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Structural characterization of a dimeric dimethylindium azide and its use as a single-source precursor for InN thin films
No full textDimeric dimethylindium azide, [Me2In(mu -N-3)](2) (1), was prepared from the reaction of Me3In with HN3. A single-crystal X-ray diffraction study reveals that 1 exists as a three-dimensional network of three symmetry-independent azide-bridged, centrosymmetric dimers. In each dimer, the two azido groups lying in the same plane of the (In-N)(2) ring have a linear geometry, and the two indium atoms exhibit a distorted octahedral geometry. InN thin films were grown with 1 on Si(lll) substrates in the temperature range 350-450 degreesC in the absence of carrier gas by a low-pressure chemical vapor deposition method. The stoichiometry of the resulting films was determined by X-ray photoelectron spectroscopy (XPS). The films are nitrogen-deficient InN (In:N approximate to 1:0.60) with:high surface impurity concentrations (C approximate to 20%, O approximate to 27%). The film structure was examined by X-ray diffraction (XRD) and: scanning electron microscopy (SEM). The films appear to be polycrystalline and show diffraction patterns characteristic of the expected hexagonal wurtzite structure. (C) 2000 Elsevier Science B.V. All rights reserved
Trans-cis isomerization and structure of dimeric [Me2M-mu-N(H)NPh2](2) (M = Al, Ga)
No full textReaction of MMe3 (M = Al, Ga) with one equivalent of NH2NPh2 affords a dimeric complex [Me2M-mu-N(H)NPh2](2) [M = Al (1), M = Ga (2)] as a mixture of trans and cis isomers. Purification of 1 and 2 by recrystallization gives only trans isomers 1a and 2a, respectively. Variable-temperature H-1-NMR studies reveal that both 1 and 2 in solution undergo trans --> cis isomerization with free energy of activation (Delta G(c)double dagger) values of 15.9 for 1 and 18.3 kcal mol(-1) for 2. The solid state structures of trans isomers 1a and 2a have been determined by single-crystal X-ray diffraction studies. The molecular geometries of la and 2a consist of a centrosymmetric and dimeric unit (M-N)(2) with two bridging hydrazido groups and two terminal methyl groups bound to each metal atom. The two N-NPh2 groups are trans to each other with respect to the (M-N)(2) core plane. The coordination geometry of both metal and nitrogen atoms are distorted tetrahedral. (C) 1999 Elsevier Science S.A. All rights reserved
Reaction of GaMe3 with H2NCH2CH2NMe2: Synthesis and characterization of adducts and imidogallane
No full textReaction of GaMe3 with N,N-dimethylethylenediamine (DMEDA) produces adducts, Me3Ga:NH2C2H4NMe2 (1:1, 1) and Me3Ga:NH2C2H4NMe2:GaMe3 (2:1, 2). A fast concerted intermolecular: exchange of the two inequivalent GaMe3 moieties occurs in 2 at room temperature. Thermolysis of 2 at 210 degreesC in a sealed ampule produces a mixture of cis and trans isomers of tetrametallic imidogallane, [Me2Ga(mu -NC2H4NMe2GaMe2)](2) (3). Slow recrystallization of 3 affords only the cis isomer as colorless crystals, but in solution the cis isomer equilibrates with the trans isomer. The cis --> trans isomerization of 3 has been studied by H-1 NMR spectroscopy. The equilibrium has been observed to follow reversible first-order kinetics with DeltaH degrees = - 3.35 +/- 1.70 kJ mol(-1) and DeltaS degrees = - 10.4 +/- 1.7 J mol(-1) K-1. The activation parameters for the cis --> trans conversion are DeltaH(1)double dagger = 144.6 +/- 6.2 kJ mol(-1) and Delta S(1)double dagger = 105.6 +/- 1.5 J mol(-1) K-1, and those for the reverse process are DeltaH(-1)double dagger = 147.6 +/- 6.4 kJ mol(-1) and DeltaS(-1)double dagger = 115.4 +/- 1.5 J mol(-1) K-1. The isomerization process has been proposed to be assisted by the NMe2 end of DMEDA as an internal Lewis base. The molecular structures of 2 and cis-3 have been determined by single-crystal X-ray diffraction studies. The geometry about the gallium atom of 2 is distorted tetrahedral with a Ga-C (av) distance of 1.98(1) Angstrom and a Ga-N distance of 2.16(1) Angstrom. The molecular geometry of cis-3 involves one four-membered Ga2N2 ring and two five-membered GaN2C2 rings of cis conformation, and it has approximate C-2 symmetry with the C-2 axis going through the center of the Ga2N2 ring
Trans-cis isomerization and structure of [R2Ga(mu-NHSiEt3)](2) (R = Me, Et)
No full textReactions of Et3SiNH2 With R3Ga in a 1:1 ratio have produced dimeric silylamidogallanes, [R2Ga(mu-NHSiEt3)](2) (1, R= Me, trans:cis = 1.5:1; 2, R = Et, trans:cis = 1.3:1), as a mixture of trans (a) and cis (b) isomers. Purification of 1 by either recrystallization or sublimation gives only trans isomer la as colorless crystals. Colorless liquid 2 has been obtained only as a mixture of the two isomers. The trans --> cis isomerization of 1 has been studied by H-1 NMR spectroscopy. The equilibrium has been observed to follow reversible first-order kinetics with Delta H degrees = - 0.64 +/- 0.03 kJ mol(-l) and Delta S degrees = - 4.4 +/- 0.2 J K-1 mol(-1). The activation parameters for the trans (1a) --> cis (1b) conversion are Delta H-1(double dagger) = 72.4 +/- 1.3 kJ mol(-1) and Delta S-1(double dagger) = - 38.8 +/- 4.5 J K-1 mol(-1), and those for the reverse process are Delta H--1(double dagger) = 73.0 +/- 0.4 kJ mol(-1) and Delta S--1(double dagger) = - 34.4 +/- 1.7 J K-1 mol(-1). The isomerization is markedly accelerated in the presence of Lewis bases. A crossover experiment indicates that the isomer interconversion is a unimolecular process. The thermodynamic and kinetic data have been explained based on the solvent effect and the silyl substituent effect on the nitrogen atom. The molecular structure of 1a has been determined by a single-crystal X-ray diffraction study. The molecular geometry of la consists of a centrosymmetric and dimeric unit with two bridging (triethylsilyl)amido groups and two terminal methyl groups bound to each gallium atom. The two silyl groups are trans to each other with respect to the planar (Ga-N)(2) ring framework. The coordination geometry of both gallium and nitrogen atoms is distorted tetrahedral
Cyclopentadienyl-hydrazido titanium complexes: Synthesis, structure, reactivity, and catalytic properties
No full textAn eta(1)-hydrazido bis(dimethylamido) complex, [(eta(5)-C5Me4)SiMe2(eta(1)-NNMe2)]Ti(NMe2)(2) (2), has been prepared from a neat reaction of (C5Me4H)SiMe2(NHNMe2) (1) with Ti(NMe2)(4) at 130 degrees C via amine elimination. Treatment of 2 with Me3SiCl gives an eta(2)-hydrazido dichloro compound, [(eta(5)-C5Me4)SiMe2(eta 2-NNMe2)]TiCl2 (3). The reaction of 2 with excess AlMe3 (ca. 5 equiv) affords an AlMe3 adduct, [(eta(5)-C5Me4)SiMe2(eta(1)-NNMe2AlMe3)]TiMe2 (4), which is converted to a dimethyl complex, [(eta(5)-C5Me4)SiMe2(eta(2)-NNMe2)]TiMe2 (5), upon treatment with NEt3. The reaction of 5 with B(C6F5)(3) affords a methyl-triarylborate complex, [(eta(5)-C-5-Me-4)SiMe2(NNMe2)]TiM(mu-Me)B(C6F5)(3) (6). The reaction of 2 with H2O produces a tetrameric organotitanoxane [(eta(5)-C5Me4)(SiMe2NHNMe2)Ti](4)O-6 (7) with an adamantane-like cage structure. However, the reaction of 2 with H2O in the presence of HCl affords a dimeric complex [mu-(OSiMe2-eta(5)-C5Me4)TiCl2](2) (8). Compounds 1-8 have been characterized by spectroscopic and analytical data. Molecular structures of 2, 3, 4, 5, 7, and 8 have been determined by single-crystal X-ray diffraction studies. Complexes 2-5 contain eta(1)-hydrazido (2 and 4) and eta(2)-hydrazido (3 and 5) groups depending on the electronic nature of the titanium center. Reversible interconversion between eta(1)- and eta(2)-hydrazido moieties has been observed upon addition of either NEt3 (4 --> 5) or AlMe3 (5 --> 4). Complexes 2-5 exhibit moderate catalytic activities for the polymerization of ethene in the presence of either methyaluminoxane (MAO) or (AlBu3)-Bu-i/Ph3C+B(C6F5)(4)(-)
Reactions of AlR3 (R = Me, Et) with H2NCH2CH2NMe2: Synthesis and characterization of amido- and imidoalanes
No full textThe reactions of R3Al (R = Me, Et) with N,N-dimethylethylenediamine (DMEDA) in a 1:1 molar ratio produce adducts, R3Al . NH2C2H4NMe2 (1a, R = Me; Ib, R = Et), which upon heating in refluxing toluene result in alkane elimination to afford the chelated monomers R2AlNHC2H4NMe2 (2a, R = Me; 2b, R = Et), respectively. Compounds 2a,b form adducts with 1 equiv of R3Al through the 3-coordinate nitrogen atom to produce R3AlN(H)C2H4NMe2AlR2 (3a, R = Me; 3b, R = Et), which can be alternatively prepared by the reaction of R3Al with DMEDA in a 2:1 molar ratio by alkane elimination at room temperature. Thermolysis of 3a,b at 140 degrees C in sealed ampules produces a mixture of cis and trans isomers of tetrametallic imidoalanes, [R2Al(mu-NC2H4NMe2AlR2)](2) (4a, R = Me; 4Cb, R = Et). Slow recrystallization of the isomeric mixture affords crystals of only the cis isomer for both 4a and 4b, but in solution cis-4a and cis-4b equilibrate with the respective trans isomers. The cis --> trans isomerization of 4a has been studied by H-1 NMR spectroscopy. The equilibrium has been observed to follow reversible first-order kinetics with Delta H degrees = 1.67 +/- 0.60 kJ mol(-1) and Delta S degrees = 9.07 +/- 2.80 J mol(-1) K-1. The activation parameters for the cis --> trans conversion are Delta H-1 = 125.8 +/- 9.3 kJ mol(-1) and Delta S-1 = 89.7 +/- 1.2 J mol(-1) K-1, and those for the reverse process are Delta H-1 = 124.2 +/- 9.3 kJ mol(-1) and Delta S-1 = 80.7 +/- 1.2 J mol(-1) K-1. Pyrolysis of 4a,b in the presence of 2 equiv of DMEDA at 190 degrees C in sealed ampules gives hexameric imidoalanes, (RAlC2H4NMe2)(6) (5a, R = Me; 5b, R = Et), by alkane elimination. Compounds 5a,b can be also prepared by pyrolysis of 2a,b under similar conditions, but in lower yields. The molecular structures of cis-4b and 5b have been determined by single-crystal X-ray diffraction. The molecular geometry of cis-4b involves one four-membered Al2N2 ring and two five-membered AlN2C2 rings of cis conformation, and it has approximate C-2 symmetry with the C-2 axis going through the center of the Al2N2 ring. The hexameric imidoalane 5b consists of a hexagonal prism of the (AlN)(6) cage formed by two flat six-membered (AlN)(3) rings linked together by six transverse Al-N bonds
Growth of AlN films using hydrazidoalane single-source precursors
No full textAluminum nitride (AIN) films were grown on Si(100) and S(111) substrates by a low-pressure chemical vapor deposition method in the temperature range 400-800 degrees C using two hydrazidoalane dimers, [Me2Al-mu-N(H)NMe2](2) (1) and [Et2Al-mu-N(H)NMe2](2) (2) as single-source precursors. Polycrystalline AlN films were obtained on Si(111) at 800 degrees C from precursor 1. Amorphous AlN films were observed under certain growing conditions, such as on Si(100) substrates from precursor 2, or at lower temperatures. The films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. (C) 1999 Elsevier Science S.A. All rights reserved.We are grateful to Korea Science and Engineering Foundation for financial support of this research. Additional support to Y. Kim was provided by the Ministry of Science and Technology
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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