Stability and CO₂ Capture Ability of Electrogenerated N‐Heterocyclic Carbene in Parent 1‐Butyl‐3‐methylimidazoliun Ionic Liquid (BMIm‐X): The Role of X⁻

Abstract

The electrochemical reduction of imidazolium‐based ionic liquids (BMIm‐X) leads to the formation of corresponding N‐heterocyclic carbenes (NHCs). Owing to the peculiar reactivity of NHCs, solutions containing NHC in ionic liquid (IL) can be used in CO2 capture or in organocatalyzed reactions. It is thus important to know the stability of the carbene in the parent IL. In fact, the NHC lifetime can be influenced by the nature of the anion (X−) of the IL. To better understand the relationship between NHC lifetime (and reactivity towards CO2) and IL medium, the presence of electrogenerated NHC in the parent IL is studied by using voltammetric, infrared, and thermogravimetric analyses and by isolation of the adduct between NHC and benzaldehyde. It is thus possible to estimate the actual NHC concentration and its lifetime through voltammetric peak‐current measurements. Moreover, NHC in BMIm‐X is able to catch CO2 (yielding the adduct NHC–CO2) and, triggered by heating (60‐150 °C), to release CO2. It is proven that the half‐life of NHC and the rate of CO2 capture are both affected by the nature of X−.

Advances in the Knowledge of N-Heterocyclic Carbenes Properties. The Backing of the Electrochemical Investigation

“…After a single electrolysis, in a series of 10 cycles the solution (electrogenerated NHC in imidazolium salt as solvent) was able to catch and release 4 mol of CO 2 per Faradays consumed during the electrolysis [31]. These investigations, extended to different ionic liquids BMImX, proved that the rate of CO 2 capture is affected by the nature of the counter ion X − [39].…”

“…The peak current is directly related to the NHC concentration and thus this peak analysis allows to verify the presence and the concentration of NHC. The stability of several NHCs was studied defining their half-life (τ 1/2 ) by voltammetric analysis [32,[39][40][41]. NHCs were generated by electrolysis of neat azolium salts (system A) or solutions of azolium salts in organic solvents (system B).…”

“…The NHC half-life is influenced by the nature of the counter ion X − of the parent azolium salt and by the presence and nature of organic solvents (Table 2) [39,40]. In fact, τ1/2 for an NHC is quite different when measured in ionic liquid or in organic solvent.…”

“…Recently, papers described the electrochemical synthesis of NHC-CO 2 adducts for the CO 2 catch and release [29,31,39]. A simple electrolysis of neat liquid imidazolium salts (e.g., 1-butyl-3-methyl-1H-imidazolium tetrafluoroborate) generated NHC in the catholyte and, after carbon dioxide bubbling, the NHC-CO 2 adduct.…”

Stability of electrogenerated 1-butyl-3-methylimidazol-2-ylidene in DMF. Part 2. Role of acid substrates. [1]

“…In previous papers [1,[29][30], we reported the use of the voltammetric technique for the determination of the half-life time of imidazolium derived NHCs in organic solvents and in ionic liquids. The half-life time τ 1/2 is defined as the time to halve the NHC 8 concentration with respect to the initial one, irrespectively to the processes (one or more) involved in the NHC 8-transformation.…”