Elsevier

Dyes and Pigments

Volume 178, July 2020, 108342
Dyes and Pigments

Tautomerism of azo dyes in the solid state studied by 15N, 14N, 13C and 1H NMR spectroscopy, X-ray diffraction and quantum-chemical calculations

https://doi.org/10.1016/j.dyepig.2020.108342Get rights and content

Highlights

  • The azo–hydrazo tautomerism of azo dyes in the solid state is studied by solid-state NMR spectroscopy.

  • 1H, 13C, 14N and 15N NMR experiments are used for the determination of the tautomeric form.

  • Indirect detection of 14N nuclei via 1H provides a rapid determination of the tautomeric form.

  • Quantum-chemical computations of NMR parameters help to determine the tautomeric form.

Abstract

The azo–hydrazo tautomerism of four model azo dyes in the solid state is investigated by multinuclear solid-state nuclear magnetic resonance (SS-NMR) spectroscopy, X-ray diffraction and quantum-chemical calculations. The measurement of 15N SS-NMR spectra is the most straightforward way to determine the tautomeric state of solid azo compounds. Unfortunately, the sensitivity of these nuclei for SS-NMR is very low and the acquisition of 15N spectra in a reasonable time may be challenging. It is demonstrated that a natural abundance of almost 100% of another nitrogen isotope, 14N, can be exploited for the investigation of the tautomerism. Indirect detection of 14N nuclei via protons is an elegant approach for the rapid and unequivocal determination of the tautomeric form. Furthermore, a comparison of experimental carbon SS-NMR chemical shifts with those obtained in solution or by quantum-chemical computations can be used for the same purpose.

Introduction

Azo dyes are the largest chemical class of dyes frequently used in industry [1,2]. Apart from their use as colorants, azo compounds have found many applications in the rapidly developing field of molecular devices and artificial molecular machines [[3], [4], [5], [6], [7]], where the easy E/Z photoisomerisation of the azo group is exploited.

Azo compounds bearing functional groups with NH, OH or SH hydrogens are tautomeric [8], leading to an equilibrium of azo and hydrazo forms (Fig. 1). The colour and properties of azo compounds are strongly influenced by their tautomerism [9].

Azo dyes in solution can easily be characterised – including azo/hydrazo tautomerism and E/Z isomerism – by using multinuclear magnetic resonance in solution [[10], [11], [12], [13], [14]]. Their characterisation in the solid state is, however, much more complicated.

The most often applied techniques for the determination of crystal structures with atomic resolution are diffraction methods. Particularly, single crystal X-ray diffraction (XRD) has been referred to as the ‘gold standard’ for solid-state structure determination [15]. Unfortunately, many samples do not provide suitable single crystals of appropriate size and quality for X-ray diffraction. Furthermore, hydrogen atoms, which are important for the determination of the tautomeric form, are very difficult to characterise by this technique [16].

Another possible experimental technique for the atomic-level characterisation of solid materials is solid-state NMR spectroscopy (SS-NMR). For example, 13C SS-NMR is commonly applied in structural studies of polymorphism [17,18]. Hydrogen atoms have the most sensitive nuclei for NMR spectroscopy. Furthermore, SS-NMR does not require a long-range order in the materials studied and can thus be used for the characterisation of disordered or amorphous samples. Therefore, SS-NMR is a complementary technique to diffraction in many aspects. The development of ultra-fast magic-angle spinning (MAS) NMR probes and advances in modern pulse techniques have transformed the resolving power and sensitivity of SS-NMR [19]. There is only a limited number of SS-NMR studies of azo dyes; they describe mostly the use of 13C experiments [[20], [21], [22]] or 15N SS-NMR experiments of 15N labelled compounds [23].

In this paper, we investigate the tautomerism of solid azo dyes 14 (Fig. 1) by SS-NMR. We discuss the use of 15N, 14N, 13C and 1H nuclei and various experimental approaches for the unequivocal determination of the tautomeric form. Furthermore, we combine the experimental data with quantum-chemical computations to assist the assignment of the signals and to determine the tautomeric form easily. Compounds with a broad range of proton T1 relaxation times have been selected on purpose as the model azo dyes studied here. Compounds with very long proton T1 relaxation time (such as compound 1) represent the most difficult systems for SS-NMR experiments because very long experimental times are required. However, we demonstrate that the tautomeric form can easily and quickly be determined even in these ‘difficult’ cases if modern SS-NMR experiments are applied.

Section snippets

Synthesis of compounds 14

Appropriate diazonium salts were prepared according to procedures described in literature [24] and they were added to 5-amino-3-methyl-1-phenylpyrazole and 3-methyl-1-phenylpyrazol-5-one, respectively, dissolved in acetic acid to give compounds 1 and 2 in high yields (>85%). Compounds 3 and 4 were prepared according to a published procedure [25]. Solid products were separated and crystallised from ethanol.

NMR experiments

High-resolution 1H, 13C and 15N solid-state NMR spectra were obtained using a JEOL ECZ600R

X-ray diffraction

The crystal structures of compounds 1 and 2 have recently been determined [38]. Compound 1 was found to crystallise as the azo form whereas compound 2 as the hydrazo form (Fig. 1). We determined the crystal structure of compounds 3 and 4 here. Both compounds crystallise in their hydroxyl–azo form with an intramolecular hydrogen bond between the hydroxyl group and one of the azo nitrogen atoms (Fig. 2). The hydrogen atoms in the hydroxyl group were unambiguously found on the difference Fourier

Conclusions

We investigated SS-NMR methods for the determination of the azo–hydrazo tautomeric state of azo dyes. We selected four model compounds, whose structure was determined by X-ray diffraction; three of them crystallise as the azo forms and one as the hydrazo form.

15N CP-MAS spectra provide straightforward evidence of the azo/hydrazo group, because their nitrogen chemical shifts are substantially different. Unfortunately, the 15N nucleus has a very low natural abundance and low gyromagnetic ratio,

CRediT authorship contribution statement

Kateřina Bártová: Investigation. Ivana Císařová: Investigation. Antonín Lyčka: Conceptualization, Investigation. Martin Dračínský: Conceptualization, Methodology, Validation, Writing - review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The work was supported by the Czech Science Foundation (grant no. 20-01472S) and the University of Hradec Králové. We thank Dr. Yusuke Nishiyama for help with setting up the 1H–14N NMR experiments.

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