Recognition with metallo cavitands

Abstract

We describe here the effects of metal complexation on the molecular recognition behavior of cavitands with quinoxaline walls. The nitrogen atoms of the quinoxalines are near the upper rim of the vase-like shape and treatment with Pd(II) gave 2:1 metal:cavitand derivatives. Characterization by 1H, 13C NMR spectroscopy, HR ESI-MS, and computations showed that the metals bridged adjacent quinoxaline panels and gave cavitands with C2v symmetry. Both water-soluble and organic-soluble versions were prepared and their host/guest complexes with alkanes, alcohols, acids, and diols (up to C12) were studied by 1H NMR spectroscopy. Analysis of the binding behavior indicated that the metals rigidified the walls of the receptive vase conformation and enhanced the binding of hydrophobic and even water-soluble guests, compared to related cavitands reported previously. The results demonstrated that the conformational dynamics of the cavitand were slowed by the coordination of Pd(II) and stabilized the host’s complexes.

Highly Selective Radical Monoreduction of Dihalides Confined to a Dynamic Supramolecular Host

“…A characteristic signal pattern is observed in the 1 H NMR spectra and increasing the length of the alkyl spacer does not affect the signals in the upfield region for these guests (see Figure 2; magenta letters). The limited space affects the orientation of bound guests and their movement: they are positioned in an extended conformation in the cavity as reported for similar linear guests in 1 [16] . The NMR signals for external −CH 2 − are observed between 1.5 and 2 ppm (see Figure 2 and Supporting Information).…”

“…Applications of open‐ended container compounds play an expanding role in biomimetic literature, [12–15] and water‐soluble cavitand 1 is one of the actors (see Figure 1). The cavitand exists in solution in the receptive vase shape or unreceptive flattened shape, called the kite or velcrand form [16] . The equilibrium between vase and kite form in solution is dynamic and regulated by experimental conditions [17] .…”

“…Host–guest complexes, kinetically stable (on the NMR timescale), must have a K A value higher than 1.2×10 3 m −1 to guarantee that the reaction, involving the guest molecules takes place in the host cavity [27] . In a previous work, isothermal titration calorimetry (ITC) was used to determine a K A value of 1.5×10 5 m −1 for n BuOH in 1 [16] . Binding competition experiments on 1 with 7 in the presence of n BuOH showed a comparable affinity of 7 to that of n BuOH for 1 (see SI47–48).…”

“…[27] In ap revious work, isothermal titrationc alorimetry (ITC) was used to determine a K A value of 1.5 10 5 m À1 for nBuOH in 1. [16] Binding competition experiments on 1 with 7 in the presenceo fnBuOH showed ac omparable affinity of 7 to that of nBuOH for 1 (see SI47-48). Since 7 showed the lowest K A value among alkyl dihalidesi nb inding to ar elated watersoluble cavitand, [27] we assumed that reactions outside the host 1 are also prevented for other alkyl halides (2-6).…”

“…The cavitand exists in solution in the receptive vase shapeo ru nreceptivef lattened shape, called the kite or velcrand form. [16] The equilibrium between vase and kite form in solution is dynamic and regulated by experimental conditions. [17] The vase form is identified by characteristic methine proton signals that appear at 5.6 ppm in the 1 HNMR spectrum, while as ignal around 4ppm is observed for the kite form.…”