Dynamic equilibrium between a supramolecular capsule and bowl generated by inter- and intramolecular metal clipping

Abstract

The metal-induced self-assembly of a resorcin[4]arene derivative 1 that has four pyridine units as pendent groups and two equivalents of [M(dppp)(OTf) 2] (M=Pd, Pt) results in a dynamic equilibrium between an interclipped supramolecular capsule 3 and an intraclipped bowl 4 in nitromethane, although the interclipped capsule 3 is formed as a sole adduct in chloroform/methanol and the intraclipped bowl 4 is formed exclusively in an aqueous phase. This demonstrates how metal-induced self-assembly can be tuned by subtle changes in the solvent system. The coexistence of the two structures in nitromethane was characterized by NMR spectroscopy and coldspray ionization mass spectrometry (CSI-MS). The crystal structure of the interclipped capsule 3b, which is composed of two units of ligand 1 and four PtII ions, reveals the capsule cavity to have nanoscale dimensions of 15 × 20 A. NMR spectra show that the dynamic equilibrium between 3 and 4 is dependent on concentration and temperature. Temperature-dependent 1H NMR spectroscopy was carried out from 273 to 343 K to verify the thermodynamic parameters that control the dynamic equilibrium process; the conversion from the interclipped supramolecular capsule 3 a to the intraclipped bowl 4 a is entropically favored and enthalpically disfavored. The rotational barrier of the restricted rotation of pyridine units in the intraclipped bowl 4 was determined by line-shape analysis.