pH-dependent formation of different coordination cages based on Co4-TC4A secondary building units and bridging ligands

Abstract

Two kinds of calixarene-based metal–organic coordination cages were obtained at different pH values despite using the same bridging ligand, either 5-(pyridin-3-yl)isophthalic acid (L¹) or 5-(5-fluoropyridin-3-yl)isophthalic acid (L²), namely, {[Co₄(TC4A)Cl]₄(Lⁿ)₄(HCOO)₄(H₂O)₄]}·26DMF (CIAC-117 and CIAC-119) and H₄{[Co₄(TC4A)Cl]₄(Lⁿ)₈}·18CH₃OH·33DMA (CIAC-118 and CIAC-120), where TC4A represents deprotonated p-tert-butylthiacalix[4]arene, and DMF and DMA represent N,N-dimethylformamide and N,N-dimethylacetamide, respectively. Tetrahedral coordination cages (CIAC-117 and CIAC-119) with shuttlecock-like Co₄-TC4A secondary building units (SBUs) as the vertices and L¹ and L² as the three-connected linkers were prepared in the presence of tetramethylammonium hydroxide, while truncated octahedral coordination cages (CIAC-118 and CIAC-120) with L¹ and L² as the two-connected linkers were generated upon the addition of HCl. For CIAC-117, two types of activation methods, solvent exchange-heating and direct heating, were used to investigate the removal of guest molecules in the structure. N₂ adsorption properties of activated samples were studied. Moreover, selective dye sorption of CIAC-120 was investigated.