Synthesis of polydentate ligands and their self-assembly into helicates, meso-helicates and cyclic helicates

Described here is the synthesis and coordination chemistry of various ligands, L1 – L17. Some of the ligands presented form interesting supramolecular assemblies upon reaction with selected metal ions.

Chapter 1 provides a general introduction to supramolecular chemistry and self-assembly.

Chapter 2 introduces a new class of potentially hexadentate symmetrical ligands, L1 – L5. These ligands consist of two tridentate binding sites separated by a 1,3-phenylene spacer unit. Reaction of L1 with Zn(II) ions results in the formation of a pentanuclear circular helicate [Zn5(L1)5]10+, within the structure all five zinc ions are six-coordinate arising from coordination of two tridentate domains from two different ligand strands. This structure was shown to exist in both the solid state and in solution. Incorporation of various enantiopure units allowed variation of the terminal functional group of the ligand, L2 – L5. These ligands, upon coordination with Zn(II) ions, were shown to from supramolecular assemblies analogous to the pentanuclear species observed for L1. Additionally these ligands were shown to be diastereoselective, controlling the resulting supramolecular architecture giving up to 80% diastereomeric excess.

Described in Chapter 3 are a number of potentially hexadentate N-donor ligands, L6 – L14. Each ligand possesses the same thiazole-pyridyl-pyridyl tridentate domains, with variation of the spacer unit. Upon coordination with selected transition metal ions these ligands resulted in the formation of dinuclear species. Reaction of L9 with Cd(II) results in the formation of a dinuclear double helicate, in which the two tridentate domains coordinate each
metal ion and the ligands twist in the centre generating an ‘over and under’ arrangement. However, reaction of L9 with Co(II) results in the formation of a dinuclear meso-helicate, in which the ligands adopt a side-by-side configuration. This difference in structure is attributed to unfavourable steric interactions which prevent the formation of the Co(II) double helicate. Reaction of two of these ligands L10, which possesses an ethylene glycol chain, and L11, containing an amine group, with Cd(II) and camphorsulfonic acid results in the formation of a heteroleptic one-dimensional chain. Hydrogen bonding interactions between the protonated amine of L11 and the glycol chains of L10 results in a structure which contains both of these meso-helicate structures in an extended one-dimensional arrangement (([Cd2(L10)2][Cd2(L11-H)2])(ClO4)10)n.

Chapter 4 reports the synthesis of three ligands, L15 – L17, each containing the same central phenol unit, and either a hydroxyl, pyridine or pyridine-N-oxide terminal unit. Reaction of each ligand with various trivalent lanthanide ions results in the formation of a dinuclear double helicate. In each structure the central phenol unit is deprotonated and bridges the two lanthanide ions giving [L2M2]4+. L17, which possesses the pyridine-N-oxide as the terminal group, effectively encompasses the cations minimising access for the coordination of any anions or solvent molecules. Photophysical measurements show that this ligand forms emissive complexes with a number of lanthanide ions, whilst the magnitude of the lifetime for [(L17)2Yb2]4+ (= 21.0 s) suggests that both Yb(III)
ions are well-shielded from excited state quenching phenomena.