Unusually weak binding interactions in tetrazole-amidine complexes.

[reaction: see text] Tetrazoles frequently replace carboxylic acids in pharmaceutical drugs. However, while the binding modes of tetrazolate and carboxylate anions in amidinium complexes turns out to be similar, the association constant of the former is 2-3 orders of magnitude smaller in DMSO. Crystal structures revealed that the N...H-N hydrogen bonds in amidinium tetrazolates are bent (162 degrees and 169 degrees ) and noticeably longer (N...N 2.96 A) than corresponding hydrogen bonds in both amidinium carboxylates and ammonium tetrazolates.