Synthesis, properties and complexation studies on 3-amino-6,6′-dimethyl-2,2′-bipyridine

Abstract

The new unsymmetrical bipyridyl ligand 6,6′-dimethyl-3-nitro-2,2′-bipyridine was prepared via coupling of 6-methyl-2-trimethylstanniopyridine and 2-chloro-6-methyl-3-nitropyridine in the presence of [Pd(PPh₃)₂Cl₂]. Reduction of the nitro group afforded 3-amino-6,6′-dimethyl-2,2′-bipyridine (L), a model for the central subunit of the antitumour drug streptonigrin. At low temperatures, in [²H₆]acetone, L is planar, held in place by a hydrogen bond from the amino group to the pyridyl nitrogen in the adjacent ring. From ¹H NMR lineshape analysis the barrier to rotation about the amino–bipyridyl bond (ΔG_b^‡) was estimated to be ≈38 kJ mol^–1 at 200 K. This value is significantly lower than the barrier to rotation about the biaryl bond connecting the aryl rings. In solution, L co-ordinates to Cd^II, Cu^I and Zn^II as a bipyridyl ligand; in these complexes the chemical shift of the amino group protons shifts upfield to ca.δ 5 compared to L where they resonate at δ 6.5. The crystal structure of [(CdLCl₂)₂] was determined by X-ray diffraction methods and refined to a residual of 0.027 for 1895 independent observed reflections. The crystals are monoclinic, space group P2₁/n, a= 9.560(2), b= 16.886(2), c= 9.577(3)Å, β= 118.37(2)°. The complex crystallized as a dimer in which each cadmium binds three chloride ligands and a bipyridyl ligand in a distorted trigonal-bipyramidal arrangement. The relevance of these results to the structure and properties of the antitumour drug streptonigrin is discussed.