Activation of carbon dioxide by a terminal uranium–nitrogen bond in the gas-phase: a demonstration of the principle of microscopic reversibility

Abstract

Activation of CO₂ is demonstrated by its spontaneous dissociative reaction with the gas-phase anion complex NUOCl₂⁻, which can be considered as NUO⁺ coordinated by two chloride anion ligands. This reaction was previously predicted by density functional theory to occur exothermically, without barriers above the reactant energy. The present results demonstrate the validity of the prediction of microscopic reversibility, and provide a rare case of spontaneous dissociative addition of CO₂ to a gas-phase complex. The activation of CO₂ by NUOCl₂⁻ proceeds by conversion of a UN bond to a UO bond and creation of an isocyanate ligand to yield the complex UO₂(NCO)Cl₂⁻, in which uranyl, UO₂²⁺, is coordinated by one isocyanate and two chloride anion ligands. This activation of CO₂ by a uranium(VI) nitride complex is distinctive from previous reports of oxidative insertion of CO₂ into lower oxidation state U(III) or U(IV) solid complexes, during which both C–O bonds remain intact. This unusual observation of spontaneous addition and activation of CO₂ by NUOCl₂⁻ is a result of the high oxophilicity of uranium. If the computed Gibbs free energy of the reaction pathway, rather than the energy, is considered, there are barriers above the reactant asymptotes such that the observed reaction should not proceed under thermal conditions. This result provides a demonstration that energy rather than Gibbs free energy determines reactivity under low-pressure bimolecular conditions.