Structure and Dynamics of Bis(triphenylphosphine)-3,6-di-tert-butyl-o-semiquinone Radical Anion Complexes of Ag⁺ and Cu⁺

Complexes of Ag+ and Cu+ with 3,6-di-tert-butyl-o-semiquinone radical anion (SQ•-) and two triphenylphosphine (PPh3) ligands have been investigated by electron paramagnetic resonance (EPR) in organic solvents. High-temperature EPR spectra of the Ag+ and Cu+ complexes contain lines of hyperfine splittings due to two equivalent 1H nuclei in the 4,5-positions of the aromatic ring of the SQ•- ligand, two equivalent 31P nuclei from the two PPh3 ligands, and 107,109Ag and 63,65Cu nuclear isotopes, respectively. On the basis of the temperature dependence of the 31P hyperfine interaction constant a(31P), the structure of the Ag+ complex at low temperature was determined to be planar, while at higher temperatures the increase in the values of a(31P) shows that phosphorus atoms of the two PPh3 ligands move out of the π-plane of the SQ•- ligand. EPR spectra of the Ag+ complex with variable temperature exhibit alternating line width effect in hyperfine splitting due to the two aromatic protons in the SQ•- ligand. At low temperatures these two protons become nonequivalent. This effect reveals a dynamic charge redistribution within Ag+ and two oxygen atoms of the SQ•- ligand. In the Cu+ complex, we have found a temperature-independent symmetric charge distribution within two oxygen atoms of the radical anion and Cu+, but different positions of the two PPh3 ligands at low temperatures. EPR spectra of the Cu+ complex at variable temperature reveal the dynamic exchange in the positions of PPh3 ligands. The rates of this process are on the order of solvent reorientation frequencies, while intramolecular electron exchange rates in the Ag+ complex are about 2 orders of magnitude lower. The rate of the electron exchange reaction dramatically decreases in viscous mineral oil, which shows that the intramolecular charge redistribution reaction is controlled by nonpolar solvent dynamics.