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

摘要

9,10-Phenanthrenequinone (PQ) and 1,10-phenanthroline-5,6-dione (PTQ) form 1:1 and 2:1 complexes with metal ions (Mn+ = sc(3+), Y3+, 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 Mn+, and PQ(-) also forms 1:1 complexes with Sc3+, 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 pi-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) ore largely positively shifted in the presence of Mn+. The rate constant of electron transfer from CoTPP (TPP2- = 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 ore 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-phenyl-pyridine)iridium(m) [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).