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.     

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.     

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.     

Oxomolybdenum(III, IV and V) complexes of thiofunctional ligands

Reaction of [MoO₂(acac)₂] with (S is a thioether, S′ a thiophenolate function) yielded the compound Li₇(thf)₁₇{MoO}₈ · 10thf · hexane, where {MoO}₈ represents one 1, three (2, linked, via the oxo group, to [Li(thf)₃]⁺) and two (3a, linked by two [Li(thf)₂]⁺).A mixed-valent variant of 3, (3b, with an additional[Li(thf)₃]⁺ attached to S′), was also identified. The compounds model features pertinent to oxo-transferases containing the molybdopterin cofactor.     

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.     

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.     

Anation kinetics of pentacyanoaquachromate(III) by thiocyanate and azide

The kinetics of the reaction of Cr(CN)₅(H₂O)²⁻ with NCS⁻ and were studied at pH 5.0 and at pH 6.3-7.0, respectively, as a function of the temperature between 25.0 and 55.0 °C, and at various ionic strengths. Anation occurs in competition with aquation of CN⁻, with rate constants that exhibit less-than-first-order dependence on the concentration of the entering anions. The results are interpreted in terms of ligand interchange in a context of association of the two reacting anions mediated by the Na⁺ or Ca²⁺ counterions. The degree of aggregation depends mainly on the total cationic charge rather than on the ionic strength, and is ca. 2-fold larger for than for NCS⁻. Within the associated species, is a better entering ligand than NCS⁻ by a factor of 4.5. The Cr(CN)₅(NCS)³⁻ and Cr(CN)₅(N₃)³⁻ complexes were also synthesized, and the rates of aquation of NCS⁻ and were measured at pH 5.0 and between 55.0 and 80.0 °C, over the same range of ionic strengths. The ionic strength enhances the anation rates but has little effect on the aquation rates. The average activation enthalpies of the interchange step are 80 ± 3 and 76 ± 3 kJ mol⁻¹ for entry of NCS⁻ and , respectively. Those of the corresponding aquation reactions are 94 ± 4 and 107 ± 4 kJ mol⁻¹. Within error limits, all ΔH^‡ values are independent of the ionic strength. The results are consistent with an I_d mechanism for substitution in Cr(CN)₅X^z− complexes.     

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.     

Investigation into the reaction of (t-BuC5H4)2Nb(η-Te2)H with CH3Li: Hydride abstraction versus telluride methylation

The reaction of (Cp′ = t-BuC₅H₄) with CH₃Li in THF was examined by variable temperature ¹H NMR, ESR and mass spectroscopic means. From these methods it is evident that the diamagnetic compounds and as well as the paramagnetic compound form simultaneously. In the subsequent reaction of the intermediate solution with [Co₂(CO)₈] compound 4 was consumed and the compound (5) formed in good yield. Complex 5 was characterized by IR and variable temperature ¹H NMR spectroscopies. Electrochemical two-electron reduction of 1 leads, in a quasi-reversible process, to products that are not stable in solution.     

Copper·Lys-Gly-His-Lys mediated cleavage of tRNA: Studies of reaction mechanism and cleavage specificity

The reactivity of [Cu²⁺·Lys-Gly-His-Lys-NH₂]²⁺ and [Cu²⁺·Lys-Gly-His-Lys]⁺ toward tRNA^Phe has been evaluated. The amidated and carboxylate forms of the copper peptides display complex binding behavior with strong and weak sites evident (, for the amide form; and , for the carboxylate form), while Cu²⁺(aq) yielded and . The time-dependence of the reaction of [Cu²⁺·Lys-Gly-His-Lys]⁺ and [Cu²⁺·Lys-Gly-His-Lys-NH₂]²⁺ with tRNA^Phe yielded k_obs ∼ 0.075 h⁻¹ for both complexes. HPLC analysis of the reaction products demonstrated guanine as the sole base product. Mass spectrometric data shows a limited number of cleavage fragments with product peak masses consistent with chemistry occurring at a discrete site defined by the structurally contiguous D and TΨC loops, and in a domain where high affinity magnesium centers have previously been observed to promote hydrolysis of the tRNA^Phe backbone. This cleavage pattern is more selective than that previously observed by Long and coworkers for nickel complexes of a series of C-terminally amidated peptides (Gly-Gly-His, Lys-Gly-His, and Arg-Gly-His), and may reflect variations in structural recognition and a distinct reaction path by the nickel derivatives. The data emphasizes the optimal positioning of the metal-associated reactive oxygen species, relative to scissile bonds, as a major criterion for development of efficient catalytic nucleases or therapeutics.     

Kinetics and mechanism of the reaction between the di-μ-cyanobis[tetracyanoferrate(III)] complex ion and sulfite in aqueous solution

The kinetics of the stepwise reduction of the title complex [Fe₂(CN)₁₀]⁴⁻ by sulfite have been studied in the presence of air as a function of pH, sulfite concentration, temperature and ionic strength using stopped-flow and conventional spectrophotometric techniques. The k_obs versus pH profile shows a marked increase in rate with increase of pH over the range 3.7 ? pH ? 6.1 due to the increase in concentration of the more reactive sulfite species . The reaction proceeds in several stages, the first of which involves a one electron transfer process with the formation of the radical anion This then adds on in a rapid stage to form a species . The second and third stages also involve one electron transfer. In the third, or possibly a fourth stage cleavage occurs, the final product being [Fe^II(CN)₅(SO₃)]⁵⁻. The reaction rate is sensitive to the nature of the cation present with a reactivity sequence .     

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 .     

Argentophilic interaction and anionic control of supramolecular structures in simple silver pyridine complexes

The synthesis and characterization of three simple 1:2 silver(I) pyridine adducts of different counter-anions, [Ag(py)₂]⁺ · X⁻ (X = ClO₄, 1; BF₄, 2; PF₆, 3), are reported. The structural studies for 1-3 reveal the presence of strong ligand-unsupported argentophilic interactions between [Ag(py)₂]⁺ ions, forming pairs of . The Ag?Ag contact distances are 2.96-3.00 Å. In 1 and 2, pairs of are further linked into 1-D infinite chains by a combined set of multiple Ag?Ag close contacts (3.34-3.37 Å), offset ‘head to head’ π-π stacking, and anion bridging interactions. Such combined set of interactions is anion-dependant with 1 and 2 containing anions of tetrahedral geometry and , affording essentially the same supramolecular architecture. Metal-anion interactions are crucial in organizing the 1-D chains into 3-D networks. The ES-MS studies of 1 and 2 provide positive evidence for the aggregation of silver(I) ions in solution. In contrast, for 3 with the counter-anion of octahedral , pairs of are organized into a 3-D network via a combined set of Ag?F contacts, C(H)?F hydrogen bonds, and ‘head to tail’ π-π stacking interactions. No extended 1-D polymeric chains of silver ions are present in 3.     

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.     

Synthesis, spectroscopy, computational evaluation and first X-ray structural characterization of a primary amine containing boronium ion

Stable (CF₃SO₂)₂N⁻, I⁻ and salts of the boronium ion [(tert-butylamine)(1-methylimidazole)BH₂]⁺ have been isolated and characterized. A single-crystal X-ray structure of the salt provides the first unambiguous proof for a boronium ion supported by a primary amine ligand.     

Synthesis of bimetallic fulvalene and bis(cyclopentadienyl)methane derivatives of niobium and tantalum tetracarbonyl

Ring coupled bimetallic derivatives (μ-η^5:5-C₅H₄C₅H₄)[Nb(CO)₄]₂ and [μ-CH₂(η⁵-C₅H₄)₂][M(CO)₄]₂, where M = Nb and Ta have been prepared. The molecular structures of the latter two compounds have been determined: , triclinic, , a = 8.028(2) Å, b = 11.414(1) Å, c = 12.711(2) Å, α = 75.020(8)°, β = 80.34(2)°, γ = 79.46(2)°, V = 1097.3(4) ų, Z = 2, R(F) = 2.79%; [μ-CH₂(η⁵-C₅H₄)₂][Ta(CO)₄]₂, triclinic, , a = 7.815(3) Å, b = 10.275(4) Å, c = 13.135(4) Å, α = 104.25(3)°, β = 100.26(4)°, γ = 96.86(3)°, V = 991.2(6) ų, Z = 2, R(F) = 3.00%.