Iron Complexes for the Catalytic Transfer Hydrogenation of Acetophenone: Steric and Electronic Effects Imposed by Alkyl Substituents at Phosphorus

A series of iron(II) complexes, trans-[Fe(NCMe)2(PR2CH2CHNCH2CH2NCHCH2PR2)][BPh4]2 (5, R = Cy; 7, R = iPr; 9, R = Et) were prepared via the template synthesis in one-pot involving air-stable phosphonium dimers, [cyclo-(-PR2CH2CH(OH)−)2](Br)2 (4, R = Cy; 6, R = iPr; 8, R = Et), KOtBu, [Fe(H2O)6][BF4]2 and ethylenediamine in acetonitrile. In the synthesis of 9, a methanol/acetonitrile solvent mixture was required; otherwise an intermediate iron bis(tridentate) complex, [Fe(PEt2CH2CHNCH2CH2NH2)2]2+, formed as determined by electrospray ionization mass spectrometry (ESI-MS). The crude iron(II) complexes from a template synthesis with ethylenediamine or (S,S)-1,2-diphenylethylenediamine are stirred in acetone under a CO atmosphere (∼2 atm) overnight to displace a NCMe ligand; however, in addition to this, bromide displaces an NCMe ligand as well to form a new class of the iron complexes trans-[Fe(CO)(Br)(PR2CH2CHNCHR′CHR′NCHCH2PR2)]+ (10 R = Cy, R′ = H; (S,S)-11, R = Cy, R′ = Ph; 12, R = iPr, R′ = H; (S,S)-13, R = iPr, R′ = Ph; 14, R = Et, R′ = H; (S,S)-15, R = Et, R′ = Ph). These complexes were isolated in moderate yields (55−84%) as tetraphenylborate salts. Complexes 10−15 were tested for the catalytic transfer hydrogenation of acetophenone in basic iso-propanol at 25 and 50 °C. The complexes 10−13 (where R = Cy or iPr) were inactive while the complexes 14 and (S,S)-15 (where R = Et) were active at 25 °C but had better activity at 50 °C. Complex (S,S)-15 was higher in activity than complex 14, achieving turnover frequencies as high as 4100 h−1, conversions of acetophenone to (R)-1-phenylethanol as high as 80% and an enantiomeric excess (e.e.) of 50% in the product. As catalysis progressed, the e.e. diminished to as low as 26%.