Controlling the lability of uranyl(vi) through intramolecular π-π stacking

A reaction of UO₂²⁺ with cyclohexyldiphenylphosphine oxide (OPCyPh₂) in ethanol resulted in a perchlorate salt of the 4-fold homoleptic complex, [UO₂(OPCyPh₂)₄](ClO₄)₂·EtOH. X-ray structure determination revealed that [UO₂(OPCyPh₂)₄]²⁺ shows a highly symmetric molecular structure supported by the intramolecular π-π stacking interactions between the phenyl groups of the neighbouring OPCyPh₂ in the equatorial plane of UO₂²⁺. To clarify whether the reactivity of UO₂²⁺ is affected by such a unique coordination structure, the ligand exchange kinetics of [UO₂(OPCyPh₂)₄]²⁺ in non-coordinating solvents has been studied using an NMR line-broadening method. In CD₂Cl₂ and CD₃NO₂ solutions, both signals of coordinated and free OPCyPh₂ appeared distinctively even at 298 K, while 2D EXSY experiment provided evidence that a chemical exchange between [UO₂(OPCyPh₂)₄]²⁺ and free OPCyPh₂ occurs in these systems. Thus, this ligand exchange reaction is quite slow on the NMR time-scale. The dependency of the apparent first-order rate constant (k_obs) on temperature and the free OPCyPh₂ concentration suggested that this ligand exchange reaction involves associative and dissociative mechanisms both governed by large negative ΔS^‡ terms predominantly. Compared with the kinetic data of ligand exchange reactions of other UO₂²⁺ complexes reported so far, the lability of UO₂²⁺ in [UO₂(OPCyPh₂)₄]²⁺ is found to be significantly suppressed due to the intramolecular π-π stacking interactions as observed in the crystal structure. The DFT calculations successfully reproduced the intramolecular π-π stacking in [UO₂(OPCyPh₂)₄]²⁺ by considering the empirical dispersion corrections, and provided theoretical rationales to the A and D mechanisms of the ligand exchange reaction of [UO₂(OPCyPh₂)₄]²⁺.