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 [H-2(6)]acetone, L is planar, held in place by a hydrogen bond from the amino group to the pyridyl nitrogen in the adjacent ring. From H-1 NMR lineshape analysis the barrier to rotation about the amino-bipyridyl bond (DELTAG(b)double dagger) was estimated to be almost-equal-to 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. Delta 5 compared to L where they resonate at delta 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 P2(1)/n, a = 9.560(2), b = 16.886(2), c = 9.577(3) angstrom, beta = 118.37(2)degrees. 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.