Lotus-Root-like One-Dimensional Polymetallocages with Drastic Void Adaptability Constructed from 4,4′-Bis(1,2,4-triazol-1-ylmethyl)biphenyl and Zn(II) or Co(II) and Their Fluorescein Encapsulation Properties

Abstract

The reaction of 4,4′-bis(1,2,4-triazol-1-ylmethyl)biphenyl (L) with Zn(NO3)2·6H2O or Co(NO3)2·6H2O in the presence of NH4PF6 in water−methanol or water−acetone produced both {[ZnL3](PF6)2(H2O)2}n (1) or {[CoL3](PF6)2(H2O)2}n (2). The metal ions in both complexes coordinate with six identical N atoms from six different ligands in an ideal octahedral environment where each ligand binds to two metal ions. Both complexes are one-dimensional triple-helical chains each containing an empty lotus-root-structured polymetallocage. The metallocage in 1 is fully empty, while the cage in 2 contains two water molecules. However, metallocages in 1 have a void volume of ∼200 Å3, which is much smaller than the 1020 Å3 void in 2. The void in 2 is comparable to the size of fluorescein. Both complexes are capable of encapsulating fluorescein anion in pH 9 aqueous solution to form {C20H10O5 ⊂ [ML3]}n, where C20H10O52− is fluorescein dianion, and M is Zn or Co. The encapsulation occurs via anion exchange. The cages in 1 are increased drastically to fit the size of fluorescein due to the flexibility of ligand, while in 2 the cages have no discernible change after fluorescein encapsulation due to a perfect match between the cavity size in 2 and the fluorescein. Fluorescein encapsulated 1 and 2 have identical powder X-ray diffraction patterns, indicating that they have similar structures after fluorescein encapsulation. Both complexes show fluorescence after fluorescein encapsulation.