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 center dot center dot center dot O/O-H center dot center dot center dot 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 center dot center dot center dot N form. Furthermore, they display the following features: i) strongly electron-donating substituents in p-position on the N-phenyl group shorten the N center dot center dot center dot O contact distance but not enough to produce the O-H center dot center dot center dot N to O-H center dot center dot center dot N/N-H center dot center dot center dot 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 center dot center dot center dot O contacts with the phenolic oxygen which, stabilizing the N-H center dot center dot center dot O bond produce the O-H center dot center dot center dot N to O-H center dot center dot center dot N/N-H center dot center dot center dot O transition and, sometimes, the complete proton transfer to the pure N-H center dot center dot center dot 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 center dot center dot center dot N/N-H center dot center dot center dot 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 center dot center dot center dot O form, while substituents with increasing electron-donating properties transform the system into a dynamic, or static, N-H center dot center dot center dot O reversible arrow O-H center dot center dot center dot N tautomeric equilibrium in the solid with an ever increasing population of the O-H center dot center dot center dot N azoenol tautomer.