Cocrystal or Salt: Solid State-Controlled Iodine Shift in Crystalline

Article pubs.acs.org/crystal

Cocrystal or Salt: Solid State-Controlled Iodine Shift in Crystalline Halogen-Bonded Systems Olena Makhotkina,† Julien Lieffrig,† Olivier Jeannin,† Marc Fourmigué,*,† Emmanuel Aubert,‡ and Enrique Espinosa*,‡ †

Institut des Sciences Chimiques de Rennes (ISCR), Université Rennes 1, UMR CNRS 6226, Campus de Beaulieu, 35042 Rennes, France ‡ Laboratoire CRM2, UMR CNRS 7036, Institut Jean Barriol, Université de Lorraine, BP 70239, 54506 Vandoeuvre-les-Nancy, France S Supporting Information *

ABSTRACT: The distinction between cocrystals and salts is usually investigated in hydrogen-bonded systems as A−H···B ⇆ [A]−···[H−B]+, where the position of the hydrogen atom actually defines the ionicity of the complex. The same distinction, but in halogen-bonded systems, is addressed here, in complexes formed out of N-iodoimide derivatives as halogen bond donors, and pyridines as halogen-bond acceptors, anticipating that the position of the iodine atom in these A−I···B ⇆ [A]−···[I−B]+ systems will also define their degree of ionicity. We show that the crystalline halogen-bonded complexes of N-iodosuccinimide (NIS) with pyridine, 4-methylpyridine, and 4-dimethylaminopyridine can be described as “close-to-neutral” cocrystals while the crystalline halogen-bonded complex of N-iodosaccharin (NISac) with 4dimethylaminopyridine adopts a “close-to-ionic” structure. Theoretical calculations were performed (i) in gas phase on isolated NIS···Py-NMe2 and NISac···Py-NMe2 complexes, and (ii) on the periodic crystal phases, and combined with the topological analysis of the electron density distribution ρ(r). We demonstrate unambiguously that the crystal environment actually plays a crucial role in the stabilization of the “close-to-ionic” structure of the NISac···Py-NMe2 complex. An external homogeneous electric field ε applied to this complex (all atoms frozen at the crystalline geometry, except iodine) in either gas phase (ε = 3.7 GV m−1) or periodic pseudo-isolated configuration (ε = 2.8 GV m−1) is able to shift the iodine atom at the crystal geometry, miming the polarization effect induced by the local crystal electric field. The strong influence of the crystalline environment on the iodine position is demonstrated by using plane wave DFT periodic calculations on optimized NIS·Py-NMe2 and NISac·PyNMe2 crystal structures, as well as by applying this plane wave basis set formalism to a hypothetical solid where the halogenbonded complexes are pushed apart from each other within a periodic system.

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INTRODUCTION The definition of cocrystals and salts is currently the subject of an active debate,1−3 in connection with applications in the pharmaceutical field.4,5 Indeed, the modifications of the solid state properties (solubility, morphology, hygroscopicity, and so forth) of a given active pharmaceutical ingredient (API) often involve the formation of ionic salts between hydrogen-bonded acids and bases. When the ΔpKa (ΔpKa = pKa(base) − pKa(acid)) exceeds 2 or 3, a salt is indeed formed, while when it is smaller than 0, a neutral cocrystal is most often isolated.6,7 The distinction between cocrystals and salts become more difficult in the intermediate ΔpKa region,8 but in all situations, one is faced with a hydrogen-bonded system, A−H···B ⇆ [A]−···[H−B]+, where the position of the hydrogen atom between A and B atoms actually defines the nature (neutral or ionic) of the bimolecular complex.9−11 Another intermolecular interaction, namely, halogen bonding,12−14 has been shown to compare in many ways with hydrogen bonding.15 Halogen bonding interactions with Lewis bases find their origin in the anisotropic charge density © XXXX American Chemical Society

distribution around a covalently bound halogen atom, with an electrophilic zone which develops in the prolongation of the C−X bond, called the σ-hole.16−20 A recent definition of halogen bonding is now available from IUPAC.21 This interaction was investigated in the 1950s by Hassel and others,22,23 from the reaction of dihalogens with Lewis bases. More recently, combined theoretical and experimental efforts have given to halogen bonding a strongly renewed interest, 24−27 as illustrated by numerous reviews and books.28−33 This interaction today finds uses in many different domains, such as crystal engineering,34−36 molecular materials (conducting,37−42 magnetic,43−45 photoactive,46,47 liquid crystals,48,49 gels,50 and polymers51), supramolecular chemistry,52 as well as organic catalysis,53−58 molecular biology, and drug design.59−62 From a thermodynamic point of view, this interaction has been shown to compare in many instances Received: April 17, 2015 Revised: May 25, 2015

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DOI: 10.1021/acs.cgd.5b00535 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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In crystalline NIS,88 a short N−I···OC XB is indeed found (2.58 Å, 26% reduction with respect to the sum of the van der Waals radii), while several halogen-bonded cocrystals are also reported with halides,89 amines,90 and imines.91 Similarly, NiSac crystallizes with water, THF, pyridine, or pyrazine, within every structure a very short and linear N−I···O or N−I···N′ halogen bond.92 We therefore postulated that such strong halogen bond donors, when faced with Lewis bases such as pyridines, also known to exist as N−iodonium cations, could be the most appropriate candidates to eventually observe, in the solid state, a continuous evolution from cocrystal to salt, with an iodine shift analogous to a proton shift, according to the equation: A−I···B ⇆ [A]−···[I−B]+. We describe below our first results along these lines, from a combination of structural and theoretical investigations.

with normal hydrogen bonds, and a halogen bond basicity scale toward I2 (acting as XB donor) has been developed63 and correlated to structural data.64 Considering these close analogies between hydrogen and halogen bonding, and the many examples of competitive situations associating both interactions,65−73 we wondered if a situation comparable to the hydrogen-bonded cocrystal vs salt system (Scheme 1a) could be extended to an analogous, halogen-bonded, cocrystal vs salt system, where the moving hydrogen atom would be replaced by a moving halogen atom, according to Scheme 1b. Scheme 1

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RESULTS AND DISCUSSION Cocrystals of NIS and NISac were investigated with three pyridines of different halogen acceptor strength, namely, pyridine itself (Py), 4-picoline (Py-Me), and 4-dimethylaminopyridine (DMAP, Py-NMe2). It is indeed anticipated that the electron releasing substituents in the para position of pyridine will increase the charge on the sp2 nitrogen atom of the pyridine derivative and favor a stronger halogen bonding.64 With NIS, we systematically obtained halogen-bonded complexes with a 1:1 stoichiometry (Figure 1), a very short

This situation is essentially observed only in the product of the reaction of dihalogens such as I2, Br2, Cl−I, or Br−I with thiones74−77 or pyridines,78−81 affording for example the ionic N-iodopyridium salts, halogen bonded to the halide anion (Scheme 2a). Other related examples involve strongly covalent, Scheme 2. Ionic, Halogen-Bonded Systems

Figure 1. Detail of the structure of the halogen-bonded NIS···PyNMe2 complex, as a representative example of the NIS complexes (see Table 1). N···I distances are given in Å.

symmetrical systems, but with charge conservation (Scheme 2b), as found for example with the central iodine atom in the triiodide anion, or in bis(pyridine)iodonium cations.82−85 Our strategy to favor an ionic form through halogen transfer between the neutral halogen-bonded cocrystal and the ionic, halogen-bonded salt, that is, A−I···B ⇆ [A]−···[I−B]+, is based on the analogy with the hydrogen bonded systems. The ability to delocalize the positive charge brought by the moving iodine should indeed play an important role in stabilizing the [I−B]+ species. Similarly, the stability of the [A]− species after iodine transfer is another important point.86 These combined requirements led us to consider the solid state properties of N-iodoimide derivatives in their interaction with pyridines of varying Lewis base character. Indeed, as noted by Rissanen,87 N-iodosuccimide (NIS) and N-iodosaccharin (NiSac) (Scheme 3) are powerful halogen bond donors: “the structures presented (with these N-iodoimides) contain some of the strongest halogen bonds found up to now. The strong interactions are a consequence of the strong polarization of the nitrogen-bound halogen by the imide group”.

and linear N−I···N′ halogen bond (Table 1), corresponding to an averaged reduction ratio (relative to van der Waals radii taken at 1.55 for N and 1.98 Å for I)93 of 0.70 with Py to 0.68 with Me2N-Py. This strengthening can be noted also with other complexes of Py-NMe2, for example, with perfluorodiodobenzene derivatives.94,95 Altogether, the three NIS systems behave at first sight as strong but “normal” halogen-bonded cocrystals. Moving now to the NISac complexes, the pyridine derivative, NISac•Py, had already been reported earlier,92 but the authors noted that “the modest quality of the crystal of NISac•Py resulted in large R1 and wR2 residualsLarge residual electron density was found in the final difference electron density maps”. Our own investigations on single crystals of NISac•Py confirm the reported unit cell parameters but showed a very complex diffraction pattern with two sets of incommensurate superstructures.96 Clearly thus, the reported structure cannot be fully trusted, albeit it indicates a tendency toward a much stronger halogen bond than with NIS (Table 2). In order to clarify this point, the NISac complexes with Py-Me and Py-NMe2 were also prepared and structurally characterized (Figure 2). As shown in Table 2, the I···N′ distances are now extremely short (2.2−2.3 Å), when compared with the NIS complexes (2.4−2.5 Å). Also, the N···N′ distances are much shorter (4.50 Å) than in the NIS complexes (4.55−4.60 Å). This is also accompanied by a concomitant lengthening of the initially “covalent” N−I bond of N-iodosaccharin. All data confirm that the N-

Scheme 3. Structures of the Strong Halogen Bond Donors

B



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Table 1. Room Temperature Geometrical Distances (Å) and Angles (deg) of the NNIS−I···N′R‑Py Halogen Bond in the Cocrystals with NISa Pyb Py-Meb Py-NMe2

d1 (N−I)

d2 (I···N′)

d1 − d2

N−I···N′

d1 + d2

2.116(5) 2.144(8) 2.116(4) 2.142(4) 2.146(4)

2.493(8) 2.430(8) 2.483(4) 2.428(4) 2.407(4)

−0.377(9) −0.286(11) −0.367(6) −0.286(6) −0.261(6)

180 180 180 180 178.9(1)

4.609(9) 4.574(11) 4.599(6) 4.570(6) 4.553(6)

a

d1 = N−I distance, d2 = I···N′ distance, d1 + d2 = N···N′ distance (the interaction is linear). bTwo crystallographically independent NIS•Py or NIS•Py-Me complexes, both located on twofold axes.

Table 2. Geometrical Distances (Å) and Angles (deg) of the NNISac−I···N′R‑Py Halogen Bond in the Cocrystals with NISaca T (K) b

Py Py-Me Py-NMe2 a

293 293 150 293 150

d1 (N−I) 2.254(11) 2.220(3) 2.223(4) 2.292(1) 2.292(2)

d1 − d2

N−I···N′

d1 + d2

−0.084(4) −0.078(6) +0.064(1) +0.074(3)

174.5(4) 178.2(1) 178.1(1) 178.8(1) 178.5(1)

4.52(2) 4.523(4) 4.523(6) 4.520(1) 4.509(3)

d2 (I···N′) 2.279(11) 2.304(3) 2.301(4) 2.228(1) 2.218(2)

a

Distances are defined as in Table 1. bFrom ref 92. X-ray data quality is poor, due to the presence of superstructures in the diffraction figure. See text.

d1 + d 2 = 2d02 + (d1 − d 2) + 2b ln[1 + exp{(d01 − d02 − d1 + d 2)/b}]

(1)

where d1, d2, (d1 + d2), and (d1 − d2) are distances given in Tables 1 and 2, and d01, d02, and b are adjustable parameters. This correlation, which follows from the bond valence-bond length concept well established in crystallography,97 was originally developed for hydrogen-bonded systems.98 Recently, the same expression has been extended to theoretically calculated halogen-bonded systems.99,100 While very good correlations were found for F−Cl···CNX (R2 = 0.994)99 and F−Cl···CNY (R2 = 0.989)100 complexes involving chlorine atom as halogen bond donor, only rough and bad correlations were respectively observed for bromo- and iodo complexes F− Br···CNY (R2 = 0.945) and F−I···CNY (R2 = 0.804) complexes. Attempts to improve the quality of the fit switching from the original model (eq 1) with a four-parameter model afforded only side improvements.100 Here, the Steiner-Limbach plot for the five complexes (Figure 3) leads to an experimental data fitting having a very good correlation coefficient (R2 = 0.986, with b = 0.37(6) Å, d01 = 2.00(3) Å, and d02 = 2.01(6) Å), indicating that eq 1 can be extended to halogen-bonded systems even when involving highly polarizable halogen atoms. To our knowledge, this is the first experimental assessment in the halogen bonding context. The fitted reference values of d01 and d02 would correspond to the N−I and I−N′ distances in isolated N-iodoimide and N-iodopyridinium. In the monomers optimized with DFT calculations (see Supporting Information for full details), the latter distances are found to be 2.046 Å in NISac, 2.059 Å in NIS, 2.094 Å in N-iodopyridinium, 2.091 Å in N-iodo-4-methylpyridinium, and 2.080 Å in N-iodo-4dimethylaminopyridinium, respectively. This situation is strongly reminiscent of the cocrystal-salt distinction found in hydrogen-bonded systems. For example, solid state associations of saccharin itself acting as a Brønsted acid with pyridine derivatives adopt almost invariably an ionic form,101−103 while the only reported example of an association of succinimide with pyridine derivatives is a neutral, hydrogen bonded system.104 As stated by Bond,5 this distinction “can also be problematic, especially for chemically similar crystals where there might be a transition from neutral to charged molecular

Figure 2. Details of halogen-bonded complexes obtained between NISac and (a) 4-picoline (Py-Me) and (b) 4-dimethylaminopyridine (Py-NMe2), at room temperature. N···I distances are given in Å.

iodosaccharin is a much stronger halogen bond donor than Niodosuccinimide. Furthermore, we note also that in NISac•Py-NMe2, the socalled I···N′ halogen bond is actually shorter than the so-called N−I “covalent” bond of NISac. In other words, the iodine atom is closer to the sp2 nitrogen atom (N′) of Py-NMe2 than to the nitrogen atom of saccharin, providing a very appealing situation with a potentially predominant ionic form, when compared with the halogen-bonded neutral cocrystal form (Scheme 4). The two interatomic distances N···I (d1) and I···N′ (d2) have been correlated, for the six systems except NISac•Py, with the Steiner-Limbach relationship (eq 1) Scheme 4. Analogy between Iodine Transfer in the NISac•Pyridine System and Hydrogen Transfer in a Carboxylic Acid•Pyridine System

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Table 3. N−I and I···N′ Distances with Optimized Position of the Iodine Atom in NIS•Py-NMe2 and NISac•Py-NMe2 Bimolecular Complexes from Various DFT Functionals and at Experimental Geometries With NIS With NISac

functional

N−I (Å) vs I···N′ (Å)

B3LYP-D3 Experimental B3LYP-D3 TPSSTPSS-D3 PBE1PBE-D3 PBEPBE-D2 Experimental

2.1263 < 2.4260 2.146(4) < 2.407(4) 2.1625 < 2.3468 2.1697 < 2.3395 2.1509 < 2.3582 2.1724 < 2.3369 2.292(2) > 2.218(2)

bond.106−111 Accordingly, the iodine position in the NISac•PyNMe2 outlier was also optimized upon the influence of varying external ε fields oriented along the N···N′ direction (−3 < ε < +5 GV m−1), showing that this atom behaves as a positive charge following the applied electric field. The linear dependence of the Ndonor···I distance with ε is clearly established from Figure 4, and this trend can be brought close to the observed

Figure 3. Steiner-Limbach relationship applied to the halogen-bonded N···I···N′ systems described in this work (see room temperature data in Tables 1 and 2). d1 = N···I distance; d2 = I···N′ distance. Error bars represent the standard uncertainties of (d1 + d2) and (d1 − d2) in Tables 1 and 2. Dashed lines represent the confidence band at 95% level.

components partway through a series, or for cases where proton transfer might be dynamic”. This has also been described as a “salt−cocrystal continuum”, to consider the possibility for continuously variable degrees of proton transfer in hydrogen-bonded molecular crystals.105 This possibility of continuous halogen transfer has been theoretically investigated in the prototype halogen-bonded system F−Cl···CN−X, with X ranking from strongly electron-withdrawing to strongly electron-releasing groups (X = CN, NC, NO2, F, CF3, Cl, Br, CCF, CCH, CH3, SiH3, Li, Na).99 Depending on the nature of X, the halogen bond was described as evolving from traditional to “chlorine-shared” to ion pair. In order to rationalize the experimental observations described above, Density Functional Theory calculations were conducted with the NIS and NISac complexes with the strongest halogen bond acceptor, namely, Py-NMe2, as potential prototypes of the cocrystal and salt systems, respectively (see details in Supporting Information). These calculations were done in three different ways: (i) in gas phase on isolated NIS and NISac molecules, (ii) in gas phase on isolated NIS•Py-NMe2 and NISac•Py-NMe2 bimolecular complexes as extracted from the crystalline phase, and (iii) on periodic crystal phases, aimed at reflecting the actual effects of the whole crystalline environment on the bimolecular motif and iodine transfer ability. In the gas phase DFT calculations of isolated halogenbonded NIS•Py-NMe2 and NISac•Py-NMe2 complexes extracted from the crystal phases, the experimental geometries were fixed for all atoms but iodine. As shown in Table 3, whatever the nature of the density functionals used, the iodine atom was systematically found close to the N-imide molecules for both systems, at variance with the experimental observation of the opposite situation in NISac•Py-NMe2. This demonstrates that the full picture of the relationships between interaction energies, bonding distances, and properties of the electron distribution in such crystal phases cannot be simply derived from calculations on isolated systems, but depends strongly on the environment, as a polar solvent or a crystal. In hydrogen-bonded systems, it has been shown that these environmental effects can be successfully described in terms of a homogeneous external field ε parallel to the hydrogen

Figure 4. NNISac−I bond distance as a function of the amplitude ε of the applied external electric field. Black squares: gas phase isolated NISac•Py-NMe2 bimolecular complex (PBEPBE-D2 functional with the aug-cc-pVTZ basis set); open diamonds: periodic pseudoisolated NISac•Py-NMe2 bimolecular complexes (Castep calculations). Experimental NNISac−I bond distance and corresponding ε values are indicated as dashed lines. Black lines represent the least-squares linear fitting against each data set. Linear fittings are given with the corresponding equations and correlation coefficients, as well as the confidence band at 95% level (dot-dashed lines).

effect of an external electric field on the equilibrium distance deq(F···H) of the hydrogen bonded dimer H−F···H−F, for which the linear dependence of deq with ε was analytically demonstrated from the interaction potential Ei[d(F···H), ε] developed in the study.109 It is noteworthy that the calculated external electric field needed to place the iodine atom at the experimental geometry in NISac•Py-NMe2 (ε = 3.7 GV m−1) falls well in the range of values felt by molecules in crystals (∼3−20 GV m−1),112−114 and is close to those observed in proteins and enzymes (∼5 GV m−1),115,116 indicating that the crystal environment indeed plays a crucial role in the actual charge distribution within the bimolecular halogen-bonded complexes. Theoretical calculations performed on the frozen NIS•PyNMe2 and NISac•Py-NMe2 complexes at the experimental geometry (without iodine position optimization) have been D



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0.013 Å] is opposite in sign and significantly different than that experimentally observed [Δdcrystal(N−I) = dNIS − dNISac = −0.146 Å]. Furthermore, as previously pointed out, while d(N···I) > d(I···N′) is true for NISac•Py-NMe2, the opposite is found for NIS•Py-NMe2. These features seem to indicate that iodine has been transferred and should be therefore associated with the Py-NMe2 molecule in the crystal phase of the NISac complex, while in the case of NIS, even though iodine is far from the nitrogen atom [ΔdNIS(N−I) = dcrystal − dmonomer = 0.087 Å], the intermolecular distances indicate that the transfer is not already done. The topological properties ρbcp and ∇2ρbcp at both N···I and I···N′ bonding interactions parallel the previous conclusion (Figure 5). Indeed, whereas the relative variation of those quantities in the NISac•Py-NMe2 complex are respectively of +14.5% and +28.4% from N···I to I···N′ (both simultaneous increases point the strengthening of I···N′ with respect to N···I, in a region of charge depletion that is characterized by ∇2ρ > 0), in the NIS•Py-NMe2 complex, the corresponding values (−40.9% and −0.4%) indicate the opposite.118 In addition, from the isolated halogen bond donors to the corresponding complexes, the topological values calculated at the N···I bcp indicate a relative variation of ρbcp and ∇2ρbcp of −38.5% and −13.1% for NISac, and of −16.1% and +8.4% for NIS. Thus, while the negative variation of ρbcp corresponds to the weakening of the N−I bonding interaction for both NISac and NIS, the negative variation of ∇2ρbcp for NISac and the positive one for NIS indicates that iodine is involved in a N···I interaction with pure-like closed-shell character in NiSac only (both ρbcp and ∇2ρbcp magnitudes decrease with the increase of the internuclear distance), pointing out a geometry where iodine could be already transferred. The transfer of iodine in NISac•Py-NMe2 and the weakening of the N···I bonding interaction only in NIS•Py-NMe2 is definitely supported by the value of the potential energy density at bcp (Vbcp), which can be interpreted as the pressure exerted on the electrons to accumulate charge between nuclei at the interatomic surface,119 and is therefore a quantitative measure of the bonding. Indeed, whereas Vbcp is significantly larger in magnitude for N···I than for I···N′ in NIS•Py-NMe2 (−326 and −152 kJ/mol/a03, respectively), the opposite is observed for NISac•Py-NMe2 (−204 and −268 kJ/mol/a03, respectively), as found in the case of the proton shift between two fluorine atoms in the [F···H···F]− system.118 It should be noted that whether transfer takes place (in NISac•Py-NMe2) or not (in NIS•Py-NMe2), both N···I and I···N′ interactions exhibit a partially covalent character, as measured by the 1 < |Vbcp|/Gbcp < 2 descriptor118 (Gbcp being the kinetic energy density at bcp) in both complexes (N···I and I···N′: 1.52 and 1.53, and 1.64 and 1.37, for NISac•Py-NMe2 and NIS•Py-NMe2, respectively). The asymmetry of the Vbcp and |Vbcp|/Gbcp values for N···I and I···N′ interactions in NISac•Py-NMe2 is reduced with respect to those in NIS•Py-NMe2, pointing that iodine transfer has just been taking place in the former complex while iodine is clearly trapped in an intermediate position of a starting transfer process, yet belonging to NIS, in the latter. In conclusion, these calculations on isolated halogen-bonded complexes extracted from the crystal phases show that the NISac•Py-NMe2 complex can indeed be described as an ionic salt with a charge transfer of q = 0.68e, whereas NIS•Py-NMe2 should still be rather considered as a cocrystal with q = 0.16e. In order to analyze the influence of the crystal environment on the transfer of iodine, geometry optimizations were also

also used to evaluate their partial charges by applying the topological analysis of the electron density distribution ρ(r) in the framework of the Quantum Theory of Atoms in Molecules (QTAIM) methodology.117 As shown in Figure 5, the iodine

Figure 5. Laplacian maps derived from PBEPBE-D2 aug-cc-pVTZ calculations in the Ndonor···I···Nacceptor planes for (a) NISac•Py-NMe2 and (b) NIS•Py-NMe2 isolated complexes at frozen experimental geometries. Topological QTAIM charges (Q) are indicated for the various fragments. At the Ndonor···I and I···Nacceptor bcp’s (green dots) the topological values are (a) ρ = 0.53 and 0.61 eÅ−3, and ∇2ρ = 2.36 and 3.03 eÅ−5; and (b) ρ = 0.71 and 0.42 eÅ−3, and ∇2ρ = 2.60 and 2.59 eÅ−5. Positive Laplacian contours are shown as continuous blue lines and negative contours as dashed red lines.

atom indeed bears in both cases a positive charge, varying from +0.42e (NIS complex) to +0.45e (NISac complex). The partition of charges among the moieties in the complexes (Figure 5) depends on which of them the iodine atom is belonging to, a conclusion that leads to the definite classification (cocrystal vs salt) of the corresponding crystal forms, as explained hereafter. The nature of both bonding interactions within the N···I···N′ motif can be further investigated from the topological properties of ρ(r) at each N···I and I···N′ bond critical point (bpc). To undertake this analysis, it is suitable to refer first to the topological properties of the N−I bonds in the fully optimized halogen bond donor alone, namely, NISac, NIS, and iodo-benzene, the latter being the archetypal C−I covalent bond of iodine. The polarization of ρ(r) along the X−I (X = C, N) covalent bond in these molecules increases from iodobenzene to NIS and NISac, as shown by their corresponding Laplacian values at bcp’s (∇2ρbcp = −1.07, +2.40 and +2.72 eÅ−5, respectively). Along the same series, the electron density values remain closely similar (ρbcp = 0.84, 0.85, and 0.86 e Å−3), indicating that in the (C,N)−I bonding region, a similar amount of charge is progressively more depleted. In addition, the observed iodine net positive charge (+0.06, +0.39, and +0.43 e, respectively) shows the increase of its acidic character along the same series (see also Table S1 and Figures S3−S5). Hence, all ρ(r) properties together point to NISac as the best halogen bond donor in the calculated series of molecules. The difference in the N−I distance from NIS to NISac optimized complexes [Δdcomplex(N−I) = dNIS − dNISac = E



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performed using plane wave DFT periodic calculations120 (CASTEP 7.01), either by atom position optimization in a fixed unit cell or with simultaneous optimizations of unit cell and atom positions. The PBE functional,121 completed with D2 Grimme dispersion correction,122 was used, leading to calculation parameters similar to those used for the study of isolated complexes (see above). With or without cell optimization, results are very close to each other (detailed values are given in Table S2 of SI). The main conclusions are collected in Table 4 and show that such calculations now Table 4. Optimized Structural Characteristics of NIS•PyNMe2 and NISac•Py-NMe2 Based on Periodic DFT Calculationsa NIS···Py-NMe2 Optimization A Optimization B Experimental NISac•Py-NMe2 Optimization A Optimization B Experimental

N−I (Å) vs I···N′ (Å)

N···N′ (Å)

2.214 < 2.360 2.214 < 2.373 2.146(4) < 2.407(4)

4.574 4.587 4.553(6)

2.314 > 2.247 2.312 > 2.253 2.292(2) > 2.218(2)

4.560 4.564 4.509(4)

Figure 6. NNISac···I bond distance as a function of the expansion factor applied to the NISac•Py-NMe2 crystal unit cell.

full details). Thus, while f = 1 corresponds to the actual periodic situation with the iodine atom closer to the acceptor molecule at the crystal geometry, for f = 3 the situation is very close to that found in the case of the isolated bimolecular complex calculations with the iodine atom closer to the halogen bond donor molecule. This trend unambiguously demonstrates the strong influence of the molecular packing or, conversely, the high sensitivity of the iodine position toward the molecular environment in this system. Looking at the crystal structures, one can see (Figure 7) that the iodine atom of NISac is also engaged in a contact with one

a

Optimization A: Unit-cell and structural parameters are refined. Optimization B: Unit-cell is fixed while structural parameters are refined.

reproduce very well the difference between the “close to neutral” NIS•Py-NMe2 system and the “close to ionic” NISac•Py-NMe2 system, demonstrating unambiguously that the solid state organization within the crystal plays a crucial role in the actual charge delocalization within these systems. In order to further check the crystal environment effect on the stabilization of the iodine transfer, iodine position optimizations (all other atoms being kept fixed) were also performed by applying the plane wave basis set formalism to a pseudo-isolated NISac•Py-NMe2 complex. With this aim, a hypothetical crystal composed of a unique halogen-bonded complex centered in a rectangular unit cell of increasing parameter values was considered. It corresponds to a hypothetical solid where halogen-bonded complexes are pushed apart from each other within a periodic system (see SI for full details). As shown in Table S3, the increase of the distance between the halogen-bonded complexes leads to a shift of the iodine atom back to the imide side, converging toward the same geometry observed in the molecular calculations of the isolated complex (see above), therefore indicating that this geometrical characteristic is not an artifact of the computational method used. It is noteworthy that, using similar periodic calculations with the largest unit cell considered, the application of an electric field along the molecular axis (ε = 2.79 GV m−1, Figure 4) is again able to recover the experimental position of iodine. A clear illustration of the effects of the crystal packing and environment on the iodine position in NISac•Py-NMe2 is given in Figure 6, which shows the evolution of the calculated N···I distance vs the expansion factor f of the unit cell. These calculations started with the optimized crystal structure of NISac•Py-NMe2 and then the original a, b, and c unit-cell parameters were multiplied by the same f value. In such expansions, the halogen-bonded complexes are progressively pushed away from each other, whereas the donor and acceptor entities within each complex are kept mutually fixed (see SI for

Figure 7. Geometry of the secondary intermolecular OCO···I contact in NISac•Py-NMe2.

oxygen atom of an inversion-related NISac molecule. This additional OCO···I contact may be a key point in the environmentally assisted iodine atom shifting toward PyNMe2. Albeit, the structural characteristics of this “secondary interaction” are not very favorable [d(OCO···I) = 3.632 Å vs ΣvdW = 3.5 Å and α(NPy‑NMe2···I···OCO) = 58.70°], the integrated atomic charges derived from isolated molecules [Q(OCO) = −1.07; Q(I) = +0.43] show that an attractive electrostatic interaction can indeed occur between these two atoms, the oxygen atom assisting the displacement of iodine from NISac toward Py-NMe2. In the NIS•Py-NMe2 crystal structure, such a OCO···I contact is also present but in a least favorable way: the charge of the iodine atom is lower [Q(I) = +0.39] and the larger contact angle is less effective for OCO to attract the I atom [d(OCO···I) = 3.622 Å; α(NPy‑NMe2···I··· OCO) = 74.61°].123

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CONCLUSION The distinction between cocrystals and salts in crystalline hydrogen-bonded systems, formulated as A−H···B ⇆ [A]−··· [H−B]+, has been addressed here in analogous halogen-bonded systems, i.e., A−I···B ⇆ [A]−···[I−B]+, through structural and theoretical investigations of the solid state associations between F



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Table 5. Crystallographic Data

a

Twinned data (see cif file for details).

NMe2 complex in either gas phase or in periodic pseudoisolated configuration is able to shift the iodine atom at the crystal geometry (ε = 3.7 or 2.8 GV m−1, respectively), miming the polarization effect induced by the local crystal electric field. The linear dependence of the distance Ndonor···I with ε clearly indicates the quantitative effect of a polarizing environment on the I atom position, leading to the salt formation. Finally, the strong influence of the crystalline environment on the iodine position has been demonstrated by using plane wave DFT periodic calculations on optimized NIS•Py-NMe 2 and NISac•Py-NMe2 structures, as well as by applying the plane wave basis set formalism to a hypothetical solid where the halogen-bonded complexes are pushed apart from each other within a periodic system.

N-iodoimide derivatives as halogen bond donors, and pyridines as halogen-bond acceptors. We have shown that the crystalline halogen-bonded complexes of N-iodosuccinimide (NIS) with pyridine, 4-methylpyridine, and 4-dimethylaminopyridine can be described as “close-to-neutral” cocrystals, while the crystalline halogen-bonded complex of N-iodosaccharin (NISac) with 4-dimethylaminopyridine adopts a “close-toionic” salt structure. An estimation of the charge transfer between the moieties is deduced from theoretical calculations with isolated NIS•Py-NMe2 and NISac•Py-NMe2 complexes at frozen experimental geometries to amount to qtransfer = 0.16 and 0.68 e, respectively. Theoretical calculations and topological analysis of the electron density distribution ρ(r) demonstrate unambiguously that the intermolecular interactions between neighboring molecules in the crystal actually play a crucial role in the stabilization of the “close-to-ionic” structure of the NISac···Py-NMe2 complex. An external homogeneous electric field ε applied along the N···N′ direction of the NISac•Py-

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EXPERIMENTAL SECTION

Crystal Growth. Starting materials are commercially available and were used as received. G



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NIS•Py. NIS (5 mg, 0.02 mmol) was dissolved in CH2Cl2 (2 mL). Pyridine (0.003 mL, 2.9 mg, 0.03 mmol) was added to the solution. The solution was filtered to remove nondissolved particles. Crystals were obtained by slow evaporation over 24 h. White crystals were obtained. Mp 128 °C. Elem. Anal. Calcd for C9H9IN2O2: C 35.5, H 2.98, N 9.21%. Found C 33.6, H 2.77, N 8.49%. NIS•Py-Me. As above from NIS (5 mg, 0.02 mmol) and 4-picoline (0.02 mL, 2 mg, 0.02 mmol) in CH2Cl2. Mp 93−94 °C. Elem. Anal. Calcd for C10H11IN2O2: C 37.76, H 3.49, N 8.91%. Found C 39.24, H 3.69, N 9.01%. NIS•Py-NMe2. NIS (13 mg, 0.057 mmol) and 4-dimethylaminopyridine (7 mg, 0.057 mmol) were dissolved in ethyl acetate and the solution was left to evaporate over 1 week. White crystals were collected by filtration. Mp 85 °C. Elem. Anal. Calcd for C11H14IN3O2: C 38.06, H 4.06, N 12.1%. Found C 34.5, H 3.7, N 10.4%. NISac•Py. Slight modification of the previously described procedure:92 N-iodosaccharin (5 mg, 0.016 mmol) was dissolved in 1 mL of ethyl acetate. Pyridine (0.002 mL, 1.9 mg, 0.02 mmol) was added and the mixture was left for slow evaporation for 1 week. Mp 152 °C. Elem. Anal. Calcd for C12H9IN2 O3S: C 37.13, H 2.34, N 7.22%. Found: C 37.51, H 2.23, N 7.02%. NISac•Py-Me. The solution was made from N-iodosaccharin (10 mg, 0.03 mmol) dissolved in CH2Cl2 (3 mL) and 4-picoline (0.003 mL, 0.03 mmol). The crystals were obtained by slow evaporation after 1 week. Mp 110 °C. Elem. Anal. Calcd for C13H11IN2 O3S: C 38.82, H 2.76, N 6.96%. Found: C 42.77, H 3.18, N 7.07%. NISac•Py-NMe2. A solution of N-iodosaccharin (5 mg, 0.016 mmol) in ethyl acetate (0.5 mL) was transferred in a long thin tube (internal diameter 5 mm), layered with pure ethyl acetate (0.5 mL) and then with a solution of 4-dimethylaminopyridine (1.9 mg, 0.015 mmol) dissolved in ethyl acetate (0.2 mL). Thin needle-like crystals were formed by slow diffusion. Mp 173 °C. Elem. Anal. Calcd for C14H14 IN3O3S: C 38.99, H 3.27, N 9.74%. Found: C 47.16, H 4.68, N 8.65%. X-ray Crystallography. X-ray crystal structure collections were performed on a Nonius FR590 diffratometer or on an APEXII BrukerAXS diffractometer equipped with a CCD camera and a graphitemonochromated Mo Kα radiation source (λ = 0.71073 Å). Details of the structural analyses are summarized in Table 5. Absorption corrections were performed with SADABS. Structures were solved by direct methods using the SIR97 program,124 and then refined with full-matrix least-squares methods based on F2 (SHELXL-97)125 with the aid of the WINGX program.126 All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. H atoms were finally included in their calculated positions. Theoretical Calculations. Density Functional Theory calculations on isolated NISac•Py-NMe2 and NIS•Py-NMe2 complexes were performed using Gaussian09 software (revision D.01; Frisch et al., 2013).127 Geometry of the XB donor•acceptor couple was taken from the experimental X-ray structures, and hydrogen atoms were moved to standard neutron distances. The triple-ζ quality basis set aug-cc-pVTZ augmented with diffuse functions was used for all elements; in the case of iodine atom the core electrons were modeled through the corresponding pseudopotential.128 Different functionals were used in the framework of these DFT calculations, as detailed in Supporting Information. Periodic Density Functional Theory calculations were performed within the Castep code (v 7.0.1)120 using plane wave basis and built-in ultrasoft pseudopotentials. The PBE functional was used,121 as completed with the semiempirical dispersion correction of Grimme.122 All details are available in SI.

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ing Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00535.

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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 This work was supported at its start by ANR (France) under contract n°ANR-08-BLAN-0091-02. The authors thank the CINES/CEA CCRT for allocation of computing time (project c2015087449 and the CDIFX (Rennes, France) for access to Xray facilities.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Details on the theoretical calculations and coordinates of optimized structures, cif files for the single crystal X-ray diffraction experiments. Crystallographic information files are also available from the Cambridge Crystallographic Data Center (CCDC) upon request (http://www.ccdc.cam.ac.uk CCDC deposition numbers 1060256−1060262). The SupportH



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same P-1 space group symmetry than NISac•Py-NMe2. This crystal structure was optimized with Castep and was found to be 29.6 kJ mol−1 less stable (and less dense, by 6.6%) than the original orthorhombic structure (Table S5). In that triclinic polymorph, the NNIS···I distance increased up to 2.256 Å, a value intermediate between the bond distance observed in its orthorhombic crystal structure (2.214 Å) and in the NISac•Py-NMe2 triclinic crystal structure (2.314 Å), whereas the NNIS···N′ distance is slightly shortened to 4.559 Å (corresponding values for NIS•Py-NMe2 and NISac•Py-NMe2 crystal structures are 4.574 Å and 4.560 Å). In the triclinic polymorph of NIS•Py-NMe2, the OCO···I contact is weakened compared to NISac•Py-NMe2, in line with the smaller elongation of the NNIS···I bond distance (d(OCO···I) = 3.994 Å; α(NPy‑NMe2···I···OCO) = 54.05°). This demonstrates again the influence of the molecular packing on the positioning of the iodine atom in these systems. (124) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115−119. (125) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122. (126) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−838. (127) Frisch, M. J. et al. Gaussian 09, revision D.01; Gaussian, Inc., Wallingford, CT, 2013. (128) Peterson, K. A.; Shepler, B. C.; Figgen, D.; Stoll, H. J. Phys. Chem. A 2006, 110, 13877−13883.

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Cocrystal or Salt: Solid State-Controlled Iodine Shift in Crystalline

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