Ketone CO Research Articles

Improvement of peroxymonosulfate utilization efficiency for sulfamethazine degradation by photo-electron activating peroxymonosulfate: Performance and mechanism

Enhancing the utilization efficiency of oxidant is of great importance for advanced oxidation processes (AOPs). Herein, nitrogen-doped titania dioxide/carbon (NTC7) catalyst was fabricated via pyrolyzing NH2-MIL-125 under nitrogen atmosphere at 700 °C. Excitation of NTC7 under visible light can successfully achieve efficient activation of peroxymonosulfate (PMS) (NTC7 + PMS + Vis). Degradation performance and PMS activation mechanism were systematically investigated using sulfamethazine (SMT) as the target pollutant. It was found that the photo-generated electrons excited from NTC7 under visible light played the dominant role in enhancing the productive consumption of PMS. Its utilization increased by 66 % (Δ[PMS]/Δ[SMT] = 7.0) in NTC7 + PMS + Vis process and the degradation rate was 2.14 times higher than that of NTC7 + PMS process. The ketonic CO groups and structural defects were responsible for the generation of 1O2 in dark activation while radicals (•OH, O2•–) were more inclined to be continuously produced in NTC7 + PMS + Vis process. The involved degradation pathways, intermediates, and toxicity assessment have been studied in detail. This work provides an effective approach to enhance the utilization efficiency of oxidant for pollutant degradation by AOPs.

Nitrogen-doped carbon nanosheets for efficient degradation of bisphenol A by H2O2 activation at neutral pH values

Metal-free carbonaceous materials exhibit great potentials as heterogeneous catalysts for Fenton-like degradation of organic pollutants, because of their high stability and free of secondary pollution of leaching metals. However, the low catalytic activity of carbonaceous materials toward H2O2 activation limits their applications. Herein, we report on a facile two-step thermal annealing method of preparing a kind of nitrogen-doped carbon nanosheets (NCN), which shows efficient degradation of bisphenol A (BPA) via H2O2 activation under neutral conditions. Almost 100% of BPA was removed in 30 min by NCN catalysts in the presence of 12 mmol/L H2O2 at neutral pH values. This NCN catalyst shows good stability with negligible activity decay during recycle use. The influencing factors including the H2O2 addition, NCN dosages, initial BPA concentration, and initial pH values to the removal of BPA by NCN were explored. The radical quenching experiment coupled with an electron spin resonance (ESR) spectrometer demonstrated that singlet oxygen (1O2), and superoxide radical (·O2–) served as primary active species for the pollutant degradation. Pyrrolic N served as adsorption active sites for BPA; Ketonic CO, unsaturated carbon bonds of CC and CC, and graphic C/N are supposed to be active sites for the catalytic reaction. This work provides a new sight in the preparation of carbon-based heterogeneous catalysts and their activation of H2O2 for the removal of organic pollutants from water.

A comprehensive neural network model for predicting flash point of oxygenated fuels using a functional group approach

In the present work, artificial neural networks (ANN) has been used for developing a comprehensive model for predicting flash point (FP) of petroleum fuels containing the following oxygenated chemical classes: alcohols, ethers, aldehydes, ketones and esters. 474 pure compounds and 314 blends comprising of various compounds were used for model development. The fuels were dissembled into eleven constituent functional groups namely, paraffinic CH3, CH2 and CH groups, olefinic –CH = CH2 groups, naphthenic –CH-CH2, aromatic C-CH groups, alcoholic OH groups, ether O groups, aldehydic CHO groups, ketonic CO groups and ester COO groups. These eleven groups were treated as model inputs along with molecular weight (MW) and branching index (BI) which is a structural parameter. These 13 inputs were calculated for each of the 788 fuels to generate a dataset, which was used to train the model. Two ANN models were developed, one using Matlab and other using Keras, an interface for ANN library. GridSearchCV and RandomSearch were used to optimize the network in the Keras model. The developed models showed satisfactory results when applied against the entries in the test set which comprised 20% of the dataset that was not used for model training. The regression coefficient for the comparison between the experimental and predicted data was found to be 0.981 (Matlab model) and 0.979 (Keras model). The developed models have low mean absolute errors of 3.12 K (Matlab model) and 3.55 K (Keras model) and can be used to predict (and screen) FP’s of various complex oxygenated compounds and their mixtures.

Active sites decoration on sewage sludge-red mud complex biochar for persulfate activation to degrade sulfanilamide

Active sites on catalyst surface play significant roles in oxidative species formation. The work focused on the regulation of main active sites on catalyst surface and oxidative species formation. Herein, sewage sludge (SS)-red mud (RM) complex biochar (SRCB) and N-functionalized SRCB (NSRCB) were served as activators of peroxymonosulfate (PMS) for sulfanilamide (SMX) degradation. Specially, NSRCB-1 showed excellent catalytic performance with 97.5% removal of SMX within 110 min. Additionally, the effects of N incorporation on the reconstruction of N species, conversion of intrinsic Fe species and ketonic CO groups in SRCB were studied systematically. Both radical (hydroxyl radicals (OH), sulfate radicals (SO4−) and superoxide radical (O2−)) and non-radical (electron transfer and singlet oxygen (1O2)) pathways were confirmed by quenching experiments, electron paramagnetic resonance (EPR) testing and electrochemical measurements. Ketonic CO groups, pyridinic N and pyrrolic N were responsible for non-radical pathway in SMX degradation process. Besides, Fe(II) modulated by N-doping was the main actives site for radicals generation. The contribution of active sites on catalyst surface to oxidative species formation provided fundamental basis for practical water treatment in PMS process.

Co/N co-doped carbonized wood sponge with 3D porous framework for efficient peroxymonosulfate activation: Performance and internal mechanism

Renewable wood sponge with lamellar structure, compressibility and three-dimensional porous frameworks exhibits excellent functionalization application potential in various fields. Herein, cobalt and nitrogen (Co/N) co-doped carbonized wood sponge (CoNCWS800) was prepared successfully for peroxymonosulfate (PMS) activation to degrade sulfamethoxazole (SMX). The CoNCWS800 material exhibited admirable catalytic activity in PMS activation to oxidize SMX molecules (99.7% within 60 min). Electron paramagnetic resonance (EPR) analysis, quenching tests and electrochemical experiments confirmed the existence of both radical (SO4·−,·OH and O2·−) and non-radical (1O2 and direct charge transfer) pathways during the SMX degradation process. Co species were verified as major contributors for the generation of multiple radicals via activating PMS. Surface defective structure and ketonic CO groups performed the positive linear correlation with reaction kinetic constants, revealing the critical role of the two active sites in PMS activation via non-radical process. This study provides a unique insight in PMS activation mechanism via both radical and non-radical pathways of wood sponge-based functional materials.

Tailoring collaborative N–O functionalities of graphene oxide for enhanced selective oxidation of benzyl alcohol

Benzaldehyde (BzH) is an important chemical for industrial feedstock. However, the conventional production processes heavily rely on precious metal catalysts and bring about unavoidable pollution issues. Here we report a new catalytic process to use N-doped graphene oxide (NGO) as the catalyst for oxidation of benzyl alcohol (BzOH) into BzH by an environmentally benign oxidant, peroxymonosulfate (PMS). Different approaches are developed to synthesize carbonylated NGO, which shows higher catalytic efficiency than CO-deficient NGO. The optimized catalyst could achieve 96% BzOH conversion and unprecedented 82% BzH yield under mild temperature (50 °C), superior to some precious metal catalysts reported. Experimental studies unveil the indispensable roles of pyridinic N and ketonic CO groups in forming an electron bridge between BzOH and PMS for non-radical oxidation, or separately inducing radical oxidation. The non-radical oxidation process is more selective in yielding BzH. Furthermore, the yielded BzH is free from over-oxidation into benzoic acid under various reaction conditions, because the radicals preferentially react with BzOH other than BzH and the BzH can hardly adsorb on graphene sheets, thus inhibiting its further oxidation. This work provides a cheap material and mechanic understanding of carbocatalysis for green organic synthesis with environmental and economic perspectives.

Robust tuning metal/carbon heterointerfaces via ketonic oxygen enables hydrogen evolution reaction outperforming Pt/C

Large-scale synthesis of metal/carbon hybrids with tunable metal/carbon heterointerfaces is vital for practical application of such hybrids in electrocatalysis. Herein, we developed a facile route for large-scale crafting of ultrafine Ru nanoparticles (NPs) anchored on the ketonic CO groups of carbon nanotubes (CNTs) (9.6 g in one batch). From both experimental results and theoretical calculations, we demonstrate that CO (rather than C–O) groups can be exploited to engineer the heterointerface of CNTs and Ru NPs. In fact, the electronic structure of Ru becomes considerably improved because the high polarity of CO facilitates the interface electron transfer of Ru/CNTs. Consequently, the obtained Ru-O-CNTs hybrids with low Ru loading of 1.5 wt% display a small overpotential of 25 mV at 10 mA cm−2 and with fast kinetics as deduced from a small Tafel slope of 20.4 mV dec−1 for hydrogen evolution reaction (HER) in 1 M KOH. Impressively, at 70 mV overpotential, the hybrids exhibit a record mass activity of 20.4 mA μg−1Ru, which is more than 50-fold of that for commercial Pt/C. Therefore, for the first time, we identified the vital role of CO groups for engineering metal/carbon heterointerfaces towards robust and efficient electrocatalysis.

Solvent effects in hydrodeoxygenation of furfural-acetone aldol condensation products over Pt/TiO2 catalyst

The solvent effects on hydrodeoxygenation (HDO) of 4-(2-furyl)-3-buten-2-one (F-Ac) over Pt/TiO2 catalyst were investigated at T=200°C and P(H2)=50bar. The initial reactant is the main product of aldol condensation between furfural and acetone, which constitutes a promising route for the production of bio-based chemicals and fuels. A sequence of experiments was performed using a selection of polar solvents with different chemical natures: protic (methanol, ethanol, 1-propanol, 2-propanol, 1-pentanol) and aprotic (acetone, tetrahydrofuran (THF), n,n-dimethylformamide (DMF)). In case of protic solvents, a good correlation was found between the polarity parameters and conversion. Consequently, the highest hydrogenation rate was observed when 2-propanol was used as a solvent. In contrast, the hydrogenation activity in presence of aprotic solvents was related rather to solvent-catalyst interactions. Thus, the initial hydrogenation rate declined in order Acetone>THF>DMF, i.e. in accordance with the increase in the nucleophilic donor number and solvent desorption energy. Regarding the product distribution, a complex mixture of intermediates was obtained, owing to the successive hydrogenation (aliphatic CC, furanic CC and ketonic CO bonds), ring opening (via CO hydrogenolysis) and deoxygenation reactions. Based on the proposed reaction scheme for the conversion of F-Ac into octane, the influence of the studied solvents over the cascade catalytic conversion is discussed. A significant formation of cyclic saturated compounds such as 2-propyl-tetrahydropyran and 2-methyl-1,6-dioxaspiro[4,4]nonane took place via undesirable side reactions of cyclization and isomerization. The best catalytic performance was found when using acetone and 2-propanol as solvents, achieving significant yields of 4-(2-tetrahydrofuryl)-butan-2-ol (28.5–40.4%) and linear alcohols (6.3–10.4%). The better performance of these solvents may be associated with a lower activation energy barrier for key intermediate products, due to their moderate interaction with the reactant and the catalyst. In case of methanol and DMF, undesired reactions between the reactant and the solvent took place, leading to a lower selectivity towards the targeted hydrodeoxygenated products.

Influence of waste cooking oil biodiesel on oxidation reactivity and nanostructure of particulate matter from diesel engine

The objective of this work is to investigate the influence of waste cooking oil biodiesel on oxidation reactivity and nanostructure of particulate matter (PM). The test was carried out in a small agricultural diesel engine. PM samples were collected by MOUDI and subjected to TGA, Raman, NEXAFS and TEM. According to TGA, when waste cooking oil biodiesel quantity increases in the fuel, PM shows a higher volatile organic fraction (VOF). When the blend ratio of biodiesel increases to 50%, lower ignition temperature and burnout temperature are observed, signifying higher oxidation reactivity. Raman spectroscopy shows a higher graphitic-like structure and a lower polyene-like structure for pure biodiesel compared to pure diesel. With the addition of biodiesel, the crystallite dimension of PM increases. C(1s) NEXAFS spectra shows that there are many types of functional groups on the surface of PM. Pure biodiesel PM has lower phenolic, aromatic COH and ketone CO groups than pure diesel and biodiesel/diesel blend PM samples. The analysis of TEM indicates the morphology of PM samples looks like chain shape. The primary particle coheres more closely and the average size of primary particles exhibits a decrease with the increase in biodiesel blend ratios.

Discovery of selective N-[3-(1-methyl-piperidine-4-carbonyl)-phenyl]-benzamide-based 5-HT₁ F receptor agonists: Evolution from bicyclic to monocyclic cores.

Preclinical experiments and clinical observations suggest the potential effectiveness of selective 5-HT1F receptor agonists in migraine. Identifying compounds with enhanced selectivity is crucial to assess its therapeutic value. Replacement of the indole nucleus in 2 (LY334370) with a monocyclic phenyl ketone moiety generated potent and more selective 5-HT1F receptor agonists. Focused SAR studies around this central phenyl ring demonstrated that the electrostatic and steric interactions of the substituent with both the amide CONH group and the ketone CO group play pivotal roles in affecting the adopted conformation and thus the 5-HT1F receptor selectivity. Computational studies confirmed the observed results and provide a useful tool in the understanding of the conformational requirements for 5-HT1F receptor agonist activity and selectivity. Through this effort, the 2-F-phenyl and N-2-pyridyl series were also identified as potent and selective 5-HT1F receptor agonists.

Efficient tuning of microstructure and surface chemistry of nanocarbon catalysts for ethylbenzene direct dehydrogenation

A facile and scalable approach to efficiently tune microstructure and surface chemical properties of N‐doped carbocatalysts through the controlled glucose hydrothermal treatment with diverse parameters and subsequent pyrolysis of pretreated carbonaceous materials with melamine (GHT‐PCM) was presented. Various characterization techniques including high resolution transmission electron microscopy (HRTEM), N2 adsorption desorption (BET), X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), Raman spectroscopy (Raman), and fourier transform infrared spectroscopy (FTIR) were employed to investigate the effect of prior GHT on the microstructure and surface chemical properties of N‐doped carbocatalysts, as well as to reveal the relationship between catalyst nature and catalytic performance in oxidant‐ and steam‐free direct dehydrogenation (DDH) of ethylbenzene for styrene production. It was found that the GHT process and its conditions significantly affect microstructure and surface chemical properties of the N‐doped carbocatalysts, which subsequently influences their catalytic performance in DDH reaction dramatically. Interestingly, the prior GHT can remove the carbon nitride layer formed on parent nanocarbon in the process of melamine pyrolysis, produce structural defects, and tune surface element component, through the “detonation” of polysaccharide coating on nanocarbon. The as‐prepared N‐doped CNT (M‐Glu‐CNT) by the established GHT‐PCM approach demonstrates higher catalytic performance (4.6 mmol g−1h−1 styrene rate with 98% selectivity) to the common N‐doped CNT (M‐CNT, 3.4 mmol g−1 h−1 styrene rate with 98.2% selectivity) as well as to pristine CNT (2.8 mmol g−1 h−1 styrene rate with 96.8% selectivity), mainly ascribed to increased structural defects, enriched surface ketonic CO groups, and improved basic properties from N‐doping on the M‐Glu‐CNT, those strongly depend on GHT conditions. The excellent catalytic performance of the developed M‐Glu‐CNT catalyst endows it with great potential for future clean production of styrene via oxidant‐ and steam‐free conditions. Moreover, the directed GHT‐PCM strategy can be extended to the other N‐doped carbonaceous materials with enhanced catalytic performance in diverse reactions by tuning their microstructure and surface chemistry. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2543–2561, 2015

Rhodium-Catalyzed Acyl-Transfer Reaction between Benzyl Ketones and Thioesters: Synthesis of Unsymmetric Ketones by Ketone CO–C Bond Cleavage and Intermolecular Rearrangement

In the presence of catalytic amounts of RhH(CO)(PPh3)3 and 1,2-bis(diphenylphosphino)benzene (dppBz), acyl groups were transferred between benzyl ketones and thioesters/aryl esters. The rhodium complex catalyzed the cleavage of ketone CO-C bonds and intermolecular rearrangement giving unsymmetric ketones. The acyl-transfer reaction also occurred with 1-(p-chlorophenyl)-3-(p-cyanophenyl)propane-2-one giving unsymmetric ketones.

Mechanistic Insights into the Rhodium-Catalyzed Intramolecular Ketone Hydroacylation

[Rh((R)-DTBM-SEGPHOS)]BF(4) catalyzes the intramolecular hydroacylation of ketones to afford seven-membered lactones in large enantiomeric excess. Herein, we present a combined experimental and theoretical study to elucidate the mechanism and origin of selectivity in this C-H bond activation process. Evidence is presented for a mechanistic pathway involving three key steps: (1) rhodium(I) oxidative addition into the aldehyde C-H bond, (2) insertion of the ketone CO double bond into the rhodium hydride, and (3) C-O bond-forming reductive elimination. Kinetic isotope effects and Hammett plot studies support that ketone insertion is the turnover-limiting step. Detailed kinetic experiments were performed using both 1,3-bis(diphenylphosphino)propane (dppp) and (R)-DTBM-SEGPHOS as ligands. With dppp, the keto-aldehyde substrate assists in dissociating a dimeric precatalyst 8 and binds an active monomeric catalyst 9. With [Rh((R)-DTBM-SEGPHOS)]BF(4), there is no induction period and both substrate and product inhibition are observed. In addition, competitive decarbonylation produces a catalytically inactive rhodium carbonyl species that accumulates over the course of the reaction. Both mechanisms were modeled with a kinetics simulation program, and the models were consistent with the experimental data. Density functional theory calculations were performed to understand more elusive details of this transformation. These simulations support that the ketone insertion step has the highest energy transition state and reveal an unexpected interaction between the carbonyl-oxygen lone pair and a Rh d-orbital in this transition state structure. Finally, a model based on the calculated transition-state geometry is proposed to rationalize the absolute sense of enantioinduction observed using (R)-DTBM-SEGPHOS as the chiral ligand.

Surface-Modified Carbon Nanotubes Catalyze Oxidative Dehydrogenation of n -Butane

Butenes and butadiene, which are useful intermediates for the synthesis of polymers and other compounds, are synthesized traditionally by oxidative dehydrogenation (ODH) of n-butane over complex metal oxides. Such catalysts require high O2/butane ratios to maintain the activity, which leads to unwanted product oxidation. We show that carbon nanotubes with modified surface functionality efficiently catalyze the oxidative dehydrogenation of n-butane to butenes, especially butadiene. For low O2/butane ratios, a high selectivity to alkenes was achieved for periods as long as 100 hours. This process is mildly catalyzed by ketonic CO groups and occurs via a combination of parallel and sequential oxidation steps. A small amount of phosphorus greatly improved the selectivity by suppressing the combustion of hydrocarbons.

The substitution chemistry of the tris(3,5-dimethylpyrazolyl)methanerhodium complex [Rh(CO)2{HC(pz′)3}]+

The complex [Rh(CO)2{HC(pz′)3}][PF6], 1+[PF6]− {HC(pz′)3 = tris(3,5-dimethylpyrazolyl)methane}, prepared by reacting [{Rh(CO)2(μ-Cl)}2] with HC(pz′)3 in the presence of Tl[PF6], has a distorted square pyramidal structure with a κ3-HC(pz′)3 ligand. Carbonyl substitution with Lewis bases gives [Rh(CO)L{HC(pz′)3}][PF6] {L = PPh3, 2+[PF6]−; L = AsPh3, 3+[PF6]−; L = P(o-tolyl)3, 4+[PF6]−}, which have square planar κ2 structures, confirmed by X-ray crystallography for 2+[PF6]−. The cations 2+ and 3+ have the third pyrazolyl ring orientated pseudo-parallel to the square planar metal whereas 4+ more likely has the third ring orientated exo to that plane. One-electron oxidation of 2+ and 3+ gives the Rh(II) dications [Rh(CO)(PPh3){HC(pz′)3}]2+, 22+, and [Rh(CO)(AsPh3){HC(pz′)3}]2+, 32+, characterised by ESR spectroscopy. Complex 1+[PF6]−reacts with PhCCPh to give [Rh(CO)(η2-PhCCPh){HC(pz′)3}][PF6], 5+[PF6]−, in which the two-electron donor alkyne occupies an equatorial position in a trigonal bipyramidal κ3 structure. With MeCCR (R = Me or Et), 1+[PF6]− gives the κ2 square planar complexes [Rh{η4-C4Me2R2C(O)}{HC(pz′)3}][PF6] (R = Me, 6+[PF6]−; R = Et, 7+[PF6]−) in which the cyclopentadienone ligands are coordinated via two Rh–monoalkene bonds; the structurally characterised form of 7+ has the two alkyne units linked head-to-head with the CEt termini bound to the ketonic CO group. With HCCPh or HCCH, 1+ gives the octahedral, κ3 rhodium(III) metallacyclopentadienes [Rh(CO)(η1:η1′-CHCRCHCR){HC(pz′)3}][PF6] (R = Ph, 8+[PF6]−; R = H, 9+[PF6]−) with the two alkynes linked head-to-tail in 8+. The reaction of 1+ with HCCH also gives the cycloheptatrienone (tropone) derivative [Rh{η4-C6H6C(O)}{HC(pz′)3}][PF6], 10+[PF6]−, with a κ3 ligand and the cycloheptatrienone ligand bound to the metal via two Rh–C σ-bonds and one Rh–monoalkene interaction.

Ruthenium(II)‐Catalyzed [2 + 2 + 2] Cycloaddition of 1,6‐Diynes with Tricarbonyl Compounds.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Kinetic and synthetic influences of water and solvent-free conditions on 1,3-dipolar cycloaddition reactions: the phthalazinium and pyridazinium dicyanomethanide 1,3-dipoles: surprisingly successful synthetic methodsElectronic supplementary information (ESI) available: sample kinetic graphs and optimised transition state structures. See http://www.rsc.org/suppdata/p2/b2/b206028k/

The influence of water on the kinetic and synthetic 1,3-dipolar cycloaddition reactions of phthalazinium-2-dicyanomethanide 1, and pyridazinium dicyanomethanide 2, with a wide range of dipolarophiles is reported. Water enhanced the rates of all reactions. The dipolarophiles were classified into two groups, water-normal and water-super. The former displayed rate enhancements of 45 times, but more often some hundred times, on changing the solvent from acetonitrile to water. A ketone CO conjugated to an alkene or alkyne constitutes a water-super dipolarophile. Esters, ethers, sulfones, nitriles and aryl rings conjugated to an alkene are water-normal dipolarophiles. The causes of these water effects are explored experimentally and with high level DFT. Hydrophobic effects and special hydrogen bonding interactions are the main factors involved. Synthetic implications are examined. Despite insolubility of the reactants in water successful high-yield reactions were achieved in water and under solvent-free conditions.

Intramolecular sensitisation of lanthanide(iii) luminescence by acetophenone-containing ligands: the critical effect of para-substituents and solvent

Tetraazamacrocyclic ligands have been prepared in which three of the four nitrogen atoms are functionalised with carboxylate donors and the fourth is alkylated with a para-substituted acetophenone group {–CH2C(O)C6H4–X, where X = H, OMe, NMe2}. The europium(III), gadolinium(III) and (for X = H) terbium(III) complexes of these potentially octadentate ligands have been prepared. The unsubstituted and methoxy-substituted ligands sensitise the europium(III) emissive state with high efficiency in aqueous solution. Calculation of the pure radiative lifetime, based on the contribution of the 5D0 → 7F1 transition to the total integrated emission intensity, has allowed an estimate of the efficiency of energy transfer to be made. Sensitisation of europium luminescence also occurs in the complex of the dimethylamino-substituted ligand but the process is highly solvent-dependent: very efficient sensitisation is observed in dichloromethane and DMSO, but the complex is non-emissive in water and only slightly emissive in acetonitrile. UV-Visible absorption spectra indicate that in all the solvents investigated, the carbonyl oxygen of the ligand is directly coordinated to the metal ion. The intensity of the hypersensitive 5D0 → 7F2 transition in the three complexes mirrors the degree of polarisation of the ketone CO bond.

Adsorption of Ethyl Pyruvate on Pt(111) Studied by XPS and UPS

The adsorption of ethyl pyruvate on Pt(111) at low temperature was investigated by XP and UP spectroscopy. The assignment of the photoelectron spectra was assisted by calculation of correlated ionization potentials. Comparison of the XP and UP spectra of the condensed and chemisorbed layer indicates a strong ethyl pyruvate adsorption bond in the latter. Upon chemisorption, the HOMO of ethyl pyruvate, which is a lone-pair orbital delocalized over both CO groups, is stabilized by about 0.7 eV with respect to the other orbitals, which is characteristic for a lone-pair bonding mechanism. The same bonding mechanism was found for coverages far below saturation. The XP spectra further indicate that the ketone CO is more strongly involved in the chemisorption bond than the carboxyl CO of ethyl pyruvate. The packing density of the saturated chemisorbed ethyl pyruvate layer, as determined by XPS, is high. This points toward an upright or tilted orientation of ethyl pyruvate in this layer, in line with the observed bonding mechanism.

Oxidative-Addition Reactions of Diiodine to Dinuclear Rhodium Pyrazolate Complexes.

The pyrazolato (Pz) rhodium(I) complexes [{Rh(&mgr;-Pz)(CO)(L)}(2)] (L = CNBu(t), P(OMe)(3), PMe(2)Ph, P(OPh)(3), P(p-tolyl)(3)) result from the reaction of [{Rh(&mgr;-Pz)(CO)(2)}(2)] with the appropriate L ligand in a trans:cis ratio ranging from 60:40 (L = CNBu(t)) to 95:5 (L = P(p-tolyl)(3)). The pure trans isomers add 1 molar equiv of diiodine to give the dirhodium(II) complexes [{Rh(&mgr;-Pz)(I)(CO)(L)}(2)] (L = CNBu(t) (6), P(OMe)(3) (7), PMe(2)Ph (8), P(OPh)(3) (9)). These complexes incorporate two iodide ligands trans to the rhodium-rhodium bond, as substantiated by the X-ray structure for 7, while the complex [(P{p-tolyl}(3))(CO)(I)Rh(&mgr;-Pz)(2)(&mgr;-CO)Rh(I)(P{p-tolyl}(3))] (10) contains a bridging ketonic CO ligand, due to the insertion of a terminal CO into the metal-metal bond. The metal-metal bond formation involves a 2e oxidation, since identical compounds (6-9) are obtained by oxidation with [Fe(Cp)(2)](PF(6)) followed by addition of potassium iodide. Further reactions of the dirhodium(II) complexes 6-9 with diiodine leading to the metal-metal rupture are electrophilic additions, as exemplified by the reactions with the positive iodine complex [I(Py)(2)](+). They start at the "endo site" (the metal-metal bond) if it is sterically accessible to the electrophile, to give directly the dirhodium(III) complexes [{Rh(&mgr;-Pz)(I)(CO)(L)}(2)(&mgr;-I)](+) (L = CNBu(t), CO). Otherwise, as for the complexes with P-donor ligands, abstraction of a iodide ligand trans to the metal-metal bond (the "exo site") occurs first, to give the dirhodium(II) cationic complexes [(PR(3))(CO)(I)Rh(&mgr;-Pz)(2)Rh(CO)(PR(3))](+) and triiodide. These react again with diiodine to give dirhodium(III) complexes [{Rh(&mgr;-Pz)(I)(CO)(PR(3))}(2)(&mgr;-I)](+) similar to those described above, but with triiodide or pentaiodide as counterion, as substantiated by the X-ray structure of [{Rh(&mgr;-Pz)(I)(CO)(PMe(2)Ph)}(2)(&mgr;-I)]I(5) (18). The diiridium(II) complexes [{Ir(&mgr;-Pz)(I)(CO)(PR(3))}(2)] (PR(3) = P(OPh)(3), PMe(2)Ph) also react with diiodine to give the cationic diiridium(III) complexes [{Ir(&mgr;-Pz)(I)(CO)(PR(3))}(2)(&mgr;-I)]I(3) through a reaction pathway involving the "exo site", while no reaction is observed for [{Ir(&mgr;-Pz)(I)(CO)(2)}(2)]. Finally, replacement of a carbonyl ligand in [{Rh(&mgr;-Pz)(I)(CO)(L)}(2)(&mgr;-I)](+) (L = CNBu(t), CO) by iodide gives the compounds [(CO)(L)(I)Rh(&mgr;-Pz)(2)(&mgr;-I)Rh(I)(2)(L)].