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Copper(II) complexes with N‐oxyl reactants such as TEMPO can selectively oxidize alcohols to aldehydes or ketones. The proposed copper intermediates of the oxidation reaction were extensively theoretically studied, but they were never experimentally detected. Here, we present an analysis of "frozen" intermediates that contain alcohols without α‐hydrogen atoms, thus preventing oxidation. The copper(II)‐TEMPO complexes with a bipyridine‐type ancillary ligand were isolated by electrospray ionization mass spectrometry and investigated spectroscopically by cryogenic photodissociation spectroscopy. The vibrational characteristics of the complexes suggest that TEMPO retains its unpaired electron even upon coordination with copper(II). In agreement, the electronic photodissociation spectra of the TEMPO‐copper(II) complexes show a characteristic band for gaseous copper(II) complexes. These results contradict interpretations of some previous density functional theory (DFT) analyses of possible reaction intermediates. The experimental data were confronted with theoretical results obtained by pure DFT (OPBE, TPSS) and hybrid DFT (TPSSH, B3LYP) calculations. The methods favor different ground states and capture various aspects of the experimental results demonstrating a multiconfigurational character of the copper(II)‐TEMPO complexes. [ABSTRACT FROM AUTHOR]

Reaction monitoring by electrospray ionization mass spectrometry (ESI‐MS) is a popular method for investigation of reaction mechanisms. Here, we present a new approach based on a coupling between a modular capillary flow reactor and ESI‐MS. The flow reactor allows a sequential adding of the reactants. We demonstrate the approach for oxidation of Ph2S by ArIO catalyzed by [(TPA)Fe(TfO)2] (ArIO=2‐(tBuSO2)C6H4IO, TPA=tris(2‐pyridylmethyl)amine, TfO−=triflate). In steps, we could follow the formation of the reactive iron(IV)oxo complexes, then the oxygen transfer reaction from the iron(IV)oxo to the sulfide, and finally the reoxidation of the iron complexes. The flow reactor also allows kinetic experiments by changing relative concentrations of the reactants. We could analyze in detail the formation of various [(TPA)FeIVO(X)]2+/+ complexes (X=MeCN, ArI, ArIO, TfO−) and compare their relative reactivity with Ph2S. The [(TPA)FeIVO(MeCN)]2+ complex is the most reactive complex for the oxygen atom transfer reaction whilst [(TPA)FeIVO(ArIO)]2+ prevails in solution under catalytic conditions. [ABSTRACT FROM AUTHOR]

Electrospray ionization mass spectrometry (ESI-MS) is becoming an important tool for mechanistic studies in organic and organometallic chemistry. It allows investigation of reaction mixtures including monitoring of reactants, products, and intermediates, studying properties of the intermediates and their reactivity. Studying the reactive species in the gas phase can be advantageously combined with theoretical calculations. This review is focused on ESI-MS studies of copper-catalyzed reactions. Possible effects of the electrospray process on the transfer of the copper complexes to the gas phase are discussed. The plethora of mass spectrometric approaches is demonstrated on copper mediated C-H activations, cross coupling reactions, rearrangements, organocuprate chemistry, and other examples. [ABSTRACT FROM AUTHOR]

Electrospray ionization/mass spectrometry (ESI-MS) is used to monitor a Cu-catalyzed aerobic between thiol esters and arylboronic acids. The ESI spectra show the formation of Cu-complexes with the starting thiol ester 1 and the coupling product. The formation of an ionic complex at m/z 305 is observed, most likely occurring upon the elimination of a mixed anhydride from [(1)CuOAc]+. An online monitoring of the reaction using ESI-MS was carried out allowing calculation of rate constants and thermodynamic parameters (ΔH‡, ΔS‡, and ∆G‡) for the title reaction. [ABSTRACT FROM AUTHOR]

Reductive eliminationof ethane from the palladium(IV) complex[PdMe3(bpy)I] (bpy = 2,2′-bipyridine) is studiedby electrospray ionization mass spectrometry. Palladium(IV) complexescan be detected as binuclear clusters [Pd2Me6I(bpy)2]+or as complexes [PdMe3(bpy)(L)]+stabilized by an electron-donating ligand L.Fragmentation of all palladium(IV) complexes is dominated by eliminationof ethane which corresponds to the reductive elimination couplingof the methyl groups. The associated energy demands for differentcomplexes reveal that the mononuclear complexes with poorly electron-donatingligands provide the fastest reaction. [ABSTRACT FROM AUTHOR]

Electrospray ionization of uranyl nitrate dissolved in water generates gaseous species containing either hydroxo-uranyl [UO[sub2](OH)][sup+] or nitrato-uranyl [UO[sub2](NO[sub3])][sup+] contract ion pairs solvated by up to four water molecules. Furthermore, uranyl clusters of the general type [U[subm]O[sub2m](X,Y)[sub2m-1](H[sub2]O)[subn]][sup+] (X,Y = OH, NO[sub3]) with m = 1-5 and n = 2-4 are formed. Collision-induced dissociation experiment are used to probe the structures and the stoichiometries of the uranyl ions generated. A detailed investigation of the concentration-dependent behavior of the formed gaseous uranyl complexes reveals a preference for nitrate- over hydroxide-containing species with increasing concentration of the sprayed solution. This behavior reflects changes in the pH value of the bulk solutions that can be attributed to solvolysis of UO[sub2][sup2+] in water. Further, the tendency for generation of polynuclear cluster ions is amplified with increasing concentration and can be explained by a mechanism which involves the association of cations present in solution with neutral species such as UO[sub2](OH)[sub2], UO[sub2](OH)(NO[sub3]), and UO[sub2](NO[sub3])[sub2]. The observed dependences of the cluster-ion intensities in the mass spectra from the concentration of the solutions fed to the electrospray source are used to suggest a scheme for a quantitative correlation between the gas-phase and solution-phase data. The results inter alia indicate that the effective concentrations of the spraying solution can be several orders of magnitude larger than those of the feed solutions entering the electrospray ionization source. [ABSTRACT FROM AUTHOR]

The complexation of copper(II) to acridine derivatives has been studied by means of electrospray ionization (ESI) mass spectrometry. Under soft conditions of ionization, the ESI mass spectra of methanolic solutions of copper(II) chloride and the acridine ligands show abundant signals of the mononuclear complexes formed from the metal and ligand. Depending on the position of the N-benzoylamino substituent in the acridinic heterocycle, however, the copper atom involved in the complexation process adopts different oxidation states in the resulting cations. Hence, the metal is reduced to copper(I) in the monocationic complex with the compound substituted in position 2, whereas it keeps its divalent state in the monocation formed with the compound substituted in position 4. As a consequence, the regioisomers lead to monocations with different masses in the ESI spectra. In order to understand this unusual behavior of two isomeric compounds, additional experiments have been performed with quinoline as a model. Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Electrospray ionization (ESI) of aqueous iron(III) nitrate solutions can be used as an easy route for the generation of bare iron-oxide and hydroxide cations such as FeO+, FeOH+, and Fe(OH)2 + in the gas phase. Analysis of the ESI mass spectra obtained under variable conditions in combination with complementary collision experiments suggest that the iron-oxo species are formed by collision-energy driven replacements of nitrato-ligands by water stemming from the sprayed solution. It is demonstrated that the gas-phase reactivity of methane with the iron-oxide cation FeO+ generated via ESI is in good agreement with the well-documented literature data for this reaction. [Copyright &y& Elsevier]

Highlights from the article: B The Cover Feature b shows spectro-scopic probing of intermediates in lanthanide catalyzed Knoevenagel condensation. The lanthanide reaction complexes were fished out from the reaction mixture by electrospray ionization mass spectrometry. The lanthanide reaction complexes were fished out from the reaction mixture by electrospray ionization mass spectrometry.

Despite the simplicity of the molecule, the site of single deprotonation of p-hydroxybenzoic acid upon electrospray ionization (ESI) has recently formed a subject of debate in this journal. By means of NMR experiments in solution and gas-phase studies employing ion-mobility mass spectrometry (IM-MS), the apparent controversy is resolved. It is shown that irrespective of the solvent the carboxylate tautomer is preferred in solution, while the opposite holds true for isolated ions in the gas phase. The tautomer distribution sampled in the gas phase very much depends on the actual solvent used in ESI, the pH value, as well as the total concentration. Moreover, the occurrence of gas-phase reactions in the course of the ESI process influences the tautomer ratio. Implications for correlations between ESI mass spectra and solution-phase chemistry are discussed. [ABSTRACT FROM AUTHOR]

Complexes of MnII with 1,10-phenanthroline (phen) and 2,2′-bipyridine (bipy) are investigated by means of electrospray ionization (ESI) mass spectrometry. Under the conditions used, [MnLn]2+ with n = 2 and 3, [MnLnCl]+ with n = 0–2, and [Mn2LnCl3]+ with n = 2 and 3 are produced (where L = phen or bipy). The collision-induced dissociation (CID) spectra of the mass-selected ions show various dissociation pathways, most notable among them is the reduction of the ligated MnII to MnII by intracomplex electron transfer. CID experiments of mixed-ligand complexes formed upon ESI from solutions which contain both phen and bipy exhibit preferential eliminations of bipy, indicating that bipy is a significantly weaker ligand for MnII than phen. This effect is mainly attributed to the flexibility of the bipy ligand concomitant with thermodynamic control in ion dissociation. To support this hypothesis, mixed complexes with some methylated derivatives as well as those containing 4,5-diazafluorene (daf) are examined also. Interestingly, the differences between the ligands diminish in charge-separation reactions of dicationic MnII complexes, due to the joined operation of thermodynamic as well as kinetic effects. In addition, the complexes [Mn(bipy)]+, [Mn(phen)]+, [Mn(bipy)]2+, [Mn(phen)]2+, and [Mn(bipy)(phen)]2+ are computed using the mPW1PW91 hybrid density functional along with the Stuttgart-Cologne-type pseudopotential and basis-set suite, and relative energies for charge-separation reactions and losses of neutral ligands are evaluated. [ABSTRACT FROM AUTHOR]

We report a new method for determination of rate constants of processes in solution using electrospray ionization mass spectrometry (ESI-MS). The investigated reaction is C-H activation of acetanilides by palladium(II)trifluoroacetate leading to stable organopalladium complexes. The rate constants can be determined from an experiment with a couple of differently substituted acetanilides being in competition for being activated by the palladium salt. The formed organopalladium complexes can be detected by ESI-MS. The time dependence is achieved by adding one of the acetanilides to the reaction mixture with a time delay. The kinetics can be then evaluated from the evolution of the ratio of the ESI-MS signals of differently substituted complexes as a function of the time delay. The Hammett analysis of the rate constants obtained for a series of meta- and para-substituted acetanilides provides a ρ value of -1.5, which is in agreement with values reported for similar C-H activations. We have also investigated the very same reaction with UV-vis spectroscopy that gave us about three times smaller rate constants but the same trend with the ρ value of -1.6. The rate constants determined by ESI-MS are directly linked to the occurrence of organopalladium complexes, whereas the UV-vis data are associated with an absorption spectra change that could involve more reaction steps. DFT calculations support the interpretation of the reaction mechanism as cyclopalladation and provide the ρ value in the same range. The rate-determining step corresponds to the agostic C-H transition structure. [ABSTRACT FROM AUTHOR]

A new method to investigate the reaction kinetics of intermediates in solution by electrospray ionization mass spectrometry is presented. The method, referred to as delayed reactant labeling, allows investigation of a reaction mixture containing isotopically labeled and unlabeled reactants with different reaction times. It is shown that we can extract rate constants for the degradation of reaction intermediates and investigate the effects of various reaction conditions on their half-life. This method directly addresses the problem of the relevance of detected gaseous ions toward the investigated reaction solution. It is demonstrated for geminally diaurated intermediates formed in the gold mediated addition of methanol to alkynes. Delayed reactant labeling allows us to directly link the kinetics of the diaurated intermediates with the overall reaction kinetics determined by NMR spectroscopy. It is shown that the kinetics of protodeauration of these intermediates mirrors the kinetics of the addition of methanol demonstrating they are directly involved in the catalytic cycle. Formation as well as decomposition of diaurated intermediates can be drastically slowed down by employing bulky ancillary ligands at the gold catalyst; the catalytic cycle then proceeds via monoaurated intermediates. The reaction is investigated for 1-phenylpropyne (Ph-CC-CH3) using [AuCl(PPh3)]/AgSbF6 and [AuCl(IPr)]/AgSbF6 as model catalysts. Delayed reactant labeling is achieved by using a combination of CH3OH and CD3OH or Ph-CC-CH3 and Ph-CC-CD3. [ABSTRACT FROM AUTHOR]

Highlights: [•] Ca2+ interactions with DMSO in water environment using ESI-MS and DFT calculations. [•] The number of DMSO ligands in [Ca(DMSO) x ]2+ depends on the temperature. [•] With increasing capillary temperature the number of DMSO ligands decreases. [•] With increasing capillary temperature water molecules substitute the DMSO ligands. [•] The bonding energy of DMSO to Ca2+ is strongly influenced by the presence of water. [Copyright &y& Elsevier]

Abstract: Structure and unimolecular reactivity of protonated para-aminophenol generated by the electrospray ionization were studied by means of mass spectrometry, IRMPD spectroscopy and DFT calculation. It is revealed that protonation of para-aminophenol takes place at the nitrogen atom. Under the conditions of the IRMPD experiment, thermodynamically favored elimination of the water molecule is observed, whereas under the CID experiment kinetically favored elimination of ammonia prevails. The IRMPD spectrum as well as all fragmentation pathways are rationalized based on DFT calculations. [Copyright &y& Elsevier]

By means of selective deuterium labeling combined with separation of enantiomers, chiral discrimination in the proton-bound dimers of 1-aza[6]helicene is probed by electrospray mass spectrometry. The analysis of the results reveals a pronounced preference for the formation of homochiral dimers (P,P and M,M) over the heterochiral variant (P,M). [ABSTRACT FROM AUTHOR]