Self-assembly of multinuclear complexes with enantiomerically pure chiral binaphthoxy imine ligands: effect of the alkyl spacer connecting two binaphthyl units on the metal binding

Metal complexing behavior of enantiomerically pure alpha,omega-diiminoalkanes possessing the two terminal binaphthyl units (L1 and L2) was studied. The ligands L1 and L2 were prepared by the reaction of optically pure 2'-butoxy-3-formyl-2-hydroxy-1,1'-binaphthyl with propane and pentane diamines. Reactions of L1 and L2 with equimolar amount of Cu(OAc)2 afforded quantitatively multinuclear complexes 1. The structure of 1 was confirmed by MALDI-TOF MS spectroscopy, X-ray single-crystal-structure analysis, and UV/vis and CD spectroscopic analyses. The reaction of L1 having a 1,3-propanediyl spacer resulted in the formation of a self-assembled product, which was assigned as enantiopure trinuclear circular helicate 1a, while the ligand having a 1,5-pentanediyl spacer L2 gave a different self-assembled product, dinuclear side-by-side complex 1b. The circular dichroism (CD) spectrum of 1a in solution showed intense Cotton effects in both the pi-pi* transition of the naphthalene units and the LMCT region of the N,O-chelate moieties. The CD spectrum of 1b was completely different from that of 1a; in particular the Cotton effects in the LMCT region were very weak, contrary to those of 1a. These results suggest that 1a retains some chirality induced on the N,O-chelating moieties even in solution, while the induced chirality on the N,O-chelating moieties in 1b is not very significant, being consistent with the consequences of the X-ray single-crystal-structure studies.