Homochiral imidazolium-based dicarboxylate compounds: Structure and solution behaviour

Abstract

Treatment of (S,S)- and (R,R)-1-(1-carboxy-2-methylpropyl)-3-(1-carboxylate-2-methylpropyl)imidazolium compounds, (S,S)-HLiPr and (R,R)-HLiPr, with the appropriate base yielded sodium, ammonium and lithium salts of the corresponding chiral anion in good yields. Compounds {Na[(S,S)-LiPr]}n (1s), {Na[(R,R)-LiPr]}n (1r), {NH4[(S,S)-LiPr]}n (2s), {NH4[(R,R)-LiPr]}n (2r), {Li[(S,S)-LiPr]}n (3s) and {Li[(R,R)-LiPr]}n (3r) were characterised by analytical and spectroscopic (IR, 1H and 13C NMR and polarimetry) methods. Specifically, 1r-3r are the first examples of homochiral compounds containing imidazolium-dicarboxylate ligands that were prepared from non-proteinogenic amino acids. In solution, NMR showed dissociation and this fact was confirmed by the good agreement found between the experimental and calculated NMR chemical shifts of the [LiPr]− anion. Interestingly, a transient carbene intermediate was proposed by DFT calculations in order to explain the solution H-D exchange observed by NMR at the C2-H atom of imidazolium ring. These compounds were also characterised by single-crystal (1s,r and 2s,r) or powder diffraction methods (3s). At the solid state, they are novel one-dimensional coordination polymers in which cations are connected via chiral 2,2′-(imidazolium-1,3-diyl)bis(3-methylbutanoate) anion. For this anion, unprecedented μ4-κ2O1, κ2O3, κ1O4 coordination mode was observed in 1s.