Synthesis of Air‐Stable,Volatile Uranium(IV) and (VI) Compounds and Their Gas‐Phase Conversion To Uranium Oxide Films

Four air‐stable, volatile uranium heteroarylalkenolates have been synthesized and characterized by three synthetic approaches and their gas phase deposition to uranium oxide films has been examined.     

Intramolecular ditryptophan crosslinks enforce two types of antiparallel beta structures.

BACKGROUND: Two types of biaryl crosslinks can be formed with natural protein sidechains: ditryptophan and dityrosine. Biaryl crosslinks have the same topology as disulfide crosslinks, yet little is known about their effect on local peptide structure. RESULTS: Three ditryptophan-linked peptide dimers based on the sequence Ac-Leu-Trp-Ala-COX were prepared. The tripeptide dimer with -CONH(2) termini was too insoluble to study, but the tripeptide dimer with -COOMe termini crystallized from methanol/chloroform as an antiparallel beta-sheet. The tripeptide dimer with a -CONMe(2) termini adopted a slipped antiparallel beta structure in methanol/chloroform. CONCLUSIONS: These results suggest that intermolecular ditryptophan crosslinks that join the middle of peptide chains can confer a preference for antiparallel beta-sheet structure. The effect is most dramatic when both the inside and outside edges of the dimer can form hydrogen bonds as in the crystal structure of dimer 3b.     

Importance of energy level matching for bonding in Th(3+)-Am(3+) actinide metallocene amidinates, (C(5)Me(5))(2)[(i)PrNC(Me)N(i)Pr]An

The synthesis of a rare trivalent Th(3+) complex, (C(5)Me(5))(2)[(i)PrNC(Me)N(i)Pr]Th, initiated a density functional theory analysis on the electronic and molecular structures of trivalent actinide complexes of this type for An = Th, Pa, U, Np, Pu, and Am. While the 6d orbital is found to accommodate the unpaired spin in the Th(3+) species, the next member of the series, Pa, is characterized by an f(2) ground state, and later actinides successively fill the 5f shell. In this report, we principally examine the evolution of the bonding as one advances along the actinide row. We find that the early actinides (Pa-Np) are characterized by localized f orbitals and essentially ionic bonding, whereas the f orbitals in the later members of the series (Pu, Am) exhibit significant interaction and spin delocalization into the carbon- and nitrogen-based ligand orbitals. This is perhaps counter-intuitive since the f orbital radius and hence metal-ligand overlap decreases with increasing Z, but this trend is counter-acted by the fact that the actinide contraction also leads to a stabilization of the f orbital manifold that leads to a near degeneracy between the An 5f and cyclopentadienyl π-orbitals for Pu and Am, causing a significant orbital interaction.     

Reduction of dinitrogen with an yttrium metallocene hydride precursor, [(C5Me5)2YH]2

Treatment of [(C(5)Me(5))(2)YH](2), 1, with KC(8) under N(2) in methylcyclohexane generates the unsolvated reduced dinitrogen complex, [(C(5)Me(5))(2)Y](2)(μ-η(2):η(2)-N(2)), 2, and extends the range of yttrium and lanthanide LnZ(2)Z'/M (Z = monoanion; M = alkali metal) dinitrogen reduction reactions to (Z')(-) = (H)(-). The hydride complex, 1, is unique in this reactivity compared to other alkane-soluble yttrium metallocenes, [(C(5)Me(5))(2)YX](x) {X = [N(SiMe(3))(2)](-), (Me)(-), (C(3)H(5))(-), and (C(5)Me(5))(-)} which did not generate 2 when treated with KC(8). [(C(5)Me(5))(2)LnH](x)/KC(8)/N(2) reactions with Ln = La and Lu did not give isolable dinitrogen complexes. Complex 2 and the unsolvated lutetium analogue, [(C(5)Me(5))(2)Lu](2)(μ-η(2):η(2)-N(2)), 3, were obtained using benzene as a solvent and [(C(5)Me(5))(2)Ln][(μ-Ph)(2)BPh(2)] as precursors with excess KC(8). Complex 2 functions as a reducing agent with PhSSPh to form [(C(5)Me(5))(2)Y(μ-SPh)](2), 4, in high yield.     

Organolutetium vinyl and tuck-over complexes via C-H bond activation

The vinyl C-H bond of tetramethylfulvene is activated in the presence of [(C5Me5)2LuH]x, 1, to form a vinyl organolutetium complex, (C5Me5)2Lu(CH=C5Me4), 2. Also formed in the reaction is the "tuck-over" complex, (C5Me5)2Lu(mu-H)(mu-eta1:eta5-CH2C5Me4)Lu(C5Me5), 3, containing a (CH2C5Me4)2- moiety long postulated to exist in organolutetium chemistry but never crystallographically characterized. Evidence for these C-H bond activations by a "(C5Me5)3Lu" intermediate, 4, is presented. Complex 3 can also be made in high yield by thermolysis of 1. Under H2, 1 catalytically hydrogenates TMF to C5Me5H.     

Tetrahedral versus square planar arrangement of cyclopentadienyl ligands in bimetallic organosamarium complexes. X-ray crystal structure of [(C5H4Me)2(THF)Sm(μ-Cl)]2

The effects of cyclopentadienyl ring size on the geometry of bimetallic organosamarium complexes have been studied by comparing the X-ray crystal structure of [(C₅H₄Me)₂(THF)Sm(μ-Cl)]₂, prepared from KC₅H₄Me and SmCl₃ in THF, with C₅Me₅ analogs. The complex crystallizes from THF at −30°C in space group Pbcn with a = 20.312(5), b = 9.626(2), c = 16.225(3) Å, V = 3172.5(12) ų and D_calc = 1.74 g cm⁻³ for Z = 4. Least-squares refinement of the model based on 1759 reflections [|F_o| > 2.0σ(|F_o|)] converged to a final R_F = 5.0%. The complex adopts a geometry which has a molecular two-fold rotation axis perpendicular to the Sm₂Cl₂ plane and a crystallographic inversion center. Hence, both methyl groups of each (C₅H₄Me)₂Sm unit are located on the side opposite of the THF ligands, which are trans to each other, and the four C₅H₄Me ring centroids define a square plane. The Sm---Cl distances are 2.759(3) and 2.819(3) Å.     

Flexibility in the coordination chemistry of the 2,3-dimethylindolide ligand with potassium,yttrium, and samarium

The coordination chemistry of the 2,3-dimethylindolide anion (DMI), (Me(2)C(8)H(4)N)(-), with potassium, yttrium, and samarium ions is described. In the potassium salt [K(DMI)(THF)](n), 1, prepared from Me(2)C(8)H(4)NH and KH in THF, the dimethylindole anion binds and bridges potassium ions in three different binding modes, namely eta(1), eta(3), and eta(5), to form a two-dimensional extended structure. In the dimethoxyethane (DME) adduct [K(DMI)(DME)(2)](2), 2, prepared by crystallizing a sample of 1 from DME, DMI exists as a mu-eta(1):eta(1) ligand. Compound 1 reacts with SmI(2)(THF)(4) in THF to form the distorted octahedral complex trans-(DMI)(2)Sm(THF)(4), 3, in which the dimethyindolide anions are bound in the eta(1) mode to samarium. Reaction of 2,3-dimethylindole with Y(CH(2)SiMe(3))(3)(THF)(2) afforded the amide complex (DMI)(3)Y(THF)(2), 4, in which the dimethylindolide anions are also bound in the eta(1) mode to yttrium. Compound 1 also reacts with (C(5)Me(5))(2)LnCl(2)K(THF)(2) (Ln = Sm, Y) to form unsolvated amide complexes (C(5)Me(5))(2)Ln(DMI) (Ln = Sm, 5; Y, 6), in which DMI attaches primarily through nitrogen, although the edge of the arene ring is oriented toward the metals at long distances.     

Zum Scherverhalten unzerstörter adsorbierter tensidmodifizierter Gelantineschichten

Zusammenfassung Untersucht wurde das grenzflächenrheologische Verhalten unzerstörter 0.2% Gelatine/Natrium-dodecylsulfat (10^–4, 10^–5 M/l)-Adsorptionsschichten bei Scherung an der Phasengrenze Luft/Lösung. Bei der Ermittlung der rheologischen Parameter muß eine Fließgrenze berücksichtigt werden, unterhalb derer lediglich spontan- und verzögert-elastisches Verhalten zu beobachten ist. Das Kriechdeformationsverhalten wurde mit einem Schofield-Scott-Blair-Modell beschrieben. Eine Analyse zur Modellierung des Fließvorganges ergab, daß Messungen bei diskontinuierlichen Scherbedingungen, bei denen die Fließgrenze nicht im gesamten Bereich der Scherung überschritten wird, wegen des dabei realisierten großen Meßeffekts für die Parameterbestimmung am geeignetsten sind. Die das Fließverhalten der Phasengrenzschicht beschreibenden Parameter weisen eine geringe Streuung (< 10%) auf, Die Ignorierung der Fließgrenze führt zu einer scheinbaren Erhöhung der plastischen Grenzflächenscherviskosität.Der Zusatz von Natrium-dodecylsulfat zu Gelatine bewirkt im untersuchten Konzentrationsbereich mit steigender Tensidkonzentration neben dem Auftreten der Fließgrenze eine erhebliche Erhöhung aller rheologischen Parameter, welche sich nicht additiv aus den Parametern der Einzelkomponenten ergibt. Das Ansteigen der rheologischen Parameter wird als Strukturverfestigung interpretiert.

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The surface-rheological behaviour of undestroyed 0.2% gelatine/sodium-dodeylsulfate (10^–4, 10^–5 M/l)-adsorption-layers at the interface air/liquid was investigated. For the determination of the rheological parameters a yield value must be taken into account. By stressing the structure below this yield value only instantaneous and retarded-elastic deformations were detected. The creep behaviour of the investigated structures was described by a Schofield-Scott-Blair-model. At discontinuous shear-conditions the yield value is not reached in the whole range of shearing. Analysing the flow-process it was shown, that using those conditions a high measuring effect is realized for the determination eached of the rheological parameters.The rheological parameters describing the creep-process deviate from its average by < 10%. Ignoring the yield value causes an apparent increasing of the plastic surface viscosity.The addition of SDS to the gelatine causes the yield value and an increasing of all rheological parameters in the investigated concentration-range. This increasing is not additiv and leads to an interpretation of a strong rise of solidity of the surface structure.

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Abkürzungen A ₁ eine die Fließgrenzef ₁ berücksichtigende Scherspannung, def. durch (30) - A ₂ eine die Fließgrenzef ₂ berücksichtigende ScherspannungF — Kraft - G ₁ ^s Schermodul der spontanen Elastizität - G ₂ ^s Schermodul der verzögerten Elastizität - M Drehmoment - S effektive Scherspannung - f ₁ Fließgrenze der plastischen Viskosität - f ₂ Fließgrenze der elastischen Viskosität - r Radialkoordinate - r ₁ Ringradius - r ₂ Gefäßradius - r _O1 kritischer Radius fürf ₁ - r _O2 kritischer Radius fürf ₂ - t Zeit - t _O Expositionszeit, Zeitpunkt der Entlastung des Systems - x Deformation - Deformation in rad - _O spontan-elastische Deformation - ^* irreversible Deformation - ₁ ^s plastische Grenzflächenscherviskosität - ₂ ^s elastische Grenzflächenscherviskosität - Retardationszeit ₂ ^s /G ₂ ^s