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

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Abstract
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.
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