Abstract
The majority of reported metallo-supramolecules are highly symmetric homoleptic assemblies of M (x) L (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 M(8)L(un) (4) molecular barrel (UNMB). Four potential orientational isomeric (HHHH, HHHT, HHTT, and HTHT) molecular barrels can be anticipated for the M(8)L(un) (4) type metallo-assemblies. However, the formation of an orientational isomer (HHTT) of the barrel was suggested from single-crystal X-ray diffraction and (1)H 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 C(70) and C(60) 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 C(70) more strongly than its lower analogue C(60). The stronger affinity of UNMB towards C(70) was exploited to separate C(70) from an equimolar mixture of C(70) and C(60). Moreover, C(70) can be extracted from the C(70)⊂UNMB complex by toluene, and therefore, UNMB can be reused as a recyclable separating agent for C(70) extraction.