SMM behaviour in a {CoIII4CoII2DyIII2} complex: Co(II)-based thermal barrier and Dy(III) spectator role

We report the synthesis, structure, and magnetic properties of a novel Co/Dy heterometallic complex, [Co^III₄Co^II₂Dy^III₂(tea)₄(CH₃COO)₄(OH)₂(NO₃)₄]·2H₂O (1), the first structurally characterized example of a mixed-valent {Co^III₄Co^II₂Dy^III₂} motif. It comprises two butterfly-like {Co^III₂Co^IIDy^III} units linked by carboxylate and alkoxide bridges. X-ray diffraction shows unique Dy(III) and Co(II) sites, with Dy(III) nine-coordinated and Co(II) in a rhombic tetrahedral geometry. Magnetic data and quantum chemical calculations reveal weak Co(II)–Co(II) antiferromagnetic exchange, negligible Co(II)–Dy(III) interactions, and moderate Co(II) zero-field splitting (D ≈ 12–20 cm⁻¹), which drives single-molecule magnet behavior below 14 K. AC studies indicate multiple tunneling pathways and an Orbach barrier (U_eff ≈ 65 cm⁻¹) arising from Co(II), while Dy(III) ions remain largely magnetically decoupled.