Assembly of [2 + 2] bimetallic macrocyclic complexes from bis(bipyridyl) ligands and metal ions

Abstract

Two new ligands L¹ and L², in which 6,6′-dimethyl-2,2′-bipyridyl groups are attached at the 6-position to 2,6-naphthalene or 2,7-pyrene via ether bridges, have been synthesised. In the presence of Cu¹, L¹ forms equal amounts of two [2 + 2] complexes [Cu₂L¹₂][PF₆]₂, which have been assigned tentatively to helical (1a) and non-helical (1b) forms. With Zn^II, L¹ forms two complexes [Zn₂L¹₂][CF₃SO₃]₄, tentatively assigned to the helical (3a) and non-helical (3b) forms, in the ratio ≈7:4. The relative populations of 3a and 3b are temperature and solvent dependent. Ligand L² forms equal amounts of helical and non-helical complexes [Cu₂L²₂][PF₆]₂2a, 2b with Cu^I. However, in the presence of Zn^II, L² forms exclusively one complex, which has been assigned as the non-helical species [ZnL²₂][CF₃SO₃]₄4b, on the basis of NMR data. In this complex, the pyrene rings are rotating rapidly on the NMR time-scale at 300 K. Molecular modelling suggests that the size and interior of the cavities of these macrocycles are ideal for encapsulation of charged aromatic substrates.