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Adamantane-Based Crystals with Rhythmic Morphologies Vasily A. Migulin and Fredric M. Menger* Department of Chemistry, Emory University, Atlanta, Georgia 30322 Received September 12, 2000. In Final Form: December 14, 2000 Fibrous morphologies were obtained from two tetrasubstituted adamantanes. One fiber consisted of crystals having aspect ratios of 2-3 orders of magnitude. According to X-ray analysis, the crystals are endowed with a series of weak hydrogen bonds which, through cooperative action born of a precise surfaceto-surface fit, leads to a preferred one-dimensional growth. The second adamantane derivative (a tetracation) forms flexible fibers up to 2 cm long when grown by the solvent diffusion method. Scanning electron microscopy shows a “rhythmic” morphology in which the fibers are composed of thin submicron plates, with rectangular or hexagonal geometries, stacked one upon another. Because the plates are congruent, the terminal plate of a growing fiber must act as a template that controls the dimensions of a plate that is in the process of forming contiguously. This unprecedented form of matter is explained on the basis of Liesegang ring theory. Anecdotal evidence supporting a Liesegang mechanism comes from yet another adamantane compound that crystallizes from solution in the form of horizontal rings. Interest in Liesegang phenomena stems in part from their possible relationship to natural periodicity as found, for example, in banded agates and gallstones.
Liesegang rings are periodic bands produced during precipitation of inorganic salts in gels such as gelatin, agar, or silicic acid.1 For example, horizontal copper chromate bands form over time when a 1 N copper sulfate solution is placed over silica gel containing 0.1 N sodium chromate.2 Interest in Liesegang rings stems in part from their possible relationship to natural periodic phenomena such as banded agates, gold veins in quartz rock, concentric waves of bacterial cell populations, and gallstones.3 Nonequilibrium states manifesting periodicity (“dissipative structures”) are also an increasingly important component of modern thermodynamics.4 In the present article, we describe adamantane-based tetracationic salts which form Liesegang rings in liquid systems rather than in the common gel systems.1,2 Even more noteworthy, fiberlike crystals were obtained with “rhythmic” morphologies. These crystals, which possess rectangular or hexagonal cross sections, are composed of thin congruent plates stacked one upon another. To our knowledge, such a form of matter is unprecedented. Tetracationic salt 2, synthesized according to Scheme 1, belongs to a class of compounds called “gemini surfactants”5 which have been the subject of intense publication and patenting in the past few years.6-8 A recent review testifies to the worldwide interest in geminis with regard to skin care, antibacterial regimens, construction of highporosity materials, analytical separations, and solubilization processes.9 The solution properties of our tetra* To whom correspondence should be addressed. E-mail:
[email protected]. (1) Henisch, H. K. Crystals in Gels and Liesegang Rings; Cambridge University Press: Cambridge, 1988. (2) Sharbaugh, A. H., III.; Sharbaugh, A. H., Jr. J. Chem. Educ. 1989, 66, 589. (3) Weiser, H. B.; Gray, G. R. J. Phys. Chem. 1932, 36, 286. (4) Kondepudi, D.; Prigogine, I. Modern Thermodynamics; John Wiley & Sons: Chichester, 1999. (5) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (6) Rosen, M. J.; Liu, L. J. Am. Oil Chem. Soc. 1996, 73, 885 (7) Chen, K.; Locke, D. C.; Maldacker, T.; Lin, J.-L.; Aawasiripong, S.; Schurrath, U. J. Chromatogr., A 1998, 822, 281. (8) Van Der Voort, P.; Mathieu, M.; Mees, F.; Vansant, E. F. J. Phys. Chem. B 1998, 102, 8847. (9) Menger, F. M.; Keiper, J. S. Angew. Chem. 2000, 39, 1907
Scheme 1. Synthesis of 1 and 2
cationic salt and its homologues with even longer radiating chains have already been published10 and include the following: (a) formation of small aggregates (i.e., micelles of only a few molecules each), (b) critical micelle concentrations 2 orders of magnitude lower than those of conventional single-chained surfactants, and (c) much lower solubility in water than in chloroform for geminis with 4 chains of 12 or more carbons. However, it was the solid-state properties of the adamantane-based geminis and their precursors that warranted particular attention. Their surprising modes of crystal growth are the subject of this letter. Crystals of precursor 1,3,5,7-tetrakis(bromoacetoxy)adamantane, 1 (Scheme 1), were grown by slow evaporation of a saturated ethanol solution at 24 °C in an NMR tube. Branchless needles formed that were as long as 6 cm and of uniform width (ca. 100 µm) for an unusually high aspect ratio of 600. Only the dimensions of the NMR (10) Menger, F. M.; Migulin, V. A. J. Org. Chem. 1999, 64, 8916.
10.1021/la001311q CCC: $20.00 © 2001 American Chemical Society Published on Web 01/18/2001
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Figure 1. Photograph of a fiber of 1 (broken off from a much longer crystal) with a ruble and nickel as reference.
tube limited the linear growth (Figure 1). The crystals were inflexible, fragile, and transparent, and they tended to separate into component filaments, giving aspect ratios running into the thousands! Scanning electron microscopy on the crystals showed no surface structure worthy of note. Slow evaporation of a 5:1 methanol/acetone solution of 1 in an NMR tube over 3 weeks produced high-quality crystalline needles from which X-ray pictures were obtained (Figure 2). A series of favorably positioned intraand intermolecular CH‚‚‚OdC hydrogen bonds is seen to be responsible, in part, for the one-dimensional growth. Thus, two intramolecular hydrogen bonds (2.31 and 2.63 Å H/O distances) are formed between each of the four carbonyl oxygens and the hydrogens of the adamantane methylenes. These serve to immobilize the four acyl substituents into a correct orientation for linear stacking. A total of eight intermolecular CH‚‚‚OdC hydrogen bonds stabilize each adamantane/adamantane juncture (Figure 2a). The carbonyls are either hydrogen-bonded to a neighbor’s adamantane methylene (2.73 Å) or to a bro-
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moacetyl methylene (2.75 Å). Although the hydrogen bonds are weak,11,12 their cooperative action, born of a precise surface-to-surface fit, leads to crystals with high aspect ratios. Only weak van der Waals attractions were observed between neighboring chains (Figure 2b). Bromines do not participate in substantial bonding, and, in fact, they display disorder while confined within “bromine channels”. Thus, a combination of multiple hydrogen bonding in one direction and weak interactions in the others leads to the highly preferred linear morphology. The solid-state properties of tetratosylate gemini 2 (Scheme 1) are completely different from those of precursor 1. Crystalline 2 was prepared by a solvent diffusion method: A 4 mL vial containing a saturated acetonitrile solution of 2 was set inside a 20 mL vial containing acetone, and the larger vial was then covered. Overnight diffusion of the acetone into the acetonitrile produced thin fibers up to 2 cm long. These fibers were more opaque than those of 1, and they were as flexible as a hair in either the wet or dry state (Figure 3). Shorter, less opaque, and seemingly more crystalline needles could be obtained when the acetonitrile solution was made much more dilute. Fiber formation is critically dependent upon structure (the benzenesulfonate salt and the tetraoctyl analogue of 2 giving only crystalline needles under any crystallization conditions). Scanning electron micrographs of fibrous 2 (coated with 30 nm 60/40 Au/Pd) showed both rectangular and hexagonal cross sections. The factors controlling crosssectional geometries and dimensions are not known. Interestingly, about 5-10% of the fibers were rhythmic in the sense that they consisted of thin congruent plates (Figure 4) that were not amenable to X-ray analysis. The term rhythmic was first used in 1935 to refer to crystals with alternating thick and thin sections.13 Our system, which possesses an alternative form of periodicity, can also be considered rhythmic. Figure 5 reveals a plate that has partially escaped its lattice, suggesting that plateto-plate association is relatively weak. Fiber flexibility
Figure 2. X-ray structure of 1 showing (a) linear stacking of molecules with eight CH‚‚‚OdC hydrogen bonds per adamantane/ adamantane juncture and (b) a top view showing bromine channels between the linear stacks.
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Figure 3. Light microscope picture of hairlike fiber of 2.
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Figure 5. Rhythmic crystal of 2 in which one plate has escaped the stacks.
Figure 6. Liesegang rings of 2-didodecaborate grown by ether diffusion into acetonitrile solution.
Figure 4. Rhythmic crystals of 2 showing rectangular and hexagonal cross sections.
may be related to cumulated small dislocations at the plate/plate interfaces. Because the plates are congruent, the terminal plate of a growing fiber apparently acts as a template that controls the dimensions of a plate that is in the process of forming contiguously. We attribute the rhythmic crystalline fibers to a type of Liesegang ring phenomenon. To qualitatively explain Liesegang rings and, by analogy, our fibers, we seek refuge in the oversimplified but appealing theory developed by Ostwald over 70 years ago.14 The theory is based upon the fact that a precipitate will form only if a compound’s concentration exceeds its solubility. When a mobile electrolyte travels down a gel and forms a precipitate with a stationary electrolyte, the ions near the resulting band
are depleted for a finite amount of time. Meanwhile, the diffusing front of the mobile electrolyte travels a certain distance in the gel before the solubility product is again exceeded, and a second ring is deposited. Thus, the two rings will be separated by a space. No analytic solutions have been found for various mathematical constructs describing band position and width in terms of variables such as precipitate solubility, diffusion constants, nucleation rates, crystal growth rates, concentration gradients, and the effects of local temperature, gravity, and convection currents.1,15,16 Our rhythmic crystals can be qualitatively interpreted via a dynamic competition between a mobile acetoneinduced supersaturation and a solute-depleting crystallization. Thus, crystallization begins near the top of a concentrated acetonitrile solution of 2 as acetone diffuses downward to create a supersaturated region. When a plate forms within this region, the region becomes lean in solute. The acetone-rich front continues its journey down the acetonitrile (below the solute-depleted band), whereupon the precipitation conditions once again become fulfilled.
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A second plate is deposited adjacent to the first provided that the acetone-rich front has not moved too far beyond the first plate. With dilute acetonitrile solutions of 2, crystallization rates are reduced relative to solvent diffusion, and short unbanded crystals should form (as was observed). Plate formation likely depends on a subtle balance of forces, including surface nucleation rates, that controls the precision with which the second plate duplicates the first. Irregularities in plate duplication should occur most often at the edges, and indeed it was these irregularities that allowed the plates to be resolved in the SEM photos. Anecdotal evidence supporting a Liesegang mechanism comes from distinct horizontal (11) Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311. (12) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond; Oxford University Press: New York, 1999. (13) MacMasters, M. M.; Abbott, J. E.; Peters, C. A. J. Am. Chem. Soc. 1935, 57, 2504. (14) Ostwald, W. Kolloid-Z. 1926, 40, 144. (15) Flicker, M.; Ross, J. J. Chem. Phys. 1974, 60, 3458. (16) Rodriguez-Hornedo, N.; Murphy, D. J. Pharm. Chem. 1999, 88, 651.
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rings encircling the crystallizing vial when 2-didodecoborate was crystallized by ether diffusion into acetonitrile (Figure 6). It is possible that rhythmic crystals are more common than might be supposed. Crystallization by the solventdiffusion method and subsequent examination of the crystals by SEM might uncover additional examples. Acknowledgment. This work was supported by the Army Research Office. We are grateful to Jason Keiper for the light microscopy, to Kevin Caran and Dr. Robert Apkarian (Integrated Microscopy and Microanalytical Facility) for the electron microscopy, to Scott Childs, Bao Do, and Dr. Ken Hardcastle for the X-ray analysis, and to Dr. David Goldsmith for the photography. Supporting Information Available: Crystal data on 1 as well as spectroscopic and elemental analyses for cited compounds. This material is available free of charge via the Internet at http://pubs.acs.org. LA001311Q
