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.