Search

Non-aqueous redox flow batteries (RFBs) have been gaining increased attention in the energy storage arena. Some of their attractive features include the promise for high energy densities, wider voltage windows compared to aqueous systems, as well as wider operating temperature ranges. One of the major challenges in the development of these systems is the lack of electroactive materials that can undergo reversible redox events within the larger potential window of organic solvents. Herein, we present the design, synthesis, and measurement of electrochemical properties of some iron-based imine- and iminopyridine complexes as promising candidates for RFB applications. Synthesized complexes afforded three reversible redox couples over a ~3 V range. The redox events were also computationally explored and are in good agreement with experimental data. Theoretical calculations show that the redox event at the positive potential can be characterized as metal-centered event corresponding to Fe3+→ Fe2+ reduction, while the two redox events at negative potentials are associated with the consecutive reductions of iminopyridine or diimine moieties. This work has led to the identification of a promising anolyte material, [tris(imino)pyridine)Fe][OTf]2 (1), which is soluble in acetonitrile and is synthesized in two simple steps. This species shows outstanding performance when cycled between 1.0 V and 2.35 V, with 93% coulombic efficiency, 84% energy efficiency, and a capacity decay rate of 0.61% per cycle at 5 mA cm−2. Further modifications to this kind of charge carrier may lead to the development of high energy density materials for grid scale energy storage applications.

Actinium-225 (225Ac) is an excellent candidate for targeted radiotherapeutic applications for treating cancer, because of its 10-day half-life and emission of four high-energy α2+ particles. To harness and direct the energetic potential of actinium, strongly binding chelators that remain stable in vivo during biological targeting must be developed. Unfortunately, controlling chelation for actinium remains challenging. Actinium is the largest +3 cation on the periodic table and has a 6d05f0 electronic configuration, and its chemistry is relatively unexplored. Herein, we present theoretical work focused on improving the understanding of actinium bonding with macrocyclic chelating agents as a function of (1) macrocycle ring size, (2) the number and identity of metal binding functional groups, and (3) the length of the tether linking the metal binding functional group to the macrocyclic backbone. Actinium binding by these chelators is presented within the context of complexation with DOTA4-, the most relevant Ac3+ binding agent for contemporary radiopharmaceutical applications. The results enabled us to develop a new strategy for actinium chelator design. The approach is rooted in our identification that Ac3+-chelation chemistry is dominated by ionic bonding interactions and relies on (1) maximizing electrostatic interactions between the metal binding functional group and the Ac3+ cation and (2) minimizing electronic repulsion between negatively charged actinium binding functional groups. This insight will provide a foundation for future innovation in developing the next generation of multifunctional actinium chelators.

Copper dissolution in nitric acid is a historic reaction playing a central role in many industrial processes, particularly for metal recovery from the electronics to nuclear industries. The mechanism through which this process occurs is debated. In order to better understand this process, quantum chemical calculations were performed to elucidate the key steps in the mechanism of copper dissolution in nitric acid. We combine both Kohn-Sham density functional theory and ab initio molecular dynamics simulations to understand the mechanism of the formation of the key products: NO2, HNO2, and NO. Our calculations suggest that the mechanisms of formation of NO2, HNO2, and NO are interconnected.

Using theoretical modeling to guide our approach, substituents were selected to improve the negative and positive potentials associated with a representative metal-based redox carrier, a phenyl spaced nickel bispicolinamide complex, [Ni(bpb)], 1. To broaden the cell potential, electron donating groups were selected and installed on the pyridyl moieties, [Ni(bpb-(NMe2)2)], 2, while electron withdrawing groups were incorporated on the phenyl linker, [Ni(bpb-R)] (R = –F, –CF3, and –NO2), 3–5. Our model predicts an increase of ∼300 mV and ∼500 mV for the first and second negative waves of Ni(bpb-NMe2), 2, and up to ∼330 mV and ∼210 mV increase to the first and second positive waves in 3–5. The modeled complexes were synthesized in good yields using a modification of the literature procedure. Due to limited solubility, the differential pulse voltammetry was measured on complexes 2–5 using a drop cast technique. Comparison of the observed and predicted potentials is discussed in acetonitrile (MeCN) and dimethylformamide (DMF) solvents.

A redox pair of Ce4+ and Ce3+ complexes has been prepared that is stabilized by the [(NP(1,2-bis-tBu-diamidoethane)(NEt2))]1− ligand. Since these complexes are isostructural to the recently reported isovalent terbium analogs, a detailed structural and spectroscopic comparative analysis was pursued via Voronoi–Dirichlet polyhedra analysis, UV-vis-NIR, L3-edge X-ray absorption near edge spectroscopy (XANES), cyclic voltammetry, and natural transitions orbital (NTO) analysis and natural bond orbital (NBO) analysis. The electrochemical studies confirm previous theoretical studies of the redox properties of the related complex [K][Ce3+(NP(pip)3)4] (pip = piperidinyl), 1-Ce(PN). Complex 1-Ce(PN*) presents the most negative Epc of −2.88 V vs. Fc/Fc+ in THF of any cerium complex studied electrochemically. Likewise 1-Tb(PN*) has the most negative Epc for electrochemically interrogated terbium complexes at −1.79 V vs. Fc/Fc+ in THF. Complexes 1-Ce(PN*) and 2-Ce(PN*) were also studied by L3-edge X-ray absorption near edges spectroscopy (XANES) and a comparison to previously reported spectra for 1-Tb(PN*), 2-Tb(PN*), 1-Ce(PN), and, [Ce4+(NP(pip)3)4], 2-Ce(PN), demonstrates similar nf values for all the tetravalent lanthanide complexes. According to the natural bond orbital analysis, a greater covalent character of the M–L bonds is found in 2-Ce(PN*) than in 1-Ce(PN*), in agreement with the shorter Ce–N bonds in the tetravalent counterpart. The greater contribution of Ce orbitals in the Ce–N bonding and, specifically, the higher participation of 4f electrons accounts for the stronger covalent interactions in 2-Ce(PN*) as compared to 2-Tb(PN*).

Californium (Cf) is currently the heaviest element accessible above microgram quantities. Cf isotopes impose severe experimental challenges due to their scarcity and radiological hazards. Consequently, chemical secrets ranging from the accessibility of 5f/6d valence orbitals to engage in bonding, the role of spin–orbit coupling in electronic structure, and reactivity patterns compared to other f elements, remain locked. Organometallic molecules were foundational in elucidating periodicity and bonding trends across the periodic table1–3, with a twenty-first-century renaissance of organometallic thorium (Th) through plutonium (Pu) chemistry4–12, and to a smaller extent americium (Am)13, transforming chemical understanding. Yet, analogous curium (Cm) to Cf chemistry has lain dormant since the 1970s. Here, we revive air-/moisture-sensitive Cf chemistry through the synthesis and characterization of [Cf(C5Me4H)2Cl2K(OEt2)]n from two milligrams of 249Cf. This bent metallocene motif, not previously structurally authenticated beyond uranium (U)14,15, contains the first crystallographically characterized Cf–C bond. Analysis suggests the Cf–C bond is largely ionic with a small covalent contribution. Lowered Cf 5f orbital energy versus dysprosium (Dy) 4f in the colourless, isoelectronic and isostructural [Dy(C5Me4H)2Cl2K(OEt2)]n results in an orange Cf compound, contrasting with the light-green colour typically associated with Cf compounds16–22.

The complex polar oxide heterojunction of ${\mathrm{SrTiO}}_{3}/{\mathrm{LaAlO}}_{3}$ (STO/LAO) is of great interest due to the emergent physical phenomena observed at the interface. STO and LAO separately are wide band-gap insulators. However, upon joining them at the 001, 110, and 111 crystallographic planes, the interface undergoes a transition to a conductive state. Although first-principles modeling of the 001 plane interface has been widely studied, there is a lack of reports regarding the 110 and 111. This paper expands the theoretical model of the STO/LAO heterointerface to the three crystallographic planes (001, 110, and 111) where the conductivity has been experimentally reported. The calculations showed that whereas at the 001 interface the conductivity appears at a critical thickness of 4 monolayers of LAO, the 110 and 111 planes have no clear critical thickness; these two interfaces were always conductive. Nevertheless, the number of conductive electrons per unit cell increases with the thickness of the LAO layer in the 110 and 111 interfaces. This is related to the energy levels downshifting due to the electrostatic potential buildup (which was in the opposite direction respect to the 001 interface), increasing the number of conductive sates below the Fermi level. Given the absence of a critical thickness and the fact that chemical intermixing and oxygen vacancies at the interface were not considered, the main mechanism responsible for the conductivity in the 110 and 111 planes was attributed to the large structure reconstruction that locally changes the energy levels at the interface causing charge transfer and accumulation at the layers close to the interface.

Bis[o-(trifluoromethyl)phenyl]dithiophosphinate is a sulfur-donating ligand capable of providing the largest reported trivalent lanthanide (Ln3+)–actinide (An3+) group separation factors. Literatur...

Electrospray ionization of solutions containing a tripodal hydroxylamine ligand, H3TriNOx ([((2-tBuNOH)C6H4CH2)3N]) denoted as L, and a hydrogen halide HX: HCl, HBr and/or HI, yielded gas-phase anion complexes [L(X)]- and [L(HX2)]-. Collision induced dissociation (CID) of mixed-halide complexes, [L(HXaXb)]-, indicated highest affinity for I- and lowest for Cl-. Structures and energetics computed by density functional theory are in accord with the CID results, and indicate that the gas-phase binding preference is a manifestation of differing stabilities of the HX molecules. A high halide affinity of [L(H)]+ in solution was also demonstrated, though with a highest preference for Cl- and lowest for I-, the opposite observation of, but not in conflict with, what is observed in gas phase. The results suggest a connection between gas- and condensed-phase chemistry and computational approaches, and shed light on the aggregation and anion recognition properties of hydroxylamine receptors.

A predictive model based on first principles calculations has been proposed to study the solid–liquid equilibria comprising of metal complexes and ionic liquids. The model is based on first principle COSMO calculation followed by post statistical thermodynamical treatment of self-consistent properties of solute and solvent molecules. The metal complex and ionic liquid have been treated as a simple binary mixture. The ionic liquid has been treated here as a single intact molecule. The experimentally observed dual-solute relationship between the ionic liquid and redox active species in presence of a third organic solvent has been established using our model in this work. Within the model, the dual-solute relationship appeared as a simple Gibbs–Duhem relationship between these two species at ambient condition. The dual-solute relationship between the metal complex (V(acac)3, Cr(acac)3 and Mn(acac)3) and ionic liquid ([Tea][BF4]) has been validated by calculating the Gibbs–Duhem relationship, xsolute vs. xsolvent(IL) and 1/γsolute vs. xsolvent(IL) plots. The present model has been applied to a set of ionic liquids, metal complexes and organic solvent (acetonitrile) for which experimental study has been done. The solvation mechanism of the metal complexes in those ionic liquids was obtained using the model. According to our findings, the ionic liquid containing imidazolium cation and [NTf2]− anion is appeared as a suitable solvent for the non-aqueous redox flow cell. We have compared our results with the already reported experimental results where they were available for the non-aqueous solvents.

The separation of trivalent lanthanides and actinides is challenging because of their similar sizes and charge densities. S-donating extractants have shown significant selectivity for trivalent actinides over lanthanides, with single-stage americium/lanthanide separation efficiencies for some thiol-based extractants reported at >99.999%. While such separations could transform the nuclear waste management landscape, these systems are often limited by the hydrolytic and radiolytic stability of the extractant. Progress away from thiol-based systems is limited by the poorly understood and complex interactions of these extractants in organic phases, where molecular aggregation and micelle formation obfuscates assessment of the metal-extractant coordination environment. Because S-donating thioethers are generally more resistant to hydrolysis and oxidation and the aqueous phase coordination chemistry is anticipated to lack complications brought on by micelle formation, we have considered three thioethers, 2,2'-thiodiacetic acid (TDA), (2R,5S)-tetrahydrothiophene-2,5-dicarboxylic acid, and 2,5-thiophenedicarboxylic acid (TPA), as possible trivalent actinide selective reagents. Formation constants, extended X-ray absorption fine structure spectroscopy, and computational studies were completed for thioether complexes with a variety of trivalent lanthanides and actinides including Nd, Eu, Tb, Am, Cm, Bk, and Cf. TPA was found to have moderately higher selectivity for the actinides because of its ability to bind actinides in a different manner than lanthanides, but the utility of TPA is limited by poor water solubility and high rigidity. While significant competition with water for the metal center limits the efficacy of aqueous-based thioethers for separations, the characterization of these solution-phase, S-containing lanthanide and actinide complexes is the most comprehensively available in the literature to date. This is due to the breadth of lanthanides and actinides considered as well as the techniques deployed and serves as a platform for the further development of S-containing reagents for actinide separations. Additionally, this paper reports on the first bond lengths for Cf and Bk with a neutral S donor.

Alkali-based semiconducting photocathodes, due to their high quantum efficiency (QE) in the visible light spectrum, are promising candidates to replace traditional metal photocathodes for high-brightness beam applications such as x-ray free electron lasers (XFELs). However, they suffer from rapid degradation which significantly limits their operational lifetimes. Coating them with two-dimensional (2D) materials has been proposed as a possible avenue to prevent the degradation. Ideally, the 2D coating layer should not increase the work function of semiconducting photocathodes, thus maintaining the high QE of semiconducting photocathodes in visible light. Herein, we report a computational screening of over 4000 2D materials in the Computational 2D Materials Database (C2DB). The assessment of their potential to be good coating layers is based on their effects to the surface electronic properties. We discover several candidate materials that are even capable of decreasing the work function of semiconducting photocathodes. Some of the experimentally synthesized 2D materials, such as hydrogenated graphene (graphane) and several hydroxylated transition metal carbides/nitrides (MXenes), are particularly appealing for this application.

A series of metallocene thorium complexes with mono- and bis(phosphido) ligands have been investigated with varying hues: (C5Me5)2Th(Cl)[P(Mes)2] (Mes = mesityl = 2,4,6-(CH3)3C6H2; dark red-purple), (C5Me5)2Th[P(Mes)(CH3)]2 (dark red-purple), (C5Me5)2Th(CH3)[P(Mes)2] (dark red-purple), (C5Me5)2Th(CH3)[P(Mes)(SiMe3)] (orange), (C5Me5)2Th(Cl)[P(Mes)(SiMe3)] (orange), (C5Me5)2Th[P(Mes)(SiMe3)]2 (orange), and (C5Me5)2Th[PH(Mes)]2 (pale yellow). While all of these complexes bear a mesityl group on phosphorus, the electronic structure observed differs depending on the other substituent (mesityl, methyl, trimethylsilyl, or hydrogen). This sparked an investigation of the electronic structure of these complexes using 31P NMR and electronic absorption spectroscopy in concert with time-dependent density functional theory calculations.

Previous studies suggested that in Nishibayashi’s homogenous catalytic systems based on molybdenum (Mo) complexes, the bimetallic structure facilitated dinitrogen to ammonia conversion in comparison to the corresponding monometallic complexes, likely due to the through-bond interactions between the two Mo centers. However, more detailed model systems are necessary to support this bimetallic hypothesis, and to elucidate the multi-metallic effects on the catalytic mechanism. In this work, we computationally examined the effects of dimension as well as the types of bridging ligands on the catalytic activities of molybdenum-dinitrogen complexes by using a set of extended model systems based on Nishibayashi’s bimetallic structure. The polynuclear chains containing four ([Mo]4) or more Mo centers were found to drastically enhance the catalytic performance by comparing with both the monometallic and bimetallic complexes. Carbide ([:C C:]2−) was found to be a more effective bridging ligand than N2 in terms of electronic charges dispersion between metal centers thereby facilitating reactions in the catalytic cycle. The mechanistic modelling suggests that in principle, more efficient catalytic system for N2 to NH3 transformation might be obtained by extending the polynuclear chain to a proper size in combination with an effective bridging ligand for charge dispersion.

The homoleptic complexes of cerium with the tris(piperidinyl)imidophosphorane ligand, [NP(pip)3]-, present the most negative Ce3+/4+ redox couple known ( 4.0 V shift from the Ce3+/4+ couple in 1 M HClO4(aq)] is established through reactivity studies. Spectroscopic studies (UV-vis, electron paramagnetic resonance, and Ce L3-edge X-ray absorption spectroscopy), in conjunction with density functional theory studies, reveal the dominant covalent metal-ligand interactions underlying the observed redox chemistry and the dependence of the redox potential on the binding of potassium in the inner coordination sphere.

Complexation of Pu(IV) with the actinide extractant CyMe4-BTPhen (2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline) was followed by UV/vis spectroscopy in acetonitrile solution. The solid-state structure of the crystallized product suggests that Pu(IV) is reduced to Pu(III) upon complexation. Analysis by DFT modeling is consistent with metal-based rather than ligand-based reduction.

The speciation of NO2, NO, and N2O gases formed from the dissolution of copper in nitric acid is measured and characterized using Fourier-transform infrared (FTIR) spectroscopy. This study describes analysis of the gas phase species formed and ingrowths of their concentrations as a function of time. Acid concentrations range from 14 to 4 M and the NO2, NO, and N2O are analyzed using FTIR between 1585 and 2209 wavenumbers. The concentrations of the gases range from 0 to 230 ppm and change over time and as a function of acid concentration.

The speciation of gases formed in the vapor phase above HNO3-H2O mixtures at room temperature is measured and characterized using Fourier-transform infrared (FTIR) spectroscopy. Although studies of speciation of compounds have been performed in the solution phase, this study focuses solely on the gas phase species formed over HNO3-H2O mixtures and their concentrations as a function of time. The nitric acid concentration in the experiments range from 14 to 4 M. The NO2, NO, and N2O (collectively known at NOx) gases are measured by FTIR at 1585, 1850, and 2208 wavenumbers, respectively. The concentrations of these NOx species ranges from 0 to 3 ppm and change over time as a function of acid concentration. Two purities of commercially available HNO3 are used to prepare the HNO3-H2O mixtures. An analysis of thermodynamic data shows that the formation of NOx gases without the presence of a catalyst are thermodynamically favorable.

Nanocrystals (NCs) with size and shape dependent properties are a thriving research field. Remarkable progress has been made in the controlled synthesis of NCs of stable elements in the past two decades; however, the knowledge of the NCs of actinide compounds has been considerably limited due the difficulties in handling them both experimentally and theoretically. Actinide compounds, especially actinide oxides, play a critical role in many stages of the nuclear fuel cycle. Recently, a non-aqueous surfactant assisted approach has been developed for the synthesis of actinide oxide NCs with different morphologies, but an understanding of its control factors is still missing to date. Herein we present a comprehensive study on the low index surfaces of thorium dioxide (ThO2) and their interactions with relevant surfactant ligands using density functional calculations. A systematic picture on the thermodynamic stability of ThO2 NCs of different sizes and shapes is obtained employing empirical models based on the calculated surface energies. It is found that bare ThO2 NCs prefer the octahedral shape terminated by (111) surfaces. Oleic acid displays selective adsorption on the (110) surface, leading to the shape transformation from octahedrons to nanorods. Other ligands such as acetylacetone, oleylamine, and trioctylphosphine oxide do not modify the equilibrium shape of ThO2 NCs. This work provides atomic level insights into the anisotropic growth of ThO2 NCs that was recently observed in experiments, and thus may contribute to the controlled synthesis of actinide oxide NCs with well-defined size and shape for future applications.

Separations of f-block elements are a critical aspect of nuclear waste processing. Redox-based separations offer promise, but challenges remain in stabilizing and differentiating actinides in high oxidation states. The investigation of new ligand types that provide thermodynamic stabilization to high-valent actinides is essential for expanding their fundamental chemistry and to elaborate new separation techniques and storage methods. We report herein the preparation and characterization of Th and U complexes of the pyridyl-hydroxylamine ligand, N- tert-butyl- N-(pyridin-2-yl)hydroxylamine (pyNO-). Electrochemical studies performed on the homoleptic complexes [M(pyNO)4] (M = Th, U) revealed significant stabilization of the U complex upon one-electron oxidation. The salt [U(pyNO)4]+ was isolated by chemical oxidation of [U(pyNO)4]; spectroscopic and computational data support assignment as a UV cation.

Introduction The use of α-emitting isotopes for radionuclide therapy is a promising treatment strategy for small micro-metastatic disease. The radioisotope 213 Bi is a nuclide that has found substantial use for targeted α-therapy (TAT). The relatively unexplored aqueous chemistry of Bi 3+ , however, hinders the development of bifunctional chelating agents that can successfully deliver these Bi radioisotopes to the tumor cells. Here, a novel series of nitrogen-rich macrocyclic ligands is explored for their potential use as Bi-selective chelating agents. Methods The ligands, 1,4,7,10-tetrakis(pyridin-2-ylmethyl)-1,4,7,10-tetraazacyclododecane (L py ), 1,4,7,10-tetrakis(3-pyridazylmethyl)-1,4,7,10-tetraazacyclododecane (L pyd ), 1,4,7,10-tetrakis(4-pyrimidylmethyl)-1,4,7,10-tetraazacyclododecane (L pyr ), and 1,4,7,10-tetrakis(2-pyrazinylmethyl)-1,4,7,10-tetraazacyclododecane (L pz ), were prepared by a previously reported method and investigated here for their abilities to bind Bi radioisotopes. The commercially available and commonly used ligands 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and N -[( R )-2-amino-3-( p -isothiocyanato-phenyl)propyl]- trans -( S,S )- cyclohexane-1,2-diamine- N , N , N ', N ", N "-pentaacetic acid (CHX-A′′-DTPA) were also explored for comparative purposes. Radio-thin-layer chromatography (TLC) was used to measure the binding kinetics and stabilities of the complexes formed. The long-lived isotope, 207 Bi (t 1/2 =32years), was used for these studies. Density functional theory (DFT) calculations were also employed to probe the ligand interactions with Bi 3+ and the generator parent ion Ac 3+ . Results In contrast to DOTA and CHX-A′′-DTPA, these nitrogen-rich macrocycles selectively chelate Bi 3+ in the presence of the parent isotope Ac 3+ . Among the four tested, L py was found to exhibit optimal Bi 3+ -binding kinetics and complex stability. L py complexes Bi 3+ more rapidly than DOTA, yet the resulting complexes are of similar stability. DFT calculations corroborate the experimentally observed selectivity of these ligands for Bi 3+ over Ac 3+ . Conclusion Taken together, these data implicate L py as a valuable chelating agent for the delivery of 213 Bi. Its selectivity for Bi 3+ and rapid and stable labeling properties warrant further investigation and biological studies.

Although we can efficiently convert bioderived furans into linear alkanes, the most energy-intensive step in this approach is the hydrodeoxygenation of the intermediate polyketone. To fully understand this process, we have examined the hydrodeoxygenation of a model compound, 3-pentanone, which allows us to follow this process stepwise using Pd/C, H2 (200 psi), and La(OTf)3 in acetic acid to remove the oxygen atom at temperatures between 25 and 200 °C. We have found that ketone reduction to an alcohol is followed by acetoxylation, which provides a more facile route to C–O bond cleavage relative to the parent alcohol.

We describe DFT computations that address the regioselective preference toward the five-membered ring product 1,3-dioxolane (solketal) over the six-membered-ring product (1,3-dioxane) during Lu(OTf)3-catalyzed ketalization of acetone with glycerol. When ketalization occurs via the internal (secondary) −OH group of glycerol, only solketal production should be possible due to the symmetry of the intermediates. Ketalization via the terminal −OH group of glycerol is predicted to occur in a different manner than the conventionally proposed ketalization mechanism. A constrained hemiketal intermediate is invoked to explain the selectivity for solketal formation.

We report an experimental and theoretical investigation of the decomposition (partial oxidation) of deuterated methanol (CD3OD) on a single-crystal Fe3O4(111) surface. The crystal surface contains ...

© 2017 American Chemical Society. The electron density distribution of [PPh4][UF6] was obtained from high-resolution X-ray diffraction data measured at 20 K. The electron density was modeled with an augmented Hansen-Coppens multipolar formalism. Topological analysis reveals that the U-F bond is of incipient covalent nature. Theoretical calculations add further support to the bonding description gleaned from the experimental model. The impact of the uranium anomalous dispersion terms on the refinement is also discussed.

Recently, Nishibayashi et al. reported a dimolybdenum-dinitrogen complex that is catalytic for complete reduction of dinitrogen to ammonia. This catalyst is different from the Schrock molybdenum catalyst in two fundamental aspects: it contains two metal centers, and the oxidation state is Mo(0) instead of Mo(III). We show that a remarkable feature of the bimetallic complex is the bond-mediated delocalized electronic states, resulting from the two metal centers bridged by a dinitrogen ligand. Using first-principles calculations, we found that this property makes the bimetallic complex the effective catalyst, as opposed to the originally postulated monometallic fragment. A favorable reaction pathway is identified, and the nature of the intermediates is examined. Furthermore, studies of the intermediate states led us to propose possible deactivation processes of the catalyst. The finding that the central bimetallic unit (Mo-N2-Mo) is relevant for catalytic activity may provide a guideline for the development of more efficient dinitrogen-reducing catalysts.

Evidence for metal–carbon orbital mixing in thorocene and uranocene was determined from DFT calculations and carbon K-edge X-ray absorption spectra (XAS) collected with a scanning transmission X-ray microscope (STXM). Both the experimental and computational results showed that the 5f orbitals engaged in significant δ-type mixing with the C8H82− ligands, which increased as the 5f orbitals dropped in energy on moving from Th4+ to U4+. The first experimental evidence for extensive ϕ-orbital interactions has been provided by the C K-edge XAS analysis of thorocene; however, ϕ-type covalency in uranocene was negligible. The results highlighted two contrasting trends in orbital mixing from one pair of highly symmetric molecules, and showed that covalency does not increase uniformly for different molecular orbital interactions with later actinides.

We describe studies of Bronsted acid catalyzed ring opening of substituted furans contained within biomass derived C8- and C9-molecules. Ring opening occurs homogeneously under relatively mild conditions of 80 °C using catalytic hydrochloric acid. In the case of 4-(5-methyl-2-furyl)-2-butanone (1a), the reaction proceeds to a single product in up to 92% yield after 24 hours. For 4-(2-furanyl)-2-butanone (1b) and 4-(5-hydroxymethyl)-2-furanyl-2-butanone (1c), however, multiple products are observed, illustrating the significant influence of furan ring substituents on the reactivity of this class of compounds. The generality of these reaction pathways was tested using several other similar substrates. Kinetics experiments indicate that ring opening of 1a occurs via specific acid catalysis, and computations elucidate the effect of initial protonation on the reaction pathway. Calculated pKa values were calibrated against experimentally measured values and are consistent with observed reactivities. Inclusion of explicit, hydrogen-bonded water molecules in addition to the SMD solvent model is necessary when studying protonation of alcohol and ketone groups.

Room-temperature ionic liquids (RTILs) are regarded as green solvents due to their low volatility, low flammability, and thermal stability. RTILs exhibit wide electrochemical windows, making them prime candidates as media for electrochemically driven reactions such as electro-catalysis and electro-plating for separations applications. Therefore, understanding the factors determining edges of the electrochemical window, the electrochemical stability of the RTILs, and the degradation products is crucial to improve the efficiency and applicability of these systems. We present here computational investigations of the electrochemical properties of a variety of RTILs covering a wide range of electrochemical windows. We proposed four different approaches with different degrees of approximation and computational cost from gas-phase calculations to full explicit solvation models. It was found that, whereas the simplest model has significant flaws in accuracy, implicit and explicit solvent models can be used to reliably predict experimental data. The general trend of electrochemical windows of the RTILs studied is well reproduced, showing that it increases in the order of imidazoliumammoniumpyrrolidiniumphosphonium giving confidence to the methodology presented to use it in screening studies of ionic liquids.

Novel cis- and trans-bis(imido) uranium disulfonamide derivatives have been prepared from iodide metathesis reactions between two equivalents of K[N(Me)(SO2Ar’)] (Ar’ = 4-Me-C6H4) and U(NtBu)2(I)2(L)x (L = OPPh3, x = 2; Me2bpy, x = 1; Me2bpy = 4,4’-dimethyl-2,2’-bipyridyl). These bis(amide) derivatives serve as useful precursors for the synthesis of the trans-diphenolate complex U(NtBu)2(O-2-tBuC6H4)2(OPPh3)2 (5), cis- and trans-dithiolate complexes U(NtBu)2(SPh)2(L)x (L = OPPh3 (6); Me2bpy (7)), and cis- and trans-dihalide complexes with the general formulas U(NtBu)2(X)2(L)x (X = Cl, L = OPPh3 (8), L = Me2bpy (10); X = Br, L = OPPh3 (9), L = Me2bpy (11)). DFT calculations performed on the trans-dihalide series U(NtBu)2(X)2(L)2 and the UO22+ analogues UO2X2(OPPh3)2 suggest that the uranium centers in the [U(NtBu)2]2+ ions possess more covalent character than analogous UO22+ derivatives but that the U-X bonds in the U(NtBu)2X2L2 complexes possess a more ionic nature.

Spatial localization of excited-state electrons in transition-metal complexes used as photocatalysts or dye sensitizers in solar cells is important for efficient electron injection into the metal oxide nanoparticles. We use density functional theory to investigate the excited states in a prototype catalyst-chromophore assembly [(bpy)(H(2)O)Ru(tpy-tpy)Ru(tpy)](4+) ([Ru(tpy)(bpy)(H(2)O)](2+) = catalyst, [Ru(tpy)(2)](2+) = chromophore, tpy = 2,2':6',2''-terpyridine, and bpy = 2,2'-bipyridine) and a series of related compounds. We explore several bridging ligand and terminal tpy ligand modifications of the prototype assembly, with the aim of inducing electronic excitations into the terminal tpy ligand upon irradiation with visible light. The excitations into the terminal ligand (i.e., ligand covalently attached to the semiconductor in the photocatalytic synthetic cell) should, in turn, enhance electron injection into the semiconductor. Our results suggest that both introduction of a spacer group (such as phenylene or alkane) into the tpy-tpy bridge and replacement of the terminal tpy group by a more extended pi-conjugated ligand are necessary to shift the electronic excitations from the bridging ligand into the terminal ligand. These results have implications for the design of photocatalysts and dye-sensitizer assemblies based on ruthenium(II) terpyridine compounds.

Studies of interfacial electron transfer (IET) in TiO(2) surfaces functionalized with (1) pyridine-4-phosphonic acid, (2) [Ru(tpy)(tpy(PO(3)H(2)))](2+), and (3) [Ru(tpy)(bpy)(H(2)O)-Ru(tpy)(tpy(PO(3)H(2)))](4+) (tpy = 2,2':6,2''-terpyridine; bpy = 2,2'-bipyridine) are reported. We characterize the electronic excitations, electron injection time scales, and interfacial electron transfer (IET) mechanisms through phosphonate anchoring groups. These are promising alternatives to the classic carboxylates of conventional dye-sensitized solar cells since they bind more strongly to TiO(2) surfaces and form stable covalent bonds that are unaffected by humidity. Density functional theory calculations and quantum dynamics simulations of IET indicate that electron injection in 1-TiO(2) can be up to 1 order of magnitude faster when 1 is attached to TiO(2) in a bidentate mode (tau approximately 60 fs) than when attached in a monodentate motif (tau approximately 460 fs). The IET time scale also depends strongly on the properties of the sensitizer as well as on the nature of the electronic excitation initially localized in the adsorbate molecule. We show that IET triggered by the visible light excitation of 2-TiO(2) takes 1-10 ps when 2 is attached in a bidentate mode, a time comparable to the lifetime of the excited electronic state. IET due to visible-light photoexcitation of 3-TiO(2) is slower, since the resulting electronic excitation remains localized in the tpy-tpy bridge that is weakly coupled to the electronic states of the conduction band of TiO(2). These results are particularly valuable to elucidate the possible origin of IET efficiency drops during photoconversion in solar cells based on Ru(II)-polypyridine complexes covalently attached to TiO(2) thin films with phosphonate linkers.

Communication is important: The dimeric bis(imido) uranium complex [{U(NtBu)(2)(I)(tBu(2)bpy)}(2)] (see picture; U green, N blue, I red) has cation-cation interactions between [U(NR)(2)](+) ions. This f(1)-f(1) system also displays f orbital communication between uranium(V) centers at low temperatures, and can be oxidized to generate uranium(VI) bis(imido) complexes.

The first mononuclear molecular mixed-ligand U(VI) solid containing hydroxide and peroxide ligands, Na6[UO2(O2)2(OH)2](OH)2·14H2O (I), was synthesized and structurally characterized using single crystal X-ray diffraction. The crystal structure of I consisted of [UO2(O2)2(OH)2]4− molecular units, with a uranyl(VI) moiety perpendicular to 6 equatorial O atoms belonging to two side-on trans peroxo groups and two terminal trans hydroxo groups. Density functional theory (DFT) calculations determined that the trans -conformer of the [UO2(O2)2(OH)2]4− molecular unit found in I was 24 kcal/mol lower in energy than the previously proposed cis -conformer. Crystal data: monoclinic, space group P21/n with a=13.357(4) Å, b=5.8521(15) Å, c=15.948(6) Å, β=112.292(4)°, and Z=2.

Density Functional calculations have been performed at the uB3LYP and uBP86 levels to calculate the one-electron redox potentials for a series of small models based on the diiron hydrogenase enzymes in the presence of acetonitrile (MeCN). The solvation effects in MeCN are incorporated via a self-consistent reaction field (SCRF) using the polarized continuum model (PCM). The calculated redox potentials reproduce the trends in experimental data with an average error of only 0.12 V using the BP86 functional, whereas comparing results with the B3LYP functional require a systematic shift of -0.82 and -0.53 V for oxidation and reduction, respectively. The bonding orbitals and d-electron populations were examined using Mulliken population analysis, and the results were used to rationalize the calculated and observed redox potentials. These studies demonstrate that the redox potential correlates with the empirical spectrochemical series for the ligands, as well as with the amount of electron density donated by the ligand onto the Fe centers.

For 3-5d transition-metal ions, the (C5R5)2MCl2 (R = H, Me for M = Ti, Zr, Hf) bent metallocenes represent a series of compounds that have been central in the development of organometallic chemistry and homogeneous catalysis. Here, we evaluate how changes in the principal quantum number for the group IV (C5H5)2MCl2 (M = Ti, Zr, Hf; 1- 3, respectively) complexes affects the covalency of M-Cl bonds through application of Cl K-edge X-ray Absorption Spectroscopy (XAS). Spectra were recorded on solid samples dispersed as a thin film and encapsulated in polystyrene matrices to reliably minimize problems associated with X-ray self-absorption. The data show that XAS pre-edge intensities can be quantitatively reproduced when analytes are encapsulated in polystyrene. Cl K-edge XAS data show that covalency in M-Cl bonding changes in the order TiZrHf and demonstrates that covalency slightly decreases with increasing principal quantum number in 1-3. The percent Cl 3p character was experimentally determined to be 26, 23, and 18% per M-Cl bond in the thin-film samples for 1-3 respectively and was indistinguishable from the polystyrene samples, which analyzed as 25, 25, and 19% for 1-3, respectively. To aid in interpretation of Cl K-edge XAS, 1-3 were also analyzed by ground-state and time-dependent density functional theory (TD-DFT) calculations. The calculated spectra and percent chlorine character are in close agreement with the experimental observations, and show 20, 18, and 17% Cl 3p character per M-Cl bond for 1-3, respectively. Polystyrene matrix encapsulation affords a convenient method to safely contain radioactive samples to extend our studies to include actinide elements, where both 5f and 6d orbitals are expected to play a role in M-Cl bonding and where transition assignments must rely on accurate theoretical calculations.

The discovery that imido analogs of actinyl dioxo cations can be extended beyond uranium into the transuranic elements is presented. Synthesis of the Np(V) complex, Np(NDipp)2((t)Bu2bipy)2Cl (1), is achieved through treatment of a Np(IV) precursor with a bipyridine coligand and lithium-amide reagent. Complex 1 has been structurally characterized, analyzed by (1)H NMR and UV-vis-NIR spectroscopies, and the electronic structure evaluated by DFT calculations.

Variable temperature scanning tunneling microscopy (VT STM) and theoretical ab initio computer simulations were used to study dissociative chemisorption and competitive surface chemistry of CCl 4 on an iron-terminated Fe 3 O 4 (111) 2 × 2 surface in an ultrahigh vacuum. The Fe 3 O 4 (111) surface was exposed to CCl 4 molecules at 224 K, slowly annealed to 500 K (0.2 K/s), and scanned at room temperature. Two different chlorine species were observed only on the iron-terminated Fe 3 O 4 (111) 2 × 2 surface due to chemisorption of CCl 4 , one on top of surface-terminating iron atoms (Cl bound to surface irons) and the other at 3-fold oxygen vacancy sites (Cl bound to subsurface irons). The ratio of the number of chlorine species on top sites to the number at 3-fold oxygen vacancy sites is approximately 9:1, which is dramatically different from the 1:10 ratio observed when CCl 4 is dosed at room temperature. The difference in the ratio of these two chlorine species can be explained with a competitive surface reaction picture in which phosgene evolution/surface oxygen atom abstraction, leading to chlorine species at 3-fold oxygen vacancy sites, can only favorably compete with recombination and association reactions, leading to chlorine atoms on top sites, near room temperature. Theoretical calculations were performed that predict an activation barrier of 0.16 eV for the production of phosgene from CCl 4 reacting with the iron-terminated Fe 3 O 4 (111) 2 x 2 surface.

Here we describe the synthesis of two imido analogs of the uranyl ion, UO 2+ 2 , in which the oxygens are replaced by divalent alkyl or aryl nitrogen groups: U(N t Bu) 2 I 2 (THF) 2 (1) and U(NPh) 2 I 2 (THF) 3 (2) (where t Bu is tert -butyl and THF is tetrahydrofuran). Both compounds have been fully characterized by standard analytical techniques, including x-ray crystallography, and the chemical bonding between the metal center and the nitrogen ligands was quantified by using hybrid density functional theory calculations. As expected for a uranyl analog, these complexes exhibit linear N-U-N linkages and very short U-N bonds. In addition, the theoretical calculations show strong involvement of the 5f and 6d electrons in the U-N bonding.

Uranium nitride [U[triple bond]N](x) is an alternative nuclear fuel that has great potential in the expanding future of nuclear power; however, very little is known about the U[triple bond]N functionality. We show, for the first time, that a terminal uranium nitride complex can be generated by photolysis of an azide (U-N=N=N) precursor. The transient U[triple bond]N fragment is reactive and undergoes insertion into a ligand C-H bond to generate new N-H and N-C bonds. The mechanism of this unprecedented reaction has been evaluated through computational and spectroscopic studies, which reveal that the photochemical azide activation pathway can be shut down through coordination of the terminal azide ligand to the Lewis acid B(C(6)F(5))(3). These studies demonstrate that photochemistry can be a powerful tool for inducing redox transformations for organometallic actinide complexes, and that the terminal uranium nitride fragment is reactive, cleaving strong C-H bonds.

Actinide complexes of the redox-active ligand (dpp-BIAN)(2-) (dpp-BIAN = 1,2-bis(2,6-diisopropylphenylimino)acenaphthylene), U(dpp-BIAN)(2) (1), U(dpp-BIAN)(2)(THF) (1-THF), and Th(dpp-BIAN)(2)(THF) (2-THF), have been prepared. Solid-state magnetic and single-crystal X-ray data for complex 1 indicate a ground-state U(IV)-pi*(4) configuration, whereas a (dpp-BIAN)(2-)-to-uranium electron transfer occurs for 1-THF, resulting in a U(III)-pi*(3) ground configuration. The solid-state magnetic data also indicate that interconversion between the two forms of the complex is possible, limited only by the ability of tetrahydrofuran (THF) vapor to penetrate the solid upon cooling of the sample. In contrast to those in the solid state, spectroscopic data acquired in THF indicate only the presence of the U(IV)-pi*(4) form for 1-THF in solution, evidenced by electronic absorption spectra and by measurement of the solution magnetic moment in THF-d(8) using the Evans method. Also reported is the electrochemistry of the complexes collected in CH(2)Cl(2), CF(3)C(6)H(5), and THF. As expected from the solution spectroscopic data, only small differences are observed in half-wave potentials of ligand-based processes in the presence of THF, consistent with the solution U(IV)-pi*(4) configuration of the complexes in all cases. Density functional theory calculations were undertaken for complexes 1 and 1-THF to determine if intrinsic energetic or structural factors underlie the observed charge-transfer process. While the calculated optimized geometries agree well with experimental results, it was not possible to arrive at a convergent solution for 1-THF in the U(III)-pi*(3) configuration. However, perturbations in the orbital energies in 1 versus 1-THF for the U(IV)-pi*(4) configuration do point to a diminished highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap in 1-THF, consistent with the solid-state magnetic data. These results represent the first example of a stable and well-defined, reversible intramolecular electron transfer in an actinide complex with redox-active ligands.

Density Functional calculations have been performed to calculate the one-electron oxidation potential for ferrocene and the redox couples for a series of small transition metal compounds of the first-, second-, and third-row elements. The solvation effects are incorporated via a self-consistent reaction field (SCRF), using the polarized continuum model (PCM). From our study of seven different density functionals combined with three different basis sets for ferrocene, we find that no density functional method can reproduce the redox trends from experiment when referencing our results to the experimental absolute standard hydrogen electrode (SHE) potential. In addition, including additional necessary assumptions such as solvation effects does not lead to any conclusion regarding the appropriate functional. However, we propose that if one references their transition metal compounds results to the calculated absolute half-cell potential of ferrocene, they can circumvent the additional assumptions necessary to predict a redox couple. Upon employing this method on several organometallic and inorganic complexes, we obtained very good correlation between calculated and experimental values (R(2) = 0.97), making it possible to predict trends with a high level of confidence. The hybrid functional B3LYP systematically underestimates the redox potential; however, the linear correlation between DFT and experiment is good (R(2) = 0.96) when including a baseline shift. This protocol is a powerful tool that allows theoretical chemists to predict the redox potential in solution of several transition metal complexes a priori and aids in the rational design of redox-active catalysts.

Addition of 1 and 2 equiv of ArNCO (ArPh, 2,4,6-Me3C6H2) to U(NtBu)2(I)2(OPPh3)2 yields the aryl−imido complexes U(NAr)(NtBu)(I)2(OPPh3)2 and U(NAr)2(I)2(OPPh3)2, respectively. Unlike analogous transition metal reactions this imido exchange reaction does not proceed through a metal oxo intermediate. Density functional theory calculations and 15N-labeling studies suggest this transformation involves the [2 + 2] cycloaddition of the aryl isocyanate CN bond across the UN imido ligand to form an N,N-bound ureato intermediate.

We investigate the role of intra-molecular conformational disorder and inter-molecular electronic interactions on the electronic structure of disorder clusters of poly-p-phenylene vinylene (PPV) oligomers. Classical molecular dynamics is used to determine probable molecular geometries, and first-principle density functional theory (DFT) calculations are used to determine electronic structure. Intra-molecular and inter-molecular effects are disentangled by contrasting results for densely packed oligomer clusters with those for ensembles of isolated oligomers with the same intra-molecular geometries. We find that electron trap states are induced primarily by intra-molecular configuration disorder, while the hole trap states are generated primarily from inter-molecular electronic interactions., 4 pages, 4 figures. Compile with pdflatex

Addition of 1.5 equiv of I2 to a THF solution of UI3(THF)4, containing either 6 equiv of tBuNH2 or 2 equiv of RNH2 (R = Ph, 3,5-(CF3)2C6H3, 2,6-(iPr)2C6H3) and 4 equiv of NEt3, generates orange solutions containing U(NtBu)2I2(THF)2 (1) or U(NAr)2I2(THF)3 (Ar = Ph, 2; 3,5-(CF3)2C6H3, 3; 2,6-(iPr)2C6H3, 4), respectively, all of which can be isolated in good yields. Alternatively, 1 can be prepared by reaction of uranium metal with 3 equiv of I2 and 6 equiv of tBuNH2, also in good yield. Complexes 1-4 have been characterized by X-ray crystallography, and each of these complexes exhibits linear N-U-N linkages and short U-N bonds. Using density functional theory simulations of complexes 1 and 2, two triple bonds between the metal center and the nitrogen ligands were identified. Complexes 1 and 2 readily react with neutral Lewis bases such as pyridine or Ph3PO to form U(NR)2I2(L)2 (R = tBu, L = py, 5; Ph3PO, 7; R = Ph, L = py, 6; Ph3PO, 8), and with PMe3 to form U(NR)2I2(THF)(PMe3)2 (R = tBu, 9; Ph, 10). The solid-state molecular structures of 5, 7, and 9 have been determined by X-ray crystallography, and these complexes, like their parent compounds, exhibit linear N-U-N angles and short U-N bonds. Complexes 1 and 2 also react with AgOTf in CH2Cl2, forming U(NR)2(OTf)2(THF)3 (R = tBu, 11; Ph, 12) after recrystallization from THF. Crystals of 12 grown from CH2Cl2 were found to contain a dimer, [U(NPh)2(OTf)2(THF)2]2, a complex possessing bridging triflate groups.

We present a theoretical study of the low-lying electronic excitations in the [Pt(P((n)Bu)(3))(2)(ethynylbenzene)(2)] molecule. Although the ground electronic state possesses D(2)(h) symmetry, with both ethynylbenzene ligands being equivalent, in the excited state the molecule breaks the symmetry deforming along a b(3u) direction and localizing the excitation on a single ligand. This localized exciton is of (3)pipi* nature with the unpaired electron and hole spread over one of the benzene rings and the ethyne linkage. The localization indicates an activated hopping mechanism for transport. Our estimate of the barrier is approximately 0.61 eV.

The electronic structure of the ruthenium blue dimer was studied with hybrid density functional theory (DFT) and complete action space self-consistent field (CASSCF) calculations. Hybrid DFT was found to overestimate the stability of the lowest triplet which leads to predicting this state as the ground state. However, CASSCF gives a singlet ground state in agreement with experiment. The density of the CASSCF ground state is in good agreement with the broken symmetry state obtained from DFT. These two results lead to the interpretation that the ground state of the ruthenium blue dimer is a partially localized antiferromagnetically coupled singlet state that lies three-quarters of the way toward the weak coupling limit. The electrons were found to localize in the dπ manifold.

A new tetrathiafulvalene-salphen uranyl complex has been prepared. The system was designed to study the electronic coupling between actinides and a redox active ligand framework. Theoretical and experimental methods – including DFT calculations, single crystal X-ray analysis, cyclic voltammetry, NMR and IR spectroscopies – were used to characterize this new uranyl complex.