New multidentate ligands for supramolecular coordination chemistry: double and triple helical complexes of ligands containing pyridyl and thiazolyl donor units

Abstract

Four new multidentate N-donor ligands L¹–L⁴ have been prepared which contain a combination of pyridyl and thiazolyl donor units. The syntheses of these ligands are facile and high-yielding, being based on reaction of an α-bromoacetyl unit with a thioamide to form the thiazolyl ring. The extended linear sequence of ortho-linked N-donor heterocycles (four for L¹, six for L²; five for L³; and six for L⁴) is reminiscent of the well-known linear oligopyridines, although these new ligands are much easier to make and have significantly different geometric coordination properties because the presence of the five-membered thiazolyl rings results in natural breaks of the ligand backbone into distinct bidentate or terdentate domains. Thus, the tetradentate ligand L¹ partitions into two bidentate domains to give dinuclear triple helicates [M₂(L¹)₃]⁴⁺ with six-coordinate first-row transition metal dications (M = Co, Cu, Zn). The hexadentate ligand L² partitions into two terdentate domains to give dinuclear double helicates [M₂(L²)₂]⁴⁺ with six-coordinate metal ions (M = Cu, Zn). In the double helicate [Cu₂(L³)₂]⁴⁺ the pentadentate ligand L³ only uses its two terminal bidentate binding sites, resulting in four-coordinate Cu(II) centres and a non-coordinated pyridyl residue in the centre of each of the two ligand strands. These pendant pyridyl residues are directed towards each other to give a potentially two-coordinate cavity between the metal ions in the centre of the helicate. Similarly, in the double helicate [Cu₂(L⁴)₂]⁴⁺ the metal ions are only four-coordinate, with each ligand having its central bipyridyl unit un-coordinated. This results in a potentially four-coordinate cavity between the two metal ions in the centre of the helicate. These easy-to-prepare ligands offer a great deal of scope for the development of multinuclear helicates.