Scott R. Daly

Synthesis, characterization, and multielectron reduction chemistry of uranium supported by redox-active α-diimine ligands.

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.

Synthesis and properties of a fifteen-coordinate complex: The thorium aminodiboranate [Th(H3BNMe2BH3)4]

The concept of coordination number is extremely useful and is widely employed to describe the local chemical environments of atoms. Originally defined by Alfred Werner in 1893, the coordination number is closely related to many other important properties such as atomic radius, molecular and electronic structure, and chemical reactivity. An important modification of Werner s original concept was the recognition that, for certain ligands such as ethylene, two linked atoms jointly occupy a single coordination site. This modified definition is widely used to describe both transitionmetal (d-block) and inner-transition-metal (f-block) complexes. An interesting question is: what is the largest possible coordination number? This question has recently been considered theoretically, and the 15-coordinate ion PbHe15 2+ has been predicted to be a bound species. The complexes tetrakis(cyclopentadienyl)uranium [UCp4] and its thorium analogue [ThCp4] are each connected to 20 atoms, [10] but the Werner coordination number of 12 (counting p bonds as occupying one site) is widely acknowledged to be more appropriate to describe the metal–ligand bonding in these compounds. Very high Werner coordination numbers are seen for metal complexes of the borohydride anion BH4 , which can coordinate to a single metal through as many as three hydrogen atoms. From an electronic perspective, each B-H-M interaction involves a separate electron pair, 14] and each BH-M interaction can be considered as a separate bond. Accordingly, [Zr(BH4)4], [15–17] [Hf(BH4)4], [15,16, 18] [Np(BH4)4], [19] and [Pu(BH4)4], [19] all have coordination numbers of 12, and [Th(BH4)4], [15, 16] [Pa(BH4)4], [19] and [U(BH4)4], [20] all of which are polymers in the solid state, have coordination numbers of 14. Some derivatives of these compounds also have high coordination numbers, such as the 14-coordinate tetrahydrofuran complex [U(BH4)4(thf)2]. [21] No complex of any kind, however, has been definitively shown to adopt a Werner coordination number higher than 14. Herein, we report the synthesis, single-crystal X-ray and neutron diffraction studies, and DFT investigations of the first 15-coordinate complex. DFT calculations suggest that this complex may adopt a 16-coordinate structure in the gas phase. This compound extends our recent studies of a new class of chelating borohydride ligands, that is, the aminodiboranates, some of which form highly volatile complexes that are useful as precursors for the chemical vapor deposition of thin films. Reaction of ThCl4 with four equivalents of sodium N,Ndimethylaminodiboranate, Na(H3BNMe2BH3), in tetrahydrofuran produced [Th(H3BNMe2BH3)4] (1), which could be isolated as colorless prisms by crystallization from diethyl ether. The IR spectrum of 1 contains strong bands at 2420 cm 1 that arise from terminal B–H stretches, and at 2264 and 2208 cm 1 that arise from bridging B-H···Th stretches. For comparison, [Th(BH4)4] contains a strong terminal B–H band at 2530 cm 1 and bridging B-H-M bands at 2270, 2200, and 2100 cm . The H NMR spectrum of 1 (C6D6 at 20 8C) contains peaks at d = 2.11 ppm (s, NMe2) and d = 4.23 ppm (br 1:1:1:1 q, JBH = 90 Hz, BH3); the terminal and bridging B–H units thus exchange rapidly on the NMR time scale. The B NMR spectrum consists of a binomial quartet at d = 2.75 ppm, which arises from coupling of the B nuclei with the three rapidly exchanging H nuclei (JHB = 90 Hz). For comparison, the B spectrum of [Th(BH4)4] consists of a quintet at d = 8.0 ppm (JBH = 86.5 Hz). Single-crystal X-ray and neutron diffraction studies of 1 reveal that it is monomeric with four chelating aminodiboranate ligands. The eight boron atoms describe a distorted D2d dodecahedral structure, in which boron atoms B1, B2, B2A, and B1A describe one planar trapezoidal array, and atoms B3, B4, B5, and B6 describe the other (Figure 1). The B2-Th1B2A and B4-Th1-B6 angles between wingtip boron atoms are almost linear at 172.61(12)8 and 171.85(13)8, respectively. Interestingly, seven of the eight Th···B distances (those for boron atoms B1–B5) range from 2.882(3) to 2.949(3) , but [*] S. R. Daly, Prof. G. S. Girolami School of Chemical Sciences University of Illinois at Urbana-Champaign 600 South Matthews Avenue, Urbana, IL 61801 (USA) Fax: (+ 1)217-244-3186 E-mail: girolami@scs.illinois.edu

Covalency in Metal–Oxygen Multiple Bonds Evaluated Using Oxygen K-edge Spectroscopy and Electronic Structure Theory

Advancing theories of how metal-oxygen bonding influences metal oxo properties can expose new avenues for innovation in materials science, catalysis, and biochemistry. Historically, spectroscopic analyses of the transition metal MO(4)(x-) anions have formed the basis for new M-O bonding theories. Herein, relative changes in M-O orbital mixing in MO(4)(2-) (M = Cr, Mo, W) and MO(4)(-) (M = Mn, Tc, Re) are evaluated for the first time by nonresonant inelastic X-ray scattering, X-ray absorption spectroscopy using fluorescence and transmission (via a scanning transmission X-ray microscope), and time-dependent density functional theory. The results suggest that moving from Group 6 to Group 7 or down the triads increases M-O e* (π*) mixing; for example, it more than doubles in ReO(4)(-) relative to CrO(4)(2-). Mixing in the t(2)* orbitals (σ* + π*) remains relatively constant within the same Group, but increases on moving from Group 6 to Group 7. These unexpected changes in orbital energy and composition for formally isoelectronic tetraoxometalates are evaluated in terms of periodic trends in d orbital energy and radial extension.

Lanthanide N,N-dimethylaminodiboranates: Highly volatile precursors for the deposition of lanthanide-containing thin films

New lanthanide CVD precursors of stoichiometry Ln(H(3)BNMe(2)BH(3))(3) have been prepared, where Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. The ligand is N,N-dimethylaminodiboranate, a new kind of multidentate borohydride. The structures of the Ln(H(3)BNMe(2)BH(3))(3) complexes are highly dependent on the size of the lanthanide ions: the coordination number decreases from Pr (CN = 14) to Sm (CN = 13) to Er (CN = 12) corresponding with the decrease in ionic radii. The Ln(H(3)BNMe(2)BH(3))(3) complexes are highly volatile and sublime at temperatures as low as 65 degrees C in vacuum. These complexes are useful CVD precursors; for example, Y(H(3)BNMe(2)BH(3))(3) has been used to deposit Y(2)O(3) on silicon at 300 degrees C by CVD using water as a coreactant.

Field-directed sputter sharpening for tailored probe materials and atomic-scale lithography

Fabrication of ultrasharp probes is of interest for many applications, including scanning probe microscopy and electron-stimulated patterning of surfaces. These techniques require reproducible ultrasharp metallic tips, yet the efficient and reproducible fabrication of these consumable items has remained an elusive goal. Here we describe a novel biased-probe field-directed sputter sharpening technique applicable to conductive materials, which produces nanometer and sub-nanometer sharp W, Pt-Ir and W-HfB(2) tips able to perform atomic-scale lithography on Si. Compared with traditional probes fabricated by etching or conventional sputter erosion, field-directed sputter sharpened probes have smaller radii and produce lithographic patterns 18-26% sharper with atomic-scale lithographic fidelity.

Synthesis, characterization, and structures of uranium(III) N,N-dimethylaminodiboranates.

The reaction of UCl(4) with sodium N,N-dimethylaminodiboranate, Na(H(3)BNMe(2)BH(3)), in diethyl ether affords the uranium(III) product U(H(3)BNMe(2)BH(3))(3), which has been crystallized as two different structural isomers from pentane and toluene, respectively. The isomer crystallized from pentane is a polymer in which each uranium center is bonded to three chelating H(3)BNMe(2)BH(3)(-) (DMADB) ligands and to one hydrogen atom from a neighboring molecule so as to form an intermolecular B-H-U bridge; each uranium center is coordinated to 13 hydrogen atoms. The isomer crystallized from toluene is also polymeric, but the uranium atoms are coordinated by two chelating DMADB ligands and two bridging DMADB ligands bound in a U(kappa(3)H-H(3)BNMe(2)BH(3)-kappa(3)H)U fashion, so that each uranium atom is 14-coordinate. When the reaction of UCl(4) with Na(H(3)BNMe(2)BH(3)) is conducted in tetrahydrofuran (thf) or 1,2-dimethoxyethane (dme), the adducts U(H(3)BNMe(2)BH(3))(3)(thf) and U(H(3)BNMe(2)BH(3))(3)(dme) are obtained. The rate of reduction from U(IV) to U(III) is correlated with the donor ability of the solvent, the relative rates being Et(2)O > thf > dme. The addition of trimethylphosphine to U(H(3)BNMe(2)BH(3))(3)(thf) generates U(H(3)BNMe(2)BH(3))(3)(PMe(3))(2). This compound slowly decomposes at room temperature over several months to yield the new borane PMe(3)BH(2)NMe(2)BH(3), mu-(N,N-dimethylamido)pentahydro(trimethylphosphine)diboron. Single crystal X-ray diffraction studies, (1)H and (11)B NMR spectra, IR data, and field ionization mass spectra for the uranium complexes are reported.

Synthesis, characterization, and structures of divalent europium and ytterbium N,N-dimethylaminodiboranates.

Treatment of the trichlorides EuCl(3) and YbCl(3) with Na(H(3)BNMe(2)BH(3)) in tetrahydrofuran (THF) results in a reduction to the corresponding divalent europium and ytterbium N,N-dimethylaminodiboranate (DMADB) complexes Eu(H(3)BNMe(2)BH(3))(2)(THF)(2) (1) and Yb(H(3)BNMe(2)BH(3))(2)(THF)(2) (2), which can be separated from trivalent Ln(H(3)BNMe(2)BH(3))(3)(THF) byproducts by extraction and crystallization from pentane. No other lanthanide trihalides react with Na(H(3)BNMe(2)BH(3)) to afford divalent products. Compounds 1 and 2 can also be prepared from the divalent lanthanide iodides EuI(2) and YbI(2) in higher yield and without the need to separate them from trivalent species. Treatment of 1 and 2 with an excess of 1,2-dimethoxyethane (DME) in pentane affords the new species Eu(H(3)BNMe(2)BH(3))(2)(DME)(2) (3) and Yb(H(3)BNMe(2)BH(3))(2)(DME) (4). Compound 1 is dinuclear: each metal center is bound to two chelating DMADB ligands, one of which also bridges to the other metal. Overall, the coordination geometry about each Eu atom can be described as a distorted pentagonal bipyramid, with five B atoms from the DMADB ligands occupying the equatorial sites and two THF molecules occupying the axial sites. Unlike 1, compound 2 is monomeric owing to the smaller radius of Yb(II) versus Eu(II); the B and O atoms describe a distorted cis octahedron. The Eu(DME) complex 3 is also monomeric; both DMADB ligands and both DME molecules chelate to the metal center. The four B atoms and the four O atoms describe a distorted square antiprism, with the O atoms occupying one square face and the B atoms occupying the other. In addition to X-ray crystallographic studies, IR, NMR, and mass spectrometric data are reported for all four new compounds.

Growth inhibition to enhance conformal coverage in thin film chemical vapor deposition.

We introduce the use of a growth inhibitor to enhance thin film conformality in low temperature chemical vapor deposition. Films of TiB(2) grown from the single source precursor Ti(BH(4))(3)(dme) are much more highly conformal when grown in the presence of one of the film growth byproducts, 1,2-dimethoxyethane (dme). This effect can be explained in terms of two alternative inhibitory mechanisms: one involving blocking of surface reactive sites, which is equivalent to reducing the rate of the forward reaction leading to film growth, the other analogous to Le Chateliers principle, in which the addition of a reaction product increases the rate of the back reaction. The reduction in growth rate corresponds to a reduction in the sticking probability of the precursor, which enhances conformality by enabling the precursor to diffuse deeper into a recessed feature before it reacts.

Direct Writing of Sub-5 nm Hafnium Diboride Metallic Nanostructures

Sub-5 nm metallic hafnium diboride (HfB(2)) nanostructures were directly written onto Si(100)-2 × 1:H surfaces using ultrahigh vacuum scanning tunneling microscope (UHV-STM) electron beam induced deposition (EBID) of a carbon-free precursor molecule, tetrakis(tetrahydroborato)hafnium, Hf(BH(4))(4). Scanning tunneling spectroscopy data confirm the metallic nature of the HfB(2) nanostructures, which have been written down to lateral dimensions of ∼2.5 nm. To our knowledge, this is the first demonstration of sub-5 nm metallic nanostructures in an STM-EBID experiment.

Probing Ni[S2PR2]2 electronic structure to generate insight relevant to minor actinide extraction chemistry.

A method to evaluate the electronic structure of minor actinide extractants is described. A series of compounds containing effective and ineffective actinide extractants (dithiophosphinates, S(2)PR(2)(-)) bound to a common transition metal ion (Ni(2+)) was analyzed by structural, spectroscopic, and theoretical methods. By using a single transition metal that provides structurally similar compounds, the metal contributions to bonding are essentially held constant so that subtle electronic variations associated with the extracting ligand can be probed using UV-vis spectroscopy. By comparison, it is difficult to obtain similar information using analogous techniques with minor actinide and lanthanide complexes. Here, we demonstrate that this approach, supplemented with ground state and time-dependent density functional theory, provides insight for understanding why high separation factors are reported for the extractant HS(2)P(o-CF(3)C(6)H(4))(2), while lower values are reported and anticipated for other HS(2)PR(2) derivatives (R = C(6)H(5), p-CF(3)C(6)H(4), m-CF(3)C(6)H(4)). The implications of these results for correlating electronic structure with the selectivity of HS(2)PR(2) extractants are discussed.