Homoconjugated hydrogen bonds with amidine and guanidine bases: Osmometric, potentiometric and FTIR studies

Abstract

Five very strong N bases, 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), pKa = 23.4; 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), pKa = 23.9; tetramethylguanidine (TMG), pKa = 23.3; 2-phenyl-tetramethylguanidine (PhTMG), pKa = 20.6; and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), pKa = 24.97; have been studied by osmometric measurements which showed that they are monomeric in acetonitrile solutions. The constants of the formation of homoconjugated complexes were determined by potentiometric measurements. In the IR spectra of the semi-protonated complexes of DBN, DBU and TMG, the homoconjugated N+-H⋯N⇌ N⋯H-+N hydrogen bonds cause broad band complexes in the region 3200-2500 cm-1 instead of the expected continua. This spectral peculiarity is discussed.