Microscopic Reactivity of Phenylferrate Ions toward Organyl Halides.

Despite its practical importance, organoiron chemistry remains poorly understood due to its mechanistic complexity. Here, we focus on the oxidative addition of organyl halides to phenylferrate anions in the gas phase. By mass-selecting individual phenylferrate anions, we can determine the effect of the oxidation state, the ligation, and the nuclearity of the iron complex on its reactions with a series of organyl halides RX. We find that Ph2 Fe(I)- and other low-valent ferrates are more reactive than Ph3 Fe(II)- ; Ph4 Fe(III)- is inert. The coordination of a PPh3 ligand or the presence of a second iron center lower the reactivity. Besides direct cross-coupling reactions resulting in the formation of RPh, we also observe the abstraction of halogen atoms. This reaction channel shows the readiness of organoiron species to undergo radical-type processes. Complementary DFT calculations afford further insight and rationalize the high reactivity of the Ph2 Fe(I)- complex by the exothermicity of the oxidative addition and the low barriers associated with this reaction step. At the same time, they point to the importance of changes of the spin state in the reactions of Ph3 Fe(II)- .

Oxidation States, Stability, and Reactivity of Organoferrate Complexes.

We have applied a combination of electrospray-ionization mass spectrometry, electrical conductivity measurements, and Mössbauer spectroscopy to identify and characterize the organoferrate species R nFe m- formed upon the transmetalation of iron precursors (Fe(acac)3, FeCl3, FeCl2, Fe(OAc)2) with Grignard reagents RMgX (R = Me, Et, Bu, Hex, Oct, Dec, Me3SiCH2, Bn, Ph, Mes, 3,5-(CF3)2-C6H3; X = Cl, Br) in tetrahydrofuran. The observed organoferrates show a large variety in their aggregation (1 ≤ m ≤ 8) and oxidation states (I to IV), which are chiefly determined by the nature of their organyl groups R. In numerous cases, the addition of a bidentate amine or phosphine changes the distributions of organoferrates and affects their stability. Besides undergoing efficient intermolecular exchange processes, several of the probed organoferrates react with organyl (pseudo)halides R'X (R' = Et, iPr, Bu, Ph, p-Tol; X = Cl, Br, I, OTf) to afford heteroleptic complexes of the type R3FeR'-. Gas-phase fragmentation of most of these complexes results in reductive eliminations of the coupling products RR' (or, alternatively, of R2). This finding indicates that iron-catalyzed cross-coupling reactions may proceed via such heteroleptic organoferrates R3FeR'- as intermediates. Gas-phase fragmentation of other organoferrate complexes leads to ß-hydrogen eliminations, the loss of arenes, and the expulsion of organyl radicals. The operation of both one- and two-electron processes is consistent with previous observations and contributes to the formidable complexity of organoiron chemistry.

Electronic and Steric Effects on the Reductive Elimination of Anionic Arylferrate(III) Complexes.

Arylferrate(III) complexes Ph3 FeR- (R=para- and ortho-substituted aryl) are proposed as model systems for the in-depth investigation of reductive eliminations from organoiron(III) species. Electrospray ionization transfers the arylferrate complexes prepared in situ from solution into the gas phase, where mass selection ensures a well-defined population of reactant ions. Upon gas-phase fragmentation, the arylferrate complexes undergo reductive elimination of the cross-coupling product PhR as well as the homo-coupling product Ph2 . The measured branching ratios between the two competing reaction channels are used to construct a Hammett plot, which shows that electron-donating aryl groups R favor the formation of the cross-coupling product. In this way, the complexes avoid the build-up of too much electron density at the iron center during the reductive elimination. ortho Substitution in R increases the fraction of the homo-coupling product, presumably by hindering the approach between the two aryl groups participating in the reductive elimination. The obtained mechanistic insight substantially advances our understanding of one of the central elementary steps of transition-metal-catalyzed cross-coupling reactions.

Solution and Gas-Phase Reactivity of Me12 Fe8- and Related Cluster Ions.

The cluster ion Me12 Fe8- is an unprecedented representative of organoiron species, which are of great interest because of their possible role as intermediates in iron-catalyzed cross-coupling reactions. To learn more about its behavior in solution, the possible formation of related cluster ions, and their reactivity, electrospray-ionization mass spectrometry and gas-phase experiments were performed. Me12 Fe8- adopts a highly dynamic behavior in solution and disappears in the presence of the chelating ligand N,N,N',N'-tetramethylethylenediamine. Besides homoleptic Me12 Fe8- , its heteroleptic analogues Me12-n Fe8 Phn- , n=1-5, are also accessible. Me12 Fe8- undergoes iron-halogen exchange reactions with aryl halides. These substrates, as well as their alkyl counterparts, mediate the formation of new homoleptic cluster ions up to Me18 Fe12- . In contrast, no evidence was found for oxidative additions or related reactions. Gas-phase fragmentation of the cluster ions results in numerous different reactions, ranging from the loss of single methyl radicals to the reductive elimination of MePh.

Ate Complexes in Iron-Catalyzed Cross-Coupling Reactions.

Iron-catalyzed cross-coupling reactions have an outstanding potential for sustainable organic synthesis, but remain poorly understood mechanistically. Here, we use electrospray-ionization (ESI) mass spectrometry to identify the ionic species formed in these reactions and characterize their reactivity. Transmetalation of Fe(acac)3 (acac=acetylacetonato) with PhMgCl in THF (tetrahydrofuran) produces anionic iron ate complexes, whose nuclearity (1 to 4 Fe centers) and oxidation states (ranging from -I to +III) crucially depend on the presence of additives or ligands. Upon addition of iPrCl, formation of the heteroleptic FeIII complex [Ph3 Fe(iPr)]- is observed. Gas-phase fragmentation of this complex results in reductive elimination and release of the cross-coupling product with high selectivity.

Substitution reactions of gaseous ions in a three-dimensional quadrupole ion trap.

Substitution reactions between gaseous ions and neutral substrate molecules are of ongoing high interest. To investigate these processes in a qualitative and quantitative manner, we have constructed a device, with which a defined amount of a volatile substrate can be mixed with a defined amount of helium gas and added into a three-dimensional quadrupole ion trap. From the known inner volume of the device, the known ratio nsubstrate :nHe of the mixture, and the determined absolute partial pressure of helium in the ion trap, we can derive the partial pressure of the substrate in the ion trap and, thus, convert the directly observable pseudo-first-order rate constants of the substitution reactions into absolute bimolecular rate constants. We have tested the device by investigating a series of SN 2 reactions of Br- and CF3 CH2 O- anions as well as ligand exchange reactions of ligated Na+ cations. As the obtained results suggest, the described device makes it possible to determine the bimolecular rate constants of substitution reactions as well as other ion-molecule reactions with satisfactory accuracy and reliability.

Mössbauer and mass spectrometry support for iron(ii) catalysts in enantioselective C-H activation.

A combination of electrospray-ionization mass spectrometry and Mössbauer spectroscopy was used to investigate the species generated in situ in highly enantioselective Fe/NHC-catalyzed C-H alkylations. The findings indicate an organometallic iron(ii)-NHC species to be of key relevance in the asymmetric catalysis.