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(MesPDIMe)­(THF) (3) (MesPDIMe = 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-MesPDIMe)­(THF) (3-tBu) (tBu-MesPDIMe = 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)­(MesHPDIMe)]2 (5) and Cp*U­(BCC)­(tBu-MesPDIMe) (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)­(MesPDIMe) (7), was synthesized via salt metathesis from Cp*UI­(MesPDIMe) (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.

关于还原态铀物种（reduced uranium species）对偶氮类化合物（diazenes）的反应性研究，揭示了偶氮苯（azobenzene）四电子裂解过程中的关键中间体。三价配合物Tp*₂U(CH₂Ph)（1a，其中Tp*为氢三(3,5-二甲基吡唑基)硼酸根（hydrotris(3,5-dimethylpyrazolyl)borate））与Tp*₂U(2,2′-联吡啶)（1b）均可实现偶氮类化合物的两电子还原，生成η²-肼基配合物Tp*₂U(AzBz)（2-AzBz，AzBz=偶氮苯）与Tp*₂U(BCC)（2-BCC，BCC=苯并[c]噌啉（benzo[c]cinnoline）），这与双(五甲基环戊二烯基)（bis(Cp*)）配体骨架的前驱体反应路径截然不同，其中Cp*为1,2,3,4,5-五甲基环戊二烯基（1,2,3,4,5-pentamethylcyclopentadienide）。使用Cp*U(MesPDIMe)(THF)（3，其中MesPDIMe=2,6-双((2,4,6-三甲基苯基)亚胺基异丙基)吡啶（2,6-((Mes)N=CMe)₂-C₅H₃N），Mes=2,4,6-三甲基苯基（2,4,6-trimethylphenyl），THF为四氢呋喃（tetrahydrofuran））可实现偶氮类化合物的四电子裂解，生成反式双(亚胺基)物种；该体系由高度还原的三阴离子螯合物配体支撑，可发生电子转移。该反应通过单铀中心的协同加成过程进行，这一结论得到了交叉实验以及不对称取代偶氮化合物Ph-N=N-Tol加成实验的佐证。对3及其取代类似物Cp*U(tBu-MesPDIMe)(THF)（3-tBu，其中tBu-MesPDIMe=2,6-双((2,4,6-三甲基苯基)亚胺基异丙基)对叔丁基吡啶（2,6-((Mes)N=CMe)₂-p-C(CH₃)₃-C₅H₂N））与苯并[c]噌啉的进一步研究表明，四电子裂解首先通过偶氮类化合物的单电子还原启动，氧化还原化学仅在氧化还原活性吡啶(二亚胺)配体上发生，生成二聚体[Cp*U(BCC)(MesHPDIMe)]₂（5）与Cp*U(BCC)(tBu-MesPDIMe)（6）。在5的形成过程中，瞬态吡啶(二亚胺)三重态双自由基会引发氢原子攫取以及对位吡啶环偶联，而6中的叔丁基基团可促进电子重排，从而避免了有害的吡啶自由基偶联。5的单体类似物Cp*U(BCC)(MesPDIMe)（7）可通过Cp*UI(MesPDIMe)（3-I）的复分解反应合成。所有配合物均通过¹H核磁共振波谱、电子吸收光谱、X射线衍射进行了表征，在适宜条件下还采用了电子顺磁共振（EPR）光谱。此外，通过超导量子干涉装置（SQUID）磁学法对3-I、5和7的电子结构进行了研究。