Synthesis and Surface Properties of New Ureas and Amides at

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Synthesis and Surface Properties of New Ureas and Amides at Different Interfaces Katharina Dreger,† Bo Zou,‡ Zhongcheng Mu,‡ Hans Joachim Galla,§ Lifeng Chi,‡ Harald Fuchs,‡ and Hans J. Scha¨fer*,† Organisch-Chemisches Institut der UniVersita¨t, Westfa¨lische-Wilhelms-UniVersita¨t Mu¨nster, Correns-Str. 40, 48149 Mu¨nster, Germany, Physikalisches Institut der UniVersita¨t, Westfa¨lische-Wilhelms-UniVersita¨t Mu¨nster, Wilhelm-Klemm-Str. 10, 48149 Mu¨nster, Germany, and Institut fu¨r Biochemie der UniVersita¨t, Westfa¨lische-Wilhelms-UniVersita¨t Mu¨nster, Wilhelm-Klemm-Str. 2, 48149 Mu¨nster, Germany ReceiVed August 30, 2005. In Final Form: December 12, 2005 The influence of the urea and amide group in the alkyl chain of methyl nonadecanoate on the surface properties is investigated and compared. For that purpose, the ureas CH3O2C-(CH2)m-NHCONH-(CH2)n-CH3 (n + m ) 14) [1 (m ) 2), 3 (m ) 3), and 5 (m ) 4)] and the amides CH3O2C-(CH2)m-NHCO-(CH2)n-CH3 (n + m ) 15) [2 (m ) 2), 4 (m ) 3), and 6 (m ) 4)] were synthesized. The π/A isotherms of the ureas show up to the attainable temperature of 313 K no LE phase, which indicates a very stable LC phase. The amides exhibit a two phase plateau region, with the exception of 2. The different behavior is connected with the hydrogen bond energies, which are stronger with the ureas in the LC than in the LE phase, whereas those of the amides have a similar strength in both phases. The effect of hydrogen bonds in self-assembled molecules of N,N′-dialkylurea CH3-(CH2)m-NHCONH(CH2)n-CH3 (m + n ) 14) [7 (n ) 2)] was visualized by STM at the octylbenzene/graphite interface. Compound 7 forms a lamella structure with a periodicity of one molecule length. The tilt angle of 86° ( 2° to the edge of the lamella points to a nearly orthogonal arrangement of the molecules. It indicates two equivalent bonds between the aza-hydrogens and the carbonyl oxygen. A similar arrangement is proposed for the LC phase of the ureas at the air/water interface.

Introduction Molecules can be ordered through hydrogen bonds.1 The shape of the arrangement is controlled by the geometry and strength of the hydrogen bond donor and acceptor. Hydrogen bonds are intensely used in nature to form catalytic pockets in enzymes. It appears intriguing to probe the use of hydrogen bonds in the alkyl chain of amphiphiles, which is in general the domain of van der Waals interactions. Here they could enhance noncovalent binding and serve as a platform for attaching operational groups at distinct locations. These features could be useful to modify the stability of monolayers, micelles, vesicles or membranes and to introduce groups that support the transport of ions or electrons through these structures. Polar groups in the alkyl part of amphiphiles may, however, diminish the monolayer stability in the LC phase due to weaker van der Waals attractions unless there is a compensation by hydrogen bonding. The monolayer stability at the air/water interface has been characterized by Langmuir isotherms for fatty acid derivatives bearing the hydroxy2-7 and the amide group8-11 in the alkyl part of the amphiphile. The stability of the LC phase formed by * To whom correspondence should be addressed. E-mail: schafeh@ uni-muenster.de. † Organisch-Chemisches Institut der Universita ¨ t. ‡ Physikalisches Institut der Universita ¨ t. § Institut fu ¨ r Biochemie der Universita¨t. (1) (a) Prins, L. J.; Reinhoudt, D. N.; Timmerman, P. Angew. Chem. 2001, 40, 2382-2426. (b) MacDonald, J. C.; Whitesides, G. M. Chem. ReV. 1994, 94, 2383. (2) (a) Menger, F. M.; Richardson, S. D.; Wood, Jr., M. G.; Sherrod, M. J. Langmuir 1989, 5, 833-838. (b) Huda, M. S.; Fujio, K.; Uzu, Y. Bull. Chem. Soc. Jpn. 1996, 69, 3387-3394. (3) (a) Kellner, B. M. J.; Cadenhead, D. A. J. Colloid Interface Sci. 1978, 63, 452-460. (b) Asgarian, B.; Cadenhead, D. A. Langmuir 2000, 16, 677-681. (4) (a) Vollhardt, D.; Siegel, S.; Cadenhead, D. A. Langmuir 2004, 20, 76707677. (b) Vollhardt, D.; Siegel, S.; Cadenhead, D. A. J. Phys. Chem. B 2004, 108, 17448-17456.

derivatives of octadecanoic acid is influenced by the ability of the substituents to form intermolecular H-bonds and cause little disorder in the packing of the alkyl chain. As substituted ureas form strong intermolecular hydrogen bonds,12 we integrated this group into the alkyl chain of methyl nonadecanoates and compared their monolayer stability with that of the corresponding amides. Up to now, only few pressure/area isotherms of urea derivatives are reported for the air/water interface. In these cases, the urea group serves as headgroup of an amphiphile13-17 or is inserted into a long alkyl chain.18 Langmuir isotherms of fatty acid esters with a urea group integrated into the alkyl chain to our knowledge have not been studied. To get an indication of the orientation of the urea group in a compressed monolayer, a look at self-assembled molecules of (5) (a) Vollhardt, D.; Siegel, S.; Cadenhead, D. A. Colloids Surf. A 2005, 256, 9-15. (b) Vollhardt, D.; Fainermann, V. B. J. Phys. Chem. B 2004, 108, 297302. (6) (a) Overs, M.; Fix, M.; Jacobi, S.; Chi, L.; Sieber, M.; Scha¨fer, H. J.; Fuchs, H.; Galla, H.-J. Langmuir 2000, 16, 1141-1148. (b) Fix, M.; Sieber, M.; Overs, M.; Scha¨fer, H. J.; Galla, H.-J. Phys. Chem. Chem. Phys. 2000, 2, 4515-4520. (7) Wang, L.; Jacobi, S.; Sun, J.; Overs, M.; Fuchs, H.; Scha¨fer, H. J.; Zhang, X.; Shen, J.; Chi, L. J. Colloid Interface Sci. 2005, 285, 814-820. (8) Melzer, V.; Vollhardt, D.; Brezesinski, G.; Mo¨hwald, H. Thin Solid Films 1998, 327-329, 857-860. (9) Melzer, V.; Weidemann, G.; Vollhardt, D.; Brezesinski, G.; Wagner, R.; Struth, B.; Mo¨hwald, H. Supramol. Sci. 1997, 4. 391-397. (10) Dreger, K.; Scha¨fer, H. J. Mater. Sci. Eng. 2002, C22, 327-330. (11) Dreger, K.; Wu, D.; Galla, H.-J.; Fuchs, H.; Chi, L.; Wu¨rthwein, E.-U.; Scha¨fer, H. J. Langmuir, submitted. (12) Coe, S.; Kane, J. J.; Nguyen, T. L.; Toledo, L. M.; Wininger, E.; Fowler, F. W.; Lauher, J. W. J. Am. Chem. Soc. 1997, 119, 86-93. (13) Kobayashi, T.; Seki, T. Langmuir 2003, 19, 9297-9304. (14) Seki, T.; Fukuchi, T.; Ichimura, K. Langmuir 2002, 18, 5462-5467. (15) Glazer, J.; Alexander, A. E. Trans. Faraday Soc. 1951, 47, 401-409. (16) Adam, N. K.; Dyer, J. W. W. Proc. R. Soc. (London) 1924, A106, 694709; 1922, A101, 452-472. (17) Adam, N. K. Trans. Faraday Soc. 1928, 24, 149-154. (18) Huo, Q.; Russev, S.; Hasegawa, T.; Nishijo, J.; Umemura, J.; Puccetti, G.; Russell, K. C.; Leblanc, R. M. J. Am. Chem. Soc. 2000, 122, 7890-7897.

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N,N′-dialkylurea could be helpful. Scanning tunneling microscopy (STM) allows 2D patterns to be visualized that are formed by aggregation of small organic molecules on a graphite surface. The structure of the 2D pattern is thereby determined by the interaction of the molecules with the graphite surface and by intermolecular hydrogen bonds.19-21 Alkyl amides and alkyl ureas are suitable compounds for STM investigations as both groups form stable patterns.22-26 STM investigations of N-alkyl fatty acid amides at the graphite surface show simple and complex lattices depending on the position of the amide group.27 Bisureas with a spacer containing a bisthiophene group have been used to fix this group in a distinct arrangement by way of their hydrogen bonds.26,28 In this connection, the monolayer films of several alkyl substituted bisureas29 and their phase behavior in mixtures with bisthiophene derivatives have been studied by STM.26 In methyl nonadecanoates with an integrated urea group, the ester was replaced by a methyl group to facilitate the analysis of the STM pictures. The 2D-lattice of a dialkyl urea at the octylbenzene/ graphite should give an indication on the 3D-arrangement of related urea amphiphiles at the air/water interface. Experimental Section General Synthetic Methods. Solvents and reagents were purified if necessary using literature methods. The reagents were used as supplied by Aldrich, Fluka, and Merck, if not mentioned otherwise. Thin-layer chromatography (TLC) was performed on aluminum plates (5 × 7.5 cm) coated with Merck Silica Gel 60F254. For silica gel column chromatography, Merck flash Silica Gel 60 was used. Elementary analyses were performed on a Varian EL III apparatus by the department of Microanalytical Services of the Institute of Organic Chemistry, University of Mu¨nster. FT-IR spectra were recorded on a Bruker IFS 28 spectrometer. Mass spectra (GC/MS) were obtained from a Finnigan MAT 8230 instrument, linked with a Varian GC 3400 using data system SS 300. 1H and 13C NMR spectra were recorded on Bruker ARX 300 and Bruker AMX 400 spectrometers. Melting points were determined on a Kofler heating plate microscope and are uncorrected. Film Balance Measurements. Surface pressure-area isotherms were taken on a Langmuir film balance Type 622 (Nima, Coventry) with a total trough area of 1300 cm2. The surface pressure was measured with a Wilhelmy plate system at a compression rate of 80 cm2/min and if not otherwise stated at 293 K. All isotherms were reproduced at least three times with the deviation given in the tables. Scanning Tunneling Microscopy. The MultiMode-STM of Digital Instruments, St. Barbara, USA with the Scanner Head A was used. A drop of saturated solutions of the compound 7 in octylbenzene was put on a freshly cleaved graphite surface (HOPG) of MaTeck GmbH, Ju¨lich and stripped off, so that a 20 µm thick liquid film remained at the surface. In this, the STM-tip (Pt/Ir wire) MaTeck GmbH, Ju¨lich was dipped. The STM pictures were taken in the (19) Rabe, J. P.; Bucholz, S. Science 1991, 253, 424-427. (20) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139-150. (21) Cyr, D. M.; Venkatamaran, B.; Flynn, G. W. Chem. Mater. 1996, 16001615. (22) Takeuchi, H.; Kawauchi, S.; Ikai, A. Jpn. J. Appl. Phys. 1996, 35, 37543758. (23) Gesquie`re, A.; Abdel-Mottaleb, M. M. S.; De Feyter, S.; De Schryver, F. C.; Schoonbeek, F.; van Esch, J.; Kellogg, R. M.; Feringa, B. L.; Calderone, A.; Lazzaroni, R.; Bre´das, J. L. Langmuir 2000, 16, 10385-10391. (24) Giancarlo, L.; Cyr, D. M.; Muyskens, K.; Flynn, G. W. Langmuir 1998, 14, 1465-1471. (25) Lim, R.; Li, J.; Li, S. F. Y.; Feng, Z.; Valiyaveettil, S. Langmuir 2000, 16, 7023-7030. (26) De Feyter, S.; Larsson, M.; Schuurmans, N.; Verkuijl, B.; Zoriniants, G.; Gesquiere, A.; Abdel-Mottaleb, M. M.; van Esch, J.; Feringa, B.; van Stam, J.; De Schryver, F. C. Chemistry 2003, 9, 1198-1206. (27) Zou, B.; Dreger, K.; Mu¨ck-Lichtenfeld, Ch.; Grimme, S.; Scha¨fer, H. J.; Fuchs, H.; Chi, L. Langmuir 2005, 21, 1364-1370. (28) Gesquiere, A.; De Feyter, S.; De Schryver, F. C.; Schoonbeek, F. M.; Feringa, B. L. Nano Lett. 2001 1, 201-206. (29) De Feyter, S.; Grim, P. C. M.; van Esch, J.; Kellog, R. M.; Feringa, B. L.; De Schryver, F. C. J. Phys. Chem. B 1998, 102, 8981-8987.

Dreger et al. “constant current mode” with a scan rate of 3-5 Hz, a positive tip potential between 0.4 and 0.9 V and a current of 0.2-0.8 nA. Materials. General Procedure for the Preparation of Urea DeriVatiVes from Carboxylic Acids. The solution of carboxylic acid (2.0 mmol), triethylamine (TEA, 0.45 mL [3.2 mmol]), and diphenylphosphoryl azide (DPPA, 0.47 mL, 2.2 mmol in dry toluene (10 mL) was heated under reflux for 3 h. Then at room temperature the amine (3.0 mmol), dissolved in toluene (4 mL), was added and the mixture was stirred for 16 h. After addition of dichloromethane (50 mL), the solution was washed with aqueous citric acid (10%, 50 mL) and dried (MgSO4). The solvent was rotaevaporated and the crude product purified by flash chromatography. For all compounds, correct elemental analyses were obtained. IR, 13C NMR, further MS data, and elemental analyses of all compounds are deposited in the Supporting Information. Methyl 3-{[(Tridecylamino)carbonyl]amino}-propanoate (1). 4-Methoxy-4-oxobutanoic acid and tridecylamine yield after flash chromatography (cyclohexane/ethyl acetate, 1:2) 1 (50%) as a white solid. Rf value: 0.16 (cyclohexane: ethyl acetate ) 3: 5). Mp: 370373 K. 1H NMR (CDCl3): δ (ppm) ) 0.86 (t, 3J ) 6.6 Hz, 3 H, CH2CH3), 1.20-1.33 (m, 20 H, 10 CH2), 1.47 (m, 2 H, CH2CH2CH2NH), 2.53 (t, 3J ) 5.9 Hz, 2 H, CH2CO2CH3), 3.11 (t, 3J ) 7.1 Hz, 2 H, CH2CH2CH2NH), 3.45 (t, 3J ) 6.6 Hz, 2 H, H3CO2CH2CH2NH), 3.68 (s, 3 H, CO2CH3). MS (direct inlet): m/z (%) ) 328 (54) [M+]. Methyl 4-{[(Dodecylamino)carbonyl]amino}-butanoate (3). 5-Methoxy-5-oxopentanoic acid and dodecylamine yield after flash chromatography (cyclohexane/ethyl acetate, 1:2) 3 (73%) as a white solid. Rf value: 0.11 (cyclohexane: ethyl acetate ) 3:5). Mp: 371374 K. 1H NMR (CDCl3): δ (ppm) ) 0.86 (t, 3J ) 6.6 Hz, 3 H, CH2CH3), 1.20-1.33 (m, 18 H, 9 CH2), 1.47 (m, 2 H, CH2CH2CH2NH), 1.80 (m, 2 H, H3CO2CH2CH2), 2.36 (t, 3J ) 7.1 Hz, 2 H, CH2CO2CH3), 3.12 (t, 3J ) 7.2 Hz, 2 H, CH2NH), 3.19 (t, 3J ) 6.9 Hz, 2 H, CH2NH), 3.65 (s, 3 H, CO2CH3). MS (direct inlet): m/z (%) ) 328 (54) [M+]. Methyl 5-{[(Undecylamino)carbonyl]amino}-pentanoate (5). 6-Methoxy-6-oxohexanoic acid and undecylamine yield after flash chromatography (cyclohexane/ethyl acetate, 1:4) 5 (53%) as a white solid. Rf value: 0.24 (cyclohexane: ethyl acetate ) 1:4). Mp: 371 K. 1H NMR (CDCl3): δ (ppm) ) 0.86 (t, 3J ) 6.8 Hz, 3 H, CH2CH3), 1.22-1.32 (m, 16 H, 8 CH2), 1.51 (m, 4 H, 2 CH2CH2NH), 1.65 (m, 2 H, H3CO2CH2CH2), 2.32 (t, 3J ) 7.2 Hz, 2 H, CH2CO2CH3), 3.12 (t, 3J ) 6.9 Hz, 2 H, CH2NH), 3.17 (t, 3J ) 6.9 Hz, 2 H, CH2NH), 3.64 (s, 3 H, CO2CH3), 4.42 (m, br, 2 H, NH). MS (direct inlet): m/z (%) ) 328 (56) [M+]. General Procedure for the Preparation of the Amides. To the carboxylic acid (1 mmol) in dry dichloromethane (10 mL) was slowly added at 273 K oxalyl chloride (1.8 mmol), and the solution was then stirred for 30 min. After further stirring for 20 h at room temperature, excess oxalyl chloride was removed under vacuum (12 Torr) and the residue added by way of a syringe at 273 K to a solution of the amine (1.4 mmol) and TEA (0.4 mL, 2.9 mmol) in dry dichloromethane (10 mL), and the mixture was then stirred for 30 min. After further stirring for 20 h, dichloromethane (20 mL) and water (20 mL) were added, and then the mixture was acidified with concentrated HCl, the organic phase separated, and the aqueous phase extracted with dichloromethane (2 × 20 mL). The combined organic extracts were washed with a NaHCO3 solution and dried (MgSO4). The solvent was removed under vacuum, and the residue purified by flash chromatography. Methyl 3-(Pentadecanoylamino)-propanoate (2). According to the general procedure, pentadecanoic acid and methyl 3-aminopropanoate-hydrochloride yield after flash chromatography (cyclohexane: ethyl acetate, 1:1) 2 (95%) as a white solid. Rf value: 0.19 (cyclohexane: ethyl acetate ) 1:1). Mp: 357-358 K. 1H NMR (CDCl3): δ (ppm) ) 0.86 (t, 3J ) 6.8 Hz, 3 H, CH2CH3), 1.22-1.32 (m, 22 H, 11 CH2), 1.58 (m, 2 H, CH2CH2CONH), 2.15 (t, 3J ) 7.5 Hz, 2 H, CH2CONH), 2.52 (t, 3J ) 6.0 Hz, 2 H, CH2CO2CH3), 3.50 (m, 2 H, CH2NHCO), 3.68 (s, 3 H, CO2CH3), 6.14 (m, br, 1 H, NH). MS (GC/MS-coupling): m/z (%) ) 327 (