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 (N-2) cleavage and functionalization. Herein, we compare the reaction products and mechanism of N-2 reduction of the previously reported K-2-bound dinuclear uranium(III) complex, [K-2{U-III(OSi((OBu)-Bu-t)(3))(3)(mu-O)}], B, with those of the analogous dinuclear uranium(III) complexes, [K(2.2.2-cryptand)][K{U-III(OSi((OBu)-Bu-t)(3))(3)}(2)(mu-O)], 1, and K(2.2.2-cryptand)[{U-III(OSi((OBu)-Bu-t)(3))(3)}(2)(mu-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 N-2 addition, namely the U(III)/U(IV) complex, [K(2.2.2-cryptand)][{U-III(OSi((OBu)-Bu-t)(3))(3)}(mu-O){U-IV(OSi((OBu)-Bu-t)(3))(3)}], 3, and the N-2 cleavage product, the bis-nitride, terminal-oxo complex, K(2.2.2-cryptand)[{U-V(OSi((OBu)-Bu-t)(3))(3)}(mu-N)(2){U-VI(OSi((OBu)-Bu-t)(3))(2)(kappa-O)}], 4. We propose that the formation of these two products involves a tetranuclear uranium-N-2 intermediate that can only form in the absence of coordinated alkali ions, resulting in a six-electron transfer and cleavage of N-2, demonstrating the possibility of a three-electron transfer from U(III) to N-2. 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 N-2.