Norbornene chaotropic salts as low molecular mass ionic organogelators (LMIOGs).

Jordan R Engstrom,Frederick M Pfeffer,Alessandra Sutti,Jonathan M White,Aramballi J Savyasachi,Chris S Hawes,Marzieh Parhizkar,Thorfinnur Gunnlaugsson

doi:10.1039/c8sc01798k

Abstract

Phenylalanine functionalised norbornene (9:Na) functions as a potent, low molecular-mass (MW = 333 Da) ionic organogelator with a minimum gelating concentration of 0.5 wt% in THF, i-PrOH, 1,4-dioxane and n-BuOH. Fibrous crystals form in the gel and X-ray crystallography identified a cation mediated helical assembly process controlled by the chirality of the phenylalanine. In addition to excellent gelating properties 9:Na readily forms aqueous biphasic and triphasic systems.

Highlights

Our ongoing interest in applications of functionalised norbornanes and related norbornylogous systems,[26,27,28,29,30,31,32,33] led us to synthesise simple norbornanes modi ed with amino-acids and here we report the development of such functionalised norbornenes as new low molecular mass ionic organogelators (LMIOGs) (MW < 350 Da)
Long needle-like crystals of (S)-9:Na that were suitable for X-ray analysis were obtained upon crystallisation from its i-PrOH gel (Fig. 7) and revealed that individual units of the norbornene adopt an amphiphilic structure in the solid state, with the norbornene framework and the phenylalanine aromatic positioned in close proximity (potentially through C(sp2)–H/p interactions)[67,68,69] and the carboxylate residue oriented away from the hydrophobic portions (Fig. 13)
9:Na acts as an efficient low molecular weight ionic organogelator (LMIOG) in a variety of organic solvents at concentrations as low as 0.5 wt%

Summary

Introduction

The development of efficient organogelating agents remains an important pursuit as the corresponding organogels have found use in sensing,[1,2,3] cosmetics,[4,5] water puri cation[6,7] and drug delivery applications.[8,9,10,11,12] Low molecular mass organogelators (LMOGs) are of particular interest as these can self-assemble using non-covalent interactions[9,13,14,15,16] to give rise to novel so materials with a range of entangled and solvent dependent morphologies.[13,17,18] Recently we have developed various examples of LMOGs from simple organic ligands that can, through selfassociation, or via coordination to either d- or f-metal ions, give rise to functional self-assembled gels.[19,20,21,22,23,24,25] In parallel, our ongoing interest in applications of functionalised norbornanes and related norbornylogous systems,[26,27,28,29,30,31,32,33] led us to synthesise simple norbornanes modi ed with amino-acids and here we report the development of such functionalised norbornenes as new low molecular mass ionic organogelators (LMIOGs) (MW < 350 Da).Ionic compounds capable of gelating organic solvents are rare and the examples that do exist are typically either based on steroidal frameworks or possess long alkyl chains that give rise to MW ranging from z350 up to 1000 Da (e.g. 1–4, Fig. 1).[9,13,17,34,35,36,37,38,39,40] Reports of compact LMIOGs, in which large hydrophobic groups have not been deliberately incorporated are extremely rare (e.g. 5 and 6, Fig. 1).[40,41]. When small amounts (4 mg) of the potassium salt 9:K were added to a range of these solvents (200 mL) and heated, on cooling the formation of so -materials with strong gel-like properties (withstanding the classic inversion test) was observed in both i-PrOH and n-BuOH, while a weak gel was formed in PhMe (Table 1).

Results

Conclusion