Robert J. Klingler

Metalmetal multiple bonds : IV. The chemistry of bis(cyclopentadienyldicarbonyl-molybdenum) (MoMo)

Abstract The synthesis and chemistry of Cp 2 Mo 2 (CO) 4 (I) a coplex containing a reaction MoMo triple bond, are described. The formation of I in refluxing xylene is shown to occur via odd-electron intermediates from homolysis of the MoMo bond in the Cp 2 Mo 2 (CO) 6 precursor. Numerous soft nucleophiles add to I to give complexes, e.g., trans -(CpMo(CO) 2 L) 2 (L = Ph 3 P, P(OMe) 3 ), (μ-RC 2 R′)Cp 2 Mo 2 (CO) 4 , and (μ-SR) 2 Cp 2 Mo 2 (CO) 4 . (Et 4 N)(Cp 2 Mo 2 (CO) 4 CN) may be isolated from the reaction of cyanide ion with I. Iodine and hydrogen chloride react to give Cp 2 Mo 2 (CO) 4 I 2 and (μ-H)(μ-Cl)Cp 2 Mo 2 (CO) 4 , respectively. Silver and mercuric ions are reduced to the metal by I, but Ph 2 Hg gives (CpMo(CO) 3 ) 2 Hg in addition to metallic mercury. With tetracyanoethylene, I forms [CpMo(CO) 4 ] + [TCNE] − . Complex I mimics the reaction of acetylenes with L 4 Pt (L = Ph 3 P) and forms the triangulo-cluster, (μ-L 2 Pt)Cp 2 Mo 2 (CO) 4 . With Na 2 Fe(CO) 4 , Cp 2 Mo 2 (CO) 4 I 2 forms the analogous cluster, (μ-Fe(CO) 4 )Cp 2 Mo 2 (CO) 4 . By electron counting rules, these clusters should contain MoMo double bonds. Evidence is presented which indicates I and Co 2 (CO) 8 form an unstable tetrahedrane cluster. At 100°C in toluene, I may reversibly dimerize to give a tetrahedrane Mo 4 -cluster, as shown by scrambling experiments. Complex I reacts slowly with Mn 2 (CO) 10 to give CpMo(CO) 3 Mn(CO) 5 as the only isolable mixed-metal product.

7Li NMR study of intercalated lithium in curved carbon lattices

Abstract A device was invented that permits nuclear magnetic resonance (NMR) analysis of the internal elements of a coin cell battery. The Coin Cell Battery Imager was used to record wideline 7 Li NMR spectra of the lithium ions that were electrochemically intercalated into three different types of carbon-based materials. The samples included graphite, corannulene, and carbon derived from sepiolite clay. All samples were excised from 2032-size coin cells that were cycled multiple times and left in a discharged state (i.e., fully lithiated). A comparison of the 7 Li NMR spectra recorded for the three carbons revealed that the curved carbon lattice derived from sepiolite affected the lithium resonances in a manner similar to that observed for the curved molecule corannulene, while both differed from the flat lattice of graphite. In addition, it was possible to observe lithium dendrites on the surface of a hard carbon electrode even in the presence of a large lithium counter electrode using NMR imaging techniques.

Structural and Mechanistic Revelations on an Iron Conversion Reaction from Pair Distribution Function Analysis

Not simply small particles: pair distribution function analysis yields comprehensive insights into the electrochemical reaction of α-Fe(2)O(3) with lithium. The metallic Fe formed in this reaction was found to be defect-rich nanoparticles that restructure continuously without growing-an unusual characteristic likely linked to its highly reversible capacity.

Efficient simultaneous reverse Monte Carlo modeling of pair-distribution functions and extended x-ray-absorption fine structure spectra of crystalline disordered materials.

An efficient implementation of simultaneous reverse Monte Carlo (RMC) modeling of pair distribution function (PDF) and EXAFS spectra is reported. This implementation is an extension of the technique established by Krayzman et al. [J. Appl. Cryst. 42, 867 (2009)] in the sense that it enables simultaneous real-space fitting of x-ray PDF with accurate treatment of Q-dependence of the scattering cross-sections and EXAFS with multiple photoelectron scattering included. The extension also allows for atom swaps during EXAFS fits thereby enabling modeling the effects of chemical disorder, such as migrating atoms and vacancies. Significant acceleration of EXAFS computation is achieved via discretization of effective path lengths and subsequent reduction of operation counts. The validity and accuracy of the approach is illustrated on small atomic clusters and on 5500-9000 atom models of bcc-Fe and α-Fe(2)O(3). The accuracy gains of combined simultaneous EXAFS and PDF fits are pointed out against PDF-only and EXAFS-only RMC fits. Our modeling approach may be widely used in PDF and EXAFS based investigations of disordered materials.

Silane-silanol dehydrocondensation. The microscopic reverse of hydrogen activation by an organometallic oxide complex

Abstract Hydrogen evolution from the dehydrocondensation reaction of triphenylsilane with triphenylsilanol is catalyzed by sodium trimethylsiloxide at 90°C in dioxane solvent. This system is the first example of a silane hydrolysis reaction that exhibits simple first order kinetics when investigated under second order conditions, equal concentrations of silane to proton source. Further analysis of the kinetics by the method of initial rates indicates that the reaction is first order in silane and zero order in silanol, demonstrating a multistep mechanisms process and that the proton source is not involved in the rate limiting step. The microscopic reverse process, hydrogen activation, has been investigated at 200–250°C and 330 atm of hydrogen using hexamethyldisiloxane and sodium trimethylsiloxide to yield significant, 12%, quantities of trimethylsilyl hydride; however, the reaction has limited application since the methyl groups are concurrently cleaved to the organic product methane under these conditions.

COMPARATIVE ENERGY BARRIERS FOR HYDROGEN ACTIVATION BY HOMOGENEOUS AND HETEROGENEOUS METAL OXIDE CATALYSTS

Triethylene glycol solutions of alkali and alkaline earth metal hydroxide complexes are well-defined soluble oxide water-gas shift catalysts which equilibrate the reaction of carbon monoxide and water to yield hydrogen and carbon dioxide at temperatures ranging from 150 ° to 250 °C and carbon monoxide pressures of 1 to 300 atm. Significantly, catalysis proceeds cleanly, even in the complete absence of a metal center in the soluble oxide system. Thus, the rate of hydroxide ion catalyzed hydrogen evolution is highest in the presence of a noncoordinating organic cation: BuΔN+>Cs+>Na+>H+>Ca+2. Furthermore, the activation energy for the homogeneous sodium hydroxide catalyst in triethylene glycol solution, 26±1 kcal, is comparable to that exhibited by a commercially used heterogeneous iron oxide catalyst, 27±0.2 kcal. The alkali metal hydroxide system may be modified for metal cocatalysis. Thus, lead (II) oxide dissolves in the triethylene glycol solutions to yield a new species which exhibits a207Pb NMR resonance shifted 3350 ppm downfield from lead perchlorate. The activity of this lead modified system is improved by three orders of magnitude. Yet, the activation energy is unchanged, 26±1 kcal, suggesting that entropic factors may be important in these homogeneous metal oxide hydrogen evolution/activation systems.