Guest Molecules Confined in Amphipathic Crystals as Revealed by X-ray Diffraction and MAS NMR† Angiolina Comotti,*,‡ Silvia Bracco,‡ Piero Sozzani,‡ Samuel M. Hawxwell,# Chunhua Hu,# and Michael D. Ward*,#
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 7 2999–3002
Department of Materials Science, UniVersity of Milano Bicocca, Via R. Cozzi 53, 20125 Milan, Italy, and Molecular Design Institute, Department of Chemistry, New York UniVersity, 100 Washington Square East, New York, New York 10003 ReceiVed March 10, 2009; ReVised Manuscript ReceiVed May 18, 2009
ABSTRACT: Multinuclear 1H-13C heterocorrelated (HETCOR) NMR spectroscopy combined with X-ray diffraction revealed the unusual dual properties of identical guest molecules confined in two crystallographically distinct host cavities within single crystals based on hexagonal frameworks comprising guanidinium ions and organomonosulfonates. The environments of the two cavities differ substantially, as one is lined by the highly polar quasihexagonal guanidinium-sulfonate network, while the other is defined by walls consisting of nonpolar aromatic groups. The effect of these different environments on the NMR properties of the guest molecules is evident from chemical shift data and two-dimensional HETCOR spectra. The large magnetic susceptibility effect due to ring currents of the aromatic hosts enables determination of the host-guest distances and suggests intermolecular CH · · · π interactions. The 13C relaxation times reveal the molecular dynamics of the guests in two nanoscale environments that differ with respect to shape and dimensionality. The confinement of molecular guests in crystalline host frameworks and the modulation of their properties have emerged as topics of considerable interest.1 Collectively, many investigations have revealed that host-guest binding and the effect of confinement on guest properties often are governed by delicate noncovalent interactions, ranging from strong and directional hydrogen bonding to weak and nondirectional dispersive forces.2 Such attributes are well recognized in organic crystals, as demonstrated by crystal polymorphs that form due to competition among intermolecular forces and conformational flexibility of the molecular constituents.3 Organic crystalline host frameworks can exhibit a diverse range of guest habitats, which can exert considerable influence on the properties of the confined guests and their corresponding inclusion compounds.4-6 Consequently, the characterization of the local environment in which guest molecules reside is essential for understanding various aspects of their behavior under confinement and any ensuing properties. The extraordinary resolution of NMR spectroscopy enables detection of small differences in chemical environments that arise from structural differences in polymorphs or guest locations and orientations in static or dynamic disordered structures.7 Moreover, multinuclear solid-state NMR is a powerful method for deducing key intermolecular distances and probing the dynamics of motion in crystalline systems.8 The structures of organic crystals containing aromatic moieties are particularly amenable to NMR spectroscopy characterization as ring currents can produce magnetic susceptibility effects that reveal structural features that may otherwise escape detection.9 Herein we describe a tandem X-ray diffraction and solid-state NMR investigation of two unique inclusion compounds based on guanidinium cations (G ) C(NH2)3+) and organomonosulfonates (S), in which identical guest molecules are compartmentalized in two distinct cavities of different shapes, dimensionality, and polarity within the same crystal. Two-dimensional 1H-13C heterocorrelated NMR techniques are used to discriminate between identical guest molecules embedded in distinct structural environments - one highly polar and the other nonpolar - and to identify weak host-guest interactions that characterize their individual properties. Measurements of the † This manuscript is dedicated to the memory of Samuel M. Hawxwell, deceased February 8, 2008. * Corresponding authors: E-mail:
[email protected]. Fax: 212-995-4895 (M.D.W). E-mail:
[email protected]. Fax: 02-64485400 (A.C.). ‡ University of Milano Bicocca. # New York University.
Figure 1. The hexagonal packing in G4CBS · 2/3(mesitylene), as viewed down the channel axes (right). The guests are omitted for clarity. The guanidinium cation G (left) and p-chlorobenzenesulfonate anion S are depicted at the left.
dynamics of both the host and guest molecules provide insight into disordered or multiconfigurational states. The crystal structures of G4CBS · 2/3(mesitylene) and G4CBS · 2/ 3(o-xylene) (4CBS ) 4-chlorobenzene sulfonate) reveal a hexagonal lattice endowed with two kinds of cavities, one-dimensional channels with an internal diameter of approximately 8 Å and “zerodimensional” cages (Figure 1). The G ions and sulfonate moieties (S) organize into cylindrical geometry forming channels while the organic substituents on the sulfonate moieties define the cages. The residual electron density in the cavities is attributable to guest molecules that occupy disordered sites within both the channels and cages.10 The high resolution achievable in the hydrogen domain through Lee-Goldburg homonuclear decoupling (LG),11 combined with twodimensional 1H-13C heterocorrelated NMR, reveal in G4CBS · 2/ 3(mesitylene) the different microenvironments of guests confined to cages and channels and their respective host-guest interactions (Figure 2). Two separate 13C signals of equal intensity were observed for the mesitylene methyl groups. At a short contact time (0.5 ms, Figure 2A) the 2D spectrum exhibited two cross-peaks for the methyls of mesitylene due to intramolecular correlation. One cross-peak resonates at high fields in both the hydrogen and
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Figure 3. The inclusion compound G4CBS · 2/3(mesitylene): (A) View of the mesitylene guest embedded in the cage formed by the 4CBS aromatic groups capping the top and bottom of the cage. (B) View perpendicular to the channel axis illustrating the arrangement of mesitylene guests in the channel. The carbon guests are rendered as space filled. The hydrogen atoms of the guests and all but the CR of the 4CBS are omitted for clarity.
Figure 2. (A) Contour plots of 2D 1H-13C HETCOR spectra of G4CBS · 2/3(mesitylene) compound with 15 kHz magic angle spinning and ramped cross-polarization times of 0.5 ms. (B) 2D spectrum at the contact time of 5 ms. The orange shadings denote the host-guest intermolecular correlations. (C) Shielding region about the aromatic 4CBS ring and topology of the mesitylene methyl group located above the plane of the aromatic ring. (D) A methyl group of one of the mesitylene guests enclosed in one of the three clip sites within the cage formed by the aromatic 4CBS rings. The methyl carbon nucleus resides at the intersection of the axes perpendicular to the two p-chlorobenzenesulfonate rings, suggesting weak CH · · · π interactions.
carbon domains (highlighted red, δC ) 20.1; δH ) 0.7 ppm) while the other resonates at lower fields (highlighted blue, δC ) 22.4 ppm; δH ) 2.4 ppm). The presence of two cross-peaks for the methyl groups demonstrates the existence of two distinct surroundings for the mesitylene guests in the crystal structure. Conformational isomerism can be ruled out as mesitylene is a rigid planar molecule. The 1:1 ratio of the two methyl signals in the quantitative 13 C spectrum denotes the equal partitioning of the guest between the two cavities. The remarkable upfield shift of the guest methyls in the hydrogen and carbon domains can be explained by the large magnetic susceptibility effect of the host aromatic moieties due to ring currents, demonstrating unequivocally that guest methyl groups are in proximity with the host aromatic rings. The distance between the guest methyl groups and the rings of the host can be deduced from the dependence of the chemical shift on proximity with the aromatic residues, as calculated by nucleus-independent chemicalshift maps (Figure 2C).9 The large upfield shifts for the carbon nuclei of the guest methyl groups (∆δC ) -2.3 ppm at room temperature and -3.4 ppm at 235 K) can be accounted for by the simultaneous presence of two aromatic rings facing the methyl carbon at a distance of less than 4 Å (Figure 2D). Instead, the hydrogens can explore, dynamically, the regions of space close to one aromatic ring at a time, generating an average shielding effect on the methyl hydrogen of ∆δH ) -1.7 ppm. The close contact of the methyl group with the aromatic rings suggests the occurrence of C-H · · · π interactions between the guest methyl and the 4CBS rings.12 Through space host-guest correla-
tions also are apparent in the 2D NMR spectrum at longer crosspolarization times of 5 ms (highlighted orange, Figure 2B).13 Thus, within the cage, each methyl group is nestled between two aromatic rings that combined resemble a “clip” (Figure 2D). The 3-fold symmetry of the mesitylene guest is compatible with the trigonal symmetry of the cage that contains three equivalent clip sites, thus forming CH · · · π interactions between the guest methyl group and the six aromatic rings of the cage that account for the robustness of the structure (Figure 3A). This perfect match of host-guest symmetry in the cage is supported by the measurements of 13C relaxation times of guest methyls at variable temperature, which, although in the extreme narrowing limit (13C T1 decreases from 3.1 to 0.74 s in the range 300-255 K with correlation times τc = 10-10 s), revealed restricted methyl rotation with a high activation energy Ea of 22 kJ mol-1. This large barrier can be attributed to close packing in the cage, similar to behavior observed for the methyl groups in polycrystalline L-alanine and other organic crystals.14 The presence of the favorable CH · · · π interaction between the methyl C-H vector and the phenylene group generates a deep energy minimum, and the C-H vector must reorient 180° to achieve favorable interaction with the next phenylene group in the clip. In addition, a slow dynamics of the host aromatic rings in the solid regime of motion was evident at low temperature, confirming the efficient packing of the cage structure. The assignment of the downfield signal of the guest methyls was elucidated by 2D HETCOR NMR experiments at longer contact times (Figure 2B). No correlation was observed between the downfield hydrogens of the guest methyl groups and the host aromatic moieties, not even at contact times of 5 ms that would reveal through-space intermolecular host-guest interactions at distances longer than bond distances.13 The absence of response from the downfield hydrogen signal with the host aromatic rings, when contrasted with the strong correlations from the guests within the cages, can be explained by the long distances separating these groups. This permits assignment of the downfield signal to the guests residing in the polar channels, which are entirely lined with guanidium cations and sulfonate anions (Figure 3B). The X-ray diffraction (XRD) electron density distribution within the channels is consistent with mesitylene guests that adopt a configuration in which their aromatic planes are inclined along the channel, forming a 22° angle with respect to the channel axis and occupying a 3-fold disordered site. This suggests that the face-to-face stacked configuration with the aromatic planes perpendicular to the channel axis is hampered by the large size of mesitylene as compared to the cross section of the channel. The 13C relaxation times for the guest molecules in the channel decrease with increasing temperature and approach the minimum value as the guest motion approaches the 108 Hz regime, indicating substantial mobility.15 Moreover, no NMR correlation was observed between the guests within the cages and the channels, not even at the long contact times of 10 ms (not
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Figure 4. Contour plots of 2D 1H-13C HETCOR MAS NMR spectra of G4CBS · 2/3(o-xylene) compound with 15 kHz magic angle spinning at contact times of (A) 0.5 ms and (B) 5 ms. The red and blue shadings highlight the intramolecular guest cross-signals of both methyl groups and CH aromatic groups in the 4,5-position in the cage and the channel, respectively. The orange areas denote the host-guest intermolecular correlations, and the green area denotes the signals of the methyl groups of the guest in the channel.
shown), indicating that the guests are compartmentalized in two cavities that are quite distant. The 2D 1H-13C HETCOR NMR spectra of G4CBS · 2/3(oxylene) also revealed the inclusion of the guests in two distinct environments (Figure 4). Unlike mesitylene, a chemical shift difference was apparent not only for the methyl group but also for the CH aromatic group in the 4,5-position. At a short contact time (0.5 ms) two signals, associated with intramolecular coupling, were apparent in both the carbon and hydrogen domains, indicating the selective confinement of guest molecules in two distinct cavities (Figure 4A). At longer contact times (5 ms), only the upfield methyl hydrogens of the guest correlated selectively with the aromatic carbons of the host, at δC ) 127.3 and 128.8 ppm (highlighted orange in Figure 4B). This observation permits the assignment of the upfield peaks to guests within the cage, with the methyl groups in close proximity to the host aromatic rings. The downfield methyl signal (highlighted green) was isolated from any other spin-system, demonstrating that the guest molecules within the polar channels are at a distance from the host aromatic residues, to the extent that through-space intermolecular interactions cannot occur even at long 5 ms contact times. However, like the mesitylene guest, the o-xylene methyl groups in the cage can interact with the aromatic rings of the host clip sites. The C2V point group symmetry of o-xylene and the 60° angle between the two ortho-methyls is incompatible with the trigonal symmetry of the cage, such that the two methyl carbons (dmethyl-methyl ) 2.8 Å) cannot simultaneously interact with two clip sites, and a few methyl-to-clipsite interactions remain frustrated. Consequently, only a moderate ∆δC ) -1.1 ppm magnetic susceptibility shift was observed as the guest explores multiple configurations dynamically, consistent with the disorder observed by X-ray diffraction for the o-xylene guest in the cage. Also, the o-xylene guests in the channels are highly disordered with the aromatic planes arranged both parallel and perpendicular to the channel axis. The fast dynamics of the o-xylene guests is proved by the 13C spin-lattice relaxation times with values as short as 1.5 s at room temperature. Furthermore, it was revealed that there is surprising mobility of the host aromatic rings about the axis connecting sulfur to chlorine, due to diffusional motion with a correlation time τc as short as 13 ns. In fact, at room temperature, the T1(13C) relaxation
Crystal Growth & Design, Vol. 9, No. 7, 2009 3001 times of the CH aromatic groups are 0.5 s and lie at the minimum of the T1 master curve (τcω = 1).15 In contrast, the relaxation times for the guanidinium carbon nuclei are long (130 s) indicating the rigidity and robustness of the ionic GS network lining the channel walls. In conclusion, we have realized self-assembled crystalline architectures with guest molecules compartimentalized in two amphipathic nanospaces with very distinct geometries and polarities. Advanced 2D NMR spectroscopy enabled the differentiation of identical guest molecules in chemically distinct environments within the same crystal. The molecular location of the guests and the occurrence of CH · · · π interactions can be established exploiting the magnetic susceptibility shifts exerted by the aromatic rings of the host. Moreover, the relaxation times reflect the distinct molecular dynamics of the guests in the different cavities, which explains the extent of disorder suggested by single crystal X-ray diffraction. Collectively, these observations inform directly on the magnetic properties of 1H and 13C nuclei in starkly different polar and nonpolar environments, which can be useful for the identification of molecular species sequestered by specific sites in complex architectures, such as distinctly polar and nonpolar domains that often exist in soft matter and biological systems.
Acknowledgment. The authors acknowledge financial support from the National Science Foundation (DMR-0720655), FIRB, Cariplo Foundation, and New York University. Supporting Information Available: . Preparation and X-ray crystallographic data for the G4CBS · 2/3(mesitylene) and G4CBS · 2/3(oxylene) compounds (at 100 K and 298 K), and solid-state NMR experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
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