Modulation of Multimetal Complexation Behavior of Tetraoxime Ligand by Covalent Transformation of Olefinic Functionalities

A new multimetal complexation system that can change its complexation behavior by C−C bond formation has been developed. The acyclic tetraoxime ligand H4L1 having two terminal allyl groups was synthesized. The olefin metathesis of H4L1 selectively produced trans-H4L2 while the reaction of [L1Zn2Ca] exclusively afforded cis-H4L2. The saturated analogue H4L3 was synthesized by hydrogenation. The complexation of the ligands H4L (L = L1, trans-L2, cis-L2, L3) with zinc(II) acetate (3 equiv) yielded the trinuclear complexes [LZn3] with a similar trinuclear core bridged by acetato ligands. Whereas the formation process of [L1Zn3] having an acyclic ligand was highly cooperative, the macrocyclic analogues [LZn3] (L = trans-L2, cis-L2, L3) were formed in a stepwise fashion via the intermediate 2:3 complex [(HL)2Zn3]. The trinuclear complexes [LZn3] (L = L1, trans-L2, cis-L2, L3) can recognize alkaline earth metal ions via site-selective metal exchange. The acyclic [L1Zn3] selectively recognizes Ca2+, while the cyclic [trans-L2Zn3] showed a Ba2+ selectivity. The metal exchange of [LZn3] (L = L1, trans-L2, cis-L2, L3) with La3+ efficiently occurred to give [LZn2La], but the trans-olefin linker of the [trans-L2Zn2La] significantly deforms the structure in such a way that one of the salicylaldoxime moieties does not participate in the coordination. Consequently, the chemical transformation of the olefinic moiety significantly affects the multimetal complexation behavior of the tetraoxime ligands.