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 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.