Synthesis and characterization of acyclic and cyclic aza-bridged ligands incorporating 2,2′-bipyridine subunits and their complexes with copper(II), cobalt(II), and nickel(II)

The synthesis of a series of N,N'-disubstituted acyclic (AL) and cyclic (CL) aza-bridged ligands incorporating 2,2-pipryidine subunits is described. H-1-NMR and IR spectral data support the proposed ligand structures. Dynamic H-1-NMR studies on diurea and diamide derivatives point to the presence of slowly interconverting conformers on the H-1-NMR time-scale, owing to N-H...N hydrogen bonding and/or a restricted rotation around the amide bonds. The ligands synthesized form 1:1 complexes with divalent transition metal ions. Upon complexation, bis-ester derivatives AL(5) and CL5 undergo a metal-induced hydrolysis of the ester groups to carboxyl functions, which act as additional binding sites for the metal ion, as well as hydrogen-bonding donor-acceptor binding site to produce dimeric complexes.