Combined Experimental and Theoretical Investigation of the Origin of Magnetic Anisotropy in Pentagonal Bipyramidal Isothiocyanato Co(II), Ni(II), and Fe(III) Complexes with Quaternary-Ammonium-Functionalized 2,6-Diacetylpyridine Bisacylhydrazone

Abstract

The magnetic anisotropy in pentagonal bipyramidal complexes of Co(II) (1 and 2), Fe(III) (3 and 4) and Ni(II) (5) with 2,2′-[2,6-pyridinediylbis(ethylidyne-1-hydrazinyl-2-ylidene)]bis[N,N,N-trimethyl-2-oxoethanaminium] equatorial ligand and isothiocyanato axial ligands has been investigated by magnetic susceptibility measurements, powder X-band EPR spectroscopy, Mossbauer spectroscopy, ab initio, and LFDFT calculations. The studied complexes display three distinct types of magnetic anisotropy. Co(II) complexes (1 and 2) show an easy plane anisotropy with large and positive D values and negligible rhombicity. Ni(II) complex (5) has uniaxial magnetic anisotropy with negative D value. Fe(III) complexes (3 and 4) have small ZFS parameters. Theoretical modeling was used to rationalize the magnetic anisotropy in these systems and to identify the most important excited states that are responsible for the zero-field splitting. These excitations are a consequence of the electronic structure of the central metal io...