Identification of structure–biodegradability relationships for ionic liquids – clustering of a dataset based on structural similarity

Abstract

Environmentally open applications as herbicides or active pharmaceutical ingredients are discussed for ionic liquids (ILs). Since most of the ILs are not readily biodegradable in the environment, they may persist there. To prevent the accumulation of persistent and toxic ILs, both the cation and anion need to be designed to completely mineralise in the environment. Several studies summarised structure–biodegradability relationships (SBRs) and gained rules of thumb for ILs’ biodegradability based on the available literature data. However, no study systematically analysed a dataset using an in silico tool. Therefore, to identify SBRs a dataset on the ready biodegradability of 508 ILs was clustered according to IL similarity by using the software Canvas by Schrödinger. The biodegradability was divided into three classes (biodegradation rates 0–19%, 20–59% and ≥60%). The identified SBRs were compared with the available rules of thumb from the literature. The results show that the cholinium cation and its derivatives acetylcholine, betaine and carnitine are promising candidates for designing environmentally mineralising ILs if a good biodegradable anion is chosen. Imidazolium and phosphonium ILs should be avoided. For pyrrolidinium and quaternary ammonium compounds cations containing ester or carboxyl groups in side chains and alkylsulphate anions should be tested to close gaps in SBRs and possibly design a mineralising IL. Due to the limited data of morpholinium, 1,4-diazabicyclo[2.2.2]octanium (DABCO), piperidinium, prolinium, piperazinium and thiazolium ILs, SBRs could not be clearly identified. Further research is needed on whether structural adjustments according to the findings can increase the biodegradability of not yet fully degrading (20–59%) ILs.