Synthesis and magnetostructural properties of two crystalline phases of [CuBr₂(sp)] (sp=(-)-sparteine)

Two crystalline phases, 1a, 1b, of [CuBr₂(sp)] complex (sp=(-)-sparteine) were obtained by direct synthesis. Crystal data: complex 1a, triclinic, P1, a=7.4715(6)Å, b=7.7082(7)Å, c=9.1435(6)Å, α=97.429(6)°,β=112.808(5)°,γ=110.666(6)°; complex 1b, orthorhombic, P212121, a=11.0563(7)(6)Å, b=11.9877(9)Å, c=12.8002(9)Å. The molecular locality of Cu(II) ion is tetra-coordinated in each case and show close values of angle and bonds length for 1a, 1b. The UV-Vis and IR spectra of 1a and 1b are very similar. ¹H NMR spectra of 1a, 1b, are typical of Cu(II) paramagnetic complexes with broad lines in a range between 34 to -21 ppm. ESR spectra at 300 and 77 K of a polycrystalline sample for complex 1a show a broad signal with g=2.036. Two species Cu(II) are developed by power saturation study. For 1b the ESR spectrum at 300 K is a singlet with composition Lorentzian 20%/Gaussian 80%. At 77 K the ESR singlet is still narrower with Lorentzian 90%/Gaussian 10%. The ratio areas is A ₇₇/A₃₀₀=5.22. All this features indicate clearly a weak ferromagnetic Cu⋯Cu exchange interaction. Standard magnetization measurements at low temperatures show that cupric ion pairs exchange interact weakly, obtaining J_1a=2.56cm^-1 and J_1b=1. 93cm^-1. The crystallographic data correlate quite well with ¹H NMR results such as: the line shapes, the relative areas and position of the signals, with the hydrogen atoms-spheres that interact with the central copper ion.