Probing magnetic and vibrational properties of trigonal-bipyramidal Co(ii) and Ni(ii) complexes using advanced spectroscopies

Abstract

Transition metal-based single-molecule magnets (SMMs) with local C₃ or higher symmetries on the metal centers have garnered significant interest due to their unique magnetic properties. With rhombic anisotropy parameters E = (or ≈) 0 for such highly symmetric complexes, quantum tunneling in magnetic relaxation is eliminated or reduced, enhancing the performance of the SMMs. Zero-field splitting (ZFS) in metal complexes has been probed by various advanced methods, including spectroscopies such as far-infrared magneto-spectroscopy (FIRMS), high-frequency and -field electron paramagnetic resonance (HFEPR), and inelastic neutron scattering (INS). Studies of the trigonal bipyramidal SMM complex with local C_3v symmetry, (Me₄N)[Co(MST)(OH₂)] (Co-MST-H₂O, MST³⁻ = N,N′,N′′-[2,2′,2′′-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido)), give the following spin-Hamiltonian parameters: axial (D = 25.3 cm⁻¹) and rhombic (E = 0) ZFS values, g_⊥ = 2.230, and g_‖ = 2.045. Phonon properties of Co-MST-H₂O have been studied by INS and DFT phonon calculations, yielding phonon symmetries, energies, and movies. Phonons here refer to either molecular vibrations (internal modes) or lattice vibrations (external modes or intermolecular vibrations) in the crystalline solid. Intermolecular interactions in Co-MST-H₂O have been probed by Hirshfeld surface analysis, revealing strong interactions between the [Co(MST)(OH₂)]⁻ anion and solvents (H₂O and CH₂Cl₂) in the crystal lattice. The DFT phonon calculations of Co-MST-H₂O have also generated the spin density on the Co²⁺ ion and other atoms in the molecule. For (Me₄N)[Ni(MST)(OH₂)] (Ni-MST-H₂O), FIRMS and INS studies did not show the magnetic transitions among the spin sublevels. Spin-Hamiltonian parameters (D, E, and g value) of Ni-MST-H₂O, determined by magnetometry, were reported earlier (K. A. Schulte et al., Chem. Sci., 2018, 9, 9018). In the absence of magnetic resonance or INS results for the magnetic transitions in Ni-MST-H₂O, magnetometry is the only viable technique to determine spin-Hamiltonian parameters for the compound. Hirshfeld surface analysis of Ni-MST-H₂O shows strong interactions of the [Ni(MST)(OH₂)]⁻ anion with lattice solvent H₂O and the cation Me₄N⁺.