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*U(CHPh) (1a) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) and Tp*U(2,2'-bpy) (1b) both perform the two-electron reduction of diazenes affording η-hydrazido complexes Tp*U(AzBz) (2-AzBz) (AzBz = azobenzene) and Tp*U(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(PDI)(THF) (3) (PDI = 2,6-((Mes)N═CMe)-CHN, 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(Bu-PDI)(THF) (3-Bu) (Bu-PDI = 2,6-((Mes)N═CMe)-p-C(CH)-CHN), 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)(HPDI)] (5) and Cp*U(BCC)(Bu-PDI) (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)(PDI) (7), was synthesized via salt metathesis from Cp*UI(PDI) (3-I). All complexes have been characterized by H 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.