Search of Nature of Planar Chirality for Pendent ... - ACS Publications

2790

J. Phys. Chem. B 2007, 111, 2790-2799

Search of Nature of Planar Chirality for Pendent Benzodiazacoronands in the Solid State: NMR, X-ray, and DFT Studies Justyna Pacholczyk,† Jarosław Kalisiak,‡ Janusz Jurczak,*,‡ and Marek J. Potrzebowski*,† Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland, and Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland ReceiVed: NoVember 23, 2006; In Final Form: January 10, 2007

In this work, we report the structural studies on the solid state of two benzodiazacoronads that form chiral and achiral crystals. Crystals have to be considered as a two-component system consisting of an organic unit and a water molecule in 1:1 ratio. Both components play an important role in the crystal structure. The strong (O-H...O, N-H...O) and weak (C-H...O) intermolecular hydrogen bonds are responsible for phase organization and, in consequence, formation of chiral or achiral crystals. The alignment of the water molecule with respect to the macrocycle is different for samples 1 and 2. Removal of water from the crystal lattice of 1 is reversible. Formation of chiral cocrystals from two different achiral molecules by self-assembly is well-known. However, in this paper, we show that the water molecule can be an important achiral cofactor responsible for chiral crystallization.

Introduction Chiral crystals formed from achiral molecules have recently received a great deal of attention due to their attractive structural properties and prospective applications in chemistry.1 For instance, such compounds can be used as ligands in enantioselective reactions or models for investigation of molecular recognition.2 Understanding of details, which govern the process of formation of chiral crystals, chiral induction, and the mechanism of spontaneous resolution of racemic compounds, is one of the most general questions touching the problem of the origin of life.3 Several research groups have intensively investigated this area by trying to rationalize experimental observations and establish the rules, which allow for the prediction of a tendency to form chiral crystals.4 The important contribution comes from the Koshima laboratory, who recently summarized the state of art on the field of chiral crystallization of achiral organic compounds.5 It has been concluded that there are three important requirements for formation of chiral crystals from achiral species: (1) generation of chirality by rotation of bonds, (2) generation of chirality by helical arrangement, and (3) formation of a head-to-head stacking columnar arrangement. If one or more of these requirements are fulfilled, then formation of chiral crystals is expected. As a part of our continuing interest in the problem of formation of chiral crystals with planar chirality,6 in this work, we present the structural studies for the two diazacoronands shown in Scheme 1. Molecule 1 with the N-benzoyl pendent group crystallizes in the achiral P21/n space group, and compound 2 with the O-benzyl pendent group crystallizes in * Corresponding authors. Marek Potrzebowski, Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland. Fax: +48 42 680 3261. Telephone: +48 42 680 3240. E-mail: [email protected]. Janusz Jurczak, Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. E-mail: [email protected]. † Centre of Molecular and Macromolecular Studies. ‡ Institute of Organic Chemistry.

SCHEME 1: Structure of Compounds 1 (a) and 2 (b) with Numbering System

chiral P212121 space group. It is apparent from Scheme 1 that the macrocycles are different for both compounds. The major aim of our project is to understand the origin of distinction between these compounds and answer the following question: what are the crucial molecular factors that determine formation of chiral crystals for the class of compounds under investigation? For this purpose, NMR spectroscopy appears to be a powerful technique. Our studies were carried out in several stages. In the first part of this paper, we present a full NMR assignment of signals for compounds 1 and 2. The second part shows the properties of crystals in the solid state and new applications of NMR spectroscopy. The correlation between X-ray crystallography, solid-state NMR, and theoretical calculations of NMR parameters is presented. Experimental The synthesis of samples 17 and 26d was recently published. NMR Measurements. The solid-state CPMAS experiments were performed on a BRUKER Avance DSX 300 spectrometer at 75.47 MHz frequency for 13C, equipped with a MAS probe head using 4 mm ZrO2 rotors. A sample of glycine was used for setting the Hartmann-Hahn condition, and adamantane was used as a secondary chemical shift reference δ ) 38.48 and 29.46 ppm from external TMS.8 The conventional spectra were recorded with a proton 90° pulse length of 3.5 µs and a contact

10.1021/jp0678002 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/23/2007

Chirality for Solid-State Benzodiazacoronands time of 1 ms. The repetition delay was 10 s, and the spectral width was 25 kHz. The FIDs were accumulated with a time domain size of 2 K data points. The RAMP shape pulse9 was used during the cross-polarization and TPPM decoupling10 with τp) 6.8 µs and a phase angle of 20° during the acquisition. The cross-polarization efficiency was measured with contact times between 10 µs and 12 ms. The spectral data were processed using the WIN-NMR program.11 A 5-π pulse 2D PASS scheme and a 2000 Hz sample spinning speed were used in the 2D experiments. The π-pulse length was 6 µs. Sixteen t1 increments using the timings described by Levitt and co-workers were used in the 2D PASS experiments.12 For each increment, 48 scans were accumulated. Because the pulse positions in the t1 set go back to their original positions after a full cycle and the t1-FID forms a full echo, the 16-point experimental t1 data were replicated to 256 points. After the Fourier transformation in the direct dimension, the 2D spectrum was sheared so as to align all side bands with the center bands in the indirect dimension of the 2D spectrum. One-dimensional CSA spinning side band patterns were obtained from t1 slices taken at isotropic chemical shifts in the t2 dimension of the 2D spectrum. The magnitudes of the principal elements of the CSA tensor were obtained from the best-fitting simulated spinning side band pattern. Simulations of the spinning CSA side bands spectra were carried out on a PC using the Simpson program under the Linux environment.13 Errors in δii parameters were estimated employing the WINMAS program.30 DFT GIAO calculations were carried out with the Gaussian 98 program running on a Silicon Graphics Power Challenge computer. The GIAO method with the B3PW91 hybrid method and 6-311+G* basis set was used to calculate NMR parameters. DSC measurements were carried out on TA instruments, 2920 Modulated DSC Analyzer, with a 1 deg/min heating rate in the temperature range from 173 to 353 K. Results and Discussion 1. XRD and DSC study of 1 and 2. Molecular structures and crystal packing for 17 and 26d are shown in Figure 1a, b, respectively. It is apparent from the analysis of the unit cell of 1 that there are four molecules forming two pairs of enantiomers. An interesting feature of crystals of 1 is the presence of a water molecule in the lattice. More precisely, water is located inside the macrocycle and is strongly hydrogen-bonded with both the azacoronand and the CdO group of the N-benzoyl residue. The geometry of hydrogen bonding in pictorial form is shown in Figure 2a. It is worthwhile to stress the presence of other strong and weak intermolecular interactions that influence the crystal lattice architecture. N-H forms hydrogen bonding with an adjacent O-CH2 unit of benzodiazacoronand. Weak C-H...O interactions are seen between C1, C2 carbons of phenyl group and the water molecule. The preliminary X-ray study of 2 was published recently.14 Sample 2 crystallizes in the enantiomeric space group P212121 (orthorhombic) with significant asymmetry in the hydrogen bonding of the pendent group. As in the previous case, the water molecule is occluded in the crystal lattice, but in contrast to sample 1, water is outside the macrocycle and can be considered rather as a clip bonding two molecules of 2. The geometry of hydrogen bonding is shown in Figure 2b. The differential scanning calorimetry (DSC) measurement unambiguously proves that sample 1 undergoes thermal processes below its melting point. Figure 3a shows a DSC plot demonstrating a broad endothermic profile that covers the range

J. Phys. Chem. B, Vol. 111, No. 11, 2007 2791

Figure 1. Molecular packing for compound 1 (a) and 2 (b). Red dots show position of water molecules in the crystal lattice.

from 350 to 420 K. It is assumed that the phase transition can occur in this region. More details describing the mechanism of this process will be discussed in Section 2.2 and 2.3. Finally, it is worthwhile to express that the DSC study does not reveal any phase transition for 2 (Figure 3b). The melting point is found to be 423 K. Thus, we can deduce that, for 2, thermal releasing of water results in destruction of the crystal lattice. 2. Investigation of 1 and 2 in the Solid Phase. 2.1. 13C SolidState NMR Studies of 1 and 2. The preliminary assignment of isotropic chemical shifts for 1 was done by data comparison with those obtained in the liquid phase (Figure 4a). The 13C CPMAS spectrum recorded at 7 kHz is shown in Figure 4b. It is apparent from these data that the numbers of 13C signals recorded in liquid and solid phase are different. The most important feature of the solid-state spectrum is the 3 ppm splitting between the C9 and C13 atoms of the aromatic core of the macrocycle. Moreover, a split of 1.7 ppm between C10 and C12 is observed. It is interesting to note an unusual line contour for the C8 carbon. The quaternary C6 carbon of the N-benzoyl residue is well-separated from other peaks. The C2,C4 signals are overlapped. Although the assignment of carbonyl groups is ambiguous due to overlapping, the presence of three systems is apparent. In order to verify the assignment of structures, we performed the dipolar dephasing (DD) experiment.15 This method is often used as a spectral editing technique. In the simplest approach after CP, the 1H decoupler is turned off for ca. 50 µs. This is a time sufficient for 13C-1H dipolar coupling to dephase the transverse magnetization for any 13C with a directly bonded 1H, as long as the dipolar coupling is not motionally averaged.

2792 J. Phys. Chem. B, Vol. 111, No. 11, 2007

Pacholczyk et al.

Figure 3. DSC profiles for compounds 1 (a) and 2 (b) recorded with a heating rate of 5 deg/min.

Figure 2. Distances of selected H-bonds in compounds 1 (a) and 2 (b).

Therefore, the lines for rigid CHn are effectively suppressed. Figure 4c shows the DD spectrum with a τD equal to 50 µs. In our recent papers, we have proven that the analysis of principal elements of the 13C δii chemical shift tensor (CST) compared to isotropic values 13C δiso is a source of detailed information about the electronic surrounding of each nucleus.16 However, it has to be emphasized that investigation of CST is demanding. At low spinning speed (Figure 4d), the overlap between different spinning side bands manifolds and the analysis of the spectrum is ambiguous because the deconvolution procedure is vague. Separation of the isotropic and anisotropic part of spectra with heavily overlapped systems is still a challenge for solidstate NMR spectroscopy. There are several approaches that allow achievement of this goal.17 In our project, we employed the 2D PASS sequence, which, compared with other techniques, offers good sensitivity and does not require any hardware modifications or a special probe head (see Experimental section for details). Figure 5 displays the 2D PASS spectrum of 1, recorded at a temperature of 278 K with a spinning rate of 2000 Hz. By proper data shearing (Figure 5b), it is possible to separate the spinning side bands for each carbon and employ a calculation

procedure for establishing the 13C δii parameters. It is clear from such a presentation that the F2 projection corresponds to a TOSS18 spectrum and F1 represents CSA (chemical shift anisotropy). The experimental and the best-fitting simulated 1D spinning CSA side band patterns for selected carbons of 1 are shown in Figure 6. The full set of 13C CST values is collected in Table 1. A similar methodology was employed for signal assignment and analysis of 13C CST parameters for sample 2. We have investigated chiral crystals previously reported by Kalisiak and Jurczak.7 The chirality of 2 was checked by means of CD measurements. The 13C spectra with a numbering system are shown in Figure 7b. The 13C δii values of the chemical shift tensor are attached (see the Supporting Information). In the next step, we recrystallized sample 2 in order to obtain racemic crystals. As in the previous case, the chirality of the sample was tested using the CD method. The motivation for testing of the racemic sample was the question of whether the Wallach rule, which predicts different molecular packing for enantiomers and racemates, is also valid for samples of planar chirality.19 A number of examples showing sensitivity of the solid-state NMR to distinct molecular packing of chiral samples was reported.20 Unfortunately, in the case of compound 2, we did not observe any differences between the samples having zero and plus Cotton effect. Probably, the recrystallized sample is a conglomerate of equal population of the plus and minus isomer. On the other hand, the presence of the racemate of 2 having the same symmetry as the enantiomer cannot be excluded. 2.2. Analysis of Thermal Processes for 1 by Means of SolidState NMR. Figure 8 shows the 13C CPMAS spectrum of 1 kept for 1 h at 423 K. Comparing the SS NMR spectra for freshly crystallized 1 (Figure 4b) and after thermal treatment, the differences are apparent. The most important distinction is seen for the carbonyl region and C9, C13 carbon atoms of cyclophane rings. It is interesting to note that, after thermal rearrangement, the carbonyl signals are much better separated and each signal

Chirality for Solid-State Benzodiazacoronands


Figure 4. 75.46 MHz 13C NMR spectra of compound 1 (a) recorded in liquid phase in DMSO, (b) solid-state CPMAS at spinning speed 7 kHz, (c) solid-state dipolar dephasing experiment, and (d) solid-state CPMAS at the spinning rate of 1.5 Hz.

Figure 6. Experimental (up) and best fitting (down) spinning CSA side band patterns for individual carbon nuclei (a) C16 (b) C8 (c) C6 (d) C15 for compound 1.

origin of asymmetric splitting of 13C resonances induced by 14N nuclei in the solid state was discussed in detail by Diaz and co-workers.21 13C-14N residual dipolar coupling is observed due to the fact that 14N eigenstates are not pure Zeeman states but are determined by the Zeeman-quadrupole Hamiltonian. MAS is not able to average this coupling to zero, leading to both splitting and broadening of 13C signals. The splitting S can be given by the following equation

S ) 9/20(Dζ/ZN)[(3 cos2βD - 1) + η sin2βD cos 2RD] Figure 5. 13C 2D PASS spectrum of compound 1 recorded at spinning rate of 2 kHz (a) before and (b) after data shearing.

can be individually analyzed. In contrast, the C9, C13 peaks are much closer each other. The isotropic lines are in the middle of the region observed for the sample before heating. Moreover, the change of line shape for the C8 carbon has to be stressed. Figure 8b, c shows the expanded region of the C8 signal. The

where ζ ) e2Qqzz/h is the quadrupole coupling constant, η is the asymmetry parameter of the EFG tensor, ZN ) γNH0/2π is the Zeeman frequency of 14N in the applied field, D ) γCγNh/ 4πr3 is the dipolar coupling constant, and βD and RD are the polar and azimuthal angles that define the orientation of the internuclear vector in the principal axis system of the EFG at the 14N. The value of S can be estimated with this equation if all the geometric (βD, RD, rCN) and energetic (ζ,η, ZN) factors are


Pacholczyk et al.

TABLE 1: 13C δii Parameters Obtained from PASS-2D Experiment Fitted Using SIMPSON Program for Compound 1: (a) Freshly Crystallized Sample (b) Sample after Heating and Compound 2 (c)a (a) carbon atom number 7 15 22 9 13 6 5 11 3 2 4 1 8 10 12 20 19 18 17 14 23 21 16

(b)

δIso

δ11

δ22

δ33

Ω ppm

k

169.3 170.2 168.4 152.4 155.5 133.9 131.8 130.2 129.5 127.9 127.9 127.9 115.1 106.7 103.8 72.9 72.0 69.2 68.5 66.8 66.8 40.3 38.8

244 244 244 226 230 224 223 231 230 227 227 227 179 182 176 102 101 94 93 95 95 58 55

172 169 169 160 164 154 148 145 146 145 145 145 117 117 124 81 81 85 86 73 73 58 55

91 91 91 71 72 24 13 13 12 10 10 10 48 8 11 36 33 28 26 31 31 4 6

152 153 153 155 158 200 210 218 218 217 217 217 130 174 165 66 69 66 67 64 64 54 49

0.02 -0.01 0.01 0.05 0.05 0.10 0.08 0.07 0.08 0.08 0.08 0.08 0.02 0.06 0.12 0.12 0.13 0.24 0.26 0.10 0.10 0.33 0.33

(c)

δIso

δ11

δ22

δ33

Ω

k

165.9 170.2 167.9 152.9 152.6 135.9 130.1 129.1 128.0 127.5 127.5 126.8 114.5 105.5 103.8 72.5 71.3 67.8 67.8 65.7 65.7 41.5 39.5

243 247 248 228 227 226 233 229 228 226 226 226 170 180 176 102 90 88 88 89 89 64 57

160 175 167 159 159 156 142 147 142 143 143 143 121 128 124 80 90 88 88 79 79 64 57

96 89 90 73 72 26 16 12 13 12 12 12 54 9 12 35 35 29 29 30 30 -2 6

147 158 157 155 156 200 217 217 216 214 214 214 116 170 164 67 55 58 58 58 58 65 51

-0.06 0.06 -0.02 0.04 0.04 0.10 0.06 0.08 0.07 0.07 0.07 0.08 0.05 0.13 0.13 0.12 0.34 0.34 0.34 0.23 0.23 0.34 0.34

carbon atom number 22 15 13 9 19 18 8 6 3 4 2 5 1 11 26 25 12 24 27 10 23 7 14 20 17 21 16

δIso

δ11

δ22

δ33

Ω ppm

k

168.7 166.8 154.7 154.7 147.2 146.3 138.5 136.7 130.8 130.8 128.6 128.6 125.8 125.8 119.9 117.6 112.7 110.9 110.9 110.1 75.3 74.4 72.0 68.0 65.4 39.9 37.7

248 246 218 218 210 209 187 236 236 236 230 230 236 236 228 214 191 188 188 179 111 99 103 93 79 56 51

168 163 168 168 165 165 159 157 152 152 150 150 141 141 126 128 131 135 135 136 82 94 65 62 79 56 51

90 76 66 66 67 67 70 17 -2 -2 6 6 4 4 6 11 15 10 10 15 33 30 47 48 38 8 11

159 170 152 152 143 142 117 219 237 237 224 224 232 232 222 203 176 178 178 164 78 69 56 45 41 48 40

0.00 -0.02 0.09 0.09 0.12 0.13 0.18 0.09 0.09 0.09 0.10 0.10 0.06 0.06 0.03 0.05 0.10 0.14 0.14 0.16 0.08 0.29 -0.12 -0.12 0.34 0.33 0.33

a Estimated error in δ , δ , and δ is ( 3 ppm; span is expressed as Ω ) δ - δ , and skew is expressed as k ) 3(δ 11 22 33 11 33 iso - δ22)/Ω. Chemical shifts are in ppm.

Figure 7. 13C spectra of compound 2 (a) recorded in the liquid phase (DMSO solution), (b) CPMAS at the spinning rate 7 kHz, and (c) dipolar dephasing.

known. In our case, the energetic parameters remain very similar (or the same) for both forms of sample 1 (before and after heating). Thus we can conclude that geometric factors (βD, RD) related with rotation of the pendent group with respect to benzodiazacoronand ring are changed during the phase transition. The distinction of C8 line shape is very convincing. As in the previous case (Section 3.1), the principal elements of 13C chemical shift tensors were obtained employing the 2D PASS experiment. The calculated values of 13δii parameters are given in Table 1. The appropriate 2D spectrum is attached as Supporting Information. Analyzing the differences between 13C SS NMR spectra and values of 13C δii parameters for both forms of 1, we assume

Figure 8. (a) 13C solid-state CPMAS spectra recorded at spinning speed of 8 kHz of compound 1 after heating. (b, c) the pattern of the C8 resonance line before and after heating, respectively.

that the observed changes are related to migration of water molecules from the crystal lattice. This hypothesis prompted us to carry out the experiments allowing us to observe the proton signals of water directly. 2.3. Search for a Water Molecule in the Crystal Lattice of 1: 1H and 2H SS NMR. Recording of good quality high-resolution 1H NMR spectra is still a challenge for solid-state spectroscopy. The major problem is line-broadening effects caused by strong homonuclear 1H-1H dipole-dipole coupling.22 Recent progress in the design of commercially available MAS probes allows



Figure 9. 300.13 MHz solid-state 1H MAS spectra recorded at spinning rate of 25 kHz: (a) compound 1 and (b) compound 2.

for spinning frequencies up to 35 kHz. In solids with not very tightly coupled protons, such spinning frequencies are sufficient to efficiently remove the dipolar broadening, which results in highly resolved proton spectra. The theoretical background and examples showing the recent advances of high-resolution 1H NMR spectroscopy in the solid state were exhaustively reviewed by Schnell and Spiess.23 Figure 9 shows 1H MAS spectra of 1 and 2 recorded with a spinning speed of 25 kHz. Comparing both samples, one can see that the resolution is much better for 2. For sample 1, aromatic signals are very broad and overlap other resonances expected in this region. Thus, the assignment of water signal in the crystal lattice still remains ambiguous. An important source of information about chemical shifts of protons in the case of strongly overlapped signals can be 1H13C heteronuclear correlation (HETCOR). In the 1990s, several methodological improvements in the technique were reported.24 The big achievement in this field was the application of frequency-switched Lee-Goldburg (FSLG) decoupling.25 In FSLG HETCOR experiments, the sample is spun very fast (often more than 10 kHz), which greatly improves the resolution of carbon and proton projections. Through lengthening of the contact time (CT) in the pulse sequences, it is possible to observe the long-range intermolecular interactions. The employed pulse sequence for recording of 2D spectra is shown in Figure 10 (top insert). Figure 10a shows the 1H-13C FSLG HETCOR spectra of sample 1 recorded at a spinning speed of 10 kHz and a CT equal to 100 µs. As expected, the quaternary carbons are not observed in the 2D correlation. On the other hand, the position of the cross-peak representing the C12 atom is rather unexpected. The correlation with aliphatic adjacent CH2 protons means that polarization transfer from these protons is very efficient and that the geometry of 1 is significantly distorted. Such a conclusion is consistent with X-ray data. With lengthening of CT to 300 µs, the quaternary signals (with exception of the C8 carbon) appear in the 2D spectrum (Figure 10b). It is worthwhile to note that the C13 signal is strongly correlated with the aliphatic signal of the macrocycle. Such a connection provides further evidence that confirms the correctness of the structural assignment. Analysis of the carbonyl region is rather ambiguous. The three signals C7, C15, C22 are overlapped. Moreover, the 1H cross-peak is very broad and covers the spectral range characteristic both for aliphatic and aromatic signals as well as the region where the signal of water is expected. Figure 10c shows the 2D spectrum of 1 after thermal treatment recorded with CT equal to 300 µs and other experimental parameters exactly as presented in Figure 10a, b. The differences between spectra are apparent. It is worthy to stress the very small efficiency of 1H cross-polarization to

Figure 10. 1H-13C FSLG HETCOR 2D spectra (for description, see text).

quaternary carbons C22, C15, C9, and C13. Moreover, the C7 carbonyl signal is not seen. Lack of the cross-peak for C7 unambiguously proves that for the untreated sample the water

2796 J. Phys. Chem. B, Vol. 111, No. 11, 2007 molecule, which according to X-ray data is very close to C7, is the donor of the magnetization during CP. In Figure 10b, the 1H water signal is hidden in a very broad cross-peak correlated with the carbonyl groups. The HETCOR correlation enabled us to assign all carbonyl group signals to the molecular structure of 1. The 1H-13C FSLG HETCOR correlation of 2 is attached as Supporting Information. Analysis of the 2D spectrum recorded with the CT equal to 300 µs reveals common features for samples 1 and 2. C9 and C13 carbons very well correlate with CH2 protons (C14, C23). The same protons are correlated with carbonyl groups C15, C22. It is interesting that the C8 carbon is correlated with both aliphatic and aromatic signals. From X-ray data, it is is known that C8 is close to C1-H protons of the phenyl pendent group (3.78 Å) Further results confirming the presence and molecular dynamics of the water molecule in the crystal lattice of 1 were obtained from the analysis of deuteron NMR. In the case when the proton can be easily replaced with the 2H isotope, deuteron NMR spectroscopy is often considered an alternative approach to search for the structure and dynamics of samples under investigation. However, from the point of view of NMR spectroscopy, these two nuclides are entirely different. The deuterium nucleus has a spin of 1 (as in case of 14N), which means that, in the presence of magnetic field, there are three energy levels and the NMR experiment consists of causing transition between these levels. In addition, the deuterion is quadrupolar and there is a nonspherical charge distribution at the nucleus. The interaction of the quadrupolar moment with the electric field gradient (EFG) tensor at the nucleus causes a substantial perturbation of the Zeeman splitting. Deuteron solidstate NMR spectra are completely dominated by the quadrupole coupling, and the lines are very broad. The typical static 2H NMR spectrum of a powder sample recorded employing a quadrupole echo pulse sequence is recorded as a characteristic doublet. Another approach relies on recording spectra under magic angle with fast sample spinning.26 Figure 11a shows the 1H-2H CPMAS spectrum of sample 1 recrystallized from the mixture of D2O/CH3OD. The spectrum presents a typical pattern for a static deuterium sample with splitting between singularities equal to 120 kHz. However, it has to be noted that not only water molecule is replaced with heavy water during crystallization but all labile N-H protons can be replaced with 2H isotope forming N-D bonds. The yield of N-H substitution was established by analysis of 1H spectra in solution and was found to 60%. The ambiguities related with assignment of 2H signals to N-D and/or D2O species were explained by measurement of the recrystallized sample kept for 2 h at 423 K. Figure 11b shows the 1H-2H CPMAS spectrum recorded with exactly the same experimental conditions as those reported for Figure 11a. At first glance, the spectra are the same, but the comparison of the signal-to-noise ratio and the difference spectrum proves that the intensity of 11b signals is much lower. It is due to removal of heavy water from the crystal lattice. The important information that we get from this experiment is the dynamics of water. The water molecules strongly hydrogenbonded with the crystal lattice do not undergo molecular motion. For such molecules, the 1H signal in the solid state is usually very broad. This can explain why we do not observe this resonance in 1H MAS spectra even at very fast sample spinning. 3. Theoretical Calculations of 13C NMR Shielding Parameters for 1 and 2: Correlation with Experimental Data. Many methods are currently available for computing of NMR parameters.27,28 The high reliability of the theoretical approach

Pacholczyk et al.

Figure 11. 46.07 MHz 2H CPMAS spectra of compound 1 recrystallized from CD3OD/D2O solution before (a) and after heating (b); differential spectrum shown in (c).

in relation to shielding elements of the carbon-13 nucleus has been proved.29 In our work, the GIAO B3PW91 and B3LYP hybrid methods with the 6-311G** basis set were used for calculation of the 13C parameters of 1 and 2 employing the Gaussian program.30 The X-ray diffraction data of both samples were taken as input files. In general, an advantage of such an approach is related to the fact that it is possible to compare the theoretical and experimental results for molecules having exactly the same geometry of heavy atoms. Unfortunately, in our case, the calculations for isolated molecules were not consistent with the experimental data. Hence, in the next step, the computation was carried out employing the cluster structures that preserved all important inter- and intramolecular interactions as hydrogen bonding and C-H...O contacts. The structure of clusters used for calculation is attached as Supporting Information. Employing cluster-type computing gave a satisfactory correlation between the theoretical and experimental data. Results showing the full set of 13C computed data for 1 and 2 are deposited. Having the 13C σiso values, we are in position to assign the 13C resonances to the molecular structure in the crystallographic unit. Figure 12a shows the correlation between isotropic values of shielding and chemical shift parameters for sample 1 before thermal treatment. The obtained equation σii ) -1.003δii + 186 can be used to convert shielding to chemical shift parameters. For sample 2, the appropriate equation is σii ) -0.9688ii + 181. (Figure 12b). More advanced analysis of NMR shielding, which considers all subtle intraand intermolecular interactions, is based on comparison of 13C σii theoretical values with those obtained from the 2D PASS experiment (Figure 13a, b for 1 and 2, respectively). In the next step, we have optimized the geometry of 1 (model I1) for testing the most favorable conformation of the pendent group with respect to the macrocycle and the relationship between the change of conformation and NMR shielding parameters when the water molecule was inside the lattice. The obtained results were compared with data for the isolated, fully optimized molecule (model I2, see Figure 14a). Further calculations were carried out for X-ray cluster 1 with water removed


Figure 12. Correlation between experimental isotropic chemical shift values δiso and calculated isotropic shielding values σiso: (a) compound 1 and (b) compound 2.

from the lattice (model M2). Other geometrical parameters including bond lengths and torsion angles were preserved. In such an approach, the influence of water hydrogen bonding on NMR shielding parameters for each center, which participates in the interactions with water, can be investigated and compared with experimental data. Finally, in order to better understand the mechanism of water migration from the crystal lattice of 1 and the influence on change of geometry and NMR shielding parameters, we have built two models shown in Figure 15. The first one presents the structure of 1 with a water molecule inside the ring and hydrogen-bonded dimethyl ether molecule outside the benzodiazacoronand (model M3). The ether molecule mimics the N-H...O strong hydrogen bond observed in the X-ray structure. During the optimization procedure, the N...O distance was kept constant and other geometrical parameters were optimized. The second model represents a very similar structure with a water molecule removed (model M4). As in the previous case, the N...O length was preserved during geometry optimization. The calculated values of 13C chemical shift parameters of selected carbon atoms for all computed models of 1 are collected in Table 2. Interesting conclusions can be drawn from inspection of data. First, by comparing theoretical results for the cluster (M1) and the fully optimized structure (I1), we can conclude that carbonyl groups are very sensitive to hydrogen bonding and molecular packing. The span parameter Ω is ca. 20 ppm larger compared with those obtained for the cluster. The C9, C13 carbons do not reveal such a significant sensitivity. Second, the most sensitive parameter that reflects the change of the hydrogen-bonding pattern is the δ22 element of the C7 carbonyl group. The difference between 13C δ22 values for models with and without water (I1 versus I2) is 18 ppm. The hydrogen-


Figure 13. Correlation between experimental CST values δii (ppm) and calculated shielding parameters σii (ppm): (a) compound 1 and (b) compound 2.

Figure 14. Comparison between the geometries of X-ray data (black) and fully optimized structures (gray shade): (a) compound 1 and (b) compound 2.

Figure 15. Structure of theoretical models M3 and M4.

bond breaking is responsible for shifting of the carbonyl signal of 1 after the thermal process. The orientation of the principal elements with respect to the molecular frame of the carbonyl group is known.31 The relationship between strength of hydrogen bonding and distinction of 13C δ22 values was reported by McDermott and co-workers.32 The mechanism of changes of


Pacholczyk et al.

TABLE 2: 13C Shielding Parameters Calculated by Means of GIAO DFT Using Gaussian Program: Shielding Parameters Were Converted to Chemical Shift Parameters Employing Equation: σiso ) -δiso + 186a carbon atom C7 C15 C22 C9 C13 C7 C15 C22 C9 C13

δIso I1 169.5 169.2 166.6 157.6 154.8 I2 165.7 167.5 167.7 151.7 149.6

δ11

δ22

δ33

Ω

δIso

250 262 258 233 231

179 163 156 171 168

79 83 86 69 65

171 179 172 165 166

168.3 167.9 166.7 155.6 157.8

254 260 262 223 222

161 160 159 167 158

82 84 84 67 70

172 177 178 157 152

165.8 167.5 168.1 155.0 152.8

δ11 M1 240 248 245 229 234 M2 251 260 261 231 229

δ22

δ33

Ω

δIso

176 166 164 169 168

89 90 91 68 72

150 158 154 161 162

169.5 168.7 167.4 154.0 156.1

164 160 161 168 161

83 83 82 65 68

168 177 180 166 161

166.2 167.1 167.5 156.8 155.4

δ11 M3 245 261 258 231 233 M4 246 246 248 231 231

δ22

δ33

Ω

182 164 160 167 170

81 86 88 69 69

164 175 171 161 165

162 165 167 171 163

90 90 88 69 73

156 156 160 162 158

a I1, fully optimized isolated molecule with H-bonded water molecule; I2, fully optimized isolated molecule; M1, cluster model 1 with H-bonded water molecule and all intermolecular contacts; M2, cluster model 2 with all intermolecular contacts, excluding water molecule; M3, model 3 with H-bonded water molecule and NH O contacts in the pendent group residue using mimic ether; M4, model 4 with NH O contacts in the pendent group residue using mimic ether. Chemical shifts are in ppm.

TABLE 3: Values of Selected Torsion Angles (deg) in the Models Used for Calculations torsion angle

I1

I2

M1

M2

M3

I4

C9-C8-N-C7 H-N-C7-O C9-C8-N-H

118.4 159.4 -48.0

128.0 159.3 -36.1

116.1 162.1 -56.7

116.1 173.2 -56.7

114.8 174.8 -53.6

117.1 173.3 -58.1

Figure 17. Array of two enantiomers in the crystal lattice for compound 1.

Figure 16. Weak intermolecular contacts organizing the crystal structure of compound 1. 13C

δiso values for C9 and C13 carbons is not so evident. The comparison of difference of isotropic chemical shift for the sample before and after heating shows a significant distinction that, at first glance, is not consistent with the theoretical results. A more careful analysis of 13C δiso unambiguously proves a considerable shift of C13. By comparing model M1 and M2, the difference is found to be 5 ppm. The geometry optimization of 1 without water revealed a change of torsion angles, which define the orientation of the pendent group with respect to the macrocycle plane. The obtained data are collected in Table 3. Summary Our results clearly show that benzodiazacoronand crystals under investigation are to be considered as a two-component system consisting of an organic unit and a water molecule in a 1:1 ratio. Both components play an important role in the crystal structure architecture. The strong (O-H...O, N-H...O) and weak (C-H...O) intermolecular hydrogen bonds are responsible for phase organization and, in consequence, formation of chiral or achiral crystals. The alignment of the water molecule with respect to the macrocycle is different for samples 1 and 2. For compound 1, the water molecule is inside the ring. An extended molecular array built by employing the intermolecular contacts is shown in Figure 16. Using such a procedure, we can obtain one of the enantiomers. The second array of enantiomers is built using the next water molecule. The distinction between enantiomers is shown in pictorial form in Figure 17. It is worthwhile to express that removal of water from the crystal lattice of 1 is a reversible process. The water molecules can be introduced to the lattice during a couple of hours at room

temperature by the diffusion process. The 13C CPMAS spectrum taken for a sample kept in a moist vessel shows a characteristic pattern exactly the same as that recorded for the sample after crystallization. Hence, we can define sample 1 as an “organic zeolite”. As we proved, the process of water migration is related with a large amplitude molecular motion of the pendent group. For sample 2, the water molecule is outside of ring. The pendent benzyl group is oriented in the direction of the benzodiazacoronand. The strong CdO...H-O bonds between carbonyls C15, C22, and water are responsible for linear extension of one of the enantiomers. The choice of a plus or minus enantiomer depends on the crystallization conditions. However, in many cases, a mixture of enantiomers (conglomerate) is obtained during crystallization. The formation of chiral cocrystals from two different achiral molecules by self-assembly is very well-known.5 However, in this paper, we clearly show that the water molecule can be an important cofactor responsible for chiral crystallization. In his recently published review, Gridnev concluded that “the structural factors are important for the possibility of spontaneous not yet completely understood. In the case of chiral crystallization, it is not clear what makes the achiral molecules joining chiral crystals in a stereospecific way instead of producing a racemic crystal”.4a We believe that the results presented in this paper shed a new light on the molecular mechanism of formation of chiral crystals from achiral molecules. The challenging question is whether is it possible to get chiral crystals of desired stereochemistry by controlling the conditions of crystallization (solvents, temperature) of benzodiazacoronands. The second important question regards conditions that can force achiral systems to form chiral crystals.33 In order to answer these questions, advanced experimental and theoretical studies of dynamic processes in the liquid phase preceding the crystallization and mechanism of preorganization including

Chirality for Solid-State Benzodiazacoronands interactions of both components are desired. Such a project for series of different benzodiazacoronands is actually in progress. Acknowledgment. The authors are grateful to Dr. S. Olejniczak and Mr. W. Ciesielski for technical assistance. Calculations were carried out in the Interdisciplinary Centre for Mathematical and Computational Modeling of the Warsaw University under grant No. G29-8. Supporting Information Available: Numbering system for the compounds, structure of cluster models, spectra for the compounds, and shielding parameters. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kondepudi, D. K.; Kaufman, R. J.; Singht, N. Science 1990, 250, 975. (2) a) Dahmen, S.; Brase, S. J. Am. Chem. Soc. 2002, 124, 5940. (b) Gibson, S. E.; Knight, J. D. Org. Biomol. Chem. 2003, 1, 1256. (c) Brase, S.; Dahmen, S.; Hofener, S.; Lauterwasser, F.; Kreis, M.; Ziegert, R. E. Synlett 2004, 2647. (3) Addali, L.; Lahav, M. In Origins of Optical ActiVity in Nature; Walker, D. C., Ed.; Elsevier: Amsterdam, 1979. (4) (a) Gridnev, I. D. Chem. Lett. 2006, 35, 148-153. (b) Soai, K.; Shibata, T.; Morioka, H.; Choji, K. Nature 1995, 378, 767. (5) Matsuura, T.; Koshima, H. J. Photochem. Photobiol., C 2005, 6, 7-24, and references cited therein. (6) (a) Kalisiak, J.; Jurczak, J. Synlett 2004, 9, 1616-1618. (b) Piatek, P.; Kalisiak, J.; Jurczak, J. Tetrahedron Lett. 2004, 45, 3309-3312. (c) Kalisiak, J.; Skowronek, P.; Gawronski, J.; Jurczak, J. J. Eur. Chem. 2006, 12, 4397-4406. (d) Kalisiak, J.; Jurczak, J. Cryst. Growth Des. 2006, 6, 22-24. (7) Kalisiak, J.; Piatek, P.; Jurczak, J. Synthesis 2005, 13, 2210-2214. (8) Morcombe, C. R.; Zilm, K. W. J. Magn. Reson. 2003, 162, 479486. (9) Metz, G.; Wu, X.; Smith, S. O. J. Magn. Reson., Ser. A 1994, 110, 219-227. (10) Bennett, A. W.; Rienstra, C. M.; Auger, M.; Lakshmi, K. V.; Griffin, R. G. J. Chem. Phys. 1995, 103, 6951-6958. (11) WIN-NMR, Bruker-Franzen Analytik GmbH, Version 6.0, Bremen, Germany, 1993. (12) (a) Antzutkin, O. N.; Shekar, S. C.; Levitt, M. H. J. Magn. Reson. 1995, 115, 7. (b) Antzutkin, O. N. Prog. NMR Spectrosc. 1999, 35, 203266. (13) Bak, M.; Rasmussen, J. T.; Nielsen, N. C. J. Magn. Reson. 2000, 147, 296-330. (14) Kalisiak, J.; Kalisiak, E.; Jurczak, J. Tetrahedron 2006, 62, 59055914. (15) (a) Alla, M.; Lippmaa, E. Chem. Phys. Lett. 1976, 37, 260. (b) Opella, S. J.; Frey, M. H. J. Am. Chem. Soc. 1979, 37, 260. (c) Zilm, K. W. In Spectral Editing Techniques: Hydrocarbon Solids, Encyclopedia of NMR; Grant, D. M., Harris, R. K., Eds.; John Wiley & Sons Ltd.: Chichester, U.K., 1996; Vol. VII, p 4498. (16) (a) Potrzebowski, M. J.; Assfeld, X.; Ganicz, K.; Olejniczak, S.; Cartier, A.; Gardiennet, C.; Tekely, P. J. Am. Chem. Soc. 2003, 125, 4223-

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