Weak Intermolecular Interactions and Molecular Recognition

John A. Ripmeester*,‡,§. Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Canada K1A 0R6, and. Ottawa-Carleto...
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10604

J. Phys. Chem. B 1999, 103, 10604-10616

Weak Intermolecular Interactions and Molecular Recognition: Structure and Dynamics of the Benzene and Pyridine p-tert-Butylcalix[4]arene Inclusions† Eric B. Brouwer,‡,§ Gary D. Enright,‡ Christopher I. Ratcliffe,‡ Glenn A. Facey,‡,§ and John A. Ripmeester*,‡,§ Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Canada K1A 0R6, and Ottawa-Carleton Chemistry Institute, Department of Chemistry, Carleton UniVersity, 1125 Colonel By DriVe, Ottawa, Canada K1S 5B6 ReceiVed: June 25, 1999

The host cavity of 1:1 host-guest compounds of p-tert-butylcalix[4]arene is well suited for the study of weak interactions in the solid state, as the motional freedom of the guests tests the weak intermolecular interactions in a very direct way. Benzene and pyridine are guests with a controlled number of similar (size, shape) as well contrasting (dipole moment, lone electron pair) properties that allow a meaningful comparative investigation of the structural and dynamic features. The 150 K single-crystal X-ray diffraction studies for the two host-guest compounds show that in both cases the guests occupy essentially the same orientation in the host cavity, with pyridine situated 0.11 Å deeper into the cavity than benzene. The complementarity of the diffraction and solid-state NMR techniques is illustrated, in particular, by incorporating the pyridine structural information obtained from 2H NMR studies into the diffraction data, thus resolving the ambiguity of the nitrogen atom position. Despite similar structural environments, the guests exhibit quite different dynamic behavior. Variable temperature 2H NMR spectra of the perdeuterated pyridine and benzene guests are interpreted in terms of specific motional models; benzene undergoes in-plane rotation followed by reorientation about the compound’s 4-fold axis of symmetry. In contrast, pyridine reorients about the pyridine C2 molecular symmetry axis (rather than in-plane rotation), followed by guest reorientation about the compound’s C4 axis of symmetry. A significant point is that the pyridine nitrogen has definite orientations in the cavity that cannot be explained by any specific directional electrostatic interactions between the host and guest. Both the dynamically averaged 15N NMR chemical shift tensor components and the absence of short contacts rule out a C-H‚‚‚N hydrogen bonding interaction of the host to the guest. Despite the fact that the pyridine molecule is tightly docked in the host cavity, intermolecular interactions must be ascribed strictly to steric interactions acting in concert rather than specific directional interactions. Both guests are oriented in the host cavity such that the aromatic plane minimizes rather than maximizes contact with the host CH3 groups. This result questions the ability to ascribe structural features in the solid state to isolated directional interactions, such as the importance and role of CHhost‚‚‚πguest interactions which have been suggested many times as having a stabilizing influence on this system.

Introduction The study of weak solid-state interactions continues to be a challenge from experimental and computer modeling perspectives. Moreover, since substances that need to function in a flexible manner at ordinary temperatures of necessity must depend on the presence of weak interactions, the challenge becomes of prime importance. Noncovalent interactions are generally classified according to whether they are steric or electronic, that is, as resulting primarily from either dispersive or electrostatic forces. Usually these interactions are distinguished by the more directional nature of the latter. For instance, in structural studies involving diffraction it is customary to look for short contacts that may indicate hydrogen bonding and a variety of other directional interactions (C-H‚‚‚O, C-H‚‚‚N, π-stacking, etc.). Similarly, one may use spectroscopic measurements to look for band or resonance shifts to establish specific * Corresponding author. Phone: (613) 993-2011. Fax: (613) 998-7833. E-mail: [email protected]. † Published as NRCC No. 42196. ‡ Steacie Institute for Molecular Sciences. § Carleton University.

interactions. For instance, there has been a recent interest in exploring 15N chemical shift tensors in the context of their sensitivity to hydrogen bonding and longer range interactions, and such studies are key to understanding structure and function in biomolecules such as nucleic acids, peptides, and proteins.1 One further way of exploring weak interactions, perhaps currently not used in any systematic manner, is to use the dynamic process itself as a probe. We have shown that the asymmetric cavity in p-tert-butylcalix[4]arenes is well suited for the study of weak interactions.2 In this study, we use a combination of structural and dynamic techniques to locate unequivocally the pyridine nitrogen in the calixarene cavity and show that it has preferred locations in the cavity, but without there being any particularly short contacts or significant effects on the 15N chemical shift tensor. The results indicate that weak directional interactions may in fact be transparent to many techniques and that collective steric interactions also may give the appearance of strong directional preferences. In the solid state, the conical p-tert-butylcalix[4]arene (tBC) host molecule (below) exhibits a wide range of guest inclusion.4 At room temperature, 1:1 tBC‚guest compounds typically

10.1021/jp9921598 CCC: $18.00 Published 1999 by the American Chemical Society Published on Web 11/11/1999

Benzene and Pyridine p-tert-Butylcalix[4]arene Inclusions crystallize in a tetragonal P4/n space group, and due to its chemical simplicity, the tBC cavity is an ideal test site for probing interactions with guest molecules. In the past, much information about CH‚‚‚π and cation‚‚‚π interactions4-6 has been inferred from structural studies of tBC‚guest compounds, although many of these are significantly disordered.7

Better understanding of host-guest interactions in supramolecular compounds depends on the degree to which the structural properties, including dynamics, of each component can be defined. In the solid state, these properties are determined by a delicate balance among many forces including ionic, dipole, hydrogen bonding, and van der Waals.8 Thus, to gain insight into processes that involve molecular recognition, such as templating9 and specific binding,10 the contributions of each interaction must be elucidated. Previous work has shown that pyridine guests occupy positions both inside (endo) and outside (exo) the cavity of the p-tert-butylcalix[n]arene‚pyridine (n ) 7,8) structures.11 The role of CHhost‚‚‚πguest and weak hydrogen-bonding interactions for the endo-pyridine guest has been examined most thoroughly with the 1,3-crown-5-ether-bridged tBC‚pyridine compound.5 The crown ether, attached to the calix via two phenolic oxygen atoms opposite in the calix cone, imparts an elliptical shape to the calix cavity; the pyridine plane lies along the major axis of the ellipse defined by the unbridged tert-butylphenol units. The distance from the pyridine nitrogen to the methyl hydrogen of the host tert-butyl group (dH‚‚‚N ) 2.58 Å) was related to the presence of a weak (8 kJ mol-1) C-H‚‚‚N hydrogen bond. Calculation of the total potential energy as a function of pyridine rotation angle about its C2 molecular symmetry axis led to the introduction of an attractive CHhost‚‚‚πguest interaction between the methyl groups of the host and the aromatic guest. Because the above calculations were executed without confirmation of the position of the pyridine nitrogen, we felt it necessary to examine a system where the pyridine nitrogen could be located and to evaluate the consequences of the guest motion on the characteristics of the host-guest interactions. To simplify the system, a calixarene host without the crown ether component is selected. The similarities and differences in the structure and dynamics of the similarly shaped benzene guest molecule are examined with the tBC‚benzene compound; the orientation of the guests is determined by the presence or lack of a C-H group at the ipso-ring position. Thus, the origin of these differences is identified as being primarily a result of collective weak steric interactions rather than a few specific and strong directional electronic interactions. Experimental Section p-tert-Butylcalix[4]arene (Aldrich) was used as received. tBC‚ pyridine and tBC‚benzene were prepared by crystallizing tBC (0.25 g) from a warm (T ≈ 70 °C) solution of the guest solvent (5 mL). Colorless crystals were isolated by filtration in high

J. Phys. Chem. B, Vol. 103, No. 48, 1999 10605 yields. Pyridine-15N (C/D/N Isotopes) was used to prepare tBC‚ pyridine-15N (25% enrichment). 13C CP-MAS NMR spectra were collected at 45.3 MHz on a Bruker CXP-180 spectrometer equipped with a Doty Scientific 7 mm double resonance spinning probe. Typical experimental conditions were as follows: 160-800 transients; 2 s recycle time; 3 µs π/2 pulse duration; 3 ms contact pulse duration; 2.9 kHz spinning rate; 4K data points collected and zero-filled to 16K data points without line broadening; 60 kHz 1H decoupling field strength. The 2H NMR static spectra were collected using a quadrupole echo pulse sequence12 on Bruker CXP-180 (27.6 MHz) or MSL-200 (30.7 MHz) spectrometers equipped with a Bruker static sample probe (5 or 10 mm solenoid coil). The 15N CP-MAS NMR spectra were collected at various spinning rates on a Bruker AMX-300 spectrometer equipped with a Doty 5 mm double-tuned spinning probe. Typical collection conditions were as follows: 1000 transients; 2 s recycle time; 3 µs π/2 pulse duration; 10 ms contact pulse duration; 2K data points collected and zero-filled to 4K data points with 20 Hz line broadening; 63 kHz 1H decoupling field strength. The 15N chemical shift is referenced to nitromethane δ ) 0 ppm taking the nitrate line of NH4NO3 to have δ ) -5.0 ppm. Single-crystal X-ray diffraction data were collected on an Enraf-Nonius diffractometer at λ ) 1.54056 Å using the θ/2θ scan mode, graphite monochromator, aperture of 4.0 × 4.0 mm2 at a distance of 173 mm from the crystal, stationary background counts at each end of the scan (scan/background time ratio 5:1), and peak and background counts determined with the profile analysis procedure of Gabe.13 σ(Fo) based on counting statistics, function minimized ∑w(|Fo| - |Fc|)2, where w ) [σ2(Fo) + k(Fo)2]-1, R ) ∑||Fo| - |Fc||/∑|Fo|, Rw ) (∑w(|Fo| - |Fc|)2/ ∑w|Fo|2)1/2, and GOF ) [∑w(|Fo| - |Fc|)2/(m - n)]1/2. Values given for R, Rw, and GOF are based on those reflections with I g 2.5σ(I). Results 1. 13C CP-MAS NMR Spectroscopy. The 13C CP-MAS (cross-polarization and magic-angle spinning) NMR spectra of tBC‚pyridine and tBC‚benzene at room temperature (Figure 1) are similar to those of a large number of 4-fold symmetric tBC‚guest structures with one guest per tBC cavity.14,15 The dipolar-dephased NMR spectra16 show intensity for the proton-bearing carbons of the guests and thus establish that these carbon atoms undergo motional averaging. The change in the value of the isotropic chemical shift on going from the liquid to the enclathrated guest (the complexation-induced shift, CIS ) δhost-guest - δliquid) is negligible for the benzene carbons and is insufficient to provide information on the orientation of the guest inside the host cavity.2d,7 With tBC‚pyridine, CIS values are significantly negative for guest carbons G3 (-3.2 ppm) and G4 (-2.8 ppm) and suggest that these carbons may reside deep in the calix cavity based on similar observations for the host with toluene, nitrobenzene, and aliphatic guests.2,7,14,17 2. 2H NMR Spectroscopy. 2H (I ) 1) NMR spectroscopy in solids is dominated by a first-order perturbation of the Zeeman interaction by the nuclear quadrupolar coupling. The latter arises from the interaction of the nuclear electric quadrupole moment with the electric field gradient (EFG), denoted by the tensor V.18 With a nuclear spin quantum number I ) 1, there are three energy levels and hence two transitions displaced equally on either side of the Zeeman frequency by the quadrupolar perturbation. The 2H powder line shape, arising from the summation of such resonance line pairs from all crystallite

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Brouwer et al. tain.19,21,22 The orientation of the EFG tensor must first be described in a final reference frame in terms of Euler angles R, β, and γ,

V(R,β,γ) ) RVpasR-1

(2)

where R is the Euler angle rotation matrix describing the coordinate transformation and Vpas is the tensor in its principal axis system. The fast motion line shape then represents the effective tensor Veff which is the population weighted average of the tensor components in the final frame for all the sites (orientations) visited during the motion (with pi the population factor of site i), n

Veff )

Vi(R,β,γ)pi ∑ i)1 n

(3)

pi ∑ i)1

Figure 1. The 45.3 MHz 13C CP-MAS NMR spectra of (a) tBC‚ pyridine and (b) tBC‚benzene; peaks indicated by the asterisk disappear under dipolar-dephasing conditions. Labeling of the guest carbons is consistent with that in Figure 6, except for host carbon 9 ) CTn.

orientations, has either two or three pairs of characteristic features (edges, shoulders, and peaks) separated by frequencies

∆νzz ) 3χ/2 ∆νyy ) 3χ(1 + η)/4

(1)

∆νxx ) 3χ(1 - η)/4 where χ is the quadrupole coupling constant in Hertz and the asymmetry parameter η ) (∆νyy - ∆νxx)/∆νzz. The ∆νii are proportional to the absolute magnitudes of the principal axis components of the effective quadrupole coupling tensor. Molecular motion at sufficiently high rates (κ > ∼104 s-1) reduces the EFG tensor components and changes the 2H NMR line shape. Differing degrees of line shape modification are observed depending on the rate of reorientation and the orientations of the principal axes of the EFG tensor relative to the rotation axis. In the fast motion limit (FML), where the reorientational rates are about κ g 107 jumps s-1, the line takes on a shape that can be described by an averaged effective tensor. Theory and methods for calculating both intermediate rate and fast limit line shapes are well established.19-23 The FML spectra may be used to determine models for the molecular motion since analytical expressions for the parameters governing the FML line shapes are relatively easy to ob-

a. tert-Butylcalix[4]arene‚Benzene-d6. Inclusion compounds containing benzene are well-known,24 and these molecules typically exhibit planar reorientation of the benzene guest about its 6-fold axis. Table 1 tabulates quadrupolar coupling constant χ and asymmetry η values for static benzene-d6 in a variety of crystalline environments. Selected variable temperature 2H NMR spectra of tBC‚ benzene-d6 are shown in Figure 2 with the measured values of ∆νii summarized in Table 2. At 77 K, the line shape already shows distortions arising from molecular reorientation in the intermediate motional regime (κ ) 104-107 Hz). The guest motion approaches the fast motion limit (FML; κ g 108 Hz) at 195 K; at this temperature, the line shape is axially symmetric with the value for χ ) 42.3 kHz slightly less than one-quarter the value of χ for static benzene-d6 (Table 2). As the temperature increases to 295 K, the spectra narrow slightly but retain an axially symmetric pattern. In other benzene-containing inclusion compounds, the pattern of 2H resonance line narrowing as a function of increasing temperature is assigned to two consecutive types of guest molecular reorientation. In-plane ring reorientation about the 6-fold guest axis of symmetry reduces the line width by a factor of 2 (usually rapid at low temperatures). This motion is then followed by an n-fold (n > 2) reorientation about an axis in the molecular plane which further reduces the line width by an additional factor of 2. In tBC‚benzene-d6, the pattern of 2Hline shape behavior as a function of increasing temperature occurs similarly, but with a slight variation. In-plane benzene reorientation is already rapid below 77 K, as the observed line width is one-half that in the static case. With increasing temperature, the benzene additionally reorientates about the 4-fold axis of the calixarene host but with the benzene plane tilted slightly off this axis, as indicated in the X-ray structural study (section 4). A detailed analysis of the variable temperature 2H benzene-d6 spectra in terms of a motional model is given below. When the benzene is static (κ ) 0 Hz), the quadrupole coupling tensor for each deuteron is oriented with its ZZ component along the C-D bond and the YY component perpendicular to the molecular plane. Introduction of rapid inplane reorientation does not reorient the YY component, whereas the ZZ and XX components are averaged. The YY component becomes the new ZZ′ component of the effective averaged tensor23,25 which has axial symmetry. This can be expressed in

Benzene and Pyridine p-tert-Butylcalix[4]arene Inclusions

J. Phys. Chem. B, Vol. 103, No. 48, 1999 10607 TABLE 2: Observed and Calculated Lineshape ∆νii Values for TBC‚Benzene-d6 in kHz experimentala (calculated) T/K

molecular reorientation

∆νxx

77 195

static in-plane in-plane + 4-foldb in-plane + 4-fold, θ ) 11°

291

in-plane + 4-fold, θ ) 12°

-132 (71) -31.7 -31.7 (-31.6) -30.6 (-30.9)

∆νyy -142 (71) -31.7 (-31.6) -30.6 (-30.9)

∆νzz 274 (-142) 63.4 63.4 (63.2) 61.2 (61.8)

a Estimated error of measurement: (0.5 kHz. bFour-fold rotation not at fast motion limit.

Vxx′ ) Vyy′ ) [Vxx + Vzz]/2

(5)

Vzz′ ) Vyy

Figure 2. The 27.6 MHz 2H NMR spectra of tBC‚benzene-d6 at 77, 195, and 291 K. The spectra were collected using a 10 mm coil, 150600 transients, 2 s recycle time, 4.3 µs π/2 pulse duration, 33-35 µs echo delay, 100 Hz line broadening, 1K points collected and zerofilled to 4K points, with symmetrization. Signal intensity at zero frequency is due to a small amount of uncomplexed benzene and is indicated by the asterisk.

TABLE 1: The Static Quadrupole Coupling Constant (χ) and Static Asymmetry Parameter (η) for Benzene-d6 in Different Crystalline Environments system

χ/kHz

η

C6D6 C6D624a C6D624b (η6-C6D6)Cr(CO)324c C6D6‚1,3-cyclohexanedione24a 2 C6D6‚Cd(NH3)2Ni(CN)424b 2 C6D6‚Cd(H2NC2H4NH2)Ni(CN)424b

180.7 ( 1.5 183 ( 1 183 ( 0.9 183 ( 1 181 ( 1 181.6 ( 0.9 184.4 ( 0.9

0.041 ( 0.007 0.04 ( 0.005 0.04 0.05 ( 0.01 0.04 0.036 0.047

25

terms of eqs 4-5 as follows. For n-fold symmetric reorientation (n > 3) about a given axis, the averaged tensor components (following from eqs 2 and 3) are

Vxx′ ) Vyy′ ) [(cos2 R cos2 β + sin2 R)Vxx + (sin2 R cos2 β + cos2 R)Vyy + sin2 β Vzz]/2 (4) Vzz′ ) cos2 R sin2 β Vxx + sin2 R sin2 β Vyy + cos2 β Vzz Hence for in-plane benzene motion where R ) 90° and β ) 90°,

Note that this makes all the deuterons equivalent with the same averaged tensor orientation. Also, once in-plane motion is rapid, a 2-fold motion about any axis in the molecular plane cannot be detected since it has no effect on the averaged tensor. (Note that the real signs of the components of the static tensor are not known, so for the purpose of calculating averaged tensors the static ZZ component is arbitrarily made positive, and all the rest follows from this). If the plane of the benzene ring is then tilted θ degrees away from the 4-fold axis of the cavity, the normal to the ring is tilted by (90 - θ) degrees. Four-fold rotation at this angle then causes a further averaging (R ) 0°; β ) (90 - θ)° in eqs 4). With θ ) 11°, we can reproduce the line shape at 195 K and similarly, with θ ) 12°, the line shape at 291 K. The ∆νii values for the 2H NMR spectra at different temperatures (calculated from the static ∆νii values) show excellent agreement with the observed values (Table 2). This analysis probably overestimates the angle θ since one usually expects a very small part of the narrowing to be caused by librational motions; however, there is not enough information in the spectra to determine the extent of this overestimation. b. tert-Butylcalix[4]arene‚Pyridine-d5. Recently, the dynamics of a pyridine guest in a tri-ortho-thymotide (TOT) host were studied.23,26 At low temperature pyridine undergoes restricted in-plane rotation followed by an additional 2-fold rotation about the molecular axis of the guest at higher temperatures. The 2H NMR spectra of tBC‚pyridine-d5 from 77 to 296 K (Figure 3) show different features from those for pyridine in TOT. We show that the 2H NMR spectra are consistent with pyridine undergoing three types of motion: (1) 2-fold molecular reorientation about the pyridine symmetry axis; (2) 4-fold molecular reorientation about the calixarene axis of symmetry; (3) out-of-plane librational motions. Whereas the low-temperature spectra do not have any sharp features, the resonance line at 77 K has a large asymmetry parameter η indicative of rapid 2-fold reorientation, most reasonably about the molecular 2-fold axis. At temperatures below 155 K, all spectra display weak edge features characteristic of 2-fold averaging with a width of 133 kHz, but even at 77 K, the intensity of the edge feature is low relative to the center. In the absence of other features, these observations suggest that first, the 2-fold reorientation is already fast and, second, that another slow motion is already present at 77 K. Indeed, the spectra above 77 K and up to 210 K display a steady development of new features confirming the onset of an additional motion at intermediate rates. At 210 K, the fast motion limit has been reached for this motion. We show that it is well

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Figure 3. Variable temperature 46.1 MHz 2H NMR spectra of tBC‚ pyridine-d5. The spectra were collected using a 5 mm coil, 50-200 transients, 3.5 s recycle time, 2.6 µs π/2 pulse duration, 35 µs echo delay, 1 kHz line broadening, 512 points collected and zero-filled to 1K points, with symmetrization. The 77 K spectrum was collected at 30.7 MHz with a 10 mm coil, 300 scans, 5 s recycle time, 5.0 µs π/2 pulse duration, 35 µs echo delay, 300 Hz line broadening, 512 points collected and zero-filled to 2K points, with symmetrization.

represented by a 4-fold reorientation of the molecule about the host calixarene axis of symmetry, analogous to benzene, toluene, and other guests2a-c,7,15 in this host lattice. Above 210 K and up to 295 K, there is a slight overall reduction in line width probably arising from out-of-plane librational motions of increasing amplitude, as for TOT‚pyridine.23,26 Librational averaging likely is active at all temperatures above 77 K, and its effects make the task of analyzing the spectral data rather difficult, because, at any particular temperature, we do not have a good grasp of the effective 2H quadrupole coupling tensors in the absence of the two large-amplitude molecular reorientations. Nevertheless, the FML spectra above 200 K are highly informative. Each resonance consists of three superimposed components, all of which are axially symmetric within the accuracy of the data. Two components are broad with very similar widths and the third is much narrower. (The central isotropic component in the spectra above 194 K arises from uncomplexed pyridine-d5.) The components, with a 2:2:1 intensity ratio, clearly correspond to the ortho-, meta-, and para- (o-, m-, p-) deuterons (see detail of the spectrum at 232 K and the simulation, Figure 4). Note that here, unlike in the benzene case, there cannot be any rapid guest molecular reorientation between sites ∼60° apart in the plane of the ring as this would make the three deuterons indistinguishable. Values of the observed splittings as a function of temperature are given in Table 3. i. Detailed Analysis. The static 2H quadrupolar tensor components in pure solid pyridine are known from earlier work:23

Brouwer et al.

Figure 4. Observed 46.1 MHz 2H NMR spectrum at T ) 232 K (middle) and simulation at the fast motion limit of the 2-fold and 4-fold guest molecular reorientation (bottom) for tBC‚pyridine-d5. The inset shows the ortho- and meta-D edge features upon expansion of the spectral region enclosed by the dashed box. The intensity of the isotropic component (uncomplexed pyridine-d5) in the observed spectrum is truncated for presentation purposes (cf., Figure 3). The scheme (top) indicates molecular reorientations of the pyridine: (left) guest rotation about its 2-fold molecular axis; (right) 2-fold rotation of pyridine + motion about tBC 4-fold axis where the angle between the two motional axes is β ) 52.9°. Note also the orientation of the averaged EFG tensor components Vii.

TABLE 3: Observed 2H NMR Splittings (in kHz) for Pyridine-d5 in tBC as a Function of Temperaturea splitting/kHz

a

T/K

meta-D

ortho-D

para-D

166.5 188.0 194.5 210.0 232.0 295.5

59.6 60.1 61.2 61.5 61.2 57.7

54.8 54.8 55.6 55.9 55.9 53.1

12.0 11.5 9.6 8.4 8.1 8.1

Estimated error of measurement: (1.4 kHz.

ZZ ) 273.6; YY ) -142.0; XX ) -131.6; χ ) 182.4 kHz; η ) 0.038, with YY perpendicular to the ring (again the sign of ZZ is arbitrarily set positive for the purposes of the following calculations). Two-fold reorientation leaves YY unchanged at -142 kHz, and this is significantly larger than the -133 kHz value observed for tBC‚pyridine-d5 at 77 K. The difference in

Benzene and Pyridine p-tert-Butylcalix[4]arene Inclusions SCHEME 1: Summary of Dynamic Averaging Calculations for the 2H Quadrupole Coupling Tensors for tBC‚Pyridinea

a Calculated values of the principal components (in kHz) correspond to 210 K experimental results. By convention |Vzz| > |Vyy| g |Vxx|.

observed and expected values can be attributed to out-of-plane librational averaging, and the above values can be used to give reasonable estimates for an effective librationally averaged tensor in the absence of any other motions, as follows. In the TOT‚pyridine 2H NMR study,23 equations were derived for the effective quadrupolar tensor as averaged by small angle (φ < 15°) out-of-plane librations (with φ expressed in radians):

c′11 ) [(Vxx + Vzz)(1 - 3φ 2) + (Vxx - Vzz)(1 - φ 2)]/2 c′22 ) [(Vxx + Vzz)(1-3 φ 2) - (Vxx - Vzz)(1 - φ 2)]/2 (6) c′33 ) -(Vxx + Vzz)(1 - 3φ 2) These equations account for librations with the same amplitude about two axes in the molecular plane. For tBC‚pyridine, the amplitudes are also assumed to be approximately equal in order to make the problem tractable. With φ ) 8.3°, librational averaging reduces the YY component from -142 to -133 kHz. The other tensor components are given in Scheme 1, which summarizes the calculated tensor components for the deuterons under the various motions discussed here. This estimated “initial” tensor is the same for all five deuterons and is then used as the starting point for the 2-fold and 4-fold motional averaging. The following calculations are based on the parameters obtained from the 210 K spectrum, which is just in the fast motion limit, and is less affected by the small increases in librational amplitude than the room-temperature spectrum. ii. The para-Deuteron. The p-D NMR line shape is unaffected by 2-fold molecular reorientation since all its tensor components are aligned with the molecular axes. The line shape of the p-D at 210 K thus arises purely from the effects of 4-fold reorientation on the librationally averaged tensor. The drastic narrowing observed (to ZZ′ ) 12.6 kHz) can arise only if the C-D bond (which corresponds to the ZZ component direction along the 2-fold molecular axis) is oriented close to the magic angle ()54.74°) with respect to the calixarene 4-fold axis symmetry. Equations 4 describe the 4-fold averaging, and if, for the moment, it is assumed that R ) 0°, the angle in question is calculated as β ) 52.9°. In many cases of motional averaging, the Euler angle R is 0° or 90° because of symmetry, but in the current situation this simplification is not strictly valid. However, from the other results, we show that R is small, and, in any case, the sensitivity of the values of the averaged components to the value of R is found to be very low for the p-D line. The angle β ) 52.9° establishes a unique orientation for the pyridine molecular 2-fold axis in the calixarene cavity; this reduces the

J. Phys. Chem. B, Vol. 103, No. 48, 1999 10609 choice of pyridine nitrogen atom positions from six to two. For the XRD analysis, this eliminates a common problem of locating the pyridine nitrogen that often is difficult to distinguish from C-H, such as in TOT‚pyridine in which the pyridine nitrogen position is not located unambiguously by diffraction.23,27 iii. The ortho- and meta-Deuterons. After averaging about the molecular 2-fold axis, the effective 2H quadrupolar tensors for all of the o-, m- and p-D’s will be aligned with the molecular axis. However, an important difference is that the XX components for o- and m-D will be oriented in the same direction as ZZ of p-D. Keeping this in mind, eqs 4 can also be used, along with the angle β ) 52.9° previously derived for the p-D and the parameters for the 210 K spectrum, to calculate back to the effective 2-fold averaged tensors for the o-D and m-D. These tensors, and the librationally averaged starting tensor, are related by the following expressions for 2-fold averaging (again derived from eqs 2 and 3):

Vxx′ ) Vxx sin2 θ + Vzz cos2θ Vzz′ ) Vxx cos2 θ + Vzz sin2 θ

(7)

Vyy′ ) Vyy where θ corresponds to the angle between the C-D bond and the 2-fold axis. Note that YY is unaffected. The values of θ which fit the o-D and m-D results are θortho ) 54.4° and θmeta ) 59.2° and are close to the value of 60° expected if the internal bond angles of pyridine were 120°. Note however that in this particular calculation R has been assumed to be 0°. An alternative approach to modeling the o-D and m-D line shapes begins with the assumption that the pyridine in the tBC cavity has the same structure as in the gas phase. Hence, θortho ) 57.5° and θmeta ) 62.1° are readily derived from the gasphase structural information.28 These values are then used to calculate the 2-fold motionally averaged tensors, which in turn are used, with the value β ) 52.89° determined above for the p-D, to calculate the values of R required to give the observed 4-fold averaged line shapes, namely Rortho ) 10.0° and Rmeta ) 8.7° (see Scheme 1). Alternatively, a value of R can be assumed and values of β calculated, e.g., for R ) 9°, θortho ) 52.5° and θmeta ) 52.97°. While this model does not produce unique values, as R does not have to be zero, and although specific approximations and assumptions have been made, a self-consistent picture emerges. Of significant importance is that the gross changes in line shape for all three types of deuteron (o-, m- and p-D) are explained exceptionally well using the same model and with Very similar angles R and β. Consequently, there is little doubt that a combination of 2-fold followed by 4-fold pyridine molecular reorientation does occur. Furthermore, the 15N NMR and X-ray structural results described later support this model. In closing this section, it is appropriate to mention that another motional model was initially considered in which there is no reorientation about the molecular 2-fold axis. Instead, 2-fold reorientation about the crystal axis occurs at low temperature, which becomes a 4-fold reorientation at higher temperature. (The change from 2-fold to 4-fold could be brought about by a phase transition involving a distortion of the calix at low temperature.) However, in this case, the calculated 4-fold averaged line shapes are very different from those observed; the line shape of the p-deuteron is the same as before, but two of the other deuterons in particular give much broader line shapes than observed. Likewise, the 77 K spectrum is not reproduced by 2-fold reorientation about the crystal axis. Consequently, this model is untenable.

10610 J. Phys. Chem. B, Vol. 103, No. 48, 1999

Brouwer et al. TABLE 4: The Observed 15N Chemical Shift Tensor Components (δii) in ppm for Solid Pyridine-15N (173 K)30 and tBC‚Pyridine-15N (290 K) (Errors in the Latter Estimated as (10 ppm)b sample pyridine-15N

(static)

tBC‚pyridine-15N

δiso -63 -64.4

δ11

δ22

δ33

200 (263) δxxa 119.6 (184)

33 (96) δzz -156.4 (-92)

-422 (-359) δyy -156.4 (-92)

a δ , δ , and δ following the pyridine static values correspond to xx yy zz the principal components along the molecular axes defined in the same way as for the para-D. b Values of (δii - δiso) relative to δiso are also given in parentheses. The labeling scheme follows the convention suggested by Mason.40

SCHEME 2: Summary of Dynamic Averaging Calculations for the 15N Chemical Shift Tensora

a Calculated values of the principal components (in ppm) correspond to 298 K experimental results referenced to δiso.

Figure 5. The 30.4 MHz 15N CP NMR spectra of tBC‚pyridine-15N (25% enriched): (a) the static spectrum at 173 K (from ref 30); (b) the simulated spectrum resulting from rapid 2-fold molecular reorientation about the pyridine molecular axis; (c) with MAS at 3.654 kHz; spinning sidebands are indicated by an asterisk. The scheme (top) indicates the orientation of the 15N CS tensor components for static pyridine-15N; β ) 52.9° is the angle between the pyridine 2-fold motional axis and the tBC 4-fold axis of symmetry.

3. 15N CP-MAS NMR Spectroscopy of tBC‚Pyridine-15N. The components of the 15N chemical shift (CS) tensor are sensitive to hydrogen bonding and intermolecular and long-range interactions.1d,29,30 Furthermore, CS tensor components are sensitive to molecular reorientation in much the same way as the components of the 2H quadrupolar interaction tensor. 15N (spin ) 1/2) NMR spectroscopy of tBC‚pyridine-15N probes the chemical environment around the pyridine nitrogen atom and also tests for the presence of guest molecular reorientation. tBC‚ pyridine-15N is characterized by both 13C and 15N CP-MAS NMR spectroscopy. The single 15N NMR resonance (Figure 5) is compatible with only one crystallographically unique nitrogen, and the isotropic chemical shift value of δiso ) -64.4 ppm is similar to solution values (δiso ) -63.9 ppm)31 and the value in solid pyridine at 173 K (δiso ) -63 ppm).30 This shift suggests that there is very little if any interaction between the N atom and C-H’s on neighboring tert-butyl groups. To access the CS tensor information, 15N CP-MAS NMR spectra (one spectrum is shown in Figure 5) were collected at four different spinning rates and the spinning sideband intensities were analyzed in terms of the Herzfeld-Berger technique.32 Because of the rapid reorientation about the 4-fold axis of the calix, the averaged CS tensor should have axial symmetry.

Consequently, this constraint was imposed on the spinning sideband analysis. The resulting 15N CS tensor components of tBC‚pyridine-15N at 290 K are given in Table 4. The reduction and change of sign in the anisotropy of the tBC‚pyridine-15N line shape relative to the static pyridine-15N line shape30 arise from the same dynamics as were revealed by 2H NMR and are described in detail in the following paragraphs. Although the principal values of the static CS tensor for solid pyridine33 were reported some time ago, the results of more recent work at 173 K30 are clearly superior (Table 4). Furthermore, in the latter report, ab initio calculations were used to determine the orientation of the principal components of the CS tensor with respect to the molecular frame (Figure 5).29,30,34 Thus, the CS component perpendicular to the pyridine plane is δyy ) -422 ppm, the component along the 2-fold molecular axis is δzz ) 33 ppm, and the third component is δxx ) 200 ppm. The x, y, and z axes are defined in the same way as for the p-D quadrupolar coupling tensor. For calculating the effects of dynamic averaging, it is more convenient to express the CS tensor components referenced relative to δiso ) 0 ppm, and these are also given in Table 4. The dynamic averaging equations are then directly analogous to those for the 2H case except that the tensors are given in terms of δii instead of Vii. Hence, beginning with the static 15N CS tensor, the effects of librational averaging can be calculated using equations similar to eqs 6 with φ ) 8.3°, followed by 4-fold averaging using analogues of eqs 4 with β ) 52.9° and appropriate values of R (Scheme 2). With R ) 9°, the final calculated tensor components are very close to the values obtained experimentally, especially upon consideration of the estimated error in the experimental results. Note that, as with the p-D quadrupolar tensor, the CS tensor components are invariant under 2-fold molecular reorientation about the pyridine symmetry axis. The dynamically averaged 15N NMR CS tensor for tBC‚ pyridine is consistent with the model developed from the 2H NMR data, as well as with the X-ray diffraction study (section

Benzene and Pyridine p-tert-Butylcalix[4]arene Inclusions

J. Phys. Chem. B, Vol. 103, No. 48, 1999 10611

TABLE 5: Crystallographic Data for tBC‚Benzene and tBC‚Pyridine compound empirical formula formula weight temperature wavelength crystal color crystal size crystal system, space group unit cell dimensions volume Z, calculated density F(000) µ (Cu KR) data collected maximum 2θ reflections collected/unique Rmerge no. of reflections I g 2.5σ(I) no. of variables R indices [I g 2.5σ(I)] goodness-of-fit maximum ∆/σ, final cycle residual density

tBC‚benzene C12.5H15.5O 181.76 150 K 1.54056 Å colorless 0.30 × 0.30 × 0.20 mm3 tetragonal, P4/n a ) b ) 12.8412(2) Å c ) 12.9751(4) Å 2139.56(7) Å3 8, 1.129 g cm-3 790.08 0.51 mm-1 h, k, l 139.7° 3609/2040 0.029 1750 56 R ) 0.053, Rw ) 0.049 4.32 0.100 -0.660-0.730 e Å-3

4). The 15N NMR data suggest that the pyridine is oriented in the cavity with its 2-fold molecular axis oriented at about β ) 52.9° to the compound 4-fold axis of symmetry. Moreover, the molecular reorientation of the pyridine guest sufficiently accounts for the observed 15N CS spectral features. Possible contributions from hydrogen bonding and intermolecular and long-range interactions that would modify the values of the tensor principal components from the values of solid pyridine itself do not appear to be significant. 4. Single-Crystal X-ray Diffraction Studies of tert-Butylcalix[4]arene‚Benzene and tert-Butylcalix[4]arene‚Pyridine. The XRD structures of tBC‚benzene and tBC‚pyridine at 150 K (Table 5, Figure 6) are, in general, quite similar but show subtle differences in guest positioning and unit cell dimensions. Both compounds crystallize in the tetragonal space group P4/n. The host molecules form a bilayer of open cavities facing each other along the c axis, with the layers displaced in the ab plane by a/2 and b/2. The host tert-butyl groups (CTn) display a 7:1 positional disordering, and this is interpreted in the context of the local environment of one of the four tert-butyl groups of the macro-ring being disordered over two positions in a 1:1 ratio, the remaining three tert-butyl groups occupying a single position. Spatial or temporal averaging over the entire crystal then gives the observed 7:1 ratio. The nature of this disorder will be discussed in more detail below. Disordering of the tertbutyl groups over two positions is typically observed for the tBC‚guest system,2,4,5,7,15 and is likely related to the strength of the host-guest interaction, with the stronger interaction increasing the distortion of the host tBC cavity. a. tert-Butylcalix[4]arene‚Benzene. A single molecule of benzene, positionally disordered about the 4-fold axis, occupies the C4-symmetric tBC cavity; the plot shows only one of the four symmetry-related guest positions. The view along the z-axis shows the guest in a position that minimizes contact with the four tert-butylphenol walls of the calix cavity. In fact, the plane of the benzene lies close to the plane that includes two of the four bridging methylene carbons and the z-axis. The benzene plane is oriented with respect to the tert-butyl methyl groups in a fashion that minimizessrather than maximizesscontact. This questions the role of CHhost‚‚‚πguest interactions in stabilizing the host‚guest compound that have been invoked in other calix‚guest compounds such as the 1,3-crown-5-ether-bridged tBC‚pyridine compound.5

tBC‚pyridine C12.25H15.25N0.25O 182.00 150 K 1.54056 Å colorless 0.15 × 0.21 × 0.27 mm3 tetragonal, P4/n a ) b ) 12.8314(4) Å c ) 12.8059(6) Å 2108.42(12) Å3 8, 1.147 g cm-3 790.10 0.52 mm-1 h, k, l 139.9° 4398/2003 0.023 1703 152 R ) 0.047, Rw ) 0.072 2.82 0.033 -0.310-0.260 e Å-3

Nonbonding atomic distances between the host and benzene guest (Table 6) are equal to or greater than the sum of the van der Waals radii (ΣrVDW ≈ 3.6 Å), with the exception of that between the guest carbon CB6 to the host CT1a tert-butyl methyl carbon (3.46 Å). This close approach of the guest likely pushes the tert-butyl group from its majority (CT1a) to its minority (CT1b) disordered position to minimize contact. In general, there do not appear to be any specific points of contact between the host and the guest. The plane of the guest is slightly tilted off the host axis of symmetry by θ ) 10.9°, very close to the angle determined by analysis of the NMR results. The tilting of the aromatic guest off the 4-fold axis of symmetry, as well as the positioning with respect to the host tert-butyl groups, is similar to that observed in the tBC‚toluene and tBC‚anisole structures.2a,4 b. tert-Butylcalix[4]arene‚Pyridine. Figure 6b shows the calixarene tetramer, the pyridine guest, and four tert-butyl groups of the next layer along the c-axis. As in other tBC‚guest compounds, the crystal packing, and in particular, the next layer along the c-axis is important in visualizing what “caps” the calix cavity and completes the incarceration of the guest. As with the guest in the tBC‚benzene structure, pyridine is situated in the cavity such that it appears to minimize interactions of the aromatic ring with the methyl groups of the hosting cavity. The pyridine plane is tilted off the compound 4-fold axis of symmetry by an angle of 9.7° ( 0.3°, and the C-H bond at the pyridine para ring position makes an angle of β ) 47.0° ( 0.3° with the 4-fold axis of symmetry of the calixarene. The Euler angles, calculated from the XRD data, relating the crystal frame (with the 4-fold axis as the z-axis) to the molecular frame (with the 2-fold axis as the new z-axis, and the y-axis perpendicular to the molecular plane) in the same way as in the NMR dynamic model, are R ) 13.3° and β ) 47.0°. These angles can be compared directly with the angles determined from both the 2H and 15N NMR analysis, and, considering the relevant errors in the X-ray determined numbers and the approximations made in the NMR dynamic model, the agreement is remarkably good. For tBC‚pyridine, the relevant nonbonded host/guest carbonto-carbon distances range from 3.58 to 3.79 Å (Table 6); the sum of the van der Waals radii ≈ 3.6 Å. While the CP6-toCT1a distance appears to be the closest contact, it is likely that the tert-butyl group occupies the minor disordered position.

10612 J. Phys. Chem. B, Vol. 103, No. 48, 1999

Brouwer et al.

Figure 6. X-ray structures of (a) tBC‚benzene and (b) tBC‚pyridine with hydrogen atoms omitted for clarity. The view looking into the cavity along the 4-fold axis of symmetry is shown at the top and the view perpendicular to the 4-fold axis of symmetry is shown at the bottom. For tBC‚pyridine, the four capping tert-butyl calixarene groups in the next layer along the z-axis are shown to indicate fully the guest environment.

TABLE 6: Summary of Relevant Nonbonding Host-Guest Distances for tBC‚Benzene and tBC‚Pyridine, in Angstromsa host atom guest atom

CT1a

CT2a

CB1 NP1 CB2 CP2 CB3 CP3 CB4 CP4 CB5 CP5 CB6 CP6

3.70 3.63

3.69 3.58 3.74 3.84

b

C3

C4a

C1

C5

C6

Discussion

3.57 3.62 3.58 3.65

3.66 3.65 3.79

3.46c 3.52c

two closest contacts of the host to the pyridine nitrogen atom are of interest; the CT1a methyl carbon of the hosting cavity and the CT2a methyl carbon of the capping calix are at distances from the guest nitrogen of 3.63 and 3.58 Å, respectively. Finally, the closest approach of the tert-butyl methyl of the two layers along c is 4.15 Å.

3.71 3.66

3.79

3.77 3.78 3.77

3.77 3.74

a Error in distances of (0.01 Å determined by disorder rather than diffraction resolution. b Methyl carbon belonging to the tert-butyl methyl group of the adjacent “capping” calixarene host. c Majority position at 50% occupation; minority (CT1b) position gives greater host-guest distance.

There does not appear to be any specific point of attachment between the two molecular components in this structure. The

This discussion focuses on three aspects of the experimental work: (a) the complementarity of the NMR and diffraction techniques; (b) the differences and similarities of the structure and dynamics of the two guests inside the tBC cavity; (c) the interactions between host and guest that give rise to the structural and motional observations are evaluated in terms of molecular recognition of neutral guests by the calixarene host. 1. NMR and X-ray Diffraction Complementarity. Initial refinement of the pyridine position from the diffraction data was hindered by the inability to define unambiguously the nitrogen atomic position. In other inclusion structures involving a pyridine guest, the pyridine nitrogen atom is often poorly characterized by diffraction since nitrogen and carbon atoms do not differ very significantly in electron density. Furthermore,

Benzene and Pyridine p-tert-Butylcalix[4]arene Inclusions any in-plane motion of the pyridine causes the nitrogen to be disordered over some (when restricted) or all (when unrestricted) of the aromatic atomic positions.27 The TOT‚pyridine structure typifies the problems encountered with a dynamic pyridine guest. In the XRD analysis, the highest peak in the electron diffraction map of the pyridine ring is attributed to the nitrogen atom,27 but subsequent dynamic 2H NMR work indicates that pyridine undergoes restricted in-plane motion which in fact partially disorders the nitrogen atom over three positions.23 In the present case, the disordering of the pyridine guest over four symmetry-related positions in the calix cavity further complicates the identification of the pyridine nitrogen atom because the electron densities are reduced by a factor of 4. The pyridine motional model developed from the 2H NMR data indicates a unique orientation for the pyridine 2-fold axis, and the determination of the 15N CS tensor components is consistent with this orientation. These data reduce the choice of position of the pyridine nitrogen atom from six to two. When one of these two atomic positions is entered into the X-ray refinement, the R value drops significantly, whereas an increased R value results if the nitrogen atom is placed at any other ring atom position. This example illustrates the benefit of a complementary approach to structure determination in systems displaying disorder and dynamics. Even when the XRD structure is “known”, complementary data from other techniques such as NMR can still contribute to a fuller understanding of structural features. Another issue addressed by combining XRD and NMR results is the nature of the disorder in the calix tert-butyl groups. On the basis of the X-ray results alone, the 7:1 positional disorder in each of the four tert-butyl groups of the calix could be either static or dynamic disorder. In the static case, the crystal symmetry comes from spatial averaging, and dynamic disorder creates 4-fold calix symmetry from temporal averaging. Note, however, that, for both scenarios, at any instant each individual calix host is not 4-fold symmetric. The disorder in the host is almost certainly associated with the guest disorder, i.e., the guest can only occupy one position in the calix at a time, and each position being off-axis means that the guest-calix unit has neither 4-fold nor 2-fold symmetry. The NMR results show that the pyridine and benzene motions about the calix axis are already active at 150 K, so the guest disorder is dynamic. As the guest moves to the next site, so too must the 1:1 disorder switch from one tert-butyl to the next, otherwise the 4-fold dynamic symmetry would not be achieved (and the 2H NMR would not show an axially symmetric averaged line shape in the fast motion limit!). An interesting question then arises as to what happens when the four-site motion eventually freezes out at low temperature. Does the disorder simply become static, or is there a phase transition to an ordered static phase with lower overall symmetry? 2. Comparison of Structural and Dynamic Features. Benzene and pyridine are two guest molecules that are similar in size and shape, yet quite different in terms of electronic structure. The structural data indicate that the two guests occupy essentially the same orientation in the cavity, while the dynamic data indicate that molecular reorientation of these guests is rather different. Both features will be addressed in turn. Comparison of the two structures upon looking along the z-axis (Figure 6) indicates that both guest planes are tilted off the z-axis by approximately the same amount. In both cases, the C-H bonds lying in the guest molecular plane point directly to the two “corners” of the calixarene cavity rather than to the “walls” formed by the phenol groups. This guest positional feature differs from the guest orientation in the 1,3-crown-5-

J. Phys. Chem. B, Vol. 103, No. 48, 1999 10613 ether-bridged-tBC‚pyridine compound where the guest molecular plane points directly to the “walls” of the cavity.5 In other words, the guest orientation in the tBC and the 1,3-crown-5ether-bridged-tBC hosts differs by a ∼45° rotation about the host axis of symmetry. The differences in guest orientation imply that guest interactions with the host differ, and as such, the identification of CHhost‚‚‚πguest interactions to stabilize such host-guest inclusions may be at best, a specific rather than a general phenomenon.5 Nonbonding host-guest distances fall into similar ranges for both compounds and are at or greater than the sum of the van der Waals radii (Table 6). A further similarity is that the host makes the closest approach to the guest at the very position where the two guests differ in atomic composition (the ipso ring atom). The only difference between the guest position is in the depth of inclusion into the cavity, measured by the distance from the host phenolic oxygen to the deepest guest carbon. Pyridine is “pushed” 0.11 Å deeper into the cavity than is benzene, and this feature can be explained by two steric differences between the pyridine and benzene. The first is the smaller molecular volume of pyridine due to the absence of a hydrogen atom at the ipso-ring position, and the second is the slight shortening of the pyridine molecular axis due to the presence of nitrogen in the aromatic ring. An additional slight difference between the two structures is the angle of the host phenol groups with respect to the host axis of symmetry. As expected from the deeper inclusion of the guest, tBC‚pyridine shows a larger value than tBC‚benzene indicating that the walls of the cavity are pushed out slightly further. Consideration of the capping host layer along the c-axis (i.e., the second layer of the calixarene bilayer) more fully describes the host environment surrounding the pyridine nitrogen and the benzene ipso-C-H. The two closest approaches of the host to the guest ipso ring position are from the methyl groups of the tert-butyl groups (1) above the guest and (2) from the host cavity, i.e., a close contact from both calixarene bilayers. For pyridine, C-H‚‚‚N distances are dC‚‚‚N ) 3.63 Å for the hosting calix and dC‚‚‚N ) 3.58 Å to the adjacent host (Table 6). The corresponding host-guest C-H‚‚‚C distances for benzene are only slightly longer at dC‚‚‚C ) 3.70 Å for the hosting calix and dC‚‚‚C ) 3.69 Å to the adjacent host.35 These differences are very small, especially considering the presence of the hydrogen atom at the ipso position of the benzene. In both cases, the two closest host C-H bonds point directly at the ipso ring atom. These two C-H bonds are perpendicular to each other and pinch the guest at approximately 45° above and below the plane (Figure 7). In essence, both guests occupy a cleft formed by the bilayer calix tert-butyl groups, and the host cavity shape appears to determine the similarity in benzene and pyridine orientation. The relevant host-guest distances (Table 6) characterize the origin of the host tert-butyl disordering over two positions in a 7:1 ratio. Since there are four symmetry-equivalent tert-butyl groups, this ratio is equivalent to only one of the four tertbutyl groups disordered over two positions in a 1:1 ratio. The closest nonbonding host-guest interactions appear to be between the guest carbon atoms at ring position six and the host tert-butyl carbons CT1 of the same layer. However, this close approach is only valid with the occupation of the major position, and it is likely that guest carbon 6 causes the tertbutyl group to occupy the minor position which minimizes this interaction. Full occupation of the minor position does not occur due to close contacts that result with tert-butyl carbon CT2b

10614 J. Phys. Chem. B, Vol. 103, No. 48, 1999

Figure 7. Local pyridine environment in tBC‚pyridine with location of pyridine electron-rich orbitals; distances are given for the two closest approaches of the host tert-butyl C-H groups from the hosting cavity (below, CT1a) and the capping cavity (above, CT2a) to the guest.

with the guest in adjacent layers; thus 1:1 positional disordering ratio is a compromise. Although the guest orientation features are similar, the guest dynamics differ. The 2H NMR results indicate that benzene in the host cavity undergoes in-plane rotation followed by reorientation about the compound’s 4-fold axis of symmetry as the temperature increases. In contrast, the pyridine in the same host cavity first undergoes 2-fold reorientation about its molecular axis, followed by motion about the compound’s C4 axis of symmetry in a similar temperature range. The motional models are consistent with the 2H NMR data and, for pyridine, the 15N NMR data. Note that the benzene could also be undergoing 2-fold reorientation, but, because 6-fold in-plane reorientation is rapid even at 77 K, any 2-fold flip is undetected by the 2H NMR line shape experiment. It is interesting to compare the motions of the two guests in the tBC cavity with motions of the same guests in different crystalline environments. The dynamical behavior of benzene in the tBC host is similar to that seen in other environments such as TOT, carceplexes, and Hoffman clathrates.24,26 In each case, benzene undergoes in-plane rotation plus motion of the guest about the host axis of symmetry. The study of pyridine dynamics in host environments is less common, although pyridine is not an uncommon component of inclusion compounds.5,11,23,26 In the solid state, pyridine by itself shows no motion on the 2H NMR time scale up to its melting point,34 and clearly the energetic barrier to molecular reorientation is lowered upon incorporation of this molecule into a host cavity. Comparison of the 2-fold molecular reorientation of pyridined5 in the TOT and tBC cavities seems to indicate more variation in motion as a function of host environment than that seen for benzene. In each situation, librational motions occur. In the TOT cavity, 2-fold pyridine molecular reorientation develops after the guest has reached the fast motion limit for a restricted inplane motion as the molecular reorientations are activated with increasing temperature. This motion contrasts with the sequence of dynamics in the tBC case, where C2 reorientation, already rapid at 77 K, is followed by 4-fold rotation about the cavity axis. The origin of these differences in dynamics may not be as strongly determined as the guest orientation. It is likely that, whereas the pyridine dipole (µ ) 2.19 D)36 has little effect on the guest positioning, it may contribute more significantly to the differences in the observed dynamics between benzene and pyridine. Moreover, steric effects are likely to remain prominent. For instance, the capping host layer approaches the pyridine

Brouwer et al. more closely than in the benzene case. The environment around the pyridine nitrogen is such that guest rotation about the pseudohexad axis has a significantly high energetic barrier upon rotation of a C-H group into the ipso position. Additional loss of 6-fold symmetry upon introduction of the nitrogen atom into the ring may increase the barrier to in-plane rotation enough to hinder this motion in the calix cavity. 3. Intermolecular Interactions and Molecular Recognition Features. In the context of the 1,3-crown-5-ether-bridged tBC‚ pyridine structure,5 the evidence for a weak C-H‚‚‚N hydrogen bond between the host and guest must be considered in light of the present work. The current results suggest that the orientation of the pyridine in the 2-fold symmetric host is related more to the crown ether functionality attached to the tBC rather than to any intrinsic property of the tBC host itself. As such, the role and ability of CHhost‚‚‚πguest interactions in stabilizing host‚guest compounds may be questioned for this particular example and possibly for other tBC inclusion compounds formed with aromatic guests.15 The first consideration of whether the guest interacts with the host through a hydrogen-bonding interaction is to assess the extent of molecular orbital interaction between the two molecular components. This strategy is similar to the analysis of C-H‚‚‚π hydrogen-bonding interactions in calix[4]resorcarene using the concepts of the hardness and basicity to assess the interacting molecular components.37 To participate in hydrogen bonding, the hydrogen of the interacting C-H group is expected to be acidic, i.e., an electron acceptor, whereas the guest is anticipated to be an electron donor. The basic, electron-rich surfaces of pyridine are located (a) above and below the aromatic plane (the π-orbitals) and (b) along the 2-fold molecular symmetry axis (the lone-pair orbital). It is quite telling, then, that the geometric arrangement of the pyridine guest in the cleft defined by the two C-H groups avoids orbital interaction of the acidic C-H σ-orbital with either basic site (Figure 7). From this simple geometrical evaluation of the possible electronic interactions, a C-H‚‚‚π hydrogen-bonding interaction between host and guest is highly unlikely. A second means to identify a hydrogen-bonding type of interaction is to examine the geometrical criteria. Mean contact distances and angles describing C-H‚‚‚N hydrogen-bonding interactions in the solid state have been recently established, with dN‚‚‚H ) 2.45 Å being the cutoff distance.38 Whereas the angular requirement for a hydrogen bonding description is met for tBC‚pyridine, the C-H‚‚‚N distance is of the same order as the sum of the van der Waals radii of C-H and an aromatic N (3.6 Å). Thus, unless a cooperative effect between the two host-guest nonbonding contacts is assumed, the earlier description of the C-H‚‚‚N interaction between host and guest as a hydrogen bond is marginal. The remaining host‚guest distances are distributed over different contact points, and there does not appear to be any further specific point at which the guest and host interact. The pyridine-15N isotropic chemical shift value indicates whether the guest is hydrogen-bonded to the host. The pyridine15N isotropic chemical shift δ iso values in a variety of solvents are correlated to the proton-donating ability of the solvent; pyridine δiso values shift to that of the pyridinium ion (δ ) -179.6 ppm) as the proton-donating ability of the solvent increases.31 Because the 15N NMR isotropic chemical shift of tBC‚pyridine-15N is unchanged from pyridine-15N in solution, it is unlikely that the hydrogen-bonding description of the C-H‚‚‚N interaction between host and guest is valid.39

Benzene and Pyridine p-tert-Butylcalix[4]arene Inclusions Conclusions Overall, the primary consideration for both guest position and dynamics appears to be the shape of the guest. The gross similarity between pyridine and benzene determines the positioning of the plane of the guest in the host cavity. Detailed differences in the shape at the guest ipso ring position (i.e., N: or C-H) may be the strongest factor in determining the orientation within the molecular plane. Other factors such as dipole or C-H‚‚‚N electrostatic interactions may also have an influence on the in-plane orientation of the pyridine. On the other hand, the orientation of the pyridine guest to give a C-H‚‚‚N geometry reminiscent of hydrogen bonding might simply be coincidental, since benzene has the same situation, rather than orientation-determining. Finally, the lack of CHhost‚‚‚πguest evidence based on distance considerations rules out this interaction as a determining factor. What remains is a description of molecular recognition of the guests by the tBC host based primarily on shape rather than any anisotropic or directional electrostatic interactions. Structural and dynamic features of the p-tert-butylcalix[4]arene host-guest system examined by diffraction and magnetic resonance techniques continue to give insight into weak intermolecular interactions between nonbonding molecular components in supramolecular systems. This work, in particular, contrasts the role played by collective steric (dispersive) interactions acting in concert and specific directional (electrostatic) interactions, finding little experimental evidence to support the latter. Clearly for the understanding of supramolecular solid-state materials, the study of weak interactions remains a challenge, and we are confident that investigation of the calixarene system will continue to contribute to a fuller understanding of these important interactions. Acknowledgment. This work was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) in the form of a postgraduate PGS-B scholarship (E.B.B.) and an operating grant (J.A.R.). Supporting Information Available: Tables of crystal data, structure solution and refinement, atomic coordinates, bond lengths and angles, and anisotropic thermal parameters for tBC‚ benzene and tBC‚pyridine. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Marassi, F. M.; Ramamoorthy, A.; Opella, S. J. Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 8551. (b) Marassi, F. M.; Opella, S. J. Curr. Opinion Struct. Biol. 1998, 8, 640. (c) Kim, Y.; Valentine, K.; Opella, S. J.; Schendel, S. L.; Cramer, W. A. Protein Sci. 1998, 7, 342. (d) Hu, J. Z.; Facelli, J. C.; Alderman, D. W.; Pugmire, R. J.; Grant, D. M. J. Am. Chem. Soc. 1998, 120, 9863. (2) (a) Brouwer, E. B.; Enright, G. D.; Ripmeester, J. A. Supramol. Chem. 1996, 7, 79. (b) Brouwer, E. B.; Enright, G. D.; Ripmeester, J. A. Chem. Commun. 1997, 939. (c) Brouwer, E. B.; Enright, G. D.; Ripmeester, J. A. J. Am. Chem. Soc. 1997, 119, 5404. (d) Brouwer, E. B.; Udachin, K.; Enright, G. D.; Ratcliffe, C. I.; Ripmeester, J. A. Chem. Commun. 1998, 587. (3) (a) Gutsche, C. D. Calixarenes; RSC: Cambridge, 1989. (b) Vicens, J., Bohmer, V., Eds. Calixarenes; Kluwer: Dordrecht, 1990. (c) Ungaro, R.; Arduini, A.; Casnati, A.; Ori, O.; Pochini, A.; Ugozzoli, F. Complexation of Ions and Neutral Molecules by Functionalized Calixarenes. In Computational Approaches in Supramolecular Chemistry; Wipff, G., Ed.; NATO ASI C426; Kluwer: Dordrecht, 1994; p 277. (d) Bohmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713. (4) (a) Andreetti, G. D.; Ungaro, R.; Pochini, A. J. Chem. Soc., Chem. Commun. 1979, 1005. (b) Ungaro, R.; Pochini, A.; Andreetti, G. D.; Domiano, P. J. Chem. Soc., Perkin Trans. 2 1985, 197. (5) Andreetti, G. D.; Ori, O.; Ugozzoli, F.; Alfieri, C.; Pochini, A.; Ungaro, R. J. Inclusion Phenom. 1988, 6, 523.

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