Stereospecific generation of homochiral helices in coordination polymers built from enantiopure binaphthyl-based ligands

Abstract

The novel enantiopure dipyridyl spacer 2,2′-dimethoxy-1,1′-binaphthyl-3,3′-bis(4-pyridyl-amido) (R)-L has been designed as a robust source of axial chirality to obtain helical coordination polymers. The reaction of (R)-L and the differently substituted dithiophosphato complexes [Ni((RO)₂PS₂)₂] [R = Me (1), Et (2)] efficiently yielded coordination polymers (1·L)_∞ and (2·L)_∞, respectively, consisting of helical chains in which the nickel(II) ions of the [Ni((RO)₂PS₂)₂] units are bridged by the enantiopure L ligands. The obtained polymers differ in terms of the configuration at the metal centres, which is trans and cis for (1·L)_∞ and (2·L)_∞, respectively. The cis configuration in (2·L)_∞ generates a further element of chirality around the metal center, which occurs stereospecifically, as only one enantiomeric form is present, with homochiral helices packed with opposite screw sense in the crystal. The electronic and structural features of L, (1·L)_∞, and (2·L)_∞ have been investigated by means of DFT theoretical calculations, and the theoretical results have been compared with the experimental ones coming from single-crystal X-ray diffraction. The cis/trans isomerism displayed by the metal centers in (1·L)_∞ and (2·L)_∞ has been tentatively explained on the basis of the results of theoretical calculations performed on hypothetical pentacoordinated intermediates.