Infrared study of hydrogen bonds involving N-heterocyclic bases and phenols

The hydrogen bond complexes between phenols and N-heteroaromatic bases 2,4,6-tri(2-pyridyl)-1,3,5-triazine, 2,2',2 "-terpyridine, quinoxaline, pyrido[2,3-b]pyrazine, pyzazino[2,3-f]quinoxaline and 5-nitrophenanthroline are investigated by infrared spectroscopy in 1,2-dichloroethane. The stability constants of the complexes involving N-heteroaromatic bases characterized by two vicinal nitrogen atoms having lone pairs pointing to each other are higher than predicted from their basicity. Possible differences between protonation and hydrogen bond formation are discussed. N-heteroaromatic bases such as tri(2-pyridyl)-1,3,5-triazine or phenanthrolines cannot be considered as proton sponges but their behaviour is intermediate between that of the classical heteroaromatic bases and the proton sponges.