3-Amino-6,6′-bis(methoxycarbonyl)-2,2′-bipyridine, a model for the central chelation unit of streptonigrin

Abstract

The synthesis, crystal structure and solution conformation of 3-amino-6,6′-bis(methoxycarbonyl)-2,2′-bipyridine (L²) have been determined. The crystal structure was refined to a residual of 0.046 for 2218 independent observed reflections. The crystals are monoclinic, space group P2₁/c, a= 9.229(2), b= 10.342(2), c= 13.650(2)Å, β= 90.92(2)°. The two pyridyl rings are almost coplanar as a consequence of an intramolecular hydrogen bond between the amino group and the pyridyl nitrogen in the adjacent ring. The same conformation is observed in solution. The effect of solvent, temperature and concentration of zinc(II) on the types of complexes formed by 3-amino-6,6′-dimethyl-2,2′-bipyridine (L¹) and L² was studied. For L¹ the 2 : 1 complex is more stable than the 1 : 1 complex. Substantial stabilization of the 1 : 1 complex of L² occurs due to co-ordination of the ester groups at the 6,6′ positions. The relevance of these results to the structure and properties of the antitumour drug streptonigrin is discussed.