Advanced spectroscopic studies of (PPh4)2[Co(N3)4], a field-induced single-ion magnet

Abstract

The high-spin Co^II complex (PPh₄)₂[Co(N₃)₄] (Co-N₃) has been investigated using advanced spectroscopic techniques [far-IR magneto-spectroscopy (FIRMS), high-frequency and high-field EPR (HFEPR), and inelastic neutron scattering (INS)] to study its zero-field-splitting (ZFS), giving spin-Hamiltonian (SH) parameters. The analysis of multi-frequency HFEPR reveals the easy-axis anisotropy with a D value of −10.39(5) cm⁻¹ and a rhombic ratio (E/D) of 0.21(1). The magnetic properties have also been probed by direct-current (DC) magnetometry, suggesting minor differences in anisotropy from the previously reported polymorph (Co-N₃′). Ligand-field theory (LFT) analysis indicates that the structures of Co-N₃ and Co-N₃′ are closer to D_2d symmetry than other symmetries considered. Alternate-current (AC) susceptibility reveals slow magnetic relaxation under an applied field, indicating that Co-N₃ is a field-induced single-ion magnet (SIM). While both Co-N₃ and Co-N₃′ were studied by DC magnetometry, one unusual aspect of the current work on Co-N₃ is that advanced spectroscopies HFEPR, FIRMS, and INS were used to directly observe transitions between ZFS split states, giving accurate SH parameters.