Crystal Structures and Magnetic Behavior of Pyridinium Bis(oxalato)cuprate(II)-Oxalic Acid and Bis(2-methylimidazole)copper(II) Oxalate

The crystal structure and magnetic properties of two oxalato complexes of copper(II) are reported. Pyridinium bis(oxalato)-cuprate(II)-oxalic acid (triclinic, P1, a = 8.980 (6) A, b = 14.159 (7) A, c = 3.697 (3) A, a = 92.47 (7)°, β = 101.17 (7)°, γ= 97.69 (4)°, R = 0.039) contains stacks of Cu(ox)₂^2-anions with long semicoordinate bonds between copper(II) ions in one anion to a coordinated oxygen atom in each adjacent anion in the stack. Magnetic susceptibility data reveal weak antiferromagnetic coupling (0 = -3.6 K from Curie-Weiss law behavior). Analysis of the EPR spectrum sets a lower limit of| J| ~ 10^-2 cm^-1for the intrachain coupling. Bis(2-methylimidazole)(oxalato)copper(II) (orthorhombic, Pbc2₁, a = 8.235 (2) A, b = 19,565 (5) A, c = 7.994 (2) A, R = 0.049) contains a distorted square pyramidally coordinated copper(II) ion. The equatorial plane contains two nitrogen atoms (one each from an imidazole ligand) and two oxygen atoms from a chelated oxalato ligand. The apical site is occupied by an uncoordinated oxygen atom from an oxalato group on an adjacent complex. This forms a chain structure parallel to the c axis. The magnetic susceptibility data indicate weak ferromagnetic coupling for this compound with J/k = 0.45 (4) K. Analysis of the EPR line width data yields an estimate of| J| ~ 0.1 cm^-1.