Chemistry

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The complex ion [Fe2III(μ-O)(phen) 4(H2O)2]4+ (1) (phen = 1,10-phenanthroline) and its hydrolytic derivatives [Fe2 III(μ-O)(phen)4(H2O)(OH)]3+ (1a) and [Fe2III(μ-O)(phen)4- (OH) 2]2+ (2a) coexist in rapid equilibria in the range pH 4.23-5.35 in the presence of excess phenanthroline (pKa1 = 3.71 ± 0.03, pKa2 = 5.28 ± 0.07). The solution reacts quantitatively with I- to produce [Fe(phen)3] 2+ and I2. Only 1 but none of its hydrolytic derivatives is kinetically active. Both inner and outer sphere pathways operate. The observed rate constants show second-order dependence on the concentration of iodide, while the dependence on [H+] is complex in nature. Added Cl- inhibits the formation of adduct with I- and thus retards the rate of inner sphere path, leading to a rate saturation at high [Cl-], where only the outer sphere mechanism is active. Kinetic data indicate that simultaneous presence of two l- in the vicinity of diiron core is necessary for the reduction of 1. © 2005 Wiley Periodicals, Inc.

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(SoNS) School of Natural Sciences

International Journal of Chemical Kinetics

Kinetics and mechanism, of oxidation of iodide with a (μ-oxo)Diiron(III,III) complex in weakly acidic media

The title MnIV complex, [Mn(LH2)3] 4+ (LH2 = biguanide = H2NC(=NH)NHC(=NH)NH 2), an authentic two-electron oxidant, quantitatively oxidizes hydrazine (H2NNH2) to dinitrogen in the pH interval 2.00-3.50. The net four-electron oxidation of hydrazine is provided by two MnIV as established by stoichiometric studies. The overall reaction is composed of two parallel paths: [Mn(LH2)3]4+ + N2H5+ k1 → products [Mn(LH2)2(LH)]3+ + N2H 5+ k2 → products The values of k 1 and k2 (in M-1 S-1) are, respectively, (5.50 ± 0.6) · 10-3 and 11.4 ± 1.5 at 25.0° and I = 1.0M (NaNO3). The order k2 ≫ k1 is unusual as protonated oxidants are generally more reactive than their conjugate bases. The remarkably superior reactivity of the conjugate base of the oxidant in oxidizing N2H5+ may be at least in part due to an overall positive-charge reduction in [Mn(LH 2)2(LH)]3+ as compared to [Mn(LH 2)3]4+ that favors reaction with positively charged N2H5+, and more plausibly, a change in the mechanism is responsible for this result as not k1 but k 2 is substantially lowered in D2O media indicating that proton motion is coupled with electron transfer in the k2 path. The rate-determining step appears to be a one-electron transfer from N 2H5+ to MnIV. © 2005 Verlag Helvetica Chimica Acta AG.

Helvetica Chimica Acta

Mechanistic studies on the oxidation of hydrazine by tris(biguanide) manganese(IV) in aqueous acidic media

Analysis of charge density distributions in molecular crystals has received considerable attention in the last decade both from high-resolution X-ray diffraction studies and from high-level theoretical calculations. An overview of the progress made in deriving one-electron properties, intermolecular interactions in terms of the Atoms in Molecule (AIM) approach (R.F.W. Bader. Atoms in Molecules-A Quantum Theory, Clarendon, Oxford (1990), R.F.W. Bader. J. Phys. Chem., A102, 7314 (1998)) is given with special emphasis on improvements in charge density models and development of both experimental and theoretical techniques to interpret and analyse the nature of weak intermolecular interactions. The significance of the derived results from the charge density of coumarin and its derivatives have been analysed to obtain insights into the nature of intermolecular C-H⋯O, C-H ⋯π, π⋯π, C-H⋯S, and S⋯S contacts. The appearance of a 'region of overlap' to segregate hydrogen bonds from van der Waals interactions based on the criteria proposed by Koch and Popelier (U. Koch, P.L.A. Popelier. J. Phys. Chem., 99, 9747 (1995), P.L.A. Popelier. Atoms in Molecules. An Introduction, pp. 150-153, Prentice Hall, UK (2000)) and the identification of differences in energy surfaces in concomitant polymorphs of 3-acetylcoumarin are described. © 2005 Taylor & Francis.

Crystallography Reviews

Evaluation of weak intermolecular interactions in molecular crystals via experimental and theoretical charge densities

Evaluation of intermolecular interactions in terms of both experimental and theoretical charge density analyses has produced a unified picture with which to classify strong and weak hydrogen bonds, along with van der Waals interactions, into three regions. © The Royal Society of Chemistry 2005.

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CrystEngComm

Charge density based classification of intermolecular interactions in molecular crystals

Experimental electron densities in coumarin, 1-thiocoumarin, and 3-acetylcoumarin have been analyzed based on the X-ray diffraction data at 90 K. These compounds pack in the crystal lattice with weak C-H⋯O and C-H⋯π interactions, and variations in charge density properties and derived local energy densities have been investigated in the regions of intermolecular interactions. Theoretical charge density calculations on crystals using the B3LYP/6-3 IG** method show remarkable agreement with the derived properties and energy densities from the experiment. The intermolecular interactions follow an exponential dependence of electron density and energy densities at the bond critical points. The Laplacian follows a "Morse-like" dependence on the length of the interaction line. Based on the set of criteria defined using the theory of "atoms in molecules", it has become possible to distinguish between a hydrogen bond (C-H⋯O) and a van der Waals interaction (C-H⋯π). This has resulted in the identification of a "region of overlap" in terms of electron densities, energy densities, and mutual penetration of the hydrogen and acceptor atoms with respect to the interaction length. This approach suggests a possible tool to distinguish between the two types of interactions.

Journal of Physical Chemistry A

Exploring the lower limit in hydrogen bonds: Analysis of weak C-H⋯O and C-H⋯π interactions in substituted coumarins from charge density analysis

The crystal structure of the title compound (systematic name: 2H-1-benzothiopyran-2-thione), C9H6S2, also known as dithiocoumarin, has been determined. The molecule is essentially planar and the structure is stabilized via S⋯S and C-H⋯π interactions. © 2004 International Union of Crystallography Printed in Great Britain - all rights reserved.

Acta Crystallographica Section E: Structure Reports Online

2H-Thiochromene-2-thione

In this study the kinetics of two electron transactions between the reducing substrates glyoxylic and pyruvic acids and the oxidising complex cation, tris(biguanide)manganese(IV) in aqueous acidic media have been examined. Under the experimental conditions (pH = 1.50-3.78) only the fully protonated form of the oxidant reacts with the anions of the substrates whereas the parent acids, viz. glyoxylic and pyruvic acids, were found to be unreactive. The second-order rate constants for the oxidations of glyoxylate and pyruvate by the title MnIV species are (2.24 ± 0.10) × 10-1 and (5.40 ± 0.30) × 10-2 M-1S-1, respectively, at 25.0 °C and at I = 1.0 M (NaNO3 or NaClO 4). A rate-liming one-electron change between the redox partners seems justified. © Wiley-VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2004.

European Journal of Inorganic Chemistry

Mechanistic investigation of the oxidation of glyoxylic and pyruvic acids by tris(biguanide)manganese(IV) in weakly acidic aqueous media

Tris(biguanide)manganese(IV), [Mn(LH2)3]4+ (LH2 = biguanide, C2N5H7), quantitatively oxidises Fe2+ to Fe3+ and is itself reduced to Mn2+ with almost quantitative (&gt; 95%) release of biguanide, The reaction rate strongly depends on added Fe3+; in the presence of externally added Fe3+, the reaction shows a clear first-order dependence in [MnIV], whereas in the absence of any added Fe 3+, an initial quick loss of MnIV is associated with a subsequent very sluggish decay. Two consecutive one-electron transfer inner-sphere steps are proposed for the entire redox process where [Mn(LH 2)3]3+, the initial one-electron-reduced product of MnIV, is believed to be a steady-state intermediate. © Wiley-VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2004.

Kinetics and mechanism of oxidation of Fe2+ by the tris(biguanide)manganese(IV) ion in aqueous acid media

The CCR5 chemokine receptor has recently been found to play a crucial role in the viral entry stage of HIV infection and has therefore become an attractive potential target for anti-HIV therapeutics. On the other hand, the lack of CCR5 crystal structure data has impeded the development of structure-based CCR5 antagonist design. In this paper, we compare two three-dimensional Quantitative Structure-Activity Relationship (3D-QSAR) methods: Comparative Molecular Field Analysis (CoMFA) and Comparative Molecular Similarity Indices Analysis (CoMSIA) on a series of piperidine-based CCR5 antagonists as an alternative approach to investigate the interaction between CCR5 antagonists and their receptor. Superimposition of antagonist structures was performed using two alignment rules: atomic/centroid rms fit and rigid body field fit techniques. The 3D QSAR models were derived from a training set of 72 compounds, and were found to have predictive capability for a set of 19 holdout test compounds. The resulting contour maps produced by the best CoMFA and CoMSIA models were used to identify the structural features relevant to biological activity in this series of compounds. Further analyses of these interaction-field contour maps also showed a high level of internal consistency. © 2003 Elsevier Ltd. All rights reserved.

Bioorganic and Medicinal Chemistry

Three-dimensional quantitative structure-activity relationship analyses of piperidine-based CCR5 receptor antagonists

An analysis of structural features associated with polymorphism in 3-acetylcoumarin has been made in terms of morphology, infrared spectroscopy, crystal structure, powder X-ray diffraction, and differential scanning calorimetric measurements. The appearance of concomitant polymorphe is found to depend on the crystallization temperature in the 1:1 chloroform/hexane solvent system. C-H⋯O and C-H⋯π interactions stabilize one form in a head to head configuration, while only C-H⋯O interactions stabilize the other in a head to tail configuration of the molecules in the crystal lattice.