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  • 1
    ISSN: 0947-6539
    Keywords: biphenyls ; bipyridinium salts ; conformation ; semiempirical calculations ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidation of substituted N-aminopyridinium compounds yields bipyridinium salts. After anion exchange Br- → CIO4-, it was possible to grow single crystals from the 4,4′-di(tert-butyl) derivative. The structure determined at 100 K shows a dihedral angle of 84° between the molecular halves connected by a shortened N—N bond of 143 pm in length. This result is contrary to that expected from the isoelectronic correspondence 〉C—C〈↔〉N—N〈, which would suggest planarity, as has been observed experimentally both in solid biphenyl, and in tetraalkylhydrazine dications and radical cations. Lattice packing analysis, however, reveals the presence of hydrogen bonds C(H)…O between the phenyl rings and the perchlorate anions. AM 1 enthalpy hypersurface calculations for the isoelectronic series H5C5X-YC5H5 (X-Y = C—C, +N—C, +N—N+, -B—N+, B—C, and B—B-) predict single-minimum potentials for both the dication 〉N—N〈 as well as the dianion 〉B—B〈 with the molecular halves twisted perpendicular to each other. In detailed model calculations, counteracting effects of π electron density delocalization vs. H/H repulsion of the ortho ring hydrogens adjacent to the central bond seem to dictate the delicate balance of the biphenyl twisting. For further experimental confirmation, the structure of the isosteric molecule 4,4′-di(tert-butyl)biphenyl has been determined: in contrast to the unsubstituted π hydrocarbon with a latticeenforced dihedral angle of 0°, close to the calculated value, the two molecular halves are twisted by 40° - as in biphenyl in the gas phase.
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  • 2
    ISSN: 0947-6539
    Keywords: copper compounds ; exchange coupling ; heterometallic compounds ; lanthanide compounds ; magnetic properties ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis and structural characterisation of three copper-lanthanoid complexes are reported. The compounds, of general formula [Cu3M(chp)8(NO3)(S)] [M = Gd, S = H2O in 2; M = Dy, S = H2O in 3; M = Er, S = (H2O)0.5(MeOH)0.5 in 4; chp = anion of 6-chloro-2-pyridone], are made by reaction of [Cu2(chp)4] (1) with the hydrated lanthanoid nitrate salt in methanol. Structural studies reveal the three copper atoms lie in an approximate hemisphere about a central lanthanoid atom. Magnetic studies on 2 and two further Cu-Gd complexes show ferromagnetic coupling between the 3d and 4f metals. Consideration of these results along with magnetic data previously reported for Cu-Gd compounds leads to a correlation between the magnitude of this exchange coupling and the exponential of the Cu…Gd distance. This is the first magneto-structural correlation reported for mixed d-block/f-block metal complexes.
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  • 3
    ISSN: 0947-6539
    Keywords: asymmetric synthesis ; chirality ; enolates ; hydrogen bonds ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of acetophenone with tetrabutylammonium hydroxide affords the tetrabutylammonium enolate of phenyl (2-hydroxy-2-phenyl)propyl ketone. The crystal structure of this chiral enolate shows intramolecular hydrogen bonding between the hydroxyl group and the enolate oxygen atom. Furthermore, the α-methylene units of the ammonium counterion form hydrogen bonds to the basic enolate C and O atoms and to the O atom of the hydroxy group. This three-point bonding occurs selectively on the Re,Re side, a phenomenon which may be responsible for the direction of diastereo-selectivity in the epoxide-forming reaction of the enolate with N-bromosuccinimide.
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  • 4
    ISSN: 0947-6539
    Keywords: cyclic voltammetry ; iron complexes ; redox systems ; structure elucidation ; sulfur ligands ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The effects of protonation and alkylation at the sulfur donors in metal-sulfur complexes have been investigated by taking the specific example of [Fe(CO)(“NHS4”)] (1). The 18 valence electron (VE) complex 1 consists of a low-spin FeII center and the dithioether thiolato amine ligand “NHS4”2-- ( = 2,2′-bis(2-mercaptophenylthio)diethylamine(2-)). Complex 1 can be reversibly protonated at the two thiolato donors; this results in an increase in ν(CO) of 35 cm-1 after the first protonation and 45 cm-1 after the second. Alkylation of 1 with one or two equivalents of the oxonium salts R3OBF4 (R = Me, Et) yields [Fe(CO)(“NHS4”-R)]BF4 (R = Me: 4, Et: 5), [Fe(CO)(“NHS4” -R2)](BF4)2 (R = Me: 6, Et: 7), and the methyl ethyl derivative [Fe(CO)(“NHS4” -Me-Et)](BF4)2 (8). An increase in ν(CO) of 31-32 cm-1 is observed for each successive alkylation. Due to the C1 symmetry of 1, complexes 4, 5, and 8 are formed as 1 : 1 mixtures of two diastereomers, whereas 6 and 7 are present as only one stereoisomer. Acidic hydrolysis of 4, 6, and 7 liberates the corresponding ligands [“NHS4” -Rn](BF4)n (9-11; n = 1,2), which were isolated as the ammonium tetrafluoroborate salts. The molecular structure of 8a has been elucidated by X-ray structure analysis. This shows that the Fe-N and Fe-S bonds in the [FeNS4] core do not change after alkylation of the thiolato donors despite the Δν(CO) of ca. 60 cm-1 between 1 and 8, which indicates a distinct decrease in electron density at the Fe center. This decrease can be rationalized by increased π-acceptor character of the sulfur donors upon protonation or alkylation. The change in electron density at the [Fe(“NS4”)] core is further corroborated by cyclic voltammetry. For each successive protonation or alkylation of 1, the redox couple potentials shift by 600-800 mV. As a consequence, the diethyl derivative, for example, gives rise to reduced species (19 or 20 VE) which are not observed for 1. The implications of these results for the reduction of N2 within the coordination sphere of the FeMo cofactor of nitrogenases are discussed.
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  • 5
    ISSN: 0947-6539
    Keywords: copper complexes ; imidazole ligands ; redox systems ; structure elucidation ; enzyme models ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The new diimidazole ligand, bis (1-methyl-4,5-diphenylimidazol-2-yl)ke-tone (BIMDPK), has been synthesised, characterised and shown to form four-coordinate bis(diimidazole) CuII and CuI complexes in the salts [Cu(bimdpk)2]-[BF4]2 and [Cu(bimdpk)2][PF6], the structures of which have been determined by X-ray crystallography. The cations of these salts have a very similar geometry with CuII-Nav = 1.949 and CuI-Nav = 1.999 Å; the N-Cu-N interbond angles are constrained by 1) the bite angle of the BIMDPK ligand to 94 ± 2° and 2) the interligand steric interactions, which lead to the dihedral angle of the intraligand CuN2 planes of 68.2° for CuII and 74.9° for CuI-that is, a CuN4 geometry intermediate between tetrahedral and square planar. The X-band EPR spectrum for the powdered CuII compound is typical of an approximately D2 CuN4 centre possessing a dxy ground state (gx = 2.080, gy = 2.075, gz = 2.291; Az = 112.3 × 10-4 cm-1). The UV/vis spectra are dominated by charge-transfer bands, and both the CuII and CuI systems are intensely coloured. The EPR and electronic spectra indicate that these cations have a very similar structure in the solid state and in solution; the potential of the [Cu(bimdpk)2]2+/[Cu(bimdpk)2]+ couple is 0.59 V vs. SCE in MeCN and 0.80 V vs. SCE in CH2Cl2, and the electron self-exchange constant in MeCN is 1.9 × 104M-1S-1. Comparisons are made between the properties of the [Cu(bimdpk)2]2+/+ centres and related Cu centres in chmical and biological systems; the results of this study reinforce the view that a [Cu(His)4] centre should not be precluded from consideration in biological electron transport.
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  • 6
    ISSN: 0947-6539
    Keywords: asymmetric eponidations ; Cstalysis ; manganese complexes ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The relationship between catalyst structure and enantioselectivity in the asymmetric epoxidation of unfunctionalized olefins by a series of chiral Mn(sa1en) complexes (1 - 10) was examined. The X-ray structures of 5-coordinate complexes 5, 8, of 6-coordinate 9 ([6,6′= -tBu; 4,4 = -tBu]+ClO4-), and 10 (6,6′= -tBu; 4,4′=-Br) were determined. Catalysts 1 - 9 were derived from (R,R)-1, tdiaminocyclohexane and catalyst 10 from (S,S)-1,2-diphenylethyIenediamine. Catalysts 1-9 differ in the stereoelectronic substitution of the orfho (6,6) and para (4,4) positions of the salicylidene moiety. A comparison between structures 5, 8, and 9 reveals that the ligand geometry around the metal center and the chiral diimine backbone remains remarkably constant in both five- and six-coordinate cyclohexanediamine-derived complexes; in contrast, the salicylidene regions of the complexes display a wide range of conformations. The asymmetric epoxidation of indene and 6-cyano-2,2-dimethylchromene with NaOCl catalyzed by complexes 1 - 10 was effected. Systematically increasing the steric bulk on the ortho and then the para position in the order 1 (6,6′ = -H; 4,4′ = -H),2(6,6′ = -CH3; 4,4′ = -CH3),3(6,6′=-tBu;4,4′=-H),4(6,6′=-tBu; 4,4′ =-CH3), 5 (6,6′=-tBu; 4,4′=-tBu), and 6 (6,6′=-tBu; 4,4′= -trityl), and electronically modifying the para substituents in 7 (6,6 = -tBu; 4,4 =-OMe) and 8 (6.6′ = -tBu; 4,4′=-OTIPS) resulted in enhanced enantioselectivities of the desired epoxides. The conformational variations observed in the solid state are likely to reflect accessible solution conformations and may help explain the high levels of stereoinduction obtained with these catalysts in the asymmetric epoxidation of unfunctionalized olefins.
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  • 7
    ISSN: 0947-6539
    Keywords: amino acids ; conformation ; helices ; molecular rulers ; oligopeptides ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Terminally blocked, isotactic homopeptides from the sterically demanding α-methylvaline of general formula Y-[L-(αMe)Val]n-OtBu (Y = Z, pBrBz, Ac; n = 2-8) have been prepared step-by-step in solution and fully characterized. The conformations preferred in solution (β-turn and right-handed 310-helix) have been assessed by FT-IR, 1H NMR and CD spectroscopy. The molecular and crystal structures of the Z-protected trimer, hexamer, heptamer and octamer have been determined by X-ray diffraction. In the crystal state, while the trimer is folded in a type III β-turn conformation, the longest homopeptides form well-developed, regular, right-handed 310-helices. The screw sense in the helix of the pBrBz-blocked octamer has been confirmed to be right-handed by solid-state and solution CD spectroscopy. The possible exploitation of these peptide helices as rigid and precise molecular rulers is discussed.
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  • 8
    ISSN: 0947-6539
    Keywords: NMR spectroscopy ; peptides ; protein structures ; structure elucidation ; zinc ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Besides its functional role in many hydrolytic metalloenzymes, zinc acts as a structural component by being attached to bis(cysteinyl) protein sequences in some of the same enzymes, and in other metalloproteins and zinc fingers, and by being an essential constituent in metallothioneins. It is not always obvious whether the zinc-binding proteins are pre-organized for the incorporation of the metal or whether the zinc ion provides the structurizing power and stability for the observed peptide conformations. We have addressed the coordination chemistry aspects of this question by synthesizing zinc complexes of small model peptides and by determining their structures in solution by 2D NMR spectroscopy. The peptides chosen were of the terminally protected bis(cysteinyl) type: Cys-Cys, Cys-Gly-Cys, Cys-Phe-Cys, and Cys-Gly-Ile-Cys. The zinc ions fold these peptides into structures that can be superimposed on those of the natural proteins.
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  • 9
    ISSN: 0947-6539
    Keywords: carbenes ; heterocycles ; ligand design ; structure elucidation ; transition metal complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: To date the only free carbenes of the imidazoline-2-ylidene type to have been described in the literature are those bearing simple hydrocarbon or haloalkyl and -aryl substituents. We report here a novel, versatile and high-yielding method for the synthesis of N-heterocyclic carbenes in a mixture of liquid ammonia and aprotic organic solvents. Deprotonation of the imidazolium precursor salts proceeds under mild conditions within a few minutes at temperatures below -30°C, and side reactions are thus avoided. The imidazolium salts are much more soluble in organic solvents if liquid ammonia is added. Furthermore, the acidity of the C-2 protons appears to be enhanced by hydrogen bonding. Not only are the known free ylidenes conveniently and quantitatively accessible by this procedure, but also novel functionalized derivatives that are not accessible by known procedures. Imidazoline-2-ylidenes with linear, branched, cyclic, heteroatom-substituted (O, N, P) and chiral hydrocarbon residues are accessible through the novel route. Stable carbene-metal “adducts” are conveniently obtained by treating the free carbenes with chloro- or acetato-bridged dinuclear metal complexes, or by displacement of coordinated ligands such as carbon monoxide, THF or acetonitrile by the free carbenes. The syntheses of novel imidazolium salts. N-heterocyclic carbenes and carbene adducts of RuII, RHI, W0 and sulfur are reported, and the structures of five products analysed by single-crystal X-ray diffraction. N-Heterocyclic carbenes bearing functionalized side chains are important because a number of these complexes show excellent activity in catalytic reactions. They do not show the typical reactivity of metal-carbon “double bonds” and are remarkably stable both thermally and chemically. For a number of reasons, they are best viewed as donor adducts of the highly Lewis basic imidazoline-2-ylidene ligands and the Lewis acidic organometallic fragments. The new synthetic procedure reported here makes N-heterocylic carbenes a generally accessible class of useful ligands in coordination chemistry and catalysis.
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  • 10
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 249-252 
    ISSN: 0947-6539
    Keywords: high-temperature synthesis ; network structures ; nitridosilicates ; silicon ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The novel nitridosilicate BaSi7N10 was obtained by the reaction of barium metal with Si(NH)2 in a special high-frequency furnace at 1650 °C. The single-crystal structure determination (a = 687.29(3), b = 671.29(3), c = 963.28(4) pm, β = 106.269(3)°, PC, Z = 2, R1 = 0.0497, wR2 = 0.0924) reveals a network structure of connected SiN4 tetrahedra. BaSi7N10 is the first nitridosilicate with both corner and edge-sharing SiN4 tetrahedra. Unlike the situation in normal oxosilicates, vertex-sharing of SiN4 tetrahedra is not exclusively favored over edge-sharing. With a Si:N molar ratio of 7:10, BaSi7N10 is the most highly condensed multinary nitridosilicate known and has almost the same degree of condensation as binary Si3N4.
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