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 [M2(L1)2][ClO4]4 (M = Ni, Zn; L1 is a
potentially hexadentate ligand containing a py–th–py–py–th–py sequence, where ‘py’ denotes pyridyl and ‘th’ denotes
thiazolyl) illustrates how L1 can show two different coordination modes: in [Zn2(L1)2][ClO4]4 the ligands L1 are bisbidentate
chelates (via the terminal py–th fragments, with the central bipyridyl unit not coordinated) such that the
metal ions are four-coordinate, whereas in [Ni2(L1)2][ClO4]4 the ligand coordinates in a more usual bis-terdentate
manner such that the metal ions are six-coordinate. Reaction of Ni(), Cu() or Zn() salts with a 1 : 1 mixture of the
potentially hexadentate ligands L1 and L2 (where L2 contains a phen–th–th–phen sequence, ‘phen’ denoting a 1,10-
phenanthroline unit) afforded in each case a mixture of helical complexes [M2(L1)2]4, [M2(L1)(L2)]4 and [M2(L2)2]4
in different proportions according to the preferences of the different metal ions for different coordination numbers,
and the actual denticity of the ligand. For example the mixed-ligand complex [M2(L1)(L2)]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 assembly of L1 and L2 with Zn() such that the homoleptic complexes [Zn2(L1)2]4 and [Zn2(L2)2]4
are favoured more than simple statistical considerations would suggest.