Hindered Hydrogen Bonding In 2,6-Di-Tert-Butylphenols With Para Aza-Heterocyclic Rings

A crystallographic study of several 2,6-di-tert-butylated phenols having 4-azaheterocyclic substituents showed phenolic (OHN)-N-... hydrogen bonding despite the steric hindrance of the tert-butyl groups, with O(H)N-... donor-to-acceptor distances less than 3 angstrom in several cases. The (OHN)-N-... angles are substantially bent from ideal near-linearity. Infrared spectroscopy in KBr matrixes for some of the tert-butylated systems shows broadened OH stretching bands that are shifted to lower energy than is typical of 2,6-di-tert-butylated phenols. This is consistent with hydrogen bonding, whereas narrow OH stretching bands are more typical for systems such as 2,6-di-tert-butylated phenols that are too sterically hindered for effective phenolic hydrogen bonding. Head-to-tail 1-D chains of phenolic (OHN)-N-... interactions are present in a number of the crystal lattices. Although the set of studied molecules has limited conformational flexibility to allow for optimizing hydrogen-bond geometry within the constraints of ortho-tert-butylation (save for torsion about the bond between phenol and 4-azaheterocycle units), the highly hindered phenolic OH groups still are capable of engaging in hydrogen bonding. Less hindered hydrogen bonding is preferred over interactions involving the hindered phenolic OH group, as exemplified in a system that also contains a benzimidazole group.