On the real catalytically active species for CO₂ fixation into cyclic carbonates under near ambient conditions: dissociation equilibrium of [BMIm][Fe(NO)₂Cl₂] dependant on reaction temperature

Abstract

An imidazolium based iron-containing ionic liquid [BMIm][Fe(NO)₂Cl₂] (BMIm = 1-n-butyl-3-methyl-imidazolium) has been synthesized for the first time and fully characterized employing a wide range of techniques. The iron-based containing ionic liquid was found to be an active catalyst for the cycloaddition of CO₂ to epoxides, giving high conversions for various substrates under near ambient conditions. In addition, the catalytic system showed a good recycling performance for five consecutive reaction cycles. Key mechanistic studies demonstrated that a bifunctional catalytic system is generated in situ by the partial dissociation of the iron-based ionic liquid into [BMIm][Cl], which results in a very efficient catalyst without the need of any additive or co-catalyst. The metal center plays a role as Lewis acid and activate the epoxide group, and the chloride anion, as part of [BMIm][Cl] moiety, acts as nucleophile and leads to the ring opening through a nucleophilic attack on the less sterically-hindered Cβ he process is favoured by an interaction via H-bonding between the substrate and the H–C₂ of the imidazolium ring, as was demonstrated by additional experiments. Kinetic studies indicated that the process followed first-order kinetics with respect to epoxide concentration and proved the existence of a reversible coordination/de-coordination equilibrium in which the active species are generated from the [BMIm][Fe(NO)₂Cl₂] complex.