STRUCTURES AND MAGNETIC-SUSCEPTIBILITY STUDIES OF 4 NEW HIGH-NUCLEARITY COPPER(II) HALIDE OLIGOMERS

The syntheses, crystal structures, and powder magnetic studies of several new quasi-planar bibridged Cu(n)X(2n+2)(2-) oligomers (n = 3, 4, 6, and 7; X = Cl or Br) are reported, based on the 1-methylpyridinium (C6H8N)(+) and 1,2-dimethylpyridinium (C7H10N)(+) cations. These include (C7H10N)(2)Cu3Br8, (C6H8N)(2)Cu4Cl10, (C7H10N)(2)Cu6Cl14, and (C7H10N)(2)Cu7Br16. Crystallographic data: (C7H10N)(2)Cu3Br8, triclinic, space group P (1) over bar, a = 7.947(2) Angstrom, b = 8.799(2) Angstrom, c = 9.840(2) Angstrom, alpha = 86.95(2)degrees, beta = 76.23(2)degrees, gamma = 71.54(2)degrees, V = 633.6(3) Angstrom(3), Z = 2, d(x) = 2.78 g/cm(3), and R = 0.0483; (C6H8N)(2)Cu4Cl10, monoclinic, space group P2(1)/n, a = 11.759(2) Angstrom, b = 9.056(2) Angstrom, c = 12.048(3) Angstrom, beta = 106.21(2)degrees, V = 1232.1(5) Angstrom(3), Z = 2, d(x) = 2.15 g/cm(3), and R = 0.0321; (C7H10N)(2)Cu6Cl14, triclinic, P (1) over bar, a = 8.997(3) Angstrom, b = 9.288(3) Angstrom, c = 11.540(4) Angstrom, alpha = 80.53(2)degrees, beta = 67.82(2)degrees, gamma = 60.22(2)degrees, V = 714.7(4) Angstrom(3), Z = 1, d(x) = 2.34 g/cm(3), and R = 0.0363; (C6H8N)(2)Cu7Br16, triclinic, P (1) over bar, a = 7.237(2) Angstrom, b = 10.880(2) Angstrom, c = 12.880(2) Angstrom, alpha = 89.47(2) Angstrom, beta = 75.08(2)degrees, gamma = 79.48(2)degrees, V = 962.7(3) Angstrom(3), Z = 1, d(x) = 3.35 g/cm(3), and R = 0.0520. A common feature of the structures is the existence of oligomers containing quasi-planar symmetric bibridged finite chains of edge-sharing CuX(4), (X = halide) monomeric units. The n = 3 oligomers aggregate into chains through the formation of asymmetric bibridged linkages between terminal copper ions on adjacent trimers. In the n = 4 salt, the oligomers aggregate into stacks in which pairs of the copper ions extend their coordination sphere by forming a long, semicoordinate bond to a halide ion from a neighboring oligomer. For both the n = 3 and n = 4 salts, the pyridinium cations lie parallel to and directly above and below the anionic oligomers, separating the chains. In the n = 6 rind n = 7 salts, the stacks formed in this manner interdigitate, forming two-dimensional slabs. The slabs are separated by the organic cations. The magnetic properties of compounds are dominated by antiferromagnetic intraoligomer interactions. Thus, the n = 4 and n = 6 salts depopulate into singlet ground states at low temperature. In contrast, the n = 3 and n = 7 oligomers have S = 1/2 ground states. Expressions for the magnetic susceptibility of the n = 6 and n = 7 oligomers were obtained by diagonalization of a nearest neighbor Heisenberg Hamiltonian. The data were fit to these expressions, with inclusion of a mean field correction for interoligomer exchange. The intra-oligomer exchange coupling constants are J(1)/k = -153 K for n = 3; J(1)/k = -60 K, J(2)/k = -40 K for n = 3; J(1)/k = -23 K, J(2)/k = -30 K, and J(3)/k = -52 K for n = 5 and J(1)/k = -90 K, J(2)/k = -90 K and J(3)/k = -120 K. At low temperature, the n = 3 oligomer exhibits ferromagnetic behavior. Since the oligomer has depopulated to a spin 1/2 ground state, the system can be modeled as a spin 1/2 ferromagnetic chain with J'/k = 22.8 K.