E.p.r. and spectroscopic studies of configurational changes in 1,4-diazacycloheptane copper(II) complexes

E.p.r. and spectroscopic studies of bis(1,4-diazacycloheptane)copper(II) perchlorate, bromobis(1,4-diazacycloheptane)copper(II) perchlorate and dibromo-bis(1,4-diazacycloheptane)copper(II), were carried out in MeNO₂, DMSO and H₂O solutions as well as in the polycrystalline state. The ligation of bromide ion to bis(1,4-diazacycloheptane)copper(II) along the z-axis was investigated. The e.p.r. spectra are explained in terms of energy changes involving the electronic ground state and the lowest excited state as a function of ligation of nucleophiles to the metal atom along the z-axis. In the polycrystalline state, the weaker the apical interaction (square-planar species) the lower the g-value and the higher the copper(II) hyperfine coupling constant. In the liquid state, the g-values observed in non-coordinating solvents such as MeNO₂ are similar to those for the polycrystalline material suggesting structural similarity in the two states. The electronic absorption maxima for the three complexes in the liquid state shift to higher energy with an increase in tetragonal distortion around copper(II). In strongly coordinating solvents the e.p.r. spectra and absorption maxima are independent of the anion present. The increasing strength of solvation results in an increase in the wavelength of the optical transition and the g-values, and a decrease in the copper(II) hyperfine coupling constant. It appears that an increase in the basicity of the solvent has the same effect on the d-orbital energy levels and on the unpaired electron density on copper(II)_as does increasing the electronegativity of the substituents on the ligand.