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

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 (ΔGb‡) 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 CdII, CuI and ZnII 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.