Synthesis and Solid-State Rotational Dynamics of Molecular

ARTICLE pubs.acs.org/crystal

Synthesis and Solid-State Rotational Dynamics of Molecular Gyroscopes with a Robust and Low Density Structure Built with a Phenylene Rotator and a Tri(meta-terphenyl)methyl Stator Zachary J. O’Brien, Arunkumar Natarajan, Saeed Khan, and Miguel A. Garcia-Garibay* Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States

bS Supporting Information ABSTRACT: Recent studies suggest that the rotational dynamics in crystals of molecular gyroscopes become more favorable (i.e., faster) when the packing coefficient of the corresponding lattice is decreased by increasing the steric bulk of the stator, as expected for structures with high protuberances or deep cavities. In an effort to explore the effects of increased stator size on the solid-state dynamics of these crystalline models for molecular machines, molecular gyroscope 4 with an “exploded” bis(tri(meta-terphenyl)methyl) stator was synthesized. Single crystal X-ray diffraction analysis revealed a packing structure with two crystallographically distinct gyroscope molecules and four ethyl acetate molecules per unit cell. Although a relatively low packing coefficient of 0.68 was determined for the corresponding packing motif, we noticed that rotators at the two sites have significantly different environments. The solid state rotational dynamics of the two central phenylenes in an ethyl acetate clathrate of 4 were explored by variable-temperature 13C NMR with cross-polarization with magic angle spinning (13C CPMAS NMR) and by quadrupolar echo 2H NMR measurements with isotopically labeled samples. It was found that the increased stator size does indeed allow for more free-volume and faster rotational dynamics as compared to molecular gyroscopes with smaller or more globular stators. However, the two crystallographic sites experience different rotational dynamics, suggesting that the average density available from the packing coefficient is a very crude indicator of solid-state dynamics.

’ INTRODUCTION Recent literature on artificial molecular machinery has addressed the function and dynamics of molecules supported on surfaces1,2 and within bulk solids,3 as it has been suggested that molecular machines may find important applications in materials science.4 Studies in our group have been based primarily on molecular gyroscopes with 1,4-bis(triarylpropynyl)benzene architectures5 and related 1,4-bis(9-triptycylethynyl)benzene structures,6 with much work focused on the exploration of different molecular topologies and their effects on crystal packing and rotational dynamics. A trend shown by structures 1
3 in Figure 1 is that increasing the steric bulk of the stator from trityl (1) to tris(3,5-di-tert-butylphenyl)methyl (3) results in crystals with greater rotational exchange rates, suggesting the bulkier stators offer more effective steric shielding of the central rotator component in the solid state. An average measure of the free volume that exists within a crystal can be obtained by calculating the packing coefficient, CK. The packing coefficient was defined by Kitaigorodskii in terms of the van der Waals volume of all the molecules within the unit cell (Vvdw) relative to the total volume (Vcell) of the unit cell, CK = Vvdw/Vcell.7 For most molecular crystals, CK varies from 0.64 to 0.77, and it generally correlates with the stability of the lattice. One can see from compounds 1
3 in Figure 1 that there is a r 2011 American Chemical Society

reasonable correlation between the rate of rotational exchange and the barriers to rotation determined by solid-state NMR, and with the packing coefficients calculated from the X-ray structures.8 However, it should also be pointed out that molecular crystals with very low packing densities tend to be unstable and collapse into more densely packed forms, sometimes becoming amorphous. It is common for low CK structures to crystallize with solvent molecules, and it is normal for these crystals to collapse when a certain amount of the included solvent escapes. Thus, while compound 3 was shown to form an open structure conducive to fast rotation at ambient temperature,5c crystals were found to lose solvent and collapse when variable temperature solid state NMR measurements were attempted, providing just a lower rotational exchange limit and an upper bound for the activation energy.5c As this observation suggests the need of crystals that have low CK values that are structurally robust, we decided to explore a structural modification that would give a lower packing coefficient and a more robust crystal lattice. Recognizing that the globular tert-butyl groups on the periphery of the trityl stator in 3 fail to lock molecules in place, as their Received: March 23, 2011 Revised: April 14, 2011 Published: April 15, 2011 2654

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Crystal Growth & Design

Figure 1. Examples of molecular gyroscopes with triarylmethyl stators showing a correlation between the rates (s
1) of rotational exchange (kr) at 298 K, their activation energies (kcal/mol), and the calculated packing coefficients. The data for compounds 1 and 2 correspond to the benzene clathrates of the two while the data for compounds 3 and 4 correspond to the dichloromethane and ethyl acetate clathrates, respectively. The rotational exchange of compound 3 could not be measured as a function of temperature because the crystal lattice collapses as dichloromethane escapes. The CK is listed as 0.68, which corresponds to complete dichloromethane inclusion from the X-ray crystal structure, but the effective packing coefficient of the sample used for NMR analyses is unknown because as the dichloromethane escapes, the lattice collapses to a crystal form with unknown unit cell parameters.

intermolecular interactions are weak, we decided to compare a substitution with less globular, disk-shaped, phenyl groups with the expectation that the stronger edge-to-face and π
π stacking interactions would yield a more robust crystal lattice. The required m-terphenyl unit has been used as a tecton in crystal engineering, along with other polyphenylbenzenes, in order to instill low density, rigidity, and high crystallinity.9 We report here the synthesis of compound 4 and its isotopologue 4-d4, which have the same general 1,4-bis(triarylpropynyl)benzene architecture with 12 phenyl groups on the periphery of the stator. As described in detail below, compound 4 has a strong tendency to crystallize with ethyl acetate in a low-density structure with a packing coefficient of 0.68, with two crystallographically distinct gyroscope molecules in the lattice. Crystals in the form of long, beautiful needles (Figure 2) are stable over a wide temperature range, losing solvent of crystallization at 78 °C and decomposing without melting at 374 °C. Variable temperature solid-state 2 H NMR studies between
73 and 35 °C confirmed that phenylene rotation is different at each of the two sites, with one of them experiencing a rotational exchange that is ca. 1000 times faster than the other, with activation energies of ca. 5.0 and 7.0 kcal/mol, respectively.

’ RESULTS AND DISCUSSION Synthesis and Characterization. Preparations of 1,4-bis(30 ,30 ,30 -tri(meta-terphenyl)propynyl) benzene (4) and its deuterated analog 4-d4 were accomplished via the 4-step synthesis shown in Scheme 1. Samples of the 3,5-diphenylbromobenzene derivative 5 were prepared via Suzuki cross coupling according to the procedure published by Matsumoto and co-workers.10 A Grignard reagent was prepared by reaction of 5 with magnesium ribbon and then reacted with 1/3 equiv of diethylcarbonate to

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Figure 2. Ethyl acetate clathrate crystals of 4 obtained from an ethyl acetate/methanol mixture viewed through a polarizing microscope (10 magnification).

Scheme 1

give the bulky triaryl methanol 6. Treatment of 6 with acetyl chloride, followed by reaction with ethynylmagnesium bromide, yielded alkyne 7. Sonogashira coupling of 7 with 0.5 equiv of 1,4diiodobenzene afforded compound 4. The isotopologue 4-d4 with a deuterated rotator for 2H NMR analysis was synthesized under similar conditions with 1,4-dibromobenzene-d4. All structures were confirmed with solution 1H NMR, 13C NMR, attenuated total reflectance (ATR) FT-IR (Supporting Information), and MALDI-TOF mass spectrometry. The tertiary alcohol in compound 6 was identified by a sharp 1 H NMR resonance at 3.08 ppm and a sharp O
H stretching band at 3566 cm
1. The carbinol carbon in the 13C NMR spectrum resonates at 82.7 ppm. Alkyne 7 has the characteristic terminal alkyne 1H NMR signal at 2.91 ppm and the corresponding IR signals at 3294 cm
1 and 2245 cm
1, indicating sp C—H and CtC stretches, respectively. The 13C NMR signals of the terminal alkyne occur at 89.7 ppm and 74.8 ppm, and a peak at 56.3 ppm corresponds to the quaternary carbon. Compounds 4 and 4-d4 have similar proton spectra, except that the central 2655

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Figure 4. Close up views of the rotators (red capped sticks) corresponding (a) to the site with no close contacts and (b) to the site with six close contacts and more steric hindrance. The stator portion at the front was removed for clarity, and close neighbors are represented as space filling models. Molecules of 4 are green and yellow, and ethyl acetate molecules are purple and pink.

Figure 3. Packing view of the molecular gyroscope 4 (hydrogens omitted for clarity) viewed down the b-axis. The molecules shown in gold have the triarylmethyl groups with a slight conformational disorder, which is not depicted in the figure.

phenylene signal of 4 at 7.55 ppm is not present in 4-d4. Similarly, the two carbon spectra are also very similar, except that the deuterated rotator appears as a triplet at 131.4 ppm, slightly upfield of the corresponding rotator signal in the 13C NMR spectrum of 4 (131.8 ppm). Thermal Analysis. Visual melting point observations indicated that 4 decomposes before melting at 374 °C. Differential scanning calorimetric (DSC) analysis of crystals of 4 showed a sharp endotherm at 78 °C on the heating curve, corresponding to the loss of ethyl acetate, and a broad endotherm starting at 374 °C that was associated with the sample decomposition before melting. Single X-ray Crystal Structure Analysis of 4. Good quality single crystals were obtained by slowly cooling a boiling 9:1 ethyl acetate/methanol solution of 4 down to room temperature. X-ray diffraction data were collected at 100 K, and the structure was solved in the triclinic space group P1 with two crystallographically distinct half molecules of 4 and two molecules of ethyl acetate per asymmetric unit. As required by the space group symmetry, the unit cell contains two distinct molecules of 4, shown in gold and green in Figure 3, with coincident molecular and crystallographic centers. The molecular gyroscopes shown in gold display rotational disorder with four of the six terphenyl phenyl groups adopting two slightly different conformations. The two structures are characterized by having the two triarylmethyl groups assuming anti conformations with the alkyne axles deviating from linearity, as measured by an angle of 175° from the ipso carbon of the rotator to the center of the alkyne bond to the trityl central carbon. The solvent molecules are localized between the two distinct rotor molecules (shown in pink and purple in Figure 3), and they are disordered. One ethyl acetate molecule has all of the carbon and oxygen atoms in a coplanar conformation and is disordered over two sites related by 180° rotation in the plane with occupancies in an 80:20 ratio. The other ethyl acetate molecule has the ethyl group rotated by 93° out of the plane of the acetate group and is also disordered over two sites in the ratio of 55:45. Further analysis shows that the packing coefficient for this structure is 0.68,11 indicating that the unit cell contains 32% “empty” space. Typical values of packing coefficients for

molecular crystals fall in the range 0.64
0.77,12 so that crystals of 4 are indeed toward the lower-density end of normal molecular crystals and are more robust than crystals of 3, confirming our hypothesis that this design with larger terphenyl-containing stators would yield a relatively open structure. Considering that there are two distinct gyroscope molecules in the crystal lattice, we were interested to see whether they have similar or different environments near the central phenylene rotators. Figure 4 shows space-filling model views with a close up of each of the two rotator environments, with neighboring molecules represented in green and yellow and solvent molecules in pink and purple. With the rotator in each view represented in red capped sticks, one can see that the less hindered site represented in Figure 4a has a more spherical cavity than that of the more hindered site in Figure 4b. While the rotator in the less hindered site has no neighboring atoms at a distance shorter than the sum of their van der Waal’s radii, the rotator in the more hindered site has close contacts that conform to the rectangular cross section of the phenylene rotator.13 To further illustrate the differences in the environments surrounding the two rotators, a Connolly surface14 of the crystal structure was calculated using a 1.3 Å diameter probe in Cerius2.15 A model was built with the rotators and axles removed, leaving only the stators visible. The resulting surfaces correspond to the space available to the rotator moieties, which are represented in red and blue for each of the two crystallographically distinct sites. It is evident from Figure 5 that the rotators that occupy the blue regions are sterically more restricted and likely to have a higher activation energy for internal rotation. In contrast, the rotators occupying the red regions can accommodate larger amplitude motions and should have a lower activation energy. Solid-State CPMAS 13C NMR. It has been shown that rotary dynamics in the solid state can be determined by taking advantage of line shape analysis of variable temperature CPMAS 13 C NMR spectra.16 This method requires the sites involved in the dynamic process to be magnetically nonequivalent and their exchange to occur at frequencies that fall within the range of their chemical shift differences, usually 50
1000 Hz. The solid-state CPMAS 13C NMR spectrum of crystalline 4 was obtained between 236 and 317 K with a magic angle spinning (MAS) rate of 10 kHz. We noticed that the spectrum acquired between 236 and 268 K showed signals corresponding to ethyl acetate at 15, 20, and 60 ppm but that loss of solvent occurred under fast sample spinning between ca. 285 and 295 K. The loss of ethyl acetate was accompanied by changes in the spectrum that included a redistribution of intensities in the aromatic region 2656

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Figure 5. Connolly surfaces calculated by removing the rotator and axle moieties of 4 and then using a 1.3 Å diameter probe. The red and blue regions correspond to the space occupied by the phenylene rotators of gyroscope molecules in the two crystallographic sites.

and a large displacement of the alkyne sp carbons from 82 and 100 ppm to 87 and 95 ppm, respectively (Figure SI-17 of the Supporting Information). In addition to problems associated with the loss of ethyl acetate under fast sample spinning, the strongly overlapping trityl aromatic signals centered at 124 ppm and 140 ppm conceal the signals of the phenylene rotator, expected at ca. 130 ppm. Based on these observations, we decided to study the site exchange of the phenylene rotator of 4 by taking advantage of low-temperature quadrupolar echo 2H NMR using phenylene-deuterated static samples. Variable Temperature Quadrupolar Echo 2H NMR Measurements of 4-d4. Variable temperature 2H NMR is a powerful method of studying the internal dynamics of static powder samples over the range ca. 103
108 Hz.17 The strength of this method arises from the orientation-dependent coupling of the nuclear electric quadrupole with the 2H nuclear spin, which results in a broad 2H spectrum known as the Pake or powder pattern. The method is based on the fact that the shape of the powder pattern changes in a predictable manner as a function of changes in the reorientation of the C
2H bond vector with respect to the external magnetic field.18 Crystalline samples with welldefined periodic processes display spectra that change as a function of temperature in such a manner that the spectra can be simulated with a model that considers the trajectories and frequencies of motion. In the case of phenylene rotation, the trajectory of the C
2H bonds typically involves an exchange process between sites related by 180° rotation, also known as 2-fold flips. The 2H NMR experiments were undertaken with a standard 2 H quadrupolar echo sequence using samples freshly crystallized from a 9:1 ethyl acetate/methanol solution. Spectra were collected at temperatures ranging from 200 to 308 K (Figure 6). The 2 H labels on the central phenylene represent only 0.5% of the total mass of the sample, so the spectra required the averaging of 2048 transients collected with a 20-s recycle delay. Considering that there are two crystallographically distinct gyroscope molecules with rather different environments surrounding the rotators, it seemed reasonable that we would detect two different rotational rates in the 2H spectra. The experimental spectra shown in Figure 6b were simulated using both single exchange frequencies in Figure 6c, and with a model that includes two different frequencies in Figure 6a. The simulations in Figure 6a

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Figure 6. Simulated and experimental solid state 2H quadrupolar echo NMR spectra of the ethyl acetate clathrate of 4-d4: (a) spectra simulated with a model that considered 2-fold flipping exchange in two distinct sites; (b) experimental spectra; (c) spectra simulated with a single 2-fold flipping exchange. The exchange frequencies used in the simulations and the temperature of the experimental spectra are indicated.

are the sum of the two frequencies that gave the best visual approximation to the experimental spectra. The differences between the one and two component simulations are significant and more strongly manifested in the central feature and the side shoulders. For example, fitting the spectrum collected at 200 K with any single frequency proved impossible, as it could not reproduce the combination of high shoulders and flat center. Shown at the bottom of Figure 6c, the most reasonable match was obtained at 700 kHz, yet it is highly unsatisfactory. In contrast, the high-shoulder and flat-top appearance of the experimental spectrum was simulated reasonably well with a model that includes a 1:1 sum of slow (5 kHz) and intermediate exchange components (2 MHz). Similar results were obtained at 225 and 248 and 268 K, with the spectrum collected at 308 K, largely in the fast exchange regime, simulated reasonably well by both the one and two frequency models. The top spectrum in Figure 6a was obtained with 2-fold flipping in the fast (>108 Hz) and intermediate exchange (500 kHz) exchange regimes, with the former undergoing a ( 20° libration consistent with the oscillation of the rotator at the bottom of its energy well. The corresponding spectrum in Figure 6c was obtained with a single frequency fast-exchange model with a slightly smaller librational amplitude ((15°). It should be recognized that there is a large uncertainty in the simulations in Figure 6a and that the suggested results are primarily qualitative. However, the frequencies used to model every spectrum were selected based on good visual matching and an internally consistent Arrhenius-type behavior. That is, we assumed that the high and low frequency motions follow the thermal behavior expected from two distinct activation barriers. As a criterion to support this assumption, we noticed that the best simulations resulted in reasonable pre-exponential factors of ca. 9.7  1011 s
1 and 4.3  1010 s
1, which are within the range of values observed in other crystalline phenylene rotators.19 The corresponding activation energies have values of 5 and 7 kcal/mol for the faster and slower rotators. While these values are somewhat tentative, as we cannot use data below and above the low (108 s
1) regimes, there is no question that rotational exchange at ambient temperature occurs with frequencies that are greater than 100 MHz.

’ CONCLUSIONS We have shown that a molecular gyroscope with a triarylmethyl stator based on relatively flat 3,5-diphenyl substituents 2657

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Crystal Growth & Design yields a more open and robust packing motif than the analogous one observed in a molecular gyroscope with tert-butyl groups in the corresponding 3,5-positions. However, a structure with two crystallographically distinct gyroscope molecules in the lattice highlights the limitations of an average quantity, such as the packing coefficient, given that internal rotation depends on the local environment, rather than the unit cell average. Using variable temperature 2H NMR, we found evidence for two different rotational exchange rates that we assign to the two crystallographically independent gyroscope molecules in the lattice. The activation energies of ca. 5 and 7 kcal/mol estimated from the temperature-dependence of the rotational exchange are among the lowest observed for phenylene rotators linked to triarylmethyl stators.

’ EXPERIMENTAL SECTION IR spectra were obtained with an HATR-FTIR instrument. 1H and C NMR spectra were acquired on a spectrometer at 500 and 125 MHz, respectively with CDCl3 as the solvent. Solid-state 13C CPMAS NMR spectra were obtained at 75 MHz. 2H quadrupolar echo measurements were acquired at 46.07 MHz with a 90° pulse width of 2.5 μs and spin
echo and refocusing delays of 50 μs, and a recycle delay of 20 s. DSC analyses were recorded between room temperature and the melting/decomposition temperature as determined by a standard melting point apparatus. 3,5-Diphenylbromobenzene (5). A 500-mL three-neck roundbottomed flask equipped with a reflux condenser was charged with 1,3,5tribromobenzene (13.83 g, 43.90 mol) and tetrakistriphenylphosphine palladium (0) (1.26 g, 1.10 mmol) under argon. A 125-mL suspension of phenylboronic acid (10.71 g, 87.80 mmol) in toluene was sparged with argon for 30 min and added to the flask. A 100-mL solution of aqueous 1 M sodium carbonate was also sparged with argon and added to the flask. The mixture was stirred vigorously and set to reflux for 48 h. Reaction progress was monitored by thin layer chromatography. The solvent was removed in vacuo, and the residue was taken up in 200 mL of a 1:1 ethyl acetate/water mixture. The water layer was washed with ethyl acetate (3  100 mL), and the combined organic layers were dried over MgSO4. The solvent was removed, and the crude product was purified by column chromatography (100% hexanes) to afford 8.20 g (60.4% isolated yield) of 5 as a white solid. Analysis: melting point: 103 °C. 1H NMR (500 MHz, CDCl3): δ 7.72 (s, 3H), 7.62 (dd, 4H, J = 1.5, 7.5 Hz), 7.48 (ddd, 4H, J = 1.5, 7.5, 7.5 Hz), 7.40 (tt, 2H, J = 1.5, 7.5 Hz). 13C NMR (125 MHz, CDCl3) δ 143.6, 139.8, 128.9, 128.0, 127.2, 124.8, 123.2. FTIR (solid HATR, cm
1): 3084, 3060, 3035, 1595, 1560, 1498, 754, 696. HRMS (MALDI) C18H13Br: calcd, 308.0201; found, 308.0211. Tri(50 -m-terphenyl)methanol (6). A 250-mL three-neck roundbottom flask was charged with a magnetic stir bar and magnesium ribbon (0.7821 g, 32.18 mmol) and equipped with a reflux condenser. The entire setup was flame-dried and allowed to cool under argon. A 100-mL solution of 1 (6.395 g, 20.68 mmol) was added, and the mixture was stirred at reflux until the magnesium ribbon disappeared. Diethyl carbonate (0.83 mL, 6.9 mmol) was then added dropwise, and the mixture was set to reflux for 4 h. The reaction was quenched with saturated ammonium chloride, and the organics were extracted with diethyl ether and dried over magnesium sulfate. The crude product was purified by column chromatography (hexanes/CH2Cl2 = 2:1) to afford 3.34 g (68.1% isolated yield) of 6 as a white solid. Analysis: melting point: 169
171 °C. 1H NMR (500 MHz, CDCl3): δ 7.87 (t, 3H, J = 1.5 Hz), 7.72 (d, 6H, J = 1.5 Hz), 7.61 (dd, 12H, J = 1.50, 7.5 Hz), 7.42 (dd, 12H, J = 7.5, 7.5 Hz), 7.34 (tt, 6H, J = 1.0, 7.5 Hz), 3.08 (s, 1H). 13C NMR (125 MHz, CDCl3) δ 147.9, 141.7, 141.2, 128.9, 127.6, 127.5, 126.0, 125.6, 82.7. FTIR (HATR, cm
1): 3566, 3031, 1595, 1426. MALDI-TOF MS, C55H39þ: calcd, 699.3046; found, 699.3187. 13

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3,3,3-Tri(50 -m-terphenyl)propyne (7). A dry 500-mL roundbottom flask was charged with a magnetic stir bar and 6 (2.708 g, 3.777 mmol) and equipped with a reflux condenser. Acetyl chloride (30 mL) was added, and the mixture was set to reflux for 60 min. Excess acetyl chloride was removed in vacuo, and the residue was taken up in 30 mL of dry benzene. The solvent was removed again, and 250 mL of dry benzene was added. The solution was allowed to stir for 15 min before a 0.5 M solution of ethynylmagnesium bromide in THF (20 mL, 10 mmol) was added. The mixture was set to reflux for 4 h and was quenched with saturated ammonium chloride, and the organics were extracted with diethyl ether and dried over magnesium sulfate. The crude product was purified by column chromatography (1 L of 100% hexanes followed by 1 L of 9:1 hexanes/diethyl ether) to afford 2.07 g (75.5% recovered yield) of 7 as a white solid. Analysis: melting point: decomposes above 175 °C. 1H NMR (500 MHz, CDCl3) δ 7.79 (t, 3H, J = 1.5 Hz), 7.74 (d, 6H, J = 1.5 Hz), 7.61 (dd, 12H, J = 1.0, 7.0 Hz), 7.44 (ddd, 12H, J = 1.0, 7.5, 7.5 Hz), 7.36 (t, 6H, J = 7.5 Hz), 2.91 (s, 1H). 13C NMR (125 MHz, CDCl3) δ 145.7, 141.7, 141.2, 128.9, 127.6, 127.5, 127.2, 125.3, 89.7, 74.8, 56.3. FTIR (HATR, cm
1): 3294, 3035, 2246, 1593, 1497, 1412. MALDI-TOF MS, C57H40: calcd, 724.3130; found, 724.3208.

1,4-Bis(30 ,30 ,30 -tri(500 -m-terphenyl)propynyl)benzene (4).

A 100-mL three-neck round-bottom flask was charged with a magnetic stir bar, 7 (0.446 g, 0.615 mmol), 1,4-diiodobenzene (0.101 g, 0.306 mmol), THF (40 mL), and triethylamine (20 mL). The mixture was sparged with argon for 1 h. Bis(triphenylphosphine) palladium chloride (0.053 g, 0.075 mmol) and copper(I) iodide (0.0254 g 0.133 mmol) were added, and the mixture was set to reflux for 20 h. The reaction mixture was washed with saturated ammonium chloride, and the organics were extracted with dichloromethane and dried over magnesium sulfate. The crude product was purified by column chromatography (hexanes/DCM = 4:1) and recrystallized from a hexanes/DCM mixture to afford 0.408 g (87.4% recovered yield) of 4 as a white solid. Analysis: melting point: decomposes above 374 °C. 1H NMR (500 MHz, CDCl3) δ 7.78 (d, 6H, J = 1.5 Hz), 7.76 (d, 12H, J = 1.5 Hz), 7.60 (d, 24H, J = 7.5 Hz), 7.55 (s, 4H), 7.42 (dd, 24H, J = 7.5, 7.5 Hz), 7.34 (t, 12H, J = 7.5 Hz). 13C NMR (125 MHz, CDCl3) δ 146.1, 141.8, 141.2, 131.8, 129.0, 127.6, 127.5, 127.3, 125.4, 123.4, 97.4, 86.1, 57.0. FTIR (HATR, cm
1): 3031, 1590, 1575, 1497, 1428, 1412. MALDI-TOF MS, C120H82: calcd, 1522.6417; found, 1522.4879.

1,4-Bis(30 ,30 ,30 -Tri(500 -m-terphenyl)propynyl) d4-Benzene (4-d4). A glass tube was charged with a magnetic stir bar, 7 (0.224 g,

0.309 mmol), and 1,4-dibromobenzene-d4 (0.0366 g, 0.153 mmol), THF (10 mL), and diisopropylamine (5 mL), and the mixture was sparged with argon for 1 h. Bis(triphenylphosphine)palladium(II) chloride (0.025 g, 0.036 mmol) and copper(I) iodide (0.0087 mg, 0.046 mmol) were added, and the tube was then sealed and the mixture was heated to 75 °C and stirred for 24 h. The reaction was cooled to room temperature and washed with saturated ammonium chloride, and the organics were extracted with dichloromethane and dried over magnesium sulfate. The purification was the same as that for 4 and yielded 116.2 mg (49.7% recovered yield) of 4-d4 as a white solid. Analysis: melting point: decomposes above 374 °C. 1H NMR (500 MHz, CDCl3) δ 7.81 (s, 6H), 7.79 (d, 12H), 7.63 (d, 24H, J = 7.5 Hz), 7.44 (dd, 24H, J = 7.5, 7.5 Hz), 7.36 (t, 12H, J = 7.5 Hz). 13C NMR (125 MHz, CDCl3) δ 146.2, 141.8, 141.2, 131.4 (t, 1JCD = 25.3 Hz), 129.0, 127.6, 127.5, 127.3, 125.4, 123.2, 97.4, 86.1, 57.0. FTIR (HATR, cm
1): 3032, 1590, 1575, 1497, 1428, 1412. MALDI-TOF, C120H78D4: calcd, 1526.6668; found, 1526.7817.

’ ASSOCIATED CONTENT

bS

Supporting Information. Spectroscopy data for compounds 4 and 4-d4 and intermediates 5, 6, and 7; variable

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Crystal Growth & Design temperature 13C CPMAS NMR of compound 4 and Arrhenius plot from the variable temperature 2H NMR of compound 4; and cif data. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support by the National Science Foundation (Grants DMR0605688 and CHE0551938 and creativity extension DMR0937243) is gratefully acknowledged. Z.O. acknowledges the National Science Foundation IGERT MCTP Program (DGE0114443). We thank Dr. Fernando Uribe-Romo for valuable discussions and advice regarding X-ray crystallography. ’ REFERENCES (1) (a) Horinek, D.; Michl, J. Proc. Natl Acad. Sci. U. S. A. 2005, 102, 14175. (b) de Jonge, J. J.; Ratner, M. A.; de Leeuw, S. W. J. Phys. Chem. C 2007, 111, 3770. (c) Shirai, Y.; Morin, J.-F.; Sasaki, T.; Guerrero, J. M.; Tour, J. M. Chem. Soc. Rev. 2006, 35, 1043. (d) Tierney, H. L.; Baber, A. E.; Jewell, A. D.; Iski, E. V.; Boucher, M.; Sykes, E. C. H. Chem.—Eur. J. 2009, 15, 9678. (2) Thibeault, D.; Auger, M.; Morin, J.-F. Eur. J. Org. Chem. 2010, 3049. (3) (a) Khuong, T.-A. V.; Nu~nez, J. E.; Godinez, C. E.; GarciaGaribay, M. A. Acc. Chem. Res. 2006, 39, 413. (b) Garcia-Garibay, M. A. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10771. (4) (a) Garcia-Garibay, M. A. Angew Chem., Int. Ed. 2007, 46, 8945. (b) Garcia-Garibay, M. A. Nat. Mater. 2008, 7, 431. (5) (a) Dominguez, Z.; Dang, H.; Strouse, M. J.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2002, 124, 2398. (b) Dominguez, Z.; Dang, H.; Strouse, J. M.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2002, 124, 7719. (c) Khuong, T. A. V.; Zepeda, G.; Ruiz, R.; Kahn, S. I.; Garcia-Garibay, M. A. Cryst. Growth Des. 2004, 4, 15. (6) (a) Godinez, C. E.; Zepeda, G.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2002, 124, 4701. (b) Godinez, C. E.; Zepeda, G.; Mortko, C. J.; Dang, H.; Garcia-Garibay, M. A. J. Org. Chem. 2004, 69, 1652. (c) Godinez, C. E.; Garcia-Garibay, M. A. Cryst. Growth Des. 2009, 9, 3124. (7) Kitaigorodskii, A. I. Molecular Crystals and Molecules; Academic Press: New York, 1973. (8) Molecular volumes were calculated using the group increment approach with the values reported in(a) Gavezzotti, A. J. Am. Chem. Soc. 1983, 105, 5220 for all groups, with the exception of quaternary carbons and ethereal oxygens, which were reported in (b) Bondi, A. J. Phys. Chem. 1964, 68, 441. These results agree closely with volumes calculated using (c) Spartan’08; Wavefunction, Inc.: Irvine, CA. (d) Shao, Y.; Molnar, L. F.; Jung, Y.; Kussmann, J.; Ochsenfeld, C.; Brown, S. T.; Gilbert, A. T. B.; Slipchenko, L. V.; Levchenko, S. V.; O’Neill, D. P.; DiStasio, R. A., Jr.; Lochan, R. C.; Wang, T.; Beran, G. J. O.; Besley, N. A.; Herbert, J. M.; Lin, C. Y.; Van Voorhis, T.; Chien, S. H.; Sodt, A.; Steele, R. P.; Rassolov, V. A.; Maslen, P. E.; Korambath, P. P.; Adamson, R. D.; Austin, B.; Baker, J.; Byrd, E. F. C.; Dachsel, H.; Doerksen, R. J.; Dreuw, A.; Dunietz, B. D.; Dutoi, A. D.; Furlani, T. R.; Gwaltney, S. R.; Heyden, A.; Hirata, S.; Hsu, C.-P.; Kedziora, G.; Khalliulin, R. Z.; Klunzinger, P.; Lee, A. M.; Lee, M. S.; Liang, W. Z.; Lotan, I.; Nair, N.; Peters, B.; Proynov, E. I.; Pieniazek, P. A.; Rhee, Y. M.; Ritchie, J.; Rosta, E.; Sherrill, C. D.; Simmonett, A. C.; Subotnik, J. E.; Woodcock, H. L., III; Zhang, W.; Bell, A. T.; Chakraborty, A. K.; Chipman, D. M.; Keil, F. J.; Warshel, A.; Hehre, W. J.; Schaefer, H. F.; Kong, J.; Krylov, A. I.; Gill, P. M. W.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2006, 8, 3172. (9) (a) Wright, R. S.; Vinod, T. K. Tetrahedron Lett. 2003, 44, 7129. (b) Maly, K. E.; Gagnon, E.; Maris, T.; Wuest, J. D. J. Am. Chem. Soc.

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2007, 129, 4306. (c) Vergadou, V.; Pistolis, G.; Michaelides, A.; Varvounis, G.; Siskos, M.; Boukos, N.; Skoulika, S Cryst. Growth Des. 2006, 6, 2486. (d) Moorthy, J. N.; Natarajan, R.; Venugopalan, P. J. Org. Chem. 2005, 70, 8568. (10) Matsumoto, K.; Hatano, K.; Umezawa, N.; Higuchi, T. Synthesis 2004, 13, 2181. (11) Kitaigorodskii, A. Molecular Crystals and Molecules; Academic Press: New York, 1973. (12) Dunitz, J. D.; Gavezotti, A. Acc. Chem. Res. 1999, 32, 677. (13) Karlen, S. D.; Ortiz, R.; Chapman, O.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2005, 127, 6554. (14) Connolly, M. L. Science 1983, 221, 709. (15) Cerius2Modeling Environment, version 4.2; Molecular Simulations Inc.: San Diego, CA, 1999. (16) (a) Fyfe, C. A. Solid State NMR for Chemists; C. F. C. Press: Guelph, 1983. (b) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973, 59, 569. (c) Taylor, R. E. Concepts Magn. Reson. Part A. 2004, 22, 37. (17) Hoatson, G. L.; Vold, R. L. NMR 1994, 32, 1. (18) (a) Kamihira, M.; Naito, A.; Tuzi, S.; Saito, H. J. Phys. Chem. A 1999, 103, 3356. (b) Hiraoki, T.; Kogame, A.; Norio, N.; Akihiro, T. J. Mol. Struct. 1998, 441, 243. (c) Zhang, H.; Bryant, R. G. Biophys. J. 1997, 72, 372. (d) Naito, A.; Izuka, T.; Tuzi, S.; Price, W. S.; Hayamizu, K.; Saito, H. J. Mol. Struct. 1995, 355, 55. (19) Kawski, A. Crit. Rev. Anal. Chem. 1993, 23, 459.

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