Valence detrapping accompanied by intramolecular and intermolecular halogen exchange in the solution of the mixed-valence binuclear ruthenocenes

Abstract

Biruthenocene(RcRc) was oxidized with halogen to form [Rc(II)-Rc(IV)-X]⁺Y⁻ (X = I, Br; Y = I₃, Br₃, BF₄, PF₆). The variable temperature (VT) ¹H NMR showed the intermolecular effect on the valence detrapping only for [RcRc-I]⁺I₃⁻ by changing the concentration of the solution, while the other complexes did not show solution concentration dependence. This reveals that the intramolecular halogen exchange is a main mechanism for valence detrapping, while the intermolecular halogen exchange is added only for [RcRc-I]⁺I₃⁻. This was suggested to be due to the equilibrium such as I₃⁻ ⇌ I₂ + I⁻. This I⁻ is thought to mediate I⁻ exchange among mixed-valence cations (I⁻ + I₃⁻ ⇌ I₃⁻ + I⁻, Grotthuss mechanism). On the other hand, [RcRc-Br]⁺Br₃⁻ did not show concentration dependence. The difference between the two adducts was suggested to be due to the difference in σ-hole density of halogen. VT ¹H NMR of mixed compounds (each [Rc(II)-Rc(IV)-X]⁺Y⁻ (X = I, Br; Y = I₃, Br₃, BF₄, PF₆) and RcRc) was also measured. The broadening of RcRc signal was observed in the mixed compounds for iodine adducts. It is considered that iodine adducts have an intermolecular exchange reaction with RcRc due to easier intermolecular transfer of adducted I⁻. In this case also the intermolecular exchange was shown to be significant for the valence detrapping for the mixture of [RcRc-I]⁺I₃⁻ and RcRc. On the other hand, bromine adducts with RcRc did not show the broadening of RcRc signal in the mixed compounds. The intramolecular valence detrapping in the mixed-valence [RcRc-Br]⁺ was dominant even in the mixture of [Rc(II)-Rc(IV)-Br]⁺Y⁻ (Y = Br₃, BF₄, PF₆) and RcRc. The difference between iodine adducts and bromine adducts in the mixture with RcRc agrees with the halogen trend for valence detrapping reported for the mixture of RcH and RcH-X⁺ (X = I, Br). It was suggested that σ-hole density of the halogen has an important role in the halogen transfer between RcRcX⁺ and RcRc. The solvent effect on halogen exchange was also discussed by considering the solvation of [Rc(II)-Rc(IV)-X]²⁺and Y⁻.