Exchange interactions in di-µ-hydroxo-bis[bis(1,2-diaminoethane)-chromium(III)] salts. Structural, magnetochemical, and optical spectroscopic investigations

Abstract

Chloride, bromide, iodide, and dichloride diperchlorate salts of the title complex have been prepared. The crystal structures of [Cr₂(OH)₂(en)₄]Cl₄·2H₂O and [Cr₂(OH)₂(en)₄]Br₄·2H₂O have been determined by single-crystal X-ray measurements on an automatic diffractometer, and refined to R 0.023 for the chloride and 0.031 for the bromide. Lattice constants are a= 8.165 0(8), b= 8.740 0(9), c= 9.790 2(7)Å, α= 67.196(7), β= 87.937(7), and γ= 64.587(7)° for the chloride, and a= 8.211(2), b= 8.980(1), c= 10.024(1)Å, α= 67.57(1), β= 88.90(1), and γ= 65.07(2)° for the bromide. Both structures belong to space group P and are strictly isomorphous. The symmetry of the dichromium complex is 2/m in both salts. Moreover, a normal probability plot analysis shows the cations to be identical in the two compounds. Powder magnetic-susceptibility data have been collected for all the four complexes between 1.5 K and room temperature. Single-crystal and Nujol-mull absorption spectra have been recorded between 7 and 40 K in the region of transitions from the electronic ground state ⁴A₂⁴A₂ to the lowest levels of the first singly excited state ⁴A₂²E, and powder luminescence spectra obtained at <10 K. From the absorption and luminescence spectra the exchange splitting of the electronic ground state can be accurately determined. The exchange parameters J thus derived deviate by up to 10% from those obtained by fitting the magnetic-susceptibility data. These discrepancies indicate a slight temperature dependence of J. The values obtained for the biquadratic exchange parameter j from the magnetochemical fit have little if any physical significance. The exchange parameters (cm^–1) for the four salts, determined spectroscopically at <40 K, are: J= 31.4 ± 0.4 and j= 0.08 ± 0.02 (chloride), 30.0 ± 0.3 and j= 0.11 ± 0.08 (bromide), 24.3 ± 0.9 and j= 0.08 ± 0.2 (iodide), and 22.2 ± 0.2 and j= 1.12 ± 0.6 (dichloride diperchlorate dihydrate).