Afif M. Seyam

Observations on the thermal decomposition of some uranium(IV) tetraalkyls

Abstract The thermally unstable products of the reactions 4 RLi + UCl 4 (R = various alkyl groups), presumed to be uranium tetraalkyls, decompose readily at room temperature in ethereal or hydrocarbon solvents. When R contains a β-hydrogen, comparable quantities of alkane (RH) and alkene (RH-H 2 ) are produced along with trace amounts of the dimer, RR. This result indicates that uranium alkyl compounds readily decompose via β-hydrogen elimination under the appropriate circumstances (coordinative unsaturation). When a β-hydrogen is not present, the alkane, RH, is the major product. That stereochemistry is retained at the α-carbon atom of the 2- cis - and 2- trans -2-butenyl compounds suggests that 2-butenes are not formed from free R − radicals.

A new approach to measuring absolute metal-ligand bond disruption enthalpies in organometallic compounds. The [(CH3)3SiC5H4]3 U-system

Absolute uranium—ligand bond disruption enthalpies in the series Cp3UR(Cp = η5-Me3SiC5H4) have been measured by halogenolytic isoperibol titration calorimetry of Cp3U/Cp3UI/Cp3UR ensembles. Derived D(Cp3UR) values in toluene solution are (kcal mol−1, R): 62.4(0.4), I; 28.9(1.7), n-Bu; 25.6(3.1), Bz; 39.3(2.3), CH2SiMe3; 44.8(1.1), Me; 48.5(2.2), vinyl; 86.7, CCPh. D(Cp3U ← L) values (kcal mol−1, 95% confidence limits) in toluene solution were also determined for L = CO [10.3(0.2)] and THF [9.8(0.2)]. The magnitudes of the D(UR) values appear to reflect a combination of steric and electronic factors, and suggest that D(UI) is less sensitive to ancillary ligation (more transferable) then D(U-alkoxide). The complex Cp3U(vinyl) crystallizes in the triclinic space group P with two molecules in a unit cell of dimensions a = 11.298(1), b = 13.997(2), c = 9.460(2) A, and α = 97.00(1), β = 105.98(1) and γ = 96.94(1)°. Least-squares refinement led to a value for the conventional R index (on F) of 0.0196 for 4373 reflections having 2θMo-Kα 3σ(I). The molecule possesses a conventional pseudotetrahedral Cp3MX geometry with UC(ring) = 2.759(4) A (average), UC(α, vinyl) = 2.436(4) A, < UC(α, vinyl)C(β, vinyl) = 137.7(3)°, < CgUCg = 116.4, 117.2, 120.0 and < CgUC(α, vinyl) = 95.1, 100.0, 100.1°. All hydrogen atoms were located and UH(Cα, vinyl) = 2.93(4) A. The metrical parameters evidence severe non-bonded repulsions between the vinyl ligand and the Cp ligands, as well as between different Cp ligands. The quantity D(MI)—D(MCH3) is proposed as a gauge of metal—ligand bonding.

What can metal-ligand bonding energetics teach us about stoichiometric and catalytic organometallic chemistry?

This contribution surveys recent progress in the thermochemistry of lanthanide, actinide, and early transition element organometallic compounds. General trends in metal-ligand bond enthalpy patterns across the transition series can be under- stood largely on the basis of straightforward electronegativity concepts. Metal-ligand bonding energetics in such unusual species as metallacycles, early transition element carbonyls and benzynes, as well as in actinide carbonylsare also quantified. Such data furthermore provide a deeper insight into do,fo- centered p-hydride/p-alkyl elimination, cyclometalation, and hydrocarbon functionalization processes, as well as into U(1V)- centered elimination reactions. New reactions which are examined in the light of metal-ligand bond enthalpy data include binuclear organolanthanide-centered hydrocarbon activation and organo- lanthanide-catalyzed hydroamination of olefins.

Thermal studies of ‘dialkyldioxouranium(VI)’

Abstract Dichlorodioxouranium(VI) reacts with RLi or RMgCl (R = hydrocarbyl) in polar and nonpolar solvents to form (presumably) the thermally unstable ‘UO 2 R 2 ’ species, which upon warming to room temperature either 1) reductively eliminates RR leaving UO 2 (R = phenyl), or 2) decomposes by β-hydride elimination to give the corresponding alkane and alkene and UO 2 , (when R = i-propyl, n-butyl, and t-butyl), or 3) affords the corresponding alkane (R = methyl) or alkene (R ] vinyl) via hydrogen abstraction. Microanalysis, and spectroscopic and gas chromatographic data are discussed.

Some dioxouranium(VI) complexes of azo-oximes

Abstract Reactions of arylazobenzaldoximes, X-(C6H4)C( NOH)N NPh(HL), where X H, p-Cl, p-CH3O, p-NO2, or arylazoformaldoximes, HC( NOH)N N(C6H4Y Y(HL), where Y p-CH3C( O) , p-Cl, p-CH3O, p-CH3, p-NO2, with UO2(OAc)2.·2H2O in methanol yielded complexes with the formula UO2(L)2·ηH2O, where η 1–5. These dioxouranium(VI) complexes were characterized by elemental analyses, conductivity measurements, IR and 1H NMR spectroscopy. In these complexes, the azo-oximes act as bidentate ligands by bonding through the oxime-oxygen and an azo-nitrogen.

Synthesis and Characterization of Silver(I) Pyrazolylmethylpyridine Complexes and Their Implementation as Metallic Silver Thin Film Precursors

A series of light- and air-stable silver(I) pyrazolylmethylpyridine complexes [Ag(L(R))]n(BF4)n (L = pyrazolylmethylpyridine; R = H, 1; R = Me, 2; R = i-Pr, 3) and [Ag(L(R))(NO3)]2 (L = pyrazolylmethylpyridine; R = H, 4; R = Me, 5; R = i-Pr, 6) has been synthesized and structurally and spectroscopically characterized. In all of the molecular structures, the pyrazolylmethylpyridine ligands bridge two metal centers, thus giving rise to dinuclear (2, 4, 5, and 6) or polynuclear structures (1 and 3). The role played by the counteranions is also of relevance, because dimeric structures are invariably obtained with NO3(-) (4, 5, and 6), whereas the less-coordinating BF4(-) counteranion affords polymeric structures (1 and 3). Also, through atoms-in-molecules (AIM) analysis of the electron density, an argentophilic Ag···Ag interaction is found in complexes 2 and 4. Thermogravimetric analysis (TGA) shows that the thermolytic properties of the present complexes can be significantly modified by altering the ligand structure and counteranion. These complexes were further investigated as thin silver film precursors by spin-coating solutions, followed by annealing at 310 °C on 52100 steel substrates. The resulting polycrystalline cubic-phase Ag films of ∼55 nm thickness exhibit low levels of extraneous element contamination by X-ray photoelectron spectroscopy (XPS). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) indicate that film growth proceeds primarily via an island growth (Volmer-Weber) mechanism. Complex 4 was also evaluated as a lubricant additive in ball-on-disk tribological tests. The results of the friction evaluation and wear measurements indicate a significant reduction in wear (∼ 88%) at optimized Ag complex concentrations with little change in friction. The enhanced wear performance is attributed to facile shearing of Ag metal in the contact region, resulting from thermolysis of the silver complexes, and is confirmed by energy-dispersive X-ray analysis of the resulting wear scars.

Further observations on the reaction of uranium tetrachloride with simple lithium alkyls

A number of years ago, we reported a brief investigation of the thermally unstable products of the reaction of uranium tetrachloride with alkyl lithium reagents [1] (eqn. (1)). The purpose of this investigation was to ascertain whether β-hydride elimination might occur in a uranium hydrocarbyl with potential coordinate unsaturation. This being the object of the investigation, no attempt was made to characterize the intermediate organometallics nor was any structure or stoichimetric formulation specifically claimed for them (historically such species have been presumed to be tetrahydrocarbyls [2, 3]). Subsequent to this work, several groups have reported the successful use of eqn. (1) to generate finely divided uranium metal for synthetic purposes [4, 5], and evidence has been presented that, under certain conditions [6], greater than four alkyl groups may coordinate to uranium. In hydrocarbon or ether solvents, eqn. (1) is obviously a highly complex, heterogeneous reaction, and the course of the transformation should be critically dependent on the state and history of the UCl4. During a recent study of ‘stabilized’ actinide tetrahydrocarbyls [7], the sensitivity of reactions such as eqn. (1) to parameters involving the heterogeneity became apparent and stimulated a brief reinvestigated of our earlier work, using improved analytical techniques and a wider range of reaction conditions. We report here for two representative lithium reagents and the ‘innocent’ solvent heptane, further observations of eqn. (1) as regards optimization of RLi-derived products and, ultimately, metallic uranium. Results The principal goal of this investigation was to determine how, for constant solvent and lithium reagent, the course of eqn. (1) depends on the exact state of the UCl4 and the reaction conditions. In Table I are compiled data for the gaseous organic products of eqn. (1) as a function of these t001. Gases Evolved in the UCl4 + 4RLi Reaction. Experiment Lithium Reagent UCl4 Treatmenta Time (h) Agitation Cumulative Yield Rh + (RH)-H (%)b Butene:Butane 1 R = n-C4H9 none 153 stirring 20 0.7 ultrasound 34 105 stirring 44 52:48c 2 R = n-C4H9 grinding, SOCl2 110 stirring 48 26 ultrasound + 9 stirring 77 38 ultrasound 90 55 ultrasound 90 55:45c 3 R = t-C4H9 none 87 stirring 11 2.3 ultrasound + 33 stirring 28 3 ultrasound 30 30 stirring 30 56:44d 4 R = t-C4H9 SOCl2 90 stirring 38 1 ultrasound + 24 stirring 52 1 ultrasound + 48 stirring 60 1 ultrasound + 38 stirring 63 1 ultrasound + 24 stirring 63 62:38d 5 R = t-C4H9 grinding, SOCl2 118 stirring 58 1 ultrasound + 9 stirring 68 8 ultrasound + 37 stirring 81 21 ultrasound + 59 stirring 96 35 ultrasound + 47 stirring 98 47 ultrasound 98 60:40d a None indicates that UCl4 was employed as obtained from the procedure of ref. 9. b Estimated uncertainty: ±5%. c 1-Butene: n-butane. Estimated uncertainty in yields: ±3%. d Isobutene:isobutane. Estimated uncertainty in yields: ±3%. Full-size table Table options View in workspace Download as CSV parameters. For all experiments, the distribution of organic products (n-butane:1-butene, isobutane: isobutene) is similar to that reported in eqn. (1) for the longest reaction periods. The nature of the products is evidence that the intermediate uranium hydrocarbyls readily suffer β-hydride elimination. The yields are, however, found to be quite sensitive to the history of UCl4 and the agitation procedure. Thus, for practical reaction times, yields of butane and butene are significantly below stoichiometric if UCl4 is employed as obtained from the synthesis, and simple stirring is carried out. These low yields are due to the heterogeneous nature of the conditions and incomplete reaction. Thus, finely pulverizing the UCl4 and drying it with SOCl2 both increase the hydrocarbon yields substantially1 Furthermore, the use of the ultrasonic agitation, which is known to accelerate many types of heterogeneous reactions [8], increases the butane:butene yield to near quantitative. In the case of experiment 4, where the butane:butene yield is not quantitative, the additional equivalents of t-butyl functionality can be readily accounted for an unreacted lithium reagent.

Some lanthanide chloride complexes of pyridinaldazine and pyrrolaldazine schiff bases

Abstract Lanthanide chloride complexes with pyridinaldazine (PAA) and pyrrolaldazine (PyAA-H 2 ): [Ln(L)Cl 2 (H 2 O) n ]Cl· m H 2 O, where Ln = Ce, Nd, Sm, Yb; L = PAA, PyAA-H 2 ; n = 2, 4; m = 0, 1, 2, 3.5, have been prepared by the reaction of the corresponding lanthanide chloride with PAA or PyAA-H 2 (YbCl 3 ·6H 2 O with PyAA-H 2 gave the 2: 1 complex, [Yb 2 (PyAA)Cl 4 (H 2 O) 4 ]·2H 2 O) and characterized by spectral data, electrical conductance, magnetic susceptibility, thermal and elemental analysis.

Energy efficient siloxane lubricants utilizing temporary shear-thinning

This study investigates the rheologic properties, elastohydrodynamic film, and friction coefficients of several siloxane-based lubricants to assess their shear stability and their potential for energy efficient lubrication. Several siloxane-based polymers with alkyl, aryl, and alkyl-aryl branches were synthesized in order to examine the relationship between their molecular structures and tribological performance. Nuclear magnetic resonance spectroscopy and gel permeation chromatography were used to characterize the molecular structures and masses, respectively. Density, viscosity, elastohydrodynamic film thickness, and friction measurements were measured from 303 to 398 K. Film thickness and friction measurements were made at loads and speeds that cover the boundary, mixed, and full film lubrication regimes. These results illustrate that the shear characteristics of siloxane lubricants vary significantly with polymer length as well as branch structure and content. The findings provide quantitative insight into the features of siloxane molecular structure conducive to optimum film formation with minimum wear and elastohydrodynamic friction to enhance energy efficiency.

Observations on the reaction of uranium tetrachloride and dichlorodioxouranium(VI) with lithium alkyls

Abstract Both uranium tetrachloride and dichlorodioxouranium(VI) react with lithium alkyls in polar and non-polar solvents to form (presumably) the thermally unstable ‘UR 4 ’ and ‘UO 2 R 2 ’ species respectively, which upon warming to room temperature both decompose by β-hydride elimination (when a β-hydrogen atom is present on the alkyl group). The course of the transformations in such a highly complex, heterogeneous reaction is critically dependent on the state and history of the uranium compound.