Iron Phosphine Catalyzed Cross-Coupling of Tetraorganoborates and Related Group 13 Nucleophiles with Alkyl Halides

Iron
phosphine complexes prove to be good precatalysts for the
cross-coupling of alkyl, benzyl, and allyl halides with not only aryl
triorganoborate salts but also related aluminum-, gallium-, indium-,
and thallium-based nucleophiles. Mechanistic studies revealed that
while Fe­(I) can be accessed on catalytically relevant time scales,
lower average oxidation states are not formed fast enough to be relevant
to catalysis. EPR spectroscopic studies reveal the presence of bis­(diphosphine)­iron­(I)
complexes in representative catalytic reactions and related processes
with a range of group 13 nucleophiles. Isolated examples were studied
by Mössbauer spectroscopy and single-crystal X-ray structural
analysis, while the electronic structure was probed by dispersion-corrected
B3LYP DFT calculations. An EPR study on an iron system with a bulky
diphosphine ligand revealed the presence of an S = 1/2 species consistent with the formation of a mono­(diphosphine)­iron­(I)
species with inequivalent phosphine donor environments. DFT analysis
of model complexes allowed us to rule out a T-shaped Fe­(I) structure,
as this is predicted to be high spin.