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

摘要

Comparison of the crystal structures of the dinuclear double helicates [M_2(L min)_2][ClO_4]_4 (M=Ni, Zn; L~1 is a potentially hexadentate ligand containing a py-th-py-th-py sequence, where 'py' denotes pyridyl and 'th' denotes thiazolyl)illustrates how L~1 can show two different coordination modes: in [Zn_2(L~1)_2][ClO_4]_4 the ligands L~1 are bis-bidentate chelates (via the terminal py-th fragments, with the central bipyridyl unit not coordinated) such that the metal ions are four-coordinate, whereas in [Ni_2(L~1)][ClO_4]_4 the ligand coordinates in a more usual bis-terdentate manner 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 ligands L~1 and L~2 ( where L~2 contains a phen-th-th-phen sequence, 'phen' denoting a 1,10-phenanthroline unit) afforded in each case a mixture of helical complexes [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 unmbers, and the actual denticity of the ligand. For example the mixed-ligand complex [M_2(L~1)(L~2)]~(4+) was formed tot he 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 assembly 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 considerations would suggest.