Ligand reprogramming in dinuclear helicate complexes: A consequence of allosteric or electrostatic effects?

The ditopic ligand 6,6′‐bis(4‐methylthiazol‐2‐yl)‐3,3′‐([18]crown‐6)‐2,2′‐bipyridine (L1) contains both a potentially tetradentate pyridyl‐thiazole (py‐tz) N‐donor chain and an additional “external” crown ether binding site which spans the central 2,2′‐bipyridine unit. In polar solvents (MeCN, MeNO2) this ligand forms complexes with ZnII, CdII, HgII and CuI ions via coordination of the N donors to the metal ion. Reaction with both HgII and CuI ions results in the self‐assembly of dinuclear double‐stranded helicate complexes. The ligands are partitioned by rotation about the central pypy bond, such that each can coordinate to both metals as a bis‐bidentate donor ligand. With ZnII ions a single‐stranded mononuclear species is formed in which one ligand coordinates the metal ion in a planar tetradentate fashion. Reaction with CdII ions gives rise to an equilibrium between both the dinuclear double‐stranded helicate and the mononuclear species. These complexes can further coordinate s‐block metal cations via the remote crown ether O‐donor domains; a consequence of which are some remarkable changes in the binding modes of the N‐donor domains. Reaction of the HgII‐ or CdII‐containing helicate with either Ba2+ or Sr2+ ions effectively reprogrammes the ligand to form only the single‐stranded heterobinuclear complexes [MM′(L1)]4+ (M=HgII, CdII; M′=Ba2+, Sr2+), where the transition and s‐block cations reside in the N‐ and O‐donor sites, respectively. In contrast, the same ions have only a minor structural impact on the ZnII species, which already exists as a single‐stranded mononuclear complex. Similar reactions with the CdII system result in a shift in equilibrium towards the single‐stranded species, the extent of which depends on the size and charge of the s‐block cation in question. Reaction of the dicopper(I) double‐stranded helicate with Ba2+ shows that the dinuclear structure still remains intact but the pitch length is significantly increased.