Nitrile Coupling Mediated by Cyclometalated Uranium Alkyl Complex: The Ketimide-Enamine Seesaw Mechanism

Metal alkyls play a crucial role in synthetic chemistry. Although s-, p-, and d-block metal alkyls have been extensively investigated, the reactivity of f-block actinide alkyls has received relatively limited attention. In this work, we systematically investigate the insertion reactivity of a four-membered cyclometalated uranium(IV) alkyl complex (1) with various nitriles, including 2-naphthonitrile, 4-cyanopyridine, and 2-cyanopyridine, yielding coupled products 2-4, respectively. Mechanistic studies reveal that stoichiometric nitrile addition generates a uranium-ketimide intermediate (5), which undergoes facile 1,3-H migration at room temperature to produce a uranium-enamine complex (6). Further reactions of the uranium-ketimide complex 5a with 2-naphthonitrile, 4-cyanopyridine, and 2-cyanopyridine yield complexes 2, 7, and 8, respectively. In contrast, reactions of the uranium-enamine complex 6a with the same nitriles result in complexes 9, 10, and 11, all of which feature a β-diketiminate ligand formed by nitrile coupling with the 2-naphthonitrile motif of 6a. Theoretical analyses highlight the critical role of the 1,3-migration intermediate 6a in facilitating nitrile coupling reactions. These results suggest a ketimide-enamine seesaw mechanism for the nitrile coupling in this system. Moreover, the uranium-enamine complex 6a exhibits broad electrophile compatibility, efficiently engaging in cross-coupling reactions with benzophenone and CS₂.