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(O t Bu) 3 ) 3 ] 2 (μ-O)}], B, with those of the analogous dinuclear uranium(iii) complexes, [K(2.2.2-cryptand)][K{U III (OSi(O t Bu) 3 ) 3 } 2 (μ-O)], 1, and [K(2.2.2-cryptand)] 2 [{U III (OSi(O t Bu) 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 N 2 addition, namely the U(iii)/U(iv) complex, [K(2.2.2-cryptand)][{U III (OSi(O t Bu) 3 ) 3 }(μ-O){U IV (OSi(O t Bu) 3 ) 3 }], 3, and the N 2 cleavage product, the bis-nitride, terminal-oxo complex, [K(2.2.2-cryptand)] 2 [{U V (OSi(O t Bu) 3 ) 3 }(μ-N) 2 {U VI (OSi(O t Bu) 3 ) 2 (κ-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 .