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 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.