Geometrical and optical isomers of the nickel(II) complexes of chiral, tetradentate unmixed and mixed schiff bases: CD and NMR spectroscopic studies

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Electronic, circular dichroïsm (CD) and NMR spectroscopic studies (1D 1H and 13C, 2D 1H COSY and NOESY, 1H 13C correlation) of several chiral unmixed Schiff base nickel complexes and chiral mixed nickel complexes have been carried out. In such Schiff base complexes, the Cotton effect associated with a given transition reflects the conformation of the chelate five-membered ring of the diamino chain. The entire set of mixed and unmixed NiN 2O 2 ( S) complexes involve a chelate ring adopting, at least preferentially, a λ conformation, with the methyl substituent taking an axial orientation. This result is confirmed by the NMR study of the vicinal coupling constants 3 J HH in the C(H α )(H β )C(H γ )R fragments of the diamino chain. On the contrary, the unmixed NiN 4 ( S) complex and the mixed NiN 3O ( S) complex are clearly different. This change comes from the presence of a 5,5,5- or a 5,5,6-chelate ring arrangement around the metal atoms that leads to a larger CNC angle involving the imine function and, eventually, allows a shift of the methyl substituent towards a more equatorial position. Furthermore, use of 2D 1H 13C NMR correlations allows a straightforward attribution of the entire set of signals. In one case, these results lead to a different assignment from previously published data.