelate ligands in inorganic and organometallic chemistry, synthesis of unsymmetrical salen derivatives consisting of two different salicylideneimine moieties is difficult because of the C=N bond recombination. To develop stable analogues of salen-type ligands, we synthesized a series of new ligands salamo (=1,2-bis(salicylideneaminooxy)ethane) on the basis of O-alkyl oxime instead of the imine moiety. Eight salamo ligands 1a-h were prepared in 64-88% yields as colorless crystals from the corresponding salicylaldehydes 2a-h. The crystal structure of 1a-c suggests that the oxime-OH form is more predominant than the keto-NH form. The reaction of 2a-e with excess 1,2-bis(aminooxy)ethane gave monooximes 3a-e in 59-86%, which further reacted with a different salicylaldehyde to afford unsymmetrical salamo ligands 4-8 as stable crystals in 51-70%. No reaction took place when a mixture of salamo derivatives 1a and 1b was treated at 40 degrees C in H2O/MeCN (5:95). However, the metathesis reaction of salen derivatives 9a and 9b completed in 2 h to give a statistical mixture. Monooxime 3b was much more stable than monoimine 11 which is difficult to be isolated. These results indicate the extremely high stability of the salamo derivatives 1 and precursors 3.

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 theta of the helix depend on the size of the guest. The La3+ complex has the smallest theta (288 degrees), and the Sc3+ complex has the largest theta (345 degrees). 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.

Novel ligands 1,2-bis(salicylideneaminooxy)ethanes were synthesized. They were much more stable against exchange reactions of aldehyde units than the corresponding salen derivatives. They form stable complexes with a copper(II) ion, whose structures were crystallographically determined. Spectroscopic and electrochemical studies indicated that the complexes have lower LUMO levels than salen analogues.

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A 36-membered macrocyclic hexaoxime was quantitatively obtained by [3 + 3] condensation of dialdehyde 2 with diamine 3 using La3+ (core metal) and Zn2+ (shell metal) as a novel core/shell template, while the yield was very low in the absence of the metal ions. The high yield can be attributed to the efficient formation of a 3:3:1 complex of dialdehyde 2, Zn2+, and La3+, which readily gives the macrocycle keeping the Zn3La core/shell tetranuclear cluster structure.