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Spectroscopic and DFT Study of Ferraaziridine Complexes Formed in the Transfer Hydrogenation of Acetophenone Catalyzed Using trans -[Fe(CO)(NCMe)(PPh 2 C 6 H 4 CH═NCH 2 −) 2 -κ 4 P,N,N,P ](BF 4 ) 2

2012; American Chemical Society; Volume: 31; Issue: 8 Linguagem: Inglês

10.1021/om201170f

1520-6041

Demyan E. Prokopchuk, Jessica F. Sonnenberg, Nils Meyer, Marco Zimmer-De Iuliis, Alan J. Lough, Robert H. Morris,

Synthesis and Catalytic Reactions

The reaction of the iron complex trans-[Fe(CO)(MeCN)(PPh2C6H4CH═NCH2−)2-κ4P,N,N,P](BF4)2 (1) with KOiPr in benzene produced the unusual complex [Fe(CO)(PPh2C6H4CH═NCH2CH2NHCHC6H4PPh2)-κ5P,N,C,N,P][BF4] (2), which has been characterized by spectroscopy and by single-crystal X-ray diffraction. The C–N bond length in this complex indicates that it is best viewed as an iron(II) ligand-folded ferraaziridine-κ2C,N complex instead of an iron(0) η2-iminium complex. Density functional theory (DFT) calculations have been employed on simplified structural models to support a mechanism of formation of this complex via the transfer of a hydride from the alkoxide complex trans-[Fe(CO)(OCHMe2)(PH2C6H4CH═NCH2−)2-κ4P,N,N,P]+ (4DFT) to an imine carbon on the ligand to produce the amide complex trans-[Fe(CO)(OC(CH3)2)(PH2C6H4CH═NCH2CH2NCH2C6H4PH2-κ4P,N,N,P)]+ (5DFTacet) followed by liberation of acetone to afford 5DFT. Two energetically similar pathways have been proposed in which deprotonation of the PNNP ligand of 5DFT by strong base produces the experimentally observed ferraaziridinido complex Fe(CO)(PH2C6H4CH═NCH2CH2NCHC6H4PH2)-κ5P,N,C,N,P (3DFT) or the square-pyramidal Fe(0) complex Fe(CO)(PH2C6H4CH═NCH2−)2-κ4P,N,N,P (7DFT). Protonation of 3DFT by free isopropyl alcohol produces the ferraaziridine complex 2DFT. Nuclear magnetic resonance and infrared spectroscopy data show that during the transfer hydrogenation of acetophenone catalyzed by 1 in basic isopropyl alcohol, free ligand is observed along with one major iron-containing species identified as 3. On the basis of our calculations of relative free energies and a CO scale factor, we predict that 2 is easily deprotonated to form the electron-rich iron complex 3 and the square-pyramidal Fe(0) complex 7, which are responsible for the two observed CO stretches below 1900 cm–1 in catalytic mixtures. Mass balance studies indicate that the catalytically active species is not observable by NMR. Although 2 and 3 are poor transfer hydrogenation catalysts, we present experimental and theoretical evidence that ligand folding/distortion is feasible.