Oxidation States, Stability, and Reactivity of Organoferrate Complexes

Abstract

We have applied a combination of electrospray-ionization mass spectrometry, electrical conductivity measurements, and Mössbauer spectroscopy to identify and characterize the organoferrate species R Fe formed upon the transmetalation of iron precursors (Fe(acac), FeCl, FeCl, Fe(OAc)) with Grignard reagents RMgX (R = Me, Et, Bu, Hex, Oct, Dec, MeSiCH, Bn, Ph, Mes, 3,5-(CF)-CH; 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, Pr, Bu, Ph, p-Tol; X = Cl, Br, I, OTf) to afford heteroleptic complexes of the type RFeR'. Gas-phase fragmentation of most of these complexes results in reductive eliminations of the coupling products RR' (or, alternatively, of R). This finding indicates that iron-catalyzed cross-coupling reactions may proceed via such heteroleptic organoferrates RFeR' as intermediates. Gas-phase fragmentation of other organoferrate complexes leads to β-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.

Evolution of Ate‐Organoiron(II) Species towards Lower Oxidation States: Role of the Steric and Electronic Factors

“…The nuclear parameters of speciesI II are reminiscento f those recently reported after the addition of 4equivalents of PhMgBrt oF eCl 2 in THF (d = 0.50 mm s À1 , DE Q = 1.06 mm s À1 ). [18] This broad signal-theL orentzian fullwidth at half-height is no lower than 0.90 mm s À1 -was said to be indicative of at least one Fe III species, which would result from the decomposition of [Ph 3 Fe II ] À ,a se vident from ESI-MS. In the present case, EPR studies exclude the formation of such ah igh quantity of S = 5/2 or S = 1/2 species (see above), suggesting that if Fe III species are generated, they should be presenta sd inuclearc omplexes with as trong exchange interaction between the two Fe III ions, leadingt oE PR-silent integer spin ground-states pecies.…”

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

“…Another possibility is the in situ formation of organoferrates, in agreement with recent reports that the nature of the iron precursor has very little effect on the transmetallation reactions with organometallic species to form such complexes. 12 These observations raised the question as to the nature of the active catalyst and its mode of action, and highlight the need for detailed, comprehensive mechanistic studies to unravel fundamental aspects of iron-catalyzed C-H activations. Such mechanistic insights have recently been gained for iron-catalyzed Kumada-Corriu-type cross-coupling reactions via Mössbauer spectroscopy and mass spectrometry.…”

Zur Rolle von Organoferraten in eisenkatalysierten Kreuzkupplungen

“…Tatsächlich gingen beide Ferrate eine quantitative Kreuzkupplung mit Alkenylbromid ein; 8 reagierte auch bereitwillig mit 4‐Chlorbenzoat (Schema ) . In Abwesenheit von NMP wurden Alkylferrate höherer Ordnung der Formel [Fe 8 R 12 ] n − isoliert, die einen topologisch neuartigen [Fe 8 ]‐Kern mit einer mittleren Oxidationsstufe von 1.4 und 12 μ 2 ‐Alkylgruppen enthalten: [MgCl(thf) 5 ][Fe 8 Me 12 ] ( 9 ) und [MgX(thf) 5 ] 2 [Fe 8 Et 12 ] ( 10 , X=Cl/Br) (Schema , unten) …”

“…[13,14] Et 12 ](10,X= Cl/Br) (Schema 3, unten). [14][15][16] Fast 50 Jahre nach den Pionierarbeiten von Kochi über Fekatalysierte Kreuzkupplungen scheinen diese Ferrat-Cluster den lang ersehnten Beweis fürd as 1976 aufgestellte Postulat aktiver Fe-Katalysatoren mit S = 1 = 2 -Signatur im EPR-Spektrum zu erbringen. [17] Die Cluster 9 und 10 gingen allerdings keine produktive Kupplung mit elektrophilen Organohalogeniden ein; [14,15] die Reaktion zwischen 9 und Alkenylbromid wurde nur nach Zugabe von 1 ¾quiv.M eMgBr beobachtet (Schema 4).…”