Popivker Inna, Meyerstein Dan, Gitin Dalia, Avraham Elad N, Maimon Eric, Zidki Tomer, Cohen Haim, Yardeni Guy, Moisy Philippe, Pevzner Svetlana, Zilbermann Israel
Chemistry Department, Nuclear Research Centre-Negev, P.O. Box 9001, Beer-Sheva 84190, Israel.
Department of Chemical Sciences and the Schlesinger Family Center for Compact Accelerators, Radiation Sources and Applications, Ariel University, Ariel 40700, Israel.
J Phys Chem A. 2021 Feb 25;125(7):1436-1446. doi: 10.1021/acs.jpca.0c09134. Epub 2021 Feb 10.
The redox chemistry of CeDOTA in carbonate solutions was studied using electrochemistry and radiolysis techniques (continuous radiolysis and pulse radiolysis). Spectroscopic measurements point out that the species present in the solutions at high bicarbonate concentrations are [CeDOTA(CO)] (or less plausible [CeDOTA(HCO)]) with the carbonate (bicarbonate) anion as the ninth ligand versus [CeDOTA(HO)] present in the absence of bicarbonate. Electrochemical results show a relatively low increase in the thermodynamic stabilization of the redox couple Ce in the presence of carbonate versus its aqueous analogue. [CeDOTA(CO)] and [CeDOTA(HO)], prepared electrochemically, decompose photolytically. However, kept in the dark, both are relatively long lived; [CeDOTA(HO)], though, is orders of magnitude kinetically more stable (a considerably longer half-life). Thus, one concludes that the carbonate species have a different mechanism of decomposition depending also on the presence of dioxygen after its preparation (in deaerated/aerated solutions). The [CeDOTA(CO)] species is produced radiolytically by oxidation of the trivalent species by CO with a rate constant, measured using pulse radiolysis, of 3.3 × 10 M s. This rate constant is at least 1 order of magnitude smaller than most of the rate constants so far reported for the reaction of CO with transition metal/lanthanide (cerium)/actinide complexes. This result together with the bulkiness of the reactants might suggest an electron transfer rather than the one so far proposed. The lifetime of the tetravalent cerium species obtained radiolytically in the presence of carbonate is shorter than the electrochemical one, suggesting a different conformer involved.
采用电化学和辐射分解技术（连续辐射分解和脉冲辐射分解）研究了碳酸溶液中CeDOTA的氧化还原化学。光谱测量指出，在高碳酸氢盐浓度下溶液中存在的物种是[CeDOTA(CO)]（或不太可能的[CeDOTA(HCO)]），其中碳酸根（碳酸氢根）阴离子作为第九个配体，而在没有碳酸氢盐的情况下存在的是[CeDOTA(H₂O)]。电化学结果表明，与水相类似物相比，在碳酸存在下氧化还原对Ce的热力学稳定性相对较低。通过电化学制备的[CeDOTA(CO)]和[CeDOTA(H₂O)]会发生光解分解。然而，保存在黑暗中时，两者寿命都相对较长；不过，[CeDOTA(H₂O)]在动力学上要稳定几个数量级（半衰期长得多）。因此，可以得出结论，碳酸物种的分解机制不同，这也取决于其制备后（在脱气/曝气溶液中）是否存在氧气。[CeDOTA(CO)]物种通过CO对三价物种的氧化以辐射方式产生，使用脉冲辐射分解测量的速率常数为3.3×10⁹ M⁻¹ s⁻¹。该速率常数至少比迄今为止报道的CO与过渡金属/镧系元素（铈）/锕系元素配合物反应的大多数速率常数小1个数量级。这一结果连同反应物的体积较大可能表明是电子转移而不是迄今为止提出的那种转移。在碳酸存在下通过辐射分解获得的四价铈物种的寿命比电化学产生的要短，这表明涉及不同的构象体。
