Dinitrogen cleavage by a dinuclear uranium(iii) complex

Understanding the role of multimetallic cooperativity and of alkali ion-binding in the second coordination sphere is important for the design of complexes that can promote dinitrogen (N₂) cleavage and functionalization. Herein, we compare the reaction products and mechanism of N₂ reduction of the previously reported K₂-bound dinuclear uranium(iii) complex, [K₂{[U^III(OSi(OBu)₃)₃]₂(μ-O)}], B, with those of the analogous dinuclear uranium(iii) complexes, [K(2.2.2-cryptand)][K{U^III(OSi(OBu)₃)₃}₂(μ-O)], 1, and [K(2.2.2-cryptand)]₂[{U^III(OSi(OBu)₃)₃}₂(μ-O)], 2, where one or two K⁺ ions have been removed from the second coordination sphere by addition of 2.2.2-cryptand. In this study, we found that the complete removal of the K⁺ ions from the inner coordination sphere leads to an enhanced reducing ability, as confirmed by cyclic voltammetry studies, of the resulting complex 2, and yields two new species upon N₂ addition, namely the U(iii)/U(iv) complex, [K(2.2.2-cryptand)][{U^III(OSi(OBu)₃)₃}(μ-O){U^IV(OSi(OBu)₃)₃}], 3, and the N₂ cleavage product, the bis-nitride, terminal-oxo complex, [K(2.2.2-cryptand)]₂[{U^V(OSi(OBu)₃)₃}(μ-N)₂{U^VI(OSi(OBu)₃)₂(κ-O)}], 4. We propose that the formation of these two products involves a tetranuclear uranium-N₂ intermediate that can only form in the absence of coordinated alkali ions, resulting in a six-electron transfer and cleavage of N₂, demonstrating the possibility of a three-electron transfer from U(iii) to N₂. These results give an insight into the relationship between alkali ion binding modes, multimetallic cooperativity and reactivity, and demonstrate how these parameters can be tuned to cleave and functionalize N₂.