Unexpected stable dimerisation of an anionic imidopyrrolecarboxylate in polar solution.

The imidopyrrolecarboxylate 3(-) unexpectedly forms stable dimers (K(ass) = 130 M(-1) in CHCl(3)/DMSO, 1 : 1, v/v) despite the fact that two anions have to interact. The dimer is more stable than an analogous neutral amidopyrrolecarboxylic acid dimer (K(ass) < 10 M(-1)) underlining the importance of charged H-bonds compared to neutral ones.

Self-association of an indole based guanidinium-carboxylate-zwitterion: formation of stable dimers in solution and the solid state.

The indole based zwitterion 2 forms stable dimers held together by H-bond assisted ion pairs. Dimerisation was confirmed in the solid state and studied in solution using dilution NMR experiments. Even though zwitterion 2 forms very stable dimers even in DMSO, their stability is lower than of an analogous pyrrole based zwitterion 1. As revealed by the X-ray crystal structure the two binding sites in 2 cannot be planar due to steric interactions between the guanidinium group and a neighbouring aromatic CH. Hence the guanidinium moiety is twisted out of planarity from the rest of the molecule forcing the two monomers in dimer 2.2 to interact in a non-ideal orientation. Furthermore, the acidity of the NHs is lower than in 1 (as determined by UV-pH-titration) also leading to less efficient binding interactions.