Post-modifiable trapezoidal cage and selective recognition of SO₄^2- from HPO₄^2--containing environments

Selective recognition of SO₄^2- from HPO₄^2--containing environments is highly challenging, as SO₄^2- and HPO₄^2- not only share similar structures and sizes, but also exhibit similarities in many characteristics such as charge density, acidity, and hydration energy. In this contribution, a post-modifiable trapezoidal cage (1a) was developed to address the selective recognition of SO₄^2- from HPO₄^2-, as well as to cope with the difficulties of trapezoidal cages in post-modification and property variation. Coupled with the newly explored [4 + 4] cyclization strategy, the synthesis efficiency of producing trapezoidal cages has also been greatly improved. Afterward, by taking advantage of the tetrahedrally deployed binding sites of the trapezoidal cage 1a, selective recognition of SO₄^2- from HPO₄^2- can be realized even in complex environments containing many other anions. Through NMR, fluorescence, nonlinear fitting analysis, and HRMS experiments, the binding affinity and binding stoichiometry of 1a + anion were extensively studied. The results demonstrate that 1a + SO₄^2- follows a 1:1 host-guest binding mode and exhibits a much higher binding affinity (K ~ 1.7 × 10⁸ M^-1) than HPO₄^2- (K = 2.6 × 10⁶ M^-1) or any other anions (K = 10⁴-10⁵ M^-1) in 5% methanol/chloroform. The selective recognition of SO₄^2- in complex environments including HPO₄^2- can provide valuable considerations for the precise design of receptors that can distinguish subtle structural differences in substrates, while the post-modification strategy may also help improve the synthesis and extendibility of other covalent cages.