Binding modes in metal ion complexes of quinones and semiquinone radical anions: Electron-transfer reactivity

9,10-Phenanthrenequinone (PQ) and 1,10-phenanthroline-5,6-dione (PTQ) form 1:1 and 2:1 complexes with metal ions (Mⁿ⁺ = Sc³⁺, Y ³⁺, Mg²⁺, and Ca²⁺) in acetonitrile (MeCN), respectively. The binding constants of PQ-Mⁿ⁺ complexes vary depending on either the Lewis acidity or ion radius of metal ions. The one-electron reduced species (PTQ^.-) forms 1:1 complexes with M ⁿ⁺, and PQ^.- also forms 1:1 complexes with Sc ³⁺, Mg²⁺, and Ca²⁺, whereas PQ^.- forms 1:2 complexes with Y³⁺ and La³⁺, as indicated by electron spin resonance (ESR) measurements. On the other hand, semiquinone radical anions (Q^.- and NQ^.-) derived from p-benzoquinone (Q) and 1,4-naphthoquinone (NQ) form Sc³⁺-bridged π-dimer radical anion complexes, Q^.--(Sc³⁺)_n-Q and NQ ^.--(Sc³⁺)_n-NQ (n = 2 and 3), respectively. The one-electron reduction potentials of quinones (PQ, PTQ, and Q) are largely positively shifted in the presence of Mⁿ⁺. The rate constant of electron transfer from CoTPP (TPP^2- = dianion of tetraphenylporphyrin) to PQ increases with increasing the concentration of Sc³⁺ to reach a constant value, when all PQ molecules form the 1:1 complex with Sc³⁺. Rates of electron transfer from 10,10′-dimethyl-9,9′-biacridine [(AcrH)₂] to PTQ are also accelerated significantly by the presence of Sc³⁺, Y³⁺, and Mg²⁺, exhibiting a first-order dependence with respect to concentrations of metal ions. In contrast to the case of o-quinones, unusually high kinetic orders are observed for rates of Sc³⁺-promoted electron transfer from tris(2-phenylpyridine)iridium(III) [Ir(ppy)₃] to p-quinones (Q): second-order dependence on concentration of Q, and second- and third-order dependence on concentration of Sc³⁺ due to formation of highly ordered radical anion complexes, Q^.-(Sc³⁺) _n-Q (n = 2 and 3).