The case of a Cu-4 rhombus in molecular magnetism: Preparation, crystal structure, and magnetic properties of [Cu-4(dpk center dot CH3O)(4)(CH3O)(2)](ClO4)(2) (dpk center dot 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

Treatment of [Cu-2(O2CCH3)(4)(H2O)(2)] with 2 equiv of di-2-pyridyl ketone (dpk)! in methanol followed bg addition of I equiv of NaClO4 yields [Cu-4(dpk . CH3O)(4)(CH3O)(2)](ClO4)(2) (1) in similar to 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: 1 stoichiometries. The 1:2 reaction of Cu(ClO4). 6H(2)O with dpk in methanol affords the mononuclear complex [Cu(dpk . CH3OH)(2)](ClO4)(2) . 2CH(3)OH (2) in similar to 70% yield, Complex 1, C50H50Cl2Cu4N8O18, crystallizes in the tetragonal space group I4(i)/acd with a=b=15.551(1) Angstrom, c = 48.938(3) Angstrom and Z=8, Crystal data for 2: C26H32Cl2CuN4O14, monoclinic, space group P2(i)/a, a=12.923(1) Angstrom, b=12.969(2) Angstrom, c=9.655(1) Angstrom, beta= 92.78(1)degrees, Z=2, The four Cu-II atoms in the cation of 1 are all coplanar, exhibiting a rhombic arrangement. The dpk . CH3O- ion behaves as a eta(1):eta(2):eta(1):mu(2) 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 (J(2)=-490(9) cm(-1)), a weak ferromagnetic interaction (J(1)=+22(4) cm(-1), and a very weak antiferromagnetic interaction (J(3)=-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.