Electrostatic control of the formation of heteroleptic transition metal helicates

Abstract

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.