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, Fe-57 M & ouml;ssbauer 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.

Automated summary

This study describes the synthesis, characterization, and reactivity of a series of quinoline pyridine(imine) iron dichloride complexes. The complexes were found to be effective catalysts for the hydrovinylation reaction of butadiene with ethylene, showing high selectivity for the hexa-1,4-diene isomer.