Water-Soluble and Ultrastable Ti₄L₆ Tetrahedron with Coordination Assembly Function

Abstract

We have successfully constructed a tetrahedral Ti₄L₆ cage with calixarene-like coordination-active vertices. It further features high solubility and stability in H₂O and DMF/H₂O solution, affording an interesting stepwise assembly function with other metal ions. Through trapping of different amounts of Co or Ln ions, the Ti₄L₆ tetrahedra can be organized into various dimensional architectures, including a Ti₄L₆-Co₃ cage, a Ti₄L₆-Ln₂ cage, a Ti₄L₆-Ln₂ chain, and a three-dimensional Ti₄L₆-Ln framework. An unusual mixed-valence phenomenon was observed in the Ti₄L₆ cage, whose Ti^3+/4+ compositions were adequately identified by electron spin resonance and X-ray photoelectron spectroscopy analyses. More remarkably, the calixarene-like oxygen vertices of the Ti₄L₆ cage can also be used for the recognition of C₃-symmetric dye molecules through N-H···O hydrogen bonding. Accordingly, driven by visible light, selective and efficient homogeneous photodecomposition of acid blue 93 and alkali blue 4B were successfully achieved. Therefore, this work not only represents a milestone in constructing symmetric Ti-based cages with interesting coordination assembly function but also provides a new method for preparing technologically important soluble photoactive cages.