Model selection for complex systems involving polynuclear species

1-hydroxyethylidene-1,1-bisphosphonic acid (HEDP) is a phosphonate chelate with several diagnostic and therapeutic applications. Previous studies have indicated that calcium complex aggregation occurs in the basic range of pH; results from this study indicate that complex aggregation occurs throughout the physiological range of pH as well. Calcium complex formation data were used for PQ analyses and plots of the derivative of Z with respect to total diphosphonate at constant free calcium ion concentration were shown to provide valuable information with regard to model selection. A link-addition stepwise complex formation model was shown to be most appropriate; a robust feature of all aggregate formation models examined is a strong pH dependence of polynuclear complex formation in the pH range 5.5–7.0.     

The reactions between tetrachloroplatinate(II) anion and bidentate sulphur-nitrogen chelating agents: A direct route for the synthesis of the neutral dichloro species

A direct route for the synthesis of the neutral dichloro complexes [Pt(NS)Cl₂], (NS = 2-[(ethylthio)methyl]pyridine and 2-[(phenylthio)methyl]pyridine) in dimethylformamide, which avoids the formation of bis-chelate cations, is reported.     

Novel species in the electrochemical reduction of a metalmetal bond bridged by a bidentate fluorophosphine

The purple bridged bimetallic complex [CH₃N(PF₂)₂]₃Co₂(CO)₂ undergoes successive chemically and electrochemically reversible one-electron reductions to the corresponding green radical anion and pale-yellow dianion. The radical anion is relatively unreactive towards oxygen and methyl iodide. The dianion is not only reactive towards oxygen and methyl iodide but also captures small positively charged species (e.g. Li⁺ and H⁺) with significant alteration of its chemical properties.     

The enantiomers of N,N-dimethylthiobenzamides: Chromatographic behaviour and rotational barriers

N,N-Dimethylthiobenzamides bearing hydrogen atoms in both orthopositions were shown by ¹H NMR in the presence of an optically active alcohol to have non-coplanar π-systems. The barrier to rotation about the C(sp²-C(sp²) bond amounts to 43 ± 2 kJ/mol. Liquid chromatography on triacetylcellulose served for the enrichment of enantiomers of 2-substituted thiobenzamides. In several cases considerable enantioselectivity (relative retention up to 8.7) was observed. The barriers to enantiomerization (Tables 3, 4 and 5), determined by thermal racemization, were discussed in terms of non-bonding interactions, electrostatic repulsions and buttressing effects in the transition state of rotation.     

Enantiomers of polarized alkenes: chromatographic enrichment and thermal interconversion

(+)- and (?)-$\text{1}$ as well as (+)- and (?)-$\text{2}$ were partially separated by liquid chromatography on triacetylcellulose; racemizations resulted in the barriers (Table 2) to rotation via planarized transition states.     

Organometallic and organobimetallic complexes of platinum(II) containing 2,2′-bipyrimidyl: syntheses and spectroscopic characteristics

Preparations are described of several monometallic complexes (bipym)PtR₂ [bipym = 2,2′-bipyrimidyl; R = Me, CF₃, Ph, 1-adamantylmethyl (adme); R₂ = (CH₂)₄] and bimetallic analogues R₂Pt(μ-bipym)PtR′₂ [R = R′ = CH₃, C₆H₅, adme; R = CH₃, R′ = Ph, adme, CF₃]. IR, ¹H NMR and UV/visible spectroscopic characteristics of the two modes of bipyrimidyl coordination are discussed.     

Potassium fluoride and phosphorous acid: ab initio calculations and spectroscopic investigations

Ab initio calculations including the effects of solvation on the hydrogen bonding interactions between F^? and phosphorous acid, HPO₃H₂, have been performed, resulting in a value of 61 kJ mol^?1 for the hydrogen bond energy of [HPO₃H₂F]^?. Attempts to show that this species exists in aqueous solution have been made using ¹⁷O, ¹⁹F and ³¹P NMR spectroscopy and pH and conductance studies, but these indicate that the principal reaction is an acid-base neutralization. Crystals of KF.HPO₃H₂ grow from aqueous solution but these are not the same as those from methanol solution which are known to be strongly hydrogen bonded.     

The diterpene alkaloids: the structure and stereochemistry of heteratisine

Chemical and spectroscopic studies have shown that heteratisine (C₂₂H₃₃NO₅), a diterpene lactone alkaloid occurring as a monobenzoyl ester in the roots of Aconitum heterophyllum Wall, has stereostructure 1. Heteratisine thus bears a closer structural relationship to the highly toxic and polyoxygenated alkaloids of the lycoctonine-aconitine type than to its companion bases of A. heterophyllum, atisine, atidine and hetisine. An unusual cleavage reaction occurring when 19-oxodehydroheteratisine (28) is treated with strong base is described.     

Telluronium salt chemistry: Some fluorides and carboxylates

Ph₃TeF is ionic and no evidence is obtained for a covalent form in solvents of relatively low polarity. By contrast significant covalent interaction is seen for Ph₂(CH₃)Te(OOCR)(R=Ph, o-CH₃O·C₆H₄-, m-NO₂·C₆H₄). Factors influencing the formation and stability of covalent forms of telluronium “salts” are discussed. Phenyl(methyl)telluronium-ortho-phthalate is shown to be a monomer in chloroform solution. The structure of the compound is discussed in relation to NMR and IR data and against the background of previous literature reports of both dimeric and monomeric diorganotellurium-ortho-phthalates.     

Organotellurium cations: Organotellurium perchlorates and their complexes with Lewis bases

Solutions of diorganotellurium(IV) diperchlorates R₂Te(ClO₄)₂ (where R = Ph, p-CH₃OC₆H₄, R₂Te = $\text{C}{}_4\text{H}{}_8\text{Te}$) may be prepared by the reaction of R₂TeCl₂ and AgClO₄ or by the reaction of diorganyltelluroxide with excess of 70% HClO₄.IR and conductivity data indicate that Ph₂Te(ClO₄)₂ possesses covalently bonded perchlorate groups in solution. Interaction of the freshly prepared solutions with Lewis bases affords cationic complexes of the types [R₂Te(ClO₄)(L)][ClO₄] and [R₂Te(L)₂][ClO₄]₂ which have been characterised by IR elemental analyses and conductivity measurements.     

Preparation and characterization of graphite-coated metallic electrodes: the graphite-sprayed electrode

Graphite-coated metal electrodes were constructed and evaluated for use in voltammetry. Aluminium, copper and platinum electrodes were sprayed with a colloidal solution of graphite particles dispersed in methyl methacrylate polymer. The polishing step was omitted for anodic stripping voltammetry with a mercury film. The resistance is about 1 Ω. Electrode areas are readily reproduced by utilizing metal supports with equal areas. Background currents are very low and useful potential ranges are extended, with −1.3 to +1.7 V vs. SCE being possible, depending on the electrolyte. It is possible to achieve ±0.1% precision for the peak-currents in the electrochemical oxidation of ferrocyanide. For phenol, which is strongly adsorbed, a precision of 2.5% can be achieved by polishing the electrode before each determination.     

Infrared spectra of ν-cyclopentadienylberyllium chloride and bromide and of dicyclopentadienylberyllium: a comparative study

The IR spectra of η⁵-cyclopentadienylberyllium chloride and bromide and of dicyclopentadienylberyllium in solution are compared. Evidence is given for pentahapto- and monohapto-attachment of the cyclopentadienyl rings for the structure of dicyclopentadienylberyllium in solution.     

Electronic structure and reactivity of ylidic systems : V. Ylid structure information form NMR data

Bond systems in triphenylphosphorus ylids, methylenetrimethylphosphorane and their tricarbonylnickel salts are discussed by interpretation of ¹³C NMR results.     

Formation constant and stoichiometry of the acetonitrile:18-crown-6 complex by nuclear magnetic resonance, raman and infrared spectroscopy

Crown ethers are increasingly used in a variety of chemical applications. While crown ether complexes with alkali-metal cations have been extensively studied, relatively little is known about their complexes with neutral molecules to form so-called host: guest complexes. The use of NMR is reported for the determination of the formation constant (2.1 +/- 0.1) for the acetonitrile: 18-crown-6 complex. The suitability of Raman spectroscopy for studies of the solid-phase complex is demonstrated and limitations in the use of infrared spectroscopy are discussed.     

On the pH dependence of Cu(II) binding to insulin: ESR and electronic studies

The ESR and electronic absorption spectra have been used to investigate co-ordination of Cu(II) to insulin in aqueous solutions at different pH. Two series of complexes with low- and high copper content were examined and the values of the magnetic tensor components and the shape of the diffuse reflectance transitions suggested that these copper-insulin derivatives have tetragonal symmetry with Cu(II) in a (N_xO_y) ligand field, where oxygen donor groups are predominant at low pH and nitrogenous ligands at high pH. Such a trend was further supported by the presence of superhyperfine structure at pH = 13. Oxygen of the carboxylato groups, nitrogen of α- and ε-amino groups and of imidazoles, all contribute to the coordinations field. At very high pH only, a preferential binding site for Cu(II) is found, which probably involves deprotonated peptide nitrogens.     

Circular dichroism of olefins—IV: A semiquantitative prediction of the intensity of the cotton effect of allyl alcohols and ethers

A semi quantitative prediction of the intensity of the Cotton effect of a few allyl alcohols and ethers in the steroid and diterpene series has been derived. It relies on a calculation of the weight of the various conformations and on the allylic bond polarization model.     

Cyclobutadieneiron tricarbonyl carboxylic acids: Synthesis and acidity

Two carboxylic acid derivatives of the cyclobutadieneiron tricarbonyl (I) system have been synthesized and their pK_a values determined. It appears that I tends to be electron withdrawing by induction and electron releasing by resonance.     

Mixed-ligand lanthanide complexes—II : Complexes of β-diketone and heterocyclic amine. Synthesis and spectroscopic studies

The imidazole adducts of tris(heptafluorooctanedionato)lanthanide(III), [Ln(fod)₃ · im] have been synthesized and characterized. The electronic spectra in the visible region have been analysed and spectral parameters ($\text{β}$, b^1/2, δ and T_λ) and oscillator strengths calculated and discussed. These complexes are more covalent than the analogous [Ln(fod)₃ · pz] complexes, pz = pyrazole. The covalency is more in solution than in the solid state. The experimental and the calculated oscillator strengths are in good agreement. The band shape for the hypersensitive transition in the solid and solution spectra is compared with the other known systems and discussed. The difference in the band shape of the hypersensitive transition in the solid and in solution is indicative of solvent molecule coordination. The Mössbauer spectrum of [Eu(fod)₃ · im] is reported. The dipolar shift produced in the NMR spectrum of [Eu(fod)₃ · im] is upfield in comparison to the signal of the organic moiety itself as well as in its diamagnetic analogues. The coordinated base does not dissociate even in DMSO solution. Due to their poor solubility these compounds are not suitable for use as induced NMR shift reagents.     

Metal chelates of azo-pyridazine dyes-II : Chelating tendencies of diacetyl-, benzil- and benzoylethane-monohydrazone-3-hydrazino-4-benzyl-6-phenylpyridazine

The diacetyl-(DAHP), benzil-(BHP) and benzoylethane-(BEHP) derivatives of 3-hydrazino-4-benzyl-6-phenylpyridazine have been prepared and characterized. Their acid-base properties and their equilibria with transition metal ions in 75% dioxan-water solvent at 30°C have been investigated by pH titrimetry. The role of proton and metal ion solvation by the organic solvent has been discussed in view of the results obtained for the lanthanide-BHP systems in different media. Probable structures of the metal chelates are given based on potentiometric data and spectral results for the solid copper chelates. The possibility of forming MHL species were inferred from electronic absorption measurements at different pH values. The use of BHP as an analytical reagent for the determination of copper spectrophotometrically or as a metallochrom indicator in the complexometric titration of Cu(II) ion is also discussed.     

Organoboranes : XVII. Reaction of organometallics with dialkylborane derivatives: The synthesis of mixed organoboranes not available via hydroboration

The reaction of organolithium reagents with methyl-dialkylborinates in hydrocarbon solvents proceeds readily with precipitation of lithium methoxide and the formation of the corresponding mixed organoborane:

Consequently, simple filtration of the reaction mixture gives the mixed organoborane in high yield and purity. Other dialkylborane derivatives, such as the chloride and the hydride, are less desirable as substrates, frequently resulting in isomerization of the new alkyl groups.