Synthesis, structure and solution chemistry of (5,5′-dimethyl-2,2′-bipyridine)(IDA)copper(II) and structural comparison with aqua(IDA)(1,10-phenanthroline)copper(II) (IDA = iminodiacetato)

The protolytic equilibria of copper(II), 5,5'-dimethyl-2,2'-bipyridine (bpy) and iminodiacetate (IDA) have been investigated by potentiometric titration between pH 2 and 8. Potentiometric titration suggests the presence of [Cu(IDA)(bpy)](0) at a fraction of 28 % of all copper species in solution at pH 8 with a stability constant logbeta = 13.9. The crystal structures of [Cu(IDA)(bpy)].4H(2)O (1) and [Cu(IDA)(H2O)(phen)] . 4H(2)O (2) show the IDA ligand coordinated in the fac-NO+O mode. The structure of I features "dimeric" entities of symmetry-independent five-coordinated Cu-complexes through a long Cu...O contact, hydrogen bonding and pi-stacking, including a "pyridyl-metal-chelate ring" pi-contact as evidence for metalloaromaticity in aromatic alpha,alpha'-diimine-copper(II) chelates. The structure of 2 has the tetragonally six-coordinated molecular Cu-complexes arranged by hydrogen bonding and pi-stacking. The crystal water in 2 forms infinite corrugated double chains composed of alternating four- and six-membered, edge-sharing rings of hydrogen-bonded water molecules.