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Copper(II) acetate is frequently used as a catalyst for bond-forming reactions in organic synthesis. Unlike aqueous solutions, in which complete heterolysis to CuOAc+(aq) and AcO–(aq) prevails at low concentrations, it is clear that copper(II) acetate shows a large degree of aggregation in typical organic solvents. Here, the speciation behavior of Cu(OAc)2 in organic solvents is probed by electrospray ionization mass spectrometry (ESI-MS), which reveals an extensive clustering of copper acetate species to form ions of the general composition [Cun(X)2n–1]+ and [Cun(X)2n+1]– (X = OAc and OCH3), along with (solvated) monomeric species such as [Cu(OAc)(CH3OH)n]+. The ESI-MS measurements are complemented by solution-phase studies of Cu(OAc)2 by electron paramagnetic resonance spectroscopy, which support an extensive aggregation of Cu(OAc)2 in organic solvents. Collisional experiments reveal that either simple degradation or redox processes can be observed, depending on the coordinative saturation of the copper clusters. The formation of the copper clusters can be efficiently suppressed by contamination of the organic solvents by a small amount of water.

Electrospray ionization/mass spectrometry (ESI-MS) is used to monitor a Cu-catalyzed aerobic cross-coupling reaction 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.

Tebuconazole ( 1 ) belongs to a class of widely utilized plant fungicides, mostly in vineyards. Consequently, traces of this compound are present in plant tissues, wine as well as in soil. The aim of this study was in situ generation of Cu cations and creation of very stable Cu complexes with 1 . The proposed method involves a hyphenation of electrochemistry with electrospray ionization mass spectrometry (ESI-MS), based on the high affinity of 1 for dissolved copper cations. We demonstrate that at appropriate potentials (above 500 mV vs. silver reference electrode) Cu cations can be generated in an electrochemical cell, which subsequently form complexes with 1 present in the sample. The resulting complex [Cu( 1 )] + as well as its fragment [Cu( 1 -C 4 H 10 )] + are then detected with online-coupled ESI-MS. This finding is useful for a creation of an analytical method based on the detection of trace amounts of 1 in complex matrices (e.g., soil solution), in which the analyte may be spread among various species.

The reaction mechanism of the intermolecular oxidative formation of pyrazole 2 via a C–C/N–N bond-formation cascade of the enaminone 1 is investigated by means of ESI-MS. No direct reaction intermediates are observed; however, the formation of an unexpected imidazolid-3-one derivative X is observed that involves an oxidative dimerization of 1 in the presence of dioxygen. The derivative X is isolated and characterized by means of multidimensional 1H and 13C NMR spectroscopy.

Electrospray ionization of dilute solutions of phenylpyridines (phpy) in the presence of nickel(II) chloride leads to gaseous ions of the type [Ni(phpy)(m)](2+) with m = 3-5 and [NiCl(phpy)(n)](+) with n = 1-3, which are characterized by various gas-phase experiments in combination with calculations using density functional theory. Of the regioisomeric phpy's, 2-phpy behaves drastically different compared to 3- and 4-phpy. Ion mobility mass spectrometry allows a differentiation of the gaseous ions and an elucidation of characteristic properties of the metal complexes. For 2-phpy, C-H bond activation in the [NiCl(phpy)(2)](+) complex is significant, whereas this route is almost suppressed for the corresponding complexes of 3- and 4-phpy and only occurs at elevated energies.

RATIONALE Heavy metals are both a problem for the environment and an important resource for industry. Their selective extraction by means of organic ligands therefore is an attractive topic. The coordination of three thiacrown ethers to late 3d-metal ions was investigated by a combination of electrospray ionization mass spectrometry (ESI-MS) and electron paramagnetic resonance (EPR). METHODS The mass spectrometric experiments were carried out in an ion trap mass spectrometer with an ESI source. Absolute binding constants were estimated by comparison with data for 18-crown-6/Na(+). EPR spectroscopy was used as a complementary method for investigating the Cu(I) /Cu(II) redox couple. RESULTS The study found that thiacrown ethers preferentially bind traces of copper even at an excess of other metal ions (Co(II), Ni(II), and Zn(II)). The absolute association constants of the Cu(I) complexes were about 10(8) M(-1), and about two orders of magnitude lower for the other 3d-metal cations. The EPR spectra demonstrated that the reduction from Cu(II) to Cu(I) upon formation of the [(thiacrown)Cu](+) species takes place in solution. CONCLUSIONS ESI-MS demonstrated that the three thiacrown ligands examined had high binding constants as well as good selectivities for copper(I) at low concentrations, and in the presence of other metal ions. By a combination of ESI-MS and EPR spectrometry it was shown that the reduction from Cu(II) to Cu(I) occurred in solution.

Experimental data on the reactions of argon with BF n + monocations ( n = 0–3) and BF n 2+ dications ( n = 1, 2) and dications reveal the formation of the new argon species ArBF 2+ and ArBF 2 + . Theoretical calculations on the stability of these ions using density functional theory confirm the formation of the rare-gas compounds, but their formation is only exothermic for the reaction Ar + BF 2 2+ → ArBF 2+ + F. According to theory, the corresponding neon compounds all involve endothermic pathways and could hence only be formed via involvement of excited states.

Electrospray ionization was used as a technique for the characterization of the interactions between cadmium(II) ions and malic acid (1) in aqueous solution. Particular attention was paid to the nature of the species formed, which generally correspond to complexes of CdX(+) cations with neutral malic acid, where X either is the counterion of the metal salt used as a precursor (i.e. X=Cl, I) or corresponds to singly deprotonated malic acid. In pure water solutions, also highly coordinated complexes [Cd(1-H)(1)(2)](+) and [CdCl(1)(2)](+) were detected, whereas the most abundant complexes detected in a sample of soil solution were: [Cd(1-H)(1)](+) and [CdCl(1)](+). With respect to possible application in environmental analysis, the effects of (i) metal salts present in solution, (ii) modest mineralization, and (iii) the matrices of real soil solutions were probed. While the presence of other metals leads to additional complexes, the characteristic species containing both cadmium(II) and malic acid can still be detected with good sensitivity.

The microhydrated cations [Co(NO 3 )(H 2 O) n ] + ( n = 2, 3) generated via electrospray ionization of aqueous cobalt(II) nitrate are studied by means of infrared-multiphoton dissociation (IRMPD) spectroscopy in an ion-trap mass spectrometer (IT-MS) at the free electron laser CLIO. While [Co(NO 3 )(H 2 O) 3 ] + shows a simple photo-induced fragmentation upon IRMPD, the situation is more complex for [Co(NO 3 )(H 2 O) 2 ] + because during storage in the IT-MS the ion associates with background water to afford [Co(NO 3 )(H 2 O) 3 ] + , and both species adsorb IR photons in the same range. By means of kinetic modeling, it is demonstrated that also in such a more complicated situation useful insights can be achieved. For all [Co(NO 3 )(H 2 O) n ] + cations studied ( n = 0–3) in parallel theoretical studies, a bidentate coordination of the nitrato ligand is preferred, and this conclusion finds support by a comparison of the computed and experimental IR spectra. In addition, the binuclear cluster [Co 2 (NO 3 ) 3 (H 2 O) 3 ] + is probed by means of IRMPD.

The formation of complexes between tebuconazole (Teb) and cadmium in simplified model solutions as well as soil solutions was studied using electrospray ionization mass spectrometry. Teb and cadmium form two types of complexes with the general formulas [Cd(Teb)n]2+ (n = 1–4) and [CdI(Teb)m]+ (m = 1–3), where iodine corresponds to the counterion used. The most intense Teb/cadmium complex is [CdI(Teb)2]+, and the most stable one is [Cd(Teb)(Teb − H)]+. Another detected complex, the dication [Cd(Teb)4]2+, was considered as the origin complex for the iodine-free complexes and was found in a sample prepared from forest soil solution naturally contaminated with cadmium ions.

Electrospray ionization mass spectrometry (ESI-MS) is used for monitoring the progress of the Suzuki–Miyaura cross-coupling reaction between bromobenzene and arylboronic acids using a palladium dichloro-bis(aminophosphine) complex as a precatalyst. The ESI-MS studies demonstrate that initiation of the catalytic reaction requires the presence of a base and that the selectivity for cross-coupling is favorable at elevated temperatures. Interestingly, after completion of the coupling reaction, the palladium is present as a dinuclear Pd0/PdII cluster, which still acts as an active catalyst when a new aliquot of reactants is added to the reaction mixture.

Electrospray ionization mass spectrometry (ESI-MS) is used to probe the complex formation between tebuconazole (1) and copper(II) salts, which both are commonly used fungicides in agriculture. Experiments with model solutions containing 1 and CuCl2 reveal the initial formation of the copper(II) species [(1)CuCl]+ and [(1)2CuCl]+ which undergo reduction to the corresponding copper(I) ions [(1)Cu]+ and [(1)2Cu]+ under more drastic ionization conditions in the ESI source. In additional experiments, copper/tebuconazole complexes were also detected in samples made from soil solutions of various origin and different amount of mineralization. The direct sampling of such solutions via ESI-MS is thus potentially useful for understanding of the interactions between copper(II) salts and tebuconazole in environmental samples. Copyright © 2011 John Wiley & Sons, Ltd.

The bimolecular reactivity of xenon with C(7)H(n)(2+) dications (n=6-8), generated by double ionization of toluene using both electrons and synchrotron radiation, is studied by means of a triple-quadrupole mass spectrometer. Under these experimental conditions, the formation of the organoxenon dications C(7)H(6)Xe(2+) and C(7)H(7)Xe(2+) is observed to occur by termolecular collisional stabilization. Detailed experimental and theoretical studies show that the formation of C(7)H(6)Xe(2+)+H(2) from doubly ionized toluene (C(7)H(8)(2+)) and xenon occurs as a slightly endothermic, direct substitution of dihydrogen by the rare gas with an expansion to a seven-membered ring structure as the crucial step. For the most stable isomer of C(7)H(6)Xe(2+), an adduct between the cycloheptatrienyldiene dication and xenon, the computed binding energy of 1.36 eV reaches the strength of (weak) covalent bonds. Accordingly, electrophiles derived from carbenes might be particularly promising candidates in the search for new rare-gas compounds.

Electrospray ionization of an aqueous solution of nickel(II) sulfate provides direct experimental evidence for the formation of triple ions of the type [Ni(2)(SO(4))(H(2)O)(n)](2+) and [Ni(SO(4))(2)](2-), whose existence in aqueous solution has previously been proposed based on relaxation spectroscopy [Chen et al. J. Sol. Chem. 2005, 34, 1045]. Formally, these triple ions are formed by aggregation of the solvated ions Ni(2+) and SO(4)(2-), respectively, with the neutral ion pair NiSO(4). In addition, also higher adducts are observed, e.g. the "pentuple ions" [Ni(3)(SO(4))(2)(H(2)O)(n)](2+) (n = 7-9) and [Ni(2)(SO(4))(3)](2-), of which the dicationic is extensively hydrated, whereas the anionic is not. The structures of the dinuclear nickel clusters are derived from ab initio calculations and their infrared spectra are compared with experimental data obtained for the gaseous ions [Ni(2)SO(4)(H(2)O)(5)](2+) and [Ni(2)(SO(4))(3)](2-), respectively. The calculations show that the structures are crucially controlled by the degree of solvation of nickel ion. Explicit consideration of solvating water molecules within the first coordination sphere suggest that the dicationic triple ion [Ni(2)SO(4)](aq)(2+) is bent and thus bears a permanent dipole moment, whereas the [Ni(SO(4))(2)](aq)(2-) dianion tends to be quasi-linear. The experimental and theoretical data for the gaseous ions thus support the elegant, but indirect, deductions previously made based on solution-phase studies.

Mono- and dications of microsolvated nickel complexes of acetonitrile are probed by means of ion-mobility mass spectrometry. Specifically, the complexes [(CH3CN)nNi]+, [(CH3CN)nNi]2+, [(CH3CN)nNiOH]+, and [(CH3CN)nNiCl]+ (n = 0–6) are compared to each other and their reactions with background water are probed. In general, the arrival times of the ions in the ion-mobility experiment linearly increase with the mass-to-charge ratio, but for the smaller, more reactive complexes, the arrival times are notably larger than expected from their mass. This effect is attributed to the markedly larger reactivity of these particular ions, as reflected in both charge-separation processes as well as adduct formation upon interaction with background water.

The gas-phase reactivity of dications generated by dissociative electron ionization of several aromatic CmHnNo precursors with 4 ≤ m ≤ 13, 4 ≤ n ≤ 21, and 0 ≤ o ≤ 2 with rare gases is investigated. Whereas most of these reactions lead to monocations via simple electron transfer, proton transfer, or Coulomb explosion, the formation of organo rare-gas dications is observed in a few cases. Specifically, dications generated from 2,4,6-trimethylpyridine react with krypton and xenon to form organo rare-gas species as major products and under maintenance of the two-fold positive charge. Such a reactivity is not observed in the presence of lighter rare gases. The formation of organo rare-gas dications are also observed for dications generated from 3-vinylpyridine, N,N-dimethylaniline, isopropylbenzene, and 4-ethyltoluene as neutral precursors. In some cases, isomeric dications are characterized by very different reactivity toward rare gases, suggesting that the structure of the precursors is crucial and that electron ionization does not lead to a total scrambling of the structures of the doubly charged ions obtained.

Mono- and binuclear complexes of N,N-dimethylformamide (DMF) with chlorides of the divalent, late 3d metals M = Co, Ni, Cu, and Zn are investigated by means of electrospray ionization (ESI). Specifically, ESI leads to monocations of the type [(DMF)(n)MCl](+) and [(DMF)(n)M(2)Cl(3)](+), of which the species with n = 2 and 3 were selected for in-depth studies. The latter include collision-induced dissociation experiments, gas-phase infrared spectroscopy, and calculations using density functional theory. The mononuclear complexes [(DMF)(n)MCl](+) almost exclusively lose neutral DMF upon collisional activation with the notable exception of the copper complex, for which also a reduction from Cu(II) to Cu(I) concomitant with the release of atomic chlorine is observed. For the dinuclear clusters, there exists a competition between loss of a DMF ligand and cluster degradation via loss of neutral MCl(2) with decreasing cluster stability from cobalt to zinc. For the specific case of [(DMF)(n)ZnCl](+) and [(DMF)(n)Zn(2)Cl(3)](+), ion-mobility mass spectrometry indicates the existence of two isomeric cluster ions in the case of [(DMF)(2)Zn(2)Cl(3)](+) which corroborates parallel theoretical predictions.

Electrochemical studies of the newly synthesized bis(triphenyl phosphine) ruthenium(II) complex, cis-[RuCl2(L)(PPh3)2] (1, with L = 2-(2′-pyridyl)quinoxaline, C13N3H9), were performed in acetonitrile (ACN). For this purpose, cyclic voltammograms (CVs) as well as electrochemical impedance spectra (EIS) were recorded on either glassy carbon (GC), platinum (Pt), gold (Au), or multi-walled carbon nanotube (MWCNT) electrodes. Qualitative examination of solutions of 1 in ACN was performed on the basis of conductivity measurements and electrospray ionization mass spectrometry (ESI–MS). The conductivity data suggest that 1 is a 1 : 1 type electrolyte in ACN. The ESI spectra further demonstrate that upon dissolution of 1 in ACN progressive replacement of chloro- and PPh3-ligands by ACN occurs, leading to formation of [RuCl(L)(PPh3)(CH3CN)2]+Cl−, [2 + Cl − ]. The CVs recorded for [2 + Cl − ] on various working electrodes demonstrate that the reversibility of the redox couple 22 +/+ enhances with the order: Au < Pt ...

The reactivity of molecular dications C(m)H(n)N(2+), generated by (dissociative) double ionization of different heteroaromatic compounds, with methane, ethane, and nitrogen is investigated using a triple-quadrupole mass spectrometer. About a quarter of the set of dications studied undergo carbon-carbon coupling reactions with methane, whereas only nonspecific reactions leading to the formation of monocations (such as proton-, electron- and hydride transfer as well as Coulomb explosions) occur in the presence of either ethane or nitrogen. Here, we report the reactivity observed for 42 different C(m)H(n)N(2+) dications formed via electron ionization of eleven different heteroaromatic precursors. The potential relevance of these results is discussed in the context of the growth of hydrocarbon species and their nitrogen-containing homologues at extremely low temperatures and pressures, such as in interstellar environments or the ionospheres of planets and moons, Saturn's moon Titan in particular.

The C-H bond activation of small alkanes by the gaseous MgO(+*) cation is probed by mass spectrometric means. In addition to H-atom abstraction from methane, the MgO(+*) cation reacts with ethane, propane, n- and iso-butane through several pathways, which can all be assigned to the occurrence of initial C-H bond activations. Specifically, the formal C-C bond cleavages observed are assigned to C-H bond activation as the first step, followed by cleavage of a beta-C-C bond concomitant with release of the corresponding alkyl radical. Kinetic modeling of the observed product distributions reveals a high preference of MgO(+*) for the attack of primary C-H bonds. This feature represents a notable distinction of the main-group metal oxide MgO(+*) from various transition-metal oxide cations, which show a clear preference for the attack of secondary C-H bonds. The results of complementary theoretical calculations indicate that the C-H bond activation of larger alkanes by the MgO(+*) cation is subject to pronounced kinetic control.

Thermal reactions of the CF32+ dication with argon lead to the formation of an ArCF22+ dication, a new type of metastable species with an argon−carbon bond. None of the other rare gases undergo a similar reaction with CF32+. For the lighter rare gases (He and Ne), no reactions with CF32+ other than those due to electronically excited reactant ions are observed, whereas for the heavier rare gases (Kr and Xe), the prevailing reactive pathways involve single-electron transfer. At elevated collision energies, single-electron transfer predominates for collisions with all rare gases (He−Xe).

The ion/molecule reaction (IMR) of “roll-over” cyclometalated [Pt(bipy−H)] + ( 1 ) (bipy = 2,2′-bipyridine) with dimethyl sulfide ( 2 ) leads to the loss of ethene in the main reaction channel (55%); this corresponds to a formal dehydrosulfurization accompanied by the dehydrogenative (“oxidative”) C–C coupling of the two methyl groups in dimethyl sulfide to form ethene. In order to obtain deeper insight into this rare reaction, the IMRs of 1 with several symmetric, asymmetric as well as cyclic thioethers and some thiols are probed. Common to many substrates is the formal transfer of H 2 S from the organic sulfur compound to the [Pt(bipy−H)] + core accompanied by the expulsion of one or more alkene units. In contrast to the behavior of [Pt(bipy−H)] + ( 1 ), bare Pt + , diatomic PtH + , and also the complex [Pt(phpy−H)] + (phpy = 2-phenyl pyridine) are not able to bring about dehydrosulfurization. It is accordingly suggested that the (bipy−H)-ligand does not only affect the electronic structure of the platinum(II) core but, moreover, plays an active role as an acceptor in the initial hydrogen transfer from the thioether ligand to the LPt + core (L = heterocyclic ligand).

Electrospray ionization (ESI) of aqueous cobalt(II) and nickel(II) nitrate solutions inter alia affords the solvated, mono- and oligonuclear nitrato complexes [Mm(NO3)2m-1(H2O)n]+ (M = Co, Ni; m = 1-5; n = 1-4). The collision-induced dissociation spectra of the mass-selected ions imply that these ions correspond to genuine hydrated metal(II) nitrato complexes in that either cluster degradation through expulsion of neutral M(NO3)2 or sequential loss of water ligands take place. In the case of the lowest member of the series (m, n = 1), however, loss of water competes with homolytic cleavage of the N-O bond, which leads to the formation of [M,O2,H2]+ cations containing formal MIII. For m > 1, similar activation of the nitrato ligand was observed under harsher ionization conditions in ESI and provides access to cationic transition-metal oxide clusters in the gas phase. In addition to the collision experiments, thermal reactions of the solvated nitrato complexes with nitrogen monoxide reveal bond-activation pathways, which shed some light on the NO-assisted pyrolysis of solid metal nitrates for the preparation of metal oxide catalysts.

Mass spectrometric experiments are used to examine the size-dependent interactions of bare vanadium cluster cations V(n)(+) (n = 1-7) with methanol. The reactivity patterns exhibit enormous size effects throughout the range of clusters investigated. For example, dehydrogenation of methanol to produce V(n)OC(+) is only brought about by clusters with nor = 3. Atomic vanadium cation V(+) also is reactive, but instead of dehydrogenation of the alcohol, expulsions of either methane or a methyl radical take place. In marked contrast, the reaction efficiency of the dinuclear cluster V(2)(+) is extremely low. For the cluster cations V(n)(+) (n = 3-7), complete and efficient dehydrogenation of methanol to produce V(n)OC(+) and two hydrogen molecules prevails. DFT calculations shed light on the mechanism of the dehydrogenation of methanol by the smallest reactive cluster cation V(3)(+) and propose the occurrence of chemisorption concomitant with C-O bond cleavage rather than adsorption of an intact carbon monoxide molecule by the cluster.

The close interaction between experiment and theory is a key motif in contemporary transition-metal chemistry with mechanistic orientation. As an illustration of this aspect, we present selected examples from the gas-phase chemistry of the coinage metals (Cu, Ag, Au). The topics range from seemingly simple systems, such as hydrated metal ions, via the role of formal and real charge states in gaseous species to several case studies which demonstrate that modern mass spectrometry can contribute to the understanding of important processes in applied chemistry, such as C C bond formations using organocuprates, the epoxidation of ethylene on silver contacts, and the recent achievements in catalysis with gold compounds.

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.

The reactions of methane with the dications C7H62+, C7H72+, and C7H82+ generated by electron ionization of toluene are studied using mass-spectrometry tools. It is shown that the reactivity is dominated by the formation of doubly charged intermediates, which can either eliminate molecular hydrogen to yield doubly charged products or undergo charge-separation reactions leading to the formation of a methyl cation and the corresponding C7Hn+1+ monocation. Typical processes observed for dications, like electron transfer or proton transfer, are largely suppressed. The theoretically derived mechanism of the reaction between C7H62+ and CH4 indicates that the formation of the doubly charged intermediate is kinetically preferred at low internal energies of the reactants. In agreement, the experimental results show a pronounced hydrogen scrambling and dominant formation of the doubly charged products at low collision energies, whereas direct hydride transfer prevails at larger collision energies.

Motivated by the search for ways of a more efficient usage of the large, unexploited resources of methane, recent progress in the gas-phase activation of methane by ligated transition-metal ions is discussed. Mass spectrometric experiments demonstrate that the ligands can crucially influence both reactivity and selectivity of transition-metal cations in bond-activation processes, and the most reactive species derive from combinations of transition metals with the electronegative elements fluorine, oxygen, and chlorine. Furthermore, the collected knowledge about intramolecular kinetic isotope effects associated with the activation of C–H(D) bonds of methane can be used to distinguish the nature of the bond activation as a mere hydrogen-abstraction, a metal-assisted mechanism or more complex reactions such as formation of insertion intermediates or σ-bond metathesis.

This study aims towards a rationalization and generalization of the fragmentation patterns of medium-sized hydrocarbon dications by tandem mass spectrometry. Phenanthrene (C14H10) serves as a precursor for the generation of CmHn2+ dications with m = 9–14 and n = 1–10. Four characteristic channels occur in the metastable-ion spectra: loss of a hydrogen atom, elimination of molecular hydrogen, loss of acetylene, and charge-separation fragmentation (Coulomb explosion). The first two channels dominate the spectra and their relative intensities show some regularities for ions with an even number of carbon atoms. Further, parent ions with an odd number of hydrogen atoms exhibit loss of atomic hydrogen, whereas loss of molecular hydrogen is preferred for dications with an even number of hydrogen atoms. The ions with odd number of carbon atoms do not show any evident regularity for the H and H2 losses. The channels involving C C bond cleavages occur for larger dications with m > 9, but only for some combinations of m and n without an apparent regularity.

Bare vanadium oxide cluster cations VmOn+ (m = 1–4; n =1–10) generated by electrospray ionization are investigated with respect to their reactivity toward water and molecular oxygen by using mass spectrometric techniques. Besides ion hydration, the ion/molecule reactions of VmOn+ with oxygen-labeled water (H218O) also lead to 16O/18O exchange reactions of the vanadium oxide clusters cations. Although the probability of degenerate 16O/18O exchange between VmOn+ and water is fairly high for the cluster cations with a medium valence state of vanadium, oxygen-atom exchange reactions between VmOn+ and 18O2 can only be accomplished by VO+, V3O6+, and V4O8+. Particularly interesting is the fact that not only oxygen atoms from vanadyl units are exchanged in the cluster cations, but bridging oxygen atoms are also most likely involved in the processes. Other reaction channels for the interaction of VmOn+ cluster cations with molecular oxygen are reported as well, such as oxidative degradation of the low-valent cluster cations upon collision with O2 and formation of association complexes for the high-valent cluster cations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

In the presence of secondary alcohols, electrospray ionization of dilute methanolic solutions of nickel(II) salts and 1,1'-bis-2-naphthol (BINOL) leads to complexes of the formal composition [(BINOLato)Ni-(CH 3 CH(OH)R)] + (BINOLato refers to a singly deprotonated (R)- or (S)-1,1'-bis-2-naphthol ligand; R=CH 3 , C 2 H 5 , n-C 3 H 7 , n-C 4 H 9 , n-C 5 H 11 , n-C 6 H 13 , c-C 6 H 11 , and C 6 H 5 ). Upon collision-induced dissociation, each mass-selected nickel complex either loses the entire secondary alcohol ligand or undergoes bond activation followed by elimination of the corresponding ketone, as revealed by deuterium labeling. When enantiomeric BINOLato ligands (R or S) are combined with chiral secondary alcohols (R or S), differences in the branching ratios between these channels for the two stereoisomers of the secondary alcohols provide insight into the chiral discrimination operative in the C-H- and O-H-bond activation processes. For saturated alkan-2-ols, the chiral discrimination is low, and if any preference is observed at all, ketone elimination from the homochiral complexes (R,R and S,S) is slightly favored. In contrast, the diastereomeric (BINOLato)Ni + complexes of 1-phenylethanol exhibit preferential ketone losses for the heterochiral systems (S,R and R,S).

The "bare" complex [Cu(PhOH)(PhO)](+) with a phenol (PhOH) and a phenoxy (PhO) ligand bound to copper is studied both experimentally and computationally. The binding energies and structure of this complex are probed by mass spectrometry, infrared multi-photon dissociation, and DFT calculations. Further, the monoligated complexes [Cu(PhO)](+) and [Cu(PhOH)](+) are investigated for comparison. DFT calculations on the [Cu(PhOH)(PhO)](+) complex predict that a phenolate anion interacts with copper(II) preferentially through the oxygen atom, and the bonding is associated with electron transfer to the metal center resulting in location of the unpaired electron at the aromatic moiety. Neutral phenol, on the other hand, interacts with copper preferentially through the aromatic ring. The same arrangements are also found in the monoligated complexes [Cu(PhO)](+) and [Cu(PhOH)](+). The calculations further indicate that the bond strength between the copper atom and the oxygen atom of the phenoxy radical is weakened by the presence of neutral phenol from 2.6 eV in bare [Cu(PhO)](+) to 2.1 eV in [Cu(PhOH)(PhO)](+).

The solvation of copper(II) sulfate in binary mixtures of water and N,N-dimethylformamide (DMF) is studied by a combined approach using electrochemical studies in solution and a mass spectrometric assay of the solvated ions formed from these solutions upon electrospray ionization (ESI). In the condensed phase, the limiting transference numbers (t(+/-)(o)) and the apparent ion association constants (K(A)'s) of CuSO(4) have been determined in water/DMF solutions at 20 degrees C. The t(+)(o) values decrease with increasing DMF content, demonstrating a gradual solvation of Cu(2+) by DMF molecules. The association constants indicate that aggregation becomes more pronounced as the DMF content increases. In order to achieve complementary insight, the intrinsic interactions among the ions and solvent molecules are investigated in gas-phase experiments of the CuSO(4)/water/DMF system using ESI mass spectrometry. Under the conditions used, the dications [Cu(DMF)(n)](2+) (n = 3-6), [Cu(2)(DMF)(n)SO(4)](2+) (n = 2-7), and [Cu(3)(DMF)(n)(SO(4))(2)](2+) (n = 2-7), and the monocations [Cu(OH)(DMF)(n)](+), [Cu(DMF)(n)(HSO(4))](+) (both, n = 1-3), and [Cu(DMF)(n)](+) (n = 1, 2), are formed as the leading copper-containing cations. Likewise, polynuclear copper clusters observed in the anion ESI spectra support partial aggregation occurring in solution. The gas-phase studies clearly support the conclusions that (i) DMF is a highly preferred ligand for CuII in comparison to water and that (ii) DMF supports ion association for which the mass spectrometric data suggest the formation of polynuclear copper clusters.

The mechanism of oxidative coupling of two naphthol molecules to form binaphthol catalyzed by Cu(OH)ClTMEDA (TMEDA=N,N,N',N'-tetramethylethylenediamine) was approached by means of a gas-phase model system. Concise evidence is provided that the coupling reaction proceeds in clusters with two Cu(II) centers, whereby the intermediacy of free naphthoxy radicals in the coupling step is avoided. In the absence of TMEDA, the cluster is bound via a bridging counterion and the coupling reaction is followed by cluster cleavage. The coordination of one or two TMEDA molecules to the reactive complex results in more efficient coupling of naphthol molecules, and moreover, the binuclear cluster is also conserved after the reaction is completed. The effect of TMEDA is twofold: First, it supports clustering of copper and, second, as a ligand bound to a copper center in the reactive complex, it weakens the bond between copper and the naphtholato ligand such that the naphtholato unit is more prone to undergo C--C coupling. Furthermore, a pronounced counterion effect is found that correlates well with condensed-phase data: weakly bridging counterions (e.g., NO3(-)) yield less stable dicopper clusters and the coupling reaction hardly occurs, whereas better bridging counterions (e.g., Cl(-) or Br(-)) provide more stable clusters that make the coupling reaction more efficient.

Electrospray ionization of methanolic solutions of nickel(II) nitrate, 1,1′-binaphthalene-2,2′-diol (BINOL), and secondary alcohols (ROH) inter alia affords monocationic complexes of the type [(BINOLate)Ni(ROH)]+, where BINOLate stands for singly deprotonated BINOL. Upon collision-induced dissociation (CID), the mass-selected ions undergo competing fragmentations involving loss of the alcohol ligand and expulsion of the corresponding carbonyl compound. The latter reaction leads to the hydride complex [(BINOL)Ni(H)]+ and can thus be regarded as the reversal of the reduction of ketones with metal hydrides. The possibility of the occurrence of enantioselective gas-phase reactions is probed for combinations of chiral BINOLate ligands with chiral alkan-2-ols. Whereas aliphatic alkan-2-ols do not show pronounced chiral effects, enantioselective bond activation is observed for 1-phenylethanol, indicating an interaction of the aromatic ring of the alkanol with the positively charged metal center.

Complexes of vanadium(V) with C4-dicarboxylic acids, i.e. maleic, fumaric, and succinic acid, are investigated by means of electrospray ionization (ESI) mass spectrometry. Under soft conditions of ionization, ESI of a diluted solution of OV(OC2H5)3 in methanol gives rise to complexes, which are best described by the general formula [(XCOO)2VO(C,H4,O)]+ (X = CH, CH2), where the solvent employed delivers the (C,H4,O) unit, whose components are either attached to vanadium as an intact methanol ligand or are distributed over the other atoms. In the complexes containing maleic or succinic acid, the acids appear to act as chelating ligands with coordination of the vanadium center by two oxygen atoms from two different carboxylic groups. In contrast, the trans-configuration in fumaric acid prevents the formation of similar chelate complexes. The fragmentation reactions of the complexes are studied by collision-induced dissociation and labeling experiments. For the vanadium(V) complex derived from maleic acid, a preferential elimination of CO2 is observed, followed by combined losses of CO2, CO, and C2H2 or alternatively an intact C4H2O3 unit, which most likely corresponds to maleic anhydride. Instead, evaporation of water followed by loss of carbon monoxide predominates for the corresponding VV cation generated in the presence of succinic acid. Due to the fact that the loss of water involves the central methylene groups of succinic acid, a connection to the vanadium-oxide-mediated formation of maleic acid upon partial oxidation of C4-hydrocarbons is proposed.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The ion/molecule reactions of selected alcohols with the vanadium oxide cations VO+ and VO2+ are studied by Fourier-transform ion-cyclotron resonance (FT-ICR) mass spectrometry. Dehydrogenation is the dominating reaction pathway for methanol and allyl alcohols. With larger or less unsaturated alcohols, dehydration and carbocation formations prevail. While the valence in VO+ remains unchanged during alcohol dehydrogenation, VO2+ is reduced to VIII. Thermochemical data for VO2H0/+, VO3H and VO3H2+ are derived by means of ICR bracketing. The experimental results are further complemented by ab initio calculations using density functional theory. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Electrospray ionization of solutions of dimethyl(1,5-cyclooctadiene)platinum(II) in methanol with traces of nitrogen-containing ligands L provides gaseous complexes of the type [(CH3)Pt(L)]+ with L = pyridine (py), 2,2′-bipyridine (bipy), and 1,10-phenanthroline (phen). These [(CH3)Pt(L)]+ cations are capable of activating the C-H bond in methane as shown by H/D exchange when using CD4 as a neutral reactant. Most reactive is the complex [(CH3)Pt(py)]+ bearing a monodentate nitrogen ligand. The cationic complexes [(CH3)Pt(bipy)]+ and [(CH3)Pt(phen)]+ also bring about activation of methane, though at a lower rate, whereas the bipyridine complex [(CH3)Pt(py)2]+ does not react with methane at thermal conditions. A detailed analysis of the experimental data by means of kinetic modeling provides insight into the underlying mechanistic steps, but a distinction whether the reaction occurs as σ bond metathesis or via an oxidative addition cannot be made on the basis of the experimental data available.

The bimolecular reactions of several hydrocarbon dications C(m)H(n)(2+) (m = 6-10, n = 4-9) with neutral benzene are investigated by tandem mass spectrometry using a multipole instrument. Not surprisingly, the major reaction of C(m)H(n)(2+) with benzene corresponds to electron transfer from the neutral arene to the dication resulting in the pair of monocationic products C(m)H(n)(+) + C(6)H(6)(+). In addition, also dissociative electron transfer takes place, whereas proton transfer from the C(m)H(n)(2+) dication to neutral benzene is almost negligible. Interestingly, the excess energy liberated upon electron transfer from the neutral arene to the C(m)H(n)(2+) dication is not equally partitioned in the monocationic products in that the cations arising from the dicationic precursor have a higher internal energy content than the monocations formed from the neutral reaction partner. In addition to the reactions leading to monocationic product ions, bond-forming reactions with maintenance of the two-fold charge are observed, which lead to a condensation of the C(m)H(n)(2+) dications with neutral benzene under formation of intermediate C(m+6)H(n+6)(2+) species and then undergo subsequent losses of molecular hydrogen or neutral acetylene. This reaction complements a recently proposed dicationic route for the formation of polycyclic aromatic hydrocarbons under extreme conditions such as they exist in interstellar environments.

The formation and fragmentation of the indene dication C 9 H 8 2 + is studied using synchrotron radiation. The double-ionization energy of indene amounts to 21.8 ± 0.1 eV. The appearance energies of the fragments C 9 H 7 2 + and C 9 H 6 2 + are 25.8 ± 0.1 eV and 25.7 ± 0.2 eV, respectively. Although the appearance energies are almost the same, the dicationic fragment C 9 H 6 2 + is formed in a higher yield than C 9 H 7 2 + . It is suggested that the elimination of atomic hydrogen is associated with a considerable energy barrier, whereas the H 2 elimination proceeds via very small barrier or is barrier-less process with respect to the reaction endothermicity.

The competition between proton and electron transfer in reactions of mass-selected dications CHX2+ (X = F, Cl, Br, and I) with rare gas atoms (Ne, Ar, Kr, and Xe) and selected molecular reagents (N2, O2, CO, H2O, and HCl) is studied in the gas phase. In the ion-molecule reactions of CHX2+ dications with atoms and nonpolar molecules, it is the energy balance of electron transfer that acts as the decisive factor: when the exothermicity of electron transfer exceeds 2 eV, this process predominates at the expense of bond-forming proton transfer. In marked contrast, the reactions between these triatomic dications and polar molecules are governed for the benefit of the thermochemically more favored products resulting from proton transfer.

Bare vanadium-oxide and -hydroxide cluster cations (V(m)O(n)H(o)+, m = 2-4, n = 1-10, o = 0, 1) were generated by electrospray ionization in order to examine their intrinsic reactivity toward isomeric butenes and small alkanes using mass spectrometric techniques. Two of the major reactions described here concern the activation of C-H bonds of the alkene/alkane substrates resulting in the transfer of two hydrogen atoms and/or attachment of the dehydrogenated hydrocarbon to the cluster cations; these processes are classified as oxidative dehydrogenation (ODH) and dehydrogenation, respectively. For the dehydrogenation of butene, it evolved as a general trend that high-valent clusters prefer ODH resulting in the addition of two hydrogen atoms to the cluster concomitant with elimination of neutral butadiene, whereas low-valent clusters tend to add the diene with parallel loss of molecular hydrogen. Deuterium labeling experiments suggest the operation of a different reaction mechanism for V2O2(+) and V4O10(+) compared to the other cluster cations investigated, and these two cluster cations also are the only ones of the vanadium-oxide ions examined here that are able to dehydrogenate small alkanes. The kinetic isotope effects observed experimentally imply an electron transfer mechanism for the ion-molecule reactions of the alkanes with V4O10(+).

The gas-phase reactions of methane with the platinum(II) ions PtX+ with X = H, Cl, Br, and CHO are studied by mass spectrometry. The PtX+ ions are generated by electrospray ionization of methanolic solutions of hexachloroplatinic acid and hexabromoplatinic acid, respectively. Small to moderate intramolecular kinetic isotope effects determined for the C—H(D) bond activation of CH2D2 suggest that the activation of methane by gaseous PtX+ cations is subject to thermochemical control by the product channels. In addition, the PtCl2+ cation is also able to activate methane, whereas PtCl3+ is unreactive under the conditions chosen. Key words: gas-phase reactions, mass spectrometry, methane activation, platinum bromide, platinum chloride.

Electrospray ionization (ESI) mass spectrometry is used to investigate the gas-phase dissociation behavior of vanadium(V) complexes [(C6H4O2)V(OR1)(OR2)]+ containing two identical or different alkoxy groups (R1, R2 = CH3, C2H5, n-C3H7, i-C3H7 and t-C4H9) and a catecholato ligand (C6H4O2). The fragmentation reactions of the complexes are studied by collision-induced dissociation (CID) and labeling experiments. For [(C6H4O2)V(OR1)(OR2)]+ cations with alkoxo groups larger than methyl, a trend for preferential evaporation of hydrocarbon fragments is observed, followed by expulsion of neutral alcohols. Further, the CID spectra of all complexes show a signal which can be assigned to the complex [(C6H4O2)VO]+. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The dinuclear metal dioxide Ni2O2+ is prepared by the reaction of gaseous Ni2+ with N2O in a Fourier-transform ion-cyclotron-resonance (FT-ICR) mass spectrometer. The title compound oxidizes CO, thus proving the catalytic activity of Ni2+ for mediating oxygen-atom transfer from N2O to CO. Whereas Ni2O2+ does not react with H2 and CH4, it accomplishes C–H bond activation of ethane, propane, and n-butane. In comparison to the previously studied metal-oxide clusters Mn2O2+ and Fe2O2+, Ni2O2+ exhibits an enhanced reactivity, which is ascribed to a lower oxygen-binding energy. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Photoionization tandem-mass spectrometry of selectively deuterium labeled precursor molecules is used to probe the structure of protonated benzene which had been questioned in a provocative publication by Mason and co-workers (J. Chem. Soc., Chem. Commun. 1995, 1027). Specifically, we address the intriguing aspect of a postulated delayed hydrogen ring-walk by modulating the internal energy content of mass-selected C6H7-nDn+ ions. To this end, ionization of selectively deuterated precursors by tunable synchrotron photons is combined with chemical monitoring of H+/D+ transfer from C6H7-nDn+ to a strong base B. The resulting BH+/BD+ ratios monotonically decrease with increasing internal energy content and, due to the virtue of the regioselective deuterium label incorporations, thereby disprove a delayed hydrogen ring-walk or any other “structural memory effect” in C6H7-nDn+ ions. As a consequence, the experimental studies of Mason et al. were reconsidered using sector-field mass spectrometry. These extensive...

Electrospray ionization mass spectrometry of copper(II) nitrate solution in the presence of phenanthroline (phen) leads to the generation of the monocation (phen)Cu(NO3)+ from which NO2 is lost at ...

Electrospray ionization-mass spectrometry (ESI-MS) is used to investigate the complexation of nickel(II) by N,N-dimethylformamide (DMF) in different water/DMF mixtures. The types of solvated cations observed are independent of the solvent composition and are part of the series [Ni(DMF) n ] 2 + with n = 2-6 and [Ni-(DMF) n X] + with n = 1-4 (where X = Cl, Br, NO 3 ). The number of ligands, n, and thus the extent of solvation depends on the cone voltage (U C ) of the ESI source. At low U C multiply ligated ions prevail, whereas ligation drops with rising U C and further increase of U C causes dissociation of the solvent molecules as well as reduction of Ni(II) to Ni(I). The collision-induced dissociation (CID) spectra of the multiply ligated ions [Ni(DMF) n ] 2 + with n ≥ 3 and [Ni(DMF) n X] + with n ≥ 2 show only losses of neutral DMF ligands. Quite different is the behavior of the bisligated ions [Ni(DMF) 2 ] 2 + and [Ni(DMF)X] + . In the CID spectrum of [Ni(DMF) 2 ] 2 + , electron transfer from DMF to the metal leads to the reduced complex [Ni(DMF)] + concomitant with ionized DMF + . Charge-stripping spectra of mass-selected monocations [Ni(DMF) n ] + (n = 1, 2) confirm that the dication [Ni(DMF) 2 ] 2 + is stable, whereas monoligated [Ni(DMF)] 2 + appears to be intrinsically unstable toward Coulomb explosion. In the CID spectra of [Ni(DMF)Cl] + and [Ni(DMF)Br] + , bond activation of the solvent takes place, and in the case of [Ni(DMF)NO 3 ] + the formation of a cationic species is observed which formally corresponds to the solvated metal-oxide cation [(DMF)NiO] + .

The gas-phase reactions of the platinum−carbene clusters PtnCH2+ (n = 2−5) with the substrates O2, CH4, NH3, and H2O have been investigated by FT-ICR mass spectrometry and compared with previous results for the mononuclear homologue PtCH2+. The ion−molecule reactions of the clusters PtnCH2+ with O2 and CH4 are similar to the corresponding processes observed for PtCH2+. In contrast, a surprising difference evolves in the reactions with NH3 and H2O. Whereas PtCH2+ mediates carbon−heteroatom bond formation as an attractive way toward methane functionalization, the homologous clusters fail in this regard and exclusively yield the carbide complexes PtnC+·NH3 and PtnC+·H2O, respectively. The differences in reactivities have been analyzed further by means of kinetic isotope effects, H/D-exchange reactions, and energy-dependent collision-induced dissociations of PtnCH2+. Essentially, the higher stabilities of the platinum−carbide clusters PtnC+ compared to PtC+ cause the change in reactivity. This conclusion is c...

Ion-cyclotron resonance mass-spectrometric experiments are used to examine the reactions of small cationic vanadium clusters Vn+ and vanadium-oxide clusters VnOm+ (n ≤ 5, m ≤ 3) with molecular oxygen. Most of these reactions are very fast (k ≥ 3 × 10-10 cm3 molecule-1 s-1) and strongly exothermic. Oxidative degradation of the clusters (decreasing n) is generally more favored than oxygen-transfer resulting in VnOm+ cations (constant n). Consequently, the mononuclear oxide ions VO+ and VO2+ evolve as the major final products at the end of a reaction cascade. For V2+ and V3+, the complete network of elementary steps is explored by MS/MS experiments. Furthermore, some thermochemical brackets are derived, e.g., for the enthalpies of formation of the ions V2Om+ (m = 1−3): 920 kJ/mol ≤ ΔfH (V2O+) ≤ 1130 kJ/mol; 670 kJ/mol ≤ ΔfH (V2O2+) ≤ 880 kJ/mol, and 420 kJ/mol ≤ ΔfH (V2O3+) ≤ 630 kJ/mol.