Three tetracoordinate Co(ii) complexes [Co(biq)X2] (X = Cl, Br, I) with easy-plane magnetic anisotropy as field-induced single-molecule magnets

Abstract

Three mononuclear complexes [Co(biq)X₂] (biq = 2,2′-biquinoline; X = Cl, Br, I) were prepared by a solvothermal method and characterized by single-crystal X-ray diffraction. In all three complexes the Co(II) atom is tetrahedrally coordinated by one biq ligand bonded in a chelate manner and two halogenido ligands. Hydrogen bonding interactions (C–H⋯X) along with the π–π interactions contribute to the stability of the formed packing. Magnetic measurements as well as ab initio calculations revealed that the complexes possess a sizable easy-plane magnetic anisotropy (D > 0). They display a superparamagnetic behaviour in an applied external field that culminates between B_DC = 0.2–0.3 T. Two relaxation processes are observed; the faster can be analysed in terms of the direct and Orbach processes yielding U/k_B = 42.6 K and τ₀ = 1.9 × 10⁻¹⁰ s for X = Cl, U/k_B = 39.6 K and τ₀ = 1.2 × 10⁻¹⁰ s for X = Br and U/k_B = 57.0 K and τ₀ = 3.2 × 10⁻¹³ s for X = I at B_DC = 0.2 T.