How Hydrogen Bonds in Hydroxyl-Functionalised Ionic Liquids Stabilise Cation-Networks and Lend a Hand to Molecular Components
Johanna BUSCH1, Dietmar PASCHEK1, Tristan YOUNGS2, Ralf LUDWIG1,3
1Department of Chemistry, University of Rostock, Rostock, Germany
2ISIS Faculty, STFC, Rutherford Appleton Laboratory, Didcot, United Kingdom
3Department Life, Light and Matter, University of Rostock, Rostock, Germany
Functionalisation is a novel way of adding specific chemical interactions to Ionic Liquids (ILs), thus offering the possibility to fine-tune their physical and chemical properties. In particular, hydroxyl-functionalised cations allow anchoring molecular components with hydrogen bond (HB) donating and accepting capabilities to the cations. Moreover, the cations themselves may form hydrogen-bonded contacts, thus creating cation clusters, which are stabilised via localised HB networks [1-2].
This concept of attraction between like-charged ions may seem counterintuitive at first glance, but a variety of spectroscopic techniques allowed us to find evidence for the existence of HB-stabilised cation clusters in 1-(4-hydroxybutyl)-pyridinium bis(trifluoromethanesulfonyl)imide, [HOC4Py][NTf2]. Interestingly, the HBs between cations seem to be stronger than their cation-anion counterparts [3-4]. This double-faced nature of HBs in hydroxyl-functionalised ILs can be characterised by a combination of neutron diffraction experiments (ND) and molecular dynamics (MD) simulations. The methodology also allows an exploration of the influence of an OH-catcher such as DMSO as an OH-catcher on the delicate balance of HBs. DMSO acts as an OH-acceptor through its strongly proton accepting S=O group. One important question is whether DMSO prefers to steal the hydroxyl groups from the cation-anion or from the cation-cation clusters. From ND and MD data, we observe three types of HB situations. Their individual strengths are indicated by characteristic bond lengths r(H???O) and r(O???O). The conceptual knowledge allows us to anchor DMSO within the IL and to control nano-scale structures, being particularly interesting for ILs containing the [NTf2]--anion, which can be quite apolar.
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