Oxidative nucleophilic substitution of hydrogen in the sapphyrin dioxouranium(VI) complex: a relativistic DFT study

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.