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 Pt-II ions, reveals the capsule cavity to have nanoscale dimensions of 15 x 20 Angstrom. NMR spectra show that the dynamic equilibrium between 3 and 4 is dependent on concentration and temperature. Temperature-dependent H-1 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 3a to the intraclipped bowl 4a 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.