Electron spin resonance and optical studies of doped copper (II) Tetrahedral complexes

This report presents an investigation of a pseudotetrahedral bromocuprate (II) compound. The bromocuprates exhibit a great structural diversity, however, the comparatively rare compressed tetrahedral compounds have been somewhat neglected. The determination of the electronic and geometric structure of these compounds has implications in the analysis of active sites in copper blue proteins. Electron Spin Resonance (E.S.R.) and optical spectra have been measured at low temperature, for single crystals of copper (II) doped into bis( ethylenediammoniummonobromide) tetrabromozincate (II), ( enH.HBr)2 ZnBr4. The room temperature powder reflectance spectrum has also been measured for this compound, along with that of the pure zinc analogue. The transition energies derived from the electronic spectra have been assigned and compared to the respective transitions based on previous studies. With the aid of a computer program, a set of theoretical transition energies has been calculated utilizing the Angular Overlap Model. Adjusting these to fit the observed values confirms the presence of a distorted tetrahedral geometry for the guest ion. At 77K, the E.S.R. spectrum of (enH2)2Zn[Cu]Br6 exhibits well resolved copper hyperfine and bromine superhyperfine couplings. Computer simulation techniques have produced optimum values for these parameters, which suggest that the principal z-axis of the superhyperfine tensor does not coincide with the copper-bromine bond direction. The bonding in the guest [CuBr4]2- ion has been described in terms of the relative unpaired electron density within the bromine 4s and 4p orbitals, which shows an increased covalency when compared with the chlorine counterpart. A highly rhombic g tensor results in an unusual groundstate. The calculated mixing coefficients and corresponding wavefunction parameters show that the dz2 orbital makes a small but significant contribution by mixing with the pure dx2-y2 orbital.