Electrostatic control of the formation of heteroleptic transition metal helicates

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.