Computational and Crystallographic Studies of Pseudo-Polyhalides

Article pubs.acs.org/crystal

Computational and Crystallographic Studies of Pseudo-Polyhalides Published as part of the Crystal Growth & Design virtual special issue on Halogen Bonding in Crystal Engineering: Fundamentals and Applications Rosa D. Walsh,† Jessica M. Smith,‡ Timothy W. Hanks,*,‡ and William T. Pennington*,† †

Department of Chemistry, Clemson University, Clemson, South Carolina 29634-0973, United States Department of Chemistry, Furman University, Greenville, South Carolina 29613, United States

‡

S Supporting Information *

ABSTRACT: Pseudo-polyhalides represent a relatively new class of compounds in which the dihalogen molecules of the polyhalide ions are replaced by organohalogen molecules. Like polyhalides, the anion structure is strongly dependent on the size, shape, and charge of the countercation, but with additional variables provided by the specific organohalogen used. The charge transfer contribution to the halogen bonding interaction in polyhalides can be quite large but is much less so in the pseudopolyhalides. Density functional calculations show that the interaction is primarily electrostatic. The charge-transfer contribution generally increases with increasing halogen bond strength but is not enough to cause significant angular dependence at the halide ion. The structures of 10 pseudo-polyiodide salts with either cryptand[2.2.2] (K222) or hexaethyleneglycol (HEG) encrypted alkali metal cations and tetraiodoethylene as the organohalogen are reported. Three salts obtained with M(K222)+ have widely different structures ranging from steeply corrugated layers to more planar layers with pendant groups to a three-dimensional network. Identical stoichiometries with different cations (M(K222)+ or M(HEG)+) give a layered or network solid, respectively, which are directly related. The latter has the unusual property of a completely disordered cationic phase, located within a perfect honeycomb anionic network.

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INTRODUCTION The triiodide ion (I3−) was discovered in 18191 and is the simplest member of the large polyiodide family. Complexes of these ions have found many uses over the years, and they continue to attract attention, as evidenced by a recent comprehensive review.2 Polyiodides are held together by halogen bonds (XB), with one or more iodide ions donating electrons to one or more iodine molecules. By convention, the anions are halogen bond “acceptors”, while the iodine is generally the halogen bond donor. I3− is linear, but higher polyiodides adopt a variety of geometries in the solid state. For example, pentaiodides (I5−) are usually “V” or “L” shaped, while heptaiodides are often pyramidal or “Z” shaped. Distortions from these idealized geometries are common, indicating that the directionality of the bonding is relatively weak. In fact, the exact size and shape of a given polyiodide is strongly dependent on the size, shape, charge, and packing requirements of the associated cation. Bond lengths also vary, with I3− ranging from a symmetric species with a bond length of 2.92 Å to asymmetric structures with lengths ranging from about 2.85 to 3.00 Å.2 In higher polyiodides, I−I distances frequently are considerably longer. Recent computational studies of I3− and related ions show that density functional theory (DFT) approaches offer both good computational efficiency and chemical accuracy.3 The interaction between the iodide ion and I2 can be understood as © 2012 American Chemical Society

the donation of electron density from an ion lone pair into both the I2 σ and σ* orbitals, resulting in a three-center, fourelectron system. As there is considerable orbital overlap in this depiction, there is a significant preference for a linear geometry.3 Like the dihalides, organohalides can also serve as halogen bond donors to halide ion acceptors (X−···X−R, where X = Cl, Br, I, and R = alkyl, aryl, or fluorinated derivatives). We will call this class of compounds pseudo-polyhalides in recognition of their apparent similarities to the polyhalide family.4 The pseudo-polyhalides have received less attention than XB complexes containing neutral acceptors, but there have been recent reports of their deliberate synthesis for crystal engineering applications,5 ion receptors,6 and molecular conductors.7 The solid-state architectures of pseudo-polyhalides have recently been reviewed,8 while a series of model complexes has been examined by ab initio methods at the MP2 level.9 Given the ubiquity of halide ions in the natural environment and their role in both natural and synthetic processes, it is certain that pseudo-polyhalides will receive increasing attention as the significance of HB becomes more widely recognized. Received: September 19, 2011 Revised: February 16, 2012 Published: February 22, 2012 2759

dx.doi.org/10.1021/cg201231t | Cryst. Growth Des. 2012, 12, 2759−2768

Crystal Growth & Design

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In this paper, we explore the effects of the nature of the halide ion and the electronic influence of the organohalide on the strength and directionality of the resulting halogen bonding interactions. We also report the structures of a series of pseudopolyiodides based on tetraiodoethylene (TIE) as the halogen bond donor, with alkali metal cations sequestered by either cryptand[2.2.2] (K222) or hexaethylene glycol (HEG).

Article

COMPUTATIONAL METHODOLOGY

All computations were performed with the Gaussian 03 suite of programs.10 The interaction energies and geometrical parameters were computed using the DFT method with the M052X11 functional and the aug-cc-pVDXZ basis set for all atoms except iodine, where a pseudo-potential was required (aug-cc-pVDXZ-PP).12 All structural minima were confirmed by the absence of imaginary frequencies using vibrational frequency calculations and Natural Bond Order (NBO) analysis was performed on all structures using the NBO 3.1 program embedded in Gaussian 03. Basis Set Superposition Error (BSSE) corrections were found to be less than 5% of the interaction energy using this approach and did not affect the observed trends. Therefore, they are not reported.

Scheme 1

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X-RAY DIFFRACTION ANALYSIS Specific details of the crystallographic experiments and results for each compound are given in Table 1. The data were measured at room temperature (294 ± 2 K) with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) on a threecircle Rigaku AFC8 diffractometer equipped with a Mercury Table 1. Crystal Data [Na(K222)]I·1.5TIE

[K(K222)]I·1.5TIE

[Rb(K222)]I3·1.5TIE

C21H36N2O6NaI7 1323.81 rhombohedral R3̅ (No. 148) 11.818(2)

C21H36N2O6KI7 1339.92 rhombohedral R3̅ (No. 148) 12.0446(8)

C21H36N2O6NaI7 1386.29 rhombohedral R3̅ (No. 148) 12.2050(5)

44.264(9)

43.880(4)

43.361(4)

5354(2) 6 2.46 6.13 0.42−1.00 2978 2749 (0.059) 2525 0.0786 (0.0819) 0.1815 (0.1836) [Rb(HEG)2]I·1.5TIE

5512.9(8) 6 2.42 6.06 0.36−1.00 3208 2807 (0.027) 2097 0.0494 (0.0657) 0.1320 (0.1387) [K(K222)]I3·TIE

5593.8(8) 6 2.47 7.16 0.29−1.00 8027 2552 (0.038) 2182 0.0630 (0.0700) 0.1508 (0.1551) [Rb(K222)]I3·TIE

C27H52O14KI7 1528.09 hexagonal P6/mmm (No. 191) 14.643(5)

C27H52O14RbI7 1574.46 hexagonal P6/mmm (No. 191) 14.660(2)

8.9118(5)

8.933(3)

8.9500(18)

C20H36N2O6KI7 1327.91 orthorhombic P212121 (No. 19) 9.7118(14) 14.9503(19) 25.800(3)

C20H36N2O6RbI7 1374.28 orthorhombic P212121 (No. 19) 9.7339(10) 14.9557(4) 25.7708(7)

1620.7(5) 2 3.10 6.79 0.56−1.00 15729 722 (0.038) 720 0.1357 (0.1360) 0.2961 (0.2962)

1658(1) 2 3.06 6.75 0.47−1.00 15942 726 (0.038) 726 0.1175 (0.1175) 0.2548 (0.2548)

1665.8(5) 2 3.14 8.04 0.61−1.00 16012 728 (0.122) 725 0.2294 (0.4117) 0.2294 (0.4117)

3746.0(8) 4 2.36 5.94 0.62−1.00 31186 7711 (0.045) 7223 0.0750 (0.0809) 0.1807 (0.1890)

3751.6(4) 4 2.43 7.11 0.52−1.00 36445 7671 (0.041) 7303 0.0871 (0.0932) 0.1582 (0.1612)

[K(K222)]I·TIE

[Rb(K222)]I·TIE

formula Mw crystal system space group a, Å b, Å c, Å β, (°) V, Å3 Z Dcalc, g cm−3 μ, mm−1 transmission coefficients reflections collected reflections unique (Rmerge) reflections observed (I > 2σ) R1 a wR2b

C20H36N2O6KI5 1074.11 monoclinic C2/c (No. 15) 20.5950(10) 12.2615(8) 26.1223(18) 92.572(3) 6589.9(7) 8 2.16 4.88 0.48−1.00 30814 6749 (0.045) 5799 0.0448 (0.0626) 0.1045 (0.1146) [Na(HEG)2]I·1.5TIE

C20H36N2O6RbI5 1120.48 monoclinic C2/c (No. 15) 20.573(3) 12.2683(14) 26.225(3) 92.017(3) 6615(1) 8 2.25 6.20 0.26−1.00 30018 6890 (0.061) 4281 0.0686 (0.1174) 0.1610 (0.1806) [K(HEG)2]I·1.5TIE

formula Mw crystal system space group a, Å b, Å c, Å β, (°) V, Å3 Z Dcalc, g cm−3 μ, mm−1 transmission coefficients reflections collected reflections unique (Rmerge) reflections observed (I > 2σ) R1a wR2b

C27H52O14KI7 1511.98 hexagonal P6/mmm (No. 191) 14.491(1)

R1 = Σ∥Fo| − |Fc∥/Σ|Fo| for observed data (I > 2σ(I)); number in parentheses is for all data. bwR2 = {Σ[w(Fo2 − Fc2)2]/Σ[w(Fo2)2]}1/2 for observed data (I > 2σ(I)); number in parentheses is for all data. a

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Table 2. Computational Details Obtained at the M052X/aug-cc-pVDXZ Level for the Interaction of Trifluormethyl- and Methyl- Halogen Bond Donors and Halide Ion Halogen Bond Acceptorsa donor (RX) CF3Cl

CF3Br

CF3I

CH3Cl

CH3Br

CH3I

acceptor (X−) −

Cl Br− I− Cl− Br− I− Cl− Br− I− Cl− Br− I− Cl− Br− I− Cl− Br− I−

carbon (Δ charge)

donor (Δ charge)

acceptor (Δ charge)

donor (WBI)

acceptor (WBI)

RX···X− (Å)

ΔE (kcal/mol)

−0.055 −0.049 −0.043 −0.083 −0.074 −0.064 −0.113 −0.112 −0.100 −0.039 −0.033 −0.029 −0.062 −0.055 −0.047 −0.106 −0.095 −0.081

0.109 0.098 0.085 0.118 0.104 0.088 0.112 0.095 0.071 0.082 0.073 0.064 0.093 0.082 0.070 0.105 0.091 0.075

0.032 0.028 0.024 0.072 0.068 0.062 0.144 0.145 0.146 0.011 0.007 0.005 0.028 0.024 0.022 0.070 0.067 0.061

1.224 1.218 1.211 1.173 1.171 1.168 1.097 1.102 1.107 1.112 1.107 1.103 1.100 1.097 1.095 1.071 1.072 1.072

0.065 0.056 0.050 0.144 0.136 0.124 0.280 0.279 0.278 0.023 0.015 0.013 0.057 0.050 0.046 0.141 0.134 0.122

2.97 3.18 3.44 2.88 3.07 3.33 2.82 3.00 3.24 3.31 3.63 3.91 3.20 3.40 3.66 3.08 3.26 3.54

−10.55 −9.04 −7.44 −14.81 −12.76 −10.68 −23.96 −20.96 −17.84 1.22 1.52 1.77 −1.58 −0.84 −0.11 −7.79 −6.17 −4.56

a

Details include changes upon bonding of the atomic charges (Δ charge) for the carbon and halogen of the haloalkane and of the halide ion; the Wiberg bond indexes (WBI) for the donor halogen and acceptor ion; halogen bond distances (Å); and halogen bond interaction energies (kcal/ mol). Synthesis of [K(K222)]I·TIE. K222 (0.013 g; 0.035 mmol), TIE (0.018 g; 0.034 mmol), and KI (0.0065 g; 0.040 mmol) were dissolved in acetone (∼25 mL) and heated to reflux with stirring (1.5 h). Slow evaporation of the solvent yielded yellow parallelepiped crystals (0.023 g; 0.021 mmol) of [K(K222)]I·TIE (62% yield). Elemental analysis, calculated (observed): %C 22.36 (22.73), %N 2.61 (2.32), %H 3.38 (3.44). Synthesis of [Rb(K222)]I·TIE. K222 (0.014 g ; 0.037 mmol), TIE (0.021 g; 0.040 mmol), and RbI (0.0080 g; 0.038 mmol) were dissolved in acetone (∼ 25 mL) and heated to reflux with stirring (1.5 h). Slow evaporation of the solvent yielded yellow parallelepiped crystals (0.027 g; 0.024 mmol) of [Rb(K222)]I·TIE (65% yield). Elemental analysis, calculated (observed): %C 21.44 (21.17), %N 2.50 (2.27), %H 3.24 (3.12). Synthesis of [Na(K222)]I·1.5TIE. K222 (0.048 g; 0.127 mmol), TIE (0.100 g; 0.188 mmol), and NaI (0.024 g; 0.160 mmol) were dissolved in ethanol (∼70 mL) and heated to reflux with stirring (1.5 h). Slow evaporation of the solvent yielded yellow hexagonal platy crystals (0.163 g; 0.123 mmol) of [Na(K222)]I·1.5TIE (98% yield). Elemental analysis, calculated (observed): %C 19.05 (19.17), %N 2.12 (2.30), %H 2.74 (3.12). Synthesis of [K(K222)]I·1.5TIE. K222 (0.049 g; 0.130 mmol), TIE (0.110 g; 0.206 mmol), and KI (0.022 g; 0.133 mmol) were dissolved in ethanol (∼70 mL) and heated to reflux with stirring (1.5 h). Slow evaporation of the solvent yielded yellow hexagonal platy crystals (0.102 g; 0.076 mmol) of [K(K222)]I·1.5TIE (55% yield). Elemental analysis, calculated (observed): %C 18.82 (19.45), %N 2.09 (2.12), % H 2.71 (2.92). Synthesis of [Rb(K222)]I·1.5TIE. K222 (0.014 g; 0.037 mmol), TIE (0.021 g; 0.040 mmol), and RbI (0.0080 g; 0.038 mmol) were dissolved in ethanol (∼30 mL) and heated to reflux with stirring (1.5 h). Slow evaporation of the solvent yielded yellow parallelepiped crystals (0.027 g; 0.024 mmol) of [Rb(K222)]I·1.5TIE (65% yield). Elemental analysis, calculated (observed): %C 18.19 (17.88), %N 2.02 (2.28), %H 2.62 (2.43). Synthesis of [Na(HEG)]I·1.5TIE. HEG (∼30 μL; 0.120 mmol), TIE (0.107 g; 0.201 mmol), and NaI (0.021 g; 0.141 mmol) were dissolved in ethanol (∼65 mL) and heated to reflux with stirring (1.5 h). Slow evaporation of the solvent yielded yellow hexagonal column crystals (0.093 g; 0.075 mmol) of [Na(HEG)]I·1.5TIE (56% yield).

CCD area detector, except for [Na(K222)]I·1.5TIE, which was measured on a four-circle Nicolet R3mV diffractometer with serial detector and [K(K222)]I·1.5TIE, which was measured on a four-circle Rigaku AFC-7R diffractometer with serial detector. An absorption correction, based on either a multiscan technique (for the CCD data)13 or on azimuthal scans of several intense reflections (for the serial detector data), was applied to the data for each compound. All structures were solved by direct methods and refined (on F2) using full-matrix, least-squares techniques. Non-hydrogen atoms were refined with anisotropic displacement parameters, and hydrogen atoms were included in the structure factor calculation at optimized positions with isotropic displacement parameters fixed at values 20% greater than that of their host atom. The TIE carbon atoms in [M(HEG)2]I·1.5TIE are equivalently disordered over two sets of sites related by a 90° rotation of this group about the axis perpendicular to plane of the molecule. A similar disorder is seen for [Rb(K222)]I3·TIE; the two orientations are not equally present. All partial occupancy carbon atoms were refined with isotropic thermal displacement parameters. All three HEG structures have badly disordered M(HEG)2+ domains, which could not be modeled. Only the atoms of I−···TIE network were included and refined, which explains the large residual values reported for these three structures. It should be noted that elemental analysis results for these compounds do agree with the reported formulas. Structure solution, refinement, and calculation of derived results was performed with the SHELXTL-Plus package of computer programs.14 Neutral atom scattering factors and the real and imaginary anomalous dispersion corrections were taken from International Tables for X-ray Crystallography, Vol. C.15

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EXPERIMENTAL SECTION

Starting materials were purchased from Sigma-Aldrich and used as received. Commercial grade solvents were dried and purified by standard techniques and were stored over activated sieves. Carbon, nitrogen, and hydrogen elemental analyses were performed in house using a PerkinElmer 2400 Series II CHNS/O Elemental analyzer. 2761



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Table 3. Computational Details Obtained at the M052X/aug-cc-pVDXZ Level for the Interaction of Phenyliodine and Iodoacetylene Halogen Bond Donors and Halide Ion Halogen Bond Acceptorsa donor (RX) C6H5I

HCCI

acceptor (X−) −

Cl Br− I− Cl− Br− I−

carbon (Δ charge)

donor (Δ charge)

acceptor (Δ charge)

donor (WBI)

acceptor (WBI)

RX···X− (Å)

ΔE (kcal/mol)

−0.057 −0.044 −0.037 0.021 0.022 0.022

0.122 0.106 0.088 0.035 0.023 0.012

0.091 0.085 0.080 0.130 0.126 0.120

1.139 1.140 1.142 1.203 1.210 1.216

0.179 0.168 0.159 0.249 0.241 0.231

2.97 3.17 3.43 2.85 3.04 3.29

−12.09 −9.96 −7.76 −21.47 −18.56 −15.51

a

Details include changes upon bonding of the atomic charges (Δ charge) for the carbon and halogen of the haloalkane and of the halide ion; the Wiberg bond indexes (WBI) for the donor halogen and acceptor ion; halogen bond distances (Å); and halogen bond interaction energies (kcal/ mol).

Elemental analysis, calculated (observed): %C 21.45 (21.42), %H 3.47 (3.52). Synthesis of [K(HEG)]I·1.5TIE. HEG (∼30 μL; 0.120 mmol), TIE (0.104 g; 0.196 mmol), and KI (0.024 g; 0.144 mmol) were dissolved in ethanol (∼65 mL) and heated to reflux with stirring (1.5 h). Slow evaporation of the solvent yielded yellow hexagonal column crystals (0.116 g; 0.093 mmol) of [K(HEG)]I·1.5TIE (71% yield). Elemental analysis, calculated (observed): %C 21.22 (21.06), %H 3.43 (3.59). Synthesis of [Rb(HEG)]I·1.5TIE. HEG (∼30 μL; 0.120 mmol), TIE (0.103 g; 0.193 mmol), and RbI (0.028 g; 0.132 mmol) were dissolved in ethanol (∼65 mL) and heated to reflux with stirring (1.5 h). Slow evaporation of the solvent yielded yellow hexagonal column crystals (0.115 g; 0.075 mmol) of [Rb(HEG)]I·1.5TIE (69% yield). Elemental analysis, calculated (observed): %C 20.60 (20.22), %H 3.33 (3.32). Synthesis of [K(K222)]I3·TIE. K222 (0.073 g; 0.195 mmol), TIE (0.107 g; 0.201 mmol), KI (0.035 g; 0.211 mmol), and I2 (0.053 g; 0.209 mmol) were dissolved in acetone (∼70 mL) and heated to reflux with stirring (1.5 h). Slow evaporation of the solvent yielded orange hexagonal platy crystals (0.226 g; 0.170 mmol) of [K(K222)]I3·TIE (87% yield). Elemental analysis, calculated (observed): %C 18.09 (18.03), %N 2.11 (2.12), %H 3.73 (2.82). Synthesis of [Rb(K222)]I3·TIE. K222 (0.070 g; 0.187 mmol), TIE (0.100 g; 0.188 mmol), RbI (0.043 g; 0.200 mmol), and I2 (0.052 g; 0.202 mmol) were dissolved in acetone (∼ 70 mL) and heated to reflux with stirring (1.5 h). Slow evaporation of the solvent yielded orange hexagonal platy crystals (0.238 g; 0.173 mmol) of [Rb(K222)]I3 (92% yield). Elemental analysis, calculated (observed): %C 17.48 (18.00), %N 2.04 (2.13), %H 2.64 (2.55).

larger halogen atoms that are electron-poor make better donors. Increased interaction energy is accompanied by increased charge transfer from the halide ion to the organohalide. The decrease in the negative charge of the halide ion upon halogen bond formation is not accompanied by an increased negative charge on the halogen of the donor molecule, but by the carbon and attached fluorines or protons. Indeed, the electron density of the covalently bound halogen is decreased by the formation of the halogen bond, indicating a disruption of the carbon−halide bond. The trifluoromethyl moiety is better able to accept the increased electron density caused by the formation of the halogen bond than is a methyl group, resulting in stronger halogen bonds as has been well-established. In addition, the greater the s-character of the carbon holding the donor halogen, the stronger the interaction (Table 3). The trends in these models systems are consistent with the results listed in Table 2 and with those reported by Lu and coauthors using the MP2 method.18 Compared to the symmetrical I3− ion where 0.5 electron is transferred from the acceptor ion to the donor, the charge transfer to organoiodides is small (Figure 1). In most cases, the charge-transfer interaction increased with increasing interaction

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RESULTS AND DISCUSSION Computational Studies of Pseudo-Polyhalides. Virtually all investigations of pseudo-polyhalides have been in the solid state, where the contribution of the halogen bonding to the overall system is difficult to extract. DFT methods have been widely used in recent years to provide insight into the nature, strength, and directionality of XB systems.16 Here, we make use of the M052X functional, which has been shown to be both efficient and highly accurate with a variety of systems, including the modeling of noncovalent interactions.11,17 The basis set is a Dunning correlation consistent-type that includes diffuse functions required for effectively modeling XB interactions18 and a pseudo-potential that permits analysis of iodine-containing systems at far less computational cost than all-electron calculations, but that introduces negligible error.12 Like I3− and all other halogen bonded systems, the simplest pseudo-polyhalides form a linear geometry, with the halogen bond acceptor opposite the existing covalent bond to the halogen. As shown by the data in Table 2, the halide anions are good halogen bond acceptors and the interaction energies follow the trends expected from the study of polyhalide ions. The smaller anions are better halogen bond acceptors, while

Figure 1. Calculated transfer of charge from a halide ion to an organoiodide halogen bond donor as compared to the halogen bond strength. Within each series, the XB bond energy is strongest for the chloride ion and weakest for the iodide ion. The series “2-CH3I” and “2-CF3I” represent the total interaction energy of a single halide ion interacting with two XB donors. 2762



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of two trans iodine atoms to extend the structure in one dimension or of all four iodine atoms to extend the structure in two dimensions is observed, but co-crystals with interaction to three iodine atoms have also been observed. Several pseudopolyhalides with tetraiodoethylene have been reported.4,7,21,22 In order to form pseudo-polyhalides, halogen bonding must overcome the cation−anion interactions of the halide salt. This can be accomplished through the use of sequestering agents such as crown ethers, calixarenes, etc. to conceal metal cations within a lipophilic host23 or through the use of large and/or lipophilic cations with very a diffuse positive charge, such as TTF radicals24 or tetraalkylammonium or phosphonium ions.25 We have approached this problem through the use of either the cryptand, K222, or the polyol, hexaethylene glycol (HEG). While K222 has frequently been used for this purpose in crystal design applications, it has not previously been coupled with TIE. All distances and angles within the cryptand and with the encrypted metal are similar to those of reported compounds.26 To the best of our knowledge, we are reporting the first instance of HEG-based cation encapsulation for the formation of pseudo-polyhalides. As will be discussed, we were not able to refine parameters for this group. Halogen bond contacts are given in Table 5, along with those of closely related compounds for comparison. Structure of [M(K222)]I·TIE, M = K, Rb. The potassium and rubidium salts are isomorphous, crystallizing in the monoclinic

energy. However, when the halogen bond donor was CF3I, the extent of charge transfer was nearly constant across the halide ion acceptors, despite a decrease in interaction energy with large ions. Since halide ions are capable of simultaneously interacting with multiple halogen bond donors, we also examined the optimized geometries when two or three donors were present. Only the CH3I and CF3I donors interacting with I− were considered. We found a slight preference for a 90° angle at the central ion, but increasing this to a linear geometry had little impact on the energy or bond lengths. With three donors present, a pyramidal geometry was preferred, but again, other geometries gave only slightly higher energies. In each case, the presence of the second or third donor decreased the halogen bond strength (ca. 7 and 15% for CH3I, respectively and 15 and 28% for CF3I, respectively). When two organohalides were placed around the ion, the total charge decreased, though the charge transferred to each halogen bond donor was less. When two CH3I donors were bonded to a single ion, the identity of the acceptor ion made essentially no difference in the extent of charge transferred. When two CF3I ions were used, the extent of charge transfer increased with increasing ion size. Crystallographic Studies of Pseudo-Polyiodides. A search of the Cambridge Structural Database19 reveals a large number of structures with X−···X−R interaction distances shorter than the sum of their van der Waals radii.20 Average halogen bond distances and angles with standard deviations and the number of occurrences for these are given in Table 4. As

Table 5. Halogen Bond Parameters for Reported and Related Structures

Table 4. Metric Parameters for Reported Pseudo-Polyhalides X−\X F−

Cl

Cl−

X−···X (Å), normalized valuea − X ···X−C (°) #compounds, #parameters X−···X (Å)

Br−

X−···X-C (°) #compounds, #parameters X−···X (Å)

I−

X−···X−C (°) #compounds, #parameters X−···X (Å) X−···X−C (°) #compounds, #parameters

3.32(11), 0.949 166(8) 86, 101 3.46(12), 0.961 168(8) 29, 32 3.61(8), 0.968 169(7) 26, 30

Br

3.39(11), 0.942 166(9) 31, 35 3.38(17), 0.914 171(7) 84, 109 3.59(12), 0.937 171(8) 19, 25

I 2.51(4)b, 0.728 177(1) 3, 4 3.18(13), 0.853 172(5) 44, 87 3.36(17), 0.880 171(6) 39, 86 3.51(14), 0.886 173(5) 85, 173

[K(K222)]I·TIE

[Rb(K222)]I·TIE

[Na(K222)]I·1.5TIE [K(K222)]I·1.5TIE [Rb(K222)]I·1.5TIE [Na(HEG)]I·1.5TIE [K(HEG)]I·1.5TIE [Rb(HEG)]I·1.5TIE [K(K222)]I3·TIE

a

Obtained by dividing the observed value by the vdw sum for the appropriate atoms. bNumbers in parentheses are standard deviations based on the statistical range of values reported.

[Rb(K222)]I3·TIE

expected, interaction strength of the halogen bond donor increases down the group (I > Br > Cl), due to increasing polarizability, and interaction strength of the halogen bond acceptor generally decreases down the group (Cl > Br > I), due to the decrease in polarizing power of the halide. The 10 structures reported here all involve iodide halogen bond acceptors interacting with an organoiodine halogen bond donor, namely, tetraiodoethylene, and all of the interaction distances and angles fall within the range of those included in Table 4. Tetraiodoethylene is a versatile halogen bond donor for crystal design applications.4 In most cases, participation either

(EDT-TTF)4BrI2·5TIE (EDST)4I3·5TIE

(MDT-TTF) 4BrI2·5TIE [N(C4H9)4]I·TIE

a

2763

I−···I (Å)

I−···I−C (°)

CNa

ref

3.5465(6) 3.6144(5) 3.5606(5) 3.5423(12) 3.6331(11) 3.5683(10) 3.5332(11) 3.5244(9) 3.5035(11) 3.588(3) 3.612(3) 3.622(5) 3.686(2) 3.698(2) 3.836(2) 3.928(2) 3.690(2) 3.712(2) 3.863(2) 3.937(2) 3.749(5) 3.472(5) 3.800(6) 3.552(6) 3.730 3.504(6) 3.539(6) 3.564(3) 3.424(3) 3.602(3)

167.64(18) 172.11(17) 156.89(17) 167.9(3) 171.8(3) 156.3(3) 170.7(3) 172.69(19) 173.4(3) 167(2) 170(1) 175(8) 167.5(4) 172.7(5) 174.0(8) 175.5(9) 168.2(6) 172.4(7) 174.6(7) 177.0(7) 177(1) 166(1) 177(1) 166(1) 174(1) 165(1) 175(1) 174(1) 175(1) 173(1)

4 4 2 4 4 2 3 3 3 6 6 6 4 4 4 4 4 4 4 4 6 6 6 6 8 3 3 3

b

b

b b b b b b b

b

21 21

21 22

CN = coordination number of the iodide. bThis paper. dx.doi.org/10.1021/cg201231t | Cryst. Growth Des. 2012, 12, 2759−2768


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molecule sitting upon an inversion center at (1/3 1/6 1/6), and an iodide anion sitting upon a 3-fold rotation axis (0 0 z). The iodide interacts with three different TIE molecules to form a steeply corrugated two-dimensional layer of −3 symmetric fused rings, shown in Figure 4. The two iodine atoms not

space group, C2/c. The asymmetric unit contains a K222 encrypted alkali metal cation in a general position within the unit cell, one TIE molecule, and two unique iodide anions, one sitting upon a crystallographic 2-fold rotation axis (0 y 1/4), and the other upon an inversion center (1/4 1/4 1/2). The iodide on the rotation axis interacts with four different TIE molecules to form a two-dimensional layer shown in Figure 2. Iodide ions on

Figure 2. Two-dimensional I−···TIE layer parallel to {0 0 1} in [M(K222)]I·TIE.

inversion centers extend on either side of the resulting plane and each interact with two TIE molecules to join the planes into a three-dimensional network with intersecting tunnels running parallel to the planes. The [M(K222)]+ ions occupy these tunnels (see Figure 3). One iodine atom of each TIE molecule does not participate in halogen bonding. Structure of [M(K222)]I·1.5TIE, M = Na, K, Rb. The sodium, potassium, and rubidium salts are isomorphous, crystallizing in the rhombohedral space group, R3̅. The asymmetric unit contains a K222 encrypted alkali metal cation sitting upon a crystallographic 3-fold rotation axis (2/3 1/3 z), which also passes through the nitrogen atoms of the cryptand, a TIE

Figure 4. Two dimensional I−···TIE layer parallel to {0 0 1} in [M(K222)]I·1.5TIE.

involved in halogen bonds extend into either side of the plane to form cavities, into which the [M(K222)]+ ions sit (see Figure 5). The resulting structure bears resemblance to a carton of eggs, with the cations representing the eggs and the I−···TIE layers providing “protective” packing. Structure of [M(HEG)]I·1.5TIE, M = Na, K, Rb. The sodium, potassium, and rubidium salts are isomorphous, crystallizing in a hexagonal space group with an iodide anion occupying a − 6m2 site, and the TIE molecule situated about an mmm site, with the carbon atom on an mm2 site and the iodine atom on a mirror plane. Each iodide anion coordinates to iodine atoms of six different TIE molecules, and all iodine atoms are involved in halogen bonds. The resulting three-dimensional network is a perfect honeycomb structure (Figure 6). Determination of the structural contents within the channels proved impossible. Difference Fourier maps based on all of the data and on a low angle subset revealed only a uniform distribution of peaks, which when coupled with the crystallographic symmetry indicated smooth rings of electron density situated about the 6/mmm sites at z = 0 and 1/2. Although there are no reported alkali metal salts with HEG, reported M(HEG) salts27 reveal the molecules’ propensity to wrap around the metal in a near cyclic structure, with the two terminal hydroxyls either disposed equally to either side of the approximate ligand plane or with one in and the other out of the plane. A smooth, continuous rotational disorder of either of these models would agree with the observed electron-density distribution, as would a completely random, but static orientational disorder of the molecule. The smooth walls of the honeycomb channels apparently offer no orientational energy well to anchor the structure, although the electrostatic cation−anion do localize the HEG-encased metal ions along the c-axis. For the sodium

Figure 3. Network structure of [M(K222)]I·TIE, viewed parallel to the b-axis. 2764



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additional disorder of the metal throughout the channel, as the structures are not charge balanced. The ordered anionic pseudo-polyiodide network coupled with channels filled with cationic liquid is reminiscent of a class of polyiodide canal complexes, the Clover phases.28 These clathrated alkali metal salts contain a polyiodine chain included as a guest within channels formed by alkali metal coordinated with carbonyl compounds such as benzophenone or courmarin.29 Clover phases have been shown to exhibit both electronic and fast ion conductivity.30 Structure of [M(K222)]I3·TIE, M = K, Rb. The potassium and rubidium salts are isomorphous, crystallizing in the orthorhombic space group, P212121. The asymmetric unit contains a K222 encrypted alkali metal cation, one TIE molecule, and triiodide anion, each occupying a general position in the unit cell. The triiodide is asymmetric with I−I distances of 2.9951(18) and 2.8435(18). The more weakly bonded terminal iodine of the anion interacts with four iodine atoms of four different TIE molecules to form a two-dimensional layer, shown in Figure 7a. The pendant I2 molecules, assuming an I−···I2 description of the triiodide, extend to either side of the undulating layer (Figure 7b). The layers stack along the c-axis, and the M(K222)+ ions occupy the space between them, with pendant I2 molecules interposed between cations along the aand the b-directions (Figure 8). The compound represents only the second example of an I3− anion acting as a halogen bond acceptor with TIE.4,31 In the other compound the anion interacts at both ends of a symmetric I3− ion. Structural Comparison and Discussion. It is often observed that a given cation will crystallize with polyiodides of several different stoichiometries,32 as is the case with [M(K222)]+ crystallizing with I−···TIE, I−···1.5TIE, and I3−···TIE. It is interesting that for all three stoichiometries, the cations are localized into planes separated by anionic layers, and further separated within their domains by either a second I- anion bridging the anionic layers in [M(K222)]I·TIE, or “folds” of the anionic layer in [M(K222)]I·1.5TIE, or by pendant I2 molecules in [M(K222)]I3·TIE. Of further interest is the similarity of anionic networks for the two different cationic systems in [M(K222)]I·1.5TIE and [M(HEG)]I·1.5TIE. A portion of the anionic layer of [M(K222)]I·1.5TIE and of the anionic network of [M(HEG)]I·1.5TIE is shown in Figure 9. Rotation of the TIE molecules of the ring shown in Figure 9a, such that they are parallel with a 3fold axis axis running through the center of the ring, gives a ring that is essentially identical to the one shown in Figure 9b. This places atoms I1 and I2 (and their 3-fold rotationally related partners) in proper position to interact with I3 of the next layer below it. Likewise I4 and I5 are brought into proper position to interact with I6 of the next layer above. These transformations coupled with the extended nature of the layers in [M(K222)]I·1.5TIE would produce the perfect honeycomb network structure of [M(HEG)]I·1.5TIE. The latter is another example of the “pea-in-a-pod” structures described by Yamamoto and co-workers in their work to produce nanowires insulated with pseudo-polyiodide sheaths.21 Apparently, the [M(K222)]+ ions are too large for the “pod”, but the similarity of the two structures suggests a similar nucleation assembly, possibly consisting of a layer as observed in [M(K222)]I·1.5TIE, with cations tucked into the cavities within the hexameric rings. The [M(HEG)]+ ions are small enough to fully insert into these cavities, leading to the building block unit suggested in Figure

Figure 5. Egg crate structure of [M(K222)]I·1.5TIE, viewed parallel to the a-axis.

Figure 6. Honeycomb anionic I−···TIE network in [M(HEG)]I·1.5TIE viewed parallel to the c-axis.

salt, there are reasonably large peaks in general positions situated just off the 6/mmm site, presumed to be associated with the cation. Disorder of the cation away from the center of the channel indicates a size mismatch, with the [Na(HEG)]+ moiety being slightly too small to fill the channel, while for potassium and rubidium salts apparent metal peaks are coincident with the symmetry site, suggesting a better fit. The metal ions could not be refined for the sodium salt, but could for the other two; however, their refinement suggests 2765



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Figure 7. (a) View down the c-axis of two-dimensional I3−···TIE layer in [M(K222)]I3·TIE (b) Side view of the layer showing the pendant I3− groups and the undulating nature of the layer.

Figure 9. Comparison of rings in (a) [M(K222)]I·1.5TIE and (b) [M(HEG)]I·1.5TIE.

stronger with stronger overall interaction energies. The exception to this is in the case of the very good donor system CF3I. Like polyiodides, pseudo-polyhalides exhibit a wide variety of structures, and this is strongly dependent on the size, shape, and charge of the countercation. Tetraiodoethylene adds an additional variable with the presence of four potential halogen donor sites. With a K222 encrypted alkali metal cation, TIE forms layers and networks with iodide, and these have widely different structures, while with hexaethylene glycol a network structure closely related to one of the layered structures with K222 is formed. The similarity of [M(HEG)]I·1.5TIE to Clover phases suggests the possibility of interesting electrical properties, and investigation of these is underway. We are also continuing to survey the structural diversity of pseudo-polyhalide systems through systematic variation of the halide anion, the organohalogen molecule, and the cation.

Figure 8. Packing view of stacked layers in [M(K222)]I3·TIE viewed down the a-axis.

9b, while the larger [M(K222)]+ ion cannot, leading to a bumpy layer with no halogen bonds possible in the third dimension. Further work in this area with a variety of cationic systems is being pursued.

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CONCLUSION DFT calculations predict that smaller halide ions are better XB acceptors, while species featuring larger halides are better XB donors. The interaction is primarily electrostatic, but there is a non-negligible charge-transfer component that is generally 2766



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(13) Jacobson, R. A. Empirical Absorption Correction, Version 1.1; Rigaku/Molecular Structure Corp.: The Woodlands, TX, 1988. (14) Sheldrick, G. M. SHELXTL, Crystallographic Computing System, version 6.12; Bruker AXS: Madison, WI, 2003. (15) International Tables for X-ray Crystallography; Wilson, A. J. C., Ed.; Kluwer Academic Publishers: Dordrecht, 1992; Vol. C, Table 6.1.1.4, pp 500−502, and Table 4.2.6.8, pp 219−222. (16) (a) Pennington, W. T.; Hanks, T. W.; Arman, H. D. Halogen Bonding with Dihalogens and Interhalogens. In Halogen Bonding, Fundamentals and Applications; Resnati, G., Metrangolo, P., Eds.; Springer: London, 2008; Structure and Bonding Series, Vol. 126, Chapter 3. (b) Ruiz, T. P.; Gomez, M. F.; Gonzalez, J. J. L.; Koziol, A. E. Chem. Phys. 2008, 320, 164. (17) Hohenstein, E. G.; Chill, S. T.; Sherrill, C. D. J. Chem. Theor. Comput. 2008, 4, 1996−2000. (18) (a) Lu, Y.-X.; Zou, J.-W.; Wang, Y.-H.; Yu, Q.-S. J. Mol. Struct. Theochem. 2006, 776, 83−87. (b) Lu, Y.-X.; Zou, J.-W.; Wang, Y.-H.; Jiang, Y,-J.; Yu, Q.-S. J. Phys. Chem. A 2007, 111, 10781−10788. (19) Cambridge Structural Database (Version 5.32, August 2011 update): Allen, F. H. Acta Crystallogr. 2002, B58, 380 Surveys were based on a VDW radii cut-off for ordered structures determined from single crystal X-ray analysis (no powder), with 3-D coordinates determined, crystallographic residual R < 0.1, and monovalent, neutral XB donors bonded to carbon (i.e., organohalogens). (20) Bondi, A. J. Phys. Chem. 1964, 68, 441−451 vdW values (Å): F, 1.47; Cl, 1.75; Br, 1.85; I, 1.98. (21) (a) Yamamoto, H. M.; Yamaura, J. I.; Kato, R. J. Am. Chem. Soc. 1998, 120, 5905−5913. (b) Yamamoto, H. M.; Kosaka, Y.; Maeda, R.; Yamaura, J. I.; Nakao, A.; Nakamura, T.; Kato, R. ACS Nano 2008, 2, 143−155. (22) Bock, H.; Holl, S. Z. Naturforsch., B: Chem. Sci. 2002, 57, 713− 725. (23) (a) Navarrini, W.; Metrangolo, P.; Pilati, T.; Resnati, G. New J. Chem. 2000, 24, 777−780. (b) Gattuso, G.; Pappalardo, A.; Parisi, M. F.; Pisagatti, I.; Crea, F.; Liantonio, R.; Metrangolo, P.; Navarrini, W.; Resnati, G.; Pilati, T.; Pappalardo, S. Tetrahedron 2007, 63, 4951− 4958. (c) Cavallo, G.; Biella, S.; Lü, J.; Metrangolo, P.; Pilati, T.; Resnati, G.; Terraneo, G. J. Fluorine Chem. 2010, 131, 1165−1172. (24) (a) Yamamoto, H. M.; Yamaura, J. I.; Kato, R. Synth. Met. 1999, 102, 1448−1451. (b) Yamamoto, H. M.; Maeda, R.; Yamaura, J. I.; Kato, R. J. Mater. Chem. 2001, 11, 1034−1041. (c) Fourmigué, M.; Batail, P. Chem. Rev. 2004, 104, 5379−5418. (d) Imakubo, T.; Shirahata, T.; Hervé, K.; Ouahab, L. J. Mater. Chem. 2006, 16, 162− 173. (25) (a) Grebe, J; Geiseler, G.; Harms, K.; Dehnicke, K. Z. Naturforsch., B: Chem. Sci. 1999, 54, 77−86. (b) Bock, H.; Holl, S. Z. Naturforsch., B: Chem. Sci. 2002, 57, 713−725. (c) Liantonio, R.; Metrangolo, P.; Pilati, T.; Resnati, G. Cryst. Growth Des. 2003, 3, 355− 361. (d) Metrangolo, P.; Meyer, F.; Pilati, T.; Resnati, G.; Terraneo, G. Chem. Commun. 2008, 1635−1637. (e) Metrangolo, P.; Carcenac, Y.; Lahntinen, M.; Pilati, T.; Rissanen, K.; Vij, A.; Resnati, G. Science 2009, 323, 1461−1464. (26) (a) M = Na+: Rosokha, S. V.; Lu, J.; Rosokha, T. Y.; Kochi, J. K. Phys. Chem. Chem. Phys. 2009, 11, 324−332. (b) M = K+: see ref 25d. (c) M = Rb+: Link, C.; Pantenburg, I.; Meyer, G. Z. Anorg. Allg. Chem. 2008, 634, 616−618. (27) (a) Yamaguchi, I.; Miki, K.; Yasuoka, N.; Kasai, N. Bull. Chem. Soc. Jpn. 1982, 55, 1372−1375. (b) Rogers, R. D.; Rollins, A. N.; henry, R. F.; Murdoch, J. S.; Etzenhouser, R. D.; Huggins, S. E.; Nunez, L. Inorg. Chem. 1991, 30, 4946−4954. Rogers, R. D.; Jezi, M. L.; Bauer, C. B. Inorg. Chem. 1994, 33, 5682−5692. (d) Rogers, R. D.; Bond, A. H.; Roden, D. M. Inorg. Chem. 1996, 35, 6964−6973. (28) Clover, A. M. J. Am. Chem. Soc. 1904, 31, 256. (29) (a) Labes, M. M.; Jones, M.; Kao, H. I.; Nichols, L.; Hsu, C. Mol. Cryst. Liq. Cryst. 1979, 52, 115−120. (b) Bolton, B. A.; Prasad, P. N. Mol. Cryst. Liq. Cryst. 1981, 76, 309−317. (c) Wu, C.; Kim, B.; Kao, H. I.; Griffin, C. W.; Jones, M.; Labes, M. M. Mol. Cryst. Liq. Cryst. 1982, 88, 317−330.

ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic information files (CIF) for [M(K222)]I·TIE, M = K, Rb; [M(K222)]I·1.5TIE, M = Na, K, Rb; [M(HEG)]I·1.5TIE, M = Na, K, Rb; [M(K222)]I·TIE, M = K, Rb. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.T.P.). Email: Tim.Hanks@ furman.edu (T.W.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Research Corporation. REFERENCES

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Computational and Crystallographic Studies of Pseudo-Polyhalides

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