Further insights into controlling the anisotropy of pentacoordinate Co(II) field-supported single-molecule magnets

To gain a better insight into understanding the factors affecting the enhancement of magnetic anisotropy in single-molecule magnets, two pentacoordinate cobalt(II) complexes [CoCl₂(R¹-terpy)] (R¹-furan-2-yl) (1) and [CoCl₂(R²-terpy)] (R²-1-methyl-1H-pyrrol-2-yl) (2) based on the tridentate ligand 2,2':6',2''-terpyridine (terpy) functionalised with two different heterocyclic groups have been synthesised and fully characterized by X-ray crystallography and physicochemical methods. Complex 1 is an interesting example of a compound with two crystallographically independent molecules (1A and 1B) differing by the structural parameters and magnitude of magnetic anisotropy, confirmed experimentally by HFEPR and FIRMS spec troscopy. The ab initio calculations (CASSCF + NEVPT2 + SOC) confirm that the spin-Hamiltonian formalism can be applied. The calculated energy gap (at the SOC level) between the lowest Kramers doublets is δ = 107 and 58 cm^-1 for 1A and 1B units, and 85 cm^-1 for 2. The axial zero-field splitting parameter is predicted as D = -51 and -26 cm^-1 for 1A and 1B, and +42 cm^-1 for 2, accompanied by some rhombic component. The FIRMS spectra reveal the energy gaps of 89 and 60 cm^-1 for 1, and 89 cm^-1 for 2. The last value matches the ab initio data, as well as the analysis of susceptibility and magnetisation (δ = 2D = 80 cm^-1). In case of 1, the agreement is worse. Both samples exhibit a field-supported slow magnetic relaxation with two relaxation modes. The high-frequency relaxation time at higher temperature follows a temperature dependence characteristic for the Raman process, 1/τ(HF) = CTⁿ with n ∼ 6; at low temperature the temperature coefficients are characteristic of the phonon-bottleneck or direct process, n ∼ 2.5 and 1.5, respectively.