The Case of a Cu₄ Rhombus in Molecular Magnetism: Preparation, Crystal Structure, and Magnetic Properties of [Cu₄(dpk·CH₃O)₄(CH₃O)₂](ClO₄)₂ (dpk·CH₃OH = Monomethylated Diol of Di-2-pyridyl Ketone), an Example of a Cluster Allowing the Determination of All Its Exchange Parameters, Ranging from Very Strong to Very Weak

Abstract

Treatment of [Cu2(O2CCH3)4(H2O)2] with 2 equiv of di-2-pyridyl ketone (dpk) in methanol followed by addition of 1 equiv of NaClO4 yields [Cu4(dpk·CH3O)4(CH3O)2] (ClO4)2 (1) in ∼75% yield (dpk·CH3O- = the monoanion of the monomethylated diol of dpk). The same complex has also been isolated from the 1:2:2 and 1:4:4 stoichiometries. The 1:2 reaction of Cu(ClO4)·6H2O with dpk in methanol affords the mononuclear complex [Cu(dpk·CH3OH)2](ClO4)2·2CH3OH (2) in ∼70% yield. Complex 1, C50H50Cl2Cu4N8O18, crystallizes in the tetragonal space group I41/acd with a = b = 15.551(1) Å, c = 48.938(3) Å, and Z = 8. Crystal data for 2: C26H32Cl2CuN4O14, monoclinic, space group P21/a, a = 12.923(1) Å, b = 12.969(2) Å, c = 9.655(1) Å, β = 92.78(1)°, Z = 2. The four CuII atoms in the cation of 1 are all coplanar, exhibiting a rhombic arrangement. The dpk·CH3O- ion behaves as a η1:η2:η1:μ2 ligand. In the cation of 2, the dpk·CH3OH ligand functions as a tridentate chelate toward the copper center, with the pyridyl nitrogens and the methylated oxygen being the ligated atoms. The magnetic properties of compound 1 are characteristic of prevailing antiferromagnetic interactions. The susceptibility data of 1 were fitted with a 3-J magnetic model, affording a very strong antiferromagnetic interaction (J2 = −490(9) cm-1), a weak ferromagnetic interaction (J1 = +22(4) cm-1), and a very weak antiferromagnetic interaction (J3 = −0.36(1) cm-1). The energy spectrum of the states, the magnetization, and the EPR data of complex 1 are in line with an almost degenerate ground state with a magnetic component. An investigation of this degeneration is attempted.