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Preparation and Characterisation of Iodo-Functionalised Azobenzene

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Preparation and Characterisation of Iodo-Functionalised Azobenzene Preparation and Characterisation of Iodo-functionalised Azobenzene p p Derivatives of the Type I– -C6H4–N=N– -C6H4–X Ulrich Siemeling, Clemens Bruhn, Mario Meier, and (in part) Christian Schirrmacher Institut f¨ur Chemie, Universit¨at Kassel, Heinrich-Plett-Straße 40, D-34132 Kassel, Germany Reprint requests to Prof. Dr. Ulrich Siemeling. Fax: +49 561 804 4777. E-mail: [email protected] Z. Naturforsch. 2008, 63b, 1395 – 1401; received October 17, 2008 A broad range of azobenzene derivates of the general type I–p-C6H4–N=N–p-C6H4–X (1)have been prepared. In the case of X = Ph (b), C≡C–Fc (e, Fc = ferrocenyl), OMe (g), Oi-Pr (i), and NMe2 (m), these compounds have been characterised by single-crystal X-ray structure analysis. In addition, the closely related 4-dimethylamino-1-(4-iodophenylazo)naphthalene 2 and 8-(4-iodophen- ylazo)quinoline 3 have also been prepared. Furthermore, the ferrocene derivative Fc–C≡C–p-C6H4– NH2 (4), which served as a starting material for the synthesis of I–p-C6H4–N=N–p-C6H4–p-C6H4– C≡C–Fc (1e), was prepared and structurally characterised by X-ray diffraction. Key words: Azobenzene, Azo-coupling, Crystal Structure, Ferrocene, Nitroso-Amine Condensation Introduction n-BuLi and subsequent reaction with SiCl4. By apply- ing an optimised protocol, we have almost doubled the Self-assembled monolayers (SAMs) [1] which con- yield originally reported [6]. In order to be able to fab- tain azobenzene-based tailgroups are of great current ricate SAMs with enhanced optically switchable prop- interest, since these SAMs can exhibit photoswitch- erties such as, for example, the binding and release of able properties [2], which are due to the photochromic metal ions or Lewis acidic target molecules, tailgroups behaviour of azobenzene derivatives [3]. The synthe- containing a terminal functional group X are essential. sis of azobenzene derivatives suitable for chemisorp- Iodo-substituted azobenzene derivatives of the type I– tion on solid substrates is a prerequisite for the fab- p-C6H4–N=N–p-C6H4–X (1) are therefore key com- rication of such ‘smart’ surfaces. Gold and silicon pounds in this context. We here report on the prepara- have been used most extensively as substrate materi- tion and characterisation of a wide range of such com- als in this context. In the case of gold, reactive sulfur- pounds. containing headgroups (thiols, disulfides, thioacetates) are most popular for chemisorption [1b – e]. In the Results and Discussion case of hydrogen-terminated silicon surfaces, hydrosi- lylative chemisorption of terminal alkene or alkyne The iodo-substituted azobenzene derivatives of groups is the standard approach [4], while with hy- type 1 prepared in our study are collected in Scheme 1. droxylated silicon surfaces the use of reactive silyl The nature of the substituent X varies considerably, groups, usually SiCl3 or Si(OR)3, is preferred [1a]. ranging from simple hydrocarbyl (X = t-Bu, Ph,) to The synthesis of azobenzene derivatives containing re- extended rigid-rod type units (X = C≡C–Ph, C≡C– active headgroups suitable for anchoring on gold or p-C6H4–C≡C–Ph), including also the redox-active silicon is flourishing [2, 5]. We recently reported very organometallic C≡C–Fc unit. Polar groups relevant to promising switchability results for SAMs fabricated the binding and release of Lewis acidic moieties have by chemisorption of Cl3Si–p-C6H4–N=N–Ph on hy- also been included, viz.ORandNMe2 [7]. droxylated silicon [2m]. Saliently, this system exhib- The OR substituted derivatives were obtained in ited an exceptionally large reversible water contact an- high yields by the alkylation of I–p-C6H4–N=N–p- gle change upon irradiation. The trichlorosilyl deriva- C6H4–OH [8] with the respective alkyl halide RY tive is prepared from 4-iodoazobenzene in a one-pot (Y = Br, I) in DMF in the presence of a base procedure, which involves metal-halide exchange with (K2CO3/Cs2CO3). All other derivatives were pre- 0932–0776 / 08 / 1200–1395 $ 06.00 c 2008 Verlag der Zeitschrift f¨ur Naturforschung, T¨ubingen · http://znaturforsch.com 1396 U. Siemeling et al. · Iodo-functionalised Azobenzene Derivatives Fig. 1. Molecular structure of compound 1b in the crystal. Fig. 2. Molecular structure of compound 1e in the crystal. Scheme 1. Azoben- zene derivatives and related compounds prepared in this investigation. pared by the condensation [9] of the readily avail- able 4-iodonitrosobenzene[10] with the respective ani- line derivative H2N–p-C6H4–X in acetic acid. Iso- Fig. 3. Molecular structure of compound 1g in the crystal. lated yields were generally good to excellent. In addi- tion, the dimethylamino-functionalised 1-(4-iodophen- ylazo)naphthalene 2 and 8-(4-iodophenylazo)quinol- ine 3 were also prepared. The heterocyclic compound 3 was obtained from the condensation of 4-iodonitr- osobenzene with 8-aminoquinoline. Compound 2 was conveniently synthesised by azo-coupling from 1-(di- methylamino)naphthalene and the diazonium species + Fig. 4. Molecular structure of compound 1i in the crystal. I–p-C6H4–N2 (generated in situ from the corre- sponding aniline derivative by diazotisation). Such azo-coupling reactions, although restricted to rather electron-rich, nucleophilic arenes, represent the most frequently used method for the preparation of aromatic azo compounds [11]. Single crystal X-ray structure analyses were per- formed for the azobenzene derivatives 1b, 1e, 1g, 1i ≡ and 1m. In addition, the crystal structure of Fc–C C– Fig. 5. Molecular structure of compound 1m in the crystal. p-C6H4–NH2 (4), which served as a starting material for the preparation of 1e, was also determined. The pares well with the value of ca. 1.25 A˚ reported for molecular structures of these compounds are shown in pristine azobenzene [12]. Essentially the same holds Figs. 1 – 6. true for the azobenzene nitrogen bond angles, which Bond parameters are unexceptional in all cases. The are ca. 114◦ in each case. The azobenzene cores are N–N double bond lengths are ca. 1.26 A˚ and are in- almost planar. The amino nitrogen atoms of com- distinguishable within experimental error. This com- pounds 1m and 4 are in an essentially trigonal pla- U. Siemeling et al. · Iodo-functionalised Azobenzene Derivatives 1397 interactions are well documented for organometallic crystals [15]. Experimental Section The following starting materials were prepared accord- ing to, or by slight modification of, published procedures: I– p-C6H4–NO [10], I–p-C6H4–N=N–p-C6H4–OH [16], Fc– C≡C–H [17], Ph–C≡C–p-C6H4–NH2 [18], Ph–C≡C–p- C6H4–C≡C–p-C6H4–NH2 [19]. All other starting materials were commercially available. NMR spectra: Varian Unity INOVA 500 spectrometer op- erating at 500.13 MHz for 1H. Mass spectra: Thermoquest Finnigan LCQ Deca mass spectrometer and Bruker Dalton- ics micrOTOF mass spectrometer (HRMS). Melting points (uncorrected): Stuart Electric SMP 3 melting point appara- tus. Elemental analyses: microanalytical laboratory of the University of Halle, Halle (Germany). X-Ray crystal struc- ture analyses: For each data collection a single crystal was mounted on a glass fibre and all geometric and intensity data were taken from this sample. Data collection using MoKα ra- diation (λ = 0.71073 A)˚ was made on a Stoe IPDS2 diffrac- tometer equipped with a 2-circle goniometer and an area de- tector. Absorption correction was done by integration using X-RED [20]. The data sets were corrected for Lorentz and polarisation effects. The structures were solved by Direct Fig. 6. Molecular structure and aggregation of compound 4 Methods (SHELXS-97) and refined using alternating cycles in the crystal. of least squares refinements against F2 (SHELXL-97) [21]. All non H atoms were found in difference Fourier maps nar bonding environment, which indicates an efficient and were refined with anisotropic displacement parameters. conjugation of the p-type nitrogen lone pair with the π H atoms were placed in constrained positions according to respective aromatic system. In the case of 4, one the riding model with the 1.2 fold isotropic displacement of the NH2 hydrogen atoms (H1A) exhibits a con- parameters. Pertinent crystallographic data are collected in tact to the two adjacent carbon atoms C6 and C10 Table 1. of a cyclopentadienyl ring of a neighbouring molecule CCDC 705442 – 705447 contain the supplementary crys- in the crystal. H1A has an almost perpendicular ori- tallographic data for this paper. These data can be obtained entation with respect to the C6 –C10 bond axis of free of charge from The Cambridge Crystallographic Data the cyclopentadienyl ring. The corresponding C···H Centre via www.ccdc.cam.ac.uk/data request/cif. distances are below the sum of the estimated van der Waals radii of 2.9 A˚ for carbon and hydrogen. General procedure for the preparation of the azobenzene These C···H distances are probably systematically derivatives of type I–p-C6H4–N=N–p-C6H4–X by condensa- overestimated in view of the fact that the experimen- tion of 4-iodonitrosobenzene with H2N–p-C6H4–X tally determined N–H bond lengths of 0.88(2) and 0.90(2) A˚ are shorter than the sum of the covalent radii The aniline derivative (1.0 mmol) and 4-iodonitrosobenz- ene (233 mg, 1.0 mmol) were dissolved in a 1 : 1 mix- (1.02 A)˚ [13]. The closest C···H distance of this type ture of acetic acid and ethyl acetate (ca. 5 mL; in the is ca. 2.54 A˚ and occurs with C6, the corresponding ˚ case of less soluble aniline derivatives: a minimal amount). carbon-nitrogen distance is ca. 3.63 A and the NHC an- The reaction mixture was stirred at 40 ◦C. The progress gle is ca.
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