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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 a...

This study details the surface reaction chemistry relevant to the vapor deposition mechanism of SnO2 thin films by atomic layer deposition. The mechanism was elucidated by combining different spectroscopic studies. Initial nucleation of cyclic N2,N3-di-tert-butylbutane-2,3-diamidotin(II) (1) consists of surface SiOH protonation of ligands as shown by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This SiO2-surface-bound stannylene was further characterized by X-ray absorption (XAS) and resonance Raman spectroscopy. XAS, DRIFTS, and Raman spectroscopy were then used to follow the further reaction of the surface-bound stannylene with different oxygen sources and a second equivalent of 1. It was observed that water does not oxidize the initial surface-bound tin site, and a well-defined, three-coordinate tin(II) species, with two surface oxygen bonds and one coordinated water molecule, was characterized. Treatment of the surface stannylene with protic oxidants such as H2O2 or tBuOOH ful...

We report a comparative study of isolated FeII, iron oxide particles, and metallic nanoparticles on silica for non-oxidative propane dehydrogenation. It was found that the most selective catalyst was an isolated FeII species on silica prepared by grafting the open cyclopentadienide iron complex, bis(2,4-dimethyl-1,3-pentadienide) iron(II) or Fe(oCp)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(oCp)2 to release one diene ligand (oCpH), generating a SiO2-bound FeII(oCp) species, 1-FeoCp. Subsequent treatment with H2 at 400 °C leads to loss of the remaining diene ligand and formation of nanosized iron oxide clusters, 1-C. Dispersion ...

Five different first-row transition metal precursors (V(III), Cr(III), Mn(II), Co(II), Ni(II)) were successfully incorporated into a catechol porous organic polymer (POP) and characterized using ATR-IR and XAS analysis. The resulting metallated POPs were then evaluated for catalytic alkyne hydrogenation using high-throughput screening techniques. All POPs were unexpectedly found to be active and selective catalysts for alkyne semihydrogenation. Three of the metallated POPs (V, Cr, Mn) are the first of their kind to be active single-site hydrogenation catalysts. These results highlight the advantages of using a POP platform to develop new catalysts which are otherwise difficult to achieve through traditional heterogeneous and homogeneous routes.

A single-site, rhodium(I) catecholate containing porous organic polymer was prepared and utilized as an active catalyst for the hydrogenation of olefins in both liquid-phase and gas-phase reactors. Liquid-phase, batch hydrogenation reactions at 50 psi and ambient temperatures result in the formation of rhodium metal nanoparticles supported within the polymer framework. Surprisingly, the Rh(I) complex is catalytically active and stable for propene hydrogenation at ambient temperatures under gas-phase conditions, where reduction of the Rh(I) centers to Rh(0) nanoparticles requires at least 200–250 °C under a flow of hydrogen gas. After high-temperature treatment, the Rh(0) nanoparticles are active arene hydrogenation catalysts that convert toluene to methylcyclohexadiene at a rate of 9.3 × 10–3 mol g–1 h–1 of rhodium metal at room temperature. Conversely, single-site Rh(I) is an active and stable catalyst for the hydrogenation of propylene (but not toluene) under gas-phase conditions at room temperature.

The oxidation state and coordination number of immobilized iron catecholate EtO2Fe(CAT-POP) were determined by X-ray absorption spectroscopy (XAS) under a variety of conditions. We find the as-prepared material to be three-coordinate Fe2+ that readily oxidizes to Fe3+ upon exposure to air but remains three-coordinate. Both the reduced and oxidized Fe(CAT-POP) catalyze olefin hydrogenation in batch and flow reactors. We determined the catalytic rates for both species and also observed by means of XAS that the oxidation state of the iron centers does not change in hydrogen at the reaction temperature. Therefore, we postulate that the mechanism of hydrogenation by Fe(CAT-POP) proceeds through one of several possible nonclassical mechanisms, which are discussed.

The insertion chemistry of U(CH2C6H5)4 (1) was explored with acetone, benzophenone, mesityl azide, and 1-azidoadamantane. Using 2 equiv of acetone affords the double-insertion product U[OC(CH3)2(CH2C6H5)]2(CH2C6H5)2(THF)2 (2), while using 4 equiv results in the tri-inserted enolate product U[OC(CH3)2(CH2C6H5)]3[OC(CH3)CH2](THF)3 (3). Deuterium labeling experiments aided in the assignment of 2 and 3. With 4 equiv of benzophenone, insertion at all U–C bonds is noted, forming U[OC(C6H5)2(CH2C6H5)]4 (4) and the THF adduct U[OC(C6H5)2(CH2C6H5)]4(THF) (4-THF). Addition of 4 equiv of N3Mes to 1 forms the tetrakis(triazenido)uranium(IV) complex U[CH2(C6H5)NNN(Mes)-κ2N1,2][CH2(C6H5)NNN(Mes)-κ2N1,3]3 (5), while the same reaction with 1-azidoadamantane generates the uranium(VI) trans-bis(imido) complex U(NAd)2[CH2(C6H5)NNN(Ad)-κ2N1,3]2(THF) (6). All species were characterized by 1H NMR and infrared spectroscopy, with select examples being structurally characterized using single-crystal X-ray diffraction.

A single-site, iron catecholate-containing porous organic polymer was prepared and utilized as a stable and remarkably active catalyst for the hydrosilylation of ketones and aldehydes. In some instances, catalyst loadings of 0.043–2.1 mol % [Fe] were sufficient for complete hydrosilylation of aldehydes and ketones within 15 min at room temperature. The catalyst can be recycled at least three times without a drop in catalytic activity. This system is an example of an immobilized homogeneous catalyst with no homogeneous analogue.

The synthesis, characterization, and reactivity of the homoleptic uranium(IV) alkyls U(CH(2)C(6)H(5))(4) (1-Ph), U(CH(2)-p-CH(3)C(6)H(4))(4) (1-p-Me), and U(CH(2)-m-(CH(3))(2)C(6)H(3))(4) (1-m-Me(2)) are reported. The addition of 4 equiv of K(CH(2)Ar) (Ar = Ph, p-CH(3)C(6)H(4), m-(CH(3))(2)C(6)H(3)) to UCl(4) at -108 °C produces 1-Ph in good yields and 1-p-Me and 1-m-Me(2) in moderate yields. Further characterization of 1-Ph by X-ray crystallography confirmed η(4)-coordination of each benzyl ligand to the uranium center. Magnetic studies produced an effective magnetic moment of 2.60 μ(B) at 23 °C, which is consistent with a tetravalent uranium 5f(2) electronic configuration. Addition of 1 equiv of the redox-active α-diimine (Mes)DAB(Me) ((Mes)DAB(Me) = [ArN═C(Me)C(Me)═NAr]; Ar = 2,4,6-trimethylphenyl (Mes)) to 1-Ph results in reductive elimination of 1 equiv of bibenzyl (PhCH(2)CH(2)Ph), affording ((Mes)DAB(Me))U(CH(2)C(6)H(5))(2) (2-Ph). Treating an equimolar mixture of 1-Ph and 1-Ph-d(28) with (Mes)DAB(Me) forms the products from monomolecular reductive elimination, 2-Ph, 2-Ph-d(14), bibenzyl, and bibenzyl-d(14). This is confirmed by (1)H NMR spectroscopy and GC/MS analysis of both organometallic and organic products. Addition of 1 equiv of 1,2-bis(dimethylphosphino)ethane (dmpe) to 1-Ph results in formation of the previously synthesized (dmpe)U(CH(2)C(6)H(5))(4) (3-Ph), indicating the redox-innocent chelating phosphine stabilizes the uranium center in 3-Ph and prevents reductive elimination of bibenzyl. Full characterization for 3-Ph, including X-ray crystallography, is reported.

The electronic structures of two uranium compounds supported by redox-active α-diimine ligands, ((Mes)DAB(Me))(2)U(THF) (1) and Cp(2)U((Mes)DAB(Me)) (2) ((Mes)DAB(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.

Uranium compounds supported by redox-active α-diimine ligands, which have methyl groups on the ligand backbone and bulky mesityl substituents on the nitrogen atoms {(Mes)DAB(Me) = [ArN═C(Me)C(Me)═NAr], where Ar = 2,4,6-trimethylphenyl (Mes)}, are reported. The addition of 2 equiv of (Mes)DAB(Me), 3 equiv of KC(8), and 1 equiv of UI(3)(THF)(4) produced the bis(ligand) species ((Mes)DAB(Me))(2)U(THF) (1). The metallocene derivative, Cp(2)U((Mes)DAB(Me)) (2), was generated by the addition of an equimolar ratio of (Mes)DAB(Me) and KC(8) to Cp(3)U. The bond lengths in the molecular structure of both species confirm that the α-diimine ligands have been doubly reduced to form ene-diamide ligands. Characterization by electronic absorption spectroscopy shows weak, sharp transitions in the near-IR region of the spectrum and, in combination with the crystallographic data, is consistent with the formulation that tetravalent uranium ions are present and supported by ene-diamide ligands. This interpretation was verified by U L(III)-edge X-ray absorption near-edge structure (XANES) spectroscopy and by variable-temperature magnetic measurements. The magnetic data are consistent with singlet ground states at low temperature and variable-temperature dependencies that would be expected for uranium(IV) species. However, both complexes exhibit low magnetic moments at room temperature, with values of 1.91 and 1.79 μ(B) for 1 and 2, respectively. Iodomethane was used to test the reactivity of 1 and 2 for multielectron transfer. While 2 showed no reactivity with CH(3)I, the addition of 2 equiv of iodomethane to 1 resulted in the formation of a uranium(IV) monoiodide species, ((Mes)DAB(Me))((Mes)DAB(Me2))UI {3; (Mes)DAB(Me2) = [ArN═C(Me)C(Me(2))NAr]}, which was characterized by single-crystal X-ray diffraction and U M(4)- and M(5)-edge XANES. Confirmation of the structure was also attained by deuterium labeling studies, which showed that a methyl group was added to the ene-diamide ligand carbon backbone.

The uranium(IV) terminal oxo species Tp*(2)U(O) has been synthesized by oxygen-atom transfer from pyridine-N-oxide to Tp*(2)U(2,2'-bipyridine), a trivalent uranium species with a monoanionic bipyridine ligand. Full characterization of the oxo species using (1)H NMR and IR spectroscopies, X-ray crystallography, and computational studies was performed.

A series of tris- and tetrakis(phosphinoamide) U/Co complexes has been synthesized. The uranium precursors, (η(2)-Ph2PN(i)Pr)4U (1), (η(2)-(i)Pr2PNMes)4U (2), (η(2)-Ph2PN(i)Pr)3UCl (3), and (η(2)-(i)Pr2PNMes)3UI (4), were easily accessed via addition of the appropriate stoichiometric equivalents of [Ph2PN(i)Pr]K or [(i)Pr2PNMes]K to UCl4 or UI4(dioxane)2. Although the phosphinoamide ligands in 1 and 4 have been shown to coordinate to U in an η(2)-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(Ph2PN(i)Pr)3U[η(2)-Ph2PN(i)Pr] (5), ICo((i)Pr2PNMes)3U[η(2)-((i)Pr2PNMes)] (6), ICo(Ph2PN(i)Pr)3UI (7), and ICo((i)Pr2PNMes)3UI (8). Structural characterization of complexes 5 and 7 reveals reasonably short Co-U interatomic distances, with 7 exhibiting the shortest transition metal-uranium distance ever reported (2.874(3) Å). Complexes 7 and 8 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.

Hydrotris(3,5-dimethylpyrazolyl)borate uranium(III) diiodide derivatives have been prepared as an entry into low-valent uranium chemistry with these ligands. The bis(tetrahydrofuran) adduct, Tp*UI(2)(THF)(2) (1) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), was synthesized by addition of sodium hydrotris(3,5-dimethylpyrazolyl)borate (NaTp*) to an equivalent of UI(3)(THF)(4). Addition of 2,2'-bipyridine (2,2'-bpy) to 1 displaced the THF molecules producing Tp*UI(2)(2,2'-bpy) (2). Both derivatives were characterized by (1)H NMR and IR spectroscopies, magnetic measurements, and X-ray crystallography. Reduction of both species was attempted with two equivalents of potassium graphite. The reduction of 1 did not result in a clean product, but rather decomposition and ligand redistribution. However, compound 2 was reduced to form Tp*(2)U(2,2'-bpy), 3, which is composed of a uranium(III) ion with a radical monoanionic bipyridine ligand. This was confirmed by X-ray crystallography, which revealed distortions in the bond lengths of the bipyridine consistent with reduction. Further support was obtained by (1)H NMR spectroscopy, which showed resonances shifted far upfield, consistent with radical character on the 2,2'-bipyridine ligand. Future studies will explore the reactivity of this compound as well as the consequences for redox-activity in the bipyridine ligand.