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

Abstract

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 N-H…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 Nsubstituent, 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.

Molecular structure of (Z)-6-[(5-chloro-2-hydroxyphenylamino) methylene]-3-(diethylamino)cyclohexa-2,4-dienone: A combined experimental and theoretical study

“…These properties were investigated by Senier and Shepheard in 1909 for the first time [2,3]. The interest on ortho-hydroxy Schiff base compounds maintains its freshness for years and there are many published studies reporting the related features [4][5][6][7][8][9][10][11]. They have wide application in many fields, e.g., as ligands in the field of coordination chemistry [12,13], as starting materials in synthesis of important drugs (antibiotics, antiallergics, antitumors and antifungals) in pharmacy [14][15][16][17], as new organic materials in nanotechnology [18,19] and as elements for the design of various electronic devices such as optical switches and optical data storage devices [20,21].…”

Joint Electrical, Photophysical, and Photovoltaic Studies on Truxene Dye‐Sensitized Solar Cells: Impact of Arylamine Electron Donors

“…At atom C10 of the molecule, a carbonyl group is substituted. X‐ray study further reveals that the keto–enol tautomerisation [ 14, 15] occur in the molecule due to intramolecular proton sharing between nitrogen of ring IV and oxygen atom of carbonyl group (N1H1…O1) and this affects in elongation of single bond distance (N1H1 = 1.164 Å) and double bond of carbonyl group(C10O1= 1.29 Å). The crystal data of keto‐8 , enol‐8 , C 22 H 16 N 2 O, M = 324.37, triclinic, P 1, a = 9.260(1) Å, b = 9.930(2) Å, c = 10.820(2) Å, α = 79.37(3)°, β = 65.46(3)°, γ = 68.97(3)°, V = 844.0(3) Å 3 , Z = 2, D c = 1.276 g cm −3 , μ(Mo‐Kα) = 0.08 mm −1 , F (0 0 0) = 340.0, rectangular block, yellowish, size = 0.2 × 0.3 × 0.35 mm 3 , 9790 reflections measured ( R int = 0.021), 3881 unique, w R 2 = 0.1394 for all data, conventional R = 0.0473 for 2891 Fo > 4σ(Fo) and 0.0634 for all 3881data, Goodness of fit S = 1.039 for all data and 230 parameters and zero restraints.…”

Eco‐friendly HClO4–SiO2 catalyzed synthesis of mono‐ and bis‐diaryl‐2h‐1,4 benzothiazines

“…At atom C10 of the molecule, a carbonyl group is substituted. X-ray study further reveals that the keto-enol tautomerisation [14,15] (9). The reaction was performed in the similar manner as described for compound 6 using diaroylethylene and compound 2 in equimolar quantities.…”