Reactions of Atomic Metal Anions in the Gas phase: Competition between Electron Transfer, Proton Abstraction and Bond Activation

Abstract

Bare metal anions K(-), Rb(-), Cs(-), Fe(-), Co(-), Ni(-), Cu(-), and Ag(-), generated by electrospray ionization of the corresponding oxalate or tricarballylate solutions, were allowed to react with methyl and ethyl chloride, methyl bromide, nitromethane, and acetonitrile in the collision hexapole of a triple-quadrupole mass spectrometer. Observed reactions include (a) the formation of halide, nitride, and cyanide anions, which was shown to be likely due to the insertion of the metal into the C-X, C-N, and C-C bonds, (b) transfer of H(+) from the organic molecule, which is demonstrated to most likely be due to the simple transfer of a proton to form neutral metal hydride, and (c) in the case of nitromethane, direct electron transfer to form the nitromethane radical anion. Interestingly, Co(-) was the only metal anion to transfer an electron to acetonitrile. Differences in the reactions are related to the differences in electron affinity of the metals and the Δ(acid)H° of the metals and organic substrates. Density functional theory calculations at the B3-LYP/6-311++G(3df,2p)//B3-LYP/6-31+G(d) level of theory shed light on the relative energetics of these processes and the mechanisms by which they take place.

Halide anions are formed from reactions between atomic metal anions and halogenated aromatic molecules

“…These metal‐containing ions were observed only at very low collision energy, suggesting the reactions are exothermic to thermoneutral and that kinetically they cannot compete with X − formation at higher internal energies. The transition states for the formation of the metal‐containing ions must be more constrained than those leading to X − , consistent if the latter is because of the oxidative addition of the metal anion to the C―X bond in the reagent molecule, as was observed previously for chloromethane and chloroethane . The driving force for these reactions is the high electron affinity of the halogen atom and when the insertion intermediate is formed, charge natural flows to the halide prior to departure.…”

“…. Therefore, NO 2 − is most likely generated by the oxidative addition of Fe − to the C―N bond, followed by the reductive elimination of NO 2 − , as was observed previously for nitromethane . This also would provide a channel for the selective detection of nitro‐containing compounds by mass spectrometry.…”

“…Previous studies have characterized the reactions of various AMAs with metal carbonyls and acids, simple thiols, sulfides and disulfides, methyl halides containing a single atom of either F, Cl, Br or I, halogenated and nitro containing alkanes and alcohols and hydrocarbons . Both electron transfer and halogen abstraction has been previously observed with large fluorinated compounds, such as pentafluorophenol and pentafluoroaniline .…”

Evaluating the Thermal Vinylcyclopropane Rearrangement (VCPR) as a Practical Method for the Synthesis of Difluorinated Cyclopentenes: Experimental and Computational Studies of Rearrangement Stereospecificity

“…Methyl halides (CH 3 X, X=F, Cl, Br, I) are frequently employed as model compounds in gas‐phase reactions1 to study at a molecular level mechanistic aspects of CH and CX bond activation. These ion/molecule reactions can be classified in four different types: 1) nucleophilic substitution when CH 3 X is reacted with anions resulting in the formation X − ;1b,d,e,i,n,p,q,r,x,z 2) halide atom abstraction by metal cations1t–v or anions;1p 3) proton, hydride or hydrogen‐atom abstraction by anions1b,k,x or cations,1v respectively; 4) dehydrogenation or dehydrohalogenation promoted by metal cations 1v. In all these processes, only one or two bonds of the methyl halide are activated.…”

Anion Recognition by Uranyl–Salophen Derivatives as Probed by Infrared Multiple Photon Dissociation Spectroscopy and Ab Initio Modeling

“…Decarboxylation processes are not uncommon in gas‐phase ion chemistry. In a recent example, twofold loss of CO 2 from alkali metal and silver oxalates has been discovered and exploited to generate bare metal anions,30, 31 whereas in another very interesting application a decarboxylation reaction promoted by CID resulted in the formation of novel organometallic ions 32. 33…”