Formation Of Dimeric Species Research Articles

Osmium dithiophosphates. Synthesis, X-ray crystal structure, spectroscopic and electrochemical properties

The reactions of ammonium salt of dialkyldithiophosphate ligands, (RO) 2PS 2 − NH 4 + (R=Me/Et) with (NH 4) 2Os IVBr 6 in methanol solvent and under dinitrogen atmosphere result in one-electron paramagnetic tris complexes, {(RO) 2PS 2} 3Os III ( 1a– 1b) in the solid state. The molecular structure of one complex ( 1a) has been determined by single-crystal X-ray diffraction. It shows the expected pseudo-octahedral geometry with reasonable strain due to the presence of four-membered chelate rings. The reflectance spectra of the solid complexes display two broad bands in the range 552–484 nm and in the solid-state complexes exhibit one isotropic EPR signal at 77 K. Although the complexes 1a– 1b are found to be stable in the solid state, in solution state the complexes are transformed selectively into the diamagnetic and electrically non-conducting metal–metal bonded dimeric species [{(RO) 2PS 2} 3Os III–Os III{S 2P(OR) 2} 3]. The formation of dimeric species in the solution state is authenticated by the electrospray mass spectrum of one representative complex where R=Et ( 1b). In dichloromethane solution the complexes show two moderately strong sulfur to osmium charge-transfer transitions in the visible region and two strong ligand based transitions in the UV region. The complexes exhibit successive two oxidations correspond to Os IV–Os IV/Os III–Os III and Os V–Os V/Os IV–Os IV processes near 0.8 and 1.9 V versus SCE, respectively. One reductive couple corresponds to the Os III–Os III/Os II–Os II couple has been observed near −0.6 V. Electrochemically generated oxidized species [{(RO) 2PS 2} 3Os IV–Os IV{S 2P(OR) 2} 3] 2+ display lowest energy ligand to metal charge-transfer transition near 550 nm which has observed to be reasonably red shifted as compared to that of the parent trivalent species. On the other hand electrochemically generated reduced species [{(RO) 2PS 2} 3Os II–Os II{S 2P(OR) 2} 3] 2− are found to be unstable even on coulometric time scale.

Ruthenium dithiophosphates: synthesis, X-ray crystal structure, spectroscopic and electrochemical properties

The reactions of ammonium salts of dialkyldithiophosphate ligands, (RO) 2PS 2 −NH 4 + (R=Me/Et), with Ru IIICl 3·3H 2O in methanol solvent and under N 2 atmosphere result in one-electron paramagnetic tris complexes {(RO) 2PS 2} 3Ru III ( 1) in the solid state. The molecular structures of both complexes were determined by single-crystal X-ray diffraction. This shows the expected pseudo-octahedral geometry with reasonable strain due to the presence of a four-membered chelate ring. The reflectance spectra of the solid complexes display two bands in the range 596–476 nm and in the solid state the complexes exhibit one isotropic EPR signal at 77 K. Although the complexes 1 are stable in the solid state, in solution the complexes are transformed selectively into the diamagnetic and electrically non-conducting sulfur-bridged dimetallic species [{(RO) 2PS 2} 2Ru IV(μ-S) 2Ru IV{S 2P(OR) 2} 2]. The formation of dimeric species in the solution state is authenticated by the electrospray mass spectrum of one representative complex where R=Et ( 1b). In dichloromethane solution the complexes show two moderately strong sulfur to ruthenium charge-transfer transitions in the range 514–419 nm, and two strong ligand based transitions in the UV region. The complexes exhibit two successive reversible reductions in the ranges 1.01→0.91 V and −0.44→−0.49 V versus SCE corresponding to Ru IV–Ru IV/Ru III–Ru III and Ru III–Ru III/Ru II–Ru II couples respectively. Electrochemically or chemically generated first step reduced complexes [{(RO) 2PS 2} 2Ru III(μ-S) 2Ru III{S 2P(OR) 2} 2] 2− display two ligand to metal charge-transfer transitions in the visible region and in the complexes the two one-electron paramagnetic metal centers (low-spin Ru III, t 2g 5, S=1/2) are antiferromagnetically coupled. The second step reduced species [{(RO) 2PS 2} 2Ru II(μ-S) 2Ru II{S 2P(OR) 2} 2] 4− are observed to be very unstable.

Copper(II) complexation by pituitary adenylate cyclase activating polypeptide fragments

Results are reported from potentiometric and spectroscopic (UV–Vis, CD, and ESR) studies of the protonation constants and Cu 2+ complex stability constants of pituitary adenylate cyclase activating polypeptide fragments (HSDGI–NH 2, TDSYS–NH 2, RKQMAVKKYLAAVL–NH 2). With HSDGI–NH 2, the formation of a dimeric complex Cu 2H −2L 2 was found in the pH range 5–8, in which the coordination of copper(II) is glycylglycine-like, while the fourth coordination site is occupied by the imidazole N 3 nitrogen atom, forming a bridge between two copper(II) ions. The formation of dimeric species does not prevent the deprotonation and coordination of the amide nitrogen, and in pH above 8 the CuH −2L complex is formed. Aspartic acid in the third position of peptide sequence stabilizes the CuH −2L species and prevents the coordination of a fourth nitrogen donor. Aspartic acid residue in the second position of TDSYS–NH 2 stabilizes the CuL (2N) complex but does not prevent deprotonation and binding of the second and third peptide nitrogens to give 3N and 4N complexes at higher pH. The tetradecapeptide amide forms with copper(II) ions unusually stable 3N and 4N complexes compared to pentaalanine amide.

A two-dimensional model associating fluid with spherically symmetric intracore square-well shell. Integral equations and Monte Carlo simulation study

The structure and thermodynamics of a monolayer of an associating fluid in the framework of the primitive model of Cummings and Stell is studied by using a two-dimensional approximation. The model permits formation of dimer species for small values of the bonding length parameter, the formation of chains, if the bonding length is slightly larger, and also the vulcanization of species for bonding length values close to the diameter of particles. The structure and thermodynamics of the model that are of interest for statistical mechanics of surface chemical reactions, are studied by computer simulations in the canonical, grand canonical and isobaric ensembles and from the two-dimensional Ornstein—Zernike-like or Wertheim's Ornstein—Zernike integral equation. We have shown that the theory is satisfactory for the case of dimerization if the fluid density is low. For higher densities one must apply a correction for the cavity distribution functions to describe the fraction of unbonded species more adequately. For the case of chain formation the theory just resembles trends following from the simulation data. For the model with vulcanization of species we have obtained the fractions of differently bonded particles and have shown that chemical ordering of species is manifested in the antiphase oscillations of the pair distribution functions of species.

INFLUENCE OF THE AMINE NATURE ON THE COMPOSITION OF PALLADIUM COMPLEXES IN SOLVENT EXTRACTION SYSTEMS

ABSTRACT The extraction of palladium chloro complexes by di-n-octylamine and diamines of various structure as function of the composition of the aqueous and organic phases has been studied. The compositions of the extracted species are indicated and the mechanisms of their distribution are described. It was shown that from 1 to 3 M HC1 solutions. complexes such as (R2NH2)2PdCl4 solvated by molecules of dioctvlamine chloride are extracted. With increasing the initial concentrations of palladium or decreasing acidity of the aqueous phase, a direct coordination takes place, first of one and then two molecules of amine to atom of palladium with the formation of extracted compounds such as HC1 solutions by salts of amines, diamines and QAB ionic associates such as (AmH)2PdCl4, (AmH)2)PdCl4 and (R4N)2PdCl4, respectively, are recovered into the organic phase. In systems containing trioctylamine, tetraoctylalkylenediamines (n = 4, 6) and QAB. ionic associates containing the dimeric complex anion, Pd2Cl6 2-, are also formed. When primary (n-octylaniline - OA) and secondary amines are used as extractants the formation of dimeric species in the organic phase is not observed. The extraction of palladium from weakly acidic and neutral solutions can proceed through a combination of anion-exchange and coordination mechanisms with the formation of (AmH)[Pd(Am)Cl3] in systems with primary and secondary amines, and through a coordination mechanism with the formation of complexes such as Pd(Am)2Cl2 in systems with primary, secondary and tertiary amines. When palladium is extracted by diamines with a short hydrocarbon chain (n=2) the formation of coordination compounds also takes place.

Electrochemistry of conductive polymers XVI. Growth mechanism of polypyrrole studied by kinetic and spectroelectrochemical measurements

The growth mechanism of polypyrrole has been studied in acetonitrile by kinetic and spectroelectrochemical measurements. The results indicate that the polymerization reaction rate is 1 3 ∼ 1 2 order with respect to the pyrrole concentration and 1 3 order with respect to the amount of polypyrrole. In-Situ spectroelectrochemical studies at lower pyrrole concentrations indicate that reasonably high steady state concentrations of trimeric and tetrameric species are maintained during the polymerization reaction. From these observations, we conclude that: (1) both the monomer and polymer films are important for the growth (from the rate equation), (2) the formation of dimeric species appears to present the rate limiting step for the reaction (from the reaction order), (3) polymers grow in one-dimension, and (4) further growth of polymers from trimeric and tetrameric species requires a high monomer concentration.

Quantitative structural analysis of dispersed vanadia species in TiO 2(anatase)-supported V 2O 5

V 2O 5-TiO 2(anatase) catalysts have been studied under oxidizing and reducing conditions using in situ laser Raman spectroscopy (LRS) and temperature-programmed reduction (TPR) and oxidation (TPO). Quantitative Raman and TPO analysis of the oxidized samples show that these materials are comprised of a distribution of monomeric vanadyls, polymeric vanadates, and crystallites of V 2O 5. At low loadings, the predominant species are monomeric vanadyls, with the remaining vanadia being present in the form of polymeric vanadates. As the surface concentration of V 2O 5 increases, a maximum in the concentration of the polymeric vanadates is detected. Crystallites of V 2O 5 form at the expense of the polymeric vanadates as the loading is raised above the dispersive capacity of the TiO 2(anatase) support. An equilibrium polymerization model is proposed to account for the observed concentration of vanadia species, which leads to an initial polymer size of ∼2 at low loadings, consistent with the formation of dimeric species. Raman and TPR/TPO studies of the reduction process indicate that the terminal V = 0 groups of the monomeric and polymeric vanadia species are removed preferentially to the bridging oxygen atoms of the polymeric species. The maximum loss of oxygen upon reduction is one oxygen atom per vanadium atom.

HEXAALKOXY AND HEXAARYLOXY 1,3,2λ5,4λ5-DIAZADIPHOSPHETIDINES

Abstract The reaction of the hexachlorodiazadiphosphetidine (MeNPCl3)2 with NaOR (R [dbnd] CH2CF3 or C6H5) yields the hexaalkoxy/aryloxy derivatives [MeNP(OR)3]2. On the other hand, the reaction of the N-phenyl derivative with NaOCH2CF3 affords the monophosphazene PhN[dbnd]P(OCH2CF3)3. Spectroscopic data for the new compounds are presented. The structure of the trifluoroethoxy derivative [MeNP(OCH2CF3)3]2 has been confirmed by a single crystal X-ray diffraction study. It crystallizes in the triclinic space group P1 with a = 9.024(2) A, b = 9.255(2) A, c = 9.944(2) A, α = 69.68(2)°, β = 60.16(2)°. γ = 78.88(2)°, and Z = 1. The final conventional unweighted residual is 0.044. The transformation of [MeNP(OCH2CF3)3]2 into its monomer at 110°C and its redistribution reaction with [MeNPF3]2 have been investigated by NMR spectroscopy. The formation of dimeric species appears to be favored by electron withdrawing groups at phosphorus promoting delocalization in the P[sbnd]N[sbnd]P system.

Renaturation of the allosteric phosphofructokinase from Escherichia coli.

The allosteric phosphofructokinase from Escherichia coli has been renatured after complete unfolding in concentrated guanidine hydrochloride. The enzyme regains both its catalytic and regulatory abilities quantitatively. The kinetics of reactivation are biphasic and are consistent with a two-step mechanism in which a monomolecular reaction precedes a bimolecular one. The presence of ATP during reactivation increases the rate at which phosphofructokinase is renatured; the second order rate constant of the bimolecular step increases from about 10(4) M-1 S-1 in the absence of ATP to about 2 X 10(5) M-1 S-1 in the presence of 1 mM ATP. The other ligands of the enzyme have no effect on reactivation. It is tentatively proposed that a folded monomer is the intermediate species which already possesses a functional ATP-binding site and that the rate-limiting association step is the formation of dimeric species. This interpretation is compatible with the known three-dimensional structure of another bacterial phosphofructokinase, that from Bacillus stearothermophilus.

Refolding of a bifunctional enzyme and its monofunctional fragment.

The renaturation of the bifunctional enzyme aspartokinase II-homoserine dehydrogenase II has been studied by using the reappearance of its two activities. The same kinetics of renaturation are obtained for the dehydrogenase (EC 1.1.1.3) and the kinase activity (EC 2.7.2.4). The mechanism of refolding of the enzyme apparently involves two steps, a folding step occurring within a monomer and a subsequent dimerization step. The reappearance of the two activities depends on this dimerization step, suggesting that monomeric species are inactive. A proteolytic fragment possessing full dehydrogenase activity is shown to be able to renature, as judged by the recovery of its activity. In this case also, the refolding depends on the formation of dimeric species. However, the refolding of this fragment is much faster than that of the dehydrogenase region in the intact enzyme. These results suggest that, although the dehydrogenase region can refold by itself when isolated as a fragment, refolding of this same region in the whole protein involves interactions with the remainder of the protein.

The aggregation of heavy metal soaps in non-aqueous solvents

Vapour pressure osmometry has been used to determine the molecular weight of chromium myristate, palmitate and stearate in toluene at 37 °C. Results indicate the formation of dimeric species above a critical concentration.

An electron spin resonance study of metal ion separations in dimeric vanadyl (DL)- and (DD)-tartrates

The e.s.r. spectra of frozen aqueous solutions containing vanadyl (DL)- and (DD)-tartrates are consistent with the formation of dimeric species. Computer simulation of the spectra, assuming a monoclinic dimer model, has enabled the determination of the magnetic parameters associated with vanadyl ions, the internuclear separations between the metal ions in the dimers and their relative orientation. For vanadyl (DL)-tartrate dimer, the excellent agreement between structural parameters for the solid and frozen glass was taken to indicate that the same structure exists in both states. In the case of vanadyl(DD)-tartrate, however, although the overall symmetry is maintained, a slight reduction in the vanadyl vanadyl separation is believed to occur as a result of incorporation into an ethylene glycol-water glass.

Electron spin resonance study of dimer formation by copper(II) chelates of hydroxybenzoic acids

Experimental evidence produced by e.s.r. measurements at room temperature and 77 K shows that the formation of dimeric species by combination of copper(II) with substituted o-hydroxybenzoic acids is influenced by the other functional groups in the aromatic ring. The ligands used include 5-chlorosalicylic acid, p-aminosalicylic acid, 2,4-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid, and 2,6-dihydroxybenzoic acid. The effect of other structural variations was studied by using 3,5-dihydroxybenzoic acids, salicylamide, and o-phthalic acid. The relevant e.s.r. spectra have been dealt with by consideration of a spin-Hamiltonian for the paramagnetic states in the dimeric species. In the case of dimer formation, the spectra may arise from dipole–dipole interactions between the copper(II) ions and/or from exchange. The magnetic parameters associated with the spectra due to dimeric species have been determined by comparison of computer-simulated and experimental spectra.

An electron spin resonance study of dimeric titanium(III) complexes

The e.s.r. spectra of dimethylformamide solutions of the titanium(III) complexes of mandelic, malic, citric, cyclopentane-1,2,3,4-tetracarboxylic, 5-sulphosalicylic, 5-chlorosalicyclic, p-aminosalicyclic, and phthalic acids, salicylamide and 8-hydroxyquinoline have been observed. The results provide evidence for the formation of dimeric species from the observation of spectra at 77 K due to ΔM= 1 transition in the g ca. 2 region and in most cases the observation of low-field components of the spectra at g ca. 4. The solution of an appropriate spin Hamiltonian which includes the magnetic dipole–dipole interaction between the titanium ions makes possible the computer simulation of the line shapes of the spectra associated with the dimeric species. By this procedure the magnetic parameters of the titanium(III) pairs have been evaluated, the parameter of chief interest being the distance between the metal ions in the dimeric complexes. This information has been used to suggest possible structures of these dimers.

Electron spin resonance study of exchange and dipole–dipole coupling in copper(II) chelates of cyclopentanetetracarboxylic acid and related systems

Electron spin resonance spectra of frozen solutions of the copper(II) chelates of cyclopentanetetracarboxylic acid formed by the interaction of copper(II) chloride or perchlorate in various non-aqueous solvents indicate the formation of dimeric species in which exchange coupling or dipole–dipole coupling occurs. The formation of the dimeric species is dependent on the nature of the solvent and composition of the reaction mixture. In dimethylformamide the e.s.r. spectrum indicates the formation of dipole–dipole coupled copper(II) pairs when the copper(II) chloride to ligand ratio is 1 : 1, whereas when this ratio is 1 : 2 low- and high-field signals are observed such that a mixture of dimeric species exists. When copper(II) perchlorate is used, the e.s.r. spectra point to the formation of dimeric species in which exchange coupling occurs, though the magnetic parameters associated with the signal have different values from those observed for the exchange coupled species when copper(II) chloride is used. Exchange coupling in dimeric species has been observed in the copper(II) chelates of m- and p-hydroxybenzoic acids, and in the copper(II) complexes of adenine formed in both aqueous and non-aqueous solutions. To evaluate the magnetic parameters associated with the signals due to species in which exchange coupling occurs, a general spin Hamiltonian to include the effects of exchange and dipole–dipole contributions has been used. Solution of the spin Hamiltonian has made possible the computation of the lineshape due to exchange coupled copper(II) pairs. Thus sets of magnetic parameters have been collected for exchange coupled and dipole–dipole coupled copper(II) pairs and the values obtained from the dipole–dipole coupled pairs used to suggest the possible structure of the dimeric species.

An electron spin resonance study of the formation of mixed-ligand chelates of copper(II) with amino-acids and certain 3,4-dihydroxyphenols

Room-temperature e.s.r. measurements indicate that copper(II) forms monomeric chelates with 5,6-dihydroxybenzene-1,3-disulphonates or 3,4-dihydroxyphenylalanine. With simple amino-acids such as glycine or threonine, copper(II) and 3,4-dihydroxyphenols form mixed-ligand complexes which are monomeric in aqueous solution. The ΔM= 1 and ΔM= 2 transitions indicate the formation of dimeric species in frozen aqueous solution. By a computer lineshape simulation procedure, the magnetic parameters and the copper(II)–copper(II) distances in the dimeric species have been determined and suitable structures proposed.