Effect of guest molecules, metal ions and linker length on the assembly of chiral [2 + 2] metallomacrocycles: solution studies and crystal structures

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Abstract
Eight new 2,2′-bipyridines, containing functionalised acetylenes at the 6 position, have been prepared. The azido derivatives were used to prepare three compounds L 1 , L 2 and L 3 , which contain pyromellitimide spacers connected to 6,6′-disubstituted 2,2′-bipyridines with propynyl, butynyl and propynyloxy linkers respectively, via iminophosphoranes. In the presence of an equimolar amount of zinc(II) triflate (trifluoromethanesulfonate), L 1 was assembled exclusively into the chiral [2 + 2] metallomacrocycle [Zn 2 L 1 2 ] 4+ at low concentrations (20 equivalents of m-dimethoxybenzene. In contrast, L 2 and L 3 formed zinc(II) complexes which gave only broadened NMR spectra consistent with equilibrating mixtures of oligomers. The different results obtained with L 1 , L 2 and L 3 highlight the importance of the rigid propargyl(prop-2-ynyl) linker in directing the outcome of the assembly process.

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