Fe-containing ionic liquids as catalysts for the dimerization of bicyclo[2.2.1]hepta-2,5-diene

Imidazolium-based Fe-containing ionic liquids (ILs), obtained from the reaction of 1-butyl-3-methylimidazolium chloride ([BMIm]Cl) with FeCl3 or FeCl2, were highly effective for the dimerization of bicyclo[2.2.1]hepta-2,5-diene (2,5-norbornadiene, NBD) in the presence of diethylaluminum chloride (DEAC). Fe-containing ILs produced hexacyclic endo–endo NBD dimer (Hnn) in high yield and selectivity, whereas FeCl3 yielded large amounts of side products. The yield and selectivity of Hnn were affected by the degree of reduction of Fe(III) compounds by DEAC to Fe(II) species as determined by X-ray photoelectron spectroscopy. Higher yield of Hnn was obtained with the catalytic system producing larger amounts of Fe(II) species. Theoretical calculation and experimental results support that Fe(II) is the active species for the dimerization.