GEOMETRICAL AND OPTICAL ISOMERS OF THE NICKEL(II) COMPLEXES OF CHIRAL, TETRADENTATE UNMIXED AND MIXED SCHIFF-BASES - CD AND NMR SPECTROSCOPIC STUDIES

Electronic, circular dichroism (CD) and NMR spectroscopic studies (1D H-1 and C-13, 2D H-1 COSY and NOESY, H-1-C-13 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 NiN2O2 (S) complexes involve a chelate ring adopting, at least preferentially, a lambda 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-alpha)(H-beta)-C(H-gamma)- fragments of the diamino chain. On the contrary, the unmixed NiN4 (S) complex and the mixed NiN3O (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 H-1-C-13 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.