Pyridyl-thiazole multidentate ligands: metal specific recognition of a combination of ligands from a mixture

Comparison of the crystal structures of the dinuclear double helicates [M-2 (L-1)(2)][ClO4](4) (M = Ni, Zn; L-1 is a potentially hexadentate ligand con taining a py-th-py-py-th-py sequence, where 'py' denotes pyridyl and 'th' d enotes thiazolyl) illustrates how L-1 can show two different coordination m odes: in [Zn-2 (L-1)(2)][ClO4](4) the ligands L-1 are bis-bidentate chelate s (via the terminal py-th fragments, with the central bipyridyl unit not co ordinated) such that the metal ions are four-coordinate, whereas in [Ni-2 ( L-1)(2)][ClO4](4) the ligand coordinates in a more usual bis-terdentate man ner such that the metal ions are six-coordinate. Reaction of Ni(II), Cu(II) or Zn(II) salts with a 1 : 1 mixture of the potentially hexadentate ligand s L-1 and L-2 (where L-2 contains a phen-th-th-phen sequence, 'phen' denoti ng a 1,10-phenanthroline unit) afforded in each case a mixture of helical c omplexes [M-2 (L-1)(2)](4+), [M-2 (L-1)(L-2)](4+) and [M-2 (L-2)(2)](4+) in different proportions according to the preferences of the different metal ions for different coordination numbers, and the actual denticity of the li gand. For example the mixed-ligand complex [M-2 (L-1)(L-2)](4+) was formed to the same extent (ca. 50%) for M = Ni and M = Cu, but hardly at all for M = Zn, indicating that self-self ligand recognition operates during assembl y of L-1 and L-2 with Zn(II) such that the homoleptic complexes [Zn-2 (L-1) (2)](4+) and [Zn-2(L-2)(2)](4+) are favoured more than simple statistical c onsiderations would suggest.