A Reaction‐Induced Localization of Spin Density Enables Thermal C−H Bond Activation of Methane by Pristine FeC4+

The reactivity of the cationic metal‐carbon cluster FeC4+ towards methane has been studied experimentally using Fourier‐transform ion cyclotron resonance mass spectrometry and computationally by high‐level quantum chemical calculations. At room temperature, FeC4H+ is formed as the main ionic product, and the experimental findings are substantiated by labeling experiments. According to extensive quantum chemical calculations, the C−H bond activation step proceeds through a radical‐based hydrogen‐atom transfer (HAT) mechanism. This finding is quite unexpected because the initial spin density at the terminal carbon atom of FeC4+, which serves as the hydrogen acceptor site, is low. However, in the course of forming an encounter complex, an electron from the doubly occupied sp‐orbital of the terminal carbon atom of FeC4+ migrates to the singly occupied π*‐orbital; the latter is delocalized over the entire carbon chain. Thus, a highly localized spin density is generated in situ at the terminal carbon atom. Consequently, homolytic C−H bond activation occurs without the obligation to pay a considerable energy penalty that is usually required for HAT involving closed‐shell acceptor sites. The mechanistic insights provided by this combined experimental/computational study extend the understanding of methane activation by transition‐metal carbides and add a new facet to the dizzying mechanistic landscape of hydrogen‐atom transfer.DFG, 53182490, EXC 314: Unifying Concepts in CatalysisTU Berlin, Open-Access-Mittel - 201

Frontier Orbitals and 1,2-Hydrogen Shifts in Carbenium Ions

For various carbenium ions it is shown that the energy value and the degree of localization of the LUMO have a significant influence on the energy barriers of 1,2-hydrogen shifts. The structures of the transition states are in line with the Hammond-Postulate

On the Intermediary Existence of Gaseous Ethylen Fluoronium Ions

The long-sought ethylen fluoronium ion (2) is generated as an intermediate in the dissociative ionization of 1-fluoro-2-(p-methyl)phenoxy ethane (4). However, prior to collisionally induced dissociation 2 undergoes ring-opening, associated with hydrogen migration, to form 1-fluoroethyl cation (3). Other decomposition pathways of the molecular ions of 4 involve (i) direct formation of 3 via a combination of C-O-cleavage (loss of ArO -) and [1,2] hydrogen migration (18%) and (ii) complete positional loss of the a-and β-methylene hydrogen atoms (34%). The remaining 48% of the molecular ions of 4 dissociate via anchimeric assistance of the fluorine in the elimination of ArO·, thus giving rise to the formation of 2

On the McLafferty Rearrangement of Ionized Phenyl Pyridyl Alkanones

A detailed investigation concerning the genesis of McLafferty rearrangement products from molecular ions of phenyl pyridyl alkanones reveals the following features: 1) The various products are not formed by competitive dissociations of the molecular ion. Most of the relevant fragment ions are generated from the primary McLafferty product of a hydrogen transfer to the ionized carbonyl group (M+·→m/z 163, ion k). The ion k plays a decisive role for the generation of the abundant product ions at m/z 93 (ion g) and m/z 106 (ion s) both of which are formed by further dissociation of k. Details of the mechanisms for the decomposition of k are obtained by investigating [D]-labelled isotopomers, the analysis of low and high resolution data, the application of MIKE and CA spectra. The problem of keto/enol tautomerism between some ions, relevant in this context, is discussed and it is shown that these isomerization processes are not involved. A detailed description is given for the syntheses of various [D]-labelled phenyl pyridyl alkanones

On the Intermediary Existence of Gaseous Ethylen Fluoronium Ions

The long-sought ethylen fluoronium ion (2) is generated as an intermediate in the dissociative ionization of 1-fluoro-2-(p-methyl)phenoxy ethane (4). However, prior to collisionally induced dissociation 2 undergoes ring-opening, associated with hydrogen migration, to form 1-fluoroethyl cation (3). Other decomposition pathways of the molecular ions of 4 involve (i) direct formation of 3 via a combination of C-O-cleavage (loss of ArO -) and [1,2] hydrogen migration (18%) and (ii) complete positional loss of the a-and β-methylene hydrogen atoms (34%). The remaining 48% of the molecular ions of 4 dissociate via anchimeric assistance of the fluorine in the elimination of ArO·, thus giving rise to the formation of 2

Rollover cyclometalation - early history, recent developments, mechanistic insights and application aspects

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.“Rollover” cyclometalation constitutes a special case among the well-known class of cyclometalation reactions. An overview is given that covers the very first description of this reaction type, as well as recent developments. In addition, not only condensed-phase experiments are reviewed, but also investigations based on mass spectrometric techniques, together with “in silico” studies using DFT-based calculations are considered. While the latter two methods allow for a detailed analysis of the intrinsic factors that affect the reaction mechanisms, consideration of all three regimes permits to develop a coherent mechanistic picture and to address the often noted gap between condensed- and gas-phase studies. Moreover, the quite unexpected reactivity of “rollover” cyclometalated complexes in gas-phase experiments, as well as potential applications, e.g. in synthetic procedures, are discussed in some detail.DFG, EXC 314, Unifying Concepts in Catalysi

SCF-Calculations as Tool to Interprete Reaction Pathes - The Cases of C02 Loss from Ionized ClCH2-COOCH3 and C2H4 Loss from Ionized (CH3)2N-COCH2X

The localization of transition states as saddle points having one negative eigenvalue of the force-constant matrix provides important informations on the energy hyperface of chemical reactions. It can be used for determining the reaction path of unusual experimental results and is demonstrated for the title reactions

C-N coupling in the gas-phase reactions of ammonia and [M(CH)](+) (M = Ni, Pd, Pt): a combined experimental/computational exercise

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Electrospray ionization (ESI) of methanolic solutions of monomeric nickel(II) acetate, [Ni(CH3COO)2], and tetrameric platinum(II) acetate, [Pt4(CH3COO)8], leads to the formation of the corresponding methylidyne complexes [M(CH)]+ (M = Ni, Pt), which react with ammonia under C–N coupling. While the product couples M/[CH4N]+ and [M(CH2N)]+/H2 are observed for both metals, hydrogen-atom expulsion to generate [M(CHNH2)]+/H is only observed in the case of the nickel-containing system, and the proton transfer leading to M/[NH4]+ is limited to platinum. Attempts to conduct related experiments with [Pd(CH)]+/NH3 failed. The mechanisms that explain the experimentally observed reaction channels have been investigated computationally using the B3LYP functional for all metals of the nickel group (M = Ni, Pd, Pt). In line with labeling experiments using the reaction pairs [M(CD)]+/NH3 and [M(CH)]+/ND3 (M = Ni, Pt), two different mechanistic scenarios of the dehydrogenation process are operative for the Ni and Pt systems, respectively.DFG, EXC 314, Unifying Concepts in Catalysi

Mechanistic aspects of the gas-phase coupling of thioanisole and chlorobenzene to dibenzothiophene catalyzed by atomic Ho+

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Mechanistic aspects of the novel gas-phase generation of dibenzothiophene via coupling of thioanisole and chlorobenzene, employing atomic Ho+ as a catalyst, have been investigated using Fourier-transform ion cyclotron resonance mass spectrometry in conjunction with density functional theory (DFT) calculations.DFG, EXC 314, Unifying Concepts in Catalysi