Synthesis, structural, spectral, electrochemical and spin equilibrium studies of hexaaza macrotricyclic complexes

Nickel(II) and copper(II) complexes of 14-membered hexaaza macrocyclic ligand (CBA) with C-methyl substituent have been synthesized and characterized by spectral, electrochemical and X-ray crystallographic techniques. The crystal structure of [Ni(CBA)](ClO4)2 indicates a square-planar geometry with two secondary and two tertiary amines of the macrocycle. The copper(II) complex [Cu(CBA)(ClO4)2] shows a distorted octahedral geometry and the two perchlorate ions are weakly coordinated to the metal ion in the axial position. The [Ni(CBA)](ClO4)2 complex exhibit solvent dependant high-spin/low-spin equilibrium between the triplet and singlet d8 electronic ground states. The equilibrium viz., [Ni(CBA)]2++2H2O⇌[Ni(CBA)(H2O)2]2+Ni(CBA)]2++2H2O⇌[Ni(CBA)(H2O)2]2+ studies in aqueous solution over a temperature range of 283–313 K indicates ΔH° = −23.98 ± 0.05 KJ mol−1 and ΔS° = −78.63 ± 0.1 K−1 mol−1.

Graphical abstractNickel(II) and copper(II) complexes of 14-membered hexaaza macrocyclic ligand (CBA) with C-methyl substituent have been synthesized from the template condensation reaction of formaldehyde with N,N′-bis(3-amino propyl)ethylenediamine and 1,2-propanediamine in the presence of metal ion. These complexes have been characterized by spectral, electrochemical and X-ray crystallographic techniques. The nickel(II) complex [Ni(CBA)](ClO4)2 exhibit a square-planar geometry whereas the copper(II) complex [Cu(CBA)(ClO4)2] posses a distorted octahedral geometry. The nickel(II) complex exists in solution as a mixture of octahedral, high-spin and planar, low-spin species.Figure optionsDownload full-size imageDownload as PowerPoint slide