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

Magnetic anisotropy in pentagonal bipyramidal complexes of Co(II) ( 1 and 2 ), Fe(III) ( 3 and 4 ), and Ni(II) ( 5 ) with a 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 electron paramagnetic resonance (EPR) spectroscopy, Mössbauer spectroscopy, ab initio, and ligand-field density functional theory (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. The Ni(II) complex ( 5 ) has uniaxial magnetic anisotropy with a negative D value. Fe(III) complexes ( 3 and 4 ) have small zero-field splitting (ZFS) parameters. Theoretical modeling is 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 ion in ideal pentagonal bipyramidal coordination.