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.
Rether
Carolin
C
Institute of Organic Chemistry, Faculty of Chemistry, University of Duisburg-Essen, Universitätsstrasse 7, 45141 Essen, Germany.
Sicking
Wilhelm
W
Boese
Roland
R
Schmuck
Carsten
C