Enhancing easy-plane anisotropy in bespoke Ni(II) quantum magnets

We examine the crystal structures and magnetic properties of several S = 1 Ni(II) coordination compounds, molecules and polymers, that include the bridging ligands HF2, AF(6)(2) (A = Ti, Zr) and pyrazine or non-bridging ligands F, SiF62, glycine, H2O, 1-vinylimidazole, 4-methylpyrazole and 3-hydroxypyridine. Pseudo-octahedral NiN4F2, NiN4O2 or NiN4OF cores consist of equatorial Ni-N bonds that are equal to or slightly longer than the axial Ni-L-ax bonds. By design, the zero-field splitting (D) is large in these systems and, in the presence of substantial exchange interactions (J), can be difficult to discriminate from magnetometry measurements on powder samples. Thus, we relied on pulsed-field magnetization in those cases and employed electron-spin resonance (ESR) to confirm D when J << D. The anisotropy of each compound was found to be easy-plane (D > 0) and range from approximate to 8-25 K. This work reveals a linear correlation between the ratio d(Ni-L-ax)/d(Ni-N-eq) and D although the ligand spectrochemical properties may play an important role. We assert that this relationship allows us to predict the type of magnetocrystalline anisotropy in tailored Ni(II) quantum magnets. (C) 2020 The Authors. Published by Elsevier Ltd.