Kinetics and mechanism of the oxidation of oxalic acid by tetrachloroaurate(III) ion

The oxidation of oxalic acid by tetrachloroaurate(III) ion in 0.005 ? [HClO₄] ? 0.5 mol dm⁻³ is first order in and a fractional order in [oxalic acid], the reactive entities being AuCl₃(OH)⁻ and ions. The pseudo first-order rate, k_obs, with respect to [Au(III)], is retarded by increasing [H⁺] and [Cl⁻]. The retardation by H⁺ ion is caused by the dissociation equilibrium . A mechanism in which a substitution complex, is formed from AuCl₃(OH)⁻ and ions prior to its rate limiting disproportionation into products is suggested. The rate limiting constant, k, has been evaluated and its activation parameters are reported. The equilibrium constant K₁ for the formation of the substitution complex and its thermodynamic parameters are also reported.     

Ruthenium(II) complexes possessing the η-p-cymene ligand

Complexes possessing a soft donor η⁶-arene and hard donor acetylacetonate ligand, [(η⁶-p-cymene)Ru(κ²-O,O-acac-μ-CH)]₂[OTf]₂ (1) (OTf = trifluoromethanesulfonate; acac = acetylacetonate) and {Ar′ = 3,5-(CF₃)-C₆H₃}, were prepared and fully characterized. The lability of the μ-CH linkage for complex 1 and the THF ligand of 2 allow access to the unsaturated cation [(η⁶-p-cymene)Ru(κ²-O,O-acac)]⁺. The reaction of with KTp {Tp = hydridotris(pyrazolyl)borate} produces . The azide complex forms upon reaction of with N₃Ar (Ar = p-tolyl), and reaction of with CHCl₃ at 100 °C yields the chloride-bridged binuclear complex . The details of solid-state structures of [(η⁶-p-cymene)Ru(κ²-O,O-acac-μ-CH)]₂[OTf]₂ (1), and are disclosed.     

Trichloroacetic acid dehalogenation by reductive radicals

Advanced oxidation processes, using either UVC/H₂O₂ or UVC/K₂S₂O₈, both in the presence of H₂CO₂ or CH₃OH are very efficient in mineralizing aqueous solutions of trichloroacetic acid (TCAA) leaving no toxic residues. The main reaction initiating TCAA depletion is its reduction by the radicals or CH₂OH to yield radicals and Cl⁻ anions. Further thermal reactions of lead to the formation of CO₂ and HCl. Molecular oxygen competes with TCAA for and CH₂OH radicals. However, in experiments under continuous irradiation of initially air-saturated solutions in closed reactors, the dissolved molecular oxygen concentration was depleted to low enough levels to favor the reaction of the reducing radicals with TCAA. A general reaction mechanism is proposed and discussed. The reaction between superoxide radical anions and TCAA was found to be of low efficiency.     

Synthesis, structure and reactions of a series of 1,2-dicyanoethylenedithiolate coordinated dimeric Mo(V) complexes

(where mnt = 1,2-dicyanoethylenedithiolate) (1), reacts with HX (X = SPh, Cl, Br) to form a series of complexes, . In acidic-alcoholic medium 1 with thiophenol yields another series of compounds, . Under similar conditions tertiary-butanol does not coordinate where a complex can only be isolated in the presence of bromide as . The use of excess of methanesulfonic acid in the presence of HSPh or HSEt facilitates methanesulfonate coordination in complexes, . All these complexes are structurally characterized by single crystal X-ray study. These complexes show pH dependent hydrolytic reaction leading to quantitative reversal to the starting complex, 1. Complexes 2a-c respond to hydrolysis in CH₂Cl₂ with the intermediate formation of EPR active molybdenum(V) species.     

Formation of an observable intermediate during the reduction of [Co(NH3)5CN] by CRR(OH) radicals

CR¹R²OH, Rⁱ = CH₃ or H, react with the complex [Co^III(NH₃)₅CN]²⁺ to form an observable intermediate probably via bonding to the nitrogen of the cyanide. This intermediate isomerizes to form a second intermediate. The second intermediate decomposes into Co²⁺(aq), 5NH₄⁺, CN⁻ and R¹R²CO. The plausible structures of the intermediates are discussed. The radicals CH³, CH₂CHO, , and are considerably less reactive towards this complex, the formation of intermediates in their presence is not observed.     

Photocatalysis of chloroform degradation by μ-dichlorotetrachlorodipalladate(II)

Unlike other chlorometallate complexes that catalyze the photodecomposition of haloalkanes through photodissociation of a chlorine atom, both and catalyze chloroform decomposition through a process that appears to involve C-H bond breakage from an excited state association complex with chloroform. This would account for the greatly retarded rate of decomposition in CDCl₃ and for the generation of CCl₄ as a side product. In chloroform, and are in slow equilibrium with each other. The rate for the conversion of - in chloroform at 23 °C obeys the expression (0.03 M⁻¹ s⁻¹) [][Cl⁻]. The equilibrium constant, K = [][Cl⁻]²/[]², was estimated to be 3 × 10⁻³ M in CHCl₃.     

Square-planar and trigonal prismatic silver(I) in bipyrimidine and oxalate bridged tetranuclear complexes and one-dimensional compounds: Synthesis and crystal structures

The reaction of [Ag₄(hfac)₄(THF)₂] (hfac⁻ = 1,1,1,5,5,5-hexafluoroacetylacetonate, THF = tetrahydrofurrane) with 2,2′-bipyrimidine (bpm) leads to single crystals. They crystallise in the triclinic system, space group . Their structure consists of [Ag₄(hfac)₄(μ₂-bpm)₃] tetranuclear complexes. In this complex, Ag(I) ions adopt distorted square planar and trigonal prismatic geometries. When [Ag₄(hfac)₄(THF)₂] is replaced by monohydrated silver(I) perchlorate, a one-dimensional (1D) compound with a formula of [[Ag(μ₂-bpm)]⁺]_n, is obtained as single crystals. They crystallise in the monoclinic system, space group P2₁/c. Their structure consists of [[Ag(μ₂-bpm)]⁺]_n chains separated by non-coordinated perchlorate ions. In the chains, the Ag(I) centres adopt a square planar geometry. Finally, starting from [[Ag(μ₂-bpm)]⁺]_n, and sodium oxalate , another 1D compound with a formula of [Ag(μ₂-bpm)(μ₂-ox)]_n, 4nH₂O is obtained as single crystals. They crystallise in the triclinic system, space group . In these chains, bipyrimidine and oxalate are alternate. They generate a square planar geometry around the Ag(I) cations.     

Identification of the first steps in charge separation in bacterial photosynthetic reaction centers of Rhodobacter sphaeroides by ultrafast mid-infrared spectroscopy: electron transfer and protein dynamics

Time-resolved visible pump/mid-infrared (mid-IR) probe spectroscopy in the region between 1600 and 1800 cm⁻¹ was used to investigate electron transfer, radical pair relaxation, and protein relaxation at room temperature in the Rhodobacter sphaeroides reaction center (RC). Wild-type RCs both with and without the quinone electron acceptor Q_A, were excited at 600 nm (nonselective excitation), 800 nm (direct excitation of the monomeric bacteriochlorophyll (BChl) cofactors), and 860 nm (direct excitation of the dimer of primary donor (P) BChls (P_L/P_M)). The region between 1600 and 1800 cm⁻¹ encompasses absorption changes associated with carbonyl (CO) stretch vibrational modes of the cofactors and protein. After photoexcitation of the RC the primary electron donor P excited singlet state (P*) decayed on a timescale of 3.7 ps to the state (where B_L is the accessory BChl electron acceptor). This is the first report of the mid-IR absorption spectrum of ; the difference spectrum indicates that the 9-keto CO stretch of B_L is located around 1670-1680 cm⁻¹. After subsequent electron transfer to the bacteriopheophytin H_L in ∼1 ps, the state was formed. A sequential analysis and simultaneous target analysis of the data showed a relaxation of the radical pair on the ∼20 ps timescale, accompanied by a change in the relative ratio of the and bands and by a minor change in the band amplitude at 1640 cm⁻¹ that may be tentatively ascribed to the response of an amide CO to the radical pair formation. We conclude that the drop in free energy associated with the relaxation of , is due to an increased localization of the electron hole on the P_L half of the dimer and a further consequence is a reduction in the electrical field causing the Stark shift of one or more amide CO oscillators.     

In situ spectroelectrochemical investigation of Pt(II/IV) oxidation in aqueous solution using X-ray absorption spectroscopy

The oxidation from to in HCl aq. was studied in situ by combining electrochemistry with XAFS spectroscopy. During the oxidation of , isosbestic points were observed in Pt L_III and L_II XANES spectra as a function of time, indicating that the Pt(II/IV) redox equilibrium is the only reaction in the system. The Pt L_III and L_II X-ray absorption edge energies of the initial Pt^IICl₄²⁻ are 11562.9 and 13271.8 eV, respectively, while those of the electrolyzed species are 11564.6 and 13273.7 eV which are identical with those of a reference sample. The coordination of the electrolyzed species was characterized by structural parameters derived from the EXAFS curve fit, and identified to .     

Heterodimetallic Pd-based carboxylate-bridged complexes: Synthesis and structure of single-crystalline Pd-M (M = Mn, Co, Ni, Cu, Zn, Nd, Eu, Ce) acetates

A series of crystalline Pd^II-based heterodimetallic acetate-bridged complexes containing the transition (Mn^II, Co^II, Ni^II, Cu^II), post-transition (Zn^II) and rare-earth (Ce^IV, Nd^III, Eu^III) metals were synthesized starting from Pd₃(OOCMe)₆ and the complementary metal(II, III) acetates. The crystal and molecular structures of the binuclear Pd^IIM^II(μ-OOCMe)₄L (M = Mn, Co, Ni, Zn; L = H₂O, MeCN), trinuclear and tetranuclear (M = Nd, Eu) and complexes were established by X-ray diffraction.     

Synthesis and single crystal structural investigations on quaternary salts of tellurated alkylamine derivatives

Tellurated alkylamine derivatives , , and have been synthesized by reacting appropriate organic halides with the nucleophile 4-CH₃OC₆H₄Te⁻ or Te²⁻ generated in situ by borohydride reduction of (4-CH₃OC₆H₄Te)₂ or Te powder followed by reaction with HCl of appropriate concentration. The zwitterionic species was generated when single crystals of 2 were grown in methanol at 0 °C. Complexes 1-4 exhibit characteristic ¹H NMR spectra. The single crystal structures of 1-4 and 2a have been determined. In the crystals of 1, C-H?π distances have been found to be 3.31(7)-3.59(5) Å. In both 2 and 2a, weak Te?Cl interactions (3.54(2) -3.62(2) Å) are observed. The C-H?π distance in the crystal of 2 is 3.19(0) Å. In 2a and 3, water hydrogen bonds connect the water molecules with the end groups from different molecules. In the case of 3, Te?Cl weak interactions involving the Cl⁻ ions connect together two such chains. The geometry of Te in 1 is V shaped. In 2 and 3 it is pseudo trigonal bipyramidal, and in 2a, it is square pyramidal. However, in the latter case it becomes distorted octahedral due to weak Te?Cl secondary interactions. The geometry about Te in 4 is distorted octahedral due to weak Te?Cl interactions involving Cl⁻ ions. However, there are no intermolecular Te?Cl interactions.     

Time-resolved FTIR difference spectroscopy in combination with specific isotope labeling for the study of A1, the secondary electron acceptor in photosystem 1

A phylloquinone molecule (2-methyl, 3-phytyl, 1, 4-naphthoquinone) occupies the A₁ binding site in photosystem 1 particles from Synechocystis sp. 6803. In menB mutant photosystem 1 particles from the same species, plastoquinone-9 occupies the A₁ binding site. By incubation of menB mutant photosystem 1 particles in the presence of phylloquinone, it was shown in another study that phylloquinone will displace plastoquinone-9 in the A₁ binding site. We describe the reconstitution of unlabeled (¹⁶O) and ¹⁸O-labeled phylloquinone back into the A₁ binding site in menB photosystem 1 particles. We then produce time-resolved Fourier transform infrared (FTIR) difference spectra for these menB photosystem 1 particles that contain unlabeled and ¹⁸O-labeled phylloquinone. By specifically labeling only the phylloquinone oxygen atoms we are able to identify bands in FTIR difference spectra that are due to the carbonyl (CO) modes of neutral and reduced phylloquinone. A positive band at 1494 cm⁻¹ in the FTIR difference spectrum is found to downshift 14 cm⁻¹ and decreases in intensity on ¹⁸O labeling. Vibrational mode frequency calculations predict that an antisymmetric vibration of both CO groups of the phylloquinone anion should display exactly this behavior. In addition, phylloquinone that has asymmetrically hydrogen bonded carbonyl groups is also predicted to display this behavior. The positive band at 1494 cm⁻¹ in the FTIR difference spectrum is therefore due to the antisymmetric vibration of both CO groups of one electron reduced phylloquinone. Part of a negative band at 1654 cm⁻¹ in the FTIR difference spectrum downshifts 28 cm⁻¹ on ¹⁸O labeling. Again, vibrational mode frequency calculations predict this behavior for a CO mode of neutral phylloquinone. The negative band at 1654 cm⁻¹ in the FTIR difference spectrum is therefore due to a CO mode of neutral phylloquinone. More specifically, calculations on a phylloquinone model molecule with the C₄O group hydrogen bonded predict that the 1654 cm⁻¹ band is due to the non hydrogen bonded C₁O mode of neutral phylloquinone.     

Effect of magnesium chloride (2:1 electrolyte) on the aqueous solution behavior of some saccharides over the temperature range of 288.15-318.15 K: a volumetric approach

Infinite-dilution standard partial molar volumes, , for various mono-, di-, and trisaccharides, and their derivatives (methyl glycosides) at molalities ranging from 0.04 to 0.12 mol kg⁻¹ in aqueous solutions of magnesium chloride of 0.5, 1.0, 2.0, and 3.0 mol kg⁻¹, have been evaluated over a range of temperatures from 288.15 to 318.15 K by density measurements employing a vibrating-tube densimeter. These data have been utilized to determine the corresponding standard partial molar volumes of transfer, , of saccharides and methyl glycosides from water to aqueous magnesium chloride solutions. The values have been found to be positive, and their magnitudes increase with an increasing concentration of magnesium chloride in all cases. Partial molar expansion coefficients, and second derivatives thereof, have been estimated. The magnitude of values increases with an increase in temperature, indicating that hydration effects in solutions are strongly sensitive to temperature. Pair and higher order volumetric interaction coefficients (V_AB, V_ABB) have also been obtained from values by using the McMillan-Mayer theory. The various parameters have been discussed in terms of the solute (saccharide or methyl glycoside)-co-solute (magnesium chloride) interactions and are thus used to understand the mixing effects due to these interactions. These results have been compared with those earlier reported in the presence of electrolytes. An attempt is made to interpret the volumetric properties data in terms of the stereochemistry of the solutes.     

Crystal structures and magnetic properties of Ni-bisdithiolene complexes with decamethylmetallocenium cations

Three new compounds of formula (1), (2), and (3) have been synthesised, and structurally and magnetically characterised (dmit²⁻ = 1,3-dithiol-2-thione-4,5-dithiolato; dmid²⁻ = 1,3-dithiol-2-one-4,5-dithiolato). Their structural features and magnetic behaviours are compared with those of and . The result of this is that the interactions between the Ni(dmit)₂ units in 1 are of ferromagnetic-type, as suggested previously for . The change from acetonitrile to acetone when going from to 2 results in stronger ferromagnetic intermolecular interactions. This better cooperativity is due to a significant increase of the number of contacts between the various moieties within the . On the contrary, the inclusion of larger solvents such as benzonitrile in this type of complexes results in a totally different structural arrangement, which leads to an antiferromagnetic behaviour for 3.