A cobalt (II)-based semiconductor complex with two-channel slow magnetic relaxation

The hydrothermal reaction of 2-methylimidazole (hereafter abbreviated as 2-mim) with cobalt (II) sulfate heptahydrate in methanol affords a novel sulfate cobalt (II) complex with formula [Co(C₄H₆N₂)₄](SO₄)∙(H₂O) (1). Its crystal structure was refined from X-ray diffraction data, complemented with the structural information derived from spectroscopic (IR and UV–Vis), thermal data and magnetic measurements. The crystal structure of 1 is made up of isolated [Co(2-mim)₄]²⁺ cations, (SO₄)^2- anions and lattice water molecules. An extensive network of hydrogen bonds ensures the interconnection of the different entities. Greater knowledge on these interactions has been provided based on the Hirshfeld surface analysis and 2D fingerprint plots. Heating of 1 above 97 °C initiates gradual decomposition stages, which lead to the metal oxide as a final product at 710 °C, as proven by TGA/DSC analysis. A UV–Vis spectroscopy study confirms the tetrahedral environment around the metal. The determined bandgap energy E_g = 2.6 eV from the UV–Vis spectra indicate the interesting semiconducting behavior of our compound. Static magnetic measurements and EPR study revealed the presence of the easy-axis anisotropy with axial term D = –5.45 cm⁻¹ and very small rhombicity, confirmed by the first-principle calculations. Complex 1 behaves as a field-induced single-ion magnet with two relaxation channels. The high-frequency relaxation can be described by the interplay of the two-phonon Orbach and Raman process, the relaxation barrier U_eff = 11.4 cm⁻¹ agrees well with the difference between the two lowest Kramer's doublets.