Showing papers by "Malcolm L. H. Green published in 1993"

Thinning and opening of carbon nanotubes by oxidation using carbon dioxide

Abstract: THE discovery1 and bulk synthesis2 of carbon nanotubes has stimulated great interest. It has been suggested that these structures may have useful electronic3–5 and mechanical6 properties, and these might be modified by introducing foreign materials into the nanotubes. But the tubes are invariably capped at the ends. Ajayan and lijima7 have succeeded in drawing molten material (lead or one of its compounds) into the tubes by heating them in the presence of lead and oxygen; less than 1% of the tubes in the sample studied could be filled in this way. Here we report that heating in carbon dioxide gas can result in the partial or complete destruction of the tube caps and stripping of the outer layers to produce thinner tubes. In some cases, we have thinned the extremity of tubes to a single layer. The opened tubes can be regarded as nanoscale test-tubes for adsorption of other molecules, and this controlled method of thinning may allow studies of the properties of single tubes.

A study of carbon deposition on catalysts during the partial oxidation of methane to synthesis gas

Abstract: The deposition of carbon on catalysts during the partial oxidation of methane to synthesis gas has been investigated and it has been found that the relative rate of carbon deposition follows the order Ni>Pd>Rh>Ir. Methane decomposition was found to be the principal route for carbon formation over a supported nickel catalyst, and electron micrographs showed that both “whisker” and “encapsulate” forms of carbon are present on the catalyst. Negligible carbon deposition occurred on iridium catalysts, even after 200 h.

Uncapped and thinned carbon nanotubes and process

Abstract: Uncapped and thinned carbon nanotubes are produced by reaction with a flowing reactant gas capable of reaction selectively with carbon atoms in the capped region of nanotubes. The uncapped and thinned nanotubes provide open compartments for insertion of chemicals and exhibit enhanced surface area with modified physical and chemical properties.

Diaphragmatic Dysfunction in Neuralgic Amyotrophy: An Electrophysiologic Evaluation of 16 Patients Presenting with Dyspnea

Abstract: We report 16 adult men (age, 41 to 75 yr) with neuralgic amyotrophy (NA) who presented with dyspnea due to involvement of the diaphragm. All patients developed breathlessness after a prodrome of acute severe neck and shoulder pain. Bilateral diaphragm paralysis (BDP) was confirmed in 12 patients and unilateral diaphragm paralysis (UDP) in four by the absence of electrical and mechanical responses to percutaneous phrenic nerve stimulation. Global expiratory muscle strength was well preserved in all patients, but inspiratory muscle strength was reduced in proportion to the extent of diaphragmatic involvement. Lung function showed low lung volumes with preservation of carbon monoxide transfer coefficient in all patients. Two BDP patients were hypoxic (PaO2 = 67 and 54 mm Hg, respectively) on daytime arterial blood gas analysis; the latter patient with pre-existing chronic obstructive pulmonary disease and marked obesity also had borderline hypercapnia (PaO2 = 49 mm Hg). Overnight sleep studies in three BDP and two UDP patients showed frequent intermittent arterial oxygen desaturations apparently caused by obstructive sleep apneas, but there was no evidence of alveolar hypoventilation. Follow-up muscle studies in five BDP and four UDP patients between 2 and 4 yr after initial referral showed complete recovery of diaphragmatic function in only two UDP patients, one of whom relapsed a year later. We postulate that NA may be an important but underrecognized cause of diaphragmatic paralysis in otherwise normal patients. Diaphragmatic strength returns very slowly, if at all.

Transition metal-carbonyl, -hydrido and -η-cyclopentadienyl derivatives of the fullerene C60

Abstract: Monoadduct derivatives of the fullerence C60, namely [Fe(CO)4(η2-C60)], [Mo(η-C5H4R)2(η2-C60)](R = H, Bun), [Ta(η2-C5)](η2-C60)] and Rh (ph(Ph3P)2(CO)(η2-C60)H], are described.

Mono-η-cyclopentadienyl-benzamidinato compounds of titanium, zirconium and hafnium

Abstract: The new compounds [M(η-C5R5){η-R′NC(Ph)NR′}Cl2][where R′= Me3,Si (TMS), and R = H, M = Ti,Zr, Hf or R = Me, M = Ti or Zr], [Zr(η-C5R5){(η-R′NC(Ph)NR′}(CH2Ph)Cl](R = H, Me), and [Zr(η-C5R5){η-R′NC(Ph)NR}(CH2Ph)2](R = H, Me) are described; the compound [Zr(η-C5H5){η-R′C(Ph)R′}Cl2] acts as a cocatalyst for the polymerisation of ethylene and propene.

High-resolution electron microscopy of tubule-containing graphitic carbon

Abstract: Carbon extracted from a cathodic graphite rod following arc-discharge in helium is examined using high-resolution electron microscopy. It is found that, in addition to the well known extended graphitic tubules (‘buckytubes’), carbon produced in this way contains many other novel graphitic structures. Of particular interest are small hollow particles which, although essentially tubular, appear to differ in significant ways from the extended tubules. Evidence for tetrahedral bonding in some of the graphitic particles is also presented.

New lanthanide–hydrogen–transition metal compounds: [{(PMe3)3WH5}2Yb·L3] and [{(η-C5H5)2NbH2}2Yb·L3] where L3=(MeOCH2CH2)2O

Abstract: The new lanthanide–hydrogen–transition metal compounds [{[PMe3)3WH5}2Yb·L3] and [{(η-C5H5)2NbH2}2Yb·L3], where L3=(MeOCH2CH2)2O, are prepared and their crystal structures determined.

Conversion of propanal to pentan-3-one using lanthanide oxides

Abstract: The heterogeneous reaction between propanal vapour and rare-earth-metal oxides gives as major products pentan-3-one and carbon dioxide. Residence time investigations indicate that pentan-3-one and carbon dioxide are primary products. Deactivation and high-resolution electron microscopy studies support the hypothesis that surface [O] participates in the reaction, and high oxygen mobilites in the oxide may allow defects to spread within the oxide. The carbon dioxide produced in the reaction poisons the lanthanum oxide leading to the faster deactivation and temperature-programmed desorption indicates the presence of a surface intermediate which decomposes to pentan-3-one and carbon dioxide. A mechanism is proposed in which the formation of pentan-3-one proceeds via surface-bound carboxylate species. Preliminary studies show that catalytic production of pentan-3-one can potentially be sustained by co-feeding water with propanal to the rare-earth-metal oxides.

A microporous carbon produced by arc-evaporation

Abstract: A carbon soot formed by arc evaporation and activated by heating under carbon dioxide is found to have a surprisingly high internal micropore volume (> 0.25 ml g–1) and an apparent BET surface area of ca. 700 m2 g–1, a large proportion of the pores are ca.⩽5 A; transmission electron microscopy shows a highly disordered microstructure, which electron irradiation readily transforms into quasi spherical concentric nanoparticles of diameter of ca. 60 A, the carbon material, which is highly absorbent to methane, shows molecular sieving properties and is more inert to oxidation than other forms of high surface area carbon.

Synthesis, structure and dynamic behaviour of the compound [Nb(η-C5H5)2(σ-C5H5)(NBut)]

Abstract: The new compound [Nb(η-C5H5)2(σ-C5H5)(NBut)] has been prepared and its crystal structure determined. Variable-temperature NMR studies reveal diverse dynamic processes including exchange between the σ-and η5-cyclopentadienyl rings.

Bis(η-cyclopentadienyl)imido compounds of niobium

Abstract: The compound [Nb(η-C5H5)2(NBut)Cl] has been synthesised and its crystal structure determined. It is the precursor for the new compounds [Nb(η-C5H5)2(NBut)Me], [Nb(η-C5H5)2(NBut)Br], [Nb(η-C5H5)2(NBut)(C6H5CH2)] and [Nb(η-C5H5)2(NBut)(σ-C3H5)], and the hydrido derivative [Nb(η-C5H5)2(NBut)H].

Dalton communications. ansa-Bridged tris(cyclopentadienyl) compounds of zirconium and hafnium: X-ray crystal structures of [M{Me2C(η5-C5H4)2}(η5-C5H5)Cl](M = Zr or Hf)

Abstract: The new ansa-bridged compounds [M{Me2C(η5-C5H4)2}(η5-C5H5)Cl](M = Zr or Hf) have been synthesised and their molecular structures determined by X-ray crystallography. The latter is the first example of a tris(η5-cyclopentadienyl) derivative of hafnium.

Synthesis and reactions of η-cycloheptatrienyl derivatives of molybdenum

Abstract: An improved route to [Mo(η-C7H7)(η-C7H9)]2 has been found which involves the reduction of MoCl5 with sodium amalgam in the presence of cycloheptatriene followed by thermolysis. Compound 2 can be oxidised with a half equivalent of iodine or tetrafluoroboric acid forming the corresponding cation [Mo(η-C7H7)(η-C7H9)]+3, which reacts with ethanol or an excess of iodine to yield the binuclear cations [(η-C7H7)Mo(µ-X)3Mo(η-C7H7+](X = OEt 4 or I 5 respectively). Treatment of 2 with 1 equivalent of iodine, bromine or an excess of PhlCl2 in the presence of a weak co-ordinating solvent or σ-donor affords the 17-electron compounds [Mo(η-C7H7)LX2][L = tetrahydrofuran (thf), MeCN, PPh3, PMe3 or Et2S; X = Cl, Br or I). Treatment of [Mo(η-C7H7)(MeCN)I2]11 with [NBu4]I gives [NBu4][Mo(η-C7H7)I3]18 and reduction of 11 with sodium amalgam in the presence of 1,2-bis(dimethylphosphino)ethane (dmpe), 1,2-bis(diphenylphosphino)ethane (dppe) or PMe3 gives [Mo(η-C7H7)(dmpe)I]19, [Mo(η-C7H7)(dppe)I]20 or [Mo(η-C7H7)(PMe3)2I]13, respectively. Compound 11 also reacts with Na(C5H4R)(R = H or Me) or lithium indenide to yield the mixed-sandwich compounds [Mo(η-C7H7)(η-C5H4R)](R = H 21 or Me 22) or [Mo(η-C7H7)(η-C9H7)]23. The crystal structures of [NBu4][Mo(η-C7H7)I3]18 and [Mo(η-C7H7)(η-C5H4Me)]22 have been determined. Compound 22 reacts with 1 equivalent of tetracyanoethene (tcne) to give the electron-transfer complex [Mo(η-C7H7)(η-C5H4Me)][tcne]24 while compound 21 reacts with 1 equivalent of 7,7,8,8-tetracyanoquinodimethane (tcnq) to yield [Mo(η-C7H7)(η-C5H5)]2[tcnq]25.

Thiolato-bridged η-cycloheptatrienyl molybdenum complexes

Abstract: A new series of binuclear thiolato-bridged molybdenum complexes [(η-C7H3R14)Mo(µ-SR2)3Mo(η-C7H3R41)][BF4](R1= H or Me; R2= Et, Pr, Bu, Ph or CH2Ph) have been prepared by treating the mixed-sandwich compounds [Mo(η-C6H5Me)(η-C7H3R14)][BF4](R1= H or Me) or [Mo(η-C7H7)(η-C7H9)][BF4] with the corresponding thiols R2SH. Dynamic NMR studies reveal that all of these complexes (except for R2= Ph) are fluxional due to the inversion at the pyramidal sulfur centre. The activation free energy of this intramolecular process has been estimated by the coalescence temperature method. The values range from ΔG‡= 52.9 to 58.1 kJ mol–1 and increase in the order: R2= CH2Ph < Bu < Pr < Et. Cyclic voltammetric studies of the binuclear compounds show that they undergo two reversible one-electron reductions and the ease of reduction is dependent on the electron-donating ability of the R1 and R2 groups.

One-dimensional antiferromagnetic cycloheptatrienyl molybdenum and tungsten compounds

Abstract: The 17 -electron compounds [M(η-C7H7)LX2](M = Mo, L = MeCN, X = I 1; M = Mo, L = PMe3, X = I 2; M = W, L = MeCN, X = I 3; M = W, L = PMe3, X = I 4; M = W, L = PMe3, X = Br 5) have been prepared. All exhibit antiferromagnetic interactions with temperatures at χmax ranging from 12 to 17 K. The magnetic susceptibility data have been described by one-dimensional magnetic models. The crystal and molecular structures of [Mo(η-C7H7)(MeCN)l2]1 have been determined, and confirm the one-dimensional structure.

η-1,2,4,6-tetramethylcycloheptatrienyl molybdenum chemistry

Abstract: Treatment of [Mo(CO)6] with 1,3,5,7-tetramethylcycloheptatriene in octane gives a mixture of [Mo(η6-C7H4Me4-1,3,5,7)(CO)3]1, [Mo(η6-C7H4Me4-1,2,4,6)(CO)3]2 and [Mo(η6-C7H4Me4-1,3,4,6)(CO)3]3. The compounds 2 and 3 arise as a result of sequential 1,5-shifts of the 7-endo-hydrogen atom of the initially formed 1. Hydride abstraction from compounds 1–3 by [CPh3][BF4] gives [Mo(η7-C7H3Me4-1,2,4,6)(CO)3][BF4]4. Compound 4 reacts with LiCl to yield [Mo(η7-C7H3Me4-1,2,4,6)(CO)2Cl]5 which reacts with 1 equivalent of Me2PCH2CH2PMe2(dmpe) to give [Mo(η3-C7H3Me4-1,2,4,6)(dmpe)(CO)2Cl]7. This is fluxional due to trigonal twist rearrangement and 1,2-shift of metal around the C7 ring. Photolysis of 7 in toluene produces [Mo(η7-C7H3Me4-1,2,4,6)(dmpe)Cl]9. Treatment of 4 with toluene affords the mixed-sandwich compound [Mo(η6-C6H5Me)(η7-C7H3Me4-1,2,4,6)][BF4]10 which yields [Mo(η7-C7H3Me4-1,2,4,6)(CH3COCHCOCH3)(PPh3)]11 upon treatment with sodium pentane-2,4-dionate and triphenylphosphine.

New η-cycloheptatrienyl tungsten compounds and the intercalation of [M(η-C7H7)(η-C5H4Me)](M = Mo or W) into ZrS2

Abstract: The compound [W(η-C7H7)(η-C7H9)]1 is formed in gram quantities by reduction of WCl6 with sodium amalgam in the presence of cycloheptatriene. The compound 1 with iodine in acetonitrile gives [W(η-C7H7)(MeCN)I2]2 and this reacts with PMe3 forming [W(η-C7H7)(PMe3)I2]. Oxidation of 1 with bromine in acetonitrile yields [W(η-C7H7)(MeCN)Br2] and with bromine in tetrahydrofuran followed by addition of PMe3 gives [W(η-C7H7)(PMe3)Br2]. Reduction of 2 with sodium amalgam in the presence of 1,2-bis(dimethylphosphino)ethane (dmpe) affords [W(η-C7H7)(dmpe)I]. Compound 2 also reacts with Na(C5H4R)(R = H or Me) and lithium indenide to give the mixed-sandwich compounds [W(η-C7H7)(η-C5H4R)](R = H or Me) and [W(η-C7H7)(η-C9H7)] respectively. The sandwich compounds [M(η-C7H7)(η-C5H4Me)](M = Mo or W) and ZrS2 form the intercalates {ZrS2[M(η-C7H7)(η-C5H4Me)]x}.

Conversion of methane to propanal at ambient pressure

Abstract: A continuous process is described using three catalysts in sequence for the synthesis of the C[sub 3] oxygenate propanal, from methane and air, at 1 atm in 13.2% yield. The three catalytic reactions are the methane oxidative coupling to ethene, methane partial oxygenation to synthesis gas, and the hydroformylation of ethene. The combined process implies that a higher yield of propanal can be obtained than for the formation of ethene from the oxidative coupling reaction alone. The authors have examined the effect of reaction conditions on the yield of propanal.

Redox properties of molten salts for methane activation

Abstract: Molten salt mixtures have been tested in a redox mode as catalysts for the activation of methane at 750 °C. It is found that after pre-treatment with dioxygen a transition metal halide/ sodium vanadate melt can convert methane selectively to C2+ products in the absence of molecular oxygen. The melt can be reactivated by passing dioxygen. Electron paramagnetic resonance studies of the quenched samples showed that the transition metal ions are reduced by methane and can be reoxidised by dioxygen. It is also found that higher C2+ selectivity, C2+ yield and C2H4/C2H6 ratio are promoted by added transition metal chlorides and, surprisingly, also by the corresponding metal bromides. It supports the suggestion that surface modification by halogen is more important than gas radical reactions. Comparison of the molten mixtures under redox and cofeed conditions showed that the former gave a higher C2+ selectivity, but no oxygenated products whereas formaldehyde was only detected in the cofeed conditions.

ansa-Bridged tris(cyclopentadienyl) compounds of zirconium and hafnium : X-ray crystal structures of [M{Me2C(η5-C5H4)2}(η5-C5H5)Cl] (M=Zr or Hf)

Abstract: The new ansa-bridged compounds [M{Me 2 C(η 5 -C 5 H 4 ) 2 }(h 5 -C 5 H 5 )Cl] (M=Zr or Hf) have been synthesised and their molecular structures determined by X-ray crystallography. The latter is the first example of a tris(η 5 -cyclopentadienyl) derivative of hafnium