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  • Title: Impact of lanthanide ion complexation and temperature on the chemical reactivity of N,N,N',N'-tetraoctyl diglycolamide (TODGA) with the dodecane radical cation.
    Author: Horne GP, Celis-Barros C, Conrad JK, Grimes TS, McLachlan JR, Rotermund BM, Cook AR, Mezyk SP.
    Journal: Phys Chem Chem Phys; 2023 Jun 21; 25(24):16404-16413. PubMed ID: 37294439.
    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 (Ea = 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 [LnIII(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([LnIII(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 [LnIII(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, [LnIII(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 [LnIII(TODGA)3(NO3)3] complexes, and are likely responsible for the reported radioprotection in the presence of TODGA complexes.
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