NANO LETTERS
Cucurbit[7]uril and o-Carborane Self-Assemble to Form a Molecular Ball Bearing
2002 Vol. 2, No. 2 147-149
Rodney J. Blanch,* Alex J. Sleeman, Timothy J. White, Alan P. Arnold, and Anthony I. Day School of Chemistry, UniVersity College, UniVersity of New South Wales, ADFA, Canberra ACT, Australia Received October 27, 2001; Revised Manuscript Received November 19, 2001
ABSTRACT The self-assembly of cucurbit[7]uril (Q7) with o-carborane (1) produces a molecular ball bearing nanostructure (2). While investigating the possible role of o-carborane as a template for the controlled synthesis of Q7, new synthetic reaction conditions were discovered. Both the solvent and the reaction temperature had a marked effect on the relative percentages of cucurbit[n]uril (n ) 5, 6, 7, 8) produced. The effect of the o-carborane in the reaction mixture is discussed.
The host-guest chemistry of spherical and globular molecules, such as fullerenes and carboranes, has attracted considerable interest.1-3 These supramolecular materials have a range of structural motifs ranging from the ball-and-socket to Russian dolls.4,5 In this paper we report the observation of a deep inclusion complex of o-carborane in cucurbit[7]uril, (Q7). This supramolecular system can be viewed as a model of a simple molecular bearing. o-Carborane was not observed to associate with other cucurbit[n]urils, (Qn). Q7 is one member of a new class of macrocyclic compounds produced from the condensation of glycoluril with formaldehyde6-8 (Figure 1). Simple AM1 calculations indicate that the cavity inside Q7 is ideally suited to the encapsulation of o-carborane (1),9 (Figure 2). Initial attempts to encapsulate the o-carborane into Q7 from either (Q7 in H2O/1 in organic solvent) or (Q7 in H2O/1 in solid systems) were unsuccessful. However, when a solvent system was found where both Q7 and 1 had adequate solubility, the supramolecular complex (2) formed immediately. Two distinct resonances for the o-carborane methine hydrogens were observed for a solution of o-carborane and Q7 in deuterated trifluoroacetic acid, TFA. This indicates that equilibrium is established between 1@Q7, and 1 dissolved in the TFA solution (1 + Q7 / 1@Q7). The methine resonance observed for bound 1 is shifted upfield (∼0.2 ppm) relative to the signal for free 1. This upfield shift is characteristic of molecules internally bound in cucurbiturils.7,10-12 The exchange process is slow at room temperature relative to the NMR time scale. Coalescence * Corresponding author. E-mail address:
[email protected]; fax: +61-2 6268 8002. 10.1021/nl015655s CCC: $22.00 Published on Web 12/06/2001
© 2002 American Chemical Society
Figure 1. Cucurbit[n]urils, (Qn), where n ) 5, 6, 7, 8, and 10 have now been isolated and their structures confirmed by X-ray crystallography.6,8,10 o-Carborane (1) is shown with C-H as heavy dots; every other vertex is a B-H (C2H12B10).
Figure 2. A space-filling model of the o-carborane@Q7 supramolecular complex (2).9
experiments were not performed due to material precipitating as the temperature was increased. o-Carborane can adopt a
variety of orientations inside Q7. Many of these would place the C-H’s in different magnetic environments. The observation of only a single C-H resonance for the o-carborane indicates that the two C-H’s are in similar magnetic environments. When an excess of 1 is added to Q7 in TFA, the resulting product, 2, can be precipitated with methanol. The 1H NMR spectrum of this product showed both Q7 and 1 in a 1:1 ratio when dissolved in DCl/D2O. This result suggests that Q7 encapsulates 1, as free 1 is insoluble (and not detected by NMR) in the same DCl/D2O conditions. The 1@Q7 complex is quite robust in aqueous acids, surviving ion exchange chromatography, repeated dissolution/precipitation cycles, and drying in a vacuum oven at 80 °C. However, continuous extraction of an aqueous acid solution of 2 with dichloromethane for 48 h did remove ∼20% of 1 from the complex. The complex 2 was also observed via ES-MS. All of the experimental data strongly support the view that 1 is encapsulated in Q7. The complex 2 can be seen as a molecular analogy of a ball bearing, where the ball, o-carborane, is able to rotate inside the casing, cucurbit[7]uril. Other globular molecules, such as C60 could be used to replace 1 as the ball in this type of molecular ball bearing. The polar nature of 1, and its ability to form hydrogen bonds, may make this a “sticky” bearing.4 The nonpolar nature of C60 and the lack of direct hydrogen bonding capability might allow the C60 to rotate more freely within the Qn. We are currently investigating the possible encapsulation of C60. However, C60 is larger than 1 and would therefore need to be encapsulated in either Q8 or Q10, rather than Q7.7,8 Given the favored association of 1 with Q7, we investigated the feasibility of 1 acting as a template in the synthesis of Q7. Qn are synthesized from the acid-catalyzed condensation of glycoluril with formaldehyde.6-8 Our work in this area has demonstrated that strong acids are required to produce Qn.6,7 However, o-carborane is essentially insoluble in the strong aqueous mineral acid conditions used for the synthesis of Qn. The addition of solid 1 to the reaction mixtures had no observable effect on yields of Qn, product distribution, or even the uptake of 1. Accordingly, we investigated the use of organic acids as solvents and/or reagents for this synthesis. Glycoluril, formaldehyde, Qn, and o-carborane are soluble in trifluoroacetic acid and methanesulfonic acid, and therefore we investigated these two organic acids as potential solvent/reaction systems. At temperatures
