Jana Roithová

Selective activation of alkanes by gas-phase metal ions.

The importance of the selective activation of alkanes for science and technology in the forthcoming decades does not need to be explicitly pointed out in this thematic issue of Chemical ReViews. Instead, we would like to summarize the development and the state-of-art of experimental and theoretical methods for the investigation of model reactions for alkane activation in the gas phase.1 However, before doing so let us address the question, how other scientists, both in academia as well as industry, can profit from such model studies in the gas phase, usually involving very small, charged species under conditions in a mass spectrometer which are very far from real catalysis. To this end, we refer to the synthesis of HCN as a reasonably simple example of how large-scale technical processes can be investigated in microscopic models. † In memoriam of Dr. Andreas Fiedler. * To whom correspondence should be addressed. E-mail: detlef.schroeder@ uochb.cas.cz, E-mail: roithova@natur.cuni.cz. ‡ Charles University in Prague. § Academy of Sciences of the Czech Republic. Jana Roithová (right), Ph.D., leads a group on the topic of reaction mechanisms at the Faculty of Science at the Charles University in Prague. Her research is based on the synergy of gas-phase experiments and theoretical calculations. She received her Ph.D. degree in the group of Prof. Zdenek Herman (J. Heyrovsky Institute of Physical Chemistry, Prague) on the topic of reactivity of small molecular dications in 2003. Since then she has made key contributions to superelectrophilic chemistry in the gas phase, with particular attention toward the activation of nonreactive substrates such as rare gases, nitrogen, and methane. Recently, she concentrates her research on the properties of redox-active molecules, their interactions with transition metals, and mechanisms of organometallic reactions. She is author of more than 80 papers and received, among others, a “L’Oréal for Women in Science” stipend and the Hlavka prize.

Can electrospray mass spectrometry quantitatively probe speciation? Hydrolysis of uranyl nitrate studied by gas-phase methods.

Electrospray ionization of uranyl nitrate dissolved in water generates gaseous species containing either hydroxo-uranyl [UO(2)(OH)](+) or nitrato-uranyl [UO(2)(NO(3))](+) contact ion pairs solvated by up to four water molecules. Furthermore, uranyl clusters of the general type [U(m)O(2m)(X,Y)(2m-1)(H(2)O)(n)](+) (X,Y = OH, NO(3)) with m = 1-5 and n = 2-4 are formed. Collision-induced dissociation experiments 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(2)(2+) 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(2)(OH)(2), UO(2)(OH)(NO(3)), and UO(2)(NO(3))(2). 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.

Bimolecular reactions of molecular dications: reactivity paradigms and bond-forming processes.

The bimolecular reactivity of molecular dications in the gas phase is reviewed from an experimental point of view. Recent research has demonstrated that in addition to the ubiquitous occurrence of electron transfer in the reactions of gaseous dications with neutral molecules, bond-forming reactions play a much larger role than anticipated before. Thus, quite a number of hydrogen-containing dications show proton transfer to neutral reagents as an abundant or even as the major pathway, and also the nature of the neutral reagent itself is decisive for the amount of proton transfer which takes place. Further, several hydrocarbon dications C(m)H(n)(2+) of medium size (m = 6-14, n = 6-10) undergo bond-forming reactions with unsaturated hydrocarbons such as acetylene or benzene, thereby offering new routes for the formation of larger aromatic compounds under extreme conditions such as interstellar environments. Likewise, recent results on the bimolecular reactivity of multiply charged metal ions have revealed the occurrence of a number of new bond-forming reactions which open promising prospects for further research.

Role of gold(I) α-oxo carbenes in the oxidation reactions of alkynes catalyzed by gold(I) complexes.

The gas phase structures of gold(I) complexes formed by intermolecular oxidation of selected terminal (phenylacetylene) and internal alkynes (2-butyne, 1-phenylpropyne, diphenylacetylene) were investigated using tandem mass spectrometry and ion spectroscopy in conjunction with quantum-chemical calculations. The experiments demonstrated that the primarily formed β-gold(I) vinyloxypyridinium complexes readily undergo rearrangement, dependent on their substituents, to either gold(I) α-oxo carbenenoids (a synthetic surrogate of the α-oxo carbenes) or pyridine adducts of gold(I) enone complexes in the condensed phase and that the existence of naked α-oxo carbenes is highly improbable. Isotopic labeling experiments performed with the reaction mixtures clearly linked the species that exist in solution to the ions transferred to the gas phase. The ions were then fully characterized by CID experiments and IRMPD spectroscopy. The conclusions based on the experimental observations perfectly correspond with the results from quantum-chemical calculations.

Bond-formation versuselectron transfer: C–C-coupling reactions of hydrocarbon dications with benzene

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.

Growth of Larger Hydrocarbons in the Ionosphere of Titan

Among the many fascinating results of the Cassini–Huygens mission, the mass spectrum of the ionosphere of Titan has attracted considerable attention. In brief, the ionosphere was found to be surprisingly complex, consisting of hydrocarbon ions CmHn + as well as nitrogen-containing ions CnHnNo + with mass-to-charge ratios up to the probe#s limit of m/z 100; even much heavier components have been proposed. While the formation of CmHn compounds with m 7 is reasonably well understood, routes to larger hydrocarbons are less obvious. Moreover, most of the present models rely on condensation reactions of CmHn + ions with unsaturated precursors such as acetylene, whereas methane, as the major hydrocarbon in the atmosphere of Titan, only plays a minor role in the subsequent growth processes. Here, we report carbon carbon (C C) coupling reactions of methane with medium-sized CmHn 2+ dications leading to larger hydrocarbon molecules. Despite low steady-state concentrations of the dicationic intermediates, kinetic modeling allows predictions about the larger hydrocarbon species present in the ionosphere of Titan, thereby rationalizing the results from the Cassini–Huygens mission which consideration of monocations only cannot explain. The activation of methane poses a particular challenge and usually involves energetic conditions or metal catalysis. Under the conditions of the Titan atmosphere (low temperatures and pressures), small hydrocarbon ions can indeed react with methane, but the rate constants decrease with size, and so far reaction 1 involves the largest CmHn + ion reacting with methane under thermal conditions.

Reduction from copper(II) to copper(I) upon collisional activation of (pyridine)2CuCl

Electrospray ionization of dilute aqueous solutions of copper(II) chloride-containing traces of pyridine (py) as well as ammonia permits the generation of the gaseous ions (py)(2) Cu(+) and (py)(2) CuCl(+) , of which the latter is a formal copper(II) compound, whereas the former contains copper(I). Collision-induced dissociation of the mass-selected ions in an ion-trap mass spectrometer (IT-MS) leads to a loss of pyridine from (py)(2) Cu(+) , whereas an expulsion of atomic chlorine largely prevails for (py)(2) CuCl(+) . Theoretical studies using density functional theory predict a bond dissociation energy (BDE) of BDE[(py)(2) Cu(+) -Cl] = 125 kJ mol(-1) , whereas the pyridine ligand is bound significantly stronger, i.e. BDE[(py)CuCl(+) -py] = 194 kJ mol(-1) and BDE[(py)Cu(+) -py] = 242 kJ mol(-1) . The results are discussed with regard to the influence of the solvation on the stability of the Cu(I) /Cu(II) redox couple.

Generation and dissociation pathways of singly and doubly protonated bisguanidines in the gas phase.

Para-bisguanidinyl benzene 1 and its N-permethylated derivative 2 are both sufficiently strong bases to afford not only the monocations [1+H]+ and [2+H]+, but also the doubly protonated ions, [1+2H]2+ and [2+2H]2+, in the gas phase. The title ions generated via electrospray ionization are probed by collision-induced dissociation experiments which inter alia reveal that the dicationic species [1+2H]2+ and [2+2H]2+ can even undergo fragmentation reactions with maintenance of the 2-fold charge. Complementary results from density functional theory predict PAs above 1000 kJ mol(-1) for the neutral compounds, i.e., PA(1) = 1025 kJ mol(-1) and PA(2) = 1067 kJ mol(-1). Due to the stabilization of the positive charge in the guanidinium ions and the para-phenylene spacer separating the basic sites, even the monocations bear sizable proton affinities, i.e., PA([1+H]+) = 740 kJ mol(-1) and PA([2+H]+) = 816 kJ mol(-1).

Silicon Compounds of Neon and Argon

Noble gas compounds, which were first discovered in 1962, still receive continuous interest. In recent years, this field of research experienced an additional boost by the generation of a variety of new noble gas compounds by photochemical reactions in low-temperature matrices. Challenges in noble gas chemistry that still remain are the formation of new noble gas heteroatom bonds, in which silicon has been raised as a particularly interesting case, and the generation of new compounds of the lighter noble gases argon and neon. Herein, we address both issues by the use of the SiF3 2+ dication as a superelectrophilic reagent 7] with particularly favorable properties for the generation of noble gas compounds. It is obvious that the involvement of the inert noble gases in covalent chemical bonds requires strong oxidation agents, such as F2; in 2006, we suggested that gaseous dications may be used for this purpose. Although we indeed managed to generate the organonoble gas compounds ArCH2 + and ArC2H 2+ recently using this approach, the chemical yields were disappointingly low. 10] Similarly, small amounts of the diatomic dication ArC had been observed in collisionally driven reactions of neutral CO with Ar and of neutral argon with CO. A more efficient dicationic reagent for the attack of noble gases must not only be a potent superelectrophile, but it also needs to meet the following requirements: 1) It should possess a potential leaving group that can be replaced by a noble gas atom without a significant kinetic barrier being involved, such as a homolytic bond cleavage; 2) the preferred oxidation states of noble gases mean that increased stabilities can be expected for even-electron compounds; for homolytic cleavage, the dicationic reagent should be a radical; 3) to prevent electron transfer processes during homolysis of the bond to the leaving group, which would afford an open-shell noble gas compound, the leaving group should have a high ionization energy (IE); and 4) the superelectrophilic dication should be accessible in quantities that suffice for reactivity studies in the gas phase. 15] The SiF3 2+ dication is a promising candidate that may fulfill these requirements: it can be readily generated by dissociative double ionization of SiF4 as a stable, neutral precursor, it has a very high recombination energy RE(SiF3 ) of about 22.4 eV, which allows it to be classified as a superelectrophile, it has one surprisingly weak Si F bond, with D(F2Si 2+ F) = 1.97 eV, and the IE of fluorine as the potential leaving group is exceptionally large (17.4 eV).

Preferential Activation of Primary CH Bonds in the Reactions of Small Alkanes with the Diatomic MgO+. Cation

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.