GEOMETRICAL AND OPTICAL ISOMERS OF THE DINICKEL COMPLEXES OF 2 CHIRALLIGANDS OF THE N,N'-DISUBSTITUTED OXAMIDE TYPE - 1D AND 2D NMR-STUDY

Two novel chiral dinucleating ligands have been obtained by grafting a methyl substituent onto one and both diamino arm(s) of N,N'-bis(4-met hyl-5-aza-3-hepten-2-one-7-yl) oxamide (H4L1), respectively. These lig ands, yl)-N'-(1,4(2,4)-dimethyl-6-oxo-3-azahepta-4-enyl) oxamide (H4L2 ) and N,N'-(1,4(2,4)-dimethyl-6-oxo-3-azahepta-4-enyl) oxamide (H4L3), and their related nickel complexes (Ni2L2) and (Ni2L3) exist as a var iety of geometrical and optical isomers since, on the one hand, two lo cations are possible for each substituent and on the other hand, each substituted amino chain comprises an asymmetric carbon. All but one ge ometrical isomer have been isolated via unequivocal synthetic pathways . Definitive proofs of the existence of all geometrical and optical is omers are gained from an NMR study including 2-D spectra and 1D spectr a obtained in the presence of a chiral auxiliary. In all cases the fiv e-membered ring resulting from coordination of the diamino moiety of t he ligands to the nickel ion adopts a preferred conformation which pla ces the methyl substituent in an axial or pseudoaxial position.