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

The new acyclic tetraoxime ligand H4L1, having
two allyl groups at the terminal benzene rings, was designed and synthesized.
The ligand H4L1 was converted to five kinds
of the trinuclear complexes, [L1Zn3(OAc)2], [L1Zn2La­(OAc)3], [L1Zn2Ca­(OAc)2], [L1Zn2Sr­(OAc)2], and [L1Zn2Ba­(OAc)2]. The terminal allyl groups were introduced so that the olefin
metathesis could convert the metal complexes into the dimeric macrocyclic
ligand H8L3. The X-ray crystallographic analysis
of the trinuclear complexes [L1Zn3(OAc)2(H2O)], [L1Zn2La­(OAc)3(MeOH)], [L1Zn2Ca­(OAc)2],
and [L1Zn2Sr­(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 H4L2. Indeed,
the olefin metathesis of the [L1Zn2Ca­(OAc)2] and [L1Zn2Sr­(OAc)2] followed
by demetalation with dilute hydrochloric acid afforded the dimeric
macrocycle H8L3 as the major product, while
the reaction of the uncomplexed ligand H4L1 gave
the monomeric macrocycle H4L2 as the major product.
The complexation behavior of H8L3 at the two
tetraoxime sites was investigated. Although the formation process
of the hexanuclear zinc­(II) complex [L3Zn6]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 Ca2+ and Ba2+, respectively. The first metal exchange reactions with Ca2+ made the second metal exchange more favorable. Thus, the obtained
dimeric macrocycle H8L3 has two tetraoxime coordination
sites, whose complexation behavior is remotely affected by each other.