Bis(bipyridine)-phenanthroline double-stranded helicates of the d¹⁰ metals: Zinc(II), silver(I) and copper(I) helicates

The new ligand 2,9-bis[(6-methyl-2,2'-bipyridin-6'- yl)methyleneoxymethylen-yl]-1,10-phenanthroline (1) was prepared and was shown to self-assemble into double-stranded helicates upon reaction with Cu⁺, Ag⁺ and Zn²⁺, thus affording the first trinuclear double-stranded zinc helicate. The [(1)₂M₃](3n+) [M = Cu⁺(n = 1), Ag⁺(n = 1) or Zn²⁺(n = 2)] helicates were characterized by 1D, 2D and diffusion NMR. FAB mass spectrometry corroborated the structure of the silver and the copper helicates and [(1)₂Cu₃]³⁺ was characterized by X-ray crystallography. The detailed assignments of the ¹H NMR spectra of these helicates have shown that H-10 absorbs at a much higher field when compared to the other protons of these complexes. This high-field shift is a unique characteristic of the helicates of 1 and is not found in structurally related helicates. The NMR spectra of these helicates, at room temperature, are consistent with an average solution structure having a D₂ symmetry. Variable-temperature ¹H NMR (VT-NMR) reveals that the silver helicate of 1 is much more labile than its copper helicate. From the VT-NMR spectra it seems that two, nearly separate, stages are observed in the dissociation process of [(1)₂Ag₃]³⁺. The activation energy of the first process for [(1)₂Ag₃]³⁺ in DMSO-d₆ was found to be 16.6 ± 0.2 kcal mol^-1. As expected, the energy barriers for the respective copper helicate are even higher. The crystal structure of [(1)₂Cu₃]³⁺ shows that the total length and the pitch length are ≃17 and 12 Å, respectively.