Effect of substituents, solute–solute–non-polar solvent interactions and temperature on the spectral properties of bis(dithiophosphato)copper(II) complexes

Extended studies by e.p.r. and electronic spectroscopy on the effect of different non-polar solvents, temperature and disulfide concentration on the spectral properties of bis(dithiophosphato)copper(II), Cu[(RO)2PS2]2, complexes [R = Me, Et and i-Pr] are reported. The molar absorptivity and e.p.r. intensity are very sensitive to the shape and size of the remote ligand substituents and increase in the order: Me < Et < i-Pr. The nature of the solvent and time after dissolution are also important regarding the magnitude of the e.p.r. intensity and molar absorptivity which, 1 h after dissolution, do not follow Beer's law. The molar absorptivities obtained at a given CuII(R2–dtp)2 concentration increase in the solvent order: n-hexane < n-heptane < CCl4 < PhMe < PhH < CHCl3. Twenty-four hours after dissolution the same samples exhibit: (i) linearity between absorbance and concentration of CuII(R2–dtp)2; (ii) a significant increase in molar absorptivity which is not equal for all the complexes studied and follows the same substituent and solvent orders. Beer's law is satisfied above 5 × 10−4 M for [(RO)2PS2]2Cu (R = Et, i-Pr) and above 3 × 10−3 M for [(MeO)2PS2]Cu. A significant additional increase, ca. 30–40%, of molar absorptivity, is obtained by increasing the solution temperature from 20 to 50 °C. The molar absorbtivity remains unchanged during 2–3 days after reducing the temperature. Further increase of molecular absorbtivity appears after addition of the corresponding disulfide of dithiophosphate [(RO2)PS2–S2P(RO)2] to CuII[(RO)2PS2]2. The molar absorptivity of Cu[(i-PrO)2PS2]2 increases from 4.8 × 103 cm−1 M−1, 1 h after dissolving the complex up to 2 × 104 cm−1 M−1 after addition of the corresponding disulfide. The observed effects are explained on the basis of a self-redox reaction taking place in this type of sulfur containing copper(II) complexes.