Thermochromism of metal ion complexes of semiquinone radical anions. Control of equilibria between diamagnetic and paramagnetic species by Lewis acids

Metal ion complexes of semiquinone radical anions exhibit different types of thermochromism depending on metal ions and quinones. Metal ion complexes of 1,10-phenanthroline-5,6-dione radical anion (PTQ^•-) produced by the electron-transfer reduction of PTQ by 1,1'-dimethylferrocene (Me ₂Fc) in the presence of metal ions (Mg²⁺ and Sc ³⁺) exhibit the color change depending on temperature, accompanied by the concomitant change in the ESR signal intensity. In the case of Mg ²⁺, electron transfer from Me₂Fc to PTQ is in equilibrium, when the concentration of the PTQ^•--Mg²⁺ complex (λ_max = 486 nm) increases with increasing temperature because of the positive enthalpy for the electron-transfer equilibrium. In contrast to the case of Mg²⁺, electron transfer from Me₂Fc to PTQ is complete in the presence of Sc³⁺, which is a much stronger Lewis acid than Mg²⁺, to produce the PTQ^•--Sc³⁺ complex (λ_max -631 nm). This complex is in disproportionation equilibrium and the concentration of the PTQ^•--Sc³⁺ complex increases with decreasing temperature because of the negative enthalpy for the proportionation direction, resulting in the remarkable color change in the visible region. On the other hand, the p-benzosemiquinone radical anion (Q^•-) forms a 2:2 π-dimer radical anion complex [Q ^•--(Sc³⁺)₂-Q] with Q and Sc³⁺ ions at 298 K (yellow color), which is converted to a 2:3 π-dimer radical anion complex [Q^•--(Sc³⁺)₃-Q] with a strong absorption band at λ_max = 604 nm (blue color) when the temperature is lowered to 203 K. The change in the number of binding Sc ³⁺ ions depending on temperature also results in the remarkable color change, associated with the change in the ESR spectra.