Structural Diversity in a Family of Quasi-Tetrahedral Organic Molecules

May 20, 2008 - Molecules: From Van Der Waals Solids to Helices and Molecular. Complexes. Michael G. Siskos,* Adonis Michaelides, Antonios K. Zarkadis,...
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CRYSTAL GROWTH & DESIGN

Structural Diversity in a Family of Quasi-Tetrahedral Organic Molecules: From Van Der Waals Solids to Helices and Molecular Complexes

2008 VOL. 8, NO. 6 1966–1971

Michael G. Siskos,* Adonis Michaelides, Antonios K. Zarkadis, Nikolaos I. Tzerpos, and Stavroula Skoulika* Department of Chemistry, UniVersity of Ioannina, 45110 Ioannina, Greece ReceiVed July 17, 2007; ReVised Manuscript ReceiVed February 22, 2008

ABSTRACT: The crystal structure and packing of a series of six nearly tetrahedral organic compounds 1–6, of general formula PhNH-CAr1Ar2Ar3 [1(Ph, Ph, Ph), 2(Ph, Ph, 2-Py), 3(Ph, Ph, 3-Py), 4(Ph, Ph, 4-Py), 5(Ph, 2-Py, 2-Py), and 6(2-Py, 2-Py, 2-Py)], were investigated by X-ray crystallography and supported, in some cases, by ab initio calculations, IR spectroscopy, and 1H NMR. All molecules have in common an aniline group, but they differ in the number of phenyl groups that are replaced by pyridyl ones (1, 2, 5, 6) or the position of the nitrogen heteroatom at the pyridyl ring (2–4). Purely van der Waals solids are obtained when the molecule can form an intramolecular N-H · · · N hydrogen bond (2, 5, 6), and, in this case, a correlation between melting point and structure was attempted. However, when intermolecular N-H · · · N bonds were established, helices (4) or tetrameric (two organic molecules + two solvent molecules) molecular complexes (3) were obtained. Introduction The packing of organic crystals depends on the interplay of various noncovalent interactions, through which the molecular units are mutually recognized to reach an equilibrium structure.1 The precise control of the assembly process is of fundamental importance for the rational synthesis of new molecular materials2 and for the realization of molecular devices.3 However, understanding how the various noncovalent forces cooperate to assemble a molecular edifice is a difficult task because the supramolecular behavior of a chemical functional group depends also on the nature and location of all other functional groups.4 For example, the progressive addition of nitrogen heteroatoms into a naphthalene ring has important consequences on the crystal structure and packing. Hence, systematic studies of molecules of similar or nearly similar shape differing by the number, nature, or location of functional groups or even heteroatoms would be useful for understanding the cooperative process leading to the formation of the crystal.5 In the present work, we investigate crystal packing differences brought about by (a) progressive substitution of phenyl by pyridine rings (1, 2, 5, 6 in Scheme 1) and (b) by modification of the relative position of the nitrogen heteroatom at the pyridyl ring (2, 3, 4, in Scheme 1) in a family of six nearly tetrahedrally shaped N-substituted anilines. The molecules 1–6 were initially synthesized to study the homolytic C-N bond fission as an alternative free radical generation method.6 During purification by recrystallization from methanol, it was observed that compound 3 always retained solvent in its structure. This observation prompted us to investigate, in a systematic way, their crystal chemistry and supramolecular behavior. We show that when the nitrogen atom in a pyridyl ring is in ortho position (compounds 2, 5, and 6) an intramolecular N-H · · · N hydrogen bond is always formed, and the packing is essentially governed by van der Waals interactions. In case of compounds 5 and 6, the molecular conformation is further stabilized by the formation of three additional weak intramolecular hydrogen bonds of the type C-H · · · N. As expected, the prototype triphenylmethyl derivative 1 is also a van der Waals solid. However, when the * To whom correspondence should be addressed.

Scheme 1

nitrogen atom is in meta position (3), intermolecular N-H · · · O and O-H · · · N hydrogen bonds between the molecules of the N-substituted aniline and methanol are established, leading to formation of a tetrameric (two molecules + two solvent molecules) cyclic molecular complex. Finally, when the nitrogen atom is in para position (4), an intermolecular N-H · · · N hydrogen bond is formed leading to a helical arrangement around a 21 axis. Experimental Section Compounds 16a and 2–46b,d were prepared according to our earlier published procedures. N-[Phenyldi(2-pyridyl)methyl]benzenamine (5).6b A solution of 0.40 g (1.4 mmol) of phenyl-di(2-pyridyl)methyl chloride and 1.4 cm3 (15.4 mmol) of aniline was stirred at 100 °C for 2 h and then diluted with 10 cm3 of ether. The solution was hydrolyzed with a basic solution NaOH/NaHCO3 (pH ≈ 10). The aqueous solution was extracted with Et2O (2 × 10 cm3), and the combined organic extracts were washed

10.1021/cg700661r CCC: $40.75  2008 American Chemical Society Published on Web 05/20/2008

6 5

C23H19N3 337.41 monoclinic P2(1)/n a ) 10.457(1) b ) 16.233(2) c ) 11.322(1) β ) 110.17(1) 1804.0(3) 4 1.242 712 3955/3142 [R(int) ) 0.0684] 98.7% 3142/0 /311 1.025 R1 ) 0.0608 wR2 ) 0.1378 0.216 and -0.287 3657.8(7) 8 1.222 1424 3477/3170 [R(int) ) 0.0234] 98.6% 3170/0/239 1.028 R1 ) 0.0521 wR2 ) 0.1076 0.142 and -0.200

4

C24H20N2 336.42 monoclinic C2/c a ) 19.648(2) b ) 9.126(1) c ) 20.943(2) β ) 104.48(1) 3636.0(6) 8 1.229 1472 3445/3152 [R(int) ) 0.0204] 98.6% 3152/0/239 1.015 R1 ) 0.0498, wR2 ) 0.1007 0.141 and -0.238

3

C24H20N2 · CH3OH 368.46 monoclinic P2(1)/c a ) 14.406(2) b ) 8.486(1) c ) 17.492(2) β ) 104.93(1) 2067.1(5) 4 1.184 784 3986/3523 [R(int) ) 0.0262] 90.9% 3523/0/257 1.099 R1 ) 0.0666 wR2 ) 0.1631 0.248 and -0.282

2

C24H20N2 336.42 orthorombic Pbca a ) 17.096(2) b ) 12.461(1) c ) 17.170(2)

with water (2 × 10 cm3) and dried over Na2SO4, and the solvent was evaporated. The residue was crystallized by adding few drops of MeOH, obtaining 0.33 g (70%) of 5; mp 175–177 °C (MeOH); λmax (Et2O)/nm 249 (ε/dm3 · mol-1 · cm-1 17300); νmax (KBr)/cm-1 3360 (N-H); 1H NMR(CDCl3) δH ) 6.48 ppm (3H, m, aniline-Hortho, Hpara), 6.93 (2H, t, aniline-Hmeta, 3Jm,p)7.25 Hz), 7.11–7.65 (11H, m, ArH) and 8.56 (2H, d, 2-PyH); 13C NMR(CDCl3), δC ) 70.15 ppm (C-N) and 115.38–162.95 (HCar, Car) (Found: C, 81.85; H, 5.53; N, 12.48. C23H19N3 requires C, 81.87; H, 5.67; N, 12.45%). N-[Tri(2-pyridyl)methyl]benzenamine (6).6b According to the above procedure (preparation of 5) from 0.16 g (0.57 mmol) of tri(2pyridyl)methyl chloride and 0.7 cm3 (7.68 mmol) of aniline, 0.11 g (57%) of 6 was obtained as white crystal, recrystallized from MeOH; mp 214–16 °C; νmax (KBr)/cm-1 3350 (N-H); 1H NMR(CDCl3), δH ) 6.51–6.53 ppm (3H, m, aniline-Hortho,para), 6.94 (2H, t, aniline-Hmeta), 7.12–7.14, (3H, m, 5-PyH), 7.58–7.71 (6H, dm, 3,4-PyH), 8.62 (3H, m, 6-PyH); 13C NMR(CDCl3) δC ) 68.65 ppm (C-N) and 116.26–150.62 (HCar, Car) (Found: C, 77.89; H, 5.47; N, 16.83. C22H18N4 requires C, 78.08; H, 5.36; N, 16.56%). Physical Measurements. 1H, and 13C NMR spectra were measured in CDCl3 solution at 25 °C on a Bruker AC-300 or AMX-400 spectrometer at 300 and 400 and 75 and 100 MHz, respectively. IR spectra were measured on a Perkin-Elmer FT-IR SpecrumGM as KBr pellets. Elemental analyses were performed on a Perkin-Elmer 2400CHN. Melting points were determined using a Buchi 510 apparatus. All solvents and chemicals were obtained commercially and used as received. X-ray Crystallography. The X-ray diffraction data were collected at room temperature on a Bruker P4 diffractometer, employing graphite monochromated Cu KR radiation (λ ) 1.54178 Å) for 3 and Mo KR radiation (λ ) 0.71073 Å) for 1–2 and 4–6 . The data were corrected for Lorentz and polarization effects. The structures were solved by direct methods and refined on F2 using SHELXL97. All non-hydrogen atoms were refined anisotropically. The hydrogen atom of the secondary anilino group was located from difference Fourier maps and refined isotropically. All remaining hydrogen atoms were positioned on calculated positions and refined by using the riding model. No absorption correction was applied. Theoretical Calculations. Molecular calculations on 2-pyridyl derivatives (2, 5, 6) were performed first at the semiempirical AM1, PM3 level, by using as initial parameters the atomic coordinates derived from X-ray crystallographic data. The calculated structures are used at the restricted Hartree–Fock level 3-21G and 6-31G basis set and DFT/ b3lyp/6-31G, using the Gaussian98 program.7 The optimized structures were followed by vibrational analysis in order to verify that the obtained stationary points correspond to true minima.8

Results and Discussion

largest diff peak and hole (e Å-3)

completeness to 2θ data/restraints/parameters goodness-of-fit final R indices [I > 2σ(I)]

volume (Å3) Z calculated density (g/cm-3) F(000) reflections collected/unique

1 compound

C25H21N 335.43 monoclinic P2(1)/n a )10.505(1) b ) 15.134(1) c ) 35.445(4) β ) 92.88(1) 5628.0(9) 12 1.188 2136 11511/9799 [R(int)) 0.0414] 99.0% 9799/0/715 1.011 R1 ) 0.0607 wR2 ) 0.1349 0.168 and -0.236

The preparation of compounds 5 and 6 was performed according to the reaction:

empirical formula formula weight crystal system space group unit cell dimension (Å) (deg)

Table 1. Crystallographic and Structure Refinement Data for Compounds 1-6

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C22H18N4 338.40 monoclinic P2(1)/n a )10.417(1) b ) 16.150(1) c ) 11.209(1) β ) 112.05(1) 1747.8(3) 4 1.286 728 3846/3046 [R(int) ) 0.0353] 98.8% 3046/0/239 1.050 R1 ) 0.0582 wR2 ) 0.1116 0.161 and -0.221

Family of Quasi-Tetrahedral Organic Molecules

Single crystals of compounds 1-6, suitable for X-ray analysis, were obtained by slow evaporation of methanol solutions. Crystallographic data and refinement parameters for all compounds are given in Table 1. The geometrical characteristics of the hydrogen bonds are presented in Table 2. The melting points and relevant spectroscopic data for all compounds are shown in Table 3.

1968 Crystal Growth & Design, Vol. 8, No. 6, 2008

Siskos et al.

Table 2. Experimental Hydrogen Bonding Parameters for Compounds 2-6 compd

type of bond

2 3

intra inter tetramer

4 5

inter helicoidal chain intra

6

intra

X-H · · · Y

X · · · Y (Å)

∠ X-H · · · Y (deg)

N20-H · · · N13 N20-H · · · O27 O27-H · · · N10 N20-H · · · N5 N20-H · · · N13 C9-H · · · N3 C15-H · · · N3 C7-H · · · N20 N20-H · · · N13 C9-H · · · N3 C15-H · · · N3 C7-H · · · N20

2.625(2) 3.048(3) 2.846(3) 3.257(2) 2.600(3) 3.070(4) 2.945(4) 2.801(4) 2.604(2) 3.032(3) 2.942(3) 2.795(3)

115(2) 172(3) 175(3) 138(2) 120(2) 110(2) 117(2) 103(2) 113(2) 111(2) 116(2) 103(2)

Table 3. Packing Coefficients, Melting Points and Relevant Spectroscopic Quantities for Compounds 1-6 compd

packing coefficient %

melting point (°C)

N-H stretch experimental (cm-1)

1 2 3 4 5 6

65.9 66.8 65.4 67.1 66.6 68.2

148–149 157–158 87, 143a 179–180 175–177 214–216

3411 3370 3310 (broad)b 3270 3342 3350

a

N-H stretch HF/3–21Gc (cm-1) 3360 3344 3342

1

H NMR δN-H (ppm) 4.99 in the aromatic range 4.95 5.02 in the aromatic range in the aromatic range

The first value corresponds to the departure of methanol; b This band includes also the νO-H stretching vibration; c Scale factor 0.8929.

Figure 1. Molecular structures of compounds 1 (a), 2 (b), 5 (c), and 6 (d). All molecules have the same atom numbering scheme. Nitrogen atoms and H-bonds are depicted in green. The N-H · · · N intramolecular bond is common in compounds 2, 5, and 6, possessing a nitrogen atom in the ortho position. In case of compound 1, only one of the three molecules of the asymmetric unit is shown.

All molecules are nearly tetrahedral.9 Compounds 1, 2, 5, and 6 are van der Waals solids, of which only 5 and 6 are isostructural. As the size of the molecules is essentially the same, we ascribe the differences in crystal packing to significant differences in the shape of these molecules. In compound 1

(Figure 1a), the asymmetric unit is composed of three symmetryindependent molecules possessing similar geometrical characteristics. Apart from van der Waals contacts no other intra- or intermolecular interactions are observed in this solid. This is also consistent with the νN-H stretching vibration of 1 at 3411

Family of Quasi-Tetrahedral Organic Molecules

Crystal Growth & Design, Vol. 8, No. 6, 2008 1969

Table 4. Calculated Hydrogen Bonding Parameters for the Intramolecular N-H · · · N Hydrogen Bond in Compounds 2, 5, and 6, Using Various Levels of Approximationa compound

mode of determination

N · · · N (Å)

∠ X-H · · · Y (deg)

2

X-rays HF/3-21G HF/6-31G DFT B3LYP/6-31G X-rays HF/3-21G HF/6-31G DFT B3LYP/6-31G X-rays HF/3-21G HF/6-31G DFT B3LYP/6-31G

2.625(2) 2.622 2.630 2.613 2.600(3) 2.574 2.595 2.581 2.604(2) 2.584 2.597 2.585

115(2) 113.2 113.3 114.9 120(2) 115.2 114.4 116.2 113(2) 115.3 114.5 116.4

5

6

a For comparison reasons the experimental values, obtained by X-ray crystallography, are given.

Figure 2. Crystal structure of 3. A centrosymmetric H-bonded tetramer (two organic molecules + two methanol molecules) is shown. Two pyridyl rings, belonging to different molecules, are strictly parallel between them with an interplanar distance of 3.37 Å and a slipping distance of 2.42 Å. Oxygen atoms are colored in red.

cm-1 (Table 3), which is characteristic of the secondary anilino N-H group stretching vibration, free of hydrogen bonding (3400–3500 cm-1).10

In compounds 2, 5, and 6, the asymmetric unit contains only one molecule. All three compounds have in common a nitrogen heteroatom (N13) at the ortho position of a pyridyl ring (Figure 1b,d). In all cases, we observe an intramolecular N-H · · · N hydrogen bond with a N · · · N distance of 2.625, 2.600, and 2.604 Å for 2, 5, and 6, respectively, leading to the formation of a five-membered ring. The frequency lowering of the N-H stretching vibration of the ortho-derivatives 2, 5, and 6, with respect to the “free” N-H stretching in 1 (Table 3) indicates that the N · · · N contacts observed correspond to hydrogen bonds. As expected, the frequency shift is inversely proportional to the length of the N · · · N contact, the N-H · · · N angle being essentially the same. The relatively low value of the N-H · · · N angle prompted us to calculate the energy of these bonds by ab initio Hartree–Fock and DFT methods. The equilibrium conformation was calculated first by semiempirical methods (AM1 and PM3), by using as initial parameters those obtained by X-ray crystallography, and then, the computation was completed by HF(3-21G and 6-31G) and DFT(B3LYP/ 6-31G) methods. All three molecules reproduce very well the experimental conformation found by X-ray crystallography (Tables S1-S3, Supporting Information and Table 4). Moreover, the N-H stretching frequencies calculated by HF(3-21G) are in good agreement with the results of IR spectroscopy (Table 3). An estimation of the hydrogen bond energy was obtained by evaluating the energy difference between the abovementioned conformation and the conformation obtained when the pyridyl ring which participates in the intramolecular hydrogen bond is rotated by 180° around the C1-C8 bond. By this rotation the formation of the hydrogen bond is inhibited, while the intramolecular van der Waals contacts remain essentially the same. The hydrogen bond energy thus estimated is around 7 kcal/mol consistent with the existence of a moderately weak hydrogen bond. It is noteworthy that the relative IR shift is less than 10%, in the range that characterizes usually the weak hydrogen bonds.12 This bond persists even in CDCl3 solutions as was evidenced by the investigation of the deshielding values observed for the N-H protons, by 1H NMR spectroscopy. In case of compounds 2, 5, and 6 these values

Figure 3. Crystal structure of compound 4. The molecules interact by an intermolecular N-H · · · N bond (left); In this way hydrogen bonded helices are formed around the 21 axes (right).

1970 Crystal Growth & Design, Vol. 8, No. 6, 2008

fall in the aromatic area (7–8 ppm), while for compound 1 for which no such bond is possible, the corresponding value is 4.99 ppm (Table 3). The good agreement between the experimental molecular structure and that determined by ab initio calculations, in the gas phase, indicates that the molecular conformation is essentially governed by intramolecular interactions. The abovementioned intramolecular hydrogen bond has some consequences on the shape of the molecules. Inspection of Figures 1–3 reveals that the part of the molecule consisting of the anilino group the central sp3 carbon atom and the pyridyl ring, involved in the intramolecular hydrogen bond, adopts a more planar conformation in compounds 2, 5, and 6, than in 1, 3, and 4. Moreover, the planarity appears reinforced in compounds 5 and 6. The essential difference in intramolecular contacts between 2, 5, and 6, is the presence in 5 and 6, of multiple C-H · · · N contacts (Figure 1). These contacts satisfy the geometrical criteria, generally accepted, for weak C-H · · · N hydrogen bonds (Table 2).11,12 However, the relatively low C-H · · · N angles might rise some doubt, especially in case of intramolecular contacts, whether these contacts are real hydrogen bonds.12 In the present case, we estimate that they are weak hydrogen bonds, accounting for the quasi-planarity observed in 5 and 6. As the melting point depends on the strength of the crystal lattice (intermolecular forces, molecular symmetry, and conformational degrees of freedom of the molecule),13 we started by comparing compounds 5 and 6, which are isostructural, and consequently any differences in their melting points should be ascribed to differences in the strength of intermolecular forces. As shown in Table 3, compound 6 has a significantly higher packing coefficient than 5 reflecting a decrease of intermolecular distances, when passing from 5 to 6, with a subsequent increase of intermolecular forces and melting points. These differences are due to the presence of the polarizing pyridyl nitrogen atom (N19), which, being free of intramolecular hydrogen bonding, can be fully available for intermolecular electrostatic interactions. Compound 2 is not isostructural to 5, so their comparison is not as straightforward as before. We observe that these compounds have approximately the same packing coefficient, but their melting points differ by about 18 degrees. This difference is due, at least partly, to the presence of the polarizing nitrogen atom (N3), which, in this case, is also involved in weak intramolecular hydrogen bonds. For compound 1, the absence of any nitrogen heteroatom on the rings is reflected on an even lower melting point. This is further supported by the observation that compound 1 has more conformational freedom than 2, 5, and 6, in which a part of the molecule is “locked” by intramolecular H-bonding. We observe striking structure differences when the nitrogen heteroatom is in a meta (3) or in a para (4) position. In both cases it is not possible for an intramolecular N-H · · · N bond to be formed. In compound 3 the organic molecule cocrystallizes with methanol in a 1:1 ratio, to form a centrosymmetric cyclic hydrogen bonded tetramer (Figure 2). In our opinion, methanol is included in the lattice to satisfy the H-bonding requirements of the organic molecules which, for steric reasons, cannot be directly self-assembled. Methanol acts both as a hydrogen bond donor and hydrogen bond acceptor (Table 2). Two methanols bridge two organic molecules to form the tetramer. This arrangement is further stabilized by aromatic interactions between two strictly parallel pyridyl rings (Figure 2). Solution NMR spectra show that the hydrogen bonds do not persist in solution (Table 3). The complex is decomposed at 80 °C and melts at 143 °C, but, as the structure of the noncomplexed solid is unknown, no structure/melting point correlation could be

Siskos et al.

done. The “open” molecular structure of 3 is also reflected in its relatively low packing coefficient (Table 3). In compound 4, the location of the nitrogen heteroatom on a para position allows intermolecular N-H · · · N hydrogen bonds to be established between organic molecules (Table 3 and Figure 3). In this way, helices around the 21axes, are formed. There are four helices in the unit cell but the presence of inversion centers rules out any polar property. The structure is stabilized by numerous van der Waals contacts and, as a result, there is no appreciable void volume and the melting point of this compound is relatively high (Table 3). Solution 1H NMR spectra show that the hydrogen bonds do not persist in solution (Table 3). As a conclusion, we showed that apparently small changes on a nearly tetrahedral molecular edifice can bring about severe modifications on crystal packing and structure. The main driving force for these modifications is the ability of the molecule to form intra- or intermolecular hydrogen bonds. However, in all cases numerous van der Waals contacts contribute to stabilize the structure. In particular, the molecular recognition motifs that lead to cyclic tetrameric complexes or molecular helices could provide adequate “design elements” to assemble other similar solids. Acknowledgment. We thank the Authorities of the region of Epirus for the purchase of the X-ray equipment. Supporting Information Available: Three tables containing experimental and calculated values of bond lengths, bond angles, and dihedral values for compounds 2, 5, and 6. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) (a) The Crystal as a Supramolecular Entity; PerspectiVes in Supramolecular Chemistry, Vol. 2; Desiraju, G. R., Ed.; Wiley: Chichester, UK, 1995. (b) Etter, M. C. J. Phys. Chem. 1991, 95, 2209. (c) Supramolecular Organization and Materials Design; Jones, W.; Rao, C. N. R., Eds.; University Press: Cambridge, 2002. (d) Organic Molecular Solids: Properties and Applications; Jones, W., Ed.; CRC Press: Boca Raton, FL, 1997. (e) Sharma, C. V.; Panneerselvam, K.; Pilati, T.; Desiraju, G. R. J. Chem. Soc., Perkin Trans. 1993, 2, 2209. (2) (a) Holman, K. T.; Pivovar, A. M.; Ward, M. D. Science 2001, 294, 1907. (b) Fernandez-Lopez, S.; Kim, H. S.; Choi, E. C.; Delgado, M.; Granja, J. R.; Khasanov, A.; Kraehenbuehl, K.; Long, G. Nature 2001, 412, 452. (c) Brunet, P.; Simard, M.; Wust, J. D. J. Am. Chem. Soc. 1997, 119, 2737. (d) VanDelden, R. A.; Koumura, N.; Harada, N.; Feringa, B. N. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 4945. (3) Ruben, M.; Payer, D.; Landa, A.; Comicso, C.; Lin, N.; Collin, J.-P.; Sauvage, J.-P.; DeVitto, A.; Kern, K. J. Am. Chem. Soc. 2006, 128, 15644. (4) Desiraju, G. M., Vögtle, F., Stoddart, J. F., Shibaski, M., Eds.; In Stimulating Concepts in Chemistry; Wiley-VCH: New York, 2000. (5) (a) Pigge, F. C.; Dighe, M. K.; Rath, N. P. Cryst. Growth Des. 2006, 6, 2732. (b) Vergadou, V.; Pistolis, G.; Michaelides, A.; Varvounis, G.; Siskos, M.; Boukos, N.; Skoulika, S. Cryst. Growth Des. 2006, 6, 2486. (6) (a) Siskos, M. G.; Garas, S. K.; Zarkadis, A. K.; Bokaris, E. P. Chem. Ber. 1992, 125, 2477. (b) Tzerpos, N. I. Ph.D. Thesis, University of Ioannina, Greece, 1995. (c) Tzerpos, N. I.; Zarkadis, A. K.; Kreher, R. P.; Repas, L.; Lehnig, M. J. Chem. Soc. Perkin Trans. 2 1995, 755. (d) Siskos, M. G.; Tzerpos, N. I.; Zarkadis, A. K. Bull. Soc. Chim. Belg. 1996, 105, 759. (7) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,

Family of Quasi-Tetrahedral Organic Molecules P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, ReVision A.9; Gaussian, Inc.: Pittsburgh, PA, 1998. (8) Ab initio calculations were carried out also on compound 1, and the calculated geometry is close to that obtained by X-ray crystallography. (9) In the six compounds studied, the angles C21-C1-C2, C21-C1-C14, and C21-C1-C8, are in the range of 99°-107°, 90°-97°, and 128°133°, respectively. For purely tetrahedral molecules see. (a) Lloyd,

Crystal Growth & Design, Vol. 8, No. 6, 2008 1971

(10) (11) (12) (13)

M. L.; Brock, C. P. Acta Crystallogr. Sect. B. 1997, B53, 773. (b) Lloyd, M. L.; Brock, C. P. Acta Crystallogr. Sect. B. 1997, B53, 780. For example, the prototype PhNHCH3 gives in the gas phase νN-H ) 3460cm-1 (NISTChemistryWebBook:http://webbook.nist.gov/chemistry/). Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997. Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond; Oxford University Press: New York, 1999. Katritzky, A. R.; Jain, R.; Lomaka, A.; Petrukhin, R.; Maran, V.; Karelson, M. Cryst. Growth Des. 2001, 1, 261.

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