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

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) × 10 13 M -1 s -1 ). The complexation of Nd(III), Gd(III), and Yb(III) ions by TODGA yielded [Ln III (TODGA) 3 (NO 3 ) 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 (NO 3 ) 3 ] + RH˙ + ) = (8.99 ± 0.93) × 10 10 , (2.88 ± 0.40) × 10 10 , and (1.53 ± 0.34) × 10 10 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 (NO 3 ) 3 ], toward electrophilic attack is for the coordinated nitrate (NO 3 - ) counter anions. Therefore, it is possible that radical reactions with the complexed NO 3 - counter anions dominate the differences in rates seen for the [Ln III (TODGA) 3 (NO 3 ) 3 ] complexes, and are likely responsible for the reported radioprotection in the presence of TODGA complexes.