Halogen-bonding in 3-nitrobenzaldehyde-derived dichlorodiazadienes

A set of 4,4-dichloro-1,2-diazabutadienes derived from 3-nitrobenzaldehyde was prepared by the copper catalyzed reaction of the corresponding hydrazones with CCl 4 in the presence of TMEDA. The structure of all products was confirmed by 1 H and 13 C NMR spectra and ESI-MS. X-ray diffraction revealed Cl ··· O halogen binding in the crystal form for some dienes. Moreover, Br ··· π and Cl ·· π types of halogen bonds are observed. Azo dyes absorb in the UV – Vis region with the  max located at about 234 −437 nm in CH 2 Cl 2 , DMF and MeOH. Both absorption intensity and  max is dependent on the solvent polarity and the electron-withdrawing/donating ability of para -substituents on the aromatic moiety of the dichlorodiazadienes


Introduction
Azo dyes R−N=N−R (R = aryl, heterocycle, etc.) are a well-known class of organic compounds in synthetic chemistry.They have been extensively used for ink jet printing, as food additives, for dyeing textiles, as indicators, cosmetics, in non-linear optics and dye-sensitized solar cells, as pigments, radical reaction initiators, photo-sensitizers, and therapeutic agents, etc. 1−5 Due to the industrial applications of azo dyes, the −N=N− fragment has been incorporated into polymers and supramolecular compounds to show valuable properties and the possibility of fine-tuning by variation of R. 6,7 There are many synthetic strategies for the synthesis of azo compounds, but the well-established method is azo-coupling of an aryl diazonium cation with another aryl ring, especially those substituted with electron-donating groups. 1,2Recently, some of us have developed a new method for the synthesis of azo dyes, namely a copper-catalyzed transformation of Nsubstituted hydrazones into halogenated azabutadienes. 8The attached reactive olefin moiety can contribute to π-conjugation in the design of functional materials or used for further functionalization of dyes.
−15 Halogen bonding is also an effective strategy to control molecular recognition in solution. 16The study of halogen-bonded associates has become one of the most attractive areas of research in the solution chemistry. 17−22 Herein, we demonstrate that this type of noncovalent interaction is of great importance in the design of diazadienes in terms of their intermolecular halogen bond strength (Scheme 1).The para-substituents on the aromatic moiety range from typical electron-withdrawing (−Br, −Cl and −F) to typical electron-donating groups (−OCH3 and −CH3).

Results and Discussion
A set of hydrazones 1 was prepared by the reaction of 3-nitrobenzaldehyde with hydrochlorides of 4substituted aryl hydrazines (Scheme 1).The reaction was performed in the presence of CH3COONa in EtOH at 80 o C [see Electronic Supplementary Information (ESI)].Next, the reaction of 1 with CCl4 in the presence of CuCl and tetramethylethylenediamine (TMEDA) in DMSO permitted synthesis of the corresponding dichlorodiazadienes 2. The prepared dienes 2 were isolated as E-isomers in up to 80 % yield (see ESI).The structures of all products were confirmed by 1 H and 13 C NMR spectra and ESI-MS.All diazadienes 2 have characteristic signals in their 13 C NMR spectra at 123-125 ppm which are typical for these compounds. 8The peaks corresponding to the aryl rings of dienes 2 were found in the NMR spectra.In addition, X-ray data were obtained for some products (Figure 1).Single crystal X-ray analyses of 2b, 2d and 2e show that the 3-nitroaromatic moiety rotates around the C1−C7 bond with dihedral angles of 65.0( 5 , Figure 3).Three absorption peaks were observed for all compounds (Figure 3), which are significantly influenced by the polarity, H-bond donor or H-bond acceptor ability ( or ) 24 of the organic solvents and the electron-withdrawing or electrondonating character of the substituents. 23The shortest wavelength peak observed in the range of 234−305 nm can be assigned to the excitation of the -electrons within the C=C bonds in the olefin moieties.3), featuring a short energy gap between two states (ground and excited state) with strong intermolecular halogen bonding in CH2Cl2, whereas there is no clear trend/correlation between max and p of para-substituents.

Conclusions
A set of (E)-1-(2,2-dichloro-1-(3-nitrophenyl)vinyl)-2-(para-substituted phenyl)diazenes 2 was synthesized by Cu-catalysed olefination of the corresponding N-aryl hydrazones with CCl4 in the presence of TMEDA in DMSO.Depending on the attached para-substituents in the N-aryl fragment, Cl•••O, Br•••π and Cl••π types of halogen bonds were found in these dyes.The UV spectra of 2 showed three λmax in CH2Cl2, DMF and MeOH, which can be attributed: i) the excitation of the -electrons within the C=C bonds; ii) the →* transition in the aromatic rings; iii) the n→* transition in N=N bonds as well as intermolecular halogen and hydrogen bonds.−35

Experimental Section
General.All the chemicals were obtained from commercial sources (Aldrich) and used as received.Infrared spectra (4000-400 cm −1 ) were recorded on a Vertex 70 (Bruker) instrument in KBr pellets.Carbon, hydrogen, and nitrogen elemental analyses were carried out using a 2400 CHN Elemental Analyzer (Perkin Elmer).The 1 H and 13 C NMR spectra were recorded on Bruker Advance II+ 300.13 (75.468 carbon-13) MHz (UltraShield TM Magnet) spectrometer at ambient temperature.Chemical shifts are reported in ppm using tetramethylsilane as the internal reference.Electrospray mass spectra (ESI-MS) were run with an ion-trap instrument (Varian 500-MS LC Ion Trap Mass Spectrometer) equipped with an electrospray ion source.For electrospray ionization, the drying gas and flow rate were optimized according to the particular sample with 35 p.s.i.nebulizer pressure.Scanning was performed from m/z 0 to 1100 in methanol solution.The compounds were observed in the positive mode (capillary voltage = 80−105 V).The UV-vis absorption spectra in the 200 − 500 nm regions were recorded with a scan rate of 240 nm•min −1 by using a Lambda 35 UV-vis spectrophotometer (Perkin-Elmer) in 1.00 cm quartz cells at room temperature, with a concentration of 2a-2f of 1.00•10 −6 mol L −1 in CH2Cl2, DMF or MeOH.X-ray structure determinations.X-ray diffraction patterns of 2b, 2d and 2e were collected using a Bruker SMART APEX-II CCD area detector equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 296(2) K, respectively.Absorption correction was applied by SADABS [s1,s2].The structures were solved by direct methods and refined on F 2 by full-matrix least-squares using Bruker's SHELXTL-97 [s3].All non-hydrogen atoms were refined anisotropically.The hydrogen atoms were inserted at calculated positions.The details of the crystallographic data for 2b, 2d and 2e are summarized in Table 1.Crystallographic data for the structural analysis have been deposited to the Cambridge Crystallographic Data Center (CCDC 1897671 for 2b, 2019413 for 2d and 2019414 for 2e).Copy of this information can be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: (+44) 1223-336033; E-mail: deposit@ccdc.cam.ac.uk or www.ccdc.cam.ac.uk/data_request/cif).

a
Electric permittivity; b H-bond donating ability of solvent; 24 c H-bonding acceptor ability of solvent. 24d Hammet substituent constant.UV-visible absorption spectra of dienes 2 were recorded over the wavelength range of 200−500 nm, in different solvents [dichloromethane (CH2Cl2), dimethylformamide (DMF) and methanol (MeOH)] with different polarity indices, at 298 K and a concentration of 1.00•10 −6 mol L −1 (Table 26−29 The second UV absorption peaks around 355−370 (in CH2Cl2), 297−345 (in DMF) and 264−322 (in MeOH) nm (Table) can be attributed to the →* transition in the aromatic rings. 26−29 The third maxima in the UV spectra of 2a-2f in the range 409−437 (in CH2Cl2), 349−391 (in DMF) and 361−398 (in MeOH) nm can be attributed to the n→* transition in N=N bonds as well as intermolecular hydrogen, Cl•••O, Br•••π or Cl•••π types of halogen bonds.The absorption intensity of the studied dyes increases with a decrease of the solvent polarity, MeOH < DMF < CH2Cl2 (Figure