Voltammetric study of humic and fulvic substances: Part II. Mechanism of reaction of the Pb-fulvic complexes on the mercury electrode

A model is presented for the electrode process corresponding to the reduction of Pb(II) complexed by fulvic substances. The various factors which may affect the polarographic results are discussed and a mathematical expression is given for the polarographic wave. The influence of various parameters was tested. It was found that the formation of complexes both present in solution and adsorbed at the surface of the electrode is the predominant factor which affects the potential of the wave. Moreover, the diffusion coefficient of the complexes is much lower than that of free lead. We discuss the nature of the complexes adsorbed at the electrode surface and the implications of the findings of this work when using the polarographic methods for the determination of the complexing ability of waters.     

The lithium aluminium hydride reduction of cyclohexadienylmanganese complexes

Reduction of cyclohexadienyltricarbonylmanganese or its ring-substituted derivatives with lithium aluminium hydride leads to dihydro-derivatives. Preparative and NMR spectral information is presented and interpreted on the basis of the CH-bridged cyclohexenyl structure recently established for these products by Lamanna and Brookhart [3].     

Sodium N-chlorobenzenesulfonamide as an analytical reagent: Determination of amino acids and their metal complexes

Simple, elegant, and reproducible back-titration methods for the determination of arginine, histidine, threonine, glutamine, alanine, lysine, glycine, and serine in free state and in Zn, Cd, Ni, Cu, Ba, Sr, Mn, and Ag metal complexes with chloramine-B have been developed. The oxidation reaction involves a four-electron change per mole of amino acid, but a six-electron change is noticed with glycine and serine in various solvent and buffer media. The proposed analytical procedures for the assay of amino acids are useful for computing the number of amino acid ligands present in the respective metal complexes. The aromatic sulfonyl haloamine used, CAB, was prepared and then characterized by ¹H and ¹³C FT-NMR spectral data.     

Three-coordinate mercury in (2,2′-bipyridyl)methylmercury(II) nitrate and related complexes

Methylmercury(II) forms complexes [MeHgL]⁺ [NO₃]^? (L = bidentate ligand) having three-coordinate mercury; an X-ray crystal structure analysis shows that the complex with 2,2′-bipyridyl has a planar CHgN₂ group with unsymmetrically chelated 2,2′-bipyridyl.     

Electrode kinetics of the metal complexes with aminopolycarboxylic acids: Part II. Cadmium-ethylenediamine-N,N,N′,N′-tetraacetate (EDTA) complexes

The d.c. polarographic current-potential curves of Cd(II)-EDTA complexes were examined in the pH range 0.5–10.0, to elucidate the mechanism of their electrode processes and to determine the relevant electrochemical kinetic parameters. It was shown that the first wave observed below pH 3 at ?0.58 to ?0.65 V vs. SCE is the reversible reduction wave of Cd(II) aquo-ion with kinetically-controlled limiting current, and the second wave observed above pH 1.5 at ?0.75 to ?1.21 V vs. SCE corresponds to the simultaneous irreversible reduction of four complex species, CdH₃L⁺, CdH₂L, CdHL^? and CdL^2?, where CdH_pL^(p?2)+ and L^4? denote the protonated complex species with p protons and the unprotonated EDTA ion, respectively. Analysis of the dependence of limiting current on the hydrogen ion concentration led to the conclusion that the preceding reaction determining the behaviour of limiting current is CdH₃L⁺?Cd²⁺+H₃L^? with k₃^d=6.3×10² s^?1 and k₃^f=3.3×10⁶ s^?1M^?1, where k₃^d and k₃^f are the dissociation and formation rate constants, respectively. On the other hand, from analysis of the dependence of half-wave potentials of the second wave on the hydrogen ion concentration, the kinetic parameters of the four complex species were evaluated, and are given in Table 1. Further, it was shown that the cathodic rate constants of these four charge transfer processes at some reference potential together with those of Cd(II)-HEDTA complexes fulfil the linear free energy relationship.     

Thermal studies on purine complexes. VII. 8-theophylline derivatives and their silver(I) complexes

Four theophylline derivatives: 8-ethyltheophylline (ETH), 8-isopropyltheophylline (IPH), 8-propyltheophylline (PRH), and 8-phenyltheophylline (FTH) have been synthesized and characterized by spectroscopic techniques and thermal analysis. The melting points of these theophylline derivatives increase with the molecular weight, from 263.8°C (ETH) to 384.7°C (FTH). The values of the fusion enthalpy increase in the same order, from 28.4 kJ mol⁻¹ (ETH) to 36.5 kJ mol⁻¹ (FTH).

The interaction of these theophylline derivatives with Ag(I) has been studied by means of spectroscopic techniques, TG, DTG and DSC. In all cases, the pyrolysis of these compounds gave metallic silver.     

Preparation of chlorodivinylborane transition metal complexes and their nucleophilic substitution at boron

Chlorodivinylborane (I) prepared in situ from boron trichloride and dimethydivinyltin, reactions under photochemical conditions with ironpentacarbonyl and cyclopentadienyldicarbonylcobalt to give (η⁵-chlorodivinylborane)tricarbonyliron (II) and (η⁵-chlorodivinylborane)cyclopentadienylcobalt (III), respectively, in high yield. Complexes II and III readily undergo base-assisted nucleophilic substitution at boron.     

Reactions of diphenylcyclopropenone with ketenes in the presence of nickel tetracarbonyl

Reactions of diphenylcyclopropenone with ketenes in the presence of catalytic amounts of nickel tetracarbonyl have been studied. In these reactions, 1 : 1 cycloadducts, cyclopentene-1,2-dione derivatives, were obtained in yields above 80%. The cycloaddition reactions were significantly affected by solvents, and DMF was the most suitable solvent. Iron pentacarbonyl did not act as a catalyst. Reaction mechanisms are discussed.     

The reaction of intermediate zirconocene-aryne complexes with CH bonds in the thermolysis of diarylzirconocenes

Upon thermolysis in aromatic hydrocarbon solvents diarylzirconocenes undergo successive replacement of their σ-bonded ligands by aryl groups from the solvent. Analysis of the product mixture by photochemical degradation to biphenyls reveals that intermediate aryne zirconocene complexes are formed which then react with the solvent with CH bond fission and formation of new ZrC bonds. A β-hydride elimination process can be excluded for the formation of the aryne complexes and it is suggested that they are formed via abstraction of an ortho proton from one of the aryl ligands by the other σ-bonded group.     

The solid state structure of arene-mercury(II) complexes from magic-angle spinning carbon-13 NMR and the solution carbon-13 NMR of some complexes of mercury(II) with arenes having bulky aliphatic substituents

The cross-polarization magic angle spinning ¹³C NMR spectra of Hg(SbF₆)₂ - 2 Arene (Arene = C₆HMe₅, 1,2,4,5-C₆H₂Me₄, 1,2,3,4-C₆H₂Me₄, or C₆H₆) have been measured. The spectra of the complexes of C₆HMe₅ and 1,2,4,5-C₆H₂Me₄ are consistent with static η¹-bonding of the mercury to the arene at an unsubstituted carbon atom, while the spectra of the 1,2,3,4-C₆H₂Me₄ and C₆H₆ complexes show the arene to have time-averaged C_s or C₂, and C₆ symmetry respectively, at the temperature of measurement (300 K).The reduced temperature ¹³C NMR spectra of Hg(Arene)_n²⁺ (n = 1 or 2; Arene = 1,3,5-C₆H₃R₃ (R = Me, i-Pr, or t-Bu)) in SO₂ solution are also reported and affirm that in these intramolecularly mobile species the mercury bonds in an η¹-manner, with unsubstituted aryl carbon atoms being the strongly preferred point of mercury attachment. This site preference is further demonstrated by the solution ¹³C NMR spectra of Hg(Arene)_n²⁺ (Arene = 1,2,3,4-C₆H₂-Me₄, n = 1 or 2; Arene = 1,4-C₆H₄R₂, R = Me or t-Bu, n = 1). The spectra of the 1,4-C₆H₄R₂ complexes and Hg(p-C₆H₄-t-BuMe)²⁺ provide clear evidence for steric influence of the binding site.Like Hg(C₆Me₆)₂²⁺, but unlike most of the complexes of substituted benzenes which have been studied, Hg(1,3,5-C₆H₃-i-Pr₃)₂²⁺ exchanges only slowly with excess free ligand.     

Stability and thermodynamic parameters of thiopyrimidines, 2-thioxanthine and their complexes with Zn(II) and Cd(II)

The thermodynamic protonation constants of 4-amino-1,6-dihydro-2-methylthio-5-nitroso-6-oxo-pyrimidine (MTH), 4-amino-5-nitroso-6-oxo-1,2,3,6,-tetrahydro-2-thiopyrimidine (TANH) and 2-thioxanthine (TXH) in aqueous media at 25, 30, 40 and 50 ± 0.1°C, have been determined potentiometrically by Bjerrum and Robinson's methods. The ionic strength was maintained constant by using 0.1 M NaNO₃ as the supporting electrolyte.The stability constants and thermodynamic functions for Zn(II) and Cd(II) complexes of MTH and TXH, at different ionic strengths and temperatures, have been calculated.     

An electrochemical study of the adsorption of two isomeric chromium(III) complexes on mercury electrodes: Thioether as an anchoring group

Cyclic voltammetry and chronocoulometry were utilized to examine the electrochemistry and adsorption on mercury of two isomeric complexes of Cr(III) with a multidentate ligand bearing a thioether group, thiobis(ethylenenitrilo) tetraacetic acid, S[CH₂CH₂N(CH₂COOH)₂]₂. One isomer contains a sulfur-chromium bond and is exceedingly strongly adsorbed. The second isomer lacks this bond and is adsorbed much less strongly. Back-bonding from chromium to sulfur is argued to play an important role in the adsorption of the first isomer. Upon reduction, both isomers yield the same Cr(II) product. A dimeric form of the second isomer in which the two Cr(III) centers are bridged by an acetate group is proposed to form at certain pH values. The different coordination environments of the two Cr(III) centers in the dimer cause them to be reduced at different potentials.     

Polarographic studies on the electrode kinetics of the nickel(II) complexes with glycine and some of its derivatives: I. glycine complexes

D.C. polarograms of the Ni(II)-glycine complexes were measured with varying pH values at constant total glycine concentration (0.05 M) and conversely with varying total glycine concentration at constant pH values (3.5, 4.5, 7.5 and 9.5). The current—voltage curves obtained were analysed to determine the limiting currents, the transfer coefficients, and the half-wave potentials, by using the non-linear least squares method. From the dependence of the half-wave potentials on the free glycine concentration, the mechanism of the electrode processes was elucidated. It was shown that the first wave corresponded to the reduction of the Ni²⁺ aquo-complex, the second to the NiG⁺ complex, the third to the NiG₂ complex and the fourth to the NiG^?₃ complex. The kinetic parameters for the four charge transfer processes were determined.The second and third waves were kinetic in character and the rate constants of the dissociation and association reactions between NiG⁺ and NiG₂ and between NiG₂ and NiG^?₃ were determined by analysing the dependence of the kinetic limiting currents on the free glycine concentration.     

Organocobalt complexes IX. The insertion of acetylenes into butenolide complexes; crystal structure of a dicobalt complex with a novel five-carbon bridge

Preparative thin layer chromatography has yielded the first-pure insertion products from acetylenes and butenolide dicobalt hexacarbonyls; the structure of one such product has been determined by X-ray crystallography and a related formulation is suggested for the other isomer formed in comparable amounts.     

Reactions of homobinuclear platinum and palladium complexes with carbon monoxide

The reactivity of homobimetallic complexes of platinum(II) and palladium(II) containing diethyl(diphenylphosphinomethyl)amine (ddpa = (C₆H₅)₂PCH₂N(C₂H₅)₂) as a bridging ligand has been investigated. Carbon monoxide reacts reversibly with these complexes. The species formed are binuclear carbonyl-bridged derivatives, which can isomerize to ionic terminal carbonyl complexes. Reaction of [PtCl₂(CO)]₂[(C₂H₅)₄N]₂ with ddpa in dichloromethane gives the ionic platinum(I) complex [Pt(ddpa)Cl₂]₂[(C₂H₅)₄N]₂, which reacts with carbon monoxide. Still, homobimetallic derivatives of palladium(I) are unstable, and none have been isolated.     

The synthesis of the zero-valent nickel complexes via di-π-allylnickel

A new route to zero-valent nickel complexes, (cycloocta-1,5-diene)duroquinonenickel, (endo-dicyclopentadiene)duroquinonenickel, dicycloocta-1,5-dienenickel and bis(tetracyclone)nickel, as described. The synthetic procedure involves the reaction between di-π-allylnickel and duroquinone, cyclic dienes, or tetraphenylcyclopentadienone. The replacement of the π-allyl ligand with duroquinone or a cyclic diene is suggested to follow the associative mechanism. The ¹³C NMR and mass spectra of the compounds isolated are reported.     

Reversed-phase partition high-pressure liquid chromatography of trace amounts of inorganic and organic mercury with silver diethyldithiocarbamate

Inorganic and organic mercury diethyldithiocarbamates have been separated by reversedphase partition high-pressure liquid chromatography. The mercury chelates were formed by an exchange reaction with silver diethyldithiocarbamate in chloroform, in the presence of acetate buffer (pH 5.0). The inorganic and organic mercury chelates in the extract were separated within 30 min on a 3.9 x 300 mm mu-Bondapak C(18) column. EDTA (10(-4)M) in methanol-water mixture (78:22 v v ) was used as eluent at a flow-rate of 0.5 ml min .     

N-alkyl- and N-aryl-thioformamido complexes of the platinum group metals

Alkyl- and aryl-isothiocyanates undergo insertion reactions with platinum metal hydrides to yield the corresponding N-alkyl- and N-aryl-thioformamido

derivatives, the structure and stereochemistry of which have been deduced using ¹H NMR data.