Organometallic Electron Reservoirs. 5. A Novel Mode of C-H Activation Using Dioxygen via Superoxide Radical Anion in Solution and in the Solid State with C5R5FeIC6R'6. Subsequent Bond Formation with C, Si, P, Mn, Fe, Cr, Mo, and Halogens

Article abstract of DOI:10.1021/ja00415a018

The 19-electron complexes CpFe^I(η⁵-C6Me6) (1), C5Me5Fe^I(η⁶-C6Me6) (26), (CpFe^IC6Me5CH2)2 (22), and CpFe^IC6Et6 (29) react quickly with 1/4 mol of O2/mol of sandwich in pentane, THF, or DME to give 0.5 mol of H2O and 81-96percent of thermally stable d⁶ CpFe(η⁵-C6Me5CH2) (2), C5Me5Fe(η⁵-C6Me5CH2) (27), (CpFe(η⁵-C6Me4CH2CH2))2 (28), and CpFe(η⁵-C6Et5CHMe) (30), resulting from benzylic H atom abstarction. ¹H and ¹³C NMR spectra indicate that the hexaalkylarene ligands are transformed into cyclohexadienyls with doubly bonded exocyclic methylene groups.The same reactions at -78 deg C, fast with 1, 22, and 26, consume 0.5 mol of O2 and produce 0.5 mol of H2O2, the cyclohexadienyl complexes 2, 27, and 28 in somewhat lower yields, and the hydroxides of 1⁺, 22⁺, and 26⁺. t-BuOK (in THF) or KO2 (in Me2SO or with stoichiometric 18-crown-6 ether in THF) also gives 2 by reaction with 1⁺PF6^-.With 26, the aerobic reaction is regiospecific on the arene whereas that of 22 is not so.The reaction between 29 and O2 is slow at -20 deg C (5 min), and the intermediate O2^-. could be characterized by its temperature-dependent EPR spectra.This indicates that H atom abstractions by O2 are best explained in terms of electron transfer to O2 due to the large difference (1 V) between the two reversible systems CpFe(arene)/CpFe⁺(arene) (ca. -1.8 V/SCE) and O2/O2^-. (-0.7 V/SCE), followed by deprotonation of the cationic sandwich by O2^-..The aerobic reaction 1--> 2 also proceeds in the solid state and can be monitored by Moessbauer spectroscopy on microcrystalline samples and monocrystals.In the latter case, the rhombic distortion can be used as a probe showing that aerobic oxidation proceeds on the surface rather than in the bulk. 2 reacts with H2 (Pd/C, THF, 20 deg C) to give 3 by exo hydrogenation.Replacement of the 18 methyl hydrogens by 18 deuteriums occurs (90percent) in 2 h upon contact of 1, 2, or 1⁺OH^- with Na/Hg in DME + air + D2O. 2 also gives nucleophilic substitution with CH3I, PhCOCl, SiMe3Cl, PPh2Cl, CpFe(CO)2Cl, CpMo(CO)3I, Mn(CO)5Br, Fe2(CO)9, Cr(CO)6, and Mo(CO)6 and nucleophilic addition with CO2, CS2, CpFe(η⁶-PhCl) (ortho to Cl), CpFe⁺(CO)3 (to Cp), and CpFe⁺(CO)2(C2H4) (to C2H4).Electron transfer often occurs between 2 and organometallics, giving 1⁺, and with TCNQ, giving 1⁺TCNQ^-.Coupling of 2 to 22⁺ is obtained by reaction of 0.5 mol of I2 in THF.Thus straightforward synthesis of bonds to many elements are achieved upon simple contact of 1 with air followed by reaction with these substrates in situ.