Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 2N2.
J Am Chem Soc. 2011 Mar 30;133(12):4316-29. doi: 10.1021/ja106566g. Epub 2011 Mar 7.
A potentially trianionic expanded porphyrin ligand, sapphyrin does not form a 1:1 complex with the uranyl cation. However, in the presence of methanol, a complex of uranyl and meso-methoxy-substituted iso-sapphyrin is formed [Burrel et al. J. Chem. Soc., Chem. Commun. 1991, 24, 1710]. Here we performed a relativistic DFT study on the thermodynamics and the possible mechanism of the reaction. Our results have shown that (1) the reason for the failure of sapphyrin to stabilize its 1:1 uranyl complex is the highly basic character of the trianionic form of ligand that is hard to achieve in solution, (2) a driving force for the reaction lies in the better affinity of the methanol-substituted (and isomerized) ligand dianion to the uranyl cation, compared with the unsubstituted sapphyrin dianion, and (3) for the single-stage synchronous methanol addition pathways explored in this work, there is a path corresponding to noninnocent uranium behavior, via a neutral, triplet U(IV) intermediate complex. However, if the solvation effects were taken into account, this pathway would be unfavorable compared with singlet U(VI) pathways involving anionic intermediate complexes. The later pathway can be described as classical oxidative nucleophilic substitution of hydrogen in an aromatic system.
一种潜在的三阴离子扩展卟啉配体，sapphyrin 不会与铀酰阳离子形成 1:1 配合物。然而，在甲醇存在下，形成了铀酰和间甲氧基取代的同 sapphyrin 的配合物[Burrel 等人，J. Chem. Soc.，Chem. Commun. 1991, 24, 1710]。在这里，我们对反应的热力学和可能的机制进行了相对论 DFT 研究。我们的结果表明：（1）sapphyrin 未能稳定其 1:1 铀酰配合物的原因是配体的三阴离子形式具有高度碱性，在溶液中难以实现；（2）反应的驱动力在于甲醇取代（和异构化）配体二阴离子与铀酰阳离子的亲和力更好，与未取代的 sapphyrin 二阴离子相比；（3）对于本工作中探索的单阶段同步甲醇加成途径，存在一条对应于非无辜铀行为的途径，通过中性、三重态 U(IV)中间体配合物。然而，如果考虑溶剂化效应，与涉及阴离子中间配合物的单重态 U(VI)途径相比，该途径是不利的。后一种途径可以描述为芳香系统中经典的氧化亲核取代氢的反应。
