Ligand Effects on the Reactivity of Iron (II) Cations FeX+ in the Gas Phase

CH4 activation by PtX+ (X = F, Cl, Br, I)

Reactions of PtX⁺ (X = F, Cl, Br, I) with methane have been investigated at the density functional theory (DFT) level. These reactions take place more easily along the low-spin potential energy surface. For HX (X = F, Cl, Br, I) elimination, the formal oxidation state of the metal ion appears to be conserved, and the importance of this reaction channel decreases in going as the sequence: X = F, Cl, Br, I. A reversed trend is observed in the loss of H₂ for X = F, Cl, Br, while it is not favorable for PtI⁺ in the loss of either HI or H₂. For HX eliminations, the transfer form of H is from proton to atom, last to hydride, and the mechanisms are from PCET to HAT, last to HT for the sequence of X = F, Cl, Br, I. One reason is mainly due to the electronegativity of halogens. Otherwise, the mechanisms of HX eliminations also can be explained by the analysis of Frontier Molecular Orbitals. While for the loss of H₂, the transfer of H is in the form of hydride for all the X ligands. Noncovalent interactions analysis also can be explained the reaction mechanisms.

Systematic Ligand Effects in the Reactions of Fe+(6D) and FeX+(5Δ) with CF3X (X = Cl, Br, I). Ion Mobility Measurements of FeX+(5Δ) (X = F, Cl, Br, I) in He

The gas phase reactions of Fe⁺(⁶D) and FeX⁺(⁵Δ) with CF₃X (X = Cl, Br, I) were examined using a selected-ion drift cell reactor under near-thermal energetic conditions. All reactions were carried out in a uniform electric field at a total pressure of 3.5 Torr at room temperature. In addition, reduced zero-field mobilities were measured for FeX⁺(⁵Δ) (X = F, Cl, Br, I) in He, yielding values of 14.2 ± 0.4, 13.7 ± 0.3, 13.3 ± 0.2, and 13.0 ± 0.3 cm²·V^-1·s^-1, respectively. Fe⁺(⁶D) reacts slowly with CF₃Cl and CF₃Br, producing an adduct exclusively with the former and FeBr⁺ with the latter. Conversely, Fe⁺(⁶D) exhibits efficient chemistry with CF₃I to yield FeI⁺, FeCF₃⁺, and FeFI⁺ in parallel reactions. Dependent on the halogen, FeX⁺(⁵Δ) reactions display one or more of four different processes: F^- abstraction, X^- abstraction, halogen switching, and association. In general, the presence of the halogen ligand enhances the rate of reaction over that of Fe⁺(⁶D) with the same molecular substrate. With CF₃Cl, this ligand effect is observed to vary systematically with the electron-withdrawing capability of the halogen. This is illustrated by the correlation between reaction efficiency and the charge distribution on FeX⁺(⁵Δ) as determined from DFT calculations. Specific reaction outcomes for the FeX⁺(⁵Δ) reactions lead to upper and lower bounds on XFe-Y bond strengths (X, Y = F, Cl, Br, I) that are generally consistent with one another and with known trends.

Gas-phase activation of methane by ligated transition-metal cations

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 sigma-bond metathesis.

Ligand and Substrate Effects in Gas-Phase Reactions of NiX+/RH Couples (X=F, Cl, Br, I; R=CH3, C2H5,nC3H7,nC4H9)

The reactions of small saturated hydrocarbons by gaseous nickel cations NiX+ (X=F, Cl, Br, I) are investigated by means of electrospray ionization mass spectrometry. The halide cations are obtained from solutions of the corresponding Ni(II) salts in water or methanol as solvents. NiF+ is the only Ni(II) halide complex that brings about thermal activation of methane. The branching ratios of the observed reactions with C2H6, C3H8, and nC4H10 are shifted systematically by changing the nature of both the ligand X and the substrate RH. In the elimination of HX (X=F, Cl, Br, I), the formal oxidation state of the metal ion appears to be conserved, and the importance of this reaction channel decreases in going from NiF+ to NiI+. A reversed trend is observed in the losses of small closed-shell neutral molecules, that is, H2, CH4 and C2H6, which dominate the gas-phase ion chemistry of NiI+/RH couples. Additionally, inner-sphere electron-transfer reactions take place for a few systems, that is, the delivery of hydride or methanide ions from the hydrocarbon to NiX+ in the course of which the hydrocarbon is converted to a carbenium ion and the cationic metal complex gives rise to a neutral RNiX molecule (R=H, CH3). This process gains importance with decreasing atomic number of the halides and with increasing the size of the alkane. Thus, it constitutes the major pathway in the reactions of NiF+ with propane and n-butane, whereas it is not observed for any of the NiI+/RH couples investigated. Concerning the regioselectivity of the reactions with propane and n-butane, heterolytic cleavage of secondary carbon--hydrogen bonds is clearly preferred compared to that of primary ones, as revealed by deuterium labeling studies. For the NiF+/C3H8 couple, the selectivity of the hydride transfer is as large as 360 in favor of the secondary positions. Though smaller, large preferences for the activation of secondary C--H bonds are also operative in homolytic bond activation of RH (R=nC3H7, nC4H9).

The generation of neutral derivatives of low-valence iron, RFe(I), by netralization-reionization mass spectrometry

Neutralization-reionization mass spectrometry is applied for the generation of low-valence iron derivatives. Neutralization of Rfe(+) (R=H, F, Cl, Br, I, CN, OH, NH(2), acac, C(6)H(5)) ions resulted in the corresponding neutrals having lifetimes of at least 5 micros. Atom connectivities in RFe ions and neutrals were elucidated by a variety of tandem mass spectrometry techniques. The present study provides the first experimental evidence of the intrinsic stability of neutral IFe, NCFe, acacFe and C(6)H(5)Fe.