Synthesis of Polydentate Ligands and the Formation of Heterometallic and Circular Helicates

Described herein, is the synthesis and coordination chemistry of seven novel ligands L1 - L7. These ligands form metallosupramolecular assemblies upon coordination of transition metal ions resulting in heterodi- and hetreotrimetallic double helicates and penta- and tetranuclear cyclic helicates.

Described in Chapter 2 is a new class of ditopic segmental pyridyl-thiazole(py-tz) N-donor ligands L1 - L3. Reaction of L1 with ZnII ions results in the formation of a dinuclear double helicate [Zn2(L1)2]4+. Reaction of L2 with either ZnII or HgII results in the formation of the L2-containing dinuclear double helicates [Zn2(L2)2]4+ and [Hg2(L2)2]4+. However, reaction with both ZnII or HgII results in the sole formation of the heterodimetallic helicate [HgZn(L2)2]+. Both metal ions are 6-coordinate but the HgII ion is coordinated by the two py-tz-py units whereas the ZnII ion is coordinated by the py-py-tz domain. The reason
that these isomeric sites have different preferences for each of the metal ions is due to the position of the thiazole unit within the terdentate domains, as in the central position the thiazole unit increases the “bite angle” of the donor unit making it more suitable for the larger HgII. Conversely the py-py-tz domain has a smaller bite angle and it more suited to the smaller ZnII ion. Reaction of L3 with ZnII, HgII and CuII results in the formation of a heterometallic trinuclear double helicate [HH-[HgCuZn(L3)2]5+. In a similar fashion to L2, the ZnII ion coordinated by the terdentate py-py-tz domain and the HgII coordinated by the py-tz-py domain. The central bipyridine unit
coordinates the tetrahedral CuII ion resulting in the first reported example of a heterotrimetallic double helicate.

Described in Chapter 4 is a potentially hexadentate N-donor ligand L4, which upon reaction with CdII results in the formation of a dinuclear double helicate [Cd2(L4)2]4+. In this structure the ligand partitions into two tridentate tz-py-py domains each of which coordinate a different metal ion. However, reaction of L4 with ZnII results in the formation of a pentanuclear circular helicate [Zn5(L4)5]10+, with all the five zinc ions adopting a octahedral coordination geometry arising from the coordination of the two tridentate tz-py-py domains from two different ligand strands. This difference in structure is attributed to unfavourable steric interactions which prevent the formation of [Zn2(L4)2]4+ but
these unfavourable interactions are not present with the larger Cd2+ ion.

Described in Chapter 5 are the potentially pentadentate and tetradentate ligands L5 and L6, respectively. The ligand L5 contains both a bidentate and tridentate binding site separated by a phenylene spacer unit. Reaction of L5 with CuII results in the formation of a pentanuclear circular helicate [Cu5(L5)5]10+. Each of the CuII ions adopts a 5-coordinate geometry formed by
coordination of the bidentate domain of one ligand strand and the tridentate domain of a different ligand. As a result this gives a head-to-tail pentanuclear double helicate. Reaction of L6 and L4 (Chapter 4) with CuII results in the formation of a heteroleptic pentanuclear circular helicate [Cu5(L4)3(L6)2]10+.
The cyclic array consists of five copper(II) ions, coordinated by three strands of L4 and two strands of L6. In this species four of the CuII adopt a 5- coordinate geometry arising from coordination of a tridentate domain from L4 and a bidentate domain from L6. The remaining copper ion is coordinated by two tridentate domains from L4 resulting in an octahedral coordination
geometry.

Described in Chapter 6 is the potentially hexadentate N-donor ligand L7 which comprises of two identical tridentate py-py-tz N3 binding domains separated by a pyrene unit. Reaction of L7 with ZnII results in the formation of a
tetranuclear circular helicate [Zn4(L7)4]8+ with all four zinc metal ions adopting a six-coordinate geometry arising from the coordination of two tridentate pypy- tz units from two different ligand strands. The formation of this lower nuclearity species (e.g. tetranuclear rather than pentanuclear) is attributed to the p-stacking between the pyrene unit and the py-py-tz domain.