Synthesis, spectroscopic, magnetic properties and X-ray crystal structure of di-μ-hydroxo-bis(μ-perchlorato- O, O′)-bis-[bis(2-amino-4-methylpyrimidine)copper(II)]: bridging perchlorate results in significant deviation from the predicted magnetic exchange

Abstract

A new copper(II) complex, [Cu 2L 4(OH) 2(ClO 4) 2], was prepared in a one-step synthesis, where L=2-amino-4-methylpyrimidine, and characterized by chemical analyses, infrared spectra, ligand-field spectra, magnetic susceptibility, electron paramagnetic resonance measurements and X-ray crystal structure determination. The compound crystallizes in the orthorhombic space group Pbca, Z=4, a=13.6282(2), b=12.7193(3), c=18.1459(4) Å, V=3145.43(11) Å 3. The least-squares refinement of the structure led to R 1=0.036 and wR 2=0.0848. Each copper(II) ion is in a distorted tetragonal environment consisting of two nitrogen atoms from the ligands, two oxygen atoms from the hydroxo bridges and two trans oxygens from bridging perchlorates in semi-coordination. The equatorial Cu–O and Cu–N distances vary from 1.924(2) to 1.936(2) and from 1.991(2) to 2.004(2) Å, respectively. The weak axial Cu–O coordination distances to the bridging perchlorates are 2.602(2) and 2.673(2) Å. The Cu–Cu distance is 2.9414(5) Å with a Cu–O–Cu angle of 99.30(9)°. A sharp peak at 3640 cm −1 in the IR spectrum is assigned to the bridging OH group. Cryomagnetic investigations (5–266 K) revealed the operation of antiferromagnetic spin–spin interactions through the hydroxo bridges for the complex. A singlet–triplet energy gap (2 J) of −68.1 cm −1 was determined for the compound, a value significantly higher than the value of −131 cm −1 predicted from the Hatfield–Hodgson formula. The X-band electron paramagnetic resonance studies in the solid state showed only a weak signal of a monomeric impurity. No triplet EPR signal was found between 0 and 0.8 T, suggesting a large zero-field splitting. Frozen-solution EPR spectra indicate that the complex dissociates into mononuclear species when dissolved in dmf or dmso.