Multistep oligometal complexation of the macrocyclic tris(N2O2) hexaoxime ligand.

A macrocyclic oxime ligand H(6)L, which has an O(6) cavity surrounded by three N(2)O(2) chelate sites, was synthesized and the multistep oligometal complexation behavior was investigated. Upon complexation with zinc(II), the H(6)L ligand afforded two kinds of hexanuclear complexes, L(2)Zn(6) then LZn(6). Each of the complexation steps proceeded highly efficiently. In the latter complex, a Zn(3)(μ(3)-OH) unit was incorporated into the trimetalated ligand, LZn(3). The integrated N(2)O(2) chelate coordination sites provide a unique environment for a homometallic complex. The different nature of the peripheral N(2)O(2) sites and the central O(6) site is particularly suitable for the selective formation of heterometallic complexes. Complexation with the zinc(II) ion in the presence of alkaline earth (Ca and Ba) or rare earth (La, Eu, Lu) metal ions afforded the heterotetranuclear complexes LZn(3)M (M=Ca, Ba, La, Eu, Lu), in which zinc(II) and ion M occupied the N(2)O(2) and O(6) sites, respectively. Titration experiments showed that the heterometallic complexes LZn(3) Ca and LZn(3) Ba were converted into the homometallic complex LZn(6) whereas LZn(3)La was not. As a result, the binding affinity in the central O(6) site of the LZn(3) unit is apparently in the order of Ca(2+), Ba(2+) <Zn(3)(μ(3)-OH)<La(3+). This difference in the affinities of metal ions as well as the ionic sizes makes the novel conversion efficient, particularly in the case of the three-step conversion from H(6)L to H(2)LZn(2)Ba, LZn(3)Ba, then LZn(6).