Investigation of the possible pathways for magnetic exchange interactions in homobinuclear chelates of naphthazarin

Homobinuclear complexes of the binucleating naphthazarinato ligand of the general formula [Formula: see text] where M = Cu, Ni, and Zn, C10H4O4 = naphthazarinato ligand, and C10H8N2 = 2,2′-bipyridyl, have been prepared and studied. In these complexes the naphthazarinato ligand, acting as a bridging unit between the two metal centers, supports the propagation of magnetic exchange interactions through its extensive π-electron system. The spectroscopic and magnetic data of the compounds showed that each metal ion is surrounded by two oxygen and two nitrogen donor atoms in a nearly tetrahedral configuration (C2v point group) with the naphthazarinato ligand adopting a centrosymmetrical structure of C2h symmetry. The room-temperature magnetic data of the copper(II) and nickel(II) homobinuclear chelates (1.32 and 1.73 BM per metal ion at 8000 G, respectively), as well as the slight decrease of the magnetic moments as the magnetic field strength decreases are indicative of the operation of antiferromagnetism in these chelates. Quantum mechanical calculations of the Hückel LCAO-MO type improved by ω-technique have been used to provide a qualitative guide to the possible pathways for the superexchange processes observed in the tetrahedral copper(II) and nickel(II) chelates, as well as to explain qualitatively why there is no antiferromagnetic interactions propagated by the bridging naphthazarinato ligand in analogous homobinuclear chelates with other coordination geometries such as square planar, square-pyramidal, trigonal bipyramidal, and octahedral.