Elucidating the Mechanism of Uranium Mediated Diazene N═N Bond Cleavage

Investigation into the reactivity of reduced uranium species toward diazenes has revealed key intermediates in the four-electron cleavage of azobenzene. Trivalent Tp*₂U(CH₂Ph) (1a) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) and Tp*₂U(2,2'-bpy) (1b) both perform the two-electron reduction of diazenes affording η²-hydrazido complexes Tp*₂U(AzBz) (2-AzBz) (AzBz = azobenzene) and Tp*₂U(BCC) (2-BCC) (BCC = benzo[c]cinnoline) in contrast to precursors of the bis(Cp*) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide) ligand framework. The four-electron cleavage of diazenes to give trans-bis(imido) species was possible by using Cp*U(^MesPDI^Me)(THF) (3) (^MesPDI^Me = 2,6-((Mes)N═CMe)₂-C₅H₃N, Mes = 2,4,6-trimethylphenyl), which is supported by a highly reduced trianionic chelate that undergoes electron transfer. This proceeds via concerted addition at a single uranium center supported by both a crossover experiment and through addition of an asymmetrically substituted diazene, Ph-N═N-Tol. Further investigation of 3 and its substituted analogue, Cp*U(^tBu-^MesPDI^Me)(THF) (3-^tBu) (^tBu-^MesPDI^Me = 2,6-((Mes)N═CMe)₂-p-C(CH₃)₃-C₅H₂N), with benzo[c]cinnoline, revealed that the four-electron cleavage occurs first by a single electron reduction of the diazene with the redox chemistry performed solely at the redox-active pyridine(diimine) to form dimeric [Cp*U(BCC)(^MesHPDI^Me)]₂ (5) and Cp*U(BCC)(^tBu-^MesPDI^Me) (6). While a transient pyridine(diimine) triplet diradical in the formation of 5 results in H atom abstraction and p-pyridine coupling, the tert-butyl moiety in 6 allows for electronic rearrangement to occur, precluding deleterious pyridine-radical coupling. The monomeric analogue of 5, Cp*U(BCC)(^MesPDI^Me) (7), was synthesized via salt metathesis from Cp*UI(^MesPDI^Me) (3-I). All complexes have been characterized by ¹H NMR and electronic absorption spectroscopies, X-ray diffraction, and, where pertinent, EPR spectroscopy. Further, the electronic structures of 3-I, 5, and 7 have been investigated by SQUID magnetometry.