Weak ferromagnetic behavior, crystal structure, and electronic studies of novel [Cu(II)(Br)(PhCO 2)(Sp)] (Sp=(-)-sparteine) complex

Complex [Cu(II)(Br)(PhCO ₂)(Sp)] 1 is obtained starting from copper(0), (-)-sparteine (sp) and benzoyl bromide. 1 Crystallizes in the monoclinic space group P2 ₁ with a=14.8857(11), b=8.9257(9), c=17.4456(14) Å, β=111.689(5)°, and Z=4. The UV-vis spectrum is characteristic of Cu(II) complexes with tetragonally distorted square pyramidal geometry. The far IR spectrum of 1 shows characteristic vibrations of Cu-Br (239 cm ^-1), Cu-N (437 cm ^-1) and Cu-O (466 cm ^-1) bonds. The ¹H NMR broad chemical shifts of 1 integrated for a total of 31 protons and are typical of Cu(II) complexes. ESR spectra of polycrystalline 1 at 77 and 300 K show axial spectra with areas in the ratio A₇₇/ A₃₀₀=4.02, suggesting a very weak Cu-Cu ferromagnetic interaction. Complex 1-doped with Zn(II) gives hfs with hyperfine interaction constant value A_∥=112.45×10 ^-4 cm ^-1. The magnetization vs temperature data in the 2-299 K range, show that cupric ion pairs interact through a small antiferromagnetic Heisenberg exchange energy— JS₁· S₂ with a ground singlet state S=0, separated by J=-1.3 cm ^-1 from the excited triplet state S=1. The sign of the very weak interchange interaction constant, J, does not agree with the ESR spectra areas ratio of 1 at 77 and 300 K, which is a more accurate quantification of the weak ferromagnetic interaction Cu-Cu through the space.