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  • Chemistry  (16)
  • Aircraft Stability and Control  (6)
  • esterification  (2)
  • 1995-1999  (9)
  • 1985-1989  (10)
  • 1980-1984  (3)
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Year
  • 1
    Electronic Resource
    Electronic Resource
    Complexes of 3,3′-Oxybis[(diphenylphosphino)methylbenzene] with Ni(II), Pd(II), Pt(II), Rh(I), and Ag(I). How Important is Backbone Rigidity in the Formation of trans-Spanning Bidenatate Chelates?? (1987)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 70 (1987), S. 158-170 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The bidentate diphosphine ligand, 3,3′-oxybis[(dipenylphosphino)methylbenzene] (1) forms monomeric, trans-square-planar complexes MX2(1) (M = Ni, Pd, Pt; X = Cl-, Br- I-, and, in part, N3-, NCS-, CN-, NO3-) as well as Pt(H)Cl(1), Pt(H)Br(1), and RhCl(CO)(1). Polymeric species have been observed with substitutionally inert metal centres: trans-[PtCl2(1)]2 and cis-[PtCl2(1)]n (mean value of n ≈ 4-5) 31P-NMR, and selected IR and UV/VIS parameters are reported. Ligand 1 shows a marked preference for trans-spanning and monomeric chelate formation, despite its various degrees of freedom of internal rotation in the lignad backbone. The readily available ligand 1 as well as analogues with other donor atoms, therefore, appear useful in most potential applications of trans-spanning chelate ligands. The crystal structure of AgCl(1)·0.5 (CH3)2C=O·0.39 C6H12 (space group C2/c,a = 21.02 Å, b = 14.57 Å, c = 24.79 Å, β = 99.77°, V = 7531.4 Å3, Z = 8) confirms the presence of three-coordinate Ag(I), with a coordination intermediate between a trigonal-planar and a T-shaped geometry (P-Ag-P = 145.61(8)°).
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19870700120
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  • 2
    Electronic Resource
    Electronic Resource
    Complete Racemization in the Base Hydrolysis of Optically Active mer-[Chloro(diethylene triamine) (1,3-diaminopropan-2-ol-N,N′) cobalt (III)]tetrachlorozincate (1980)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 63 (1980), S. 693-696 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The base hydrolysis of the optically active title compound proceeds with full racemization whilst the meridional arrangement of the diethylenetriamine ligand is fully retained. The optically active hydroxo complex prepared independently with retention of configuration racemizes ≳ 104 times slower. This unique result is discussed in terms of the classical π-stabilization hypothesis for the base hydrolysis mechanism.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19800630319
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  • 3
    Electronic Resource
    Electronic Resource
    Electrochemical Investigation of Potassium Heptacyanorhenate(III) in Aqueous Solution (1987)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 70 (1987), S. 375-380 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electrochemistry of potassium heptacyanorhenate(III) in aqueous solution was studied by cyclic and by rotating disk voltammetry at planar microelectrodes. The results are consistent with a single, reversible electron transfer: Re(CN)3-7 + e⇄Re(CN)4-7 with E′0 = 643 mV vs. NHE. A single protonation equilibrium is observed: Re(CN)4-7 + H+⇄ Re(CN)7H3- with pK = 1.31 determined from combined voltammetric and pH data. The Re-CN bond appears to be kinetically inert, and none of the cyano complexes in other oxidation states of Re claimed in the literature was found in the potential range - 2 V to + 1 V.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19870700215
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  • 4
    Electronic Resource
    Electronic Resource
    Morphological and Thermal Investigations of the ‘Active’ Dimer Hydroxide of Chromium(III) (1987)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 70 (1987), S. 1737-1744 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ‘active’ dimer hydroxide, [Cr2(μ-OH)2(OH)4(OH2)4]·2 H2O, has been characterised by means of IR spectroscopy, thermal analysis, X-ray powder diffraction, and electron microscopy. Thermogravimetric measurements and the Cr content of the hydroxide are in agreement with this composition. The IR spectrum supports the notion that this hydroxide is composed of unaltered dimer fragments linked through hydrogen bonds of ca. 2.8 Å. The compound is microcrystalline in nature with an X-ray powder diffraction pattern distinctly different from that of the isomeric ‘active’ monomer hydroxide. Electron micrographs of the ‘active’ dimer hydroxide indicate that this material consists of platelets the sie of which is dependent on the pH of precipitation.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19870700709
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  • 5
    Electronic Resource
    Electronic Resource
    Poly{[μ-(2,5-dicarboxypyrazine- 3,6-dicarboxylato)-trans-diaquairon(II) dihydrate]}, a Member of a New Class of Quasi-Linear Chain Compounds. Preparation, Structure, and Magnetic Susceptibility (1986)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 69 (1986), S. 1004-1011 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The title compound 1, {[Fe(H2pztc)(H2O)2] · 2 H2O}∞, has been prepared from pyrazinetetracarboxylic acid (H4pztc) and Fe(II) salt in H2O. It crystallizes in the monoclinic space group P21/a with cell parameters a = 13.56(1) Å, b = 7.17(1) Å, c = 6.48(1) Å, β = 101.0(2)°, V = 618.4 Å3. The molecules form infinite parallel chains with bis(bidentate) coordination (through N and O) of the bridging pyrazine ligands. Two trans-COOH groups of the ligand remain uncoordinated, and two trans-H2O ligands complete a pseudo-octahedral Fe(II) coordination which is distorted by the steric effect of the free COOH groups. The parallel polynuclear chains are linked through an extended network of H-bonds involving the COO functions of the bridging ligand and the coordinated and lattice H2O molecules. Mössbauer and magnetic-susceptibility data suggest normal paramagnetic behaviour of high-spin Fe2+. An observed drop below 15 K in effective magnetic moment for 1 was attributed to temperature-dependent population changes of the levels of the ground state 5Γ, and not to antiferromagnetic coupling (J 〈 1 cm-1). This result establishes that the obvious potential magnetic exchange pathway through the pyrazine bridges is inefficient, and this may be attributed, in part, to the high planarity of Fe(H2pztc) moieties.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19860690507
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  • 6
    Electronic Resource
    Electronic Resource
    Preparation and Properties of Bis[μ-S-(2-tiolatoethyl)-diphenylphosphine][(2-thiolatoethyl)diphenyl-phosphine]dinickel(+1) Ion, a Novel Unsymmetrical Thiolate-bridged complex (1984)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 67 (1984), S. 654-663 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The title cation ( = Ni2L+3) is formed in a variety of reactions (Schemes 1 and 2) in systems containing Ni2+ and (2-thiolatoethyl)-diphenylphosphine (= L-) in the absence of coordinating anions at Ni2+/L- ratios 〉 0.5 in apolar or moderately polar media. Solid [Ni2L3]CIO4 and [Ni2L3]BPh4 have been isolated. Job's plots confirm the Ni2L+3- stoichiometry in solution. 31P-NMR data are consistent with ≥ 97% Ni2L+3 (vs. ≤ 3% of hypothetical Ni3L2+4) at equilibrium and support the suggested configuration (Fig. 2). The equilibrium between NiL2 + NiL2Br2 and Ni2L+3 + Br- varies with the solvent composition in CH23Cl2/EtOH mixtures. The rate of formation of Ni2L2Br2 from Ni2L+3 and bromide (in high excess) in CH2Cl2 is first-order in [Ni2L+3]tot but depends on the ratio [Bu4NBr]tot/[Ni2L3 · ClO4]tot, even at a high excess of bromide. This is interpreted by efficient competition in ion-aggregate formation between the small perchlorate concentration introduced as the counterion of Ni2L+3, and the large excess of bromide.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19840670304
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  • 7
    Electronic Resource
    Electronic Resource
    Horeau's Coupling of Enantiomers Revisited. The Reversible Coupling of Enantiomers to Form Diastereoisomers in Kinetically Labile Metal Complexes (1987)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 70 (1987), S. 1774-1785 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Pairs of enantiomeric molecules (R,S) of a substrate of unknown, but incomplete enantiomeric purity may be coupled to each other through labile coordination to a metal center M. This results in the formation of an equilibrium mixture of diastereoisomeric complexes, viz. meso-LnMRS(=m) and a pair of enantiomers LnMRR and LnMSS(= e), where L is an achiral auxiliary ligand. General expressions are presented for determining the ratio of enantiomers [R]0/[S]0 from experimental parameters, which may be obtained from NMR measurements. Effects of diastereoselectivity are specifically considered. Limiting cases (nearly pure or nearly racemic substrates, very high or no diastereoselectivity, very large or no excess of free substrate) are discussed. The cases of additional diastereoisomerism owing to different coordination geometries and of the formation of complexes with more than two substrate molecules per metal center are also investigated. The results presented can not only be used for determining the enantiomeric excess but also for designing optimal strategies for the successive resolution of partly enriched samples.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19870700713
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  • 8
    Electronic Resource
    Electronic Resource
    A New Route to Crystalline Salts of the Hydrolytic Dimer of Chromium(III) (1989)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 72 (1989), S. 993-995 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Crystalline salts of the hydrolytic dimer of Cr(III), [Cr2(μ-OH)2(H2O)8]X4·n H2O (X = p-toluenesulfonate (tos) or mesitylene-2-sulfonate (dmtos)) have been prepared in good yields via a simple two-step procedure: H+ oxidation of Cr metal to give Cr2+ (T ≍ 70°) followed by O2 oxidation, of Cr2+ to give the dimer (T ≍ 25°). The mechanism of conversion of Cr2+ into the dimer is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19890720516
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  • 9
    Electronic Resource
    Electronic Resource
    Origin of diastereoselectivity in the thermal [4+2] cycloadditions of dienophiles derived from Oppolzer's sultams: Steric vs. Stereoelectronic InfluencesPresented in part at the IXth Eur. Symp. Org. Chem., 18-23 June 1995, Warse, Poland.Dedicated to the memory of Professor Wolfgang von Oppolzer (1997)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 80 (1997), S. 146-172 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Comparative semi-empirical PM3 and ab initio STO 3-21G calculations on bornanesultam-derived dienophiles containing the structural moiety SO2—N—C(O)—X(α) = Y(β) suggest that, among the conformers of low energy, the thermodynamically less stable SO2/C(O)-syn,C(O)/X=Y-s-cis conformation is also reactive in terms of LUMO level and atomic coefficients. Furthermore, the X(α), Y(β) LUMO atomic coefficients are nonequivalent with respect to both X(α)-re and X(α)-si faces, and thus have, depending on the conformation, a matching or mismatching stereoelectronic influence with the co-operative steric effect. This dissymmetry is believed to result from the generalized anemone effect of the N lone pair, itself anomerically stabilized and directed, in the absence of crucial steric interactions, by the pseudo-axial anti-periplanar S=O bond. Five N-acyl-substituted bornanesultams arc discussed ((-)-1a: N-acryloyl, X=CH, Y=CH2; (-)-1b; N-crotonoyl, X=CH, Y=CHMe; (-)-1c: N-N′-fumaroyl, X=CH, Y=CH(C(O)-bornanesultam); 2a: N-glyoxyloyl, X=CH, Y=O; 2b: N-acylnitroso, X=N, Y=O). In this context, differences with toluenesultams 3 are pointed out. A previous report on N-(acylnitroso)-bornanesultam 2b is revisited, and the diastereoselectivity observed is shown to result from thermodynamic control.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19970800115
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  • 10
    Electronic Resource
    Electronic Resource
    A Unified Interpretation of Kinetic Data on the Acid-Induced Cleavage and of Product-Analysis Data on Spontaneous Cleavage of the Mono-ol Cation μ-Hydroxo-bis[pentaamminecobalt(III)] ([(NH3)5CoOHCo(NH3)5]5+) (1985)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 68 (1985), S. 1914-1922 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Recent work on the spontaneous (= acid-independent) cleavage of the mono-ol cation, i.e. \documentclass{article}\pagestyle{empty}\begin{document}$$ \begin{array}{*{20}c} {[({\rm NH}_{\rm 3})_5 {\rm CoOHCo(NH}_{\rm 3})_5]^{5 +} + {\rm H}_{\rm 2} {\rm O} \to {\rm [(NH}_{\rm 3})_5 {\rm CoOH}_{\rm 2}]^{3 +} + [({\rm NH}_{\rm 3})_5 {\rm CoOH]}^{{\rm 2} +}} \\ {[({\rm NH}_{\rm 3})_5 {\rm CoOH]}^{{\rm 2} +} + {\rm H}^ + \to [({\rm NH}_{\rm 3})_5 {\rm CoOH}_{\rm 2}]^{3 +} {\rm (fast),}} \\ \end{array}$$\end{document} in Cl-/ClO4- and NO3-/ClO4- mixed-electrolyte media has established (by analysis of anion-competition experiments) the existence of reactive ion pairs of the mono-ol cation with Cl- and NO3-. Their existence must be allowed for in the analysis of the rate data for the acid-induced cleavage (pH 0-1) of the mono-ol cation in these mixed-electrolyte media. Thus, previous data for acidic Cl-/ClO4- media have been re-interpreted in this work, and new data for NO3-/ClO4- media have been analyzed in the same sense. This analysis removes an apparent discrepancy in the orders of magnitude of ion aggregate stability constants between the mono-ol and similar binuclear cations.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19850680715
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