Organometallics 2009, 28, 1377–1382
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Novel Organometallic Gelators with Enhanced Amphiphilic Character: Structure-Property Correlations, Principles for Design, and Diversity of Gelation Andreas Gansa¨uer,*,† Iris Winkler,† Thorsten Klawonn,† Roeland J. M. Nolte,‡ Martin C. Feiters,‡ Hans G. Bo¨rner,§ Jens Hentschel,§ and Karl Heinz Do¨tz*,† Kekule´-Institut fu¨r Organische Chemie and Biochemie der UniVersita¨t Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany, Department of Organic Chemistry, Institute for Molecules and Materials, Faculty of Science, Radboud UniVersity Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands, and Max Planck Institute of Colloids and Interfaces (Colloid Chemistry), Am Mu¨hlenberg 1, 14476 Potsdam-Golm, Germany ReceiVed October 24, 2008
A series of new titanocene complexes was synthesized and investigated toward their gelation abilities. The first structure-property correlations for organometallic gelators could be deduced. The gels and their structural diversity were characterized by TEM, cryo-SEM, and AFM as well as CD-spectroscopy. The organometallic gels display an enhanced amphiphilic character compared to typical organic ALS gelators. Introduction The assembly of small molecules to supramolecular aggregates and nanostructures has sparked intense interest over the last decades.1-3 An especially fascinating case in this respect is the formation of physical gels. In contrast to covalently linked polymer gels, physical gels can be formed from low molecular mass compounds that show the ability to assemble via noncovalent interactions such as hydrogen bonds, van der Waals, π-π, and ionic interactions.4,5 The structural investigation on a nano- and micrometer range reveals fibrous networks with interconnected fibers of different sizes as the crucial structural element. These soft materials have unique properties that open promising perspectives in material sciences. Practical applications may be found in medicinal products6 or pollutant capture and removal.7 Furthermore, physical gel-based electrolytes are used in solar technology.8 The polymerization of the gel net* Corresponding authors. E-mail:
[email protected]; doetz@ uni-bonn.de. † Universita¨t Bonn. ‡ Radboud University Nijmegen. § Max Planck Institute of Colloids and Interfaces. (1) Lehn, J.-M. Supramolecular Chemistry: Concepts and PerspectiVes; VCH: Weinheim, 1995. (2) For a recent compilation, see: Crego-Calama, M.; Reinhoudt, D. N. Supramolecular Chirality. Top. Curr. Chem. 2006, 265, 1–312. (3) Furhop, J.-H.; Ko¨ning, J. Membranes and Molecular Assemblies: The Synkinetic-Approach; Royal Society of Chemistry: Cambridge, 1994. (4) For recent reviews on low molecular mass gelators, see: (a) Smith, D. K. Low Molecular Weight Organic Gelators. Tetrahedron 2007, 63, 7271–7494. (b) Weiss, R. G.; Terech, P. Molecular Gels: Materials with Self-Assembled Fibrillar Networks; Springer: Heidelberg, 2006. (c) Fages, F.; Vo¨gtle, F.; Zˇinic´, M. Low Molecular Mass Gelators. Top. Curr. Chem. 2005, 256. (d) Estroff, A. L.; Hamilton, A. D. Chem. ReV. 2004, 104, 1201– 1218. (e) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133–3159. (5) (a) van Esch, J.; Feringa, B. L. Angew. Chem. 2000, 112, 2351– 2354; Angew. Chem., Int. Ed. Engl. 2000, 39, 2263-2266. (b) Abdallah, D. J.; Weiss, R. G. AdV. Mater. 2000, 12, 1237–1247. (6) (a) Lee, K. Y.; Mooney, D. J. Chem. ReV. 2001, 101, 1869–1879. (b) Xing, B.; Yu, C. W.; Chow, K. H.; Ho, P. L.; Fu, D.; Xu, B. J. Am. Chem. Soc. 2002, 124, 14846–14847. (c) Tiller, J. C. Angew. Chem. 2003, 115, 3180–3183; Angew. Chem., Int. Ed. Engl. 2003, 42, 3072. (d) Hirst, A. R.; Escuder, B.; Miravet, J. F.; Smith, D. K. Angew. Chem. 2008, 120, 8122; Angew. Chem., Int. Ed. 2008, 47, 8002.
work9 or its liquid component10 that leads to open nanoscaled architectures is of relevance in applications such as selective adsorbers and materials for chromatography.11 Considering their significance, it is not surprising that a large variety of low molecular mass gelators (LMMG) have been described.4 The most straightforward gelators contain long alkyl chains12 as present in fatty acids13 and cyclohexylalkyl amides.9b,14 In addition, aromatic entities such as anthracenes, anthraquinones15 or porphyrines16 serve as building blocks for LMMGs. Heteroatoms play an important role in aggregation processes due to their hydrogen-bonding ability, as exploited in more recent amino acid-,17 amide-,18 and urea-based19 gelators. Derivatives of other natural products such as carbohydrates20 or steroids21 are also important LMMGs.22 Steroids, in particu(7) Kiyonaka, S.; Sugiyasu, K.; Shinkai, S.; Hamachi, I. J. Am. Chem. Soc. 2002, 124, 10954–10955. (8) Yanagida, S.; Kambe, S.; Kubo, W.; Murakoshi, K.; Wada, Y.; Kitamura, T. Z. Phys. Chem. 1999, 212, 31–38. (9) (a) de Loos, M.; van Esch, J.; Stokroos, I.; Kellogg, R. M.; Feringa, B. L. J. Am. Chem. Soc. 1997, 119, 12675–12676. (b) Inoue, K.; Ono, Y.; Kanekiyo, Y.; Hanabusa, K.; Shinkai, S. Chem. Lett. 1999, 42, 9–430. (10) (a) Gu, W.; Lu, L.; Chapman, G. B. R. G.; Weiss, R. G. Chem. Commun. 1997, 543–544. (b) Hafkamp, R. J. H.; Kokke, B. P. A.; Danke, I. M.; Geurts, P. M.; Rowan, A. E.; Feiters, M. C.; Nolte, R. J. M. Chem. Commun. 1997, 545–546. (11) Svec, F.; Fre´chet, J. M. J. Chem. Mater. 1995, 7, 707–715. (12) (a) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16, 352–355. (b) Twieg, R. J.; Russel, T. P.; Siemens, R.; Rabolt, J. F. Macromolecules 1985, 18, 1361–1362. (c) Abdallah, D. J.; Weiss, R. G. Chem. Mater. 1999, 11, 2907–2911. (d) Abdallah, D. J.; Weiss, R. G. Chem. Mater. 2000, 12, 406– 413. (13) Terech, P.; Pasquier, D.; Bordas, V.; Rassat, C. Langmuir 2000, 16, 4485–4494. (14) (a) Kato, T.; Kondo, G.; Hanabusa, K. Chem. Lett. 1998, 193– 194. (b) Hanabusa, K.; Yamada, M.; Kimura, M.; Shirai, H. Angew. Chem. 1996, 108, 2086–2088; Angew. Chem., Int. Ed. Engl. 1996, 35, 19491951. (c) Kato, T.; Kutsuna, T.; Hanabusa, K.; Ukon, M. AdV. Mater. 1998, 10, 606–608. (15) Brotin, T.; Untermo¨hlen, R.; Fages, F.; Bouas-Laurent, H.; Desvergne, J.-P. J. Chem. Soc., Chem. Commun. 1991, 416–418. (16) (a) Ishi-i, T.; Iguchi, R.; Snip, E.; Ikeda, M.; Shinkai, S. Langmuir 2001, 17, 5825–5833. (b) Shirakawa, M.; Kawano, S.; Fujita, N.; Sada, K.; Shinkai, S. J. Org. Chem. 2003, 68, 5037–5044.
10.1021/om801022c CCC: $40.75 2009 American Chemical Society Publication on Web 01/27/2009
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Gansa¨uer et al. Scheme 1. Cholesterol-Functionalized Titanocenes Bearing Various Linker and Substitution Patterns
Figure 1. Basic structural motif of ALS gelators.22a
lar, are often encountered in ALS gelators, in which an aromatic unit (A) is connected via a linker (L) to a steroidal (S) component. Incorporation of a metal moiety into LMMGs is expected to result in even more attractive organometallic and coordinatively bound gelators (LMMGs), offering additional functionalities due to the variation of the oxidation state, the coordination geometry, and the coligand sphere provided by the metal center.23 To this rapidly growing field, we have contributed organometallic gels based on sugar chromium carbenes,24 functionalized titanocenes,25 and most recently CNC pincer biscarbene palladium gelators,26 which exhibit catalytic activity in the gel state. Due to their Lewis acidity, the titanocene derivatives are able to form gels with a large variety of polar and apolar solvents. (17) (a) Suzuki, M.; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K. Chem. Commun. 2002, 884–885. (b) Hanabusa, K.; Tange, J.; Taguchi, Y.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1993, 390– 392. (c) van Bommel, K. J. C.; van der Pol, C.; Muizebelt, I.; Friggeri, A.; Heeres, A.; Meetsma, A.; Feringa, B. L.; van Esch, J. Angew. Chem. 2004, 116, 1695–1699; Angew. Chem., Int. Ed. 2004, 43, 1663-1667. (d) Hirst, A. R.; Smith, D. K.; Feiters, M. C.; Geurts, H. P. M. Chem.-Eur. J. 2004, 10, 5901–5910. (18) (a) Cˇaplar, V.; Zˇinic´, M.; Pozzo, J. L.; Fages, F.; Mieden-Gundert, G.; Vo¨gtle, F. Eur. J. Org. Chem. 2004, 4048–4059. (b) Hanabusa, K.; Kawakami, A.; Kimura, M.; Shirai, H. Chem. Lett. 1997, 191–192. (19) (a) van Esch, J.; Kellogg, R. M.; Feringa, B. L. Tetrahedron Lett. 1996, 38, 281–284. (b) van Esch, J.; Schoonbeek, F.; de Loos, M.; Kooilman, H.; Spek, A. L.; Kellogg, R. M.; Feringa, B. L. Chem.-Eur. J. 1999, 5, 937–950. (20) (a) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhoudt, D. N. Chem.-Eur. J. 1999, 5, 2722– 2729. (b) Gronwald, O.; Sakurai, K.; Luboradzki, R.; Kimura, T.; Shinkai, S. Carbohydr. Res. 2001, 331, 307–318. (c) Furhop, J.-H.; Schnieder, P.; Boekema, E.; Helfrich, W. J. Am. Chem. Soc. 1988, 110, 2861–2867. (d) Hafkamp, R. J. H.; Feiters, M. C.; Nolte, R. J. M. Angew. Chem. 1994, 106, 1054–1055; Angew. Chem., Int. Ed. Engl. 1994, 33, 986-987. (e) Hafkamp, R. J. H.; Feiters, M. C.; Nolte, R. J. M. J. Org. Chem. 1999, 64, 412–426. (f) Fuhrhop, J.-H.; Svenson, S.; Boettcher, C.; Ro¨ssler, E.; Vieth, H.-M. J. Am. Chem. Soc. 1990, 112, 4307–4312. (g) Beginn, U.; Keinath, S.; Mo¨ller, M. Liquid Cryst. 1997, 1, 35–41. (21) (a) Martin-Borret, O.; Ramasseul, R.; Rassat, A. Bull. Soc. Chim. Fr. 1979, 401–408. (b) Terech, P.; Ramasseul, R.; Violino, F. J. Phys. (Paris) 1985, 46, 895–903. (c) Terech, P.; Berthet, C. J. Phys. Chem. 1988, 92, 4269–4272. (d) Lu, L.; Weiss, R. G. Langmuir 1995, 11, 3630–3632. (22) (a) Zˇinic´, M.; Vo¨gtle, F.; Fages, F. in ref 4c, 39-76, and references therein. (b) Lin, Y.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542–5551. (c) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664–6676. (23) Fages, F. Angew. Chem. 2006, 118, 1710–1712; Angew. Chem., Int. Ed. 2006, 45, 1680-1682. (24) Bu¨hler, G.; Feiters, M. C.; Nolte, R. J. M.; Do¨tz, K. H. Angew. Chem. 2003, 115, 2599–2602; Angew. Chem., Int. Ed. 2003, 42, 24942497. (25) Klawonn, T.; Gansa¨uer, A.; Winkler, I.; Lauterbach, T.; Franke, D.; Nolte, R. J. M.; Feiters, M. C.; Bo¨rner, H.; Hentschel, J.; Do¨tz, K. H. Chem. Commun. 2007, 1894–1895. (26) Tu, T.; Assenmacher, W.; Peterlik, H.; Weissbarth, R.; Nieger, M.; Do¨tz, K. H. Angew. Chem. 2007, 119, 6486–6490; Angew. Chem., Int. Ed. 2007, 46, 6368-6371.
Here we report on the elucidation of structure-property correlations of these compounds that are readily accessible by a modular titanocene synthesis.27
Results and Discussion An ideal starting point for the elaboration of structure-property correlations is complex 1 (Scheme 1). Compared to organic ALS gelators, the aromatic moiety is replaced by a metallocene unit. Metallocenes have an external surface resembling that of an aromatic nucleus, although they are thicker due to their sandwich structure. In addition, they are small, rigid molecules with a polarity depending on the metal and its oxidation number. In this study, we describe ALS gelators containing the Lewis acidic titanocene fragment. The steroidal part is provided by β-cholesterol, which is linked to the titanocene moiety via a gemdimethylated propionic acid spacer. The naturally occurring β-cholesterol represents by far the most successful steroid in this type of gels and allows for a maximum compatibility with other systems described in the literature.22a The linker was adjusted by variation of both the tether length (complex 2) and the substitution pattern of the carbon attached to the “upper” cyclopentadienyl ring (complexes 3 and 4). The structure of the titanocene moiety can be easily modified by substitution of the “lower” cyclopentadienyl ligand as demonstrated for titanocenes 5-7. Gelation Abilities. A potent LMMG is expected to form aggregates in a variety of solvents of different polarity, a requirement that is met with steroidal titanocene 1.25 It is able to immobilize apolar solvents such as toluene, benzene, or din-butyl ether as well as polar solvents such as 1,4-dioxane, acetone, and DMSO. The critical gelation concentration (cgc), defined as the minimum concentration of the gelator required to form a gel at room temperature, may be
