Probing the Limits of Tetraazamacrocycle-Glyoxal Condensates as Bidentate Ligands for Cu2+

Tetracyclic bisaminals formed by condensation of tetraazamacrocycles and glyoxal have rarely been utilized as ligands for transition metal ions. Five novel Cu2+ complexes are presented to expand this set. Glyoxal derivatives of pyruvic aldehyde and butanedione were expected to further rigidify the bisaminal skeleton because of the steric requirements of the methyl groups, while the previously unutilized homocyclam was expected to increase flexibility due to the presence of a seven-membered ring. Additionally, a nitrato complex was prepared to examine the effect of more bulky monodentate ligands on the bisaminal framework. The new complexes showed similar four-coordination of Cu2+, but with significant changes in coordination geometry caused by substitution of the glyoxal bridge by one or two methyl groups. The homocyclam derivative produced a completely different coordination mode due to the flexibility of the larger macrocycle ring. Both structural and electronic properties were affected by the use of nitrato in place of the typical chlorido monodentate ligands. Overall, the electronic properties (magnetic moment, electronic spectra, and cyclic voltammetry) were only subtly affected. However, larger changes were observed when the larger homocyclam macrocycle was used or the nitrato anion was used in place of the chlorido ligand to complete the coordination. Additionally, the recently published geometric index τδ was evaluated by using this series of closely related Cu2+ complexes.