Oligometallic Template Strategy for Synthesis of a Macrocyclic Dimer-Type Octaoxime Ligand for Its Cooperative Complexation

Abstract

The new acyclic tetraoxime ligand H(4)L(1), having two allyl groups at the terminal benzene rings, was designed and synthesized. The ligand H(4)L(1) was converted to five kinds of the trinuclear complexes, [L(1)Zn(3)(OAc)(2)], [L(1)Zn(2)La(OAc)(3)], [L(1)Zn(2)Ca(OAc)(2)], [L(1)Zn(2)Sr(OAc)(2)], and [L(1)Zn(2)Ba(OAc)(2)]. The terminal allyl groups were introduced so that the olefin metathesis could convert the metal complexes into the dimeric macrocyclic ligand H(8)L(3). The X-ray crystallographic analysis of the trinuclear complexes [L(1)Zn(3)(OAc)(2)(H(2)O)], [L(1)Zn(2)La(OAc)(3)(MeOH)], [L(1)Zn(2)Ca(OAc)(2)], and [L(1)Zn(2)Sr(OAc)(2)] revealed that the distances between the two allyl groups are 11-12 Å, which should be sufficient to suppress the intramolecular reaction leading to the monomeric macrocycle H(4)L(2). Indeed, the olefin metathesis of the [L(1)Zn(2)Ca(OAc)(2)] and [L(1)Zn(2)Sr(OAc)(2)] followed by demetalation with dilute hydrochloric acid afforded the dimeric macrocycle H(8)L(3) as the major product, while the reaction of the uncomplexed ligand H(4)L(1) gave the monomeric macrocycle H(4)L(2) as the major product. The complexation behavior of H(8)L(3) at the two tetraoxime sites was investigated. Although the formation process of the hexanuclear zinc(II) complex [L(3)Zn(6)](4+) was complicated, the metal exchange of the two trinuclear zinc(II) units proceeded in a two-step fashion. The analysis of the spectral changes indicated the positive and negative cooperative effects on the double metal exchange with Ca(2+) and Ba(2+), respectively. The first metal exchange reactions with Ca(2+) made the second metal exchange more favorable. Thus, the obtained dimeric macrocycle H(8)L(3) has two tetraoxime coordination sites, whose complexation behavior is remotely affected by each other.