Synthesis and Hydrogenation Activity of Iron Dialkyl Complexes with Chiral Bidentate Phosphines

The
activity of bis­(phosphine) iron dialkyl complexes for the asymmetric
hydrogenation of alkenes has been evaluated. High-throughput experimentation
was used to identify suitable iron–phosphine combinations using
the displacement of pyridine from py2Fe­(CH2SiMe3)2 for precatalyst formation. Preparative-scale
synthesis of a family of bis­(phosphine) iron dialkyl complexes was
also achieved using both ligand substitution and salt metathesis methods.
Each of the isolated organometallic iron complexes was established
as a tetrahedral and hence high-spin ferrous compound, as determined
by Mössbauer spectroscopy, magnetic measurements, and, in many
cases, X-ray diffraction. One example containing a Josiphos-type ligand,
(SL-J212-1)­Fe­(CH2SiMe3)2, proved
more active than other isolated iron dialkyl precatalysts. Filtration
experiments and the lack of observed enantioselectivity support dissociation
of the phosphine ligand upon activation with dihydrogen and formation
of catalytically active heterogeneous iron. The larger six-membered
chelate is believed to reduce the coordination affinity of the phosphine
for the iron center, enabling metal particle formation.