Synthesis, structure and tautomerism of two benzothiazolyl azo derivatives of 2-naphthol: A crystallographic, NMR and computational study Livio Racané a , Zlatko Mihali  c b , Helena Ceri  c c , Jasminka Popovi  c d , Vesna Trali  c-Kulenovi  c a, * a Department of Applied Chemistry, Faculty of Textile Technology, University of Zagreb, Prilaz baruna Filipovi ca 28a, 10000 Zagreb, Croatia b Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia c Pliva Croatia Ltd., TAPI Research & Development, Prilaz baruna Filipovi ca 25, 10000 Zagreb, Croatia d RuCer Bo skovi c Institute, Division for Materials Physics, Bijeni cka cesta 54, 10000 Zagreb, Croatia article info Article history: Received 19 July 2012 Received in revised form 17 October 2012 Accepted 18 October 2012 Available online 7 November 2012 Keywords: Azo dyes Synthesis Crystal structure DFT-calculations Azoehydrazone tautomerism NMR spectroscopy abstract The efficient synthesis, 1 H and 13 C NMR characterization, crystal and molecular structure, DFT calcula- tions of 6-[(2-hydroxy-1-naphthyl)diazenyl]-2-methylbenzothiazole, as well as DFT calculations of 6-[(2- hydroxy-1-naphthyl)diazenyl]benzothiazole, are reported. Single-crystal X-ray diffraction data show that crystals of the azo dye derived from 2-methylbenzothiazole is principally composed of the hydrazone form. 2D NMR techniques (COSY, HMQC and HMBC) were used to assign 1 H and 13 C chemical shifts for each azo dye in DMSO and CHCl 3 solution. Azoehydrazone tautomerism of the dyes was investigated jointly by 13 C NMR and DFT calculations. All three approaches show that in solution the equilibrium is only slightly shifted towards the hydrazone form. The very good agreement between the three differently obtained equilibrium constants suggest M06-2X/6-311þG(d,p)-SMD as the level of theory of choice for quantum-mechanical calculations of these and similar compounds in solution. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Azo dyes are the largest group of colourants with respect to number and production volume, with applications in many industrial fields for colouring different substrates [1]. In addition to classical applications of azo dyes for colouring textile fibres, paper and food, there is a number of high-tech applications such as for optical data storage and non-linear optics [1e3]. The use of heterocyclic diazo components in the design of new disperse azo dyes is due to the fact that they exhibit better tinctorial strength and brightness in comparison with aromatic analogues [4]. Besides, the replacement of an aromatic diazocomponent with a hetero- aromatic component is interesting from the toxicological point of view. Some of aromatic azo dyes can undergo a natural reductive cleavage of the azo group to potentially carcinogenic amines, especially those derived from aniline, toluidine and benzidine diazo components [5]. On the other hand, detailed knowledge of molecular structure is essential for finding the structureefunction relationships and systematic approach to the design of new dyes [6e8]. The azo dyes with a hydroxyl group in conjunction with an azo chromophore exhibit azoehydrazone tautomerism, and the inves- tigation of proton transfer between oxygen and nitrogen atoms is quite interesting from the theoretical and practical point of view. The position of tautomeric equilibrium affects the basic properties of azo dyes such as colour, tone, photostability as well as technical proper- ties of different tautomers. For instance, the present tautomer could be responsible for NeN or CeN bond cleavage during the bleaching process. A. S. Özen and co-authors [9] established by DFTcalculations that the bleaching process starts via attack to the N atom of azo form causing the cleavage of the NeN bond. The position of the tautomeric equilibrium has been studied by various spectroscopic techniques as well as X-ray crystallography and computational approaches. In order to estimate the position of azoehydrazone equilibrium, NMR spectroscopy techniques are powerful tools [10e12]. In the solid state, X-ray single-crystal diffraction provides detailed structural information for investigation of tautomerism [13,14]. We have previously reported the synthesis and structural study of azoehydrazone tautomerism in the solid state of 6-[(2-hydroxy- 1-naphthyl)diazenyl]benzothiazole (1) by X-ray crystallographic and computational methods [15]. Here, we report the synthesis, crystal and molecular structure of 6-[(2-hydroxy-1-naphthyl)dia- zenyl]-2-methylbenzothiazole (2), as well as investigation of tautomerism on the basis of crystallographic data. We also * Corresponding author. Fax: þ385 1 3712599. E-mail address: vtralic@ttf.hr (V. Trali c-Kulenovi c). Contents lists available at SciVerse ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig 0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2012.10.009 Dyes and Pigments 96 (2013) 672e678