The Coordination Chemistry of 3,3′-Diamino-2,2′-bipyridine and Its Dication: Exploring the Role of the Amino Groups by X-ray Crystallography

The synthesis and structural chemistry of a series of new divalent transition metal complexes of the bis-bidentate ligand 3,3′-diamino-2,2′-bipyridine (L1) and its dication L1H2 are described. Ligand L1 reacts with salts of divalent transition metals to afford the (1:1) metal-ligand complexes (2a−2d) as well as the tris complexes (3a−3f). All complexes were fully characterised by spectroscopic methods and the following compounds [Cu(L1)Cl2]2 (2a), [Cu(L1)(OAc)2] (2b), [Zn(L1)3][OTf]2 (3a), and [Zn(L1)3][ZnCl4] (3e and 3f) were structurally characterised. Results from single crystal X-ray diffraction measurements indicate that formation of an intramolecular hydrogen bond between the two amino groups allows the ligand to coordinate divalent metal ions through their diimine binding sites. Furthermore, the structure of compound 2a reveals that it crystallises as a dimer in which each copper ion is bound to two pyridine nitrogen atoms and two chloride ions in a distorted square planar arrangement, with a long axial contact from a neighbouring amino group completing the approximately square-pyramidal geometry at CuII. Complexation of this ligand in acidic conditions afforded the compound [Cu(L1H2)Cl4] (4), as well as the two salts [L1H2][CuCl4] (5a) and [L1H2][ZnCl4] (5b). All three compounds have been structurally characterised and results indicate that the dication (L1H2) displays a different coordination preference for the chelation of metal ions. In all three cases, both of the heterocyclic N atoms of the ligand are protonated, thus preventing chelation to the metal ion, although for compound 4 crystallographic studies reveal that the two amino functionalities coordinate the copper(II) ion. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)