An Imidazolate-Bridged Tetranuclear Copper(II) Complex: Synthesis, Magnetic and EPR Studies, and Crystal Structure of [L4Cu4(Im)4](ClO4)4·2H2O (L = 1,4,7-Triazacyclononane, Im = Imidazolate Anion)

The tetranuclear complex [L₄Cu₄(Im)₄] (ClO₄)₄·2H₂O, where L is the cyclic amine 1,4,7-triazacyclononane and Im⁻ is the imidazolate anion, has been synthesized and its structure determined by X-ray diffraction methods as having imidazolate as bridging ligands. The complex crystallizes in monoclinic space group P2₁/c with cell constants a = 15.088(4) Å, b = 14.430(3) Å, c = 14.713(5) Å, β = 102.05(2)°, V = 313.73(1.52) ų, and Z = 2. Each of the four LCu units is coordinated via two imidazolate anions (Im⁻) to two LCu units, yielding four distorted square pyramidal CuN₅ polyhedra. The four copper ions which lie on a plane form an approximate parallelogram with sides of 5.89 and 5.99 Å. The compound has also been studied with variable temperature (2–295 K) magnetic susceptibility measurements and X-band EPR spectroscopy. Analysis of the susceptibility data yields an antiferromagnetic interaction between adjacent Cu(II) centers. The following parameter values are obtained: J = 70 cm⁻¹, g = 2.16, [formula omitted]. The electronic state has been established to have S_t = 0. The X-band EPR spectra, recorded in the temperature range 2.7–295 K are governed by an isotropic, almost temperature independent resonance C1 at g = 2.10. A second component C2, which arises above 20 K, exhibits significant temperature dependence with maximum at about 40 K. A satisfactory simulation of the subspectrum C2 has been obtained using the following parameters: isotropic g₁₁₁ =2.1, and zero-field values D₁₁₁ = 0.23 cm⁻¹ and E/D₁₁₁ ≈ 0.06. The EPR subspectra C1 and C2 have been assigned to the highest quintet |112〉 and to the first excited triplet state |111〉 of the tetramer, respectively.