Quantum chemical study of C—H bond activation in methane molecule by AuIII aqua chloride complexes

The mechanism of the reactions of methane with the gold(III) complexes [AuCl_x(H₂O)_{4− x} ]^3−x (x = 2, 3, or 4) was studied by the DFT/PBE method with the SBK basis set. High activation barriers obtained for the reactions of [AuCl₄]⁻ and [Au(H₂O)Cl₃] with methane suggest these reactions cannot proceed under mild conditions. The reaction of the [Au(H₂O)₂Cl₂]⁺ complex with methane has a rather low energy barrier and proceeds through the formation of an intermediate complex. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 191–201, February, 2006.     

About the stability of Au(NH3) 4 3+ in aqueous solution

The transformations of Au(OH) ₄ ^? in aqueous solutions (T = 20°C, I = 1) containing NH₃ and NH ₄ ⁺ (pH 8.1–8.5) were studied. The most pronounced changes in the system occur in the range 0 > log [NH ₄ ⁺ ] > ?2.0 (c _Au = (1?10) × 10^?4 mol/L, the monitoring time was about two weeks). When log [NH ₄ ⁺ ] > 0, Au(NH₃) ₄ ³⁺ dominates together with the amido form Au(NH₃)₃NH ₂ ²⁺ ; when log [NH ₄ ⁺ ] < ?2.0, no changes in the spectra are observed, probably, because of the very low rate of the processes. As c _Au increases in the indicated range, the polymerization rate grows. The equilibrium constant for Au(NH₃)₃OH²⁺ + NH₃ = Au(NH₃) ₄ ³⁺ + OH is log $ K_{4 OH, NH_3 } The transformations of Au(OH)₄⁻ in aqueous solutions (T = 20°C, I = 1) containing NH₃ and NH₄⁺ (pH 8.1–8.5) were studied. The most pronounced changes in the system occur in the range 0 > log [NH₄⁺] > −2.0 (c _Au = (1−10) × 10⁻⁴ mol/L, the monitoring time was about two weeks). When log [NH₄⁺] > 0, Au(NH₃)₄³⁺ dominates together with the amido form Au(NH₃)₃NH₂²⁺; when log [NH₄⁺] < −2.0, no changes in the spectra are observed, probably, because of the very low rate of the processes. As c _Au increases in the indicated range, the polymerization rate grows. The equilibrium constant for Au(NH₃)₃OH²⁺ + NH₃ = Au(NH₃)₄³⁺ + OH is log = −4.2 ± 0.3. This constant was used together with other constants, taking into account possible ligand effects, to estimate the formation constant of Au(NH₃)₄³⁺: logβ₄ = 47 ± 1, E _3/0 = 0.64 ± 0.02 V, log = −8.5 ± 1 (substitution of 4 NH₃ for 4 OH⁻ in Au(OH)₄⁻), log = 17.5 ± 1 (substitution of 4NH₃ for 4Cl⁻ in AuCl₄⁻). Original Russian Text ? I.V. Mironov, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 4, pp. 711–715.     

Kinetics and mechanism of the reduction of gold(III) by glycine in acetate buffer

The kinetics of reduction of tetrachloroaurate(III) by glycine has been spectrophotometrically studied in NaOAc–AcOH buffer in the pH range 3.73–4.77. The reaction is first order with respect to both Au(III) and glycine. Both H⁺ and Cl⁻ ions have inhibiting effects on the reaction rate. The rate decreases with a decrease in the dielectric constant of the medium. AuCl₄⁻ and AuCl₃(OH)⁻ are presumed to be the predominant oxidizing species under the conditions of the experiment. The reaction of gold(III) and zwitterionic species of glycine proceeds with the intermediate formation of gold(I) and iminic cation and the latter subsequently hydrolyses in a fast step to produce formaldehyde and ammonium ion. Formaldehyde was identified as the only organic product by ¹H NMR spectroscopy.     

The photocatalytic decomposition of chloroform by tetrachloroaurate(III)

Abstract  

Near-UV irradiation of solutions of (Bu₄N)AuCl₄ in aerated ethanol-stabilized chloroform causes the continuous decomposition of chloroform, as evidenced by the production of many equivalents of HCl and peroxides. At the outset of irradiation, most of the AuCl₄ ⁻ is reduced to AuCl₂ ⁻, but the reduction stops and is reversed. The same experiments done in ethanol-free chloroform cause chloroform decomposition only until the irreversible reduction of the gold is complete. In deoxygenated ethanol-free chloroform, irreversible reduction to AuCl₂ ⁻ is accompanied by the formation of HCl and CCl₄, while the main decomposition products in deoxygenated ethanol-stabilized chloroform are HCl and C₂Cl₆. It is proposed that, in ethanol-free chloroform, photoreduction of AuCl₄ ⁻ begins with the concerted elimination of HCl from an association complex of CHCl₃ with AuCl₄ ⁻, and that ethanol suppresses { \textCHCl₃ ·\textAuCl₄ ^- } \{ {\text{CHCl}}_{3} \cdot {\text{AuCl}}_{4}^{ - } \} complex formation, leaving a slower radical process to carry out the photoreduction of AuCl₄ ⁻ in ethanol-stabilized chloroform. In the presence of oxygen, the radical process causes a build-up of CCl₃OOH, which reoxidizes AuCl₂ ⁻ to AuCl₄ ⁻ and allows the photodecomposition of CHCl₃ to continue indefinitely.     

Gold nanoparticle synthesis in graft copolymer micelles

 An amphiphilic poly(acrylic acid)/polystyrene graft copolymer (PAA-g-PS) has been used to form “nanoreactors” for the synthesis of gold clusters. Such copolymers tend to form stable micelles in non-polar organic solvents where the poly(acrylic acid) chains constitute the core, and the polystyrene chains, the shell. In the present study, the micellar structure of PAA-g-PS in toluene has been demonstrated by dynamic light scattering and transmission electron microscopy (TEM). The subsequent preparation of gold-graft copolymer composites involved the introduction of gold chloride (AuCl₃), either in powder form or previously dissolved in ether, into the micellar cores of the PAA-g-PS in toluene. The gold salt was then reduced by ultraviolet (UV) irradiation of the emulsion, or of dried cast films. TEM and ultraviolet-visible (UV/Vis) spectroscopy were used to characterize the resulting composites. Gold particles of less than 5 nm in diameter were observed in all cases, but the size distribution and the spatial arrangement of the clusters in the cast films were modified when diethyl ether was used to introduce AuCl₃ into the PAA-g-PS micellar cores. This was thought to be due to enhanced nucleation of the gold particles and partial disruption of the micellar cores in the presence of diethyl ether. Received: 21 January 1998 Accepted: 11 June 1998     

Receptor properties of calix[4]resorcinarenes toward tetramethylammonium and choline cations in micellar solutions of sodium dodecyl sulfate

Concentration range of solubilization of calix[4]resorcinarene (H₈L) in sodium dodecyl sulfate (SDS) micelles was found. The interaction of the deprotonated form of H8L (tetraanions [H₄L]⁴⁻) with tetramethylammonium (TMA) and choline cations in micellar solutions of SDS was studied by pH-metry and NMR spectroscopy. The concentration dependences of the change in the cloud point in a multicomponent system TMA (choline)-[H₄L]⁴-SDS-tetrabutylammonium bromide were determined. A correlation of these dependences with host-guest binding processes was found. The sharp change in the cloud points of the corresponding micellar solutions in concentration regions of TMA (0-5·10⁻⁴ mol L⁻¹) and choline (0–1.1· 10⁻³ mol L⁻¹) is caused by the formation of inclusion complexes TMA (choline)-[H₄L]⁴⁻ at the interface of the aqueous and micellar pseudophases. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1366–1371, August, 2006.     

A Density Functional Investigation on C2Aun+ (n?=?1, 3, 5) and C2Aun (n?=?2, 4, 6): from Gold Terminals, Gold Bridges, to Gold Triangles

A systematic density functional theory investigation on C₂Au _n ⁺ (n = 1,3,5) and C₂Au _n (n = 2,4,6) indicates that gold atoms serve as terminals (–Au) in the chain-like C_s C₂Au⁺ (C=C–Au⁺) and D_∞h C₂Au₂ (Au–C≡C–Au) and as bridges (–Au–) in the side-on coordinated C_2v C₂Au₃ ⁺ ([Au–C≡C–Au]Au⁺) and C_s C₂HAu₂ ⁺([H–C≡C–Au]Au⁺). However, when the number of gold atoms reaches four, they form stable gold triangles (–Au₃) in the head-on coordinated C_2v C₂Au₄ (Au–C≡C–Au₃) and the side-on coordinated C_2v C₂Au₅ ⁺ ([Au–C≡C–Au]Au₃ ⁺). Similar –Au₃ triangular units exist in the head-on coordinated C_2v C₂HAu₃ (H–C≡C–Au₃) and D_2d C₂Au₆ (Au₃–C≡C–Au₃). The existence of stable –Au₃ triangular units in small dicarbon aurides is significant and intriguing. The high stability of Au₃ triangles originates from the fact that an equilateral D_3h Au₃ ⁺ cation possesses a completely delocalized three-center-two-electron (3c–2e) σ bond and therefore is σ-aromatic in nature. The extension from H/Au analogy to H/Au₃ analogy established in this work may have important implications in designing new gold-containing catalysts and nano-materials.     

The Hydrolysis of AuCl<Stack><Subscript>4</Subscript><Superscript>−</Superscript></Stack> and the Stability of Aquachlorohydroxocomplexes of Gold(III) in Aqueous Solution

The equilibria AuCl₄⁻+jOH⁻+kH₂O⇌AuCl_4−j−k (OH) _j (H₂O) _k ^k−1+(j+k)Cl⁻, β _jk (0≤j,k≤4) have been studied spectrophotometrically at 20 °C in aqueous solution. For I=2 mol⋅dm⁻³(HClO₄) the conventional constants, β _i ^*, of the equilibria, Au^*+iCl⁻ ⇌AuCl _i ^*, are equal to log ₁₀ β ₁^*=(6.98±0.08); log ₁₀ β ₂^*=(13.42±0.05); log ₁₀ β ₃^*=(19.19±0.09); and log ₁₀ β ₄^*=(24.49±0.07), where [AuCl _i ^*]=∑[AuCl _i (OH) _j (H₂O)_4−i−j ] at i=const. The hydrolysis and other transformations of AuCl₄⁻ in aqueous solution are discussed. On the basis of new and known data, a full set of equilibrium constants, β _jk , or their estimates has been obtained.     

Equilibria in the system of aquachlorohydroxogold(III) complexes in aqueous solution

AuCl₄⁻ + jOH⁻ + kH₂O = AuCl_{4 − j − k} OH _j (H₂O) _k ^{k − 1} + (j + k)Cl⁻ equilibria at 20°C were studied spectrophotometrically, and the constants β _jk in acid aqueous solutions were determined for I = 2.0 mol/L (HClO₄).     

Gold(III) amine complexes in aqueous alkali solutions

The gold(III) 1,3-diaminopropane complex [Au(1,3-pn)(1,3-pn-H)]Cl₂ has been synthesized. Its dissociation constant has been determined: Au(1,3-pn)₂³⁺ = Au(1,3-pn-H)²⁺ + H⁺, logK _a1 = −7.03 ± 0.05 (I = 0.1 mol/L NaClO₄). Considerable spectral changes are observed for strong alkali solutions (pH 11–14) containing the monoamido forms of the gold(III) ethylenediamine, 1,3-diaminopropane, and diethylenetriamine complexes (Au(en)(en-H)²⁺, Au(1,3-pn)(1,3-pn-H)²⁺, Au(dien-H)OH⁺). These changes are attributed to the formation of the diamido species Au(en-H)₂⁺, Au(1,3-pn-H)₂⁺, and Au(dien-2H)OH⁰. The dissociation constants of the monoamido complexes have been determined: Au(en)(en-H)²⁺ (logK _a2 = −10.9 ± 0.1 at I = 0.001–0.01 mol/L NaCl); Au(1,3-pn)(1,3-pn-H)²⁺ (logK _a2 = −11.3 ± 0.1 at I = 0.1 mol/L NaCl); Au(dien-H)OH⁺ (logK _a2 = −12.4 ± 0.1 at I = 0.1 mol/L NaCl).     

The Tetrachloridoaurates(III) of Zinc(II) and Cadmium(II)

The first salt‐like compounds of dications with [AuCl₄]^– anions are reported. The compounds Zn[AuCl₄]₂ · (AuCl₃)_1.115 ( 1 ) and Cd[AuCl₄]₂ ( 2 ) are obtained from reactions of MCl₂ (M = Zn, Cd) and elemental gold in liquid chlorine at ambient temperature under autogenous pressure and subsequent annealing at 230 °C. The structure of 1 represents an incommensurately modulated composite [superspace group C2/c(α0γ)0s] built of two subsystems. The first subsystem contains chains of zinc(II) tetrachloridoaurate(III), which feature a slightly distorted octahedral coordination of Zn and can be described by the Niggli formula ¹_∞{Zn[AuCl₄]_1/1[AuCl₄]_2/2}. The second subsystem consists of Au₂Cl₆ molecules, which are located in channels built up by the first subsystem. The structural parameters of the hosted Au₂Cl₆ molecules show only small deviations from neat AuCl₃. The crystal structure of Cd[AuCl₄]₂ ( 2 ) consists of chains built of Cd²⁺ ions coordinated by bridging [AuCl₄]^– anions and alternating Cd‐Au sequence. Cd has a distorted octahedral coordination environment.     

Coordination of gold(III) in the chemisorption by cadmium diisobutyldithiocarbamate: Structure and thermal properties of the polynuclear gold-cadmium complex [Au“S2CN(iso-C4H9)2”2]2n[CdCl4]n

Gold(III) is coordinated by binuclear cadmium diisobutyldithiocarbamate (Dtc) via chemisorption to give a heteropolynuclear Au(III)-Cd complex of the formula [Au“S₂CN(iso-C₄H₉)₂”₂]_2n [CdCl₄] _n (I). According to X-ray diffraction data, structure I contains three structurally nonequivalent complex cations [Au“S₂CN(iso-C₄H₉)₂”₂]⁺. These cations are conformers. Relatively weak nonvalence interactions produce zigzag polymer chains of the type (...C...A...B...A...) _n with alternating nonequivalent cations A, B, and C in a ratio of 2: 1: 1. The anions [CdCl₄]²⁻ are localized at the side. The calculated theoretical chemisorption capacity of cadmium Dtc with respect to [AuCl₄]⁻ is 378.0 mg of Au³⁺ per gram of the sorbent. To optimize the conditions for isolation of sorbed gold, the thermal properties of complex I were studied by simultaneous thermal analysis. The two-step thermolysis of complex I involves (1) thermal decomposition of the dithiocarbamate part and [CdCl₄]²⁻ with liberation of metallic gold and cadmium dichloride and (2) evaporation of CdCl₂. The final thermolysis product of complex I is reduced metallic gold.     

The replacement of five-membered N-donor heterocycles from aminotrichlorogold(III) complexes. A comparison with pyridines

The kinetics of the process AuCl₃(nu) + Cl⁻→ AuCl ₄ ⁻ + nu (nu = one of a number of five-membered N-donor heterocycles covering a wide range of basicity, namely: oxazole, 2,4,5-trimethyloxazole, thiazole, 5-methylthiazole, 4-methylthiazole, 4,5-dimethylthiazole, 2,4-dimethylthiazole, 2,4,5-trimethylthiazole, imidazole and 2-methylimidazole) have been studied in methanol at 25 °C. The reactions follow the usual two-term rate law, rate = (k ₁ + k ₂[Cl⁻])[complex], observed in a square-planar substitution associative-mechanism. The second-order rate constants, k ₂, indicate that the discriminating ability of Au^(III) in these complexes is good and markedly influenced by the nature of the leaving group. A linear-free-energy relationship, logk ₂ = 0.53pK _a + constant, is observed between the rate constant and the basicity of the leaving group for its replacement by chloride. The results are compared with data from the literature regarding a series of complexes of the type AuCl₃(py) (py = one of a number of pyridines) reacting with the Cl⁻ anion under the same experimental conditions. The reactivity depends not only upon ligand basicity but also upon the nature of the ligand in the order: pyridines> five-membered heterocycles. Steric factor due to the presence of a single methyl group ortho to the sp² nitrogen atom in the ring has no influence on the rate of substitution while, surprisingly, when there are two ortho methyl groups a remarkable steric retardation effect is observable. The results are discussed in terms of reaction-profile in the associative-substitution reaction and bonding interactions in the ground and transition states.     

Formation of the ionic gold(III) complex [Au3{S2CN(CH2)4O}6][Au2Cl8][AuCl4] in chemisorption systems [M{S2CN(CH2)4O}2] n -[AuCl4]−/2 M HCl (M = Cd, Zn): Supramolecular structure and thermal behavior

The interaction of freshly precipitated cadmium and zinc morpholinedithiocarbamates with solutions of AuCl₃ in 2 M HCl is studied. In both cases, the heterogeneous reactions of gold(III) binding from solutions lead to the formation of the ionic gold(III) complex [Au₃{S₂CN(CH₂)₄O}₆][Au₂Cl₈][AuCl₄] (I), whose molecular and supramolecular structures are determined by X-ray diffraction analysis. Compound I includes centrosymmetric and noncentrosymmetric cations [Au{S₂CN(CH₂)₄O}₂]⁺ in a ratio of 1: 2. According to the manifested structural differences, the complex cations are related as conformers (cations A are Au(1) and cations B are Au(2)). At the supramolecular level, the isomeric cations form linear trinuclear structural fragments [Au₃{S₂CN(CH₂)₄O}₆]³⁺ [A...B...A] due to secondary bonds Au...S of 3.6364 Å. The anionic part of compound I is presented by [AuCl₄]^? and centrosymmetric binuclear [Au₂Cl₈]^2?, whose formation involved secondary bonds Au...Cl of 3.486 and 3.985 Å. The ultimate chemisorption capacity of cadmium and zinc morpholinedithiocarbamates calculated from the binding of gold(III) is 901.7 and 1010.4 mg of Au³⁺ per 1 g of the sorbent, respectively (i.e., each miononuclear fragment of the chemisorption complexes [M{S₂CN(CH₂)₄O}₂] participates in binding of two gold atoms). To establish the conditions for the isolation of bound gold, the thermal properties of compound I are studied by simultaneous thermal analysis. The thermal destruction process includes the thermolysis of the dithiocarbamate part of the complex and anions [AuCl₄]^? and [Au₂Cl₈]^2? with the reduction of gold to the metal, being the only final product of the thermal transformations of compound I.     

Hydrocarbon adsorption on gold clusters: Experiment and quantum chemical modeling

Heats of adsorption Q of n-alkanes C₆–C₉ on ZrO₂ modified with gold and nickel nanoparticles were determined experimentally. The Q values were found to be higher on average by 7 kJ/mol on the modified samples as compared to the pure support. Density functional theory with the PBE functional and the pseudopotential for gold effectively allowing for relativistic corrections was used to model the adsorption of saturated hydrocarbons on Au and Au + Ni, as exemplified by the interaction of alkanes C₁–C₃ with Au _m , Au _{m − 1}Ni (m = 3, 4, 5) clusters as well as the interaction of C₁–C₈ with Au₂₀. Based on the calculation results, the probable coordination centers of alkanes on nanoparticle surfaces were found to be vertices and edges, whereas face localization was less probable.     

Hypercoordinated gold (i) compounds

The reaction of [O(AuPPh₃)₃]⁺BF₄ ⁻ with a Li, K derivative ofo-cresol followed by the interaction of the reaction product with CO₂ gave (triphenylphosphine)gold acetate. The reaction of ClAuPPh₃ witho-LiC₆H₄SLi afforded (triphenylphosphine)gold thiophenoxide. According to the data of X-ray structural analysis, the latter occurs as a dimer formed through an intermolecular secondary Au…Au bond. The reaction of this complex with diazomethane was accompanied by insertion of carbene into the Au−S bond to form a new gold complex, PhSCH₂AuPPh₃. The reactions with PPh₃Au⁺BF₄ ⁻ or HBF₄ yielded a new tetranuclear gold thiocluster, {[PhS(AuPPh₃)₂]²⁺(BF₄ ⁻)}₂, which involves Au…Au bonds that differ in strength. The structures of the compounds obtained were established by X-ray structural analysis¹H and³¹P NMR spectroscopy, and FAB mass spectrometry. For Part 4, seeIzv. Akad. Nauk, Ser. Khim., 1997, 2244 [Russ. Chem. Bull., 1997,46 2127 (Engl. Transl)]. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2 pp. 350–355, February, 1998.     

Solubilization of Aliphatic Carboxylic Acids (C3-C6) by Sodium Dodecyl Sulfate and Brij 35 Micellar Pseudophases

The apparent dissociation constants of 1-propanoic, 1-butanoic, 1-pentanoic and 1-hexanoic acids were obtained for the first time in Brij 35 micellar solutions with concentration from 0.03 to 0.20 mol⋅L⁻¹ and sodium dodecyl sulfate (SDS) micellar solutions with concentrations from 0.01 to 0.30 mol⋅L⁻¹. A pronounced effect of Brij 35 micelles on the acid-base properties of aliphatic acids was observed. The binding constants, K _b, of carboxylic acids to micellar pseudophases of SDS and Brij 35 were estimated within the framework of the pseudophase model. The dependences of Gibbs energies of transfer from water to the micellar pseudophases were constructed, and Gibbs energies were evaluated for methylene and carboxylic group transfers into Brij 35 and SDS micelles. Comparison of the Gibbs energies of methylene group transfer from water to Brij 35 and SDS suggests that the mechanisms of hydrocarbon group transfer into the core of nonionic and anionic micelles involving the same monomer hydrophobic tail length are similar.     

Dinuclear complex of gold(III) with 1,3-diaminopropane (HL), [Au<Subscript>2</Subscript>(HL)(L)<Subscript>2</Subscript>](ClO<Subscript>4</Subscript>)<Subscript>3</Subscript>(OH): Synthesis and crystal and molecular structures

The dinuclear gold(III) complex, [Au₂(HL)(L)₂](ClO₄)₃(OH) (I) (HL = 1,3-diaminopropane), with two amide bridges has been synthesized for the first time. According to the X-ray diffraction data, the crystal structure of complex I consists of the complex cations [Au₂(HL)(L)₂]⁴⁺ and anions ClO₄⁻ and OH⁻. The coordination sites AuN₄ are insignificantly distorted squares. In the four-membered ring Au₂N₂, the gold atoms are bound by the bridging nitrogen atoms of the deprotonated primary amine.     

Thermal behavior of carbon nanotubes decorated with gold nanoparticles

Three different forms of carbon, i.e., multi-walled carbon nanotubes (CNTs), single-walled CNTs, and soot, were decorated with gold nanoparticles by a new method. In this method C₁₀H₈ ⁻ ions transfer electrons to the CNTs or soot. These electrons on the carbon surface can then reduce Au³⁺ species to form supported Au nanoparticles with a narrow particle size distribution. Thermogravimetric/differential thermal analyses (TG/DTA), XRD, Raman, and TEM show that naphthalene molecules remain trapped inside the Au nanoparticles and can only be removed by treatment at ca. 300 °C. Remarkable effect of the Au nanoparticles on the oxidation of carbon by O₂ is also observed by TG/DTA, i.e., on-set oxidation temperature and activation energy (E _a). It is shown that as the Au particle size decreases from 25 to 2 nm a linear decrease of the oxidation temperature is observed. Au particles larger than 25 nm do not produce any significant effect on carbon oxidation. These results are discussed in terms of spillover catalytic effect where Au nanoparticles activate O₂ molecules to produce active oxygen species which oxidize the different carbon supports.