Formation of a low-symmetry Pd8 molecular barrel employing a hetero donor tetradentate ligand, and its use in the binding and extraction of C70

Abstract

The majority of reported metallo-supramolecules are highly symmetric homoleptic assemblies of M_xL_y type, with a few reports on assemblies that are obtained using multicomponent self-assembly or using ambidentate ligands. Herein, we report the use of an unsymmetrical tetratopic ligand (L^un) containing pyridyl and imidazole donor sites in combination with a cis-protected Pd(II) acceptor for the formation of a low-symmetry M₈L^un₄ molecular barrel (UNMB). Four potential orientational isomeric (HHHH, HHHT, HHTT, and HTHT) molecular barrels can be anticipated for the M₈L^un₄ type metallo-assemblies. However, the formation of an orientational isomer (HHTT) of the barrel was suggested from single-crystal X-ray diffraction and ¹H NMR analysis of UNMB. Two large open apertures at terminals and the hydrophobic confined space surrounded by four aromatic panels of L^un make UNMB a potential host for bigger guests. UNMB encapsulates fullerenes C₇₀ and C₆₀ favoured by non-covalent interactions between the fullerenes and aromatic panels of the ligand molecules. Experimental and theoretical studies revealed that UNMB has the ability to bind C₇₀ more strongly than its lower analogue C₆₀. The stronger affinity of UNMB towards C₇₀ was exploited to separate C₇₀ from an equimolar mixture of C₇₀ and C₆₀. Moreover, C₇₀ can be extracted from the C₇₀⊂UNMB complex by toluene, and therefore, UNMB can be reused as a recyclable separating agent for C₇₀ extraction.