Calix[6]arene-functionalized titanium-oxo clusters for photocatalytic cycloaddition of carbon dioxide to epoxides

Abstract

Calix[n]arenes, a promising family of macrocyclic ligands, have been widely employed in the construction of polynuclear metal clusters. However, their utilization in titanium-oxo clusters (TOCs) is rare, especially in the realm of high-nuclearity TOCs. In this study, we employed tert-butylcalix[6]arene (TBC6A) as an organic “functional armor” owing to its molecular flexibility and versatile topologies to successfully synthesize two TBC6A-functionalized TOCs through a solvothermal reaction. These clusters are denoted as [Ti₄(μ₃-O)₂(TBC6A)₂(CH₃CN)₂]·CH₃CN (Ti4-TBC6A; TBC6A = tert-butylcalix[6]arene) and [Ti₂₈(μ₂-O)₁₈(μ₃-O)₁₈(TBC6A)(PA)₃₄(ⁱPrO)₂]·4CH₃CN (Ti28-TBC6A; PA = propionic acid, ⁱPrOH = isopropanol). Interestingly, the TBC6A ligand exhibits two distinct configurations: a monoconical configuration for Ti4-TBC6A and a biconical configuration for Ti28-TBC6A, which significantly enhances the structural variety of TOCs. Additionally, the incorporation of auxiliary propionic acid ligands further increases the cluster core size from {Ti₄} to {Ti₂₈} with a length of 3.55 nm. Ti28-TBC6A represents the largest calixarene-stabilized TOC reported to date. Density functional theory (DFT) calculations reveal a substantial electron transfer effect from the TBC6A ligand to the titanium-oxo core in Ti4-TBC6A and Ti28-TBC6A, attributed to the predesigned multiple coordination sites of TBC6A for chelation with titanium atoms. This phenomenon broadens the light absorption range of the two TOCs to encompass the visible region, resulting in excellent photoelectronic properties. Of note, Ti28-TBC6A exhibits superior catalytic activity and high stability for CO₂ fixation owing to the presence of more metal catalytic sites and better photoelectric performance. This work not only demonstrates the flexibility and diversity of macrocyclic ligands in constructing TOCs, but also provides an example of calixarene-stabilized high-nuclearity TOCs, contributing to a comprehensive understanding of their interfacial and metal-core structures.