MAS NMR Structural Studies of

J. Phys. Chem. B 2010, 114, 10311–10320

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X-ray Diffraction, FT-IR, and 13C CP/MAS NMR Structural Studies of Solvated and Desolvated C-Methylcalix[4]resorcinarene Rafal Kuzmicz,† Violetta Kowalska,† Sławomir Domagała,‡ Marcin Stachowicz,‡ Krzysztof Wozńiak,‡ and Waclaw Kolodziejski*,† Department of Inorganic and Analytical Chemistry, Faculty of Pharmacy, Medical UniVersity of Warsaw, Banacha 1, Warszawa 02-097, Poland, and Department of Chemistry, UniVersity of Warsaw, Pasteura 1, Warszawa 02-093, Poland ReceiVed: February 21, 2010; ReVised Manuscript ReceiVed: June 16, 2010

Solid C-methylcalix[4]resorcinarene solvated by acetonitrile and water (CAL-Me) and then modified by slow solvent evaporation (CAL-Me*) was studied using single-crystal and powder X-ray diffraction, FT-IR, and 13 C CP/MAS NMR. The CAL-Me solvate crystallizes in the monoclinic P21/n space group with three CH3CN and two H2O molecules in the asymmetric part of the unit cell. The CAL-Me molecules adopt a typical crown conformation with all of the hydroxyl groups of the aryl rings oriented up and all of the methyl groups disposed down (the rccc isomeric form). The crystalline network is formed by resorcinarene, CH3CN, and H2O molecules and assembled by intermolecular hydrogen bonds and weak C-H · · · A or C-H · · · π interactions. The desolvated CAL-Me* loses its crystalline character and becomes partly amorphous. It is devoid of CH3CN and deficient in water. However, the resorcinarene molecules still remain in the crown conformation supported by intramolecular hydrogen bonds, while intermolecular hydrogen bonds are considerably disintegrated. The work directs general attention to the problem of stability and polymorphism of resorcinarene solvates. It shows that the joint use of diffractometric and spectroscopic methods is advantageous in the structural studies of complex crystalline macromolecular systems. On the other hand, the solid-state IR and NMR spectroscopic analyses applied in tandem have been found highly beneficial to elucidate the disordered structure of poorly crystalline, desolvated resorcinarene. Introduction Calix[4]resorcinarenes are macrocyclic compounds (Figure 1) formed by acid-catalyzed condensation of benzene-1,3-diol (resorcinol) with acetaldehyde or higher aliphatic aldehydes.1-4 They are of great interest in the host-guest chemistry as molecular receptors for many useful molecules.5-8 Unsubstituted calix[4]resorcinarenes may exist in one of the five conformations: crown (C4V), boat (C2V), chair (C2h), saddle (D2d), and the least discussed diamond (Cs).1,4 Four intramolecular hydrogen bonds between the OH groups on neighboring aromatic rings support the rigid structure of the crown conformer (Figure 1).1,9-13 In the solid state, the rccc isomer (epimer allcis) of the crown conformation1 is the prevalent form (Figure 1).2,10-22 Chemical properties of unsubstituted calix[4]resorcinarene in the crown conformation are generally determined by the presence of eight hydroxyl groups in the so-called “upper rim” and by its internal hydrophobic cavity.1 Calix[4]resorcinarenes have unusual complexing properties toward organic molecules such as solvents,10-13 aliphatic and aromatic amines,16 neurotransmitter acetylcholine,7,17 and sugars.18-21 The complexes were comprehensively studied using single-crystal X-ray diffraction (XRD) and NMR in solutions. So far, little is known about the structure of desolvated resorcinarenes in the solid state.23 The reason is that they do not form single crystals suitable for the XRD studies. Hence, in order to solve this problem, one has to apply methods, which * To whom correspondence should [email protected]. † Medical University of Warsaw. ‡ University of Warsaw.

be

addressed.

E-mail:

Figure 1. Hydrogen bonds stabilizing the crown conformer of C-methylcalix[4]resorcinarene and the systematic enumeration of its carbon atoms. For other resorcinarenes, the methyl groups are replaced by various substituents.

do not rely on the material crystallinity. In this study, we use IR and NMR spectroscopic techniques in the solid state. The solid-state high-resolution 13C NMR spectroscopy comprises three major techniques: magic-angle spinning (MAS) of a sample, high-power decoupling of protons, and signal enhancement by cross-polarization (CP) from protons. However, the kinetics of 1H f 13C CP has not yet been used to investigate the structure and phase homogeneity of calixarene samples. C-Methylcalix[4]resorcinarene (Figure 1), hereafter abbreviated CAL-Me, is the simplest member of the resorcinarene compounds. A solid-state structure of CAL-Me cocrystallized with various small molecules, e.g., solvents, has been exclusively studied using X-ray crystallography.12-14,23-55 Recent X-ray diffraction studies56 showed a dramatic effect of water and CH3CN exerted on C-alkylpyrogallol[4]arene assemblies by means of intermolecular hydrogen bonding.

10.1021/jp1015565  2010 American Chemical Society Published on Web 07/26/2010

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Kuzmicz et al.

TABLE 1: Crystal data for CAL-Me data

CAL-Me

data

CAL-Me

empirical formula formula weight temperature (K) wavelength (Å)

C38H45N3O10 703.77 100 0.71073

F(000) crystal size (mm) θ range for data collection (deg) index ranges:

crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (Mgm-3) µ (mm-1)

monoclinic P2(1)/n 7.300(1) 19.456(1) 26.023(1) 90.0 97.681(3) 90.0 3663.0(3) 4 1.276 0.093

reflections collected/unique Rint refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)]: R1 wR2 R indices (all data): R1 wR2 largest diff. peak and hole (eÅ-3)

1496 0.20 × 0.10 × 0.02 1.89-27.73 h: -6, 9 k: -21, 25 l: -33, 34 39454/8597 0.0471 full-matrix least-squares on F2 8597/0/569 1.023

In this paper, we are concerned with the solid-state structure of a CAL-Me/CH3CN/H2O solvate and with the structure of desolvated solid CAL-Me (hereafter denoted CAL-Me*). Singlecrystal XRD, IR, and 13C CP/MAS NMR results are discussed with particular attention paid to solvation and hydrogen bonding. Experimental Methods Synthesis. C-Methylcalix[4]resorcinarene was synthesized using previously published procedures.2,16 The microcrystalline product (a 1:4 resorcinarene/water hydrate) was recrystallized from boiling acetonitrile to give the CAL-Me solvate. Single crystals suitable for X-ray diffraction were selected directly from the analytical samples. The desolvated substance CAL-Me* was obtained by storing CAL-Me in an open vessel at 298 K in a dry atmosphere over 2 years. Single-Crystal X-ray Diffraction. The data were collected using the BRUKER KAPPA APEXII ULTRA diffractometer controlled by the APEXII software,57 equipped with the Mo KR rotating anode X-ray source (λ ) 0.71073 Å, 50.0 kV, 22.0 mA) monochromatized by multilayer optics and with the APEXII CCD detector. The experiments were carried out at 100 K using the Oxford Cryostream cooling device. The crystal was positioned at 60 mm from the CCD camera. 2330 frames were measured at 0.3° intervals with a counting time of 15 and 40 s. The crystal was mounted on a thin cactus needle with a droplet of Pantone-N oil and immediately cooled. Indexing, integration, and initial scaling were performed with the SAINT58 and SADABS59 software. The data collection and processing statistics are reported in Table 1. The structures were solved by direct methods approach60 using the SHELXS-97 program and refined with the SHELXL-97 and WinGX Program System.61 The analytical numeric absorption correction using a multifaceted crystal model was applied in the scaling procedure. The refinement was based on F2 for all reflections except for those with negative intensities. Weighted R factors wR and all goodness-of-fit S values were based on F2, whereas conventional R factors were based on the amplitudes, with F set to zero for negative F2. The F02 > 2σ (F02) criterion was applied only for the R factors calculation, and it was not relevant to the choice of reflections for the refinement. The R factors based on F2 are for all structures about twice as large as those based on F. Most of the hydrogen atoms were located from a differential map and refined isotropically. Scattering factors were taken from Tables 4.2.6.8 and 6.1.1.4 in ref 62. CCDC 718048 contains

0.0439 0.1181 0.0759 0.1181 0.36, -0.29

the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif. Powder X-ray Diffraction. The data were collected with a Bruker D8 Discover diffractometer (KR line, parallel beam obtained with Gobel mirror, PSD detector Vantec-1) working in the reflection geometry. The powder crystal sample was deposited on the zero background holder and measured at 298 K. FT-IR Spectroscopy. The spectra were recorded at 298 K from KBr pellets using a Perkin-Elmer Spectrum 1000 FT-IR spectrometer (spectral range 700-4000 cm-1, resolution 2 cm-1). NMR Spectroscopy. Solid-state 13C CP/MAS NMR spectra were recorded at 298 K on a Bruker Avance 400 WB spectrometer at 100.6 MHz, using a Bruker double-bearing probehead and zirconia rotors (4 mm in diameter) driven by dry air at 7.5 kHz. The conventional single contact 1H f 13C cross-polarization (CP),63-65 with reversal of spin temperature in the rotating frame, was performed with the 6 s recycle delays (optimized), proton π/2 pulses of 2.85 µs duration, and highpower proton decoupling during signal acquisition. The contact time was varied in 32 steps up to 20 ms. Each FID was acquired with 400 scans and processed using exponential apodization with LB ) 5 Hz. The magic angle was set precisely by observing the 79Br resonance from KBr. The solid-state spectra were processed with the ACD program.66 The CP kinetics functions were fitted using the KaleidaGraph program,67 which employs the nonlinear least-squares algorithm with the LevenbergMarquardt gradient descent method of minimization of the error function. The liquid-state 1H and 13C spectra were measured from saturated solutions in acetone-d6 on a Varian Unity PLUS500 spectrometer, and the peak assignments, based on chemical shifts, were confirmed using DEPT, COSY, and HETCOR experiments. Results and Discussion X-ray Crystallography. The solvated CAL-Me crystallizes in the monoclinic space group P21/n with three CH3CN (S1, S2, and S3) and two H2O (W1 and W2) molecules in the asymmetric part of the unit cell (Figure 2). The CAL-Me molecules adopt a typical crown conformation with all of the hydroxyl groups of the aryl rings oriented up and all of the

Structures of Solvated and Desolvated Resorcinarene

J. Phys. Chem. B, Vol. 114, No. 32, 2010 10313

Figure 2. ORTEP drawing of the asymmetric part of the CAL-Me unit cell with carbon atoms labeled and hydrogen atoms omitted for clarity. Thermal ellipsoids are presented at 50% probability level.

methyl groups disposed down (the rccc isomeric form). Those hydroxyl groups maximize the number of intramolecular hydrogen bonds (HBs) with the neighboring resorcinol rings. The CAL-Me geometry deviates slightly from the C4V symmetry observed for the ideal crown conformation. The distances between the four bridging carbon atoms (C7, C15, C23, and C31) vary from 5.10 to 5.12 Å, whereas the separations between the carbon atoms bearing the hydroxyl groups range from 3.64 to 3.81 Å. Besides, the dihedral angles between the planes of the aryl rings and the plane defined by the bridging carbon atoms vary from 51 to 60°, with the average angle equal to 54°. The CAL-Me molecular network consists of resorcinarene, CH3CN, and H2O species, and it is assembled by intermolecular HBs and weak C-H · · · A or C-H · · · π interactions (Figure 3). All of the HBs and selected important short contacts are described in Table 2. The HBs involving the CAL-Me hydroxyl groups exhibit H · · · A distances in the range from 1.7 to 1.9 Å and are almost linear, since the DHA angles are over 169°. The CAL-Me molecules present a slightly shifted head-to-tail columnar arrangement, which creates resorcinarene channels going along the X direction. Those channels are occupied by acetonitrile molecules S1 trapped between two CAL-Me molecules, as shown in Figure 4. Each S1 molecule is located almost at the center of the CAL-Me cavity with nearly equidistant separation of the nitrile carbon from the resorcinarene ring centroids. Those C1S1 · · · Xn distances are 3.595, 3.688, 3.871, and 3.552 Å for n ) 1, 2, 3, and 4, respectively. The S1 molecules interact through the nitrile and methyl groups with the C-H groups of one CAL-Me molecule and with π-electrons of the resorcinol rings of the other neighboring CAL-Me molecule, respectively (Figure 4, Table 2). Two other acetonitrile molecules S2 and S3 are located at the adjacent sites of the CAL-Me crown (Figures 2 and 3) and participate in HBs as proton acceptors. The former takes part in the O4-H · · · N1S2 hydrogenbondwithCAL-Me,whilethelatterintheO1W1-H · · · N1S3 hydrogen bond with the water molecule W1. The water molecules W1 and W2 serve both as proton donors and acceptors toward CAL-Me and CH3CN. They provide main intermolecular links between the CAL-Me molecules. It is spectacular that there is only one type of direct HB between the CAL-Me molecules (O3-H · · · O8#1). The relative strength of molecular interactions can roughly be assessed by comparison of the H · · · A distances (Table 2). It

Figure 3. Hydrogen bond network of one layer of CAL-Me presented in the YZ projection (a) and a perpendicular view of the CAL-Me layer (b), showing intermolecular interactions.

TABLE 2: Parameters of Hydrogen Bonds and Selected Weak Interactions (Distances in Å and Angles in Degrees) D-H · · · A

D-H

H· · ·A

D· · ·A

∠DHA

O1-H1O · · · O1W1 O2-H2O · · · O3 O3-H3O · · · O8#1a O4-H4O · · · N1S2 O5-H5O · · · O4 O6-H6O · · · O1W2#2b O7-H7O · · · O6 O8-H8O · · · O1 O1W1-H1W1 · · · N1S3 O1W1-H2W1 · · · O5#3c O1W2-H1W2 · · · O2 O1W2-H2W2 · · · O7#1a C8-H8C · · · N1S1#4d C16-H16C · · · N1S1#4d C24-H24C · · · N1S1#4d C32-H32C · · · N1S1#4d C1S1-H2S1 · · · X1e C1S1-H3S1 · · · X2e C1S1 · · · X3e C1S1-H1S1 · · · X4e

0.88(2) 0.85(2) 0.88(2) 0.80(2) 0.87(2) 0.79(2) 0.81(2) 0.86(2) 0.85 0.85 0.91(3) 0.75(3) 0.96(2) 0.99(2) 0.96(2) 0.96(2) 0.98(1) 0.98(1)

1.68(2) 1.81(2) 1.84(2) 2.00(2) 1.86(2) 1.87(2) 1.84(2) 1.76(2) 1.99 1.92 1.85(3) 1.93(3) 2.74(2) 2.66(2) 2.70(2) 2.97(2) 2.89(1) 2.87(1)

172(2) 177(2) 169.3(19) 177(2) 177(2) 175(2) 175(2) 177(2) 172.6 175.4 176(2) 168(3) 174.3(2) 170.6(2) 174.7(2) 171.8(2) 129.9(1) 142.1(1)

0.98(1)

2.60(1)

2.559(2) 2.661(2) 2.710(2) 2.793(2) 2.729(2) 2.662(2) 2.643(2) 2.617(2) 2.839(2) 2.774(2) 2.762(2) 2.660(2) 3.701(2) 3.644(2) 3.654(2) 3.920(3) 3.595(2) 3.688(2) 3.871(1) 3.552(2)

164.8(1)

a

Symmetry transformations used to generate equivalent atoms: #1 -x, y - 1/2, -z + 3/2. b Symmetry transformations used to generate equivalent atoms: #2 x - 1, -y + 3/2, z - 1/2. c Symmetry transformations used to generate equivalent atoms: #3 x + 1, -y + 3/2, z + 1/2. d Symmetry transformations used to generate equivalent atoms: #4 x + 1, y, z. e The ring centroids X1, X2, X3, and X4 are defined by all carbon atoms of a given aryl ring: C1-C6, C9-C14, C17-C22, and C25-C30, respectively (see Figure 4).

seems that the HBs formed using the CAL-Me hydroxyl groups as proton donors, especially those intramolecular bonds connecting the resorcinarene crown, are stronger than the intermo-

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Kuzmicz et al. TABLE 3: Assignment of the IR Bands from CAL-Me and CAL-Me* assignmenta

Figure 4. Shortest contacts to CH3CN inside the cavity created by two neighboring CAL-Me molecules in their columnar head-to-tail arrangement. The X-axis goes from the right to the left. The ring centroids X1, X2, X3, and X4 are defined by all carbon atoms of a given aryl ring: C1-C6, C9-C14, C17-C22, and C25-C30, respectively.

lecular HBs involving the CH3CN nitrogens acting as proton acceptors or water hydroxyl groups acting as proton donors. Generally, the HBs are stronger than the C-H · · · A or C-H · · · π interactions. Thus, the short contacts between the molecules along the X-axis are due to the weak, nonpolar interactions, whereas in the perpendicular directions they are dominated by the HBs created mainly by CAL-Me and water molecules (polar interactions). Unsuccessfully, the desolvated CAL-Me* sample was not suitable for the single-crystal X-ray diffraction studies. According to powder X-ray diffraction, this material lost crystalline character and became partly amorphous (Figure 1S from the Supporting Information). FT-IR Spectroscopy. The mid-IR spectra of solvated CALMe and desolvated CAL-Me* have been assigned on the basis of the literature22,68,69 (Table 3). We shall focus our discussion on two spectral regions: over 2000 cm-1 (Figure 5) and below 2000 cm-1 (Figure 6). First of all, we note that the CAL-Me spectrum (Figure 5) contains three distinct CN stretching bands at 2293, 2263, and 2241 cm-1, in accordance with the X-ray diffraction results showing three crystallographically inequivalent CH3CN molecules in the resorcinarene lattice. The corresponding band from liquid CH3CN appears at 2254 cm-1.68,69 This hint and larger intensity of the 2263 cm-1 band comparing to the other CN bands may indicate that that band can have contribution from CH3CN molecules adsorbed at the crystal surface. Another possibility is that the CN stretching bands have different absorption coefficients. The CN bands are missing from the spectrum of desolvated CAL-Me*. The resorcinarene OH stretching bands appear in the region over 3000 cm-1. According to Alpert et al.,69 hydrogen bonding differentiates stretching frequencies of phenolic hydroxyl groups as follows: 3611 cm-1, free groups; 3599 cm-1, hydroxyls with oxygens acting as proton acceptors in HBs; 3481 cm-1, hydroxyls acting as proton donors in HBs; 3393 cm-1, hydroxyls participating in two HBs, i.e., simultaneously as proton donor and proton acceptor. Therefore, the 3398 cm-1 band of CALMe (Figure 5) can be readily assigned to the latter hydroxyl groups in agreement with the X-ray diffraction structure (Figure 3). A strong band of CAL-Me* with the maximum at 3435 cm-1

CAL-Me

νOH HB CAL

3398

δOH(ip) HB CAL νOH H2O δOH H2O νCtN CH3CN νCsO νCH3 as. νCH3 s. δCH3 as. δCH3 s. νCH of sCHRs δCH of sCHRs νbackbone of the sCHR bridge νCH arom. δCH(ip) arom. δCH(op) arom.

1288 ∼3240, ∼3290c ∼1644e 2293, 2263, 2241 1241, 1217, 1205 2970 2872 1441f 1394 2935 1341, 1325 1167, 1152 3032 1096, 1030 862, 852, 845, 835, 830 1619, 1589, 1504, 1441f

νarom. ring

CAL-Me* 3435, 3587,b 3552,b ∼3500b 1274 ∼3200d ∼1644e 1237, 1223, 1198 2967 2872 1426f 1367g 2931 1325 1172, 1149 1095, 1116, 1027 837, 825 1622, 1605, 1507, 1426f

a Wavenumbers in cm-1. Abbreviations: ν, stretching; δ, bending; s., symmetric; as., antisymmetric; ip., in plane; op., out of plane; arom., aromatic; HB, hydrogen bonded. b A weak band on the slope of the intensive band at 3435 cm-1. c Substantial overlapping with the bands at 3398 and 3240 cm-1. d Substantial overlapping with the band at 3435 cm-1. e Hidden at the base of the intensive band at ca. 1620 cm-1 from the aromatic ring. f Overlapping of the δCH3 as. and νarom. ring. bands. g Possible contribution from a δCH of sCHRs band.

Figure 5. Mid-IR spectra of solvated (CAL-Me) and desolvated (CALMe*) forms of C-methylcalix[4]resorcinarene in the spectral region over 2000 cm-1.

(located halfway between the earlier mentioned 3481 and 3393 cm-1 positions) is certainly from the OH groups, which take part in HBs. It may be indicative of the OH groups acting as proton donors and engaged only in single HBs. Another important finding for CAL-Me* is that there is no band from free OH groups at 3611 cm-1. This implies that after desolvation the hydrogen bonding is still extensive. Furthermore, it must be intramolecular, because solvent molecules (CH3CN and to a substantial extent H2O) participating in the intermolecular HB network have been removed. Then, consider that intramolecular HBs are possible only for three resorcinarene conformers, i.e., for crown, diamond, and scoop forms1,34 (4, 2, and 2 HBs, respectively). The crown conformer is the most stable because of four cooperative intramolecular HBs, and it


Figure 6. Mid-IR spectra of solvated (CAL-Me) and desolvated (CALMe*) forms of C-methylcalix[4]resorcinarene in the spectral region below 2000 cm-1.

is most frequently present in the solid state. Besides, only in this conformer all the resorcinarene OH groups are involved in HBs, so the band at 3611 cm-1 from free OH groups must be absent from its IR spectrum, as it is for CAL-Me*. Hence, it is most probable that CAL-Me* still remains in the crown conformation maintained by intramolecular HBs, while intermolecular HBs are considerably disintegrated. Considering the position of the CAL-Me* band at 3435 cm-1 (Figure 5), it must be mainly from the OH groups employed only as proton donors in the intramolecular HBs but may have some contribution from the 3398 cm-1 band assigned to the OH groups participating in two HBs (intra- and intermolecular). Three weak bands on the high-frequency side of the 3435 cm-1 band are indicative of some structural disorder in CAL-Me*. On the basis of the wavenumber, we speculate that the 3587 cm-1 band may originate from the resorcinarene OH groups, which take part in single HBs, using their oxygens as proton acceptors. In this case, a proton donor could be a hydroxyl group of the same (Figure 1) or neighboring resorcinarene molecule, and possibly from remnant water. The feeble appearance of the 3587 cm-1 band does not necessarily mean that the corresponding OH groups are rare in CAL-Me*. It is well-known that such bands are characterized by considerably smaller absorption coefficients than those from the OH groups acting in HBs as proton donors. Then, the 3552 and 3500 cm-1 bands (Figure 5) can be tentatively assigned to the resorcinarene OH groups donating protons to intermolecular HBs, formed either with residual water or with other resorcinarene molecules. As far as intracrystalline water is concerned, there are two types of water molecules in the CAL-Me structure (cf. X-ray crystallography). Each water molecule takes part in three intermolecular HBs. The oxygen atom of water is always an acceptor of a proton from one of the resorcinarene OH groups. Besides, each water molecule serves as a donor of protons into two HBs. Either both of them are formed with oxygen atoms of different resorcinarene molecules (W2) or one is created with a resorcinarene oxygen atom and the other with an acetonitrile nitrogen atom (W1). Thus, according to the crystallographic results, the water molecules W1 and W2 are involved in weaker and stronger hydrogen bonding, respectively. Consider that the stronger the HB, the farther the OH stretching band is shifted toward lower frequencies.68,69 On this basis, we assign the water

J. Phys. Chem. B, Vol. 114, No. 32, 2010 10315 bands of CAL-Me at 3290 and 3240 cm-1 to W1 and W2, respectively (Figure 5). On desolvation, CH3CN is removed together with the great bulk of water, as indicated by the disappearance of the CN bands in the 2200-2300 cm-1 range and considerable decrease of water bands in the range 3100-3350 cm-1. However, still some water is left in CAL-Me* and this remnant water produces a band at ca. 3200 cm-1. It follows that the remnant water in CAL-Me* takes part in similar hydrogen bonding as W2 in CAL-Me. There are also significant differences between the IR spectra of CAL-Me and CAL-Me* below 1700 cm-1 (Figure 6). Upon desolvation, the δOH(ip) band assigned to the OH bending vibration in the plane of the aromatic ring has moved by 14 cm-1 toward lower frequencies (Table 3). This is probably related to the disruption of the intermolecular HBs, involving the crown macrocycle of CAL-Me*. To be more specific, the oxygen atoms of the resorcinarene hydroxyl groups are less involved as proton acceptors in intermolecular hydrogen bonding, as it has already been revealed by the analysis of the OH stretching bands. The interpretation of the bands attributed to the -CH(CH3)linkages between the aromatic rings is not clear. The greatest differences in the frequencies have been found for the bending vibrations. Namely, the δCH3 frequencies of the symmetric and asymmetric modes decreased on desolvation by 27 and 15 cm-1, respectively, and the δCH band at 1341 cm-1 has completely disappeared. In addition, the bands from the -CH(CH3)backbone vibrations became considerably reduced in the CALMe* spectrum. We speculate that such severe effects must be induced by some transformation(s) of the resorcinarene crown. It is also spectacular that a new strong band emerged in the CAL-Me* spectrum at 1116 cm-1 (Figure 6). However, its identification is not straightforward. This band could be assigned either to some backbone vibration of the -CH(CH3)- bridges or to a δCH(ip) vibration of the CH groups within the aromatic rings. We opt for the latter interpretation. The 1116 cm-1 band is located too far from the reliably assigned backbone bands of the -CH(CH3)- linkages (1172 and 1149 cm-1). Then, in the CAL-Me spectrum, the δCH(ip) band at 1096 cm-1 is broader than that from CAL-Me* and it already shows some residual splitting, which for some reason is manifested in CAL-Me* by two separate bands at 1095 and 1116 cm-1. It is clear that the latter effect can result from some inequivalence of the aromatic rings, although the crown conformation is retained as is evidenced by the 3435 cm-1 band. We submit that such interpretation is consistent with the solid-state NMR results (Vide infra). 13 C CP/MAS NMR. Carbon-13 solid-state NMR spectra of CAL-Me and CAL-Me* are presented in Figure 7. The experiments were performed with CP63-65 from protons to carbon-13. The assignments were done on the basis of liquidstate chemical shifts and the CP kinetics (Tables 4 and 5). The crystals for the 13C CP/MAS NMR experiments were collected in the same manner as for the single-crystal X-ray diffraction. That is to say, a hot acetonitrile solution of resorcinarene was subjected to slow cooling, and the precipitated polycrystalline sample was taken out and cautiously dried just before measurements. We followed this procedure twice, first by crystallization of freshly synthesized resorcinarene and then by recrystallization of the desolvated material (CAL-Me*). In both cases, we have got exactly the same 13C CP/MAS NMR spectra. The CP kinetics is the dependence of the peak intensity I(t) on the contact time t. For the CAL-Me and CAL-Me* samples, the CP kinetics followed the nonclassical I-I*-S kinetic

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Kuzmicz et al. TABLE 5: Parameters of the 1H f 13C CP NMR Kinetics for Desolvated CAL-Me* Obtained by Fitting Function 1 to Experimental Peak Intensities Using the KaleidaGraph Program67 δ/ppm assignment

λ

T2/µs

Tdf/ms

H T1F /ms

18.7 20.5

C8 C8

CH3 0.45 ( 0.02 40 ( 3 0.49 ( 0.02 38 ( 2

1.1 ( 0.3 12 ( 1 1.2 ( 0.2 10.0 ( 0.6

31.7 103.2 104.9 126.7

C7 C2 C2 C5a

CH 0.44 ( 0.02 17.4 ( 0.7 0.40 ( 0.03 16.3 ( 0.8 0.34 ( 0.02 17 ( 1 0.58 ( 0.02 24 ( 2

1.1 ( 0.1 8.5 ( 0.4 1.3 ( 0.3 8.1 ( 0.5 1.0 ( 0.2 10.0 ( 0.5 0.9 ( 0.1 13.9 ( 0.8

121.3 151.3

Carbons without Adjacent Hydrogens C4, C6 0.60 ( 0.02 83 ( 5 1.7 ( 0.3 14 ( 1 C1, C3 0.61 ( 0.02 85 ( 5 1.6 ( 0.3 12.2 ( 0.8

a Admixture of the peak at 125.5 ppm from C4+C6, which is resolved only for contact times below 100 µs.

Figure 7. 13C CP/MAS NMR spectra of solvated (CAL-Me) and desolvated (CAL-Me*) forms of C-methylcalix[4]resorcinarene recorded with a contact time of 8 ms. For the enumeration of the carbon atoms, see Figure 1.

TABLE 4: Parameters of the 1H f 13C CP NMR Kinetics for Solvated CAL-Me Obtained by Fitting Function 1 to Experimental Peak Intensities Using the KaleidaGraph Program67 a δ/ppm assignment

λ

T2/µs

Tdf/ms

H T1F /ms

-4.0 -0.9 0.5 17.0 17.8 18.5 20.1 20.3

CH3CN CH3CN CH3CN C8 C8 C8 C8 C8

CH3 0.41 ( 0.01 0.44 ( 0.01 0.46 ( 0.01 0.40 ( 0.01 0.41 ( 0.01 0.40 ( 0.01 0.40 ( 0.01 0.41 ( 0.01

53 ( 2 59 ( 2 53 ( 2 59 ( 2 58 ( 2 56 ( 2 58 ( 2 56 ( 2

2.2 ( 0.2 2.1 ( 0.1 1.7 ( 0.1 2.0 ( 0.1 1.7 ( 0.1 1.4 ( 0.1 1.4 ( 0.1 1.2 ( 0.1

150 ( 30 150 ( 20 140 ( 10 100 ( 10 94 ( 7 150 ( 20 135 ( 5 150 ( 20

28.8 29.2 101.6 102.9 103.6 106.4 124.5 125.0 126.9 127.3

C7 C7 C2 C2 C2 C2 C5 C5 C5 C5

CH 0.48 ( 0.01 0.48 ( 0.01 0.49 ( 0.01 0.53 ( 0.01 0.51 ( 0.01 0.55 ( 0.02 0.53 ( 0.01 0.52 ( 0.01 0.51 ( 0.01 0.54 ( 0.02

22 ( 1 20 ( 1 19 ( 1 18 ( 1 17 ( 1 18 ( 2 19 ( 1 22 ( 2 19 ( 1 19 ( 1

1.0 ( 0.1 1.0 ( 0.1 1.0 ( 0.1 1.3 ( 0.1 1.3 ( 0.1 1.9 ( 0.3 1.7 ( 0.1 1.4 ( 0.1 1.7 ( 0.1 1.7 ( 0.1

110 ( 20 100 ( 20 150 ( 30 90 ( 20 80 ( 10 60 ( 10 180 ( 30 90 ( 10 170 ( 30 170 ( 20

124.5 125.7 126.3 126.9 127.3 128.3 128.8 149.2 150.3 151.0 152.0

Carbons without Adjacent Hydrogens C4, C6 0.53 ( 0.01 172 ( 5 1.7 ( 0.1 C4, C6 0.55 ( 0.02 169 ( 9 1.9 ( 0.1 C4, C6 0.56 ( 0.02 170 ( 10 1.8 ( 0.2 C4, C6 0.51 ( 0.01 172 ( 5 1.7 ( 0.1 C4, C6 0.54 ( 0.02 172 ( 5 1.7 ( 0.1 C4, C6 0.55 ( 0.02 179 ( 9 2.0 ( 0.1 C4, C6 0.55 ( 0.03 170 ( 10 1.9 ( 0.2 C1, C3 0.53 ( 0.02 180 ( 9 2.0 ( 0.1 C1, C3 0.54 ( 0.02 187 ( 8 2.0 ( 0.1 C1, C3 0.54 ( 0.02 172 ( 7 1.8 ( 0.1 C1, C3 0.54 ( 0.01 161 ( 6 2.0 ( 0.1

180 ( 30 180 ( 20 180 ( 20 170 ( 30 170 ( 20 190 ( 10 190 ( 10 140 ( 20 150 ( 20 147 ( 5 200 ( 20

a The peaks at 124.5, 126.9, and 127.3 contain contributions from C5 and (C4 + C6) in the ratio 69:31, 59:41, and 70:30, respectively.

model.70 In our case, the abundant spins I and the rare spins S are 1H and 13C, respectively.

According to the nonclassical I-I*-S model,70 CP begins in an isolated group of proximate, strongly coupled spins I* and S. Such a group of spins can be a spin pair I*-S or a spin cluster In*-S, consisting of n abundant spins and one rare spin. Within this model, the abundant spins do not have a common spin temperature. The initial exchange of polarization between spins I* and S proceeds in an oscillatory manner, damped by subsequent spin diffusion from distant bulk spins I. The oscillation frequency is dependent on the I*-S dipolar interaction. For rotating powder samples and/or overlapped signals from various chemical species, their CP oscillations mutually cancel out. The resulting CP kinetics is smoothed and devoid of the oscillations, and is expressed by eq 1: H I(t) ) A exp(-t/T1F )[1 - λ exp(-t/Tdf) -

(1 - λ) exp(-3t/2Tdf) exp(-t2 /2T22)] (1) H is where t is the contact time, A is the 13C peak amplitude, T1F the proton spin-lattice relaxation time in the rotating frame, Tdf is the proton spin-diffusion time constant, and T2 is the time constant of the polarization transfer. The latter parameter reflects the 1H-13C dipolar couplings71 and should not be confused with any spin-spin relaxation time. For a rigid lattice, λ ) (n + 1)-1, where n is the number of protons in close neighborhood of the observed 13C nucleus. In theory, we get λ equal to 1/2 and 1/3 for the CH and CH2 groups, respectively. In practice, λ is also dependent on a molecular and/or group motion and should be treated as an adjustable parameter.70 Function 1 corresponds to the three-stage CP kinetics (Figure 8): steep rise governed by T2 for CP within the spin cluster, slower rise governed by Tdf for CP controlled by proton spin diffusion from bulk protons to protons of the spin cluster, and H . decline due to spin-lattice relaxation governed by T1F The proton spin-lattice relaxation in the rotating frame is dependent on the material structure and molecular dynamics. It is favored by motions occurring at or near the frequency ω1 ) γ HB1H, so it probes dynamic processes in the tens of kHz range. H is a volume property averaged The proton relaxation time T1F 72 over a distance of ca. 2 nm. It follows that it has a specific value for a given solid phase. In fact, for each signal of CALH equal to ca. 11 ms (Table 5). This means Me*, we found T1F that CAL-Me* forms a single phase. In contrast, for CAL-Me, H falls in the range 60-200 ms (Table 4), which may indicate T1F


J. Phys. Chem. B, Vol. 114, No. 32, 2010 10317

Figure 8. Kinetics of the 1H f 13C NMR cross-polarization for the C1 + C3 peak of desolvated C-methylcalix[4]resorcinarene (CAL-Me*) at 151.3 ppm. For the fitted parameters, see Table 5.

that the powder sample of the solvate, analyzed using NMR, contained two or more phases. Another reason is that larger H values are measured by means of the variable-contact time T1F CP experiments with lower accuracy. As it has been expected, the CAL-Me* spectrum is devoid of the CH3CN peaks, while they are clearly displayed in the CAL-Me spectrum (Figure 7). The nitrile carbon resonances are located in the 110-120 ppm spectral region and split by the neighboring nitrogen nuclei.73 There are three methyl resonances of CH3CN, in accordance with the crystallographic results. They have similar intensities; the peaks are sharp and thereby more suitable for the structural analysis than those from the nitrile carbons. They all appear below 0.5 ppm due to considerable deshielding caused by the resorcinarene aromatic rings (Table 4). Their CP parameters are close to those of the H C8 carbons of CAL-Me. In particular, the similarity of the T1F values of CH3CN and C8 carbons indicates that the solvent molecules are well integrated into the CAL-Me phase. On the basis of the deshielding effect, the peak at -4.0 ppm (Table 4) might be assigned to the CH3CN molecules entering with their methyl groups the CAL-Me macrocycles (Figure 4), while the remaining peaks at -0.9 and 0.5 could belong to the CH3CN molecules located at the adjacent sites of the resorcinarene crown (Figures 2 and 3). We have not detected any CH3CN peak from solvent molecules adsorbed on the crystalline surface. Such NMR signal should appear at ca. 2 ppm. Since it is absent, we conclude that the CN stretching IR bands were unequal (Vide supra) because of different absorption coefficients. The CH3CN molecules of CAL-Me located at the adjacent sites of the resorcinarene crown participate in the HBs as proton acceptors, whereas those entering the crown may undergo some slow motion or suffer from orientational disorder, because they are not fixed by intermolecular HBs. Such disorder can be expected at 298 K, that is, at the temperature of the NMR measurements, and it can be associated with some CH3CN mobility and resulting dynamic macrocycle distortions. Another possible phenomenon (the authors are grateful to the anonymous referee for his valuable suggestion) would be the formation of various pseudopolymorphs, expected in complex host-guest systems.74 This could explain the above-mentioned, broad H values, and perhaps the complicated peak distribution of the T1F splittings (Figure 9), too. However, it is unclear why various pseudopolymorphs would all give the three 13C CP/MAS NMR

Figure 9. The C2, C7, and C8 regions of the 13C CP/MAS NMR spectrum of CAL-Me recorded with a contact time of 20 ms (shown on the same absolute intensity scale). The deconvolutions were done with the ACD program.66 The numbers denote relative peak areas within the specified spectral regions. The asterisk marks a spinning sideband.

peaks of CH3CN still at the same positions (-4.0, -0.9, and 0.5 ppm), unless they have a quite similar disposition of the three inequivalent CH3CN molecules against the resorcinarene macrocycles. Indeed, we have encountered in one crystallization a CH3CN/ resorcinarene solvate (further denoted as CAL-Me†; see the Supporting Information), which gave a fourth extra peak of CH3CN at -4.7 ppm, while the others appeared with the regular chemical shifts (-4.0, -0.9, and 0.5 ppm). The -4.7 ppm peak had similar parameters of the CP kinetics to the three remaining resonances and considering its chemical shift could be from a CH3CN molecule slightly deeper inserted into the resorcinarene cavity than that related to the -4.0 ppm signal. At the same time, the CAL-Me† solvate might represent a transition structure appearing in between the CAL-Me and CAL-Me* forms. In particular, in its spectrum, we found a minor peak at 31.8 ppm H value at the side of the prominent C7 peak at 28.8 ppm. The T1F of the minor peak at 31.8 ppm was at least 1 order of magnitude lower than that of the predominant peak at 28.8 ppm, and H value characfurthermore, it was practically equal to the T1F teristic for CAL-Me*. However, all reflexes of the desolvated phase were not present in the diffractogram of that particular material, so it would be inappropriate to conceive CAL-Me† to be just a solvated phase contaminated with CAL-Me*. As far as the powder X-ray diffraction is concerned, we admit that the diffractograms of the resorcinarene solvated form were

10318


considerably dependent on the crystallization condtions, in particular on the crystallization rate. This would be consistent with the concept of structural flexibility and concomitant pseudopolymorphism in this system, resulting from weak interactions between solvent molecules and resorcinarene fragments. Actually, we have collected single-crystal X-ray diffraction data at least three times and in each case we obtained the same structure although of different quality. It is likely that the solvated phase could contain monocrystals of different pseudopolymorphs. However, if it is so, we did not see significant differences in the shape of single crystals when selecting them for the crystallographic studies. The most troublesome and at the same time very informative features of the 13C CP/MAS NMR spectra of solvated CALMe and desolvated CAL-Me* are the line splittings (Figures 7 and 9). For the sake of unambiguous analysis, it is better to choose a line pattern from a single carbon, well separated from the peaks of other carbons and unaffected by any spinning sidebands. Therefore, we recommend to examine the C2, C7, and C8 regions of the CAL-Me spectrum (Figure 9). In those spectral ranges, there are four (C2 and C7) or five (C8) components with dissimilar intensities. Our X-ray diffraction studies indicated that each of those chemical positions in the CAL-Me crown (C2, C7, or C8) corresponds to four crystallographically inequivalent sites. However, this inequivalence would split the C2, C7, or C8 peaks into four components, necessarily with the same integral intensities. This is in an obvious discrepancy with the experimental NMR results. Therefore, we believe that the splitting can be reasonably rationalized in terms of the earlier discussed pseudopolymorphism and/or by some dynamic macrocycle distortions, for example, due to an orientational effect of the intramolecular HBs as proposed by Ma¨kinen et al.9 Such HB dynamics has already been postulated for tetraethylcalix[4]resorcinarene in the gas phase;75 then, similar intramolecular HB flipping was observed for C-undecylcalix[4]resorcinarene10 and calix[8]arene76 in solution, and was proved for cyclodextrins in the solid state.77 If present in solid CAL-Me, such flipping between various H-bonded structures has to be very slow, because the line splittings are not obscured by the proton dynamics. The overlapped components from those structures could produce the observed NMR line patterns. As far as CAL-Me* is concerned, the line patterns are different than those for CAL-Me (Figure 7). It is clearly seen that the C2 and C8 resonances are split into asymmetric doublets with the area ratios 26:74 and 71:29, respectively. Those ratios look utterly alike, so the reason for the splitting is probably the same. Careful analysis of the C4-C6 spectral range for contact times below 100 µs, supported by the dipolar dephasing technique, reveals a single peak from C5 at 126.7 ppm together with a doublet from C4 + C6. This doublet consists of a component at 125.5 ppm, which is overlapped with the C5 peak and resolved only for contact times below 100 µs, and a component at 121.3 ppm, clearly visible for any contact time. The C1 + C3 carbons of CAL-Me* give a single peak. In brief summary, the C1 + C3, C5, and C7 resonances are unsplit, while those from C4 + C6, C2, and C8 are split into doublets. The reasons why some carbons of CAL-Me* produce single NMR peaks while the others give doublets are rather unclear. At the moment, we can only offer and assess possible explanaH value for all of the NMR peaks tions. Almost the same T1F indicates that CAL-Me* forms a single phase. It follows that a mixture of various crystal polymorphs and/or pseudopolymorphs (hydrates) can be rather excluded. From the IR study, we know

Kuzmicz et al. that CAL-Me* remains in the crown conformation. Consequently, let us assume that CAL-Me* is composed of identical resorcinarene molecules in the crown conformation. However, it is impossible to envisage a rigid crown resorcinarene, which would give single peaks for the C1 + C3, C5, and C7 resonances and simultaneously the C2 and C8 resonances split into asymmetric doublets (Figure 7). Therefore, one has to postulate two or more different resorcinarene species and possibly a dynamic process capable of averaging out some of their 13C NMR peaks. The latter assumption is consistent with the fact that the fine structure of the CAL-Me* peaks is simpler than that of CAL-Me peaks. At first glance, this could be caused by fast intramolecular HB flipping between the crown structures, considered earlier for CAL-Me. However, a closer look at the C7 resonances from both samples (Figure 7) proves that such motional averaging could not coalesce the CAL-Me line components into the observed single CAL-Me* peak, because this peak is located outside the spectral range of those components. Therefore, if this dynamic process was operative in CAL-Me*, it alone would not produce the observed line patterns. The substantial difference in the chemical shifts of the C7 NMR peaks from the solvated and desolvated resorcinarene (ca. 3 ppm) is in accordance with the significant displacements of the IR bands assigned to the -CH(CH3)- bridges. Apparently, some important modification of the molecular structure occurred in CAL-Me* compared with CAL-Me. Perhaps the resorcinarene macrocycles released from the grip of intermolecular HBs have gained more flexibility. Besides, it cannot be overlooked that for CAL-Me* there were two δCH(ip) IR bands. We believe that at 298 K in CAL-Me* two crown resorcinarene structures in the population ratio of ca. 3:1 must coexist, with their geometries being different from those in CAL-Me, and probably undergoing some dynamic interconversion. For example, those species could differ in size of the upper rim and be interrelated by breathinglike vibration of the whole hydrogen-bonded macrocycle. However, the complete explanation requires further detailed solid-state NMR studies. One must look at the line patterns at various temperatures and monitor 13C relaxation as a function of temperature and magnetic field. Such studies are beyond the scope of this work. Overall, our present and former78-80 studies of calixarenes and resorcinarenes indicate that high-resolution solid-state NMR can provide interesting information on the structure and dynamics of those complicated systems. Conclusions The CAL-Me structure has been solved at 100 K using singlecrystal X-ray diffraction. The CAL-Me solvate crystallizes in the monoclinic P21/n space group with three CH3CN and two H2O molecules in the asymmetric part of the unit cell. The CALMe molecules adopt a typical crown conformation with all of the hydroxyl groups of the aryl rings oriented up and all of the methyl groups disposed down (the rccc isomeric form). The crystalline network is formed by resorcinarene, CH3CN, and H2O molecules, and assembled by intermolecular HBs and weak C-H · · · A or C-H · · · π interactions. The CAL-Me molecules present a slightly shifted head-to-tail columnar arrangement, which creates resorcinarene channels going along the Xdirection. Those channels are occupied by acetonitrile molecules trapped between two CAL-Me molecules. Two other acetonitrile molecules are located at the adjacent sites of the CAL-Me crown and participate in HBs as proton acceptors. The two crystallographically distinct water molecules serve both as proton donors

Structures of Solvated and Desolvated Resorcinarene and acceptors toward CAL-Me and CH3CN. The desolvated CAL-Me* sample was not suitable for the single-crystal X-ray diffraction studies. According to powder X-ray diffraction performed at 298 K, this material lost crystalline character and became partly amorphous. The spectroscopic results for CAL-Me are in fair agreement with those from diffractometry, although the studies were carried out at different temperatures: 298 and 100 K, respectively. The IR spectrum contains a band at 3398 cm-1 from the resorcinarene hydroxyls participating in two HBs, i.e., simultaneously as proton donor and proton acceptor. There are three distinct CN stretching bands at 2293, 2263, and 2241 cm-1 and two bands at 3290 and 3240 cm-1 from hydrogen-bonded structural water. The 13C CP/MAS NMR experiments showed peaks from various CH3CN molecules and revealed some contamination by the desolvated CAL-Me* phase. The NMR peaks of solvated CAL-Me exhibited splittings, which can be reasonably interpreted in terms of pseudopolymorphism and/or by some dynamic macrocycle distortions, for example, due to an orientational effect of the intramolecular HBs as proposed by Ma¨kinen et al.9 The structural information on CAL-Me* was solely provided by the spectroscopic studies. It was found from the IR spectrum that during desolvation acetonitrile had been removed together with the great bulk of water. The resorcinarene molecules remained in the crown conformation supported by intramolecular HBs, while intermolecular HBs became considerably disintegrated. The remnant water was employed for hydrogen bonding between neighboring resorcinarene molecules. The solid-state NMR studies demonstrated that CAL-Me* constituted a single phase. The detailed joint analysis of the NMR and IR spectra revealed that at 298 K in CAL-Me* two crown resorcinarene structures probably coexist in the population ratio of ca. 3:1, which undergo some dynamic interconversion. Our work directs general attention to the problem of stability and polymorphism of resorcinarene solvates. It shows that the joint use of diffractometric and spectroscopic methods is advantageous in the structural studies of complex crystalline macromolecular systems. On the other hand, the solid-state IR and NMR spectroscopic analyses applied in tandem have been found highly beneficial to elucidate the disordered structure of poorly crystalline, desolvated resorcinarene. Acknowledgment. The authors thank Medical University of Warsaw for financial support. K.W. thanks the Foundation for Polish Science for a grant for distinguished scholars (the Master Programme). Single-crystal X-ray measurements were carried out at the Structural Research Laboratory (SRL) of Chemistry Department, University of Warsaw, Poland. SRL has been established with financial support from the European Regional Development Found in the Sectoral Operational Programme “Improvement of the Competitiveness of Enterprises, years 2004-2006” project No.: WKP-1/1.4.3./1/2004/72/72/165/2005/U. Supporting Information Available: Powder X-ray diffractograms of CAL-Me† and CAL-Me* (Figure 1S), comparison of the 13C CP/MAS spectra of CAL-Me† and CAL-Me* (Figure 2S), and parameters of the 1H f 13C CP NMR kinetics for CAL-Me† (Table 1S). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Timmerman, P.; Verboom, W.; Reinhoudt, D. N. Tetrahedron 1996, 52 (8), 2663.

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MAS NMR Structural Studies of

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