AN OXALATE-LINKED COPPER(II) COORDINATION POLYMER, [CU-2(OXALATE)(2)(PYRAZINE)(3)](N), CONSTRUCTED WITH 2 DIFFERENT COPPER UNITS - X-RAY CRYSTALLOGRAPHIC AND ELECTRONIC-STRUCTURES

The new copper(II) coordination polymer [CU2(mu-ox)(2)(mu-pyz)(pyz)(2)](n) (1) (ox = oxalate; pyz = pyrazine) has been synthesized and characterized. 1 crystallizes in the triclinic space group P ($) over bar 1 with a = 10.578(4) Angstrom, b = 11.603(5) Angstrom, c = 8.027(4) Angstrom, alpha = 92.11(5)degrees, beta = 103.10(3)degrees, gamma = 76.32(4)degrees, V = 932.3(8) Angstrom(3), Z = 2, and chemical formula CU2C16N6O8H12. 1 shows an extended sheet structure of copper(II) ions bridged by oxalate anions and pyrazine. The repeating unit of (Cu-2(mu-ox)(2)(mu-pyz)(pyz)(2)) contains two types of 4 + 2 coordination environments with O4N2, which are characteristic of pyz coordination; one of the two copper atoms has only terminally coordinated pyz molecules while the other is linked by bridging pyz molecules. The Cu-ox-Cu-sequence displays as pleated ribbon, thus planes of Cu-ox-Cu form a boat conformation. In addition to the coordination bond linking, there is stack linking of pyz molecules whose column runs along the ribbon. The magnetic susceptibilities were measured to 2 K and analyzed in terms of an alternating-chain Heisenberg-exchange model (H = -2J Sigma(i=1)(n/2) [S-2i. S-2i-1 + alpha S-2i. S-2i+1]) to yield J = -20.4 cm(-1) and alpha = 0.85. The alternate arrangement of the copper geometries along a ribbon leads to that of the magnetic orbitals. This well explains the value of J smaller than that of [Cu(ox)](n) without apical ligands and comparable to that of [CU(ox)(NH3)(2)](n). The density functional MO calculation and EHMO band calculations were carried out to delineate the electronic structure and the role of pyz molecules in the extended structure.