Spectroscopic and DFT Study of Ferraaziridine Complexes Formed in the Transfer Hydrogenation of Acetophenone Catalyzed Using <i>trans</i>-[Fe(CO)(NCMe)(PPh₂C₆H₄CH═NCH₂−)₂-κ⁴<i>P,N,N,P</i>](BF₄)₂

The reaction of the iron complex trans -[Fe(CO)(MeCN)(PPh 2 C 6 H 4 CH═NCH 2 −) 2 -κ 4 P, N, N, P ](BF 4 ) 2 ( 1 ) with KO i Pr in benzene produced the unusual complex [Fe(CO)(PPh 2 C 6 H 4 CH═NCH 2 CH 2 NHCHC 6 H 4 PPh 2 )-κ 5 P, N, C, N, P ][BF 4 ] ( 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-κ 2 C, N complex instead of an iron(0) η 2 -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)(OCHMe 2 )(PH 2 C 6 H 4 CH═NCH 2 −) 2 -κ 4 P, N, N, P ] + ( 4 DFT ) to an imine carbon on the ligand to produce the amide complex trans- [Fe(CO)(OC(CH 3 ) 2 )(PH 2 C 6 H 4 CH═NCH 2 CH 2 NCH 2 C 6 H 4 PH 2 -κ 4 P, N, N, P )] + ( 5 DFT acet ) followed by liberation of acetone to afford 5 DFT . Two energetically similar pathways have been proposed in which deprotonation of the PNNP ligand of 5 DFT by strong base produces the experimentally observed ferraaziridinido complex Fe(CO)(PH 2 C 6 H 4 CH═NCH 2 CH 2 NCHC 6 H 4 PH 2 )-κ 5 P, N, C, N, P ( 3 DFT ) or the square-pyramidal Fe(0) complex Fe(CO)(PH 2 C 6 H 4 CH═NCH 2 −) 2 -κ 4 P, N, N, P ( 7 DFT ). Protonation of 3 DFT by free isopropyl alcohol produces the ferraaziridine complex 2 DFT . 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.

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