Crystal and molecular structure and magnetic properties of tetrakis-[bromo(2-diethylaminoethanolato)copper(II)]–tetrachloromethane (1/4), a complex with a ferromagnetic ground state

Abstract

The crystal and molecular structure of [{CuBr(OCH₂CH₂NEt₂)}₄]·4CCl₄(A) has been determined from three-dimensional X-ray diffractometer data and refined to R 0.061 using 1 581 independent reflections. The tetragonal unit cell of space group I4₁/a with dimensions a= 19.47(2) and c= 15.78(1)Å contains four oxygen-bridged cubane-type clusters of symmetry S₄ with a Cu₄O₄ core. Copper–copper distances are 3.523(2)(× 2), 3.176(2)Å(× 4) and copper–oxygen distances 1.924(5)(× 2), 2.524(5)(× 2), and 1.982(5)Å(× 4), respectively. The co-ordination around copper (amine nitrogen, bromine, three bridging alkoxide oxygens) is distorted square pyramidal. Sixteen molecules of tetrachloromethane act as solvent of crystallization in the unit cell. The isomorphic [{CuCl(OCH₂CH₂NEt₂)}₄]·4CCl₄(B) has cell dimensions a= 19.21(2) and c= 15.96(1)Å. The decomposition of (A) and (B) has been followed by Debye–Scherrer investigation. The magnetic susceptibility of (A) and (B) in the temperature range 3.4–300 K and of the α and β modifications of [{CuBr(OCH₂CH₂NEt₂)}_n](n= 2 or ∞) in the range 70–300 K have also been determined and interpreted in terms of the isotropic Heisenberg–Dirac–van Vleck model. Whereas the α and β modifications show strong antiferromagnetic exchange interaction [2J=–817 and –744 (±20) cm^–1], there is simultaneous antiferromagnetic and ferromagnetic exchange interaction in compounds (A) and (B) with the latter interaction dominating. The evaluated exchange parameters are: J₁₂=–4.5 ± 3, J₁₃= 40.0 ± 3 cm^–1(A); J₁₂=–5.2 ± 3, J₁₃= 46.9 ± 3 cm^–1(B). These can be understood in terms of the Cu–O–Cu bridging angle and the Cu–O bond distances.