Radical-copper wheels: structure and magnetism of hexanuclear hybrid arrays

Complexation of copper(II) bromide and chloride with 4-pyrimidinyl nitronyl nitroxide (4PMNN) as a bridging ligand gave discrete hexanuclear complexes carrying 12 spins, [CuX(2).(4PMNN)](6) [X = Br (1), Cl (2)], which crystallize in a trigonal space group. The crystallographic parameters are C(11)H(15)Br(2)CuN(4)O(2).0.3H(2)O, a = 28.172(2), c = 12.590(2) A, V = 8653(2) A(3), and Z = 18 for 1, and C(11)H(15)Cl(2)CuN(4)O(2).0.3H(2)O, a = 28.261(2), c = 12.378(1) A, and Z = 18 for 2. The hexanuclear arrays construct a perfect column perpendicular to the molecular plane. The diameter of the resultant honeycomblike channel is ca. 11.5 A defined by the interatomic distance of two opposing copper ions. Their magnetic behavior is interpreted as the simultaneous presence of ferro and antiferromagnetic couplings. The ferromagnetic couplings are attributed to the interactions between a copper spin and the axially coordinated nitronyl nitroxide spin and between nitronyl nitroxide groups through van der Waals contacts. The antiferromagnetic coupling is due to the interaction between copper ions across the pyrimidine bridges.