Tungstate ionic liquids as catalysts for CO₂ fixation into epoxides

Highlights

• Tungstate and peroxotungstate ionic liquids were synthesized by anion metathesis as well as by a greener halide-free method.

• The tungstate and peroxotungstate ionic liquids function as catalysts for CO₂ fixation in epoxides.

• The yields compare to established catalytic systems if the tungstate ionic liquids are coupled with bromide analogues.

• We propose a reaction mechanism based on the experimental data.

Abstract

Herein we describe the syntheses of a series of ammonium, phosphonium, imidazolium and diazabycicloundecenium tungstate and peroxotungstate ionic liquids, their full spectroscopic characterisation (FT-IR, ¹H-, ¹³C-and ¹⁸³W-NMR) and a comparison of their properties and possible applications in catalysis. The synthetic procedures to obtain the ionic liquids rely on anion exchange and acid-base reactions – including an innovative route for the synthesis of tungstate and peroxotungstate ionic liquids using, for the first time, a halide-free organic ionic liquid as precursor. The tungstate ionic liquids were used as catalysts for CO₂ fixation in styrene oxide as well as in a series of other epoxides to yield the corresponding carbonates. Under optimized conditions, styrene carbonate is obtained in up to 67 % yield at 90 °C with just butylmethylimidazolium tungstate, and in 91 % yield by coupling tetrabutylammonium tungstate and bromide. Preliminary tests indicate that the same catalysts can also promote epoxidation reactions, paving the way for their use in the direct oxidative carboxylation of olefins.

Introduction

Emissions of CO₂ represent a widespread concern for industrialized, emerging, and developing countries. At an average growth rate of 2.6 %/y (2000–2014) and with a yearly production of ca 35 gigatons, [1] anthropogenic CO₂ is among the major causes of global warming with temperature predicted to increase up to 2 °C by 2100 (relatively to the pre-industrial level) [2]. While chemical conversion of captured CO₂ will hardly account for more than 1% of the mitigation challenge [3]. Nonetheless CO₂ is a chemically attractive, abundant, renewable and non-poisonous C1 feedstock useful for the synthesis of various classes of compounds, such as carboxylic acids and derivatives, alcohols and organic carbonates [4]. The challenges facing CO₂ conversion are its thermodynamic stability and kinetic inertia. The former implies a large energy input that could generate more carbon dioxide than is consumed, while the latter requires the presence of catalysts able to activate CO₂ [5].

Common routes to activate CO₂ involve the use of Lewis bases such as superbases or amine-containing species, [6] N-heterocyclic carbenes, [7] or by the formation of a transition metal−CO₂ complex [8]. Activated CO₂ is then reacted with starting materials such as unsaturated compounds (e.g. alkenes or alkynes), three membered rings (e.g. epoxides and aziridines) or organometallics to yield more thermodynamically-stable oxygenated products [9].

In this field, a continuous effort has been done in the last years to identify bifunctional catalysts capable of simultaneously activating CO₂ and the substrate [5,10]. A remarkable case is represented by catalytic systems designed for the insertion of CO₂ into epoxides (Fig. 1). The accepted mechanism for the reaction involves a Lewis- or Brønsted acid (A⁺) that activates the epoxide and a strong nucleophile (Nu⁻) which plays a double role by first favouring the ring opening and then acting as a leaving group in the final step where the cyclic organic carbonate forms and the catalyst is restored.

Usually A⁺ is a tetra-alkylammonium or metal-based Lewis acid (e.g. Fe, Cr, Co, Al, Sn) [11] or a species with H-bonding ability (e.g. −OH, −COOH, -NH group) [12], while Nu⁻ is a halide (especially bromide or iodide).

Comprehensive recent reviews on the subject have further confirmed that the use of bifunctional metal- and organo-catalysts for the reaction requires a co-catalyst or a halide salt to induce the epoxide ring-opening [10,13]. In addition, very few articles have exploited halide-free processes [14].

Among metal-based catalysts, Kimura et al. have shown that monomeric tungstate salts as tetrabutylammonium tungstate ([N_4,4,4,4]₂[WO₄], were active systems for the chemical fixation of CO₂ into different compounds such as aryldiamines, primary monoamines, propargylic alcohols or 2-aminobenzonitriles. The comparably higher basicity, nucleophilicity, and H-bonding character of [WO₄]²⁻ with respect to polyoxotungstates, was accounted for the concurrent activation of CO₂ and reactant substrates [15]. Subsequently, Guo et al. also proved that silver tungstate acted as a bifunctional catalyst for the carboxylation of terminal alkynes with CO₂ under ambient conditions, in the presence of a stochiometric amount of a base and of butyl iodide [16]. Although interactions between the tungstate anion and CO₂ were known since 1985, [17] these works were the first examples of catalytic exploitation of the tungstate-carbon dioxide adduct for CO₂ fixation.

In the field of CO₂ insertion into epoxides for the synthesis of cyclic organic carbonates, there is evidence in the literature for the use of complex catalytic systems based on W such as zinc-substituted sandwich type polyoxotungstates, [18] Keggin-type zinc polioxotungstate metal organic frameworks, [19] tetracarbonyl manganese selenotungstate derivatives [20]. Surprisingly however, simple monotungstate-based catalysts have not been reported so far for this reaction. We here report for the first time the synthesis of ammonium and phosphonium tungstate and peroxotungstate ionic liquids and their use as bifunctional catalysts for CO₂ insertion in epoxides. The focal point of this work is the non-trivial synthesis of a series of tungstate and peroxotungstate ionic liquids catalysts (TILCs) and their use for the cycloaddition of CO₂ with different epoxides. The choice of the anions was functional for the development of new tandem chemistry where the tungstate moiety is expected to behave as a catalyst both for olefin epoxidation as well as for CO₂ insertion.