Linker Stoichiometry-controlled stepwise supramolecular growth of a flexible Cu₂Tb single molecule magnet from monomer to dimer to one-dimensional chain

Abstract

A trinuclear complex [(CuL)₂Tb(H₂O)(NO₃)₃]•MeOH•H₂O (1) is synthesized by using a metalloligand [CuL] (H₂L = N,N′-bis(salicylidene)-1,3-propanediamine or salpn). This complex is used as a new 3d-4f metallatecton for stepwise supramolecular oligomerization along with a simple diimine linker, 4,4′-bipyridine (bpy). A ratio of metallatecton/linker = 2:1 is conducive for the formation of a dimer [{(CuL) ₂Tb(H₂O)(NO₃)₃}₂bpy]•2MeOH•4H₂O (2), whereas a higher proportion of linker at a ratio of 1:1 gives rise to a zigzag one-dimensional coordination polymer {[(CuL) ₂Tb(NO₃)₃bpy]•MeOH•2H₂O}_∞ (3). The inherent conformational flexibility of the trinuclear (CuL) ₂Tb node which is folded (cisoid) for 1 and 2 (asymmetric TbO₉ tricapped trigonal prism) and unfolded (transoid) for 3 (a C₂-symmetric TbO₁₀ tetradecahedron) seems to stabilize the crystal packing of these coordination-driven self-assemblies. The rapidly saturated magnetization at 1.8 K for 1 (11 μ_B) and 2 (22 μ_B) clearly indicates the presence of Tb–Cu ferromagnetic coupling through the phenoxido bridges of the Cu(μ_1,1-O) ₂Tb(μ_1,1-O)₂Cu cores. A slow saturation behavior has been found for 3 (9.5 μ_B). Both in-phase and out-of-phase ac magnetic measurements of 1 under an applied dc magnetic field reveal temperature dependence of the susceptibility at different frequencies below 5 K, characteristic of Single Molecule Magnets (SMMs), which is conserved in 2 and 3 signifying oligo- and polymeric aggregates of SMMs.