The influence of the linkage mode between bipyridine units in oligobipyridine ligands on the formation of copper-(I) and -(II) helicates

Abstract

A novel asymmetric oligobipyridine ligand, 1-(5′-methyl-2,2′-bipyridin-5-yl)-2-(6′-methyl-2,2′-bipyridin-6-yl)ethane (L⁵⁶), was synthesized, and double-stranded helical complexes [Cu^I₂L⁵⁶₂][ClO₄]₂·Et₂O 1 and [Cu^II₂L⁵⁶₂(OH)(H₂O)][ClO₄]₃ 2 were obtained by reaction of the ligand with copper-(I) and -(II) ions, respectively. X-Ray data for 1 show that the two ligands intertwine around the two copper(I) ions with distorted tetrahedral geometry. The distance between the two Cu(I) ions is 7.290(2) Å. The copper(II) ions in 2 have a disordered tetragonal pyramidal geometry with a N₄O donor set. The intermetallic distance is 6.875(2) Å. In both 1 and 2 the two ligand strands are arranged in a head-to-tail configuration. The complexes were also characterized by ES-MS, ¹H NMR and ESR spectroscopies. The results show that the difference in linkage of the spacer group to the bipyridine units produces a great impact on the helix formation with Cu^II.