Design of bimetallic magnetic chains based on oxalate complexes: towards single chain magnets

Abstract

We describe the synthesis, structure and magnetic characterization of several oxalate-based bimetallic 1D systems. We will exemplify how by suitable choice of the molecular building blocks and strict control of their arrangement in the solid state, the magnetic properties of these low-dimensional materials can be tuned to finally obtain bimetallic oxalate chains behaving as single-chain magnets (SCM). First, we will focus on compounds [K(18-crown-6)][M^II(bpy)Cr(ox)₃] (1, 2; M^II = Mn, Co; bpy = C₁₀N₂H₈). The MnCr derivative behaves as a 1D ferromagnet down to 2 K, the lowest investigated temperature. The lack of magnetic ordering in this chain prompted us to prepare the more anisotropic MnCo derivative with the aim of checking whether the single-chain magnetic behaviour was observed. However, this compound exhibits long-range ferromagnetic ordering at T_c = 5.5 K due to the poor chain insulation. Next, in order to slightly change the supramolecular interactions between neighbouring chains we substituted the bpy ligand with dpy, in view of its ability to establish H bonds, yielding the bimetallic compounds [K(18-crown-6)][Co(bpy)M^III(ox)₃] (3, 4; M^III = Cr, Fe; dpy = C₁₀N₃H₉). These compounds exhibit long-range magnetic ordering at 4.6 and 11 K, respectively, and a frequency dependence of the ac susceptibility at lower temperatures. However, such behaviour cannot be attributed to a SCM relaxation, but to the movement of domain walls in the ordered state. Finally, by introducing an additional cation, which increases the interlayer separation, we obtained the compound [K(18-crown-6)]_1/2[(18-crown-6)(FC₆H₄NH₃)]_1/2[Co(H₂O)₂Cr(ox)₃] (5) exhibiting a SCM-like behaviour.