Ferromagnetic Mn(II)...Cu(II) exchange in the new bimetallic quasi-2-D compound Cu(op)(2)MnCl4 (op=1,4-diazacycloheptane)

The synthesis, crystal structure, and magnetic properties are reported for the new bimetallic compound Cu(op)(2)MnCl4, where op HN(CH2)(5)NH. The compound, C10H24N4Cl4CuMn, crystallizes in the monoclinic space group P2(1)/n. Cell dimensions are as follows: a = 15.316(3) Angstrom, b = 16.608(3) Angstrom, c = 7.141(2) Angstrom, beta = 100.01(5)degrees, Z = 4. The structure consists of well-separated and magnetically equivalent layers which are composed of chloride-bridged Cu(op)(2)MnCl4 binuclear units connected by rather loose Cu-N-H...Cl-Mn contacts. The MnCl4 fragment approximates tetrahedral symmetry. The Cu(II) geometry is (4 + 1) square-pyramidal with the apical position occupied by a bridging chloride ligand and the basal ones by the nitrogen atoms from the organic ligands. The shortest interlayer M...M separations, similar to 7 Angstrom, are of the Mn Cu type. Magnetic susceptibility and single-crystal EPR measurements for the compound have been carried out over the range 4-300 K. At room temperature the T-chi product (per MnCu unit) has a value of 4.84 emu . mol(-1). K, close to that expected for uncoupled S = 5/2 and S = 1/2 spins. When the temperature is lowered, T-chi remains almost constant until 80-90 K, slightly increases to reach a maximum at similar to 13 K (5.21 emu . mol(-1). K), and then rapidly decreases. Comparison between theory and experiment, made with use of both a mean field corrected dimer model and an approximate 2-D model, indicates that Mn(II)...Cu(II) exchange is ferromagnetic within the dimers (J(1) similar to 2.6 cm(-1)) and antiferromagnetic among dimers, with J values between -0.07 and -0.03 cm(-1) (the interaction Hamiltonian is of the form H = -2JS(A) . S-B). Single-crystal EPR spectra recorded along the a, b, and c* axes show a large temperature dependence of the g factors: at 4.2 K, g(a) = 2.10, g(b) = 1.96, and g(c)* = 2.01. This pattern substantiates the presence of a 2-D magnetic structure with ferromagnetic intradimer exchange and interdimer antiferromagnetic exchange of weaker magnitude. The opposite signs of the interactions are ascribed to the local symmetries of the Cu(II) and Mn(II) ions.