Quantifying magnetic anisotropy using X-ray and neutron diffraction.

In this work, the magnetic anisotropy in two iso-structural distorted tetrahedral Co(II) complexes, CoX 2tmtu2 [X = Cl(1) and Br(2), tmtu = tetra-methyl-thio-urea] is investigated, using a combination of polarized neutron diffraction (PND), very low-temperature high-resolution synchrotron X-ray diffraction and CASSCF/NEVPT2 ab initio calculations. Here, it was found consistently among all methods that the compounds have an easy axis of magnetization pointing nearly along the bis-ector of the compression angle, with minute deviations between PND and theory. Importantly, this work represents the first derivation of the atomic susceptibility tensor based on powder PND for a single-molecule magnet and the comparison thereof with ab initio calculations and high-resolution X-ray diffraction. Theoretical ab initio ligand field theory (AILFT) analysis finds the d xy orbital to be stabilized relative to the d xz and d yz orbitals, thus providing the intuitive explanation for the presence of a negative zero-field splitting parameter, D, from coupling and thus mixing of d xy and . Experimental d-orbital populations support this interpretation, showing in addition that the metal-ligand covalency is larger for Br-ligated 2 than for Cl-ligated 1.

Investigating Complex Magnetic Anisotropy in a Co(II) Molecular Compound: A Charge Density and Correlated Ab Initio Electronic Structure Study.

Understanding magnetic anisotropy and specifically how to tailor it is crucial in the search for high-temperature single-ion magnets. Herein, we investigate the magnetic anisotropy in a six-coordinated cobalt(II) compound that has a complex geometry and distinct triaxial magnetic anisotropy from the perspective of the electronic structure, using electronic spectra, ab initio calculations, and an experimental charge density, of which the latter two provides insight into the d-orbital splitting. The analysis showed that the d-orbital splitting satisfactorily predicted the complex triaxial magnetic anisotropy exhibited by the compound. Furthermore, a novel method to directly compare the ab initio results and the d-orbital populations obtained from the experimental charge density was developed, while a topological analysis of the density provided insights into the metal-ligand bonding. This work thus further establishes the validity of using d-orbitals for predicting magnetic anisotropy in transition metal compounds while also pointing out the need for a more frequent usage of the term triaxial anisotropy in the field of single-molecule magnetism.

High-Pressure Crystallography as a Guide in the Design of Single-Molecule Magnets.

Single-molecule magnet materials owe their function to the presence of significant magnetic anisotropy, which arises from the interplay between the ligand field and spin-orbit coupling, and this is responsible for setting up an energy barrier for magnetic relaxation. Therefore, chemical control of magnetic anisotropy is a central challenge in the quest to synthesize new molecular nanomagnets with improved properties. There have been several reports of design principles targeting such control; however, these principles rely on idealized geometries, which are rarely obtained in crystal structures. Here, we present the results of high-pressure single-crystal diffraction on the single-ion magnet, Co(SPh)4(PPh4)2, in the pressure range of 0-9.2 GPa. Upon pressurization a sequence of small geometrical distortions of the central CoS4 moeity are observed, enabling a thorough analysis of the magneto-structural correlations. The magneto-structural correlations are investigated by theoretical analyses of the pressure-dependent experimental molecular structures. We observed a significant increase in the magnitude of the zero-field splitting parameter D, from -54.6 cm-1 to -89.7 cm-1, which was clearly explained from the reduction of the energy difference between the essential dxy and dx2-y2 orbitals, and structurally assigned to the change of an angle of compression of the CoS4 moeity.

Quantification of the Magnetic Anisotropy of a Single-Molecule Magnet from the Experimental Electron Density.

Reported here is an entirely new application of experimental electron density (EED) in the study of magnetic anisotropy of single-molecule magnets (SMMs). Among those SMMs based on one single transition metal, tetrahedral CoII-complexes are prominent, and their large zero-field splitting arises exclusively from coupling between the d x 2 - y 2 and dxy orbitals. Using very low temperature single-crystal synchrotron X-ray diffraction data, an accurate electron density (ED) was obtained for a prototypical SMM, and the experimental d-orbital populations were used to quantify the dxy-d x 2 - y 2 coupling, which simultaneously provides the composition of the ground-state Kramers doublet wave function. Based on this experimentally determined wave function, an energy barrier for magnetic relaxation in the range 193-268 cm-1 was calculated, and is in full accordance with the previously published value of 230 cm-1 obtained from near-infrared spectroscopy. These results provide the first clear and direct link between ED and molecular magnetic properties.

A linear cobalt(II) complex with maximal orbital angular momentum from a non-Aufbau ground state.

Orbital angular momentum is a prerequisite for magnetic anisotropy, although in transition metal complexes it is typically quenched by the ligand field. By reducing the basicity of the carbon donor atoms in a pair of alkyl ligands, we synthesized a cobalt(II) dialkyl complex, Co(C(SiMe2ONaph)3)2 (where Me is methyl and Naph is a naphthyl group), wherein the ligand field is sufficiently weak that interelectron repulsion and spin-orbit coupling play a dominant role in determining the electronic ground state. Assignment of a non-Aufbau (d x 2 -y 2 , d xy )3(d xz , d yz )3(d z 2 )1 electron configuration is supported by dc magnetic susceptibility data, experimental charge density maps, and ab initio calculations. Variable-field far-infrared spectroscopy and ac magnetic susceptibility measurements further reveal slow magnetic relaxation via a 450-wave number magnetic excited state.