Neutral cobalt(ii)-bis(benzimidazole)pyridine field-induced single-ion magnets for surface deposition

Abstract

Two novel hexacoordinated Co(II)-based single-ion magnets were prepared and characterised. Both neutral complexes feature metal-centred coordination with one terminal and one bidentate nitrate anions along with tridentate derivatives of a 2,6-bis(1H-benzimidazole-2-yl)pyridine ligand containing either n-octyl (complex 1) or n-dodecyl (complex 2) chains. The presence of long aliphatic chains ensures their solubility in low polarity and volatile solvents frequently used for lithography patterning. This enabled the preparation of microstructural layers and patterns on technologically relevant substrates by easy-to-handle and low-cost wet lithographic techniques. On the other hand, attempts at surface deposition via sublimation were not successful due to thermal instability. The electronic structure of complexes typically features an orbitally non-degenerate ground state well-separated from the lowest excited state, which allows one to analyse magnetic anisotropy by the spin Hamiltonian approach. Zero-field splitting parameters obtained from CASSCF-NEVPT2 calculations and from the analysis of magnetic data suggest that both compounds display positive axial D parameters within a range of 17–25 cm⁻¹. Combined results from high-field electron paramagnetic resonance (X-band and HF-EPR) and Fourier-transform infrared magnetic spectroscopy (FIRMS) simulated with the spin Hamiltonian provided the axial and rhombic zero-field splitting terms D = +23.7 cm⁻¹ for complex 1 and D = +24.2 cm⁻¹ for complex 2, together with pronounced rhombicity in the range of E/D ≈ 0.15–0.19 for both compounds. Dynamic magnetic investigations have revealed the field-induced slow relaxation of magnetisation, with maximal relaxation times (τ) of 7.6 ms for 1 and 0.8 ms for 2. This relaxation is governed via a combination of several relaxation mechanisms, among which the quantum tunnelling was efficiently suppressed by the applied static magnetic field. The effective barriers of spin reversal U_eff = 77(4) K for 1 and U_eff = 70(2) K for 2 are consistent with the expected values calculated using the ZFS parameters.