Effect of solute-co-ordinating solvent interactions and temperature on the EPR and electronic spectra of bis(dithiophosphato)copper(II)

Article abstract of DOI:10.1016/S1386-1425(01)00707-7

The self-redox reaction proceeding between two molecules of the complex bis(disubstituted-dithiophosphato)copper(II), Cu^II(R₂-dtp)₂, is studied by EPR and UV-VIS spectroscopy in DMFA, DMSO and pyridine. The effect of temperature and disulphide concentration in the solutions is also evaluated. The EPR spectra show that the g-values of Cu^II(R₂-dtp)₂ increase when it is dissolved in co-ordinating solvents, whereas the copper hyperfine splitting decreases compared to the corresponding values in non-co-ordinating solvents. Under the same conditions, a hypsochromic shift is observed in the maximal absorption at 420 nm of the electronic spectra which corresponds to the ligand-to-metal charge-transfer (LMCT) transition of the complex. The results are explained with the formation of axial or equatorial adducts between Cu^II(R₂-dtp)₂ and the co-ordinating solvents used. On the other hand, the molar absorptivity of the LMCT band and the intensity of the EPR spectrum increase strongly with the nature of the used co-ordinating solvent, the time after dissolution and the quantity of added disulphide. Both also depend on the size and shape of remote ligand substituents and they increase in the order Me2P(S)S-S(S)P(RO)₂] to the Cu^II(R₂-dtp)₂ solution. As a result, the molar absorptivity value at the maximum of the LMCT band of Cu[(i-PrO)₂-dtp]₂ increases from 7.9×10³ m^-1dm³cm^-1 immediately after dissolution to 2.9×10⁴ m^-1dm³cm^-1. In DMSO and pyridine, the intensity of both the EPR signal and LMCT band of Cu^II(R₂-dtp)₂ continuously decrease after the preparation of the solutions. A small increase is only observed immediately after the addition of the corresponding disulphide of dithiophosphate. While DMFA forms stable adducts with Cu[(i-PrO)₂-dtp]₂, adduct formation with DMSO and pyridine destroys the initial complex.