Halogen bonding of N-bromophthalimide by grinding and solution

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Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

Halogen Bonding of N‑Bromophthalimide by Grinding and Solution Crystallization Published as part of a Crystal Growth and Design virtual special issue William Jones and His Contributions to Organic Solid-State Chemistry Mihael Eraković, Vinko Nemec, Tomislav Lež, Ivan Porupski, Vladimir Stilinović,* and Dominik Cinčić* Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, HR-10000 Zagreb, Croatia S Supporting Information *

ABSTRACT: A series of 10 cocrystals derived from Nbromophthalimide and nitrogen bases (pKa from 3.2 to 8.8) have been prepared by both crystallization from solution and mechanochemically (liquid-assisted grinding). All 10 structures exhibit molecular complexes interconnected by short Br···N halogen bonds involving N-bromophthalimide bromine and a nitrogen atom. The enthalpies of complexation have been calculated to range between −30 and −50 kJ mol−1, indicating fairly strong halogen bonds. Bond lengths and energies generally follow the basicity of the used halogen bond acceptor.

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halogen bond.37 Particularly potent organic halogen bond donors are N-halosuccinimides, recently introduced as halogen bond donors by Rissanen and Raatikainen.38−40 N-Halosuccinimides are commercially available compounds, commonly used for halogenation in organic synthesis,41 as they possess an extremely polarized halogen atom. Recently in our study on a large family of N-halosuccinimide cocrystals, it was demonstrated that in equivalent systemsthose that differ only in the contact (halogen/hydrogen) atom−halogen bonds with iodine atoms are stronger supramolecular interactions than hydrogen bonds.26 In spite of this, N-halosuccinimides do not seem to have been fully recognized and studied for halogen bonding, and reports of crystal structures comprising them are scarce, while other N-haloimides have received little or no attention as halogen bond donors. In this study, we now describe a family of 10 halogen-bonded cocrystals involving N-bromophthalimide (nbp) as halogen bond donor. As halogen bond acceptors, we have selected bases with a wide range of nitrogen atom basicity (pKa from 3.2 to 8.8) containing additional functional groups which may be hydrogen bond donors or acceptors (Scheme 1). Our work is based on the observation that N-bromosucinimide, as well as Niodosucinimide, forms a halogen-bonded cocrystals with piridine derivatives26,40,42,43 where N−Br···N halogen bond motif is formed.23 Up until now, nbp and its derivatives have mostly been studied and used as bromination agents or

INTRODUCTION Halogen bonding is an attractive interaction between a positively charged area of a covalently bound halogen atom (Br, I) and Lewis bases (nucleophilic atoms such as O, N, S, etc.).1−3 Over the past several decades, they have been well recognized in crystal engineering4−9 and are among the most interesting noncovalent interactions, of the family of σ-hole interactions,10−12 used for constructing supramolecular assemblies such as cocrystals,13−15 functional materials,16,17 metal− organic materials,18,19 etc. Additionally, halogen bonding is a growing area of research covering the fields of solution chemistry,20,21 biomolecular chemistry,22 as well as theoretical chemistry.23−25 It has been shown that the halogen bond strength is strongly dependent on the surroundings to which the halogen atom is bound.26,27 Most studies of halogen bonding in the solid state to date have largely focused on multicomponent solids of perfluorohalocarbons as “classic” halogen bond donors.28−32 In addition to these compounds many other alternative halogen bond donors also exist (their use in crystal design has been a subject of several recent reviews3,33), but they have generally received much less attention, even though they have demonstrated significant potential for their use in supramolecular assembly. For example, haloalkynyl moieties offer a facile means of introducing halogen bond donating ability into otherwise “halogen bond-inactive” molecules;34 dihalogen molecules (I2, Br2) form halogen bonds with Lewis bases, both as mono- and ditopic donors;35,36 aryl halides can act as halogen bond donors, but these interactions are often very weak with X···A distances significantly longer than a typical © XXXX American Chemical Society

Received: November 27, 2017 Revised: January 2, 2018 Published: January 9, 2018 A

DOI: 10.1021/acs.cgd.7b01651 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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kinetics of dehalogenation of nbp in a protic solvent (ethanol) and an aprotic one (acetonitrile). The reaction was carried out by mixing 0.5 mL of 1 mmol L−1 solution of nbp in benzene and 2 mL of ethanol (or acetonitrile) and the extent was followed by UV−vis spectrophotometry. The reaction with ethanol was found to be of the second order (pseudo-first with respect to nbp with ethanol in excess) with a rate constant of (6.83 ± 0.28) × 10−4 dm3 mol−1 s−1. We have also determined the rate constant for the systems which contained two of the pyridine derivatives used as halogen bond acceptors: 4benzoylpyridine (pKa = 3.2) and 2,4,6-trimethylpyridine (pKa = 7.3) in stoichiometric amounts, in order to observe whether their presence will affect the debromination rate, as the reaction could conceivably be base-catalyzed. The measurements have in fact shown the opposite to be the casein the presence of pyridines the reaction rate decreased, the decrease being only slight in the case of weakly basic 4-benzoylpyridine (k(4Bzpy) = (6.39 ± 0.60) × 10−4 dm3 mol−1 s−1) and more pronounced with the more basic 2,4,6-trimethylpyridine (k(col) = (4.59 ± 0.43) × 10−4 dm3 mol−1 s−1). The decrease in the rate constant observed in systems containing the pyridines could be caused by formation of stable complexes with nbp which effectively reduced the amount of nbp available for the oxidation of ethanol. Conversely, solutions of nbp in acetonitrile have proven to be stable over a period of 15−30 min, although some degradation could be observed over longer periods of time, both with and without the addition of the pyridines. Aprotic solvents can therefore be used for crystallization of nbp cocrystals, providing the crystallization is relatively fast. Ten cocrystals of nbp with the acceptors (Scheme 1) have thus been prepared by mixing high concentration solutions of nbp and the corresponding base in acetonitrile or acetone, which would yield single crystals in several minutes.

Scheme 1. N-Bromophthalimide and Halogen Bond Acceptors Used To Obtain Cocrystals

catalysts in organic synthesis.44 A cursory search of the Cambridge Structural Database45 (CSD) based on the ability of nbp to act as a halogen bond donor revealed that no cocrystal structures have been published.

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RESULTS AND DISCUSSION A possible reason for the relative under-investigation of Nbromoimides in general (and N-bromophthalimide in particular) in supramolecular chemistry lies in their chemical reactivity. Unlike perfluorohalocarbons, which are stable in most of the commonly used organic solvents, nbp is a highly reactive compound in the solution, most of all in protic solvents where it undergoes reductive solvolysis, leaving unhalogenated phthalimide as a product.46 In order to optimize our attempts to produce single crystals we have performed a study of the

Figure 1. PXRD patterns of products obtained by mechanochemical experiments (LAG) involving nbp as halogen bond donor and bases as as halogen bond acceptors. Each experimental pattern is compared to the corresponding one simulated from the crystal structures of the corresponding products (single crystals obtained from solution). B



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As the mechanochemical synthesis of N-halosuccinimide cocrystals has already been described,26,40 we have decided to screen for cocrystal formation through liquid-assisted grinding (LAG)47,48 to establish whether this method (which would circumvent the unwanted reaction of nbp with the solvent altogether) could be systematically used to obtain the same products as the solution synthesis. Powder X-ray diffraction (PXRD) patterns of all solid products obtained by grinding solid reactants for 15 min with addition of a small quantity of acetone were measured and compared with powder patterns calculated from the obtained single crystal data and found to be in good agreement, with no discernible diffraction maxima corresponding to reactants (Figure 1). Mechanochemical synthesis therefore affords quick and complete conversion of the reactants into the cocrystal, resulting in a clean product as the unwanted reactions such as halogenation and oxidation are sidestepped, avoiding unexpected products. For this reason, mechanochemical synthesis of cocrystals was also used in order to obtain the bulk product needed for thermal measurements. Molecular and crystal structure determination of the obtained single crystals revealed that in all 10 cases the molecular complexes of nbp and acceptors are formed via Br···N halogen bonds involving nbp bromine and a nitrogen atom (Figure 2). General and crystallographic data for all

weaker base than col. Unlike the halogen bond lengths (Figure 3a), the N−Br···N angles do not appear to follow an equivalent trend (smaller angles in the case of weaker bases), as some of the weakest bases employed in the study exhibit almost perfectly linear bonds (e.g., 3Brpy; pKa of 3.4 with N−Br···N angle of 179.9°) (Figure 3b). However, it is noticeable that generally there is a wider distribution of angles among the weaker bases (ca. 174−180°), while N−Br···N angles for bonds with more basic acceptors tend to cluster closer to 180° (Figure 3b). This result is hardly surprising as it can be expected that weaker bonds will be more flexible; i.e., they will be more easily distorted from the ideal geometry by the effects of crystal packing. In all cocrystals the N−Br bond in nbp molecules is extended in comparison to that observed for the pure nbp (up to 0.1 Å, Table 1). This extension of the N−Br bond correlates with the Br···N distance (Figure 4) so that structures with shorter halogen bonds show longer covalent N−Br bonds. An interesting feature appearing in structures with orthomethylated pyridines, (nbp)(2Mepy) and (nbp)(col), as well as in the structure of the quinoline cocrystal (nbp)(qin) is an apparent ancillary C−H···Br contacts between the bromine atom involved in the halogen bond and a hydrogen atom of neighboring methyl groups (i.e., other ring of the condensed bicyclic qin). It is tempting to conclude that the same bromine atom in these cocrystals acts simultaneously as a Lewis acid (XB donor) and a Lewis/Brønsted base (HB acceptor). This would be made possible by the distribution of the electron density on bromine atom with a ring of excess electron density (and therefore of negative charge) capable of acting as a hydrogen acceptor, surrounding the σ-hole responsible for the formation of the halogen bond. However, when these three complexes are analyzed by means of plotting electrostatic potential on the Hirshfeld surfaces49,50 of the nbp molecules (Figure 5), it is evident that the C−H···Br contacts in (nbp)(2Mepy) and (nbp)(col) are not in fact made with the negative region of the bromine atom, but rather that this is an incidental contact between hydrogen and the intermediate region of small (close to zero) electrostatic potential, and only slightly negative in (nbp)(qin) (Figure 5c). It therefore appears that this additional contact cannot be considered a significant contribution to the overall binding, but rather a true “nonbonding” contact brought on solely by rigidity of the acceptor molecules. In two structures, however, the negative region of the bromine atom does indeed participate in an intermolecular contact as a Lewis base. This occurs in structures of (nbp)(3Mepy) and (nbp)(nicot), where the bromine participates in contacts orthogonal to the halogen bond. In both structures the contacts are formed with carbonyl carbon atoms of parallel nbp molecules (C···Br distance 3.644 and 3.538 Å respectively) and in (nbp)(3Mepy) additional contacts with the pyridine α-carbon (3.624 Å). In both cases the electrostatic potential plotted on the Hirshfeld surfaces shows a positive value on the contact carbon (Figure 5d,e). Such an appearance of positive charge (depletion of electron density) perpendicular to a covalent bond is often referred to as a πhole,51 and has on several occasions been found to be pronounced on aromatic carbon bonded to a more electronegative atom.52,53 Therefore, (nbp)(3Mepy) and (nbp)(nicot) present two interesting instances where the same (halogen) atom acts as a Lewis acid in a σ-hole type interaction, and a Lewis base in a π-hole interaction.

Figure 2. Structure of discrete molecular complexes formed by Br···N halogen bonds.

compounds (Tables S1 and S2), as well as displacement ellipsoid plots showing the atom-labeling schemes are given in Supporting Information (Figures S1−S11). The analysis of halogen bond parameters for each of the cocrystals confirms the formation of halogen bonds, as all Br···N distances range from 2.25 to 2.49 Å and are considerably shorter than the sum of the van der Waals radii (3.4 Å) (Table 1). Generally, the halogen bond lengths decrease with the basicity of the acceptor, the longest bonds appearing in (nbp)(nicot) and (nbp)(4Bzpy) (both bases having pKa values ca. 3.2) and the shortest in (nbp)2(dabco), dabco being the strongest base employed in the study. The most notable aberration from this trend is (nbp)(col), where, in spite of relatively high basicity of 2,4,6trimethylpyridine (see above), the halogen bond is relatively long, due to steric hindrance of the two methyl groups in the ortho position. It is noteworthy, however, that (nbp)(2Mepy), having only one ortho-methyl group, does not apparently suffer from similar steric hindrance and shows markedly shorter halogen bond than (nbp)(col), in spite of 2Mepy being a C



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Table 1. Halogen Bond Lengths (d) and Angles (∠), Relative Shortenings (R.S.)a of N···Br Distances for the Prepared Cocrystals compound (nbp)2(bpy) (nbp)2(dabco)

(nbp)(nicot) (nbp)(4Bzpy) (nbp)(3Brpy) (nbp)(qin) (nbp)(2Mepy) (nbp)(col) (nbp)(3Mepy) (nbp)(35diMepy) nbp a

d(Br···N)/Å

d(N−Br)/Å

∠(N−Br···N)/deg

R.S.a/%

pKa of acceptor

2.483(2) 2.322(9) 2.257(8) 2.348(8) 2.487(4) 2.494(3) 2.449(4) 2.454(3) 2.404(4) 2.425(4) 2.476(2) 2.380(2) 2.451(2) 2.723(4)b

1.914(2) 1.939(9) 1.948(8) 1.937(8) 1.870(3) 1.880(3) 1.898(3) 1.888(3) 1.904(3) 1.906(3) 1.894(2) 1.914(2) 1.883(2) 1.836(4)

176.38(8) 179.0(4) 179.2(3) 178.9(3) 178.3(2) 174.6(1) 179.9(1) 175.4(1) 178.3(2) 179.3(2) 179.2(1) 178.60(8) 175.9(1) 168.5(2)b

27.0 31.7 33.6 30.9 26.9 26.6 28.0 27.8 29.2 28.6 27.2 30.0 27.9 19.2

3.3 8.8

3.2 3.2 3.4 4.5 5.8 7.3 5.6 6.1

Calculated as R.S. = 1 − d(X···A)/[rvdW(X) + rvdw(A)]. bFor N−Br···O contact in the structure of pure nbp

Figure 3. Geometric descriptors of Br···N halogen bonds; (a) Br···N distances and (b) N−Br···N angles in nbp cocrystals as a function of the basicity of the halogen bond acceptor.

Figure 4. Correlation between halogen bond lengths and N−Br bond lengths of the nbp molecules in the series of cocrystals.

The observed bond geometries are mirrored by halogen bond energies (computed as enthalpies for formation of nbp− acceptor complexes in vacuo) (Figure 6). The obtained enthalpies fall in the region between −30 and −50 kJ mol−1 (Table 2), the most exothermic being the formation of the 1:1 complex with the strongest base − dabco (−49.39 kJ mol−1) and least exothermic with the weakly basic 3Brpy (−29.42 kJ mol−1). As dabco is a ditopic acceptor participating in two halogen bonds with two nbp molecules in the solid state we have also computed the enthalpy for binding a second nbp molecule in order to produce the (nbp)2(dabco) complex, as found in the solid state. The enthalpy for this second stage of

Figure 5. Hirshfeld surfaces of the nbp molecules in (a) (nbp)(2Mepy), (b) (nbp)(col), (c) (nbp)(qin), (d) (nbp)(3Mepy), and (e) (nbp)(nicot) with mapped molecular electrostatic potential (calculated at HF-3-21G level of theory). Red dashed lined show the directions of closest contacts between hydrogen of the XB acceptors (in a−c) and carbonyl carbon of a neighboring nbp molecule (d and e) and nbp bromine atoms.

complexation was found to be −43.37 kJ mol−1, giving the total enthalpy of formation of the (nbp)2(dabco) complex of −92.76 kJ mol−1. The decrease in the absolute value of the enthalpy in D



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Figure 6. Geometric descriptors of Br···N halogen bonds; (a) Br···N distances and (b) N−Br···N angles in nbp cocrystals as a function of complexation enthalpy (computed for the gas phase, using B3LYP/def-2TZVP level of theory with D3 version of Grimme’s dispersion and ultrafine integration grid). The line in panel a is drawn only as a guide for the eye, and does not represent a physical correlation.

the second stage can be attributed to inductive effect of binding of the first nbp molecule which somewhat reduces the electron density on the second nitrogen atom. A similar effect is noticed also with the other ditopic acceptor used in this study (bpy) where the enthalpies for the first and second stage of the complexation are respectively −32.36 kJ mol−1 and −30.80 kJ mol−1, although the difference between the two values is much smaller (ca. 1.56 kJ mol−1 as compared to over 6 kJ mol−1 in the case of dabco), due to the reduction of the inductive effect, through the larger distance between the two donor atoms in bpy (ca. 7.1 Å, through 7 bonds) than in dabco (ca. 5.5 Å, through 3 bonds). There is an approximate linear correlation between the halogen bond lengths measured in the crystal structures and the computed energies. The three structures which were found to deviate the most from this trend are (nbp)(col), (nbp)(3Mepy), and (nbp)(nicot)the first having a substantially

Table 2. Computed Enthalpies of Complexation for nbp and Bases, in the Gas Phase with and without BSSE Correction and DSC Data, Decomposition/Melting Temperatures (Te)

a

cocrystal

ΔrH°/kJ mol−1

ΔrH°corr/kJ mol−1

Te/°C

(nbp)2(bpy) (nbp)2(dabco) (nbp)(nicot) (nbp)(4Bzpy) (nbp)(3Brpy) (nbp)(qin) (nbp)(2Mepy) (nbp)(col) (nbp)(3Mepy) (nbp)(35diMepy)

−32.36 −49.39a −31.40 −31.63 −29.42 −36.38 −37.79 −41.52 −35.04 −36.41

−30.84a −47.02a −29.86 −30.12 −27.96 −34.64 −35.94 −39.34 −33.44 −34.79

174.0 100.6 93.7 123.8 124.5 126.6 102.8 113.6 102.6 123.0

a

Values given correspond to the formation of 1:1 complex.

Figure 7. C−H···O contacts in crystal structures of (a) (nbp)2(bpy), (b) (nbp)2(dabco), (c) (nbp)(3Mepy), (d) (nbp)(35diMepy). In all figures the nbp molecules are colored orange. E



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Figure 8. C−H···O contacts in crystal structures of (a) (nbp)(nicot), (b) (nbp)(4Bzpy), (c) (nbp)(3Brpy), (d) (nbp)(2Mepy), (e) (nbp)(col), (f) (nbp)(qin). In all figures the nbp molecules are colored orange.

above, the crystal structures of all the compounds are dominated by C−H···O contacts where the nbp oxygen atoms act as hydrogen acceptors and aromatic rings (nbp, pyridine or both) act as hydrogen donors. In the structures of (nbp)(4Bzpy) and (nbp)(nicot), containing pyridine derivatives with substituents which can also act hydrogen acceptors, additional C−H···O contacts are made involving the oxygen atoms of the pyridine. In (nbp)(3Brpy), while one nbp oxygen atom participates in a C−H···O contact with a neighboring complex, the other forms a secondary Br···O halogen bonding contact of 3.075 Å with a bromine atom of the pyridine ring of another neighboring complex. These weak interactions interconnect the halogen bonded complexes into 2D in (nbp)2(bpy), (nbp)2(dabco), (nbp)(qin), (nbp)(3Mepy), (nbp)(2Mepy) or 3D networks in (nbp)(4Bzpy), (nbp)(col), (nbp)(3,5diMepy), (nbp)(nicot), (nbp)(3Brpy) (Figures 7 and 8). The absence of significant interactions between the halogen bonded complexes in the solid state has a profound effect on the thermal behavior of the cocrystals. A differential scanning

longer Br···N distance than one would expect based on bonding energy in vacuo, and the latter two displaying somewhat shorter halogen bonds (Figure 6a). In all three cases this can be brought into connection with the surroundings of the bromine in crystal packing. As described above, in (nbp)(3Mepy) and (nbp)(nicot) the halogen is in contact with the (positive) πholes of the surrounding molecules. As it has been noted earlier,26 the positive surroundings of the halogen further polarize the halogen withdrawing even more electron density, thus leading to an increase of the σ-hole and subsequently a stronger halogen bond. The opposite effect is present in (nbp)(col). Here the closest neighbors to the bromine are two (negative) nitrogen atoms above and below the plane of the complex. This pushes electron density on the bromine toward the σ-hole, thus reducing the (positive) potential of the σ-hole and yielding with a weaker (and therefore longer) halogen bond than one would expect based on the calculated bonding energy in vacuo. Apart from the supramolecular contacts involving the negatively charged portion of the bromine atom discussed F



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calorimetry (DSC) study (Table 2) of the obtained cocrystals has shown that the majority of the cocrystals melt (or decompose) in the 100−140 °C temperature range. This is considerably lower than the melting point of pure nbp (202 °C). There is no apparent correlation between either the length or the energy of the halogen bond, or the basicity of the pyridine, and the melting points of the cocrystals. Therefore, the breaking of the halogen bond does not seem to represent a significant contribution in cocrystal melting, which might indicate that halogen bonds are to a large extent preserved in the melt. This would also account for the considerable decrease of melting point upon cocrystalization, as melting of nbp necessarily involves breaking of the Br···O halogen bonds present in the crystal structure of pure nbp (Figure 9). It is

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EXPERIMENTAL SECTION

Syntheses. Single crystals of cocrystals suitable for X-ray diffraction were obtained by adding stoichiometric amounts of liquid halogen bond acceptors (qin, 2Mepy, 3Mepy, 35diMepy, 3Brpy, col) or acetone or acetonitrile solutions of solid halogen bond acceptors (bpy, dabco, nicot, 4Bzpy) to acetone solutions of nbp. Both halogen bond acceptor and nbp solutions were of high concentrations to enable fast crystallization, as over longer periods nbp decomposes in solution. Single crystals appeared over several minutes to an hour after mixing of the solutions and were immediately removed from the solution, mounted on a glass thread and covered with a protective layer of mineral oil. Crystals prepared in this way were stable at room temperature for up to several days and could be measured without noticeable decomposition during the measurement. Mechanochemical syntheses were performed by first placing 0.6 mmol of npb and a stoichiometric amount of halogen bond acceptor in a 10 mL SmartSnap stainless steel jar (produced by Form-Tech Scientific) along with two stainless-steels balls 7 mm in diameter and 40.0 μL of acetone.55 The mixtures were then milled for up to 15 min in a Retsch MM200 Shaker Mill operating at a frequency of 25 Hz. All syntheses were repeated to ensure reproducibility. Thermal Analysis. DSC measurements were performed on a Mettler Toledo DSC823e module in sealed aluminum pans (40 μL) with three pinholes in the lid, heated in flowing nitrogen (200 mL min−1) at a rate of 10 °C min−1. The data collection and analysis were performed using the program package STARe Software 14.00.56 DSC curves are given in Supporting Information, Figures S22−S32. Crystallography. Single crystal X-ray diffraction experiments. Single crystal diffraction experiments were performed at 295 K on an Oxford Diffraction Xcalibur diffractometer equipped with a Sapphire3 CCD detector, using graphite-monochromated MoKα (0.71073 Å) radiation. Programs CrysAlis CCD and CrysAlis RED were used for data collection, cell refinement, and data reduction.57 All structures were solved and refined using the SHELXT and SHELXL programs, respectively.58 Structural refinement was performed on F2 using all data. All alkyl and aryl hydrogen atoms were placed at calculated positions and treated as riding on their parent atoms. All calculations were performed using the WinGX crystallographic suite of programs.59 CCDC deposition numbers 1587752−1587762 contain crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK. Powder X-ray Diffraction. PXRD experiments were performed on a PHILIPS PW 1840 X-ray diffractometer with CuKα1 (1.54056 Å) radiation at 40 mA and 40 kV. The scattered intensities were measured with a scintillation counter. The angular range was from 3 to 40° (2θ) with a continuous step size of 0.03°, and measuring time of 0.3 s per step. Data collection and analysis were performed using the program package Philips X’Pert.60 UV−Vis Spectrophotometry. The reactivity/stability measurements of nbp in solution at 25 °C was carried out by mixing 0.5 mL of 1 mmol/L solution of nbp in benzene and 2 mL of ethanol or acetonitrile (placed in a quartz cell; l = 1 cm), and the extent was followed by UV−vis spectrophotometry by using a Varian Cary 60 spectrophotometer. We have also determined the rate constant for the systems which contained 4-benzoylpyridine and 2,4,6-trimethylpyridine used as halogen bond acceptors in stoichiometric amounts. Computational Details. All calculations were performed using Gaussian 09 (Revision D.01) software package. 61 Geometry optimization was performed using B3LYP/def-2TZVP62−64 level of theory with D3 version of Grimme’s dispersion65 and ultrafine integration grid (99 radial shells and 590 points per shell), starting from the structures obtained with single crystal X-ray diffraction experiments. This method was shown to reproduce experimental halogen bond lengths, complexation energies, and vibrational frequencies in the gas phase with good accuracy.66 Harmonic frequency calculations were performed on the optimized geometries to ensure that they are minima on the potential energy surface and to calculate Gibbs free energies and enthalpies of formation. Analogous

Figure 9. A fragment of halogen-bonded chain in the crystal structure of nbp.

noteworthy that preservation of halogen bonds upon melting of halogen bonded complexes has previously been reported by Bruce et al.54 in cocrystals of 1,4-dihalogenotetrafluorobenzenes and alkoxystilbazoles where cocrystals derived from 1,4diiodotetrafluorobenzene exhibited formation of a mesogen liquid crystalline phase, while those derived from 1,4dibromotetrafluorobenzene did not. This difference in behavior was explained by preservation of (stronger) halogen bonds in the former case, and their breaking upon melting in the latter. The preservation of the halogen bonding upon melting of nbp cocrystals, in spite of the fact that the contact atom is bromine, is yet another demonstration of the potency of N-haloimides as halogen bond donors.

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CONCLUSION N-Bromophthalimide has shown significant potential as a halogen bond donor for preparation of binary molecular solids with nitrogen bases. It forms strong (between −30 and −50 kJ mol−1) and short (26 to 33% shorter than the sum of van der Waals radii) halogen bonds, primarily dependent on the basicity of the acceptor, but also the supramolecular surroundings of the bromine. While the decomposition of nbp in solution can make the preparation of (pure) cocrystal materials somewhat difficult, and preparation of single crystals does require a little skill and some dispatch, employing mechanochemical synthesis avoids most of the problems and allows for quick and complete conversion of the reactants into the cocrystal. Mechanochemical synthesis can therefore be considered as preferred method for preparation of cocrystals derived from nbp (and N-haloimides generally). G



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calculations were performed on the monomers to obtain Gibbs free energies and enthalpies of complexation in the gas phase. Complexation energies were corrected for basis set superposition error (BSSE) using Boys−Bernardi counterpoise method67,68 without taking into account the relaxation of monomers (on the optimized geometry of the complex). Furthermore, Mulliken’s charges were determined for bromine and nitrogen atoms participating in the halogen bonding. The pKa values were calculated from molecular diagrams by ACE JChem pKa predictor.69

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01651. Experimental details, including DSC curves, PXRD patterns and single crystal diffraction data (PDF) Accession Codes

CCDC 1587752−1587762 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*(D.C.) E-mail: [email protected]; phone: +38514606362. *(V.S.) E-mail: [email protected] ; phone: +38514606371. ORCID

Vladimir Stilinović: 0000-0002-4383-5898 Dominik Cinčić: 0000-0002-4081-2420 Funding

Croatian Science Foundation (HRZZ-IP-2014-09-7367). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Croatian Science Foundation under the project IP-2014-09-7367. We are thankful to Nikola Bregović for access to UV−vis spectrophotometer.

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REFERENCES

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Halogen bonding of N-bromophthalimide by grinding and solution

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