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 pypy 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.