Fluoride Enhances Alcohol Binding Within a Trigonal-Prismatic Metal-Organic Capsule

Abstract

Herein we utilize the binding of fluoride to boron atoms to functionalize the interior of a boron-containing trigonal prismatic capsule that incorporates two triangular and three rectangular ligands, enabling the tuning of its guest binding properties. The methyl groups of the triangular ligands guide the rectangular ligands to adopt a "landscape" orientation to avoid steric hindrance. This small structural change gives rise to an enlarged interior cavity volume for guest encapsulation, as compared with a previously-reported trigonal prismatic capsule, where the same rectangular ligand took a "portrait" orientation with a non-methylated triangular ligand of similar size. The methylated triangular ligand contains a boron core, which can bind fluoride ions that point inward. These bound fluorides serve as hydrogen bond acceptors, which increases the affinity of the capsule for hydrogen-bond-donating alcohols, which are bound in preference to ketones of similar sizes. Moreover, this boron-containing trigonal prism selectively binds perrhenate over perchlorate, while fluoride binding modulates the cavity charge, leading to perrhenate ejection. These and similar endo-functionalized capsules may thus be of use in the fields of molecular recognition and separation.

Keywords: Endo‐functionalization; Host–guest chemistry; Metal‐organic cages; Post‐assembly modification; Self‐assembly.

Figure 1

a) Self‐assembly of our previously‐reported Zn^II ₆ L^A ₃ L^B ₂ trigonal prismatic cage and new Zn^II ₆ L^A ₃ L^C ₂ trigonal prism 1; b) X‐ray structure of 1 shown in stick (top, without hydrogen atoms) and space‐filling (bottom) modes (disorder and solvents are omitted for clarity). The yellow circle highlights how the steric bulk of the methyl groups in L^C is accommodated within clefts formed by the longer edge of L^A in a “landscape” orientation; c) comparison of the cavity volumes^{[ 57 ]} (in purple mesh) of the previously‐reported trigonal prismatic capsule (top) and capsule 1 (bottom).

Figure 2

a) Illustration of fluoride binding to 1 and subsequent F⁻ removal upon treatment with Ca²⁺; b) DFT^{[ 62 ]}‐minimized structures of 1·F⁻ (left) and 1·2F⁻ (right).

Figure 3

a) Prospective guests were tested for binding within 1 and 1·2F⁻. G9 and G10 were observed to bind within 1, but not 1·2F⁻, and G11–G14 bound within neither cage. b) G9 and G10 were released from 1 following the formation of 1·2F⁻.