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

Four new multidentate N-donor ligands L-1-L-4 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 alpha -bromoacetyl unit with a thioamide to form the thiazolyl ring. The extended linear sequence of ortho-linked N-donor heterocycles (four for L-1, six for L-2; five for L-3; and six for L-4) 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 5-membered thiazolyl rings results in natural breaks of the ligand backbone into distinct bidentate or terdentate domains. Thus, the tetradentate ligand L-1 partitions into two bidentate domains to give dinuclear triple helicates [M-2(L-1)(3)](4+) with six-coordinate first-row transition metal dications (M=Co, Cu, Zn). The hexadentate ligand L-2 partitions into two terdentate domains to give dinuclear double helicates [M-2(L-2)(2)](4+) with six-coordinate metal ions (M=Cu, Zn). In the double helicate [Cu-2(L-3)(2)](4+) the pentadentate ligand L-3 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 [Cu-2(L-4)(2)](4+) the metal ions are only four-coordinate, with each ligand having its central bipyridyl unit non-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.