Homochiral imidazolium-based dicarboxylate compounds: Structure and solution behaviour

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)-L-iPr]}(n) (1s), {Na[(R,R)-L-iPr]}(n) (1r), {NH4[(S,S)-L-iPr]} n (2s), {NH4[(R,R)-L-iPr]} n (2r), {Li[(S,S)-L-iPr]}(n) (3s) and {Li[( R,R)-L-iPr]}(n) (3r) were characterised by analytical and spectroscopic (IR, H-1 and C-13 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 [L-iPr](-) 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 C-2-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 mu(4)-kappa O-2(1), kappa O-2(3), kappa O-1(4) coordination mode was observed in 1s.