Tautomerism of 4,4′-dihydroxy-1,1′-naphthaldazine studied by experimental and theoretical methods

BACKGROUND: The title compound belongs to the class of bis-azomethine pigments. On the basis of comparative studies on similar structures, insight into the complex excited state dynamics of such compounds has been gained. It has been shown, for example, that only compounds that possess hydroxyl groups are fluorescent, and that the possibility for cis-trans isomerisation and/or bending motions of the central bis-azomethine fragment allows for different non-radiative decay pathways. RESULTS: The compound, 4,4'-dihydroxy-1,1'-naphthaldazine (1) was synthesized and characterized by means of spectroscopic and quantum chemical methods. The tautomerism of 1 was studied in details by steady state UV-Vis spectroscopy and time resolved flash photolysis. The composite shape of the absorption bands was computationally resolved into individual subbands. Thus, the molar fraction of each component and the corresponding tautomeric constants were estimated from the temperature dependent spectra in ethanol. CONCLUSIONS: According to the spectroscopic data the prevalent tautomer is the diol form, which is in agreement with the theoretical (HF and DFT) predictions. The experimental data show, however, that all three tautomers coexist in solution even at room temperature. Relevant theoretical results were obtained after taking into account the solvent effect by the so-called supermolecule-PCM approach. The TD-DFT B3LYP/6-31 G** calculated excitation energies confirm the assignment of the individual bands obtained from the derivative spectroscopy

(E)-1-(4-Meth­oxy­anthracen-1-yl)-2-phenyl­diazene

The title compound, C21H16N2O, has an E-conformation about the diazene N=N bond. It is reasonably planar with the phenyl ring being inclined to the mean plane of the anthracene moiety [planar to within 0.077 (3) Å] by 6.43 (10)°. The crystal structure is stabilized by C—H⋯π and weak π–π inter­actions [centroid–centroid distances of 3.7192 (16) and 3.8382 (15) Å], leading to the formation of two-dimensional networks stacking along [001] and lying parallel to (110)

Controlled tautomerism – switching caused by an ‘underground’ anionic effect

In a previous communication, we demonstrated a conceptual idea for a tautomeric switching system based on implementation of a flexible piperidine unit in 4- (phenyldiazenyl)naphthalen-1-ol (1). The results showed that a directed shift in the position of the tautomeric equilibrium can be achieved through protonation/deprotonation in a number of solvents. However, the effect of the counter ion in the process of protonation was never considered. The crystallographic analysis of protonated cyano and nitro derivatives of 4-(phenyldiazenyl)-2-(piperidin-1- ylmethyl)naphthalen-1-ol have shown an interesting and unexpected feature: the counter ion is captured in the process of protonation and the shift in the position of the tautomeric equilibrium is achieved through a bridged complex formation. To the best of our knowledge this is a rare example when controlled shift in the position of tautomeric equilibrium is achieved through anion complexation. The results from the solid state analyses are confirmed by NMR spectroscopy in solution and by quantum- chemical calculations

Tautomerism in 1-phenylazo-4-naphthols: experimental results vs quantum-chemical predictions

The reliability in the description of the tautomerism of 1-phenylazo-4-naphthol by using of HF and MP2 ab initio levels of theory and DFT methods with variety of pure GGA (OLYP), hybrid (B3LYP and B3PW91), long range corrected (LC-BLYP) and double-hybrid (B2PLYP and mPW2PLYP) functionals with large number of basis sets was estimated. In this evaluation three criteria were used: reproduction of the bond lengths in the structures of the individual tautomers, description of the non-planarity of the enol tautomer and prediction of the position of the tautomeric equilibrium (∆G value) at 298 K. The results show that in substantial number of cases HF reasonably covers all requirements. The tested pure (OLYP) and hybrid functionals (B3LYP and B3PW91) fail in the prediction of the position of the equilibrium independent on the basis set. The situation is slightly better at the long range corrected functional (LC- BLYP), which give predominance of the enol tautomer at 6-31+G** and D95++**. The double hybrid functionals give very good description with D95++** basis set, but at substantial computational costs

Betti bases from 4-(3-pyridazo)-1-naphthol: synthesis, coordination behaviour and unusual substitution reactions

A series of Betti bases from 4‐(3‐pyridyl)azo‐1‐naphthol dye, possessing flexible methylene or more constrained aryl‐methyne spaced NH‐containing side‐chains, were obtained and characterized. It was shown by NMR spectroscopy that the ligands exist in keto form. The products’ structure was confirmed by single crystal XRD of a selected sample. A study on the coordination ability of the ligands with silver(I) was performed, leading to an unexpected substitution reaction

Controlled tautomeric switching in azonaphthols tuned by substituents on the phenyl ring

A series of new tautomeric azonaphthols are synthesized and the possibilities for molecular switching are investigated using molecular spectroscopy, X-ray analysis and density functional theory quantum chemical calculations. Two opposite effects that influence switching are studied: attaching a piperidine sidearm, and adding substituents to the phenyl ring. On the one hand, the attached piperidine moiety stabilizes the enol form leading to a controlled shift of the equilibrium upon protonation. On the other hand, the relative stability of the azonaphthol tautomers strongly depends on the effects of the substituents on the phenyl ring: electron donors tend to stabilize the enol tautomer, whereas electron acceptors lead to stabilization of the keto form. However, these effects do not shift fully the equilibrium towards either of the tautomers. Nevertheless, the effect of the substituents can be an additional tool to affect the switching between “on” and “off” states. Electron-withdrawing substituents stabilize the keto form and impede switching to the off state, whereas electron donors stabilize the enol form. The effect of the piperidine unit is dominant overall, and with strongly electron-withdrawing substituents at the phenyl ring, the enol form exists as a zwitterion

Novel Quaternary Ammonium Derivatives of 4-Pyrrolidino Pyridine: Synthesis, Structural, Thermal, and Antibacterial Studies

Six novel quaternary ammonium derivatives of 4-pyrrolidino pyridine were prepared and isolated via a facile one-pot synthesis and a simple purification procedure. The purity and the molecular structure of the 4-pyrrolidino pyridine derivatives were confirmed with 1H and 13C NMR spectroscopy and powder X-ray diffraction techniques. The crystal structures of the compounds were characterized by single crystal X-ray diffraction (SCXRD) and their thermal properties were studied by Differential Scanning Calorimetry (DSC) analyses. The antibacterial properties of the title compounds against five bacterial strains were evaluated using Kirby–Bauer disk diffusion susceptibility test. The compounds crystallize in the monoclinic or orthorhombic crystal systems (space groups: P21/c, P21/n, or P212121) and their crystal structures are stabilized by a combination of intra- and intermolecular halogen bonding interactions, short contacts and π-π interactions. Above interactions, they contribute to the thermal stability and lack of phase transition effects up to 350 °C. Two of the compounds possess antibacterial effect against E. coli or S. aureus bacterial strains—similar or better than the kanamycin reference

Data on the synthesis and characterization of two novel polydentate ligands possessing unsymmetrical NH–urea fragment

The data represent the synthesis and structural characterization of two novel polydentate ligands possessing unsymmetrical NH-urea fragment: an open-chain substituted aromatic compound with unsymmetrical urea and secondary amine units (S1) and fused aryloxazinone with unsymmetrical urea fragment (S2). The data include the analyzed NMR spectra, turbo-spray mass spectra, melting points and Rf-values. 1D and 2D NMR spectra are given in the article. The efficiency of the ligands as synergists in the isolation and separation of lantanoid ions is reported in rerence. [1]