Aromatic nitrogen ligands with metal

Janiak, C., A critical account on 7t-7t stacking in metal complexes with aromatic nitrogen-containing ligands. J. Chem. Soc., Dalton Trans. 2000, 3885-3896. 

Bipyridines (1UPAC), also known as bipyridyls, dipyridyls, and dipyridines, are aromatic nitrogen heterocycles that form complexes with most transition metals.1-5 This class of compounds contains six possible regioisomers—2,2 (1), 2,3 (2), 2,4 (3), 3,3 (4), 3,4 (5), and 4,4 (6)—most common of which is the bidentate chelate, (1), (bpy = 2,2 -bipyridine). Bipyridine ligands interact... 

Aromatic substituents at C2 and C4 are to some extent hindered in rotation. This leads to a third type of isomerism in bispidine chemistry (i.e., atropisomer-ism). The activation energy for the rotation around the C2/C4—aryl bond for bispidones with various meta-substituted phenyl groups was determined by various NMR methods and found to be 70-75 kJ mol (23). For a rotation of 180°, which is usually necessary for the coordination of bispidine ligands to metal ions (e.g., 14, see Scheme 14), two energy barriers have to be overcome. The higher is the result of an interaction of the ortho-disposed proton of the aromatic ring with the proton or the alkyl substituent at the N3 amine nitrogen atom. The... 

---