Synthesis and Structural Properties of Aza[n]helicene Platinum Complexes: Control of Cis and Trans Stereochemistry

The synthesis and the structural characterization of azahelicene platinum complexes obtained from cis-PtCl2(NCEt)(PPh3) and from ligands that differ both in terms of the position of the N atom and the number of fused rings, is reported. These square planar complexes of general formula PtCl2(nHm)(PPh3) (n = 4,5, m = 5,6) display mainly a cis configuration. However, by X-ray crystallographic analysis we show that for both PtCl2(4H6)(PPh3) and PtCl2(5H6)(PPh3) there is a chirality control of the cis-trans stereochemistry. Indeed, while starting from a racemic mixture of aza[6]helicene, Pt complexes with a cis configuration are invariably obtained, the more thermodynamically stable trans isomers are formed when using enantiopure ligands. We further corroborated these results by NMR analysis in solution.