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-2(CH2Ph) (la) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) and Tp*U-2(2,2'-bpy) (lb) both perform the two-electron reduction of diazenes affording eta(2)-hydrazido complexes Tp*U-2(AzBz) (2-AzBz) (AzBz = azobenzene) and Tp*U-2(BCC) (2-BCC) (BCC = benzo[c]cinnoline) in contrast to precursors of the bis(Cp*) (Cp* = l,2,3,4,5-pentamethylcyclo-pentadienide) ligand framework. The four-electron cleavage of diazenes to give trans-bis(imido) species was possible by using Cp*U((PDIMe)-P-mes)(THF) (3) ((PDIMe)-P-mes = 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(Bu-t-(PDIme)-P-mes)(THF) (3-Bu-t) (tBu-mesPDIme = 2,64(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)((HPDIMe)-H-mes)], (5) and Cp*U(BCC)(Bu-t-(PDIMe)-P-mes) (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)((PDIMe)-P-Mes) (7), was synthesized via salt metathesis from Cp*UI(MesPDIMe) (3-I). All complexes have been characterized by H-1 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.