Charge-Assisted Halogen Bonds in Halogen-Substituted Pyridinium

Charge-Assisted Halogen Bonds in Halogen-Substituted Pyridinium...

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Charge-Assisted Halogen Bonds in Halogen-Substituted Pyridinium Salts: Experimental Electron Density Ai Wang,‡ Ruimin Wang,‡ Irmgard Kalf,‡ Angelika Dreier,§ Christian W. Lehmann,§ and Ulli Englert*,‡ ‡

Institute of Inorganic Chemistry, RWTH Aachen University, Aachen, Germany Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mülheim an der Ruhr, Germany


S Supporting Information *

ABSTRACT: Favorable electrostatic interactions dominate the packing of three halogensubstituted pyridinium salts with chloride or tetrachloridometallate counteranions. The electron density in these solids was determined based on high resolution X-ray diffraction experiments. Residues carrying opposite charges subtend short interhalogen contacts X···X in which the anionic chlorides act as electron donors and the cationic halogen substituents of the pyridinium cations act as acceptors. The X···X interactions coexist and in part compete with classical N−H···Cl hydrogen bonds. Properties derived from the experimental charge density such as the position of the bond critical point, electron density, Laplacian and energy densities help to establish the relevance of the competing interactions. Multipole-derived charge densities allow classification of short contacts more reliably than exclusively geometry-based analyses and simple promolecule densities.

INTRODUCTION Crystal engineering represents a flourishing field with its own dedicated journals. The original intermolecular synthon approach1 was first based on hydrogen bonds, the most relevant secondary interactions; their strongest representatives can be associated with a relevant degree of covalency.2−4 The last few decades have seen an ever increasing number of reports about other and less unambiguous interactions in crystals. The interpretation of such presumably structure-directing contacts affects the very idea of crystal engineering and is currently under debate.5−8 Most of the arguments put forward in favor of the relevance of specific interactions are based on geometry, whereas experimental charge density studies have been undertaken more rarely. We here cannot provide a comprehensive review but only refer to a few recent results from selected groups.9−18 Among the interactions discussed as potentially relevant in crystal engineering, halogen bonds play an important role.19−26 According to the commonly accepted model, a halogen bond denotes a directional interaction between a potential electron donor D (mostly N, O, or halogen) and a (heavy) halogen atom X as the electrophile27 with a lone pair on D directed toward the σ hole28,29 of the electrophile X bonded to the parent atom C which can often be associated with carbon. According to this model, short halogen bonds in crystalline solids should adopt a linear arrangement D···X-C as shown in Figure 1. The halogen bond model as outlined above is not only associated with a certain geometry but also involves a characteristic distribution of electron density, and electron density can be observed by suitable experiments: high resolution X-ray diffraction can provide such information and allows the electron density in the region of the alleged halogen bond to be analyzed.23,25,30−32 In this contribution, we use © 2017 American Chemical Society

Figure 1. Charge distribution in a halogen bond; D is electron-rich and interacts with the σ hole on X.

Coulomb interactions between suitably substituted ions of opposite charge to assemble crystals with short interhalogen contacts. The resulting salts are compiled in Figure 2. We chose halogen-substituted pyridinium salts as cations and crystallized their halides (1) or tetrachloridometallates (2, 3). Not surprisingly, these comparatively simple systems have been studied previously. Hydrogen and halogen bonds, the apparently most relevant interactions in crystalline halopyridinium salts, have been systematically investigated by the groups of Willett33−36 and Brammer.37−44 In the context of their work, these authors have also performed diffraction experiments for 1,33 2,35,38 and compounds isomorphous with 3.35 These earlier reports on short contacts in halopyridinium salts are based on diffraction data at standard resolution and either focus on experimental geometries or on results from theory for which diffraction results at standard resolution Received: October 24, 2016 Revised: March 15, 2017 Published: March 28, 2017 2357

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Figure 2. Composition of the pyridinium salts investigated by high resolution X-ray diffraction. SADABS.53 A summary of resolution and data completeness is listed in Table 1; additional details are provided in the Supporting Information.

provided the structural input.34,45−48 We here analyze the experimental charge density and critically evaluate its significance with respect to the short interhalogen contacts and hydrogen bonds.

Table 1. Resolution and Completeness of the Diffraction Data for 1−3


General. Commercially available reagents were used without further purification: 3-chloropyridine (99%, Aldrich), 3-bromopyridine (99%, Aldrich), 2-chloropyridine (99%, Aldrich), CuCl2·2H2O (Rein, Merck), ZnCl2 (99%, Grüssing), HCl (37%, Fisher), and acetonitrile (99.9%, VWR). IR spectra were recorded on a Nicolet Avatar 360 E.S.P. spectrometer in KBr pellets. CHN microanalyses were carried out at the Institute of Organic Chemistry, RWTH Aachen University, using a HERAEUS CHNO-Rapid. Powder diffraction experiments were performed at room temperature on flat samples with a Stoe & Cie STADI P diffractometer equipped with an imageplate detector with constant ω of 55° using germanium-monochromated Cu−Kα1 radiation (λ = 1.54051 Å). Synthesis and Crystal Growth. Procedures by Willett et al. were used to synthesize and crystallize compounds 133 and 2.35 We here give our detailed version for synthesis and crystallization of 2: 4 mmol (0.375 mL) of 3-chloropyridine was dissolved in 3 mL of acetonirile and 2 mL of concentrated hydrochloric acid was added. Two mmol (341 mg) of copper(II) chloride dihydrate was dissolved in 3 mL of acetonitrile. The 3-chloropyridine solution was added dropwise under stirring; a clear yellow solution was obtained after several minutes. After a few hours, yellow rod-shaped crystals of 2 formed in the upper part of the test tube. Larger single crystals of high quality were obtained by separating the precipitate from the mother liquor after 2 days. Synthesis and crystallization of the new compound 3: 1 mmol of ZnCl2 was dissolved in 5 mL of ethanol, and the solution was acidified with 1 mL of concentrated hydrochloric acid. Two mmol of 2chloropyridine was dissolved in 5 mL of ethanol acidified with 1 mL of concentrated hydrochloric acid. The two solutions were combined and stirred for 15 min at room temperature, filtrated, and allowed to evaporate at ambient temperature. After ca. 2 weeks, colorless platelets crystallized. Characterization. Powder diffraction (see Supporting Information, Figure S1) shows that 3 is obtained as a phase pure product. Crystalline 2 undergoes decomposition: evaporation of hydrogen chloride leads to the pyridine complex [CuCl2(3-Clpy)2]38 as indicated by the change in microanalyses. 2 [C10H10Cl6CuN2]: C, 27.65; H, 2.32; N, 6.45. Found: C, 28.02; H, 2.25; N, 6.89. 2 without 2·HCl [C10H8Cl4CuN2]: C, 33.22; H, 2.23; N, 7.75. Found: C, 33.59; H, 2.25; N, 8.04. 3 [C10H10Cl6N2Zn]: C, 27.53; H, 2.31; N, 6.42. Found: C, 27.68; H, 2.91; N, 6.76. IR for 3: υ(N−H, cm−1) = 3201;49 υ(C−Cl, cm−1) = 766.50 [see Supporting Information, Figure S2]. Data Collection and Structure Refinement Details. Suitable single crystals were cooled from ambient temperature to 100 K over a period of 1 h, using an Oxford Cryosystems 700 controller. Mo Kα radiation (λ = 0.71073 Å) was used in all data collections. Intensity data for 1 and 3 were collected on a Bruker D8 goniometer equipped with an APEX CCD area detector and an Incoatec microsource. The data were integrated using the program SAINT+.51 Intensities for compound 2 were obtained on an Enraf-Nonius KappaCCD with a rotating anode and a Helios focusing multilayer optics; EVALCCD52 was used for integration. For all compounds, scaling and absorption correction by multiscan methods were performed with the help of


resolution (Å)

2θmax (deg)

no. of reflection

completeness (%)

1 2 3 1 2 3

0.72 0.70 0.72 0.50 0.41 0.50

61.93 62.00 61.87 91.12 120.00 90.00

5097 49573 23276 24502 204186 92407

99 99 100 98 100 100

Structure Refinement. The structures were solved by direct methods.54 The independent atom model (IAM) was refined by full matrix least-squares procedures based on F2.55 Non-hydrogen atoms were assigned anisotropic displacement parameters. Coordinates for hydrogen atoms were refined freely. In the case of 1 and 3, isotropic displacement parameters for H atoms were constrained to multiples of the equivalent displacement parameter of the parent atoms; for 2, they were refined freely. Convergence parameters have been compiled in Table 2. The final IAM served as starting point for the multipole model (MM). Data were merged with the help of the program mergehklf5.56 For each reflection, internal [Σ(|(I − I)̅ |)/n n − 1 ] and external [ Σ(w·σ 2) /(n·Σw)] standard uncertainties (sus) were calculated, and the larger of these values was used as the su of the averaged intensity. Multipole refinements on F2 based on the Hansen-Coppens formalism for aspheric atomic density expansion57 were carried out with the program XD2006;58 the VM data bank59 was used. Refinement was conducted with all intensity data for 1 and 2; for 3, reflections with I > 2σ(I) were considered observed. Multipoles up to hexadecapoles for non-H atoms and up to quadrupoles for the N bonded hydrogen atoms were refined; the latter are the only donors in classical hydrogen bonds. The remaining H atoms were assigned bond-directed dipoles. In the MM, C−H and N−H distances were constrained to 1.083 and 1.009 Å, respectively. After convergence, both IAM and MM models gave satisfactory results for the rigid-bond test.60 As a general strategy, free refinement of contraction parameters was attempted; several parameters were constrained to ensure convergence to physically meaningful multipole models. Detailed information about contraction parameters can be found in Table S1 (Supporting Information). Displacement ellipsoid plots for 1−3 are shown in the Supporting Information (Figure S3) and confirm the high quality of the crystals used in the diffraction experiments.

RESULTS AND DISCUSSION Structure Descriptions. Earlier results based on X-ray diffraction experiments at standard resolution are available for 133 and 2.35,38 The tetrachloridozincate 3 is a new compound; it is, however, isomorphous to the analogous tetrachloridocuprate.35 These earlier reports comprised a series of analogous compounds and aimed at a systematic understanding of trends caused by substitution pattern and halide anions involved, with 2358

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Table 2. Crystal Data and Refinement Results of Three Compounds for 1−3 structure



empirical formula molecular weight Z crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) total Rint (all data) Rint [F2 > 2σ(F2)] (sin θ/λ)max (Å−1) crystal size (mm) μ (mm−1) Absorption Correction Detailsa Tmin (multiscan) Tmax (multiscan) Tmin (absorption) Tmax (absorption) Independent Atom Model R [F2 > 2σ(F2) ] wR2 GOF no. of reflection in refinement no. of parameters Δρmax/Δρmin (e·Å−3) CCDC Multipole Model R [F2 > 2σ(F2)] wR2 GOF no. of reflection in refinement no. of parameters Δρmax/Δρmin (e·Å−3) CCDC

C5H5BrClN 194.46 2 triclinic P1̅ 4.7750(3) 7.7459(5) 9.1691(6) 84.2844(8) 76.9703(7) 86.0817(7) 328.39(4) 24502 0.0455 0.0398 1.00 0.21 × 0.07 × 0.06 6.552

C10H10Cl6CuN2 434.45 2 triclinic P1̅ 7.1459(8) 7.6784(4) 15.4187(10) 82.451(5) 80.693(6) 71.146(5) 781.09(11) 204186 0.0288 0.0272 1.22 0.32 × 0.28 × 0.22 2.404

1639.50(5) 92407 0.0602 0.0549 1.00 0.22 × 0.21 × 0.17 2.461

0.549 0.749 0.340 0.695

0.737 0.864 0.513 0.619

0.579 0.751 0.613 0.680

0.0297 0.0626 1.048 5511 89 0.768/−0.902 1509787

0.0221 0.0568 1.108 23612 212 0.823/−0.448 1537557

0.0288 0.0745 1.048 6868 103 0.844/−0.613 1509792

0.026 0.051 0.995 5511 293 0.698/-0.666 1509788

0.016 0.045 1.062 23612 689 0.452/-0.399 1509790

0.024 0.065 0.996 6692 346 0.605/-0.694 1537624

3 C10H10Cl6N2Zn 436.27 4 orthorhombic Pccn 14.8904(3) 7.5087(2) 14.6636(3)


Tmin (multiscan) and Tmax (multiscan) are the minimum and maximum correction factors applied by the scaling program; Tmin (absorption) and Tmax (absorption) are the minimum and maximum transmission estimated from crystal dimensions and linear absorption coefficient.

an emphasis on short contacts. Before we focus on such short contacts with the help of our high resolution data and multipole refinement results, we follow an alternative approach to describe the solids 1−3 by highlighting their basic construction principle, namely, electrostatic interactions. Figure 3 shows the arrangement of pyridinium cations and chloride anions in 1: most short contacts occur between residues of opposite charge. Figure 3 also highlights the obvious exception: it is observed about the crystallographic center of inversion at the origin of the unit cell and involves shortest C···C contacts of 3.7 Å between neighboring bromopyridinium cations. The resulting antiparallel dipole arrangement can again be associated with favorable electrostatics. Favorable electrostatic contributions are readily detected for the tetrachloridometallates 2 and 3, as well. We anticipate that the above-mentioned motif, antiparallel stacking of pyridinium cations about inversion centers at distances slightly longer than van-der-Waals contacts, is also observed for 2 and 3. The

Figure 3. Distribution of cations (represented by their center of gravity, blue) and anions (red) in 1, projected on the bc plane. The inset shows the antiparallel orientation of two cations about a center of inversion.


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commonly used geometric criteria. In principle, a topological analysis of the electron density according to Bader’s Atoms In Molecules partitioning scheme62 allows details about the strength and nature of chemical bonds and short contacts to be extracted. In this context, we will also address the question to what extent the topological properties of such an advanced density model differ from that of a simple promolecule density.63 For all three pyridinium salts not only (3, −1) critical points for the covalent and coordinative bonds but also for short contacts such as hydrogen and halogen bonds could be located. The properties of the electron density in all bond critical points have been summarized in the Supporting Information (Tables S4−S6). As expected, all covalent bonds in the pyridinium cations are characterized by negative and the coordinative bonds between metal centers and chlorido ligands in the anions by positive values for the Laplacian of the electron density in the bond critical point (bcp). We will here focus on the secondary interactions, in particular, on short contacts between halogen atoms. All three compounds 1−3 show interhalogen contacts which are significantly shorter than the sum of the van der Waals radii.64 In Figure 7, the X···X interactions may be identified on the Hirshfeld surfaces65,66 of the pyridinium cations. The short contacts in 133 and 235,38 have been addressed as halogenhalide synthons or halogen bonds, and their geometric features have been discussed in detail. They occur between pyridinium cations and chloride (in 1) or tetrachloridometallate (in 2 and 3) counteranions: a strong charge assistance can be expected and is indeed reflected in the experimental electron density. According to the Hansen-Coppens multipole model, the halide substituent in the pyridinium cations represents the less, the anionic chloride the more negative partner in each X···X contact. Table 3 compiles the properties of the bond paths and the electron density (XDPROP58 and TOPXD69) in the interhalogen contacts for 1−3. Short contacts to halogen atoms in other compounds investigated by high resolution X-ray diffraction have been included for comparison. The electron density ρ in the bond critical point (bcp) shows the trend also expected for overlapping promolecule densities and is larger for short contact distances. Estimates for the kinetic energy density G and ratio between kinetic energy density and electron density in the bond critical point, G/ρ were derived following the procedure of Abramov;70 the potential energy density V was calculated according to the local virial theorem.71,72 G/ρ quite uniformly adopts numerical values between 0.6 and 0.8 when expressed in atomic units. The two independent Cl···Cl contacts in 2 (Figure 6) differ: the electron density in the bcp is higher in the slightly shorter contact Cl1···Cl6 (0.084 e· Å−3) than in Cl2···Cl3 (0.073 e·Å−3). The commonly accepted model for halogen bonds involves electrostatic interactions between suitably oriented and polarized residues. For the Cl···Cl contacts in 2 and 3, the position of the bcp along the bond path is closer to the pyridinium-bonded halide. This matches chemical intuition: the chlorido ligands in the metalate anion act as electron-rich and the chloro substituents in the pyridinium cations as electrondeficient partners in the halogen bonds. The electrostatic potential provides independent evidence for the same fact: it measures the potential energy exerted on a positively charged probe in space and was derived from the experimentally determined charge densities for 1−3 according to the approach

tetrachloridometallate anions in these salts merit a short comparison. As a consequence of its d9 electron configuration, the [CuCl4]2− moiety in 2 shows strong Jahn−Teller distortion (Figure 4 left). The chloride ligands subtend two large (146

Figure 4. Distorted CuCl42− polyhedron in 2 and ZnCl42− tetrahedron in 3.

and 138°) and four small (94−98°) angles, thus indicating a coordination environment between square-planar and tetrahedral. A database search61 (version 5.36) gave 881 (error-free, no disorder) occurences of [CuCl4]2− anions. The largest Cl−Cu− Cl angle in these building blocks adopts a clearly bimodal distribution, with maxima for square planar at ≤180 and distorted tetrahedral coordination at ca. 140° for the Cl−Cu− Cl angles (Figures S4 and S5, Supporting Information). Electrostatic interactions also dominate packing in 2 (Supporting Information, Figure S6) and 3 (Figure 5). In

Figure 5. Packing in 3; the view direction is slightly inclined with respect to b.

contrast to the dianion in 2 and to its isomorphous Cu(II) derivative,35 the tetrachloridozincate in 3 corresponds to an almost undistorted tetrahedron as expected for a d10 system (Figure 4 right). Figure 6 summarizes the short contacts in 1− 3. The geometry of halogen and hydrogen bonds in 133 and 235,38 has been extensively discussed. In the new compound 3, the 2-chloropyridinium cation interacts with two neighboring dianions by an almost linear (177°) N−H···Cl hydrogen bond and by a C−Cl···Cl halogen bond, again in almost linear geometry (173°) and with Cl···Cl = 3.24 Å. Topological Properties of the Experimental Electron Density. The resolution of our diffraction experiments enabled us to perform multipole refinements, to interpret the resulting electron density and to access information beyond the 2360

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Figure 6. Hydrogen bonds and interhalogen interactions in 1, 2, and 3. H atoms are shown as spheres with a radius of 0.10 Å. i = 1 − x, 1 − y, −z; ii = 1 − x, 1 − y, −z; iii = 2 − x, −y, −z; iv = x, 3/2 − y, 1/2 + z; v = 3/2 − x, 3/2 − y, z and vi = 3/2 − x, y, −1/2 + z.

Figure 7. Visualization of interhalogen contacts in 1, 2, and 3 with the help of Hirshfeld surfaces65,66 using CrystalExplorer;67 short contacts are coded in red on the surface. Symmetry operators: i = 1 − x, 1 − y, −z; ii = 1 − x, 1 − y, −z and vi = x, 3/2 − y, 1/2 + z.

Table 3. Topological Properties of Short X···X on Their Bond Critical Point (3, −1)a this work 1 2 3 ref 23 (1) ref 75 (1) ref 75 (2) ref 75 (4)

ref 75 (5) ref 25 (3)


dist (Å)

Rij (Å)

d1 (Å)

d2 (Å)

ρ (e·Å−3)

∇2ρ (e·Å−5)

G (a.u.)

G/ρ (a.u.)

V (a.u.)

E (a.u.)

Br1···Cl1i Cl1···Cl6 Cl2···Cl3 Cl1···Cl2

3.3652(3) 3.2219(4) 3.2871(4) 3.2411(3)

3.3653 3.2258 3.2895 3.2730

1.7227 1.5992 1.6302 1.5980

1.6426 1.6266 1.6593 1.6750

0.081(2) 0.084(2) 0.073(2) 0.109(3)

0.862(2) 1.006(2) 0.867(2) 0.930(3)

0.0078 0.0089 0.0081 0.0090

0.65 0.71 0.70 0.58

−0.0067 −0.0070 −0.0060 −0.0090

0.0013 0.0020 0.0010 0.0000

Cl1···Cl3V Cl2···Cl2II Br1···Br1I Br1···Br1II Cl2···Cl1I Cl1···Cl1I Cl2···Cl2I Br2···Br2I Br2···Br2III I1···N2 I2···O7IV I3···O1

3.1912(5) 3.7641(3) 3.7044(2) 3.5834(2) 3.7622(6) 3.8561(6) 3.8561(6) 3.906(2) 3.8973(3) 2.833(3) 3.026(6) 3.157(2)

3.1912 3.7641 3.7054 3.6082 3.7640 3.8569 3.8610 3.9070 3.8997 2.8527 3.0818 3.1702

1.5915 1.8875 1.8420 1.8956 1.7924 1.8554 1.9120 1.9557 2.0037 1.5223 1.6303 1.7442

1.5997 1.8766 1.8634 1.7126 1.9716 2.0014 1.9490 1.9513 1.8960 1.3304 1.4515 1.4260

0.086(2) 0.023(2) 0.044(2) 0.040(2) 0.036(2) 0.033(2) 0.023(2) 0.031(2) 0.023(2) 0.15(2) 0.092(7) 0.082(4)

0.986(2) 0.308(2) 0.603(2) 0.467(2) 0.455(2) 0.435(2) 0.290(2) 0.459(2) 0.280(3) 1.71(2) 0.856(9) 1.063(7)

0.0088 0.0024 0.0048 0.0038 0.0036 0.0034 0.0022 0.0035 0.0022 0.0171 0.0082 0.0092

0.69 0.69 0.74 0.64 0.68 0.70 0.65 0.77 0.63 0.75 0.60 0.76

−0.0074 −0.0015 −0.0034 0.0027 −0.0025 −0.0023 −0.0014 −0.0023 −0.0014 −0.0164 −0.0074 −0.0074

0.0014 0.0008 0.0014 0.0011 0.0011 0.0011 0.0008 0.0012 0.0007 0.0006 0.0007 0.0018

a Symmetry Operations: i = 1 − x, 1 − y, − z. I = x, y, 1 + z. II = 1 − y, x, − z. III = y, 1 − x,2 − z. IV = x − 1, y − 1, z + 1. V = 3/2 − y, 1/2 + x, 3/2 + z.

the electron donor and acceptor σ hole partners can be immediately identified. We will now address the second type of directional short contacts in 1−3, namely, the classical NH···Cl hydrogen bonds. Properties (XDPROP58 and TOPXD69) for the electron density in their H···Cl bond critical points are listed in Table 4, together with charge density results for compounds with short (nonclassical) CH···X contacts from the literature; the corresponding references have been compiled in the table. We recall that for the more popular classical hydrogen bonds with nitrogen or oxygen acceptor, G/ρ amounts to about 1.06262

of Volkov et al.73 The Laplacian of the electron density emphasizes local charge accumulations and depletions, and an isosurface of the Laplacian reflects these properties in a very sensitive way via its shape. In Figure 8, the electrostatic potentials have been mapped on Laplacian isosurfaces; the combination of both properties allows identification of regions prone to electrophilic and nucleophilic contacts and clarifies the role of electron donors and acceptors partaking in short X···X interactions. With the help of the electrostatic potential, the role of opposite charges in our salts becomes very obvious, and 2361

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Figure 8. Electrostatic potential mapped on Laplacian isosurfaces (Program Moliso,68 isosurface values −1.8 e·Å−5 for 1, −2.2 e·Å−5 for 2, and −1.3 e·Å−5 for 3). Symmetry operators: i = 1 − x, 1 − y, −z; ii = 1 − x, 1 − y, −z.

Table 4. H···X Type Interaction with Properties of Their Bond Critical Point (3, −1)a this work 1 2

3 ref 23 (1) ref 75 (1) ref 75 (2) ref 75 (3) ref 75 (4) ref 25 (5)


dist (Å)

Rij (Å)

d1 (Å)

d2 (Å)

ρ (e·Å−3)

∇2ρ (e·Å−5)

G (a.u.)

G/ρ (a.u.)

V (a.u.)

E (a.u.)

H1···Cl1 H1···Cl5iii H1···Cl6iii H6···Cl6vii H6···Cl3vii H1···Cl3vi

2.02 2.48 2.36 2.14 2.87 2.11

2.0143 2.5642 2.4463 2.1578 2.8925 2.1810

0.6537 0.9479 0.8491 0.6865 1.2318 0.7260

1.3606 1.6163 1.5972 1.4713 1.6606 1.4550

0.25(7) 0.07(2) 0.10(3) 0.13(4) 0.08(3) 0.28(6)

1.5(2) 1.38(2) 0.66(3) 2.33(2) 0.77(4) 0.6(2)

0.0221 0.0095 0.0103 0.0165 0.0061 0.018

0.60 0.46 0.90 0.58 0.55 0.44

−0.0292 −0.0115 −0.0076 −0.0198 −0.0054 −0.030

−0.007 −0.0020 0.0026 −0.0033 0.0007 −0.012

H1···Cl3VI H3···Cl1 H1···Cl1VII H1···Cl1 H3···Br1VIII H1···Br1 H6···Br1 H3···Br2IX H1···Br1

2.84 2.69 2.55 2.59 2.85 2.77 2.78 2.69 2.80

2.9138 2.7346 2.5742 2.6890 2.9100 2.8291 2.8301 2.7053 2.8392

1.0992 1.0136 0.9735 1.0749 1.0298 1.0391 1.0478 1.0017 1.0845

1.8146 1.7210 1.6008 1.6141 1.8802 1.7900 1.7824 1.7036 1.7547

0.035(5) 0.055(2) 0.094(2) 0.090(2) 0.051(5) 0.086(7) 0.055(5) 0.067(9) 0.064(9)

0.497(2) 0.760(2) 1.184(2) 1.017(2) 0.577(2) 0.851(4) 0.627(2) 0.701(3) 0.682(5)

0.0039 0.0062 0.0105 0.0092 0.0048 0.0079 0.0053 0.0062 0.0059

0.75 0.76 0.75 0.69 0.64 0.62 0.65 0.62 0.63

−0.0026 −0.0045 −0.0087 −0.0078 −0.0037 −0.0069 −0.0041 −0.0051 −0.0048

0.0013 0.0017 0.0018 0.0014 0.0012 0.0009 0.0012 0.0011 0.0011

Symmetry Operations: iii = 2 − x, − y, − z. vi = x, 3/2 − y, 1/2 + z. vii = 1 − x, − 1 − y, 1 − z. VI = −1/2 + y, 1/2 − x, 1/2 + z. VII = y, 1 − x, − z. VIII = 1 − y, x, 2 − z. IX = 1/2 + x, − 1/2 + y, − 1 + z. a

and that significantly smaller values indicate the onset of shared interactions in very short OH···O bonds.3 No such trend can be deduced for the contacts in Table 4: in view of the rather large experimental error associated with the electron density in the bcp, it is no surprise that G/ρ scatters over a wide range. All X···X contacts in the table are associated with a ratio G/ρ smaller than unity, and this value is not sensitive to the contact distance. We admit that 1−3 are not ideally suited74 for the study of hydrogen bonds with Cl acceptors. In order to test whether the observed C less than unity is a general property of NH···Cl interactions, we are planning high resolution diffraction experiments and an experimental charge density study for a compound with a NH···Cl hydrogen bond and without competing directional interactions such as short interhalogen contacts. In order to obtain a more general view about G/ρ, the ratio between kinetic energy density and electron density in the bcp of X···X and for H···X contacts, we combine our results on 1−3 with those for compounds from the literature in a common graph: Figure 9 shows G/ρ as a function of the electron density ρ and the Laplacian of the electron density. The often very strong hydrogen bonds with O as acceptor cover a wide range; cases with very high electron density in the bcp are not even contained in the figure. Much less variation can be expected

Figure 9. Ratio G/ρ as a function of the electron density ρ and its Laplacian; all quantities refer to the bond critical point. Experimental results for X···X halogen contacts are shown in yellow, for H···X in red and for H···O in orange.

from contacts to halogen. At least based on the data available to date, we conclude that G/ρ does not represent a very sensitive criterium for categorizing halogen bonds or hydrogen bonds with respect to crystal engineering. We come back to the symmetrically independent Cl···Cl contacts in 2. Table 3 shows that Cl1···Cl6 is slightly shorter 2362

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than Cl2···Cl3 and associated with a higher electron density in the bond critical point. Figure 10 summarizes the closest ionic



S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01562.. Details about data collection and structure refinement. Tables of contraction parameters, hydrogen bonds, topological properties of the electron density in the bond critical points and data completeness. Powder pattern and IR spectrum for 3. Displacement ellipsoid plots. Figures providing structural details for 2. Figures documenting data quality, refinement fit and residual electron densities (PDF) Accession Codes

CCDC 1509787−1509788, 1509790, 1509792, 1537557, and 1537624 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Figure 10. Relationship between short contacts in 2. Secondary interactions are shown as dashed lines; iii = 2 −x, −y, −z; iv = 1 − x, −1 − y, 1 − z.

residues in the neighborhood of Cl1···Cl6 and Cl2···Cl3. Cl6 is engaged in a short Cl···Cl contact and two short Cl···H−N bonds, whereas Cl3 is “only” involved in a halogen bond. Although the Cl···Cl distances are rather similar, the environments for the partaking halogen atoms differ. This may justify the different electron density distributions and electrostatic potentials (Figure 8, middle) for the halogen atoms involved in these contacts, i.e., Cl1 versus Cl2 and Cl6 versus Cl3. Geometry data support this idea: the short interhalogen contacts Cl1···Cl6 and Cl2···Cl3 are subtended by two significantly different Cu-coordinating chlorido ligands: Cu1− Cl6 is by far the longest Cu−Cl coordinative bond because Cl6 is engaged in the shorter hydrogen and halogen contacts (Figure 10).


Corresponding Author

*E-mail: [email protected]. Phone: +49 241 809 4666. Fax: +49 241 8092 288. ORCID

Ulli Englert: 0000-0002-2623-0061 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by China Scholarship Council (scholarship for Ai Wang) and DFG (Project EN309/10-1: Experimental electron density of halogen bonds and interhalogen contacts). We thank Michael Wittpahl for the synthesis of 3 and Dr. Birger Dittrich and Dr. Richard Goddard for helpful discussions.

CONCLUSION We doubt that we (and, of course, the original authors of 1 and 2 who should be given proper credit) did “engineer” our crystals based on halogen bonds and N−H···Cl hydrogen bonds as synthons. Rather, we used the lattice energy of these salts to promote necessarily close contacts between the chloride or chloridometallate anions and suitable peripheral atoms in the halopyridinium cations. Potential contact sites in the pyridinium cations are the π system, hydrogen atoms bonded to carbon or nitrogen, and the halogen substituent; a chloride anion or a tetrachloridometallate will obviously prefer the latter two options. Both halogen and hydrogen bonds are directional, and their geometries in 1−3 are essentially linear. Our pyridinium salts are well-suited for experimental charge density studies with a focus on these close contacts. The Laplacian of the electron density and the electrostatic potential allow to unambiguously assign the role of charge donors and acceptors within the halogen bonds. In the present case, they match chemical intuition, with anionic chlorides as electron donors and substituents of the pyridinium cations as acceptors. Not all properties derived from the experimental electron density are equally informative for the analysis of secondary interactions in our systems: kinetic energy densities proved rather uniform, whereas details of the bond path revealed features beyond the standard paradigm “short is meaningful”. In contrast to a geometry-based structure analysis or a database search, a charge density study may pick up the subtle differences between apparently similar short contacts.

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DEDICATION Dedicated to Professor Gerhard E. Herberich on the occasion of his 80th birthday. REFERENCES

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DOI: 10.1021/acs.cgd.6b01562 Cryst. Growth Des. 2017, 17, 2357−2364