Additive Effects in Metal/Lewis Acid Cooperativity Assessed in a Tetrahedral Copper Hydrazine Complex Featuring an Appended Borane.

Within metal/ligand cooperative systems employing acidic groups, studies that empirically assess distance relationships are needed to maximize cooperative interactions with substrates. We report the formation of two Cu(I)-NH complexes using 1,4,7-triazacyclononane ligand frameworks bearing two -butyl groups and either a Lewis acidic trialkylborane or an inert alkyl group. Metal/Lewis acid cooperativity imparts heightened acidification of the hydrazine substrate and plays a key role in the release of substrate to a competitive Lewis acidic group.

Synthesis, Characterization, and Reduction of Thorium Pyridinediimine Complexes.

A family of thorium complexes featuring the redox-noninnocent pyridinediimine ligand PDI was synthesized, including (PDI)ThCl (), (PDI)ThCl(THF) (), (PDI)ThCl(THF) () and [(PDI)Th(THF)] () Full characterization of these species shows that these complexes feature PDI in four different oxidation states. The electronic structures of these complexes have been explored using H NMR and electronic absorption spectroscopies, X-ray crystallography, and SQUID magnetometry where appropriate.

A Bidentate Ligand Featuring Ditopic Lewis Acids in the Second Sphere for Selective Substrate Capture and Activation.

We present a ligand platform featuring appended ditopic Lewis acids to facilitate capture/activation of diatomic substrates. We show that incorporation of two 9-borabicyclo[3.3.

Distinct Reactivity Modes of a Copper Hydride Enabled by an Intramolecular Lewis Acid.

We disclose a 1,4,7-triazacyclononane (TACN) ligand featuring an appended boron Lewis acid. Metalation with Cu(I) affords a series of tetrahedral complexes including a boron-capped cuprous hydride. We demonstrate distinct reactivity modes as a function of chemical oxidation: hydride transfer to CO in the copper(I) state and oxidant-induced H evolution as well as alkyne reduction.

Hydrazone and Oxime Olefination via Ruthenium Alkylidenes.

We describe the development of an efficient method for the olefination of hydrazones and oximes. The key design approach that enables this transformation is tuning of the energy/polarity of C=N π-bonds by employing heteroatom functionalities (NR , OR). The resulting hydrazones or oximes facilitate olefination with ruthenium alkylidenes.

Substrate Specific Metal-Ligand Cooperative Binding: Considerations for Weak Intramolecular Lewis Acid/Base Pairs.

Metal-ligand cooperative binding modes were interrogated in a series of zinc bis(thiophenoxide) complexes. A weak B-S binding interaction is observed in solution between the weakly Lewis basic thiophenoxide ligands and an appended trialkylborane. The energy of this binding event is dependent upon the strength of the Lewis acid and its proximity to the zinc thiophenoxide.

Mobility of Lewis acids within the secondary coordination sphere: toward a model for cooperative substrate binding.

Distance dependence of appended Lewis acids in NH binding and deprotonation was evaluated within a series of zinc complexes. Variation of spacer-length to a tethered trialkylborane Lewis acid revealed distinct preferences for binding and stabilization of the resulting deprotonated NH unit.

Examining the Generality of Metal-Ligand Cooperativity Across a Series of First-Row Transition Metals: Capture, Bond Activation, and Stabilization.

We outline the generality and requirements for cooperative NH capture, N-N bond scission, and amido stabilization across a series of first-row transition metal complexes bearing a pyridine(dipyrazole) ligand. This ligand contains a pair of flexibly tethered trialkylborane Lewis acids that enable hydrazine capture and M-NH stabilization. While the Lewis acids are required to bind NH, the identity of the metal dictates whether N-N bond scission can occur.

Tetrahedral iron featuring an appended Lewis acid: distinct pathways for the reduction of hydroxylamine and hydrazine.

We present the preparation of a nitrogen-based bidentate ligand featuring an appended boron Lewis acid as well as its tetrahedral Fe2+ and Zn2+ complexes. These complexes act as platforms for hydrazine and hydroxylamine capture and reduction chemistry.

Tuning ligand field strength with pendent Lewis acids: access to high spin iron hydrides.

Geometrically flexible 9-borabicyclo[3.3.1]nonyl units within the secondary coordination sphere enable isolation of high-spin Fe(ii)-dihydrides stabilized by boron-hydride interactions and a rare example of an isolable = 3/2 reduction product.

Requirements for Lewis Acid-Mediated Capture and N-N Bond Cleavage of Hydrazine at Iron.

An iron complex bearing a pyridine(dicarbene) pincer was designed to probe the requirements of Lewis acid-enabled NH capture and subsequent N-N bond cleavage. Appended boron Lewis acids were installed by two methods to circumvent the incompatibilities associated with Lewis acid/base quenching of free carbenes and boranes. NH capture by borane Lewis acids is dependent on both the Lewis acidity and the steric profile about boron.

Hydrogen Bonds Dictate O Capture and Release within a Zinc Tripod.

Six directed hydrogen bonding (H-bonding) interactions allow for the reversible capture and reduction of dioxygen to a trans-1,2-peroxo within a tripodal zinc(II) framework. Spectroscopic studies of the dizinc peroxides, as well as on model zinc diazides, suggest H-bonding contributions serve a dominant role for the binding/activation of these small molecules.

Tailoring the Electronic Structure of Uranium Mono(imido) Species through Ligand Variation.

Uranium mono(imido) species have been prepared via the oxidation of Cp*U(PDI)(THF) (1-Cp*) and [CpU(PDI)] (1-Cp), where Cp* = η-1,2,3,4,5-pentamethylcyclopentadienide, Cp = 1-(7,7-dimethylbenzyl)cyclopentadienide, PDI = 2,6-[(Mes)N═CMe]CHN, and Mes = 2,4,6-trimethylphenyl, with organoazides. Treating either with NDIPP (DIPP = 2,6-diisopropylphenyl) formed uranium(IV) mono(imido) complexes, CpU(NDIPP)(PDI) (2-Cp) and Cp*U(NDIPP)(PDI) (2-Cp*), featuring reduced [PDI]. The addition of electron-donating 1-azidoadamantane (NAd) to 1-Cp* generated a dimeric product, [Cp*U(NAd)(HPDI)] (3), from radical coupling at the p-pyridine position of the pyridine(diimine) ligand and H-atom abstraction, formed through a monomeric intermediate that was observed in solution but could not be isolated.

Hydrazine Capture and N-N Bond Cleavage at Iron Enabled by Flexible Appended Lewis Acids.

Incorporation of two 9-borabicyclo[3.3.1]nonyl substituents within the secondary coordination sphere of a pincer-based Fe(II) complex provides Lewis acidic sites capable of binding 1 or 2 equiv of NH.

Facile Reductive Silylation of UO to Uranium(IV) Chloride.

General reductive silylation of the UO cation occurs readily in a one-pot, two-step stoichiometric reaction at room temperature to form uranium(IV) siloxides. Addition of two equivalents of an alkylating reagent to UO X (L) (X=Cl, Br, I, OTf; L=triphenylphosphine oxide, 2,2'-bipyridyl) followed by two equivalents of a silyl (pseudo)halide, R Si-X (R=aryl, alkyl, H; X=Cl, Br, I, OTf, SPh), cleanly affords (R SiO) UX (L) in high yields. Support is included for the key step in the process, reduction of U to U .

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.

Examining the Effects of Ligand Variation on the Electronic Structure of Uranium Bis(imido) Species.

Arylazide and diazene activation by highly reduced uranium(IV) complexes bearing trianionic redox-active pyridine(diimine) ligands, [CpU(PDI)] (1-Cp), Cp*U(PDI)(THF) (1-Cp*) (Cp = 1-(7,7-dimethylbenzyl)cyclopentadienide; Cp* = η-1,2,3,4,5-pentamethylcyclopentadienide), and Cp*U(Bu-PDI) (THF) (1-Bu) (2,6-((Mes)N═CMe)2-p-R-CHN, Mes = 2,4,6-trimethylphenyl; R = H, PDI; R = C(CH), Bu-PDI), has been investigated. While 1-Cp* and 1-Cp readily reduce NR (R = Ph, p-tolyl) to form trans-bis(imido) species, CpU(NAr)(PDI) (Ar = Ph, 2-Cp; Ar = p-Tol, 3-Cp) and Cp*U(NPh)(PDI) (2-Cp*), only 1-Cp* can cleave diazene N═N double bonds to form the same product. Complexes 2-Cp*, 2-Cp, and 3-Cp are uranium(V) trans-bis(imido) species supported by neutral [PDI] ligands formed by complete oxidation of [PDI] ligands of 1-Cp and 1-Cp*.

Synthesis, Characterization, and Stoichiometric U-O Bond Scission in Uranyl Species Supported by Pyridine(diimine) Ligand Radicals.

Two uranium(VI) uranyl compounds, Cp*UO2((Mes)PDI(Me)) (3) and Cp*UO2((t)Bu-(Mes)PDI(Me)) (3-(t)Bu) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide; (Mes)PDI(Me) = 2,6-((Mes)N=CMe)2C5H3N; (t)Bu-(Mes)PDI(Me) = 2,6-((Mes)N=CMe)2-p-C(CH3)3C5H2N; Mes = 2,4,6-trimethylphenyl), have been synthesized by addition of N-methylmorpholine N-oxide to trianionic pyridine(diimine) uranium(IV) precursors, Cp*U((Mes)PDI(Me))(THF) (1), Cp*U((Mes)PDI(Me))(HMPA) (1-HMPA), and Cp*U((t)Bu-(Mes)PDI(Me))(THF) (1-(t)Bu). These uranyl complexes contain singly reduced pyridine(diimine) ligands suggesting formation occurs via cooperative ligand/metal oxidation. Treating 3 or 3-(t)Bu with stoichiometric equivalents of Me3SiI results in stepwise oxo silylation to form (Me3SiO)2UI2((Mes)PDI(Me)) (5) or (Me3SiO)UI2((t)Bu-(Mes)PDI(Me)) (5-(t)Bu), respectively.

Investigation of Uranium Tris(imido) Complexes: Synthesis, Characterization, and Reduction Chemistry of [U(NDIPP)3(thf)3].

Addition of KC8 to trivalent [UI3(thf)4] in the presence of three equivalents of 2,6-diisopropylphenylazide (N3DIPP) results in the formation of the hexavalent uranium tris(imido) complex [U(NDIPP)3(thf)3] (1) through a facile, single-step synthesis. The X-ray crystal structure shows an octahedral complex that adopts a facial orientation of the imido substituents. This structural trend is maintained during the single-electron reduction of 1 to form dimeric [U(NDIPP)3{K(Et2O)}]2 (2).

Trivalent uranium phenylchalcogenide complexes: exploring the bonding and reactivity with CS2 in the Tp*2UEPh series (E = O, S, Se, Te).

The trivalent uranium phenylchalcogenide series, Tp*2UEPh (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, E = O (1), S (2), Se (3), Te (4)), has been synthesized to investigate the nature of the U-E bond. All compounds have been characterized by (1)H NMR, infrared and electronic absorption spectroscopies, and in the case of 4, X-ray crystallography. Compound 4 was also studied by SQUID magnetometry.

Harnessing redox activity for the formation of uranium tris(imido) compounds.

Classically, late transition-metal organometallic compounds promote multielectron processes solely through the change in oxidation state of the metal centre. In contrast, uranium typically undergoes single-electron chemistry. However, using redox-active ligands can engage multielectron reactivity at this metal in analogy to transition metals.

Isolation of a uranium(III) benzophenone ketyl radical that displays redox-active ligand behaviour.

The first uranium(III) charge separated ketyl radical complex, Tp*2U(OC·Ph2), has been isolated and characterized by infrared, (1)H NMR, and electronic absorption spectroscopies, along with X-ray crystallography. Tp*2U(OC·Ph2) is a potent two-electron reductant towards N3Mes (Mes = 2,4,6-trimethylphenyl) and (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO), with reducing equivalents derived from the metal centre and the redox-active benzophenone.

Utility of a redox-active pyridine(diimine) chelate in facilitating two electron oxidative addition chemistry at uranium.

Exposure of the uranium(IV) complex, Cp(P)U((Mes)PDI(Me)) (1) ((Mes)PDI(Me) = 2,6-((Mes)N=CMe)2–C5H3N; Mes = 2,4,6-trimethylphenyl; Cp(P) = 1-(7,7-dimethylbenzyl)cyclopentadienyl), which contains a [(Mes)PDI(Me)](3−) chelate, to I2, Cl2, PhSeCl, and PhEEPh (E = S, Se, Te) results in oxidative addition to form the uranium(IV) family, Cp(P)U(XX′)((Mes)PDI(Me)) (X = X′ = I, Cl, EPh; X = SePh, X′ = Cl). Spectroscopic and structural studies support products with [(Mes)PDI(Me)](1−), indicating the reducing equivalents derive from this redox-active chelate.

Multielectron C-O bond activation mediated by a family of reduced uranium complexes.

A family of cyclopentadienyl uranium complexes supported by the redox-active pyridine(diimine) ligand, (Mes)PDI(Me) ((Mes)PDI(Me) = 2,6-((Mes)N═CMe)2-C5H3N, Mes = 2,4,6-trimethylphenyl), has been synthesized. Using either Cp* or Cp(P) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide, Cp(P) = 1-(7,7-dimethylbenzyl)cyclopentadienide), uranium complexes of the type Cp(X)UI2((Mes)PDI(Me)) (1-Cp(X); X = * or P), Cp(X)UI((Mes)PDI(Me)) (2-Cp(X)), and Cp(X)U((Mes)PDI(Me))(THF)n (3-Cp(X); *, n = 1; P, n = 0) were isolated and characterized. The series was generated via ligand centered reduction events; thus the extent of (Mes)PDI(Me) reduction varies in each case, but the uranium(IV) oxidation state is maintained.

Synthesis of terminal uranium(IV) disulfido and diselenido compounds by activation of elemental sulfur and selenium.

Rare stakes: Terminal uranium(IV) disulfido and diselenido compounds, Tp*2U(E2) (E=S, Se), were synthesized by the activation of elemental chalcogens. Structural, spectroscopic, computational and magnetic studies of these species establish their tetravalency and highly polarized U-E bonds.