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) (<b>1a</b>) (Tp* = hydrotris­(3,5-dimethylpyrazolyl)­borate) and Tp*₂U­(2,2′-bpy) (<b>1b</b>) both perform the two-electron reduction of diazenes affording η²-hydrazido complexes Tp*₂U­(AzBz) (<b>2-AzBz</b>) (AzBz = azobenzene) and Tp*₂U­(BCC) (<b>2-BCC</b>) (BCC = benzo­[<i>c</i>]­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 <i>trans</i>-bis­(imido) species was possible by using Cp*U­(^MesPDI^Me)­(THF) (<b>3</b>) (^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 <b>3</b> and its substituted analogue, Cp*U­(^<i>t</i>Bu-^MesPDI^Me)­(THF) (<b>3-</b>^{<i><b>t</b></i>}<b>Bu</b>) (^<i>t</i>Bu-^MesPDI^Me = 2,6-((Mes)­NCMe)₂-<i>p</i>-C­(CH₃)₃-C₅H₂N), with benzo­[<i>c</i>]­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 (<b>5</b>) and Cp*U­(BCC)­(^<i>t</i>Bu-^MesPDI^Me) (<b>6</b>). While a transient pyridine­(diimine) triplet diradical in the formation of <b>5</b> results in H atom abstraction and <i>p</i>-pyridine coupling, the <i>tert</i>-butyl moiety in <b>6</b> allows for electronic rearrangement to occur, precluding deleterious pyridine-radical coupling. The monomeric analogue of <b>5</b>, Cp*U­(BCC)­(^MesPDI^Me) (<b>7</b>), was synthesized via salt metathesis from Cp*UI­(^MesPDI^Me) (<b>3-I</b>). 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 <b>3-I</b>, <b>5</b>, and <b>7</b> have been investigated by SQUID magnetometry

Tris(phosphinoamide)-Supported Uranium–Cobalt Heterobimetallic Complexes Featuring Co → U Dative Interactions

A series of tris- and tetrakis­(phosphinoamide) U/Co complexes has been synthesized. The uranium precursors, (η²-Ph₂PN^<i>i</i>Pr)₄U (<b>1</b>), (η²-^<i>i</i>Pr₂PNMes)₄U (<b>2</b>), (η²-Ph₂PN^<i>i</i>Pr)₃UCl (<b>3</b>), and (η²-^<i>i</i>Pr₂PNMes)₃UI (<b>4</b>), were easily accessed via addition of the appropriate stoichiometric equivalents of [Ph₂PN^<i>i</i>Pr]K or [^<i>i</i>Pr₂PNMes]K to UCl₄ or UI₄(dioxane)₂. Although the phosphinoamide ligands in <b>1</b> and <b>4</b> have been shown to coordinate to U in an η²-fashion in the solid state, the phosphines are sufficiently labile in solution to coordinate cobalt upon addition of CoI₂, generating the heterobimetallic Co/U complexes ICo­(Ph₂PNⁱPr)₃U­[η²-Ph₂PNⁱPr] (<b>5</b>), ICo­(ⁱPr₂PNMes)₃U­[η²-(ⁱPr₂PNMes)] (<b>6</b>), ICo­(Ph₂PN^<i>i</i>Pr)₃UI (<b>7</b>), and ICo­(^<i>i</i>Pr₂PNMes)₃UI (<b>8</b>). Structural characterization of complexes <b>5</b> and <b>7</b> reveals reasonably short Co–U interatomic distances, with <b>7</b> exhibiting the shortest transition metal–uranium distance ever reported (2.874(3) Å). Complexes <b>7</b> and <b>8</b> were studied by cyclic voltammetry to examine the influence of the metal–metal interaction on the redox properties compared with both monometallic Co and heterobimetallic Co/Zr complexes. Theoretical studies are used to further elucidate the nature of the transition metal–actinide interaction

Isolated Fe^II on Silica As a Selective Propane Dehydrogenation Catalyst

We report a comparative study of isolated Fe^II, iron oxide particles, and metallic nanoparticles on silica for non-oxidative propane dehydrogenation. It was found that the most selective catalyst was an isolated Fe^II species on silica prepared by grafting the open cyclopentadienide iron complex, bis­(2,4-dimethyl-1,3-pentadienide) iron­(II) or Fe­(<i>o</i>Cp)₂. The grafting and evolution of the surface species was elucidated by ¹H NMR, diffuse reflectance infrared Fourier transform spectroscopy and X-ray absorption spectroscopies. The oxidation state and local structure of surface Fe were characterized by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure. The initial grafting of iron proceeds by one surface hydroxyl Si–OH reacting with Fe­(<i>o</i>Cp)₂ to release one diene ligand (<i>o</i>CpH), generating a SiO₂-bound Fe^II(<i>o</i>Cp) species, <b>1-Fe</b><i><b>o</b></i><b>Cp</b>. Subsequent treatment with H₂ at 400 °C leads to loss of the remaining diene ligand and formation of nanosized iron oxide clusters, <b>1-C</b>. Dispersion of these Fe oxide clusters occurs at 650 °C, forming an isolated, ligand-free Fe^II on silica, <b>1-Fe</b>^<b>II</b>, which is catalytically active and highly selective (∼99%) for propane dehydrogenation to propene. Under reaction conditions, there is no evidence of metallic Fe by in situ XANES. For comparison, metallic Fe nanoparticles, <b>2-NP-Fe</b>^<b>0</b>, were independently prepared by grafting Fe­[N­(SiMe₃)₂]₂ onto silica, <b>2-FeN*</b>, and reducing it at 650 °C in H₂. The Fe NPs were highly active for propane conversion but showed poor selectivity (∼14%) to propene. Independently prepared Fe oxide clusters on silica display a low activity. The sum of these results suggests that selective propane dehydrogenation occurs at isolated Fe^II sites

Computational Insights into Uranium Complexes Supported by Redox-Active α-Diimine Ligands

The electronic structures of two uranium compounds supported by redox-active α-diimine ligands, (^MesDAB^Me)₂U­(THF) (<b>1</b>) and Cp₂U­(^MesDAB^Me) (<b>2</b>) (^MesDAB^Me = [ArNC­(Me)­C­(Me)NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), have been investigated using both density functional theory and multiconfigurational self-consistent field methods. Results from these studies have established that both uranium centers are tetravalent, that the ligands are reduced by two electrons, and that the ground states of these molecules are triplets. Energetically low-lying singlet states are accessible, and some transitions to these states are visible in the electronic absorption spectrum

Computational Insights into Uranium Complexes Supported by Redox-Active α-Diimine Ligands

The electronic structures of two uranium compounds supported by redox-active α-diimine ligands, (^MesDAB^Me)₂U­(THF) (<b>1</b>) and Cp₂U­(^MesDAB^Me) (<b>2</b>) (^MesDAB^Me = [ArNC­(Me)­C­(Me)NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), have been investigated using both density functional theory and multiconfigurational self-consistent field methods. Results from these studies have established that both uranium centers are tetravalent, that the ligands are reduced by two electrons, and that the ground states of these molecules are triplets. Energetically low-lying singlet states are accessible, and some transitions to these states are visible in the electronic absorption spectrum

Computational Insights into Uranium Complexes Supported by Redox-Active α-Diimine Ligands

The electronic structures of two uranium compounds supported by redox-active α-diimine ligands, (^MesDAB^Me)₂U­(THF) (<b>1</b>) and Cp₂U­(^MesDAB^Me) (<b>2</b>) (^MesDAB^Me = [ArNC­(Me)­C­(Me)NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), have been investigated using both density functional theory and multiconfigurational self-consistent field methods. Results from these studies have established that both uranium centers are tetravalent, that the ligands are reduced by two electrons, and that the ground states of these molecules are triplets. Energetically low-lying singlet states are accessible, and some transitions to these states are visible in the electronic absorption spectrum