The synthesis of the potentially hexadentate ditopic ligand L1, which contains both N-donor and crown ether binding units, is described. Reaction of L1 with Zn(II) results in the formation of a dinuclear double helicate ([Zn2(L1)2](ClO4)4), which has been structurally characterized. In the solid state, each ligand splits into two tridentate binding domains with two Zn(II) ions coordinated by two bridging ligands in a double helicate arrangement. This complex can further react with s-block metal ions via the crown ether unit resulting in the structurally characterized zinc-containing helicate [Zn2(L1)2Ba2](ClO4)8. Ligand recognition studies of this helicate with a ligand that does not contain the crown ether unit but does contain the same N-donor array (L2) demonstrates, via both 1H NMR and ESI-MS studies, that a ligand recognition process does occur and the major species observed are the homoleptic species ([Zn2(L1)2](ClO4)4 and [Zn2(L2)2](ClO4)4). Addition of barium ions to the mixture increases the amount of the heteroleptic species ([Zn2(L1)(L2)](ClO4)4) present, resulting in an equal amount of homoleptic and heteroleptic species. This change in ligand recognition properties is attributed to electrostatic effects.
Baylies, C. J., Harding, L. P., Jeffery, J. C., Moon, R., Rice, C. R., & Riis-Johannessen, T. (2007). Electrostatic control of the formation of heteroleptic transition metal helicates. New Journal of Chemistry, 31(8), 1525-1529. https://doi.org/10.1039/b702608k