Synthesis of cationic dialkylgold(III) complexes: nature of the facile reductive elimination of alkane

A series of gold(III) cations of the type cis-[CH₃)₂AuL₂]⁺ X^? where L  Ph₃, PMePh₂, PMe₂Ph, PMe₃, AsPh₃, AsPh₃, SbPh₃, $\text{1}\text{2}$H₂NCH₂CH₂NH₂, $\text{1}\text{2}$ Ph₂PCH₂CH₂-PPh₂, $\text{1}\text{2}$ Ph₂AsCH₂CH₂AsPh₂, and $\text{1}\text{2}$o-C₆H₄(AsMe₂)₂ and X  BF₄^?, PF₆^?, ClO₄^?, and F₃CSO₃^? has been prepared. In addition, the cis complexes [(CH₃)(CD₃)-Au(PPh₃)₂]F₃CSO₃, [(C₂H₅)₂Au(PPh₃)₂]F₃CSO and [(n-C₄H₉)₂Au(PPh₃)₂]F₃-CSO₃ have been synthesized. All have been characterized by PMR, Raman and infrared spectroscopy. These [R₂AuL₂]X compounds yield only ethane, butane, or octane via reductive elimination, and no disproportionation is observed. The alkane eliminations have been studied in CHCl₃, CH₃Cl₂, and CH₃COCH₃ solution as a function of temperature, concentration of the complex, and concentration of added ligand L. Elimination is fastest when L is bulky (PPh₃ > PMePh₂ > PMe₂Ph > PMe₃), decreases in the sequence SbPh₃ > AsPh₃ > PPh₃, is slow with chelating ligands, is inhibited by excess ligand, and there is small anion effect as X is varied. As R is varied, the rate of elimination decreases Bu ? Et > Me. An intramolecular dissociative mechanism is proposed which involves rapid elimination of alkane from an electron deficient dialkylgold(III) complex with nonequivalent gold—carbon bonds and produces the corresponding [AuL₂]X complex.     

Synthesis and characterization of Di-π-cyclopentadienyl- metal pentasulfides of titanium(IV) and vanadium (IV):an operational test of the influence of an unpaired electron on the molecular geometry

Our preparation of Ti(h⁵?C₅H₅)₂ S₅ by the reaction of elemental sulfur with Ti(h⁵?C₅H₅)₂(CO)₂ in hexane and of V(h⁵?C₅H₅)₂S₅?$\text{1}\text{2}$ H₂O by the reacti of V(h⁵?C₅H₅)₂ Cl₂ with Na₂S₅ in THF and structural analyses by single crystal X-ray diffraction (together with infrared, solution EPR, and temperaturedependent magnetic susceptibility measurements) represent an extension of our previous work on M(h⁵?C₅H₅)₂ (SC₆H₅)₂ (M = Ti, V). The crystallographic results provide further support of our previous conclusions that the Ballhausen—Dahl model is not valid for M(h⁵?C₅H₅)₂L₂ systems. The structuralfeatures of the chair-like titanium and vanadium pentasulfide molecules are compared to the corresponding phenylmercapto analogs and to the chair-like cyclohexasulfur molecule in rhombohedral sulfur. Ti(h⁵?C₅H₅)₂S₅ was isolated as a mixture of monoclinic and orthorhombic Crystalline phases-which were both characterized by preliminary X-ray data. A complete Structural determination and refinement of the monoclinic phase, which contains two independent molecules in a cell of dimensions a 22.843(2), b 7.958(1), c 14.465(1) Å, β 90.074(4)° and symmetry P2₁/c, yielded R₁ 5.3 % and R₂ 5.9 % for 2168 independent diffractometry-collected data with I≥ 2.5o(I). V(h⁵?C₅H₅)₂S₅-$\text{1}\text{2}$ H₂O contains four V(h⁵?C₅H₅)₂S₅ molecules and two water molecules of hydration (of crystallographic site symmetry C₂-2) in onorthorhombic unit cell of symmetry P2₁2₁2 and of dimensions a 13.491(1), b 12.748(1), c 7.715(1) Å. Least-squares refinement of 750 diffractometry data with I≥2.0σ(I) gave R₁ 2.4% and R₂ 3.0% Both of these compounds were independently synthesized and Spectroscopically characterized by Köpf and co-workers, and-a Complete X-ray diffraction study was performed by Epstein and Bernal on a different monoclinic phase of Ti(h⁵?C₅H₅)₂S₅ (isolated by-Köpf). An extraction of V(h⁵?C₅H₅)₂S₅ with re fluxing benzene under nitrogen atmosphere in a Soxhlet apparatus led to the formation of the previously reported [V₂ (h⁵?C₅H₅)₂S₅]_n compound which was characterized by physical measurements including a preliminary X-ray diffraction study.     

The NiII,HgII and CuII complexes of 12‐membered‐ring mixed‐donor macrocycles

The structures of di­aqua(1,7‐dioxa‐4‐thia‐10‐aza­cyclo­do­decane)­nickel dinitrate, [Ni(C₈H₁₇NO₂S)(H₂O)₂](NO₃)₂, (I), bis­(nitrato‐O,O′)(1,4,7‐trioxa‐10‐aza­cyclo­do­decane)­mercury, [Hg(NO₃)₂(C₈H₁₇NO₃)], (II), and aqua­(nitrato‐O)(1‐oxa‐4,7,10‐tri­aza­cyclo­do­decane)copper nitrate, [Cu(NO₃)(C₈H₁₉N₃O)(H₂O)]NO₃, (III), reveal each macrocycle binding in a tetradentate manner. The conformations of the ligands in (I) and (III) are the same and distinct from that identified for (II). These differences are in agreement with molecular‐mechanics predictions of ligand conformation as a function of metal‐ion size.     

Chemistry of constrained dioxocyclam ligands with Co(III): unusual examples of C-H and C-N bond cleavage

The reactions between H(2)dc3 and Co(acac)(3) have been studied in the presence and absence of base. In the presence of base, a complex with an intramolecular Co-C bond, Co(dc3-C-(8))(H(2)O), 1, is formed, presumably through heterolytic C-H bond activation. An X-ray crystallographic study demonstrates the presence of a Co-C bond and shows that the diazacyclooctane (daco) subunit adopts the chair-boat conformation with respect to the metal. The cobalt-carbon bond induces strain in the macrocycle as demonstrated by bond angles significantly deviating from tetrahedral. The (13)C NMR resonance of the carbon atom bound to cobalt (-10.5 ppm) suggests significant ionic character in the cobalt-carbon bond. However, we were unable to cleave this bond in the presence of strong acid. In the absence of base, the reaction of Co(acac)(3) with H(2)dc3 resulted in C-N cleavage of the ligand and the formation of a complex of dioxocyclam, Co(dc)(acac), 2. This complex has subsequently been prepared in high yield by the reaction of Co(acac)(3) with dioxocyclam. An X-ray crystallographic study demonstrates that dioxocyclam adopts the heretofore unreported cis configuration, having folded along a N-Co-N axis that is perpendicular to the Co-acac plane.     

Amperometric detection of simple alcohols in aqueous solutions by application of a triple-pulse potential waveform at platinum electrodes

Application of a triple-pulse waveform is described for the anodic detection of methanol, ethanol, and ethylene glycol. The execution of the waveform incorporates the cleaning and reactivation of the platinum electrode, by alternate anodic and cathodic polarization, with measurement of the faradaic signal for the analyte at the reduced electrode surface. Some results for formic acid are also presented. The waveform is completed within approximately 2 s and the faradaic signal exhibits no decay with time as would be the case for amperometric detection at a constant applied potential. Calibration curves made by plotting —1/I vs. 1/C are linear. This is consistent with a reaction mechanism in which the analyte is adsorbed prior to anodic detection. The technique is applicable for detection in chromatographic and flow-injection systems.     

ELECTRON TRANSFER FROM CHLOROPHYLL a TO QUINONE IN MONO- AND MULTILAYER ARRAYS

Abstract— Mono- and multilayers of chlorophyll a (Chl a )– lecithin have been prepared on quartz slides, by means of the Blodgett-Langmuir technique, for fluorescence studies. Self-quenching of the Chl a fluorescence has been observed in Chl a -lecithin single layer excited with a laser light at 632.8 nm. The fluorescence yield is reduced by 50% at a concentration of 7 ± 10¹² Chl a molecules cm⁻². Chl a fluorescence quenching, by adding N,N -distearoyl-1,4-diaminoanthraquinone (SAQ), has been studied. in a single layer, in pure Chl a and also at various dilutions of Chl a in lecithin. The results are explained in terms of a dynamic quenching rather than in terms of a permanent complex formation, at the ground state, between Chl a and SAQ. The fluorescence quenching has been interpreted as the result of an electron transfer from excited Chl a to SAQ, and rate constants of 8.3 ± 10⁻⁵ cm² molecule⁻¹ S⁻¹ and 2.4 ± 10⁻⁴ cm² molecule⁻¹ s⁻¹ have been found for pure diluted Chl a , respectively. Ten per cent of the diluted Chl a fluorescence always remains unquenchable and independent of the quinone concentration. In multilayers, where SAQ and Chl a are in different layers, there is no fluorescence quenching for pure or diluted Chl a even when the chromophores are in two adjacent layers. This happens only if SAQ is not able to diffuse from one layer to another. A minimum value of 22.4 nm has been found for the singlet exciton diffusion length in pure Chl a multilayers.     

Anodic stripping voltammetry of sulphide at a nickel film: towards the development of a reagentless sensor

The determination of sulphide at an electrochemically generated nickel oxide layer at glassy carbon and screen-printed electrodes in acidic media has been examined and appraised. The NiO layer was found to produce a stripping-like signal to sulphide and gave a linear peak current response from 20 to 90 μM. The response was further enhanced by repetitive cycling allowing accumulation of nickel sulphide at the electrode surface such that lower micromolar levels of sulphide (i.e. 5 μM) can be determined. The response at the NiO layer to sulphide is shown to be reproducible over a period of 24 h, thereby offering the development of a disposable amperometric sensor for sulphide.     

Separation and quantitation of components in FD&C Red No. 3 using capillary electrophoresis

The use of capillary electrophoresis as a technique to separate and quantitate components of FD&C Red No. 3 (erythrosine, color index No. 45430) is described. The fluorescein isomers, 2',4',5'-triiodofluorescein (2,4,5-I3F) and 2',4',7'-triiodofluorescein (2,4,7-I3F), the most abundant by-products formed during the preparation of the dye, were selected for quantitation studies. The separation of other lower halogenated impurities was also demonstrated. Electrophoretic mobility of the compounds was achieved in a 50 mM borate, 25 mM sodium dodecyl sulfate buffer at pH 9.3. The limits of quantitation were found to be 0.15% (w/w) (2,4,5-I3F) and 0.14% (w/w) (2,4,7-I3F) (relative to the mass of FD&C Red No. 3). The method is linear from 0.08 to 20.0% (w/w) for 2,4,5-I3F and between 0.06 and 17.0% (w/w) for 2,4,7-13F. In addition, relative standard deviations of 2.03 and 5.11% were determined from precision studies in the repeat analysis of FD&C Red No. 3 for 2,4,5-I3F and 2,4,7-I3F, respectively. Overall, the CE method produced data in excellent agreement with the reference HPLC method, used considerably less solvent and sample, generated less waste and was found to be considerably more cost efficient.     

Voltammetric Detection of Free Sulfhydryl Compounds in Food Flours

Cyclic voltammetry has been applied to the rapid measurement of free SH‐compounds in food flours samples. The protocol is based on the electrochemical adaptation of the Ellman's test, where the DTNB reacts with SH‐compounds present in food flour extracts, resulting in the equimolar production of the electroactive compound TNBA which, in turns, reacts with phenylendiamine. The reaction is found to provide an analytical signal from which to quantify indirectly free sulfhydryl compounds in flours extracts.     

CuII Coordination Chemistry of Patellamide Derivatives: Possible Biological Functions of Cyclic Pseudopeptides

Two synthetic derivatives of the naturally occurring cyclic pseudooctapeptides patellamide A–F and ascidiacyclamide, that is, H₄pat², H₄pat³, as well as their Cu^II complexes are described. These cyclic peptide derivatives differ from the naturally occurring macrocycles by the variation of the incorporated heterocyclic donor groups and the configuration of the amino acids connecting the heterocycles. The exchange of the oxazoline and thiazole groups by dimethylimidazoles or methyloxazoles leads to more rigid macrocycles, and the changes in the configuration of the side chains leads to significant differences in the folding of the cyclic peptides. These variations allow a detailed study of the various possible structural changes on the chemistry of the Cu^II complexes formed. The coordination of Cu^II with these macrocyclic species was monitored by high‐resolution electrospray mass spectrometry (ESI‐MS), spectrophotometric (UV/Vis) and circular dichroic (CD) titrations, and electron paramagnetic resonance (EPR) spectroscopy. Density functional theory (DFT) calculations and molecular mechanics (MM) simulations have been used to model the structures of the Cu^II complexes and provide a detailed understanding of their geometric preferences and conformational flexibility. This is related to the Cu^II coordination chemistry and the reactivity of the dinuclear Cu^II complexes towards CO₂ fixation. The variation observed between the natural and various synthetic peptide systems enables conclusions about structure–reactivity correlations, and our results also provide information on why nature might have chosen oxazolines and thiazoles as incorporated heterocycles.