High rate detection of volatile products using differential electrochemical mass spectrometry: combining an electrode-coated membrane with hydrodynamic flow in a wall-tube configuration.

We present an experimental system that combines differential electrochemical mass spectrometry with hydrodynamic flow consisting of an impinging jet in a wall-tube configuration. This assembly allows simultaneous detection of electrochemical signals along with monitoring of dissolved gas species using differential electrochemical mass spectrometry under well-defined hydrodynamic conditions and over a wide range of mass transfer rates. The working electrode is deposited directly onto a thin, hydrophobic membrane, which also serves as the inlet to the mass spectrometer. This inlet provides extremely rapid mass detection as well as a high flux of products from the electrode surface into the mass spectrometer. The impinging jet is designed in a wall-tube configuration, in which the jet diameter is large compared to the electrode diameter, thus providing uniform and rapid mass transfer conditions over the entirety of the electrode surface. This combination of rapid detection and controllable flow conditions allows a wide range of hydrodynamic conditions to be accessed with simultaneous electrochemical and mass spectrometric detection of dissolved gas species, which is important in the analysis of a range of electrochemical reactions. The capabilities of this configuration are illustrated using a platinum-coated electrode and several electrochemical reactions, including ferrocyanide oxidation, proton reduction, and oxalic acid oxidation.

Catalytic reactions of carbene precursors on bulk gold metal.

Bulk gold metal powder, consisting of particles (5-50 microm) much larger than nanoparticles, catalyzes the coupling of carbenes generated from diazoalkanes (R(2)C=N(2)) and 3,3-diphenylcyclopropene (DPCP) to form olefins. It also catalyzes cyclopropanation reactions of these carbene precursors with styrenes. The catalytic activity of the gold powder depends on the nature of the gold particles, as determined by TEM and SEM studies. The reactions can be understood in terms of mechanisms that involve the generation of carbene R(2)C: intermediates adsorbed on the gold surface.

Solid-state NMR investigations of the immobilization of a BF4(-) salt of a palladium(II) complex on silica.

The structure of the silica supported palladium(II) complex [Pd(dppp)(S2C-NEt2)]BF4 (abbreviated as [Pd(dppp)(dtc)]BF4, where dppp is Ph2P(CH2)3PPh2) and interactions between the [Pd(dppp)(dtc)]+ cation, the BF4(-) anion, and the silica surface are studied using solid-state NMR spectroscopy. The unsupported, crystalline form of [Pd(dppp)(dtc)]BF4 is also investigated, both by X-ray diffraction and NMR. The structures of the cation and anion are found to be essentially the same in both unsupported and supported complex. The [Pd(dppp)(dtc)]BF4 loading has been determined by quantitative measurements of 11B, 19F, and 31P intensities, whereas the arrangement of anions and cations on the surface of silica has been established by two-dimensional heteronuclear correlation experiments involving 1H, 11B, 13C, 19F, 29Si, and 31P nuclei. At low coverages, the [Pd(dppp)(dtc)]+ cations are located near the BF4(-) anions, which in turn are immobilized directly on the surface near the Q4 sites. At higher loadings, which in this study corresponded to 0.06-0.15 mmol/g, the complexes stack on top of each other, despite the fact that the directly adsorbed molecules take up less than 10% of the silica surface. The relevance of these findings to heterogeneous catalysis is discussed.

Isocyanide ligands adsorbed on metal surfaces: applications in catalysis, nanochemistry, and molecular electronics.

Knowledge of the coordination chemistry and reactivity of isocyanide ligands in transition-metal complexes forms the basis for understanding the adsorption and reactions of isocyanides on metal surfaces. In this overview, we explore reactions (often catalytic) of isocyanides adsorbed on metal surfaces that reflect their patterns of reactivity in metal complexes. We also examine applications of isocyanide adsorption to the stabilization of metal nanoparticles, the functionalization of metal electrodes, and the creation of conducting organic-metal junctions in molecule-scale electronic devices.

Electrochemical and EPR studies of the corannulene ruthenium(II) sandwich complex [(eta6-C6Me6)Ru(eta6-C20H10)](SbF6)(2).

The dication [(eta6-C6Me6)Ru(eta6-C20H10)]2+ in propylene carbonate solution exhibits a sequence of reduction processes that is either metal-centered [Ru(II)/Ru(I)/Ru(0)] or ligand-centered. The marginally stable Ru(I) monocation [(eta6-C6Me6)Ru(eta6-C20H10)]+ has been characterized by EPR spectroscopy. The electrochemistry of C20H10 and EPR features of its stable monoanion [C20H10]- have also been revisited.

Non-nanogold catalyzed aerobic oxidation of secondary amines to imines.

Bulk gold powder (approximately 10(3) nm particle size) is a highly active catalyst for the oxidative dehydrogenation of secondary amines to imines under the mild conditions of 1 atm O2 and 60-100 degrees C.

Non-nanogold catalysis of carbon monoxide oxidative amination.

Bulk metallic gold particles ( approximately 1000 nm) catalyze the reaction of CO, O2, and primary amines (R-NH2) to give ureas (RNH)2C=O. Isocyanates (R-N=C=O) are identified as intermediates in the reactions and are shown to react with primary and secondary amines to give ureas under the conditions of the gold-catalyzed reactions. Although many recent studies indicate that nanosized particles of gold are required for the catalytic oxidation of CO, the results presented in this paper show that bulk gold is capable of catalyzing the oxidative amination of CO under mild conditions (45 degrees C, 1 atm CO and O2).

Gold metal-catalyzed reactions of isocyanides with primary amines and oxygen: analogies with reactions of isocyanides in transition metal complexes.

Despite its generally poor catalytic properties, bulk gold metal is observed to catalyze reactions of isocyanides (CN-R) with primary amines (H2N-R') and O2 to give carbodiimides (R-N=C=N-R') at room temperature and above. Detailed infrared reflection absorption spectroscopic (IRRAS) and kinetic studies show that the reaction occurs by initial eta1-adsorption of the isocyanide on the Au surface, which activates the isocyanide to attack by the amine. This attack is the rate-determining step in the catalytic cycle and has characteristics very similar to those of amine reactions with coordinated isocyanides in transition metal complexes. However, the metallic Au surface provides a pathway involving O2 to give the carbodiimide product whereas homogeneous metal ion catalysts give formamidines [HC(=NR)(NHR')].

CpRu(CO)2(BF4) and [CpFe(CO)2(THF)]+ on mesoporous silica as adsorbents for the removal of dibenzothiophenes from hydrocarbon solutions.

The complexes, CpRu(CO)2(BF4) and [CpFe(CO)2(eta2-2-methylpropene)][BF4], react with dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-Me2DBT) to give [CpRu(CO)2(DBT)][BF4] and [CpFe(CO)2(4,6-Me2DBT)][BF4], whose structures were established by X-ray diffraction studies. The same types of products are obtained when dibenzothiophenes react with CpRu(CO)2(BF4) and [CpFe(CO)2(THF)][BF4] that are adsorbed on the mesoporous silica SBA-15. DRIFT and XPS studies indicate that CpRu(CO)2(BF4) and [CpRu(CO)2(DBT)][BF4] are adsorbed on the SBA-15 by hydrogen-bonding of the BF4- anions to surface Si-O-H groups. CpRu(CO)2(BF4)/SBA-15 removes 99% of the DBT in a 45% toluene/55% hexanes simulated petroleum feedstock. This solid phase extractant is less successful for sterically-hindered 4,6-Me2DBT, as only 72% of it is removed. The results show that CpRu(CO)2(BF4) can be immobilized by adsorption on mesoporous silica and that it reacts with dibenzothiophenes in the adsorbed form, CpRu(CO)2(BF4)/SBA-15, in much the same way that it reacts in solution.

Deep desulfurization by selective adsorption of dibenzothiophenes on Ag+/SBA-15 and Ag+/SiO2.

Silver (Ag+) salts adsorbed on amorphous silica or mesoporous SBA-15 extract dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-Me2DBT) from simulated, hydrotreated petroleum feedstocks.

Hydrogenation and carbon-sulfur bond hydrogenolysis of benzothiophene promoted by Re2(CO)10 and H4Re4(CO)12.

In the search for metal complexes that promote the cleavage of C-S bonds in thiophenes, we observe that the reaction of Re(2)(CO)(10) and benzothiophene (BT) under a hydrogen atmosphere gives the trinuclear cluster Re(3)(mu-H)(2)(mu(3)-S-2-EtC(6)H(4))(mu-2,3-DHBT)(CO)(9) (1), which contains a hydrogenated BT ligand and a thiolate ligand resulting from the hydrogenation and cleavage of a C-S bond in BT. A detailed study of the reaction shows that Re(2)(CO)(10) initially reacts with H(2) to give H(3)Re(3)(CO)(12), which subsequently converts to H(4)Re(4)(CO)(12), which finally reacts with BT to give 1.

Synthetic, structural and binding studies of the 4,6-dimethyldibenzothiophene complex [(eta(5)-C(5)Me(5))Ru(CO)(2)(eta(1)(S)-4,6-Me(2)DBT)]BF(4): toward an understanding of deep hydrodesulfurization.

The synthesis of the first completely characterized transition-metal complex containing a sulfur-bound 4,6-dimethyldibenzothiophene (4,6-Me(2)DBT) ligand, [CpRu(CO)(2)(eta(1)(S)-4,6-Me(2)DBT)]BF(4) (1) (Cp = eta(5)-C(5)Me(5)), is reported. X-ray studies of 1 and its 4-methyldibenzothiophene and dibenzothiophene analogues, [CpRu(CO)(2)(eta(1)(S)-4-MeDBT)]BF(4) (2) and [CpRu(CO)(2)(eta(1)(S)-DBT)]BF(4) (3), show that the Ru-S bond distances increase in the order, 3 < 2 < 1. Equilibrium studies on the series of [CpRu(CO)(2)(eta(1)(S)-DBTh)](+) compounds, where DBTh = DBT, 4-MeDBT, 4,6-Me(2)DBT, and 2,8-Me(2)DBT, show that the relative binding strengths of the dibenzothiophene ligands increase in the order 4,6-Me(2)DBT (1) < 4-MeDBT (20.2(1)) < DBT (62.7(6)) < 2,8-Me(2)DBT (223(3)). These results are the first to quantify the steric effect of 4- and 6-methyl groups on the sulfur-coordinating ability of dibenzothiophenes to transition-metal centers. They are also consistent with the proposal that 4- and 6-methyl groups reduce the coordination of dibenzothiophenes to active metal sites on hydrodesulfurization catalysts, which could account for the slow rate of 4-MeDBT and 4,6-Me(2)DBT hydrodesulfurization in petroleum feedstocks.

Re(2)(CO)(10)-promoted S-binding, C-S bond cleavage, and hydrogenation of benzothiophenes: organometallic models for the hydrodesulfurization of thiophenes.

In hydrodesulfurization model reactions of dinuclear metal complexes with thiophenes, we observe that ultraviolet photolysis of Re(2)(CO)(10) and benzothiophenes (BT) in hexanes solution produces the ring-opened BT complexes Re(2)(CO)(7)(mu-BT) (1a-d) (BT = benzothiophene (BT) 1a, 2-methylbenzothiophene (2-MeBT) 1b, 3-methylbenzothiophene (3-MeBT) 1c, and 3,5-dimethylbenzothiophene (3,5-Me(2)BT) 1d). The eta(1)(S)-bound BT complexes Re(2)(CO)(9)(eta(1)(S)-BT) (2a-d), prepared from Re(2)(CO)(9)(THF) and BT, are readily converted into 1a-d in good yields (40-60%) during UV photolysis in hexanes solution, which suggests that the eta(1)(S)-bound complexes 2a-d are precursors to 1a-d in the reactions of Re(2)(CO)(10) with BT. Irradiation of Re(2)(CO)(10) and 3,5-Me(2)BT with UV light in decane solution under an atmosphere of H(2) produces complex 1d and the partially hydrogenated BT complex Re(2)(CO)(7)(mu-3,5-Me(2)BT-H)(eta-H) (3d). Reactions of 1a with phosphines yield further ring-opened BT-Re complexes of the types Re(2)(CO)(7)(PMe(3))(3)(mu-BT) (4) and Re(2)(CO)(7)(PR(3))(2)(mu-BT) (R = Me (5), (i)Pr (6), Cy (7), and bis(diethylphosphino)ethane (8)). Structures of 1d, 2c, 3d, and 6, which demonstrate various bonding modes of benzothiophene and its C-S cleaved derivatives to two metal centers, were determined by X-ray crystallographic studies.

Kinetic Studies of the Alkylation of Metal Cyanide Complexes: M(CO)(x)()(L)(5)(-)(x)()CN (M = Mn, Re) and CpM'(CO)(x)()(L)(2)(-)(x)()CN (M' = Fe, Ru).

Reaction rates for the alkylation of Mn(CO)(dppm)(2)CN (dppm = PPh(2)CH(2)PPh(2)), Mn(CO)(2)(tripod)CN [tripod = (PPh(2)CH(2))(3)CCH(3)], Re(CO)(3)(dppm)CN, (eta(6)-C(6)Me(6))Mn(CO)(2)CN, CpFe(dppe)CN (dppe = PPh(2)CH(2)CH(2)PPh(2)), CpRu(dppe)CN, and CpRu(CO)(PPh(3))CN with methyl 4-nitrobenzenesulfonate (MeONs) to produce complexes of the type [L(n)()M-CNMe](+) (-)ONs were investigated in 1,2-dichloroethane (DCE) at 30.0 degrees C. The reactions are first order in both the complex and the alkylating agent, which is consistent with a mechanism that involves nucleophilic attack of the cyanide nitrogen on the methyl of the MeONs. The second-order rate constants (k(2)) correlate approximately with the stretching frequency of the cyanide ligand nu(CN) in the complexes. There is a somewhat better correlation between nu(CN) of the CNCH(3) ligands in the products and k(2). There is also a good correlation between the rate constant (k(2)) and the calculated force constant (k(CO)) for the nu(CO) vibrational mode of the analogous [L(n)()M-CO](+) complex. These correlations appear to be useful for predicting nucleophilicities of metal cyanide complexes of different metals, in different oxidation states, and with ligands exhibiting a range of electronic and steric properties.

Tridentate Ligand Effects on Enthalpies of Protonation of (L(3))M(CO)(3) Complexes (M = W, Mo).

Titration calorimetry has been used to determine the enthalpies of protonation (DeltaH(HM)) for the reaction of (L(3))M(CO)(3) complexes, where M = W and Mo and L(3) = cyclic and noncyclic tridentate ligands of the N, S, and P donor atoms, with CF(3)SO(3)H in 1,2-dichloroethane solution at 25 degrees C to give (L(3))M(CO)(3)(H)(+)CF(3)SO(3)(-). The basicities (-DeltaH(HM)) increase with the ligand donor groups (X, Y, or Z) in the order S

Cyclopentadienyl Ligand Effects on Enthalpies of Protonation of the Ru-Ru Bond in Cp'(2)Ru(2)(CO)(4) Complexes.

Basicities of a series of Cp'(2)Ru(2)(CO)(4) complexes were established by measuring the heats evolved (DeltaH(MHM)) when the complexes were protonated by CF(3)SO(3)H in 1,2-dichloroethane at 25.0 degrees C. Spectroscopic studies show that the protonation occurs at the metal-metal bond to form [Cp'(2)Ru(2)(CO)(4)(&mgr;-H)](+)CF(3)SO(3)(-), in which all of the CO ligands are terminal. The basicities (-DeltaH(MHM)) increase with the Cp'(2) ligands in the following order: (C(5)Me(4)CF(3))(2) < (C(9)H(7))(2) < C(5)H(4)C(5)H(4) < C(5)H(4)CH(2)CH(2)C(5)H(4) < (C(5)H(5))(2) < (C(5)Me(5))(2) < C(5)H(4)CH(2)C(5)H(4). This trend can be understood in part by considering that more strongly donating Cp' ligands increase the basicity of the Ru-Ru bond. Another important factor is the CO-bridging or nonbridging form of each Cp'(2)Ru(2)(CO)(4) complex. A dimer with bridging CO groups is significantly less basic than another dimer with only terminal CO groups although the donor abilities of their Cp' ligands are nearly equal. The Ru-Ru bond in Cp(2)Ru(2)(CO)(4) is substantially more basic than the Ru in the related mononuclear CpRu(CO)(2)H. Molecular structures of [Cp(2)Ru(2)(CO)(4)(&mgr;-H)](+)CF(3)SO(3)(-), [(C(5)H(4)CH(2)C(5)H(4))Ru(2)(CO)(4)(&mgr;-H)](+)CF(3)SO(3)(-), and (C(5)H(4)CH(2)CH(2)C(5)H(4))Ru(2)(CO)(4) as determined by X-ray diffraction studies are also presented.

Iron Carbonyl-Promoted Isomerization of Olefin Esters to Their alpha,beta-Unsaturated Esters: Methyl Oleate and Other Examples.

Ultraviolet photolysis of stoichiometric amounts of methyl oleate and Fe(CO)(5) in hexanes solvent at 0 degrees C gives Fe(CO)(3)(eta(4)-alpha,beta-ester) in which the alpha,beta-unsaturated ester isomer of methyl oleate is stabilized by eta(4)-oxadiene pi coordination of the olefin and ester carbonyl groups to the Fe(CO)(3) unit. Treatment of the Fe(CO)(3)(eta(4)-alpha,beta-ester) with pyridine or CO liberates the free alpha,beta-ester, methyl octadec-trans-2-enoate, in 70% yield. The Fe(CO)(3) unit both catalyzes the olefin isomerization and stabilizes the alpha,beta-unsaturated ester, which results in the formation of the alpha,beta-ester in a yield far above that (3.5%) observed for simple catalyzed methyl oleate isomerization. The much smaller olefin esters, methyl 3-butenoate and ethyl 4-methyl-4-pentenoate, are isomerized under the same conditions to their alpha,beta-unsaturated esters in 94 and 90% yields, respectively. The effects of reaction conditions on the yield, the use of Fe(CO)(3)(cis-cyclooctene)(2) as a nonphotolytic catalyst, and the mechanism of this useful synthetic process are discussed.

Effects of Cyclopentadienyl and Phosphine Ligands on the Basicities and Nucleophilicities of Cp'Ir(CO)(PR(3)) Complexes.

Basicities of the series of complexes CpIr(CO)(PR(3)) [PR(3) = P(p-C(6)H(4)CF(3))(3), P(p-C(6)H(4)F)(3), P(p-C(6)H(4)Cl)(3), PPh(3), P(p-C(6)H(4)CH(3))(3), P(p-C(6)H(4)OCH(3))(3), PPh(2)Me, PPhMe(2), PMe(3), PEt(3), PCy(3)] have been measured by the heat evolved (DeltaH(HM)) when the complex is protonated by CF(3)SO(3)H in 1,2-dichloroethane (DCE) at 25.0 degrees C. The -DeltaH(HM) values range from 28.0 kcal/mol for CpIr(CO)[P(p-C(6)H(4)CF(3))(3)] to 33.2 kcal/mol for CpIr(CO)(PMe(3)) and are directly related to the basicities of the PR(3) ligands in the complexes. For the more basic pentamethylcyclopentadienyl analogs, the -DeltaH(HM) values range from 33.8 kcal/mol for the weakest base CpIr(CO)[P(p-C(6)H(4)CF(3))(3)] to 38.0 kcal/mol for the strongest CpIr(CO)(PMe(3)). The nucleophilicities of the Cp'Ir(CO)(PR(3)) complexes were established from second-order rate constants (k) for their reactions with CH(3)I to give [Cp'Ir(CO)(PR(3))(CH(3))](+)I(-) in CD(2)Cl(2) at 25.0 degrees C. There is an excellent linear correlation between the basicities (DeltaH(HM)) and nucleophilicities (log k) of the CpIr(CO)(PR(3)) complexes. Only the complex CpIr(CO)(PCy(3)) with the bulky tricyclohexylphosphine ligand deviates dramatically from the trend. In general, the pentamethylcyclopentadienyl complexes react 40 times faster than the cyclopentadienyl analogs. However, they do not react as fast as predicted from electronic properties of the complexes, which suggests that the steric size of the Cp ligand reduces the nucleophilicities of the CpIr(CO)(PR(3)) complexes. In addition, heats of protonation (DeltaH(HP)) of tris(2-methoxyphenyl)phosphine, tris(2,6-dimethoxyphenyl)phosphine, and tris(2,4,6-trimethylphenyl)phosphine were measured and used to estimate pK(a) values for these highly basic phosphines.