SYNTHESIS, CRYSTAL-STRUCTURE, AND OPTICAL AND MAGNETIC-PROPERTIES OF A NOVEL 2-DIMENSIONAL COPPER(II) NETWORK FORMED CONJOINTLY WITH MU-BIPYRIMIDINE, MU-OXALATO, AND MU-CHLORO LIGANDS

The preparation, X-ray crystal structure, and optical and magnetic pro perties of a polymeric two-dimensional mu-2,2'-bipyrimidine (C8H6N4, b pym), mu-oxalato-bridged (C2O42-, ox) and mu-chloro-bridged copper(II) network of formula [Cu-2(bpym)(ox)Cl-2](n) is reported. The compound crystallizes in the orthorhombic system, space group Pbca, with a = 9. 522(2) Angstrom, b = 10.509(2) Angstrom, c = 13.222(3) Angstrom, Z = 4 , and V = 1323.1(5) Angstrom(3). The structure consists of alternating ly mu-bpym- and mu-ox bridged copper(II) chains which again are connec ted through mono(mu-chloro) ligands, thus forming a corrugated two-dim ensional (2D) framework. Polarized optical single crystal absorption s pectra, measured at both room and liquid-helium temperatures, are pres ented and the absorption pattern is discussed using the selection rule s derived from an orbital scheme of the Cu(II) chromophore with ideali zed C-2 upsilon symmetry. A low-lying MLCT state is taken as the origi n of the strongly polarized absorption parallel to the b-axis, hence t he ox-Cu(II)-bpym direction. The temperature dependence of the magneti c susceptibility is well explained with an alternating chain model, ta king into account the strong intramolecular antiferromagnetic interact ion through the mu-bpym and mu-ox bridges. The analysis is based on th e Hamiltonian, ($) over cap H = -2J Sigma(i=1)(n/2)[($) over cap S-2i. ($) over cap S-2i-1 + alpha ($) over cap S-2i.($) over cap S-2i+1], an d the resulting exchange parameters are J = -189(1) cm(-1) for the mu- ox link and alpha J = -76(1) cm(-1) for the mu-bpym link, which corres ponds to an alternation parameter of alpha = 0.40(2). In addition, a m ean- field correction is discussed which considers the possibility of weak ''interchain'' interactions mediated by the asymmetric mono(mu-ch loro) bridges.