Structurally Reinforced Chiral Macrocyclic Polyamine Based Dinuclear Copper (II) Complexes as Studied by CW and FT Pulsed ESR and Squid Measurements

Abstract

We have attempted to synthesize structurally reinforced chiral B₂-18-membered hexaazamacrocyclic amine based dinuclear copper(II) complexes 2 and 2′ as models for studying the size-mach selectivity of metal ions. From the viewpoint of molecular based magnetism-structure relationship, these two dinuclear complexes have manifested a prominent difference in the electronic structure, although only apparent structural difference is subtle. In the dinuclear complex 2′, the fifth ligand is missing, forming slanting bi-squares with a common oxygen atom of hydroxide ion. CW ESR measurements of 2 and 2′ in polycrystalline solids have shown that a triplet fine-structure spectrum with a large zero-field splitting (ZFS) value features in 2′ and an exchange coupled copper(II) pair spectrum does in 2, respectively. These findings have been expounded in connection with the structural difference in the macrocyclic rugbyball-shaped cavity of the ligand. The ground-state spin multiplicity for the copper dinuclear complexes has been rationalized in terms of a molecular orbital picture of 90 degree superexchange interaction via the bridging oxygen linker.