Hydrazine reacts with silica-supported tantalum-hydrides [(≡SiO)2TaHx] (x = 1, 3), 1, under mild conditions (100 °C). The IR in situ monitoring of the reaction with N2H4 or (15)N2H4, and the solid-state MAS NMR spectra of the fully (15)N labeled compounds (CP (15)N, (1)H-(15)N HETCOR, (1)H-(1)H double-quantum, and (1)H-(1)H triple-quantum spectra) were used to identify stable intermediates and products. DFT calculations were used for determining the reaction pathway and calculating the (15)N and (1)H NMR chemical shifts. Combining the experimental and computational studies led to the following results. At room temperature, only hydrazine adducts, 1-N2H4, are formed. Upon heating at 100 °C, the hydrazine adducts are converted to several species among which [(≡SiO)2Ta(═NH)(NH2)], 2, [(≡SiO)2TaH(NH2)2], 3, and [(≡SiO)2TaH2(NH-NH2)], 4, were identified. The final product 2 is also formed in the reaction of N2 with the same silica-supported tantalum-hydride complexes, and the species identified as 3 and 4 had been previously suggested by DFT studies as intermediates on the reaction pathway for N-N cleavage in N2. The present computational studies (cluster models with M06 functional complemented by selected calculations with periodic calculations) show that 2 is formed via 3 and 4, with either N2 or N2H4. This strengthens the previous proposal of the existence of 3 and 4 as intermediates in the reaction of N2 with the tantalum-hydrides. However, the reaction of N2 does not imply the formation of N2H4 or its hydrazido monoanionic or dianionic ligand as an intermediate. For this reason, this study informs both on the similarities and differences of the reaction pathways involving N2 and N2H4 with tantalum-hydrides.

Rational use of novel high-performance semiconductors in field-effect transistors (FETs) requires exact knowledge of the dominating charge transport mechanisms. In particular, the distinction between contact- and semiconductor-limited transport is important in FETs with small channel lengths. Here, we analyze the relative contributions of contact limitation and intrinsic conductivity of FETs based on mechanically exfoliated multilayers of the high performance n-type semiconductor molybdenum disulfide (MoS2). Based on a lithography-free fabrication process, we realize FETs with room temperature mobility (mu) of up to 46.8 cm2/Vs and ION/IOFF ratio of up to 105. Using temperature- and bias-dependent charge transport measurements, we are able to show that the intrinsic bulk transport in the flake can be best described by a phonon-limited band transport model with a conductive bulk at room temperature that freezes out upon cooling of the sample. In addition, we notice an increase of mu by a factor of 25 when using a self-assembled monolayer (SAM)-modified SiO2/MoS2 interface. More importantly, we show that the choice of drainsource bias (VDS) is crucial when interpreting MoS2 transport measurements, while for large VDS the intrinsic semiconductor transport properties can be observed, a strong contact limitation appears at low VDS. Our combined measurements allow us to distinguish between the effect of bulk and semiconductor/dielectric interface transport and the effect of contact resistance on the electrical transport properties. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The well-defined silica-supported tantalum(V) surface complex [( SiO)(2)Ta(=NH)(NH2)] undergoes H/D exchange when exposed to deuterated benzene, toluene, 3,3-dimethyl-1-butene and methane. In the case of the two facile benzene and toluene activation reactions, a mechanism proceeding through 1,2-addition across tantalum-imido bonds of the surface species and its reverse 1,2-elimination from the transient silica supported tantalum(V) bis amido alkyl intermediate is proposed.

DFT(B3PW91) calculations have been carried out to propose a pathway for the N(2) cleavage by H(2) in the presence of silica-supported tantalum hydride complexes [(≡SiO)(2)TaH(x)] that forms [(≡SiO)(2)Ta(NH)(NH(2))] (Science 2007, 317, 1056). The calculations, performed on the cluster models {μ-O[(HO)(2)SiO](2)}TaH(1) and {μ-O[(HO)(2)SiO](2)}TaH(3), labelled as (≡SiO)(2)TaH(x) (x = 1, 3), show that the direct hydride transfers to coordinated N-based ligands in (≡SiO)(2)TaH(η(2)-N(2)) and (≡SiO)(2)TaH(η(2)-HNNH) have high energy barrier barriers. These high energy barriers are due in part to a lack of energetically accessible empty orbitals in the negatively charged N-based ligands. It is shown that a succession of proton transfers and reduction steps (hydride transfer or 2 electron reduction by way of dihydride reductive coupling) to the nitrogen-based ligands leads to more energetically accessible pathways. These proton transfers, which occur by way of heterolytic activation of H(2), increase the electrophilicity of the resulting ligand (diazenido, N(2)H(-), and hydrazido, NHNH(2)(-), respectively) that can thus accept a hydride with a moderate energy barrier. In the case of (≡SiO)(2)TaH(η(2)-HNNH), the H(2) molecule that is adding across the Ta-N bond is released after the hydride transfer step by heterolytic elimination from (≡SiO)(2)TaH(NH(2))(2), suggesting that dihydrogen has a key role in assisting the final steps of the reaction without itself being consumed in the process. This partly accounts for the experimental observation that the addition of H(2) is needed to convert an intermediate, identified as a diazenido complex [(≡SiO)(2)TaH(η(2)-HNNH)] from its ν(N-H) stretching frequency of 3400 cm(-1), to the final product. Throughout the proposed mechanism, the tantalum remains in its preferred high oxidation state and avoids redox-type reactions, which are more energetically demanding.

The thermolysis of Cp*W(NO)(Npt)(eta(3)-CH2CHCHSiMe3) (1; Cp* = eta(5)-C5Me5; Npt = CH2CMe3) in benzene at 55 degrees C generates three isomeric products having the composition Cp*W(NO)(H)(eta(3)-Me3SiCHCHCHPh) (2). These are isomers of the expected Cp*W(NO)(Ph)(eta(3)-Me3SiCHCHCH2) compound and result from an intramolecular Ph/allyl H exchange. Thermolysis of 2 in the presence of pyridine produces the eta(2)-olefin pyridine adduct Cp*W(NO)(eta(2)-Me3SiCH2CH=CHPh)(C5H5N) (3). However, when the same reaction is carried out in deuterobenzene with 10 equiv of pyridine, NMR spectroscopic data suggest that the meso hydrogen of the allyl ligand is exchanged for a deuterium atom before pyridine trapping occurs. The activation of fluorobenzenes (i.e., pentafluorobenzene, p-difluorobenzene, and o-difluorobenzene) by Cp*W(NO)-(Npt)(eta(3)-CH2CHCHSiMe3) has also been studied, and for these substrates, C-H bond activation occurs exclusively. Selectivity for the activation of these C-H bonds appears to be determined by sterics. Intramolecular migration of the newly formed fluoroaryl ligands onto the allyl ligands does not occur when there is a fluorine atom in the position ortho to the newly formed W-C bond. This behavior is probably a manifestation of the fact that metal-o-fluoroaryl bonds tend to be stronger than metal-aryl linkages. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.

Thermolysis of Cp*W(NO)(Npt)(eta(3)-CH2CHCHSiMe3) [Cp* = eta(5)-C5Me5; Npt = CH2CMe3] at 55 degrees C leads to the loss of neopentane and the formation of the 16-electron eta(2)-allene intermediate Cp*W(NO)(eta(2)-CH2=C=CHSiMe3), which activates hydrocarbons at their methyl groups In the case of linear alkanes, only terminal C-H activation occurs. This selectivity persists in the presence of an ether functionality, but not with other oxygen containing substrates such as aldehydes and alcohols With these latter substrates, the organometallic complex is oxidized to Cp*W(O)(2)(Npt). The existence of the allene intermediate has been confirmed by its reaction with PMe3 to form the 18 electron adduct and by its diagnostic reaction with cyclohexene. Carbonylation of Cp*W(NO)(Npt)(eta(3)-CH2CHCHSiMe3) with CO (550 psig) at room temperature results in the clean formation of the corresponding Cp*W(NO)(eta(1)-C(=O)Npt)(eta(3)-CH2CHCHSiMe3) complex, which exists as a mixture of two interconverting isomers differing in their modes of attachment of the (eta(3)-CH2CHCHSiMe3) ligands to the tungsten centers. The congeneric molybdenum complex, Cp*Mo(NO)(Npt)(eta(3)-CH2CHCHSiMe3), has also been synthesized, and although it generates the requisite eta(2)-allene intermediate upon thermolysis, its preferred mode of reactivity is coupling of the allyl and alkyl ligands. Consequently, the molybdenum complex is inferior to the tungsten system for effecting C-H activations. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.

Ammonia N-H bond activation and dinitrogen N N cleavage with dihydrogen on an isolated metal atom have been achieved with the silica-grafted Ta-III and Ta-V hydrides [( SiO)(2)TaH] (2a) and [( SiO)(2)TaH3] (2b), accessed through surface organometallic chemistry (SOMC). The synthesis of the starting tantalum hydrides 2a and 2b by grafting tris(neopentyl)neopentylidenetantalum(V), Ta(=CH-tBu)Np-3, on silica yields well-defined, isolated tantalum atoms. Silsesquioxane molecular modelling shows that the mechanism of the grafting reaction implies a tetraalkyl intermediate [( SiO)TaNp4]. The starting hydrides 2a and 2b react stoichiometrically and catalytically with alkanes in reactions such as alkane metathesis, cross-metathesis between ethane and toluene, and methane coupling to form ethane. Mechanistic studies show the relevance of tantalum carbenes and Chauvin-like metallacyclobutane intermediates in most of these reactions. Finally, the stoichiometric N-2 cleavage and NH3 activation to the final imido amido tantalum(V) complex [( SiO)(2)Ta(NH)(NH2)] (3) are reviewed and discussed mechanistically. In the N N cleavage reaction, dihydrogen adducts on silica-grafted isolated tantalum atoms appear to play a central role. The ammonia reaction occurs by bifunctional activation through the Lewis acid/Lewis base couple formed by a metal centre and a coordinated nitrogen atom, the so-called "NH effect". Such bifunctional activation is also observed for the heterolytic cleavage of H-2 by [( SiO)(2)Ta(NH)(NH2)] (3).

Cp*W(NO)(CH2CMe3)(eta(3)-CH2CHCHMe) (1) is known to initiate facile and selective aliphatic C-H bond activations of hydrocarbons at ambient temperatures. Its ability to effect C-H activations of unfunctionalized hydrocarbon portions of more complex molecules containing various functional groups has now been investigated in some detail. In addition, molybdenum analogues of 1 have also been examined in order to see how the C H activation chemistry is affected when the central metal is changed. Then-nolyses of 1 in neat I -chloropropane, 1-chlorobutane, and 1-bromobutane at room temperature result in activation of the terminal C H bonds at the end opposite the carbon halogen linkage and the clean formation of the alkyl-ally' complexes Cp*W(NO)(CH2CH2CH2Cl)(eta(3)-CH2CHCHMe) (2), Cp*W(NO)(CH2(CH2)(2)-CH2Cl)(eta(3)-CH2CHCHMe) (3), and Cp*W(NO)(CH2(CH2)(2)CH2Br)(eta(3)-CH2CHCHMe) (4), respectively. No reaction occurs with the C C1 or C Br bonds in the haloalkanes even though they are weaker than the C H bonds that are activated. Similarly, treatment of 1 with eta-Bu2O yields exclusively the terminal sp(3) C H activated product, Cp*W(NO)((CH2)(4)O(CH2)(3)CH3)(eta(3)-CH2CHCHMe) (5), whereas the reaction with THF results in the single activation of a secondary sp(3) C H bond alpha to the oxygen atom in THF and the formation of Cp*W(NO)(C4H7O)(eta(3)-CH2CHCHMe) (6). Consistently, reaction of 1 with ethylcyclohexane results in preferential activation of one of the primary sp3 C H linkages of the ethyl group and the formation of Cp*W(NO)(CH2CH2C6H11)(eta(3)-CH2CHCHMe) (7). The Cp*Mo(NO)(alkyl)(eta(3)-allyl) complexes analogous to 1 are generally thermally unstable and react at or slightly above room temperature. The first member of this family of complexes to be studied was Cp*Mo(NO)(CH2CMe3)(eta(3)-C3H5) (8), whose thermolysis in the presence of pyridine at 35 degrees C over 3 days leads to the formation of Cp*Mo(NO)(C5H5N)(eta(2)-CH2=CHCH2CH2-t-Bu) (9), an eta(2)-olefin complex in which the ally' and neopentyl ligands have coupled. The related Cp*Mo(NO)(CH2SiMe3)(eta(3)-CH2CHCHMe) complex (11) exists as a 2:1 mixture of isomers distinguishable by the orientation of the endo, syn allyl ligand. In the less sterically congested major isomer, the methyl group on the ally' ligand is adjacent to the NO ligand, but in the minor isomer the methyl group is adjacent to the more sterically demanding CH2SiMe3 ligand. In general, the thermal reaction of 11 is similar to that of 1. Spectroscopic monitoring indicates that the loss of TMS from 11 at room temperature results in the formation of a 16e eta(2)-diene intermediate complex that can be trapped with PMe3 as an 18e adduct, Cp*Mo(NO)(eta(2)-CH2=CHCH=CH2)(PMe3) (12). However, reactions of 11 with various substrates (e.g., pentane, Et2O, and mesitylene) all lead to a single product, Cp*Mo(NO)(eta(4)-trans-butadiene) (13). Evidently, the formation of the I 8e butadiene complex is favored over the activation of a relatively electron-poor C H bond by this molybdenum system. The results of OFT calculations on the model reaction of CpW(NO)(eta(2)-CH2=CHCH=CH2) with propane confirm that the rate-determining step is the cleavage of a propane C H bond and that the activation barrier for terminal activation is 8.6 kJ/mol lower in energy than that for internal activation. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of complexes 3, 5, 6, 8, 9, and 13 have been established by X-ray crystallographic analyses.

Thermolysis of Cp*W(NO)(CH2CMe3)(eta(3)-CH2CHCHPh) (1) at 55 degrees C leads to the loss of neopentane and the formation of the 16e eta(2)-allene intermediate complex Cp*W(NO)(eta(2)-CH2=C=CHPh) (A), which has been isolated as its 18e PMe3 adduct (2). Further support for the existence of the allene intermediate A is provided by the thermolysis of 1 in cyclohexene, which affords Cp*W(NO)(H)(eta(3)-CH2C(3-cyclohexenyl)CHPh) (3) as the principal organometallic product. In the presence of n-heptane, n-octane, or n-pentane, A effects C-H activations of the hydrocarbons exclusively at their terminal carbons and forms 18e Cp*W(NO)(n-alkyl)(eta(3)-CH2CHCHPh) complexes (4-6). Similarly, treatments of 1 with mesitylene, methylcyclohexane, and ethylcyclohexane all lead to the corresponding primary sp(3) C-H activation products (7-9). Complex mixtures of organometallic products result when 1 is thermolyzed in p-xylene and toluene, reflecting the occurrence of both aryl and benzylic C-H activations. Interestingly, the aryl C-H activations do not afford the expected Cp*W(NO)(aryl)(eta(3)-CH2CHCHPh) products but rather their Cp*W(NO)(H)[eta(3)-CH(aryl)CHCHPh] isomers resulting from aryl-H exchange. The thermal chemistry of the molybdenum analogue of 1, namely Cp*Mo(NO)(CH2CMe3)(eta(3)-CH2CHCHPh) (14), has also been investigated, and it turns out to be much more limited in scope. When 14 is heated at 35 degrees C in neat mesitylene for 22 h, it results in conversion to the mesitylene-activated product Cp*Mo(NO)(CH2C6H3-3,5-Me-2)(eta(3)-CH2CHCHPh) (15) in low yield, but thermolyses of 14 in other hydrocarbons do not produce tractable organometallic materials. The results of DFT calculations on the model reaction of CpW(NO)(eta(2)-CH2=C=CHMe) with propane confirm that the rate-determining step is the cleavage of a propane C-H bond and that the lower energy anti conformers favor terminal activation by 11.5 kJ/mol. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by X-ray crystallographic analyses.

A purification method for multi-walled carbon nanotubes (MWCNTs) has been developed to remove almost all (>98%) residual metal catalyst without introducing a significant amount of surface functionality. The process involves alternating mild acid oxidation and thermal oxidation in an iterative fashion with progressively higher temperatures for thermal treatment, in accordance with the increasing thermal stability. Thermogravimetric analysis and inductively coupled plasma mass spectrometry were employed to assess thermal stability and residual catalyst content, respectively, throughout the process. Transmission electron microscopy confirms the integrity of the nanotubes, and the degree of acid functionalization introduced by the acid oxidation is minimal, as determined by titration analysis.Key words: multi-walled carbon nanotube, purification, ICP-MS, TGA, FTIR.