Two Types of Anion-Induced Reconstruction of Schiff-Base Macrocyclic Zinc Complexes: Ring-Contraction and Self-Assembly of a Molecular Box

Two
46-membered [2 + 2] Schiff-base macrocyclic dinuclear Zn­(II)
complexes (1a and 1b) were investigated
deeply by the postmodification strategy, and two types of supramolecular
processes (ring-contraction and self-assembly) have been achieved
after the addition of specific anions as stimulus for the equilibrium
of Schiff-base macrocyclic complexes. Namely, in the presence of linear
three-atom SCN–, 1a was degraded into
two 23-membered [1 + 1] Schiff-base macrocyclic complexes simultaneously
(mononuclear Zn­(II) complex 2 and dinuclear Zn­(II) complex 3). In contrast, 1b was only transformed into
the macrocyclic mononuclear complex 5. More interestingly,
in the case of pseudolinear five-atom N­(CN)2–, supramolecular self-assembly took place instead of the above-mentioned
ring-contraction. Finally, 1a was assembled into a unique
molecular box 4 with two 46-membered [2 + 2] Schiff-base
macrocyclic heteronuclear Zn4Na4 substrates
and double μ2-N­(CN)2– bridges, while no similar assembly process was observed for 1b. The geometry of anions and pH values slightly adjusted
by the pendant arms on the macrocyclic skeletons are believed to be
the critical factors for the different supramolecular processes originating
from the dynamic covalent chemistry of Schiff-base imine bonds.