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

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 53; no. 31; pp. 12962 - 12972
• Main Authors: Juráková, Jana, Santana, Vinicius Tadeu, Pavlik, Ján, Moncoľ, Ján, Nemec, Ivan, Clemente-León, Miguel, Kuppusamy, Senthil Kumar, Ruben, Mario, Čižmár, Erik, Šalitroš, Ivan
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 06.08.2024
• Subjects: Anions; Cations; Cobalt compounds; Coordination compounds; Couplings; Data analysis; Electron spin; Fourier transforms; Magnetic anisotropy; Magnetic induction; Magnetic spectroscopy; Magnetization; Molecular structure; Parameter identification; Phonons; Pyridines; Spin dynamics
• ISSN: 1477-9226; 1477-9234; 1477-9234
• DOI: 10.1039/d4dt01509f

Four novel Co(II) coordination compounds 1-4 of the general formula [Co(L ) ][Co(NCY) ]· CH CN (where L are tridentate ligands L1 = 2,6-bis(1-hexyl-1 -benzimidazol-2-yl)pyridine for 1 and 2; L2 = 2,6-bis(1-octyl-1 -benzimidazol-2-yl)pyridine for 3; L3 = 2,6-bis(1-dodecyl-1 -benzimidazol-2-yl)pyridine for 4, Y = O for 1, 3, and 4 and Y = S for 2; = 0 for 1 and 3, = 0.5 for 2 and = 2 for 4) were prepared and characterised. The molecular structures of all four compounds consist of the hexacoordinate complex cation [Co(L ) ] and tetracoordinate complex anion [Co(NCY) ] , 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) ] exhibit small axial parameters | | spanning the range of 2.2 to 7.7 cm , while octahedral cations [Co(L ) ] display significantly larger | | parameters in the range of 37 to 95 cm . 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 ( ) of 92(2) μs for 2 at 2 K and = 0.3 T. The temperature evolution of was analysed using a combination of Orbach, direct and Raman relaxations ( = 8(1) K (5.6 cm )) or Orbach, direct and spin-phonon induced relaxations ( = 10.3(9) K (7.2 cm )). The rest of the complexes, namely 1, 3, and 4 show field-induced slow relaxation of magnetisation with smaller than 16 μs.