Isomorphism between Chiral and Racemic Crystals of rac - American

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Intriguing Case of Pseudo-Isomorphism between Chiral and Racemic Crystals of rac- and (S)/(R)2-(1,8-Naphthalimido)-2-quinuclidin-3-yl, and Their Reactivity Toward I2 and IBr Simone d’Agostino,† Dario Braga,† Fabrizia Grepioni,*,† and Paola Taddei*,‡ †

Dipartimento di Chimica G. Ciamician, Università di Bologna Via F. Selmi 2, 40126 Bologna Dipartimento di Scienze Biomediche e Neuromotorie, Università di Bologna, Via Belmeloro 8/2, 40126 Bologna

‡

S Supporting Information *

ABSTRACT: Condensation reactions between 1,8-naphthalic anhydride and racemic 3-aminoquinuclidine or chiral (S) or (R)-(−)-3aminoquinuclidine allowed preparation of three novel racemic and enantiopure aza-donor ligands, namely NMiABCO (1), (S)NMiABCO (2a), and (R)NMiABCO (2b). Racemic NMiABCO (1) crystallizes in the monoclinic space group P21/c, Z′ = 1, while enantiopure (S)NMiABCO (2a) and (R)NMiABCO (2b) crystallize in the chiral monoclinic space group P2 1 , Z′ = 2, and show significant pseudocentrosymmetry, being pseudo-isomorphous with racemic NMiABCO (1). Reactivity of both racemic and enantiopure NMiABCO toward iodine and interhalogen IBr was also investigated as a way to remove the pseudoisomorphism, yielding the three new molecular adducts [NMiABCO·I2] (3), [(S)NMiABCO·I2]·xCHCl3 (4), [(S)NMiABCO·IBr]·xCHCl3 (5) and the molecular salt [HNMiABCO][IBr2] (6). Synthesis of complexes 3 and 4 was also carried out in the solid state via kneading and vapor digestion techniques. All compounds were fully characterized via single crystal and powder X-ray diffraction and Raman spectroscopy.

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INTRODUCTION Crystal engineering is a process that leads from purposedly chosen building blocks to solid aggregates in the form of extended periodical arrays. The basic idea is that of being able to predict, from a knowledge of the characteristics of the components, not only the structural features of the aggregate but also the collective physicochemical properties of the crystalline material.1 New crystalline aggregates can be obtained from solvents or melts by conventional crystallization techniques but also by mechanical mixing of solid reactants [(grinding and kneading, this last also referred to as LAG (i.e., liquid assisted grinding)] or exposure of a solid reactant to vapors of a second reactant or to solvent vapors (vapor digestion); these latter techniques, using no or very little solvent, are alternative “green” routes for the preparation of new crystal forms or materials, often not otherwise accessible from solution.2 Over the past years, we have intensely investigated the utilization of these methods in crystal engineering for the preparation of supramolecular aggregates, cocrystals, and coordination complexes.3 In the following, we report the results of a crystal engineering effort aimed to the synthesis of new chiral ligands, their isolation as racemic mixtures, and enantiopure forms and their reactivity toward iodine and the interhalogen compound IBr. The condensation reaction between 1,8-naphthalic anhydride and racemic 3-aminoquinuclidine or chiral (S) or (R)-3aminoquinuclidine (see Scheme 1) yields the racemic and enantiopure aza-donor ligands, NMiABCO (1), (S)NMiABCO © XXXX American Chemical Society

Scheme 1. Condensation Reaction between 1,8-Naphthalic Anhydride and Racemic 3-Aminoquinuclidine or Chiral (S) or (R)-3-Aminoquinuclidine (See Scheme 1) Yields the Racemic and Enantiopure Aza-Donor Ligands NMiABCO (1), (S)NMiABCO (2a), and (R)NMiABCO (2b)

(2a), and (R)NMiABCO (2b), which, upon treatment with I2 and IBr, yield three new molecular adducts (i.e., [NMiABCO· I2] (3), [(S)NMiABCO·I2]·xCHCl3 (4), [(S)NMiABCO·IBr]· xCHCl3 (5), and the molecular salt [HNMiABCO][IBr2] (6). Aza-donor molecules are important building blocks in crystal engineering because of their ability to coordinate to metal centers and to participate in hydrogen-bonding and halogenbonding (XB) formation.4 Halogen bond has been generally described as an electrostatic noncovalent interaction that arises between the electron-deficient “sigma hole”,5 localized on the Received: November 11, 2013 Revised: January 7, 2014

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dx.doi.org/10.1021/cg401686p | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Table 1. Crystallographic Data and Details of Measurements for Compounds NMiABCO (1), [(S)NMiABCO] (2a), [NMiABCO·I2] (3), [(S)NMiABCO·I2]·xCHCl3 (4), [(S)NMiABCO· IBr]·xCHCl3 (5), and [NMiABCO·H][IBr2] (6) formula fw crystal system space group Z a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Dcalc (Mg/m3) μ (mm−1) measd reflns indep reflns R1 [on F02, I > 2σ(I)] wR2 (all data)

1

2a

3

4a

5a

6

C19H18N2O2 306.35 monoclinic P21/c 4 8.4595(7) 6.9540(6) 24.8960(2) 90 98.407(7) 90 1448.8(2) 1.404 0.092 9914 3407 0.0747 0.1575

C19H18N2O2 306.35 monoclinic P21 2 8.4200(4) 6.9970(4) 25.0660(1) 90 97.582(5) 90 1463.85(1) 1.390 0.091 14047 6652 0.0744 0.1395

C19H18N2O2I2 560.15 triclinic P1̅ 2 7.281(5) 8.032(5) 16.235(4) 94.32(5) 101.21(6) 95.38(5) 923.0(7) 2.016 3.422 14030 4416 0.1049 0.3736

C19H18N2O2I2·xCHCl3 560.15b orthorhombic P212121 4 7.1867(3) 14.4780(5) 21.8500(9) 90 90 90 2273.47(15) 1.637b 2.779b 33409 5708 0.0555 0.1292

C19H18BrIN2O2·xCHCl3 513.16b orthorhombic P212121 4 7.1230(4) 13.7984(6) 22.1697(13) 90 90 90 2179.0(2) 1.564b 3.316b 18358 5188 0.0924 0.1895

C19H19Br2IN2O2 594.08 triclinic P1̅ 2 7.159(5) 8.471(5) 16.713(5) 77.212(5) 82.243(5) 84.341(5) 976.9(9) 2.020 5.746 7510 4415 0.0530 0.1510

a

These compounds are characterized by the presence of disordered solvent molecules in their structures. bFormula weight, crystal density, and absorption coefficient calculated without the solvent contribution. 0.029 mmol, respectively) in 4 mL of dichloromethane or chloroform. The solution was stirred at room temperature for 1 h, and then it was stored in the dark and allowed to slowly evaporate. Orange crystals (prisms) were recovered after 72 h and one week, respectively. [(S)NMiABCO·IBr]·xCHCl3 (5). One equivalent of (S)NMIABCO (10.1 mg, 0.035 mmol) was added to a solution of IBr (7.3 mg, 0.035 mmol) in 5 mL of chloroform. The solution was stirred at room temperature for 1 h, and then it was stored in the dark and allowed to slowly evaporate. Yellow-orange crystals (prisms) were recovered after one week. [NMiABCO·H][IBr2](6). One equivalent of NMIABCO (22.5 mg, 0.073 mmol) was added to a solution of IBr (13.1 mg, 0.073 mmol) in 5 mL of chloroform. The solution was stirred at room temperature for 1 h, and then it was stored in the dark and allowed to slowly evaporate. Yellow-orange crystals (prisms) were recovered after one week. Solid-State Synthesis. In the solid-state reactions, iodine (I2) and NMiABCO or (S)NMiABCO were manually ground together in an agate mortar in 1:1 molar ratio for ca. 10 min. In kneading experiments, a few drops of solvent (dichloromethane or chloroform) were added to the grinding mixture. Formation of the solid products 3 and 4 was confirmed by comparison of the experimental XRPD patterns with those calculated on the basis of single crystal data. Vapor Digestion Experiments. In three different experiments ca. 20 mg of NMiABCO (1), (S)NMiABCO (2a), or (R)NMiABCO (2b) were placed onto a watch glass, which was then placed inside a glass jar containing an excess of solid I2. After a few hours, the powder color turned from pale yellow to orange-yellow. The same experiments were repeated, but this time a vial containing 2 mL of dichloromethane or chloroform was also placed inside the jar (vapor digestion): the powder color turned from pale yellow to orange, indicating the formation of (3) and (4), respectively. Crystal Structure Determination. Single-crystal data for all compounds were collected at RT on an Oxford X’Calibur S CCD diffractometer equipped with a graphite monochromator (Mo Kα radiation, λ = 0.71073 Å). Data collection and refinement details are listed in Table 1. All nonhydrogen atoms were refined anisotropically; HCH atoms for all compounds were added in calculated positions and refined riding on their respective carbon atoms. SHELX977a was used for structure solution and refinement on F2. The program PLATON7b was used to calculate hydrogen-bonding interactions. CYLview7c and Mercury7d were used for molecular graphics. Channel size in 4 was evaluated with the program PLATON:7b the channels have a diameter

top of a halogen atom (XB-donor) bound to a carbon/organic substituent or to a second halogen atom and an electron-rich species carrying a lone pair or π-electrons (XB-acceptor).6 However, it is well-known that electrostatic, dispersive, and charge transfer all contribute to halogen bond formation, and the relative relevance of the different components vary from one adduct to the other. When strong halogen bond donors are used (such as I2 and IBr), the charge transfer component cannot be neglected. Crystals of racemic NMiABCO (1) and enantiopure (S)NMiABCO (2a) and (R)NMiABCO (2b) compounds show an unexpected feature, as they crystallize in a pseudo-isomorphous fashion in the monoclinic space groups P21/c, with Z′ = 1, and P21, with Z′ = 2. The analysis of the crystal structures allows a rationalization of the packing similarities in spite of the difference in chirality.

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EXPERIMENTAL SECTION

Solution Synthesis. All solvents and chemicals were bought from Sigma-Aldrich and used without further purification; doubly distilled water was used. NMiABCO (1), (S)NMiABCO (2a), and (R)NMiABCO (2b). Two equivalents of solid KOH were added to a solution of racemic (333.3 mg, 1.6 mmol) or chiral 3-aminoquinuclidinedihydrochloride in H2O (8 mL). The solution was stirred for 20 min, and then 334.6 mg (1.6 mmol) of naphthalene monoanhydride (NMa) suspended in ca. 15 mL of EtOH were added. The cream-colored suspension was refluxed for 9 h, and complete dissolution of NMa was observed. The resulting solution was cooled to RT and aqueous K2CO3 (pH ca. 11) was added dropwise until formation of a pale-yellow precipitate. The product was recovered by filtration and washed with cold H2O (5 × 2 mL) and cold EtOH (5 × 2 mL), and then it was dried overnight in a desiccator. Recystallization from dimethylformamide yielded pale yellow crystals (needles). 1: Yield = 80%. [α]rot = 0.2 ± 0.5. ESI-MS: m/z = 307.3 [M + H+]; mp = 209−210 °C. 2a: Yield = 78%; [α]rot = 0.2 ± 0.5; ESIMS: m/z = 307.3 [M + H+]; mp = 209−210 °C. 2b: Yield = 78%; [α]rot = 58.4 ± 0.5; ESI-MS: m/z = 307.3 [M + H+]; mp = 202−203 °C. [NMiABCO·I2] (3), [(S)NMiABCO·I2]·xCHCl3 (4). One equivalent of NMIABCO (20 mg, 0.065 mmol) or (S)NMIABCO (9 mg, 0.029 mmol) was added to a solution of I2 (17 mg, 0.065 mmol and 7.5 mg, B



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of ca. 7−9 Å, with a potentially accessible volume of ca. 526 Å3 (calculated using a spherical probe of radius 1.20 Å) and corresponding to approximately 23% of the unit cell volume. Powder Diffraction Measurements. X-ray powder diffractograms in the 2θ range 5−40° (step size, 0.02°; time/step, 20 s; 0.04 rad soller; 40 mA × 40 kV) were collected on a Panalytical X’Pert PRO automated diffractometer equipped with an X’Celerator detector and in Bragg−Brentano geometry, using Cu Kα radiation without a monochromator. The program Mercury7d was used for simulation of X-ray powder patterns on the basis of single crystal data. Chemical and structural identity between bulk materials and single crystals was always verified by comparing experimental and simulated powder diffraction patterns. For variable temperature experiments, the diffractometer was equipped with an Anton Paar TTK 450 system for measurements at controlled temperature. Data were collected in open air. Thermogravimetric Analysis (TGA) and FT-IR. Thermogravimetric analyses were performed with a Mettler Toledo Stare System. Heating was performed in a nitrogen flow (20 cm3 min−1) using a platinum crucible, at the rate of 5 °C min−1 up to decomposition. Sample weights were in the range of 5−10 mg. TGA was coupled with a Thermo Nicolet 6700 spectrometer to analyze the fumes that accompanied the weight loss processes. Differential Scanning Calorimetry (DSC). Calorimetric measurements were performed with a Perkin-Elmer DSC-7 equipped with a PII intracooler. Temperature and enthalpy calibrations were performed using high-purity standards (n-decane, benzene, and indium). Heating of the aluminum open pans containing the samples (3−5 mg) was carried out at 5 °C min−1 in the temperature range of 40−350 °C. Polarimetry. Measurements were performed on an UniPol L1000 Schmidt + Haensch, operating at room temperature (ca. 20 °C), using cuvettes with a path length of 1 dm. Solvent was spectroscopic grade methanol (C. Erba) and sample concentrations were [NMiABCO] = 5.8·10−3 g mL−1, [(S)NMiABCO] = 4.2·10−3 g mL−1, and [(R)NMiABCO] = 4.7·10−3 g mL−1. Raman Spectroscopy. Raman spectra were recorded on a Bruker MultiRam FT-Raman spectrometer equipped with a cooled Ge-diode detector. The excitation source was a Nd3+-YAG laser (1064 nm) in the backscattering (180°) configuration. The focused laser beam diameter was about 100 μm, and the spectral resolution was 4 cm−1. The reported spectra were recorded with a laser power at the sample of about 60 mW. No sample degradation upon laser irradiation under these conditions was observed (actually, the reported spectra are coincident with those recorded at laser powers as low as 1 mW).

Figure 1. Optical microscope images of single crystals of NMiABCO (1), (S)NMiABCO (2a), and (R)NMiABCO (2b).

Compounds (S)NMiABCO (2a) and (R)NMiABCO (2b), on the other hand, crystallize as chiral crystals9 in the monoclinic space group P21. The asymmetric unit contains two independent molecules (Z′ = 2), which are clearly related by a pseudo-inversion center (see Figure 2b). Although this is not an uncommon feature in crystals of enantiopure compounds,10 what makes the situation more interesting is the very close structural similarity with the packing of 1. Without single crystal X-ray diffraction data (i.e., on powder data only), it would be impossible to distinguish between racemate or conglomerate9 formation. The structure of 2 is characterized by the same columnar arrangement of the naphthalimide moieties along the b axis, see figure 2d (distance between the ring planes along the column ca. 3.39 Å). Hence, the pseudocentrosymmetry in 2a and 2b and the close similarity of the packings accounts for the pseudoisomorphicity with the centrosymmetric structure of 1. The X-ray powder diffraction patterns of 1, 2a, and 2b are shown in Figure 3. For all compounds, the experimental patterns correspond to the simulated ones and are also mutually superimposable (even if some subtle difference is present at high angle), indicating, de facto, that the three compounds are essentially isostructural. It is quite uncommon for enantiopure and racemic crystals of a given compound to possess unit cell parameters and packing features so strikingly similar that they can be considered as pseudoisomorphous phases. To the best of our knowledge, there are only two other examples in the literature showing such behavior (i.e., an imine of 4-hydroxybenzohydrazide11a and carvedilol phosphate hemihydrate).11b The observed pseudoisomorphism can be explained, at least in this case, by considering that (i) the packing is dominated by π-stacking interactions between naphthalimide moieties, (ii) the ABCO substituents behave as “bulky spheres” that occupy almost the same positions within the crystal and with almost the same spacial electron density, irrespective of chirality. As a result, the packing arrangements, hence powder diffraction profiles of the racemic and enantiopure crystals, are extremely similar. The most visible difference between the two packings is in the relative position, with respect to the naphthalimide planes along one column, of the N-atoms located on the substituents (cyano spheres in Figure 2b). We decided to take advantage of the affinity of nitrogen for iodine; if halogen bonds can be formed between the NABCO atom and iodine or IBr, and if the columnar stacking arrangement of the aromatic groups is maintained also in the resulting NMiABCO···XY complex (X = I, Y = I, or Br), then it should be possible, in principle, to remove the pseudoisomorphism and obtain different crystalline forms for NMiABCO···XY and (R)- or (S)NMiABCO···XY. Reactivity toward Iodine. When compounds 1 and 2a are reacted with I2, two new crystalline materials are obtained, namely [NMiABCO·I2] (3) and [(S)NMiABCO·I2]· xCHCl3

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RESULTS AND DISCUSSION Before proceeding with the discussion of the reactivity toward I2 and IBr, we address the issue of the structural relationship between racemic and enantiopure crystals of NMiABCO. Racemic and chiral NMiABCO were synthesized (see Experimental Section) within a recent project on cocrystallization as a means to tune solid state luminescence of the 2-(1,8naphthalimido)ethanoic acid (NEaH), variously functionalized with acid or base hydrogen bonding groups.8 Single crystals of NMiABCO (1), (S)NMiABCO (2a), and (R)NMiABCO (2b) were obtained by recrystallization from dimethylformamide. Pictures of the three compounds are shown in Figure 1. Compound NMiABCO (1) crystallizes as a true racemate9 in the centrosymmetric monoclinic space group P21/c with Z′ = 1 (see Figure 2a). The main packing feature of crystalline 1 is the columnar stacking along the b axis, as shown in Figure 2c, which clearly takes advantage of the flat shape of the naphthalimide aromatic moiety (distances between the ring planes along the column ca. 3.40 and 3.35 Å). C



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Figure 2. Content of the asymmetric unit in crystalline (a) racemic 1 and (b) chiral 2a and columnar stacking of naphthalimide moieties in crystalline (c) 1 and (d) 2a; N-atoms on the quinuclidinyl substituents are depicted in cyan to highlight the difference in their spatial orientation.

In the iodo-complexes the NABCO···I distance [2.40 (2) Å for 3 and 2.355(6) Å for 4] is shorter than the sum of the van der Waals radii for nitrogen and iodine. The I−I···N angle is approximately linear [179.2(4)° for 3 and 177.8(2)° for 4], see Figure 5. As a result of electron density donation from the lone pair on nitrogen to the antibonding orbital of iodine, the I−I bond is weakened; this is the case for compounds 3 and 4, in which the I−I distance is elongated [2.840(2) Å and 2.851(8) Å for 3 and 4, respectively] with respect to elemental iodine (2.715 Å).12 An additional interaction is detected between neighboring I2 molecules in crystalline 3 (see Figure 5a); given a I···I distance of 4.149(2) Å and I−I···I angle (θ in Figure 5a) of 134.63(6)°, this can be classified as a weak halogen···halogen interaction of type I in trans geometry.13 The same kind of stacking arrangement of the naphthalimide moieties observed in the parent structures can also be detected in crystalline 3 and 4, with the stacking running parallel to the a axis direction (see Figure 6). The crystal structure of 4 is highly porous and contains disordered solvent molecules that could not be located from an electron density map; monodimensional channels (see Experimental) can be seen running parallel to the a axis direction, as it is shown in Figure 7. TGA/FT-IR (see Figure SI-1 of the Supporting Information) confirmed the presence of chloroform as the crystallization solvent. Variable temperature powder diffraction experiments were used to investigate the desolvation process (see Figure 8). Upon heating, a polycrystalline sample of 4 up to 160 °C an abrupt change of the diffraction pattern is observed, together with a color change from deep yellow to dark brown: a desolvated form 4_desolv is obtained in which the iodine is, presumably, still bound to the nitrogen atom (see Raman Spectroscopy). The original phase 4 can be restored when 4_desolv is dissolved in or exposed to vapors of chloroform (see Scheme 2). Unfortunately, the quality of diffraction data for the desolvated phase was not sufficient to allow indexing and structural resolution. Solid State Process. Solid I2 was ground with NMiABCO (1) and (S)NMiABCO (2a) either in dry conditions or in the presence of a tiny amount of solvent (kneading). Reactivity of I2 vapors with NMiABCO (1) and (S)NMiABCO (2a) was also investigated with vapor digestion

Figure 3. Comparison between X-ray powder patterns either simulated (black lines) on the basis of single crystal data and experimental (red lines) for compounds NMiABCO (1), (S)NMiABCO (2a), and (R)NMiABCO (2b).

(with x(estimated) < 1) (4). Single crystals (see Figure 4) were obtained by slow evaporation of dichloromethane (3) or

Figure 4. Optical microscope images (40×) of single crystals of (a) [NMiABCO·I2] (3) and (b) [(S)NMiABCO·I2]·xCHCl3 (4).

chloroform (3 and 4) solutions. While 3 is a true racemate9 and crystallizes in the centrosymmetric triclinic space group P1̅, 4 crystallizes in the chiral orthorhombic space group P212121. Apart from space groups considerations, the two crystals are now profoundly different in their packing (see below): as a result of iodine complexation, therefore, the pseudoisomorphism is lost. D



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Figure 5. (a) Halogen bonding involving the iodine molecule and the ABCO nitrogen in crystalline (a) 3 and (b) 4; an inversion center is located midway from the I2···I2 interaction in crystalline 3.

Figure 6. Columnar stacking observed in crystalline 3 (left) and 4 (right).

Scheme 2. Schematic Representation of the Solvation/ Desolvation Behaviour of Crystalline 4

Figure 7. Space-filling representation of crystalline [(S)NMiABCO· I2]·xCHCl3 (4) down the crystallographic a axis, evidencing the monodimensional channels accommodating the disordered CHCl3 solvent molecules (not shown here).

In all cases, the complex formation was confirmed by comparing the X-ray powder diffraction pattern of the resulting polycrystalline product with the one calculated on the basis of single crystal data (see Figures 9 and 10 for compounds 3 and 4, respectively). In both cases, grinding only resulted in a physical mixture of the starting material (the patterns are essentially those of 1 and 2a) and only the addition of a few drops of solvent followed by manual grinding (kneading) allowed for compounds 3 and 4 to be obtained. In the gas−solid reactions, exposure of polycrystalline samples of 1 and 2a to vapors of I2 resulted in the formation of an amorphous phase, which turned into crystalline 3 and 4 upon exposure to vapors of dichloromethane (3) or chloroform (4). Reactivity Toward IBr. As mentioned above, 2a and 1 were also reacted with the interhalogen compound IBr, yielding the crystalline materials [(S)NMiABCO·IBr]·xCHCl3 (5) and (b) [HNMiABCO][IBr2] (6), respectively (see Figure 11). Of these two latter complexes, 5 is the only one containing the interhalogen compound IBr. Crystalline 5 is isomorphous with the iodine complex 4, but the NABCO···I distance [2.28(1)

Figure 8. Experimental XRPD pattern for crystalline 4 at RT (in black) and for the desolvated form 4_desolv at 160 °C (in blue).

experiments, both in the presence and in the absence of solvent vapors. E



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A comparison of the experimental and calculated XRPD patterns for compound 5 is reported in Figure SI-3 of the Supporting Information. The reaction between 1 and IBr unexpectedly yielded a salt consisting of the protonated NMiABCO and the [Br−IBr]− anionic species. The IBr2− ion is usually synthesized by the reaction of I− with Br2.15 We would like to comment on the fact that, at least with the systems described here, we have experienced a general problem of reactivity concerning IBr, often leading to scarce reproducibility of results or to unexpected compounds. In this case, the formation of the IBr2− ion may be the result of the heterolytic dissociation of IBr,16 or of a series of reactions, involving moisture water, which result in formation of HIBr2 followed by the acid−base reaction with compound 1. The [IBr2]− anion is almost linear [Br−I−Br = 179.21(3)°] and with I−Br distances of 2.661(2) − 2.733(2) Å. Geometric parameters are consistent with those extracted from a search in the Cambridge Structural Database (I−Br distances in the range 2.3−2.8 Å).17 The crystal structure is also featured by bifurcated hydrogen bonds between the protonated quinuclidinyl moieties and the Br atom from the dibromoiodate anion [N−H+···Br = 2.838(1) − 2.893(1) Å], π-stackings are also present in the crystal structure, see Figure 12. Raman Spectroscopy. Raman spectra of racemic NMiABCO (1) and enantiopure (S)NMiABCO (2a) (see Figure SI-4 of the Supporting Information) are very similar, confirming their similar crystal packing. Raman spectroscopy proved a powerful tool for the structural characterization of I2-adducts of electron donor species (D); the extent of the charge transfer from D to I2 may be evaluated through the analysis of the spectral range below 200 cm−1, where the νI−I stretching modes have been reported to fall.18 On the basis of the I−I bond distance (d) and bond order (n) (which may be estimated according to the Pauling’s equation19), the I2-adducts may be classified into weak or medium-weak adducts (type 1 D···I2 adducts; d in the 2.72− 2.85 Å range, n > 0.6) and strong adducts (type 2 D−I−I threebody system adducts; d about 2.90 Å, bond orders of both D−I and I−I in the 0.4−0.6 range), according to Deplano et al.18 The XRD data obtained for the adducts 3 and 4 allow to calculate n values of 0.64 and 0.62, respectively (i.e., at the borderline between type 1 and type 2 adducts). However, the presence of two stretching bands, observed in the Raman spectra of the synthesized adducts (Figure 13 and Figure SI-5 of the Supporting Information) at about 110 cm−1 (ν1 symmetric N−I−I stretching) and 150 cm−1 (ν3 antisymmetric N−I−I stretching), not present in the donors, suggests the existence of a N−I−I three-body system, according to the assignments proposed by Arca et al.20 From this point-of-view, the I2-adducts under study appear significantly different from those obtained by other authors using other N-donors, since they show N···I distances shorter than most complexes reported in the literature,21 according to the strong n → σ* donor character of quinuclidine, related to the high pKa value of its conjugate acid.22 Pyridine-, pyrazine-, and quinoxaline-based donors have been reported to be characterized by N···I distances significantly higher than ours (i.e., 2.562, 2.817, and 2.994 Å, respectively);23−25 not unexpectedly, these adducts show in their Raman spectra a single band,23,24 in agreement with their weak nature. The ν3 wavenumber value of the three I2-adducts (Figure 13 and Figure SI-5 of the Supporting Information) decreased on

Figure 9. Crystalline 3: comparison of the XRPD patterns calculated on the basis of (a) single crystal data and measured on the solid products obtained by (b) grinding, kneading with (c) dichloromethane, (d) I2 vapor digestion, and (e) I2 vapor digestion in the presence of dichloromethane vapors.

Figure 10. Crystalline 4: comparison of XRPD patterns calculated on the basis of (a) single crystal data and measured on the solid products obtained by (b) grinding, (c) kneading with chloroform, (d) I2 vapor digestion, and (e) I2 vapor digestion in the presence of chloroform vapors.

Figure 11. Pictures of single crystals of (a) [(S)NMiABCO·IBr]· xCHCl3 (5) and (b) [HNMiABCO·H][IBr2] (6).

Å] is shorter than in the case of compound 4. Analogously to 4, also in 5, the Br−I···N angle is approximately linear [178.2°] (see Figure SI-2 of the Supporting Information). As an effect of the coordination to the nitrogen atom, the I−Br bond [2.521(5) Å]14 is elongated to a value of 2.678(2) Å. As in the case of crystalline 4, the crystal structure of 5 is highly porous and contains disordered solvent molecules (see discussion on Raman spectroscopy), that could not be located from the electron density map. F



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Figure 12. (a) Dimeric hydrogen-bonded units in crystalline [NMiABCO·H][IBr2] (6) and (b) columnar stacking observed in crystalline 6. HCH omitted for clarity.

Figure 13. Raman spectra of 1 (black, bottom) and adduct 3 (red, top). The spectra are normalized to the intensity of the band at 1589 cm−1 (aromatic νCC). The main bands of quinuclidine (■) and naphthalimide (●) moieties that underwent changes upon adduct formation are indicated.

going from 4_desolv (152 cm−1) to 3 (146 cm−1) and 4 (142 cm−1). This trend is in agreement with the XRD data obtained on 3 and 4, which disclosed in the former a shorter I−I bond distance than in the latter. The ν3 value of 4_desolv, for which no reliable XRD data are available, could suggest that the I−I bond strength was even stronger than in the adduct 3. However, this deduction is only a hypothesis, since no linear correlation has been found between the ν3 wavenumber value and the I−I bond distance for strong adducts.26 Also in the interhalogen IBr-adduct (adduct 5), two stretching bands were detected (see Figure SI-6 of the Supporting Information), suggesting that also in this case, the adduct is classifiable as strong (type 2) and should be treated as an N−I−Br three-body system. In accordance with other authors,26,27 Δd(I−Br) values [i.e., Δd(I−Br) = d(I−Br)adduct − d(I−Br)in gas phase, where d(I−Br)in gas phase = 2.485 Å28] lower than 0.18 Å are indicative of medium-weak adducts, while Δd(I−Br) values comprised between 0.18 and 0.34 Å identify strong IBr-adducts. The XRD data obtained on the adduct 5 [d(I−Br)adduct = 2.678(2) Å] allowed for the calculation of a Δd(I−Br) of about

0.19 (i.e., in the range identified as indicative of strong adducts), in agreement with the Raman findings. Upon adduct formation, several changes were observed also in the bands due to the NMiABCO donor. The CH2 stretching vibrations of the quinuclidine moiety observed at 2854−2920− 2943 and 2864−2922−2943−2952 cm−1 in NMiABCO (1) and (S)NMiABCO (2a), respectively, shifted to higher wavenumber values in all the analyzed adducts (Figure 13 and Figures SI-5 and SI-6 of the Supporting Information); in agreement with Messina et al.,29 the observed changes indicate a higher positive charge (i.e., a more acidic character) on the quinuclidine hydrogen atoms, consistent with the nitrogen atom working as Lewis base through a n → σ* electron donation to iodine in the adducts. In agreement with Santos and De Mello,30 upon adduct formation several bands assignable to quinuclidine increased in intensity (Figures 13 and Figures SI-5 and SI-6 of the Supporting Information). The symmetric CO stretching vibration, observed at 1697 cm−1 in both 1 and 2a, underwent a slight shift toward higher wavenumber values in the adducts (i.e., toward a less hydrogenbonded state); this behavior may be explained by considering G



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that in the ligands, all CO groups are involved in hydrogen bonds, while in the adducts some of them become free, leading to an increase in the νCO wavenumber value. Also other bands due to naphthalimide31 underwent changes upon adduct formation (see Figure 13 and Figures SI-5 and SI-6 of the Supporting Information).

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CONCLUSIONS In this paper, we have discussed the results of a crystal engineering investigation of halogen-bonded adducts of dihalogen (I2) and interhalogen (IBr) with the novel racemic and enantiopure ligands rac- and (S)/(R)2-(1,8-naphthalimido)-2-quinuclidin-3-y, which have been synthesized by condensation between 1,8-naphthalic anhydride and racemic 3-aminoquinuclidine or chiral (S)- or (R)-(-)-3-aminoquinuclidine. The compounds NMiABCO (1), (S)NMiABCO (2a), and (R)NMiABCO (2b) have been isolated and structurally characterized in the solid state by single crystal X-ray diffraction. Unexpectedly, the crystals of racemic NMiABCO (1) (P21/c, Z′ = 1) were found pseudoisomorphous with those of the enantiopure (S)NMiABCO (2a) and (R)NMiABCO (2b) compounds (P21, Z′ = 2). The analysis of the crystal structures has allowed for the rationalization of the similarity on the basis of the pseudo centrosymmetric arrangement of the enantiopure molecules. Although pseudoisomorphism is not new to the field,10a we envisage implications and possible applications in crystal engineering, particularly for the synthesis of mixed crystals with modulated chirality. Work is in progress concerning this aspect. Upon treatment with dihalogen (I2) and interhalogen (IBr) 1, 2a, and 2b yield four new complexes with formula [racNMiABCO·I2] (3), [(S)NMiABCO·I2]·xCHCl3 (4), [(S)NMiABCO·IBr]·xCHCl3 (5), and [NMiABCO·H][IBr2] (6), which have also been characterized by X-ray diffraction and Raman spectroscopy.

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ASSOCIATED CONTENT

S Supporting Information *

TGA/FTIR measurement for compound [(S)NMiABCO·I2]· xCHCl3 (4); crystal structure, packing diagram, comparison between simulated and experimental powder patterns for compound [(S)NMiABCO·IBr]·xCHCl3 (5); crystallographic information files (cif) for all structures described herein; Raman spectra for compounds 1, 2a, 4, 4_desolv, and 5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the University of Bologna. We are grateful to Dr. Elena Dichiarante (PolyCrystalLine s.r.l.) for the TGA-FT-IR measurements.

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REFERENCES

(1) (a) Braga, D.; Grepioni, F. Chem. Commun. 2005, 3635−3645. (b) Desiraju, G. R. Angew. Chem., Int. Ed. 2007, 46, 8342−8356. H



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Isomorphism between Chiral and Racemic Crystals of rac - American

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