Gas Phase Ion Chemistry of Transition Metal Clusters: Production, Reactivity, and Catalysis

This review focuses on the use of mass spectrometry to examine the gas phase ion chemistry of metal clusters. Ways of forming gas phase clusters are briefly overviewed and then the gas phase chemistry of silver clusters is discussed to illustrate the concepts of magic numbers and how reactivity can be size dependent. The chemistry of other bare and ligated metal clusters is examined, including mixed metal dimer ions as models for microalloys. Metal clusters that catalyze gas phase chemical reactions such as the oxidation of CO and organic substrates are reviewed. Finally the interface between nanotechnology and mass spectrometry is also considered.     

Reactions of nitrophenide and halonitrophenide ions with acrylonitrile and alkyl acrylates in the gas phase: addition to the carbonyl group versus Michael addition

Gas‐phase reactions of isomeric nitrophenide ions and p‐halonitrophenide ions with acrylonitrile, methyl acrylate, and ethyl acrylate have been studied using mass spectrometry and computational methods. Depending on the structure of the α,β‐unsaturated compound, formation of adducts to the carbonyl group of the acrylate (for methyl acrylate and ethyl acrylate) and β‐adducts (adducts of p‐halonitrophenide ions to α,β‐unsaturated compounds in β position) was observed. Further transformations of these adducts lead to the products of elimination of an alcohol molecule and the anionic products of intramolecular substitution of a halogen atom, respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

Benzylium versus tropylium ion dichotomy: vibrational spectroscopy of gaseous C8H9+ ions

Definitely different: the path towards sorting out a long-standing dichotomy in carbocation chemistry is disclosed by infrared multiple photon dissociation spectroscopy of tropylium and benzylium isomers of C(8)H(9)(+) ions.     

Imidazole and imidazolium porphyrins: gas‐phase chemistry of multicharged ions

Electrospray ionization mass spectrometry/mass spectrometry in the positive ion mode was used to investigate the gas‐phase chemistry of multicharged ions from solutions of porphyrins with 1,3‐dimethylimidazolium‐2‐yl (DMIM) and 1‐methylimidazol‐2‐yl (MIm) meso‐substituents. The studied compounds include two free bases and 12 complexes with transition metals (Cu(II), Zn(II), Mn(III), and Fe(III)). The observed multicharged ions are either preformed or formed during the electrospraying process by reduction or protonation and comprise closed‐shell and hypervalent mono‐radical and bi‐radical ions. The observed extensive and abundant fragmentation of the DMIM and MIm meso‐substituents is a characteristic feature of these porphyrins. Fragments with the same mass values can be lost from the meso‐substituents either as charged or neutral species and from closed‐shell and hypervalent radical ions. Reduction processes are observed for both the free bases and the metallated DMIM porphyrins and occur predominantly by formation of hypervalent radicals that fragment, at low energy collisions, by loss of methyl radicals with formation of the corresponding MIm functionalities. These findings confirm that, when using electrospray ionization, reduction is an important characteristic of cationic meso‐substituted tetrapyrrolic macrocycles, always occurring when delocalization of the formed hypervalent radicals is possible. For the Fe(III) and Mn(III) complexes, reduction of the metal centers is also observed as the predominant fragmentation of the corresponding reduced ions through losses of charged fragments testifies. The fragmentation of the closed‐shell ions formed by protonation of the MIm porphyrins mirrors the fragmentation of the closed‐shell ions of their DMIM counterparts. Copyright © 2014 John Wiley & Sons, Ltd.     

Gas-phase catalysis by atomic and cluster metal ions: the ultimate single-site catalysts

Gas-phase experiments with state-of-the-art techniques of mass spectrometry provide detailed insights into numerous elementary processes. The focus of this Review is on elementary reactions of ions that achieve complete catalytic cycles under thermal conditions. The examples chosen cover aspects of catalysis pertinent to areas as diverse as atmospheric chemistry and surface chemistry. We describe how transfer of oxygen atoms, bond activation, and coupling of fragments can be mediated by atomic or cluster metal ions. In some cases truly unexpected analogies of the idealized gas-phase ion catalysis can be drawn with related chemical transformations in solution or the solid state, and so improve our understanding of the intrinsic operation of a practical catalyst at a strictly molecular level.     

Effect of internal excitation on the collision-induced dissociation and reactivity of Co 2 +

The kinetic energy dependence of collision-induced dissociation (CID) of dicobalt ion (Co ₂ ⁺ ) with He, Ar, and Xe has been investigated using guided ion-beam mass spectrometry. The change in efficiency of CID as the target gas is changed is in general agreement with previous CID studies of other systems: the cross section with Ar is 0.5 that with Xe, and no product ions are found with He. By varying the conditions under which the reactant ions are formed, the degree of internal excitation of the dicobalt ions is changed. The internal energies can be characterized by a Maxwell-Boltzmann distribution. We find that CID and reactions with O₂ and CO are very sensitive to Co ₂ ⁺ internal energy. The bond-dissociation energy derived from this work is D^o(Co ₂ ⁺ )=2.75±0.10 eV (63.4±2.3 kcal/mol). The Co ₂ ⁺ results are compared with a previous study of Fe ₂ ⁺ .     

Theoretical study of the reaction of Cr+ with SCO in gas phase

To elucidate the mechanism of reaction M⁺ + SCO, the reaction of Cr⁺ + SCO has been investigated using density functional theory (DFT) with the popular hybrid functional, B3LYP, in conjunction with 6‐311+G* basis set on both the sextet and quartet potential energy surfaces (PESs). To obtain an accurate evaluation of the activation barrier and reaction energy, the coupled cluster single‐point calculations using the B3LYP structures is performed. The crossing points (CPs) of the different PESs have been localized with the approach suggested by Yoshizawa and colleagues. The involving potential energy curve‐crossing dramatically affects reaction mechanism. The present results show that the reaction mechanism is insertion‐elimination mechanism both along the C? S and C? O bond activation branches, but the C? S bond activation is much more favorable than the C? O bond activation in energy. All theoretical results not only support the existing conclusions inferred from early experiment study, but also complement the pathway and mechanism for this reaction. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Hydride abstraction of methylamine with Cu(S) in the gas phase: A density functional theory study

The gas-phase hydride abstraction of methylamine with Cu⁺(¹S) is theoretically investigated by using density functional theory. Geometries for all the stationary points involved are fully optimized at both the B3LYP/6-311++G(d,p) and B3LYP/6-311++G(3df,2p) levels and the reaction is analyzed in terms of the topology of potential energy surface. Approach of Cu⁺ towards methylamine could form either “classical” N or “nonclassical” η¹-methyl-H attached complex with the former being the global minimum. Both complexes are found to be key intermediates for the hydride abstraction, which could transfer into each other via two parallel routes, i.e., concerted metal movement and stepwise C-H activation-rearrangement. A charge-transfer process is detected for the “nonclassical” complex converting to a precursor species (CuH-NH₂CH₂⁺), which accounts for the final products by a nonreactive dissociation.     

Experimental and computational studies of intracomplex reactions in Mg+(primary, secondary alkylamine) complexes induced by photoexcitation of Mg+

We report herein a comprehensive study of photoinduced reactions in complexes of Mg+ with primary (n-propyl- and isopropylamine) and secondary amines (dipropyl- and diisopropylamine) in the spectral range of 230-440 nm. Similar to the methyl- and ethylamine complexes studied previously, N-H bond activation of these complexes is very unfavorable. Instead, the C(alpha)-C, C-N, and C(alpha)-H bond-cleavage photoproducts are observed after photoexcitation of the Mg+ complexes (3(2)P<--3(2)S). For Mg+(primary amine) complexes, for example, Mg+-NH2CH2CH2CH3, and Mg+-NH2CH(CH3)2, the photoproducts resulting from C(alpha)--C rupture prevail after P(z) and charge-transfer excitations, whereas the Mg+ photofragment is predominant upon P(x,y) excitation. However, with further N-alkyl substitution, as in Mg+(secondary amine) complexes, for example, Mg+-NH(CH2CH2CH3)2 and Mg+-NH[CH(CH3)2]2, a novel intracomplex C-C coupling photoreaction dominates on P(x,y) excitation of Mg+, which is believed to arise from Mg+* insertion into the C-N bond. With P(z) and charge-transfer excitation, the Mg-R elimination photoproducts, arising from C(alpha)-C bond cleavage, predominate. The energetics and possible mechanisms of the intracomplex photoreactions are analyzed in detail with the help of extensive quantum mechanics calculations.     

Ab initio study of the complexes of first-row transition-metal ions with CH, CH_2, and CH_3

The geometries and bonding characteristics of the complexes of the first-row transition-metal ions with CH, CH_2 and CH_3 were investigated by ab initio molecular orbital theory. MCH~ and MCH_2~ are linear and coplanar, re spectively. Both of them are with obvious treble or double bond characteristics, but these multiple bonds are mostly "im perfect". The calculated bond dissociation energies of C--M~ , C=M~ and C≡M~ are mostly close to the experi mental values, and appear in similar periodic trends from Sc to Zn.     

Generation and reactivity of enantiomeric (BINOLato)Ni+ complexes with chiral secondary alcohols in the gas phase

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(CH3CH(OH)R)]+ (BINOLato refers to a singly deprotonated (R)- or (S)-1,1'-bis-2-naphthol ligand; R=CH3, C2H5, n-C3H7, n-C4H9, n-C5H11, n-C6H13, c-C6H11, and C6H5). 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).     

Potential energy surface and thermochemistry for the direct gas phase reaction of germane and water

Gas phase reaction between germane GeH₄ and water H₂O was investigated at CCSD(T)/[aug-cc-pVTZ-pp for Ge + Lanl2dz for H and O]//MP2/6-31G(d,p) level. Only the hydrogen elimination channels are monitored. Within the energy range of 100 kcal/mol, we located nine equilibrium and six transition states on the potential energy surface (PES) of the Ge–O–H systems. GeH₄ reacts with H₂O exothermically (by 2.37 kcal/mol) without a barrier to form a non-planar complex GeH₄·H₂O which isomerizes to GeH₃OH·H₂ and H₂GeOH₂·H₂ with a barrier of 44.34 kcal/mol and 53.75 kcal/mol respectively. The first step of hydrogen elimination gives two non-planar species, GeH₃OH and H₂GeOH₂ but germinol GeH₃OH is found to be more stable. Further thermal decomposition reactions of GeH₃OH involving hydrogen elimination have been studied extensively using the same method. The final hydrogen elimination step gives HGeOH which can exist in cis and trans forms. As the trans form is more stable, only the trans form is considered on the potential energy surface (PES) of the reaction. The important thermochemical parameters (∆_rE_tot + ZPE), ∆_rH and ∆_rG for the H₂ elimination pathways are predicted accurately.     

Theoretical study on the reactions of Nb and Nb+ with CO2 in gas phase

Density functional theory calculations have been performed to explore the potential energy surfaces of C? O bond activation in CO₂ molecule by gas‐phase Nb atom and Nb⁺ cation for better understanding the reaction mechanism of second‐row metal with CO₂. The minimum‐energy reaction path is found to involve the spin inversion in the different reaction steps. This potential energy curve‐crossing dramatically affects the reaction energetic. The present results show that the mechanism is insertion‐elimination mechanism along the C? O bond activation reaction. All theoretical results not only support the existing conclusions inferred from early experiment but also complement the pathway and mechanism for this reaction. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Ion-molecule reaction of alkanenitriles and transition-metal ions in the gas phase: A study on fragmentation mechanism of the adducts

Gas-phase ion-molecule reactions between transition-metal ions (Mn +, Fe+, Co+, Ni +) and propionitri1e and acetonitrile were investigated. Ion-molecule adducts were prepared in a modified fast atom bombardment source and their metastable and collision-induced fragmentations, occurring in the frrst held-free region of an E/B configuration instrument, were studied by means of B/E linked scans. The experimental data suggest a coexistence of both “end-on” and “side-on” coordination modes; the former undergoes ligand detachment alone, whereas the latter loses methyl and ethyl radicals by insertion of M+ into organic substrates and further produces ethylene via a l3-hydrogen transfer. An order for the bonding energy of RCN-M+ is also suggested: RCN-Ni+> RCN-Co+> RCN-Fe+> RCNMn+.     

Ionic recognition by thiacalixarenes: binding of alkali and heavy metal ions by thiacalix[4]arene-bis-crown[n] ethers

The binding properties of two thiacalix[4]arene-bis-crown[n] derivatives (n = 5 and 6) were examined through extraction experiments. The stability constants of the resulting complexes in methanol were determined. The replacement of the bridging CH2 groups by sulfur atoms leads to a strong decrease in both extraction and complexation levels of alkali metal ions but does not affect the selectivity within the series of crown ethers. The stability of complexes with heavy metal ions does not change markedly on passing from thiacalix[4]arene-bis-crown[n] ethers to their calix[4]arene-bis-crown[n] counterparts; therefore no clear-cut conclusions about the possible interactions between these cations and the sulfur atoms can be drawn.