Cerium(IV) Pyrasal Complexes: A pH-Dependent 8- to 10-Coordinate Cerium Chelate Switch.

In this work, five cerium(IV) complexes were synthesized, three of which were structural isomorphs from the same pyrasal ligand with the solid-state result identified by structural analysis dependent on the initial pH of the reaction solution and the temperature at which the reaction is performed. The ligands explored here are pyrasal ligands, which are Schiff-base ligands formed by the condensation of 2,3-diaminopyrazine and a salicylaldehyde derivative. Pyrasal ligands have weaker binding than other salophen-type ligands due to the electron-withdrawing effect of the nitrogen atoms contained within the pyrazine ring. The weaker binding leaves the ligand more susceptible to the changes in pH and temperature that alternate the chelating environment from 8- to 10-coordinate. This electron-withdrawing effect of the pyrazine backbone also deactivates the second amine after the first condensation addition of salicylaldehyde. Without a metal to template the complex formation reaction, even with extended reaction times and the addition of a large excess of ligand, the result is the addition of only one salicylaldehyde.

Applications of catalysis in hydroboration of imines, nitriles, and carbodiimides.

The catalytic hydroboration of imines, nitriles, and carbodiimides is a powerful method of preparing amines which are key synthetic intermediates in the synthesis of many value-added products. Imine hydroboration has perennially featured in notable reports while nitrile and carbodiimide hydroboration have gained attention recently. Initial developments in catalytic hydroboration of imines and nitriles employed precious metals and typically required harsh reaction conditions. More recent advances have shifted toward the use of base metal and main group element catalysis and milder reaction conditions. In this survey, we review metal and nonmetal catalyzed hydroboration of these unsaturated organic molecules and group them into three distinct categories: precious metals, base metals, and main group catalysts. The TON and TOF of imine hydroboration catalysts are reported and summarized with a brief overview of recent advances in the field. Mechanistic and kinetic studies of some of these protocols are also presented.

Ensemble effects on allylic oxidation within explicit solvation environments.

Umbrella-sampling density functional theory molecular dynamics (DFT-MD) has been employed to study the full catalytic cycle of the allylic oxidation of cyclohexene using a Cu(ii) 7-amino-6-((2-hydroxybenzylidene)amino)quinoxalin-2-ol complex in acetonitrile to create cyclohexenone and H2O as products. After the initial H-atom abstraction step, two different reaction pathways have been identified that are distinguished by the participation of alkyl hydroperoxide (referred to as the "open" cycle) versus the methanol side-product (referred to as the "closed" cycle) within the catalyst recovery process. Importantly, both pathways involve dehydrogenation and re-hydrogenation of the -NH2 group bound to the Cu-site - a feature that is revealed from the ensemble sampling of configurations of the reactive species that are stabilized within the explicit solvent environment of the simulation. Estimation of the energy span from the experimental turnover frequency yields an approximate value of 22.7 kcal mol-1 at 350 K. Whereas the closed cycle value is predicted to be 26.2 kcal mol-1, the open cycle value at 16.5 kcal mol-1. Both pathways are further consistent with the equilibrium between Cu(ii) and Cu(iii) that has previously been observed. In comparison to prior static DFT calculations, the ensemble of both solute and solvent configurations has helped to reveal a breadth of processes that underpin the full catalytic cycle yielding a more comprehensive understanding of the importance of radical reactions and catalysis recovery.

New up-conversion luminescence in molecular cyano-substituted naphthylsalophen lanthanide(iii) complexes.

A new naphthylsalophen and its 3 : 2 ligand-to-lanthanide sandwich-type complexes were isolated. When excited at 380 nm, the complexes display the characteristic metal-centred emission for NdIII, ErIII and YbIII. Upon 980 nm excitation, in mixed lanthanide and the Er complexes, Er-centred upconversion emission at 543 and 656 nm is observed, with power densities as low as 2.18 W cm-2.

Pyrrophens: Pyrrole-Based Hexadentate Ligands Tailor-Made for Uranyl (UO22+) Coordination and Molecular Recognition.

Derivatives of a novel pyrrole-containing Schiff base ligand system (called "pyrrophen") are presented which feature substituted phenylene linkers (R1 = R2 = H (H2L1); R1 = R2 = CH3 (H2L2)) and a binding pocket modeled after macrocyclic species. These ligands bind neutral CH3OH in the solid state through pyrrolic hydrogen-bonding. The interaction of the uranyl cation (UO22+) and H2L1-2 yields planar hexagonal bipyramdial uranyl complexes, while the Cu2+ and Zn2+ complexes were found to self-assemble as dinuclear helicate complexes (M2L2) with H2L1 under identical conditions. The favorable binding of UO22+ over Zn2+ provides insight into the molecular recognition of uranyl over other metal species. Structural features of these complexes are examined with special attention to features of the UO22+ coordination environment which distinguish them from other related salophen and porphyrinoid complexes.

Bonding Interactions in Uranyl α-Diimine Complexes: A Spectroscopic and Electrochemical Study of the Impacts of Ligand Electronics and Extended Conjugation.

Uranyl complexes of aryl-substituted α-diimine ligands gbha (UO2-1a-f) and phen-BIAN (UO2-2a-f) [gbha (1) = glyoxal bis(2-hydroxyanil); phen-BIAN (2) = N,N'-bis(iminophenol)acenaphthene; R = OMe (a), t-bu (b), H (c), Me (d), F (e), and naphthyl (f)] were designed, prepared, and characterized by X-ray diffraction, FT-IR, NMR, UV-vis, and electrochemical methods. These ligand frameworks contain a salen-type O-N-N-O binding pocket but are redox-noninnocent, leading to unusual metal complex behaviors. Here, we describe three solid-state structures of uranyl complexes UO2-1b, UO2-1c, and UO2-1f and observe manifestations of ligand noninnocence for the U(VI) complexes UO2-1b and UO2-1c. The impacts of accessible π-systems and ligand substitution on the axial uranium-oxo interactions were evaluated spectroscopically via the intraligand charge-transfer (ILCT) processes that dominate the absorption spectra of these complexes and through changes to the asymmetric (ν3) O═U═O stretching frequency. This, in combination with electrochemical data, reveals the effects of the inclusion of the conjugated acenaphthene backbone and the importance of ligand electronic structure on uranyl's bonding interactions.

Oxidative Mannich Reactions of Tertiary Amines Using a Cu(II) 2-Quinoxalinol Salen Catalyst.

Oxidative Mannich reactions can be catalyzed using a Cu(II) 2-quinoxalinol salen catalyst and with tert-butyl hydroperoxide (TBHP). Under mild conditions, a range of both cyclic and open chain tertiary amines was tested as substrates, resulting in yields up to 98%.

Solid-state structural elucidation and electrochemical analysis of uranyl naphthylsalophen.

A salophen ligand derivative incorporating naphthalene (naphthylsalophen = [H2L]) and the corresponding uranyl (UO22+) complex have been synthesized and characterized both in solution and the solid-state. A hydrogen bonding uranyl tetramer and the electrochemical analysis of [H2L] and UO2[L] are described.

Computational Study of Reduction Potentials of Th4+ Compounds and Hydrolysis of ThO2(H2O)n, n = 1, 2, 4.

The stability of Th4+ to reduction in water is studied by DFT methods. The standard reduction potential (SRP) of homoleptic complexes including Th(H2O)94+, Th(H2O)104+, Th(NO3)4, Th(NO2)62-, Th(NO3)62-, Th(COT)2, Th(acac)4, ThCp4, ThF4, and ThCl4 have been investigated. The values vary widely (from -3.50 V for Th(OH)4 to -0.62 V for Th(NO3)4 depending on whether the ligands are redox active (noninnocent) or not. Several additional topics of thorium chemistry are explored, including the hydrolysis mechanism of ThO2(H2O)n, n = 1, 2, 4, and the solution phase nonzero dipole moment of ThCp4. Dinuclear complexes are also characterized, including Th2O4, Th2O2(OH)4, Th2O2(H2O)8, Th2(OH)8(H2O)4, and Th2(OH)2(NO3)6(H2O)4 and condensed thorium complexes as [Th4(OH)6(H2O)12]10+ and [Th6(OH)14(H2O)12]10+. For the Th2(OH)2(NO3)6(H2O)4 dinuclear complex, the first SRP is -0.82 V and the second is 1.59 V. The first SRP corresponds to the reduction of the ligand NO3-, and the second SRP corresponds to dissociative electron transfer to the NO32- ligand. The calculated formation constant of Th(EDTA)(H2O)4 is in reasonable agreement with experiment. The different stereochemistries of the bidentate ligands NO2-, NO3-, and acetylacetonate (acac) around the thorium center have very similar stabilities.

2-Quinoxalinol diamine Cu(II) complex: facilitating catalytic oxidation through dual mechanisms.

The Cu(II) complex 1, Cu(II)-6-N-3,5-di-tert-butylsalicylidene-6,7-quinoxalinol-diamine, has been developed to address problems with current methods of catalytic oxidation using tert-butyl hydroperoxide (TBHP). Complex 1 demonstrated an increased capability to utilize TBHP while limiting interference from free radical reactions and was demonstrated to be highly effective in the oxidations of a variety of olefins.

Emission, Raman spectroscopy, and structural characterization of actinide tetracyanometallates.

Three new compounds, {U2(H2O)10(O)[Pt(CN)4]3}·4H2O, {Th2(H2O)10(OH)2[Pd(CN)4]3}·8H2O, and {(UO2)2(DMSO)4(OH)2[Ni(CN)4]}, in the actinide tetracyanometallate, Anx[M(CN)4]y, class of compounds have been synthesized and characterized by confocal Raman spectroscopy and single crystal X-ray diffraction. These compounds contain unique structures illustrating dimeric actinide species. The absence of intense charge transfer emission in the visible range for {U2(H2O)10(O)[Pt(CN)4]3}·4H2O, as compared to the platinum starting material, is unusual because of the presence of pseudo-one-dimensional Pt···Pt chains in this compound. Confocal Raman spectroscopy of the cyanide stretching region provides insight into the binding domain (mono-, bi-, tri-, tetradentate) of the tetracyanometallates in these novel structures.

Coordination chemistry with f-element complexes for an improved understanding of factors that contribute to extraction selectivity.

Here, we highlight some recent accomplishments in f-element coordination chemistry aimed at probing the fundamental chemical differences between the 4f elements, lanthanides, and the 5f elements, actinides. The studies of particular interest are those that target improving our knowledge of fundamental chemistry to aid in increased selectivity for extractions of actinides. Two components key to understanding the challenges of actinide separations are detailed here, namely, previously described separation methods and recent investigations into the fundamental coordination chemistry of actinides. Both are aimed at probing the critical features necessary for improved selectivity of separations. This is considered a critical goal in the safe remediation of contaminated sites and reprocessing of nuclear fuel sources used in either civilian and noncivilian energy production.

Allylic C-H activations using Cu(II) 2-quinoxalinol salen and tert-butyl hydroperoxide.

Using a Cu(II) 2-quinoxalinol salen complex as the catalyst and tert-butyl hydroperoxide (TBHP) as the oxidant, allylic activations of olefin substrates can be converted to the corresponding enones or 1,4-enediones. Excellent yields can be achieved (up to 99%) within a very short reaction time and with great tolerance for additional functional groups. Possible mechanistic pathways have been characterized using Raman spectroscopy, cyclic voltammetry, and theoretical calculations.

Actinide tetracyanoplatinates: synthesis and structural characterization with uncharacteristic Th-NC coordination and thorium fluorescence.

Here, we report the synthesis of three novel actinide tetracyanoplatinates (AnTCP, where An = Th, UO(2) unit and TCP = [Pt(CN)(4)](2-)) along with their unique fluorescence properties and structural characterization by single crystal X-ray diffraction.

Amine templated two- and three-dimensional uranyl sulfates.

Structural variability in amine template uranyl sulfate frameworks is observed with two- and three-dimensional structures containing channels of dimensions 7.4 A x 5.1 A, 36 A x 5.8 A, and 10 A x 10 A.

An effective method for allylic oxidation of Delta5-steroids using tert-butyl hydroperoxide.

An allylic oxidation method for Delta(5)-steroids using TBHP as oxidant with a 2-quinoxalinol salen Cu(II) complex as catalyst is reported. A variety of Delta(5)-steroidal substrates are selectively oxidized to the corresponding enones. Excellent yields are achieved (up to 99% under optimized conditions) while significantly reducing reaction times required as compared to other current methods.

Hydroxy- and alkoxy-bridged dinuclear uranyl-Schiff base complexes: hydrolysis, transamination and extraction studies.

The reaction of uranyl nitrate with 1,3-bis(salicylideneamino)-2-propanol (H(3)L1) and 1,3-bis(3,5-di-tert-butylsalicylideneamino)-2-propanol (H(3)L2) in the presence of triethylamine (Et(3)N) yielded hydroxy- and alkoxy-bridged dinuclear complexes; [(UO(2))(2)(L1)(OH)(MeOH)(2)].(MeOH)(2) (.(MeOH)(2)) and [(UO(2))(2)(L2)(OH)(MeOH)(2)].(MeOH)(2) (.(MeOH)(2)). The crystal structures of .(DMF)(2) and .(DMF)(2) exhibit an unsymmetrical central U(2)O(2) core involving bridging alkoxy- and hydroxy-oxygen atoms. The geometry around the uranium center in .(DMF)(2) and .(DMF)(2) is that of a distorted pentagonal bipyramid with the solvent molecule occupying the fifth coordination site. The flexible nature of the ligand backbone is more pronounced in .(DMF)(2) compared to .(DMF)(2), yielding two molecules per unit cell in different conformations. Under similar reaction conditions, using ethylenediamine as a base, the respective Salen-based uranyl compounds, [UO(2)(Salen)(MeOH)] () and [UO(2)(Bu(t)(2)-Salen)(MeOH)] () are obtained due to transamination of the ligand backbone. Complexes .(MeOH)(2) and .(MeOH)(2) when reacted with an excess of ethylenediamine failed to yield the respective Salen-based complexes, and , respectively. The new compounds have been characterized using solution (NMR and UV-Vis) and solid-state (IR, X-ray crystallography) techniques. Hydrolysis of .(MeOH)(2) and .(MeOH)(2) in the pH range 1-14 was studied using UV-Vis spectroscopy and compared with the hydrolysis of and [UO(2)(Salophen)(MeOH)] (). A two-phase extraction study suggests quantitative removal of uranyl ions from the aqueous phase at higher pH conditions.

Regioselective synthesis of asymmetrically substituted 2-quinoxalinol salen ligands.

Diamino-2-quinoxalinols are reacted with salicylaldehyde derivatives to produce 2-quinoxalinol imines regioselectively as one isomer in good yield. Regioselectivity has been determined through the use of isotopic 15N labeling experiments. The 2-quinoxalinol imines may then be reacted without further purification with additional salicylaldehyde derivatives to yield asymmetrically substituted 2-quinoxalinol salens.

Uranyl stabilized Schiff base complex.

Uranyl Schiff base complex [(UO(2))(2)(Salpro)(OH)(Solvent)(2)] (1) in the presence of excess of ethylenediamine (EDA) does not undergo nucleophilic addition (hydrolysis) and substitution (transamination) reactions due to an extended chelation [2N, 3O + OH] by the flexible backbone.

Novel dinuclear uranyl complexes with asymmetric schiff base ligands: synthesis, structural characterization, reactivity, and extraction studies.

The reaction of uranyl nitrate with asymmetric [3O, N] Schiff base ligands in the presence of base yields dinuclear uranyl complexes, [UO2(HL1)]2.DMF (1), [UO2(HL2)]2.2DMF.H2O (2), and [UO2(HL3)]2.2DMF (3) with 3-(2-hydroxybenzylideneamino)propane-1,2-diol (H3L1), 4-((2,3-dihydroxypropylimino)methyl)benzene-1,3-diol (H3L2), and 3-(3,5-di-tert-butyl-2-hydroxybenzylideneamino)propane-1,2-diol (H3L3), respectively. All complexes exhibit a symmetric U2O2 core featuring a distorted pentagonal bipyramidal geometry around each uranyl center. The hydroxyl groups on the ligands are attached to the uranyl ion in chelating, bridging, and coordinate covalent bonds. Distortion in the backbone is more pronounced in 1, where the phenyl groups are on the same side of the planar U2O2 core. The phenyl groups are present on the opposite side of U2O2 core in 2 and 3 due to electronic and steric effects. A similar hydrogen-bonding pattern is observed in the solid-state structures of 1 and 3 with terminal hydroxyl groups and DMF molecules, resulting in discrete molecules. Free aryl hydroxyl groups and water molecules in 2 give rise to a two-dimensional network with water molecules in the channels of an extended corrugated sheet structure. Compound 1 in the presence of excess Ag(NO3) yields {[(UO2)(NO3)(C6H4OCOO)](NH(CH2CH3)3)}2 (4), where the geometry around the uranyl center is hexagonal bipyrimidal. Two-phase extraction studies of uranium from aqueous media employing H3L3 indicate 99% reduction of uranyl ion at higher pH.