Supramolecular chemistry of multinuclear zinc(II) complexes in aqueous solution

Abstract

We report novel supramolecular polyhedrons formed by three-dimensional self-assembly of multinuclear zinc(II)-12-membered tetraamine (= cyclen = 1,4,7,10-tetraazacyclododecane) complexes with potentially multidentate ligands such as cyanuric acid (CA) and trithiocyanuric acid (TCA) in aqueous solution. Two new supermolecular frameworks were isolated by self-assembly of a tris (Zn(2+)-cyclen) (Zn3L1) with di-(CA2-) or tri-deprotonated cyanuric acid (CA3-) in aqueous solution. One was a very stable 2:3 complex of Zn3L1 and CA2- formed above pH6, which was stable in aqueous solution at neutral pH. The second was an unexpected supramolecular complex formed by 4:4 self-assembly of Zn3L1 and CA3-, which was isolated by allowing a 1:1 mixture of Zn3L1 and CA to stand in aqueous solution at pH 11.5. X-ray crystal analysis showed a highly symmetric 4:4 assembly complex with a cuboctahedral exterior and an inner hollow, which was schematically represented as a truncated tetrahedron formed by binding four equilateral triangles and four scalene hexagons with each other through CA(3-)-Zn2+ bonds. The 4:4 complex was found to be stable only in solid form or in DMSO solution and tends to revert to the 2:3 complex in the presence of H2O. This problem has been overcome by replacing CA with TCA, of which the thioimide functions possess lower pKa values than those of CA. TCA acted as a tridentate donor for three Zn3L1 at neutral pH to yield a similar type of 4:4 self-assembling supercomplex, in which the deprotonated TCA3- in an aromatic 1,3,5-triazine binds to Zn3L1 through Zn(2+)-S- (exocyclic) coordination bonds, and thus the 4:4 assembly is a chiral twisted cuboctahedron. More interestingly, this supramolecular capsule was found to be stabilized by encapsulation of various size-matched and hydrophobic guest molecules such as adamantane in the twisted truncated cavity. Finally, we succeeded in synthesizing new supramolecular trigonal prisms from linear multinuclear zinc complexes such as p-Zn2L2 and p,p-Zn3L3 with TCA3- in aqueous solution at neutral pH, which are stabilized by Zn(2+)-S- or Zn(2+)-N- coordination bonds and hydrogen bonds in aqueous solution at neutral pH. Thus we discovered a new approach to the design of various supramolecular structures in aqueous solution.