Exploiting reduced-symmetry ligands with pyridyl and imidazole donors to construct a second-generation stimuli-responsive heterobimetallic [PdPtL4]4+ cage

Abstract

A new sequential metalation strategy that enables the assembly of a new more robust reduced symmetry heterobimetallic [PdPtL₄]⁴⁺ cage C is reported. By exploiting a low-symmetry ditopic ligand (L) that features imidazole and pyridine donor units we were able to selectively form a [Pt(L)₄]²⁺ “open-cage” complex. When this was treated with Pd(II) ions the cage C assembled. ¹H and DOSY nuclear magnetic resonance (NMR) spectroscopy and electrospray ionisation mass spectrometry (ESIMS) data were consistent with the quantitative formation of the cage and the heterobimetallic structure was confirmed by single crystal X-ray crystallography. The cage C was shown to bind anionic guest molecules. NMR studies suggested that these guests interacted with the cavity of the cage in a specific orientation and this was confirmed for the mesylate ion (MsO⁻) : C host–guest adduct using X-ray crystallography. In addition, the system was shown to be stimulus-responsive and could be opened and closed on demand when treated with appropriate stimuli. If a guest molecule was bound within the cage, the opening and closing was accompanied by the release and re-uptake of the guest molecule.