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. (1)H-NMR and IR spectral data support the proposed ligand structures. Dynamic (1)H-NMR studies on diurea and diamide derivatives point to the presence of slowly interconverting conformers on the (1)H-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 CL(5) 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.