Crystal Chemistry and Prototropic Tautomerism in 2-(1-Iminoalkyl)- phenols (or naphthols) and 2-Diazenyl-phenols (or naphthols)

This review reports a systematic investigation, carried out on the Cambridge Crystallographic Database (CSD), on the structural features of iminoalkyl-phenols (or naphthols) as well as azo-phenols (or naphthols). All these compounds form N-H…O/O-H…N intramolecular hydrogen bonds assisted by resonance (RAHB) in tautomeric equilibrium which can be finely tuned by the substituents. Accordingly, they can present reversible proton transfer leading to interesting properties such as thermochromism and photochromism. The phenol derivatives 2-(1-aryliminoalkyl)phenols and 2-arylazophenols display the predominance of the O-H…N form. Furthermore, they display the following features: i) strongly electron-donating substituents in p-position on the N-phenyl group shorten the N…O contact distance but not enough to produce the O-H…N to O-H…N/N-H…O transition; ii) such a transition can be however induced by strongly electron-withdrawing substituents at the phenol ring which increase the phenol acidity; iii) very similar effects are determined by intermolecular hydrogen-bonded O-H…O contacts with the phenolic oxygen which, stabilizing the N-H…O bond produce the O-H…N to O-H…N/N-H…O transition and, sometimes, the complete proton transfer to the pure NH…O form. The naphthol derivatives (1-(1-aryliminoalkyl-2-naphthols and 1-arylazo-2-naphthols) display the most interesting situation of two roughly isoenergetic O-H…N/N-H…O tautomers which can be tuned by modifying the N-substituent, the effects of substitution at the N-phenyl moiety being by far the most extensively studied. The structures of variously aryl-substituted 1-(1-aryliminoalkyl)-2-naphthols, though collected only at room temperature, indicate that almost all of them are tautomeric equilibria, even if full information on proton populations and dynamic or static nature of the proton disorder are not available. Conversely, the structures of a series of aryl-substituted 1-arylazo-2-naphthols have been recently determined at variable temperature showing that strongly electron-attracting substituents stabilize the pure N-H…O form, while substituents with increasing electron-donating properties transform the system into a dynamic, or static, N-H…O ->/<- O-H…N tautomeric equilibrium in the solid with an ever increasing population of the O-H…N azoenol tautomer.