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

Eight new 2,2′-bipyridines, containing functionalised acetylenes at the 6 position, have been prepared. The azido derivatives were used to prepare three compounds L¹, L² and L³, 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), L¹ was assembled exclusively into the chiral [2 + 2] metallomacrocycle [Zn₂L¹₂]⁴⁺ at low concentrations (<10 mmol dm^-3) or in the presence of an aromatic guest molecule S, which stabilises the complex through binding in the cavity of the metallomacrocycle. The crystal structures of diquabis{N,N′-bis[3-(6′-methyl-2,2′-bipyridine-6-yl)-prop-2- ynyl]naphthalene-1,8:4,5-tetracarboximide}dizinc tetrakis(triflate) [Zn₂L¹₂][CF₃SO₃] ₄·2H₂O, containing either p- or o-dimethoxybenzene bound in the cavity, were determined by X-ray diffraction methods. In both cases the crystals were extremely fragile and there was extensive disorder of substrate and solvate molecules and the triflate counter ions in channels in the lattice. These structures, while of limited quality, confirm the identity of the complexes and reveal several substrate-dependent differences. Comparison of the two structures provided clear evidence for the metallomacrocycle adjusting its shape and dimensions to accommodate optimum binding of the substrate through π-stacking/electrostatic interactions, and provides the first example of a metallosuprarnolecular complex in which the cavity size is determined by the bound substrate. The assembly of the chiral [2 + 2] metallomacrocycle was also achieved with L¹ and cadmium(II) and L¹ and copper(I) in the presence of >20 equivalents of m-dimethoxybenzene. In contrast, L² and L³ formed zinc(II) complexes which gave only broadened NMR spectra consistent with equilibrating mixtures of oligomers. The different results obtained with L¹, L² and L³ highlight the importance of the rigid propargyl(prop-2-ynyl) linker in directing the outcome of the assembly process.