Magnetic anisotropy and slow relaxation of magnetisation in double salts containing four- and six-coordinate cobalt(ii) complex ions

Abstract

Four novel Co(II) coordination compounds 1–4 of the general formula [Co(Lⁿ)₂][Co(NCY)₄]·mCH₃CN (where Lⁿ are tridentate ligands L1 = 2,6-bis(1-hexyl-1H-benzimidazol-2-yl)pyridine for 1 and 2; L2 = 2,6-bis(1-octyl-1H-benzimidazol-2-yl)pyridine for 3; L3 = 2,6-bis(1-dodecyl-1H-benzimidazol-2-yl)pyridine for 4, Y = O for 1, 3, and 4 and Y = S for 2; m = 0 for 1 and 3, m = 0.5 for 2 and m = 2 for 4) were prepared and characterised. The molecular structures of all four compounds consist of the hexacoordinate complex cation [Co(Lⁿ)₂]²⁺ and tetracoordinate complex anion [Co(NCY)₄]²⁻, with distorted octahedral and tetrahedral symmetry of coordination polyhedra, respectively. The electronic structures of all compounds feature an orbitally non-degenerate ground state well-separated from the lowest excited state, which allows the analysis of the magnetic anisotropy by the spin Hamiltonian model. ZFS parameters, derived from both CASSCF-NEVPT2 calculations and magnetic data analysis, indicate that tetrahedral anions [Co(NCY)₄]²⁻ exhibit small axial parameters |D| spanning the range of 2.2 to 7.7 cm⁻¹, while octahedral cations [Co(Lⁿ)₂]²⁺ display significantly larger |D| parameters in the range of 37 to 95 cm⁻¹. For 1–3, the Fourier-transform infrared magnetic spectroscopy (FIRMS) revealed a reasonable transmission with a magnetic absorption around the expected value for the ZFS accompanied by features allowing to identify phonon frequencies and simulate spin-phonon couplings. Dynamic magnetic investigations unveiled the field-induced slow relaxation of magnetisation, with maximal relaxation times (τ) of 92(2) μs for 2 at 2 K and B_DC = 0.3 T. The temperature evolution of τ was analysed using a combination of Orbach, direct and Raman relaxations (U_eff = 8(1) K (5.6 cm⁻¹)) or Orbach, direct and spin-phonon induced relaxations (U_eff = 10.3(9) K (7.2 cm⁻¹)). The rest of the complexes, namely 1, 3, and 4 show field-induced slow relaxation of magnetisation with τ smaller than 16 μs.