Ligand self-recognition in the stereoselective assembly of [2 + 2] metallomacrocycles from racemic chiral bisbipyridyl molecular clefts and zinc(II)

The synthesis of racemic and optically pure ligand L, in which two 6,6′-disubstituted bipyridines are connected by methyleneoxy linkers to the molecular cleft dibenzobicyclo[b,f][3.3.1.]nona-5a,6a-diene-6,12-dione, is reported. In the presence of 2 equivalents of zinc(II) trifluoromethansulfonate (-)-L undergoes slow reversible coordination over 24 h to form a pair of enantiomeric [2 + 2] metallomacrocycles, [Zn₂(+)L₂](OTf) ₄ and [Zn₂(-)L₂](OTf)₄ respectively, that contain either two (+)-L ligands or two (-)-L ligands. This assignment was confirmed by independent studies with either (+)-L or (-)-L which formed the same complexes but at a significantly faster rate (3 h), and circular dichroism spectra of [Zn₂(+)L₂](OTf)₄ and [Zn ₂(-)L₂](OTf)₄ which gave signals of the same intensity with the opposite sign. Treatment of (±)-L or optically pure L with copper(I) showed rapid formation of a mixture of oligomers as well as the [2 + 2] metallomacrocycle. The complex Zn₂L₂(OTf) ₄ exhibits slow exchange between two species on the NMR time scale at room temperature. The results are consistent with the formation of a library of metal complexes in which the zinc(II) binds initially to the most accessible bipyridyl binding sites in (±)-L. Equilibration over several hours results in self-recognition of enantiomeric ligands to form a pair of enantiomeric metallomacrocycles, which have been tentatively assigned as having the helical configuration. Slow exchange is attributed to the preference for both metal centres to adopt 6-coordinate geometries involving the linker oxygens, but are limited to exchanging 5-coordinate complexes due to the shape of the cleft and the short linker.