Assemblies of cucurbit[6]uril-based coordination complexes with disulfonate ligands: from discrete complexes to one- and two-dimensional polymers

Abstract

In this work, the combination of cucurbit[6]uril (Q[6]) with various naphthalene disulfonates and alkaline-earth metal ions allows for successful isolation of 9 novel Q[6]-based coordination complexes with the formulas [Ca(H₂O)₄(Q[6])_0.5(1,5-NDA)_0.5]·(1,5-NDA)_0.5·3H₂O (1), [Sr(H₂O)₄(Q[6])_0.5(1,5-NDA)_0.5]·(1,5-NDA)_0.5·4H₂O (2), [Ba(H₂O)₄(Q[6])_0.5(1,5-NDA)_0.5]·(1,5-NDA)_0.5·4H₂O (3), [Ca(H₂O)₅(Q[6])_0.5](2,6-NDA)·9H₂O (4), [Sr(H₂O)₅(Q[6])_0.5]·(2,6-NDA)·6H₂O (5), [Ba₂(H₂O)₇(Q[6])(2,6-NDA)₂]·11H₂O (6), [Sr(H₂O)₄(Q[6])_0.5(2,6-NDA)_0.5]·(2,6-NDA)_0.5·5H₂O (7), [Ca₂(H₂O)₁₀(Q[6])](2,7-NDA)·14H₂O (8) and [Sr(H₂O)₅(Q[6])(2,7-NDA)]·4H₂O (9) (H₂NDA = naphthalenedisulfonic acid). Structural analysis revealed that the carbonyl groups of Q[6] bind directly to the alkaline-earth metal ions to form 1D coordination chains. Moreover, the metal ions are also linked to the disulfonate substituents of the naphthalenes, which, in the case of 9, resulted in a simple coordination complex, for 6, a 1D Q[6]–NDA–M²⁺ chain, and in the case of 1–3 and 7, 2D networks. By contrast, for 4, 5 and 8, the disulfonate ligands do not bind to the metals, and instead, act as supramolecular linkers. The resultant 3D supramolecular architecture of these compounds was constructed through various noncovalent interactions, particularly involving outer-surface interactions of the Q[6]s. Furthermore, the fluorescence properties of species 2 and 7 were investigated, and sensing experiments indicated that both were capable of functioning as fluorescent sensors for Fe³⁺ with high sensitivity and selectivity.