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THE REDUCTION OF THE NBX5/AIX3 SYSTEM WITH ALUMINUM IN THE PRESENCE OF AROMATIC-HYDROCARBONS - AN APPROACH TO NIOBIUM(II), NIOBIUM(I), AND NIOBIUM(0) ORGANOMETALLICS
No full textThe niobium(0) compound [Nb(mes)2] (mes = 1,3,5-trimethylbenzene) was prepared not only by metal atom - ligand cocondensation, but also according to Equation (a). The chemical behavior of this compound is similar to that of the analogous complex [V(mes)2]. For example, it reacts under carbon monoxide atmosphere to give the green salt [Nb(mes)2(CO)] [Nb(CO)6]. This new, simple approach to niobium(0) compounds should facilitate their study (DME = dimethoxyethane)
ELECTRON-TRANSFER PROCESSES WITH SUBSTITUTED GROUP-5 METAL-CARBONYLS - SYNTHESIS, CRYSTAL AND MOLECULAR-STRUCTURE OF AG3M3(CO)12(ME2PCH2CH2PME2)3, M=NB, TA, THE 1ST STRUCTURALLY CHARACTERIZED CARBONYL DERIVATIVES OF NIOBIUM(0) AND TANTALUM(0)
No full textSYNTHESIS AND REACTIVITY OF MIXED ARENE HALIDES OF NIOBIUM(I) - CRYSTAL AND MOLECULAR-STRUCTURE OF NBMES2 AND [NBMES2(CO)][NB2(MU-I)3(CO)8]
No full textBis(mesitylen)niobium(0), Nbmes2, prepared via the NbCl5/Al/AlCl3/mes system by exaustive reduction in the presence of 1,2-dimethoxyethane or tetrahydrofuran as previously reported, has been studied by X-ray diffraction methods. The sandwich molecule has parallel, substantially planar, aromatic rings, in an eclipsed conformation. Nbmes2 is oxidized to Nbmes2X by reaction with 1,1'-dimethyl-4,4'-bipyridinium halides [methylviologen halides, MvX2 (X = Cl, I)]. The reaction of both halo compounds with CO yielded the niobium(I) derivatives [Nbmes2(CO)] [Nb2(mu-X)3(CO)8]. In the case of X = I an IR study at low temperature has shown the presence of intermediate carbonyl compounds, presumably Nbmes2I(CO) and [Nbmes2(CO)]+. The ionic iodide derivative was shown by X-ray diffraction methods to consist of discrete [Nbmes2(CO)]+ cations and [Nb2(mu-I)3(CO)8]- anions. The two mesitylene groups in the cation are bent and in an eclipsed conformation
LOW-TEMPERATURE SYNTHESES OF VANADIUM(III) AND MOLYBDENUM(IV) BROMIDES BY HALIDE EXCHANGE
No full textHalide-exchange reactions of VCl4 or MoCl5 with dry HBr at temperatures between ca. -50 and +20-degrees-C afforded the corresponding bromides, VBr3 or MoBr4, in good yields, presumably via the formation of unstable higher-valent bromide intermediates which undergo spontaneous loss of bromine. In corresponding reactions with HI the exchange was incomplete and mixed halides were obtained. At room temperature the VCl4-HI system gave VCl3 in an almost quantitative yield. By reaction of MoBr4 with tetrahydrofuran (thf) [MoBr3(thf)3] was isolated and its crystal structure determined: orthorhombic, space group Pbcn (no. 60), a = 8.812(2), b = 13.882(5), c = 14.279(3) angstrom, z = 4, R = 0.063. The molecule has the usual meridional geometry, typical of other [MoX3(thf)3] derivatives (X = Cl or I)
SYNTHESIS AND REACTIVITY OF ETA-6-ARENE DERIVATIVES OF NIOBIUM(II), NIOBIUM(I), AND NIOBIUM(O)
No full textThe primary reduction products in the Al/AlX3/NbX5/arene system, namely Nb(eta-6-arene)(AlX4)2, X = Cl, arene = benzene, hexamethylbenzene; X = Br, arene = hexamethylbenzene, have been isolated. Tetrahydrofuran at room temperature reacts with Nb(eta-6-hexamethylbenzene)(AlBr4)2, to give the aluminium-free complex Nb2(eta-6-hexamethylbenzene)2Br4, whose crystal structure was determined by X-ray diffraction methods. In the presence of tetrahydrofuran or dimethoxyethane, the eta-6-mesitylene-tetrahaloaluminato complexes are further reduced by aluminium to bis-mesityleneniobium(0), Nb(mes)2, which undergoes: a) disproportionation with CO to [Nb(mes)2(CO)]+[Nb(CO)6]-, b) oxidation by V(CO)6 or (4,4'-dimethyl-2,2'-dipyridyl)(BPh4)2 under carbon monoxide to [Nb(mes)2(CO)]X, X = [V(CO)6]- or BPh4-. The structure of [Nb(mes)2(CO)] BPh4 has been determined by X-ray diffraction methods. In the presence of CO, Nb(mes)2 is reduced by CoCp*2 [Cp* = eta-5-C5(CH3)5] to [Nb(CO)6]-, which was isolated as its CoCp*2+ derivative; it efficiently deoxygenates CO2 to CO in the presence of carbon monoxide acceptors such as the [Nb(mes)2(THF)]+ cation
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
A new type of alumoxane obtained by controlled hydrolysis of N,N-dialkylcarbamato complexes of aluminium: crystal and molecular structure of [Al4(μ3-O)2 (O2CNPri2)8]
No full textCyclopentadienyl-N,N-diethylcarbamato Derivatives of Zirconium(IV) and Hafnium(IV), MCp(O2CNEt2)3: Synthetic Procedures and Use as Precursors for the Chemical Implantation of Organometallic Fragments on a Silica Surface
No full textReaction of Mg(C5H5)2 with the homoleptic diethylcarbamato derivatives of zirconium(IV) or hafnium(IV),
M(O2CNEt2)4, gave the mixed cyclopentadienyl–diethylcarbamato complexes [M(C5H5)(O2CNEt2)3]. These
derivatives have also been obtained by reaction of Fe(O2CNEt2)2 with M(C5H5)4 in the appropriate stoichiometric
ratio. The compound [Zr(C5H5)(O2CNEt2)3] crystallizes in the space group P1 ̄ , Z = 2, a = 9.734(4), b = 10.234(2),
c = 12.948(6) Å; a = 83.41(4), b = 86.13(4) and g = 67.43(3)8. On reaction of [Zr(C5H5)(O2CNEt2)3] with
dehydrated silica, 1 mol of CO2 per zirconium was evolved. The organometallic fragment Zr(C5H5)(O2CNEt2)2
grafted on silica has been characterized by analytical methods and by CP MAS 13C NMR spectroscopy
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