Metal Ion and Anion-Based “Tuning” of a Supramolecular Metallogel

pubs.acs.org/Langmuir © 2009 American Chemical Society

Metal Ion and Anion-Based “Tuning” of a Supramolecular Metallogel† Marc-Oliver M. Piepenbrock, Nigel Clarke,* and Jonathan W. Steed* Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K. Received January 13, 2009. Revised Manuscript Received February 18, 2009 A bis(pyridylurea) ligand (1) forms metallogels in methanol in the presence of up to 0.5 equiv of copper(II) chloride. The metallogel has been characterized using powder X-ray diffraction, SEM, and MAS NMR spectroscopic techniques. The addition of further copper(II) chloride gives the unusual crystalline 4:3 coordination polymer [Cu3(1)4Cl4]Cl2 3 nH2O (2). In the presence of 0.5 equiv of copper(II) nitrate, the 2:1 crystalline coordination polymer [Cu(1)2](NO3)2 3 H2O 3 MeOH (5) is isolated. Both 2 and 5 have been characterized by single-crystal X-ray diffraction. Complex 5 represents a possible model for the gelator and highlights key interactions with counteranions that suggest a means to tune gel properties using anion binding. The influence of chloride and acetate anions (as their NBu+ 4 salts) on the rheological properties of the copper(II) chloride metallogels of 1 are investigated. The rheology of the anion-containing mixtures shows complex behavior with the gel structure apparently evolving over time.

Introduction The last 15 years have seen an ever increasing interest in low-molecular-weight gelators (LMWG).1-5 Today they constitute a very topical field because of their potential applications in areas as diverse as tissue engineering, drug delivery, the templated synthesis of nanoparticles and other inorganic nanostructures as well as polymers, and the capture and removal of pollutants.6,7 In contrast to chemical gels where the solid component is covalently linked through the entire system, LMWGs interact noncovalently and self-assemble into fibrous networks leading to the formation of reversible gels. By understanding the nature of these interactions (e.g., hydrogen bonding, solvophobic effects, π-π stacking, etc.), it is now beginning to be possible to move from the discovery of gelators by chance to attempting a more rational design of molecules for specific gel applications.8-10 In addition, more † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue. *Corresponding authors. E-mail: [email protected].

(1) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237–1247. (2) de Loos, M.; Feringa, B. L.; van Esch, J. H. Eur. J. Org. Chem. 2005, 3615–3631. (3) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201–1217. (4) Sangeetha, N. M.; Maitra, U. Chem. Soc. Rev. 2005, 34, 821–836. (5) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3159. (6) Smith, D. K.Molecular Gels - Nanostructured Soft Materials. In Organic Nanostructures; Atwood, J. L., Steed, J. W., Eds.; Wiley-VCH: Weinheim, Germany, 2008. (7) Hirst, A. R.; Escuder, B.; Miravet, J. F.; Smith, D. K. Angew. Chem., Int. Ed. 2008, 47, 8002–8018. (8) de Loos, M.; Friggeri, A.; van Esch, J.; Kellogg, R. M.; Feringa, B. L. Org. Biomol. Chem. 2005, 3, 1631–1639. (9) Fages, F.; Vogtle, F.; Zinic, M.Systematic Design of Amide- And Urea-Type Gelators with Tailored Properties. In Low Molecular Mass Gelators: Design, Self-Assembly, Function; Springer-Verlag: Berlin, 2005; Vol. 256, pp 77-131. (10) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263–2266. (11) Aggeli, A.; Bell, M.; Carrick, L. M.; Fishwick, C. W. G.; Harding, R.; Mawer, P. J.; Radford, S. E.; Strong, A. E.; Boden, N. J. Am. Chem. Soc. 2003, 125, 9619–9628. (12) Frkanec, L.; Jokic, M.; Makarevic, J.; Wolsperger, K.; Zinic, M. J. Am. Chem. Soc. 2002, 124, 9716–9717.

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and more attention is being given to devising LMWGs that can react to chemical and physical stimuli.11-20 Of particular interest in this context are the changes in gel properties induced by anions6,21-25 and, in parallel, the introduction of metal binding sites into the structure of the gelator molecule in order to form metallogels.26-32 In our work, we are particularly concerned with the understanding of how anions and metal cation binding affect the supramolecular structure of the gel as well as the possibility of tuning gel properties such as (13) Kawano, S.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2004, 126, 8592–8593. (14) Kawano, S.; Fujita, N.; Shinkai, S. Chem.;Eur. J. 2005, 11, 4735–4742. (15) Osada, Y.; Rossmurphy, S. B. Sci. Am. 1993, 268, 82–87. (16) Wang, S.; Shen, W.; Feng, Y. L.; Tian, H. Chem. Commun. 2006, 1497–1499. (17) Li, Y. G.; Wang, T. Y.; Liu, M. H. Tetrahedron 2007, 63 7468–7473. (18) Liu, J.; He, P. L.; Yan, J. L.; Fang, X. H.; Peng, J. X.; Liu, K. Q.; Fang, Y. Adv. Mater. 2008, 20, 2508–2511. (19) Naota, T.; Koori, H. J. Am. Chem. Soc. 2005, 127, 9324–9325. (20) Eastoe, J.; Sanchez-Dominguez, M.; Wyatt, P.; Heenan, R. K. Chem. Commun. 2004, 2608–2609. (21) Dzolic, Z.; Cametti, M.; Cort, A. D.; Mandolini, L.; Zinic, M. Chem. Commun. 2007, 3535–3537. (22) Maeda, H.; Haketa, Y.; Nakanishi, T. J. Am. Chem. Soc. 2007, 129, 13661–13674. (23) Piepenbrock, M. O. M.; Lloyd, G. O.; Clarke, N.; Steed, J. W. Chem. Commun. 2008, 2644–2646. (24) Stanley, C. E.; Clarke, N.; Anderson, K. M.; Elder, J. A.; Lenthall, J. T.; Steed, J. W. Chem. Commun. 2006, 3199–3201. (25) Yamanaka, M.; Nakamura, T.; Nakagawa, T.; Itagaki, H. Tetrahedron Lett. 2007, 48, 8990–8993. (26) Arai, S.; Imazu, K.; Kusuda, S.; Yoshihama, I.; Tonegawa, M.; Nishimura, Y.; Kitahara, K.; Oishi, S.; Takemura, T. Chem. Lett. 2006, 35, 634–635. (27) Fages, F. Angew. Chem., Int. Ed. 2006, 45, 1680–1682. (28) Hanabusa, K.; Maesaka, Y.; Suzuki, M.; Kimura, M.; Shirai, H. Chem. Lett. 2000, 1168–1169. (29) Kawano, S. I.; Fujita, N.; van Bommel, K. J. C.; Shinkai, S. Chem. Lett. 2003, 32, 12–13. (30) Kishida, T.; Fujita, N.; Sada, K.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 7298–7299. (31) Kishimura, A.; Yamashita, T.; Aida, T. J. Am. Chem. Soc. 2005, 127, 179–183. (32) Kuroiwa, K.; Shibata, T.; Takada, A.; Nemoto, N.; Kimizuka, N. J. Am. Chem. Soc. 2004, 126, 2016–2021.

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flow characteristics.24,33-36 In this context, bis(urea) ligands are a promising target whose well-known gelling ability results from hydrogen bonding interactions to give the urea R-tape motif.37-42 Ureas are also effective anion-binding functional groups,43-51 and hence the introduction of anions can disrupt or alter the gels’ hydrogen bonding network.23-25 In addition to the urea anion-binding moiety, it is possible to simultaneously introduce metal-cation binding groups such as the pyridyl functionality. These ureido-pyridines have been shown to bind both cations and anions simultaneously in crystalline networks45,50-54 and also exhibit some potential as LMWGs.34 Furthermore, such a design offers the possibility of deliberately engineering fully tunable supramolecular gels in which anion binding is in competition with gel assembly via the urea R-tape motif (and/or the common urea pyridyl bifurcated acceptor interaction40,55) whereas metal countercation binding can cross-link the gels, potentially increasing their strength. The potential competing interactions are shown in Scheme 1. Hydrogen bonding of the types shown in Scheme 1a,b is likely to result in organogelation in pyridyl ureas in nonpolar solvents. The metal- and anion-linking interactions shown in Scheme 1c,d may result in increased cross-linking within gel fibrils but are in competition with the interactions shown in Scheme 1b,a, respectively. In addition, direct coordination of the anion to the metal may also occur (Scheme 1e). Hence by varying the concentration of the added ions and by molecular design to adjust the ion

(33) Anderson, K. M.; Day, G. M.; Paterson, M. J.; Byrne, P.; Clarke, N.; Steed, J. W. Angew. Chem., Int. Ed. 2008, 47, 1058–1062. (34) Applegarth, L.; Clark, N.; Richardson, A. C.; Parker, A. D. M.; Radosavljevic-Evans, I.; Goeta, A. E.; Howard, J. A. K.; Steed, J. W. Chem. Commun. 2005, 5423–5425. (35) Kim, H. J.; Lee, J. H.; Lee, M. Angew. Chem., Int. Ed. 2005, 44, 5810–5814. (36) Steed, J. W. Chem. Commun. 2006, 2637–2649. (37) Brinksma, J.; Feringa, B. L.; Kellogg, R. M.; Vreeker, R.; van Esch, J. Langmuir 2000, 16, 9249–9255. (38) de Loos, M.; Ligtenbarg, A. G. J.; van Esch, J.; Kooijman, H.; Spek, A. L.; Hage, R.; Kellogg, R. M.; Feringa, B. L. Eur. J. Org. Chem. 2000, 3675–3678. (39) Jeong, Y.; Hanabusa, K.; Masunaga, H.; Akiba, I.; Miyoshi, K.; Sakurai, S.; Sakurai, K. Langmuir 2005, 21, 586–594. (40) Reddy, L. S.; Basavoju, S.; Vangala, V. R.; Nangia, A. Cryst. Growth Des. 2006, 6, 161–173. (41) Schoonbeek, F. S.; van Esch, J. H.; Hulst, R.; Kellogg, R. M.; Feringa, B. L. Chem.;Eur. J. 2000, 6, 2633–2643. (42) van Esch, J.; Schoonbeek, F.; de Loos, M.; Kooijman, H.; Spek, A. L.; Kellogg, R. M.; Feringa, B. L. Chem.;Eur. J. 1999, 5, 937–950. (43) Pratt, M. D.; Beer, P. D. Polyhedron 2003, 22, 649–653. (44) Snellink-Ruel, B. H. M.; Antonisse, M. M. G.; Engbersen, J. F. J.; Timmerman, P.; Reinhoudt, D. N. Eur. J. Org. Chem. 2000, 165–170. (45) Bondy, C. R.; Gale, P. A.; Loeb, S. J. J. Am. Chem. Soc. 2004, 126, 5030–5031. (46) Gomez, D. E.; Fabbrizzi, L.; Licchelli, M.; Monzani, E. Org. Biomol. Chem. 2005, 3, 1495–1500. (47) Amendola, V.; Boiocchi, M.; Colasson, B.; Fabbrizzi, L. Inorg. Chem. 2006, 45, 6138–6147. (48) Dos Santos, C. M. G.; Glynn, M.; McCabe, T.; De Melo, J. S. S.; Burrows, H. D.; Gunnlaugsson, T. Supramol. Chem. 2008, 20, 407–418. (49) Gunnlaugsson, T.; Glynn, M.; Tocci, G. M.; Kruger, P. E.; Pfeffer, F. M. Coord. Chem. Rev. 2006, 250, 3094-3117. (50) Turner, D. R.; Smith, B.; Spencer, E. C.; Goeta, A. E.; Evans, I. R.; Tocher, D. A.; Howard, J. A. K.; Steed, J. W. New J. Chem. 2005, 29, 90–98. (51) Turner, D. R.; Spencer, E. C.; Howard, J. A. K.; Tocher, D. A.; Steed, J. W. Chem. Commun. 2004, 1352–1353. (52) Custelcean, R.; Moyer, B. A.; Bryantsev, V. S.; Hay, B. P. Cryst. Growth Des. 2006, 6, 555–563. (53) Russell, J. M.; Parker, A. D. M.; Radosavljevie-Evans, I.; Howard, J. A. K.; Steed, J. W. Chem. Commun. 2006, 269–271. (54) Blondeau, P.; van der Lee, A.; Barboiu, M. Inorg. Chem. 2005, 44, 5649–5653. (55) Byrne, P.; Turner, D. R.; Lloyd, G. O.; Clarke, N.; Steed, J. W. Cryst. Growth Des. 2008, 8, 3335–3344.

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binding constant, it may be possible to realize tunable supramolecular gels. In the present study, we report preliminary results on the effect of a representative metal salt, namely, copper(II) chloride, and anions of different tetrabutylammonium (TBA) salts on the gel properties of ligand 1, which is readily prepared from the reaction of 3-aminopyridine with 1,3-bis(1-isocyanato-1-methyl-ethyl)benzene (Supporting Information). The bulky backbone constrains the ligand to commonly adopt C-shaped geometry, and we have previously reported a study of crystal structures of the free ligand, which does not form gels in common solvents but instead forms clathrates.56 We now show how the addition of copper(II) transforms this ligand into a metallogelator, and we report preliminary studies on the tuning of this metallogel using anion binding.

Results and Discussion Compound 1 was found to gel a methanol solution at 1 wt % in the presence of varying amounts of hydrated copper(II) chloride. Ligand 1 does not form any gels by itself, but the addition of a Cu(II) salt results in blue, slightly translucent gels. The sol-gel transition temperature (Tgel) determined using the dropping ball method proved to be dependent on the amount of added copper(II) chloride (Supporting Information). For 0.2 equiv of metal salt, Tgel was found to be 34 ( 2 °C, rising to 63 ( 2 °C for 0.3 equiv of copper(II) chloride. At higher copper concentrations, the gel persisted up to the boiling point of the solvent. Where observed, the sol-gel transition is thermoreversible. There are clear differences in the appearance of the gels as the concentration of CuCl2 increases (Supporting Information, Figure S1). The ligand forms only a partial gel with 0.1 equiv of CuCl2, but 0.2-0.5 equiv of copper salt is sufficient to immobilize the mixture. The addition of more than ca. 0.5 equiv of CuCl2 3 2H2O results in the precipitation of a new crystalline phase with a greenish color and the disappearance of the gel. We therefore postulate that the reaction of 1 with 0.5 equiv of copper(II) chloride gives rise to a metallogelator coordination complex. Further copper(II) addition results in the formation of a second, insoluble species. We have sought to gain insight into the structure and behavior of both the blue gelator and green crystalline phases. SEM images of the dried xerogels (Supporting Information, Figure S2) reveal the fibrillar nature of the samples. The width of the fibers varies from 20 to 100 nm, and the fibers themselves are brittle, showing signs of breakage along the individual strands, perhaps as a result of the drying process. The crystallization of ligand 1 with 1 equiv of CuCl2 3 2H2O in a MeOH/H2O mixture resulted in the formation of a coordination polymer [Cu3(1)4Cl4]Cl2 3 nH2O (2, n = ca. 16) that was characterized by single crystal X-ray diffraction (Figure 1a). (See Supporting Information for the crystal data and extended packing diagram.) With substoichiometric amounts of CuCl2, mixtures of crystals of 2 (identified by unit cell determination) and the metallogel were formed; however, no crystalline material with a 2:1 ligand/metal ratio could be isolated. We previously reported the X-ray molecular and crystal structure of free ligand 1 and showed that the molecule adopts the same C-shaped conformation as in the present case as a result of steric hindrance at the urea carbonyl group arising (56) Todd, A. M.; Anderson, K. M.; Byrne, P.; Goeta, A. E.; Steed, J. W. Cryst. Growth Des. 2006, 6, 1750–1752.

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Letter Scheme 1. (a-e) Competing Interactions in Metal- and Anion-Binding Pyridyl Urea Gelators and (f) Molecular Structures of Ligands 1 and 3

from the CMe2 backbone. This causes the urea moiety to turn away from these methyl groups.56 In a building block of the coordination polymer in 2, four of the ligands wrap around three copper centers, which in turn are each connected by two asymmetrically bridging chloride ligands. The trimetallic Cu3Cl4 unit is discrete and linked to the next via a single bridging ligand 1. The structure exhibits a triple layer of π stacking within the trimetallic unit, whereas edge-to-face π interactions support the link between one unit and the next. Of the eight urea groups per Cu3L4 unit, two form a typical six-membered hydrogen-bonded ring to the two uncoordinated chloride anions,57 and the remaining six hydrogen bond to water molecules, leading to an extensively hydrated structure with ca. 16 water molecules per trimetallic formula unit. Half of the ligands and the water network are disordered over two positions. The structure is unusual for its type in that it does not display any urea 3 3 3 urea R-tape type hydrogen bonding or any interactions from urea to coordinated chloride.40,55,57 The PXRD pattern calculated from the single-crystal data was compared with that of pure ligand 1 and those obtained from the xerogels containing varying amounts of CuCl2. The data (Supporting Information, Figure S4) shows a clear transformation undergone by the ligand-Cu aggregates with increasing CuCl2 concentrations. At 0.1 equiv, the pattern still resembles that of the pure ligand, although broadening of the features indicates a less-ordered solid and there are additional peaks. Also, a new peak starts to develop at around 2θ = 11.3°. With further increases in CuCl2 concentration, the solid appears to be more and more amorphous up to 0.5 equiv of CuCl2. The addition of more than ca. 0.6 equiv of CuCl2, however, gives a material showing a much sharper diffraction pattern that closely resembles the calculated diffraction pattern from the single-crystal data, and hence we can identify the green precipitate observed during gel formation as coordination polymer 2. The same trend is observed in solid-state

MAS 13C NMR spectra in that the mixtures form an amorphous gel phase until a ligand/Cu ratio of 2:1 is reached whereas for higher CuCl2 concentrations the samples appear to be virtually identical to isolated crystalline material 2 (Supporting Information). The PXRD pattern of the xerogel is too broad and featureless to allow structure determination even by crystal structure calculation methods.58 However, insights into the nature of this 2:1 low-molecular-weight metallogelator species come from a discrete 2:1 complex of related ligand 3 and [Co (3)2(H2O)4](NO3)2 3 H2O (4) characterized by X-ray crystallography in previous work.34 This complex exhibits both urea-anion and urea-urea hydrogen bonding of the type shown in Scheme 1 and represents a possible model for the metallogelator in the present system. Alternatively, instead of a discrete 2:1 complex with pendant pyridyl groups such as 4, a 1D coordination polymer based on a four-coordinate metal center may be involved. The crystallization of 1 with 0.5 equiv of copper(II) nitrate (as opposed to chloride) does not result in gelation, but crystals of formula [Cu(1)2](NO3)2 3 H2O 3 MeOH (5) can be isolated. The X-ray crystal structure of 5 (Figure 1b) comprises a 1D chain of linked metallomacrocycles. Each “Cu2(1)2” macrocyclic ring is joined to the two adjacent rings not only by a shared copper(II) center but also by hydrogen bonding from the urea functionalities to the nitrate anions in a fashion reminiscent of the sandwich geometry of anion-binding complexes of 1-pyridin-3-yl-3-p-tolyl-urea (TUP).50,51,53 In the case of the metallogelator removal of the rigidifying effects of this nitrate, hydrogen bonding and replacing the anion with the coordinating Cl- could result in more flexible chains, leading to gelation instead of crystallization. We reasoned, therefore, that the rheological properties of the copper(II) metallogel of 1 might well be very susceptible to tuning by anion binding. Gels of different concentrations of both CuCl2 3 2H2O and ligand 1 were prepared and measured in stress

(57) Turner, D. R.; Smith, B.; Goeta, A. E.; Evans, I. R.; Tocher, D. A.; Howard, J. A. K.; Steed, J. W. Cryst. Eng. Commun. 2004, 6, 633–641.

(58) Anderson, K. M.; Day, G. M.; Paterson, M. J.; Byrne, P.; Clarke, N.; Steed, J. W. Angew. Chem., Int. Ed. 2008, 47, 1058–1062.

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Figure 1. (a) X-ray crystal structure of [Cu3(1)4Cl4]Cl2 3 nH2O (2) obtained from a 1:1 solution of 1 and CuCl2 3 2H2O in 7:3 MeOH/H2O.

(b) X-ray crystal structure of [Cu(1)2](NO3)2 3 H2O 3 MeOH (5) showing hydrogen bonding to nitrate (CH hydrogen atoms and included methanol solvent omitted).

sweep experiments. For all samples, the elastic modulus, G0 , was higher than that of the viscous modulus, G00 , as is typically observed for gel-like materials.59 With an increase in the force applied to the sample, the induced stress will disrupt the integrity of the network, causing G0 to decrease up to a point where it becomes lower than G00 and the gel breaks. In the following discussion, the values of G0 of the initial plateau and the values of the stress at which the gel breaks (yield point) are used to elucidate the mechanical properties in terms of gel morphology. The dependence of the elastic modulus and yield stress on the concentration of CuCl2 in a series on metallogels formed from 1 wt % ligand 1 in MeOH is shown in the Supporting Information (Figure S5). Between 0.25 and 0.5 equiv, G0 is proportional to the concentration of CuCl2 3 2 H2O.8 Such a strong dependence of the modulus on the metal salt concentration suggests that there is likely to be more than one reason for this behavior, but clearly the increasing transformation of nongelator 1 to the unknown 2:1 metallogelator complex will be a major factor. We then examined the effect of added chloride and acetate (two anions generally strongly bound by urea-based anion hosts60) as their tetrabutylammonium (TBA) salts on copper (II) metallogels of 1 prepared using 1 wt % ligand 1 with 0.3 equiv of CuCl2. The presence of the TBA cation may have an effect on the gel properties; however, this cation is constant throughout the experiments and interacts only very weakly with ureas, hence we anticipate that what effect there is should (59) Larson, R. G. The Structure and Rheology of Complex Fluids; Oxford University Press: Oxford, U.K., 1999. (60) Sessler, J. L.; Gale, P. A.; Cho, W.-S. Anion Receptor Chemistry; Royal Society of Chemistry: Cambridge, U.K., 2006.

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not alter the trends observed for particular anions. For this amount of the copper salt, no precipitate is formed, which is important in order to obtain reproducible results for the rheology measurements. In a typical experiment, the ligand was dissolved in MeOH, and CuCl2 3 2H2O was added carefully to avoid precipitation. Any precipitate formed was carefully dissolved by heating and stirring. Once a clear solution was obtained, the TBA anion salts were added swiftly, and the solution was injected into the rheometer. Immediately after the addition, the time-sweep measurements were started at constant oscillation stress (0.03 Pa) and constant frequency (1 Hz). The rheology of these metallogels was carried out using a concentric cylinder geometry to accommodate weaker gels and to be able to study the gels before equilibration. Figure 2 shows the appearance of the metallogels as a function of time for different concentrations of TBA acetate and TBA Cl- at different stages after the injection of the anions. For both anions, low concentrations (0.1 equiv relative to 1) led to the formation of gels that were visually identical to the metallogel before anion addition. As more of the anions are added, the two series of samples behave very differently, according to the anion added. Samples with TBA acetate show increasingly strong green coloration with increasing concentration. One hour after the addition of the acetate, gelation can be observed for all samples up to 0.7 equiv of TBA acetate, and this continues for the next 24 h. However, the resulting metallogels have a clearly different appearance from the pure gel, showing more structured domains and a lighter blue color. Gels were left for 1 week, but even after this time, the highest concentrations of anions did not form complete gels. The addition of TBA Cl- does not lead to a significant color Langmuir 2009, 25(15), 8451–8456

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Figure 2. Anion binding studies and appearance of gels of (from left to right in each row) the metallogel of ligand 1 and 0.3 equiv of CuCl2 (I) and the metallogel with 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 equiv of TBA acetate (II) and with 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 equiv of TBA Cl- (III). Pictures are taken (from top to bottom for each column) 10 min (A), 1 h (B), 24 h (C), and 1 week (D) after anion injection.

change, and instead “lumps” of the metallogels are formed for all concentrations. However, gelation for all amounts of TBA Cl- is only partial, and it appears that the chloride inhibits the gelation process until finally precipitation is observed for higher concentrations of the anion. Both of these systems show interesting rheological behavior over time. The pure metallogel (a gel of 0.3 equiv of copper(II) chloride in 1 wt % ligand) and all samples containing TBA Cl- show a swift increase of G0 that is followed by a significant decrease (Figure 3a). A quantitative discussion of the rheology is intrinsically difficult because of the rheometer geometry used for these in situ investigations. We observed significant variations (in some cases up to ( 50 Pa) in G0 , which are possibly due to complex and uncontrollable nucleation events on the surface of the geometry and are strongest in the early stages of gel formation. However, the occurrence of the maxima is a consistent feature during the formation of the gels and was observed for all samples. After reaching the maximum value, G0 decreases gradually to a value of typically 30-10% of the maximum. A more detailed and complete investigation of the rheology behavior is currently underway. A different behavior is observed for metallogels titrated with acetate. For up to 0.3 equiv of the anion, gel formation proceed similarly to that for TBA Cl- containing metallogels, but the drop in the elastic modulus is shallower. At 0.4 equiv of the anion, no maximum in G0 is observed on the 1 h time scale, but instead it plateaus at relatively high values compared to those for the pure metallogel. These results in the early stages of gelation suggest a rapidly forming gel structure or perhaps a gel precursor, which undergoes some slower rearrangement and relaxation. The nature of this structure is unknown, but it may be related to the formation of a discrete coordination complex gelator as discussed above. Within the context of Scheme 1, there are two possible ways in which anions could interact with such a copper(II) complex metallogelator. Either the anions could hydrogen bond to the urea, weakening the urea-urea hydrogen bonds, or they could coordinate directly as ligands to the copper(II) center. In the case of anion amounts of up to 0.3 equiv relative to 1 (1 equiv relative to copper), the increased rate of gelation and elastic modulus suggest that anions initially bind to the metal center, possibly displacing the pyridyl ligand over a longer time scale. After 1 equiv relative to copper has been added, the anions apparently inhibit Langmuir 2009, 25(15), 8451–8456

Figure 3. (a) Change in the elastic mouduls, G0 , with time during the first hour of gel formation for the pure metallogels and different amounts of TBA Cl-. (b) Development of the elastic moduli during gel formation for the first 300 min for a pure metallogel (1 wt % ligand 1 + 0.3 equiv of CuCl2) and two samples with added TBA acetate. DOI: 10.1021/la900145n

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gelation in a competitive fashion with hydrogen bonding in solution, slowing down the rate of fiber formation. The gel formation for selected samples was monitored in situ over 24 h, and details of the development of the elastic moduli for the first 300 min after TBA acetate addition is shown in Figure 3b. It can be seen that after the initial drop in G0 the pure metallogel undergoes a process of rearrangement with a steady increase in the elastic modulus. This ultimately reaches a plateau of 5300 Pa after ca. 16 h. The two samples with TBA acetate also exhibit a considerable increase in their elastic moduli, reaching plateau values of 15 000 and 3700 Pa, and after 1 day for the samples with 0.2 and 0.4 equiv of acetate added, respectively. Whereas the increase in G0 for the pure metallogel and in fact all metallogels treated with TBA Cl- is relatively smooth, for TBA acetate there is a point at ca. 180 min in which the elastic modulus increases sharply. In general, the addition of small amounts of acetate anion seems to lead extremely strong gels, whereas higher concentrations of TBA acetate weaken the gels.

from a gelator of composition Cu(1)2 to crystalline complex 2 with 4:3 stoichiometry, characterized by X-ray crystallography. Broadly, metal binding by 1 increases its gelation tendency, and the rheological properties may be further tuned, albeit in a complex manner, by varying the identity and concentration of the counteranion. The nature of the interaction of anions with ureas is a complex one and is very dependent on solvation and anion binding free energy as well as geometric and directional factors.60 How these anion-specific binding effects translate into effects on the gel is a key question, and a great deal more work is required to fully understand the structural and dynamic properties of these metallogels. This will be reported in an article; however, these preliminary results establish that the introduction of both pyridyl and urea groups can lead to the tunability of gels by both metals and anions. Acknowledgment. We thank Mrs. A. Christine Richardson for SEM sample preparation, Dr. David Apperley for assistance and helpful discussions about MAS NMR interpretation, and the EPSRC for funding.

Conclusions A simple bis(pyridylurea) ligand that is not itself an LMWG has been shown to form metallogels in the presence of copper (II) chloride but not other copper(II) salts such as nitrate. The mechanical properties of these systems depend on the concentration of the copper salt and undergo a transformation

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Supporting Information Available: Experimental details for the preparation of the gelator and gels, pictures of gels, and additional supplementary figures referred to in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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