Bis(bipyridine)-phenanthroline double-stranded helicates of the d(10) 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 int o double-stranded helicates upon reaction with Cu+, Ag+ and Zn2+, thus affo rding the first trinuclear double-stranded zinc helicate. The [(1)(2)M-3](3 n+) [M = Cu+(n = 1), Ag+(n = 1) or Zn2+(n = 2)] helicates were characterize d by is, 2D and diffusion NMR. FAB mass spectrometry corroborated the struc ture of the silver and the copper helicates and [(1)(2)Cu-3](3+) was charac terized by X-ray crystallography. The detailed assignments of the H-1 NMR s pectra of these helicates have shown that H-10 absorbs at a much higher fie ld when compared to the other protons of these complexes. This high-field s hift is a unique characteristic of the helicates of 1 and is not found in s tructurally related helicates. The NMR spectra of these helicates, at room temperature, are consistent with an average solution structure having a D-2 symmetry. Variable-temperature H-1 NMR (VT-NMR) reveals that the silver he licate of 1 is much more labile than its copper helicate. From the VT-NMR s pectra it seems that two, nearly separate, stages are observed in the disso ciation process of [(1)(2)Ag-3](3+) The activation energy of the first proc ess for [(1)(2)Ag-3](3+) in DMSO-d(6) was found to be 16.6 +/- 0.2 kcal mol (-1). As expected, the energy barriers for the respective copper helicate a re even higher, The crystal structure of [(1)(2)Cu-3](3+) shows that the to tal length and the pitch length are approximate to 17 and 12 Angstrom respe ctively.