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

Four novel Co(II) coordination compounds 1-4 of the general formula [Co(L n ) 2 ][Co(NCY) 4 ]· m CH 3 CN (where L n are tridentate ligands L1 = 2,6-bis(1-hexyl-1 H -benzimidazol-2-yl)pyridine for 1 and 2; L2 = 2,6-bis(1-octyl-1 H -benzimidazol-2-yl)pyridine for 3; L3 = 2,6-bis(1-dodecyl-1 H -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(L n ) 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(L n ) 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.