Quinoline Pyridine(Imine) Iron Complexes as Catalysts for the 1,4-Hydrovinylation of 1,3-Dienes

The synthesis, characterization, and stoichiometric reactivity
of a series of quinoline pyridine(imine) (R(R’)Q(R”)PI) (R = 2,6-Me, 2,6-iPr or 2,4,6-Cy; R’
= H or Cl, R″ = H or tBu) iron dichloride complexes is described.
Treatment of (QPI)FeCl2 with two equivalents of methyl
lithium furnished two examples of the corresponding (QPI)FeCH3 complexes. The molecular structures were established by X-ray
diffraction, and the electronic structures were studied by magnetometry, 57Fe Mössbauer spectroscopy, and DFT calculations. The
combined data support overall S = 3/2 ground states
that are best described as high spin iron(II) complexes engaged in
antiferromagnetic coupling with a chelate radical anion, similar to
related pyridine(diimine) and terpyridine iron alkyl complexes. Bimolecular
reductive elimination of ethane in the presence of butadiene afforded
an iron crotyl complex bearing a cyclometalated QPI ligand, formed
upon ligand-to-ligand hydrogen transfer (LLHT) or stepwise oxidative
addition/reduction elimination from a putative (QPI) iron butadiene
intermediate. The cyclometalated QPI iron crotyl complex proved to
be a competent catalyst for the hydrovinylation of butadiene with
ethylene and produced high selectivity for the hexa-1,4-diene isomer.
Deuterium labeling experiments established H/D scrambling between
ethylene, butadiene and the QPI ligand, consistent with background
transfer hydrogenation reactivity and catalyst decomposition observed
during catalysis.