Impact of lanthanide ion complexation and temperature on the chemical reactivity of N,N,N′,N′-tetraoctyl diglycolamide (TODGA) with the dodecane radical cation

Abstract

The impact of trivalent lanthanide ion complexation and temperature on the chemical reactivity of N,N,N′,N′-tetraoctyl diglycolamide (TODGA) with the n-dodecane radical cation (RH˙⁺) has been measured by electron pulse radiolysis and evaluated by quantum mechanical calculations. Additionally, Arrhenius parameters were determined for the reaction of the non-complexed TODGA ligand with the RH˙⁺ from 10–40 °C, giving the activation energy (E_a = 17.43 ± 1.64 kJ mol⁻¹) and pre-exponential factor (A = (2.36 ± 0.05) × 10¹³ M⁻¹ s⁻¹). The complexation of Nd(III), Gd(III), and Yb(III) ions by TODGA yielded [Ln^III(TODGA)₃(NO₃)₃] complexes that exhibited significantly increased reactivity (up to 9.3× faster) with the RH˙⁺, relative to the non-complexed ligand: k([Ln^III(TODGA)₃(NO₃)₃] + RH˙⁺) = (8.99 ± 0.93) × 10¹⁰, (2.88 ± 0.40) × 10¹⁰, and (1.53 ± 0.34) × 10¹⁰ M⁻¹ s⁻¹, for Nd(III), Gd(III), and Yb(III) ions, respectively. The rate coefficient enhancement measured for these complexes exhibited a dependence on atomic number, decreasing as the lanthanide series was traversed. Preliminary reaction free energy calculations—based on a model [Ln^III(TOGDA)]³⁺ complex system—indicate that both electron/hole and proton transfer reactions are energetically unfavorable for complexed TODGA. Furthermore, complementary average local ionization energy calculations showed that the most reactive region of model N,N,N′,N′-tetraethyl diglycolamide (TEDGA) complexes, [Ln^III(TEGDA)₃(NO₃)₃], toward electrophilic attack is for the coordinated nitrate (NO₃⁻) counter anions. Therefore, it is possible that radical reactions with the complexed NO₃⁻ counter anions dominate the differences in rates seen for the [Ln^III(TODGA)₃(NO₃)₃] complexes, and are likely responsible for the reported radioprotection in the presence of TODGA complexes.