Article pubs.acs.org/crystal
Solution and Solid State Synthesis of the Discrete Polyiodide I73− under Modular Cation Templation Published as part of the Crystal Growth & Design virtual special issue In Honor of Prof. G. R. Desiraju Jingxiang Lin,† Javier Martí-Rujas,‡ Pierangelo Metrangolo,*,†,‡ Tullio Pilati,† Stefano Radice,§ Giuseppe Resnati,*,†,‡ and Giancarlo Terraneo†,‡ †
NFMLab, Department of Chemistry, Materials, and Chemical Engineering “Giulio Natta”, Politecnico di Milano, Via Mancinelli 7, I-20131 Milan, Italy ‡ Centre for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, I-20133 Milan, Italy § Solvay Specialty Polymers Italy SpA, Viale Lombardia 20, I-20021 Bollate (MI), Italy S Supporting Information *
ABSTRACT: Discrete I73¯ polyiodide is obtained in pure form through solution and solid-state processes thanks to templation by a triammonium cation which elicits the selective formation of the size matching supramolecular anion.
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As a part of our interest in optimizing charge transport in I−/ I3 -based dye sensitized solar cells (DSSCs),6 we are engaged in an ongoing project which applies the principles of supramolecular chemistry and crystal engineering to the design and synthesis of ionic organic solids wherein specific polyiodides,7 polyhalides, and mixed polyhalides8 are present. The aim is to optimize conductivity in solid and semisolid materials containing polyhalides by favoring charge transport through a hopping mechanism over an ions transport mechanism.9 If the hopping mechanism is operative, an increase in the length of polyiodides would increase the efficiency of electrical conductivity, and this is the reason for our search for new general protocols for the synthesis of large polyiodides. To the best of our knowledge, examples of precise control in the exact composition and crystalline structure of polyiodides are not numerous because the nature and structural chemistry of solid polyiodides is the result of a subtle balance of several factors, among others the steric and electronic features of the anion neighborhood (e.g., the cation nature),10 the stoichiometry of the reagents, the conditions of crystal formation (e.g., the solvent(s) and its (their) polarity), etc.11 Polyiodides being obtained through diiodine uptake by iodide salts, the positive counterion plays a key role in determining both the stoichiometry and the geometry of the formed polyiodide. A strategy in the synthesis of solid
INTRODUCTION Polyiodides are attracting continuing interest due to their fascinating structural chemistry and, more important, their diversified properties allowing for useful applications in a variety of areas, including superconductor, solar cells, optical devices, etc.1 Different synthetic approaches have afforded several hundreds of polyiodides with quite diverse structures spanning from the discrete units (zero-dimensional (0D) adducts), to 1D, 2D and 3D networks. Interaction distances and angles in solid systems cover a wide range of values and, in most cases, are the result of the so named type I and type II iodine−iodine interactions.2 The former interactions are typically weak, and rather than determining the adopted crystal packing, they are just the outcome of symmetry requirements and energy minimization in the overall crystal packing.3 Differently, type II iodine−iodine interactions usually contribute to the crystal packing stabilization. They are a direct consequence of the anisotropic electron density distribution around covalently bound iodine atoms, and result from the attraction between the electrophilic caps, namely. positive σholes,4 and the nucleophilic regions present on interacting atoms. Type II iodine−iodine interactions belong to the larger family of halogen bonding (XB).5 I2, I−, and I3− are the building blocks affording, on self-assembly, the larger polyiodides, I2 functions as the XB donor unit (Lewis acid, electrophilic site), while I− and I3− function as the XB acceptor units (Lewis bases, nucleophilic sites). Short contacts with geometric and electronic features in between the two types described above can also occur, mainly in extended polyiodide nets. © 2012 American Chemical Society
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Received: August 30, 2012 Revised: October 3, 2012 Published: October 5, 2012 5757
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Scheme 1. Structural Formulas of Diiodide Salts Used for the Templated Self-Assembly of the Described Polyiodides
CCD diffractometer using Mo−Kα radiation (λ = 0.71073 Å). Data integration and reduction were performed using SaintPlus 6.01. Absorption correction was performed with a multi-scan method implemented in SADABS.14 Space groups were determined using XPREP implemented in APEX II suite. Structures were solved using SHELXS-97 (direct methods) and refined using SHELXL-9715 (fullmatrix least-squares on F2) contained in APEX II and WinGX v1.80.0116 software packages. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in geometrically calculated positions and included in the refinement process using a riding model with isotropic thermal parameters (Table 1). The molecular diagrams were generated using Mercury 3.0.17 CIF files containing crystallographic data can be obtained free of charge from Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Powder X-ray Diffraction (PXRD). A Bruker AXS D8 powder diffractometer was used for all PXRD measurements with experimental parameters as follows: room temperature, Cu−Kα radiation (λ = 1.54056 Å), reflection mode, scanning interval: 2θ range 5−40°. Step size 0.016°, exposure time 1.5 s per step. Experimental PXRD patterns and simulated PXRD patterns from single crystal data were compared to confirm the composition of bulk materials. Differential Scanning Calorimetry (DSC). Thermal analysis was performed on a Mettler Toledo DSC 823e differential scanning calorimeter. Aluminum crucibles were used for all samples, and the instrument was calibrated using indium as standard. For reference, an empty crucible was used. The thermal analysis was performed from 25 to 250 °C at a rate of 10 °C/min. Infrared (FT-IR) and Raman Spectroscopy. The IR characterization of samples was performed on a Nicolet Nexus FTIR spectrometer equipped with Smart Endurance ATR-device. Spectra were measured over the range of 4000−550 cm−1 and analyzed using Omnic software v6.2. Peak values are given in wavenumbers and rounded to 1 cm−1 upon automatic assignment. FT-Raman spectra have been colletcted with a Thermo Nicolet Nexus 870 spectrometer equipped with a FT-Raman module. Laser exciting line: Nd-YAg laser emitting at 1064 nm. Laser power at sample in the range of 200 mW; collection parameters provided 1 cm−1 as spectral resolution, while 2048 scans have been coadded. Synthesis of 2,2,9,9,16,16-Hexamethyl-2,9,16-triazoniaheptadecane Tri-iodide (HMTAHD3+·3I−, 3a). Methyl iodide (2.27 g, 16.0 mmol) was added dropwise at room temperature to a solution of di-(6-aminohexyl)amine (0.2154 g, 1.0 mmol) and 1,2,2,6,6pentamethylpiperidine (0.908 mmL, 5.0 mmol) in 5.0 mL of DMF. The resulting solution was stirred overnight to give a white precipitate which was filtered and washed with CH3CN to yield 604 mg (85%
polyiodides with a given composition and crystalline structure is the use of cations which, on crystallization, can robustly template the formation of architectures possessing domains with a specific dimension and geometry tailored to accommodate the pursued polyiodide.1,12 For instance, long-chain alkyl ammonium cations allow for the dimensional caging of I3− and I5− anions11 as they tend to organize into lamellar structures wherein polyiodide anions are confined in the vicinity of the positive charges. In line with this strategy, bis(trimethyl ammonium) hexane diiodide (1a, BTMAH2+·2I−) selectively encapsulates iodine to give BTMAH2+·I42− 1b containing the rare polyiodide species I42− (Scheme 1).7a The formation of this uncommon anion is driven by the sizematching between the intramolecular distance of the N atoms in the dication unit and the distance between the terminal iodide ions in the self-assembled I42− unit (in 1b they are 8.9 and 9.6 Å, respectively). The reliability of the cation size to determine the nature and structure of the formed polyiodide is confirmed by the I42− polyanion formation also when the (E)1,2-bis(4,4′-bipyridinium)ethylene diiodide derivative 2a (N+− N+ distance ∼9.3 Å) reacts with iodine in solution and solid/ gas processes.8 Structural complementarity of the interacting charged moieties significantly influences the electrostatic binding strength,13 and in this paper we use this tenet to design cations enabling the synthesis of polyiodides larger than I42−. We report how 2,2,9,9,16,16-hexamethyl-2,9,16-triazoniaheptadecane triiodide (3a, HMTAHD3+·3I−) effectively templates the synthesis of discrete and V-shaped I73− polyiodides wherein diiodine is incorporated from solution and solid state conditions.
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EXPERIMENTAL SECTION
Materials and Methods. All starting materials were purchased from commercial suppliers (Apollo and Sigma Aldrich) and used without further purification. 1H NMR spectra were recorded on a Bruker AV-400 spectrometer at 400 MHz. Solid state synthesis was performed using a Retsch MM400 ball mill with 5.0 mL vessels. Single-Crystal X-ray Data Collection and Structure Determinations. Crystals suitable for single crystal X-ray diffraction analysis were selected using a polarized light optical microscope. X-ray diffraction data were collected on a Bruker-AXS KAPPA-APEX II 5758
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Table 1. Crystallographic Parameters 3b formula Fw crystal system space group a [Å] b [Å] c [Å] β [deg] V [Å3] T (K) Z F(000) ρcalcd [Mg m−3] μ [mm−1] Tmin, Tmax crystal size [mm3] Θmax [°] no. refl. collected Rint no. unique no. with I > 2σ(I) refined parameters restraints R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 [all data] wR2 [all data] Δρ [e Å−3] CCDC number
C20H48N3I7 1218.91 orthorhombic Pmmn 7.4778(12) 43.266(9) 5.7659(11) 90.00 1865.5(6) 200(2) 2 1120 2.170 5.835 0.001 × 0.25 × 0.32 22.50 6898 0.0705 1330 919 91 0.0616 0.1400 0.0932 0.1569 −1.278; 1.970 897949
0.0475 0.1120 0.0968 0.1236 −1.088; 1.125 897950
RESULTS AND DISCUSSION
As mentioned in the introduction, diiodine is a very effective XB donor. On reaction with I− it gives the linear I3− (I−·I2), the simplest polyiodide species and the best known halogenbonded (X-bonded) adduct. Under cation templation, one diiodine molecule can be X-bonded at either ends by two I− entering the two σ-holes on the extensions of the I−I covalent bond and the linear I42− is formed [2I−·I2]. Considering that iodide anions can be X-bonded to one and two diiodine molecules, we wondered about the structural features of a cation enabling the formation of the heptaiodide I 7 3− [3I−·2(I2)], namely, forcing an iodide anion to be X-bonded to two diiodine molecules having their respective endings both pinned by two other I− anions. BTMAH2+·2I− 1a templates the synthesis of linear I42− dianions, namely, one -(CH2)6-N+Me3 pendant on an ammonium moiety templates the entrance of two iodide anions on one diidodine molecule. We thus decided to verify if this strategy is modular, namely, if two -(CH2)6N+Me3 pendants on an ammonium moiety template the entrance of three iodide anion on two diiodine molecules and here we describe that HMTAHD3+·3I− 3a indeed allows for the synthesis of discrete and V-shaped I73− polyiodides [3I−·2(I2)]. Slow isothermal diffusion (23 °C) of n-hexane or diethyl ether into a solution of HMTAHD3+·3I− 3a and I2 (1:2 ratio) in MeOH/CH3CN/CHCl3 solvent mixture (1:4:7 ratio) afforded crystals of a new compound 3b having a darker color (brown vs white) and a lower melting point (156−159 °C vs 267−270 °C) than starting trisiodide 3a. Both features are consistent with the transformation of an iodide salt into a polyiodide. The single crystal X-ray analysis of 3b (Table 1) revealed that the compound is in the orthorhombic Pmmn space group and that HMTAHD3+·3I− and I2 are present in a 1:2 ratio. A half molecule of HMTAHD3+, one iodine molecule, and 1.5 iodide anions are present in the unit cell, and the trication HMTAHD3+ adopts a conformation where the central nitrogen atom lies on two orthogonal mirrors bisecting the two methyl and ammoniumhexyl pendants, respectively. The nearly perfect all trans conformation adopted by hexyl chains results in an N1−N2 distance of 8.889(2) Å. Iodide anions are hold close to the positive nitrogen atoms by electrostatic attraction and short I−···H contacts (∼3.1 Å) with methyl groups. The two I− anions close to the terminal trimethylammonium groups are at 9.624(2) Å from the I − anion close to the central dimethylammonium group, namely, they are perfectly positioned to double pin two diiodine molecules via I−···I X-bonds (Figure 1; I1···I4 distances: 3.4270(6) Å; I2···I3 distances: 3.3875(7) Å; I2···I3−I4 angle: 177.86(9)°, I3−I4···I1 angle 178.5(1)°). The I−···I X-bonds are fairly strong as suggested by their linearity and length (the ‘normalized contact’18 for I1···I4 and I2···I3 are 0.83 and 0.82, respectively). As a likely consequence of the fine balance of electrostatics/polarization contribute and n → σ* character of XBs,19 the covalent bond of I2 molecule in 3b is significantly elongated (2.8127(5) Å) relative to elemental iodine (2.72 Å). The two I2 molecules work as connectors of the three iodide anions and an I73− polyiodide, i.e., I−···I2···I−···I2···I−, is constructed. As it is often the case when iodide anions work as bidentate acceptors,1c,5a the angle formed by the two entering iodine molecules is nonlinear (I4···I1···I4′ angle: 136.22(9)°) and the I73− polyiodide has a V-shape. This
3c C20H48N3I9 1472.71 monoclinic C2/c 35.3094(15) 9.7861(4) 11.8691(5) 91.926(2) 4098.9(3) 298(2) 4 2664 2.386 6.820 0.5621, 0.7461 0.01 × 0.16 × 0.45 30.66 59598 0.0407 6176 3764 150
Article
yield) of the pure 3a as a white powder product. Mp 267−270 °C. 1H NMR (400 MHz, CD3OD): δ 1.49−1.54 (m, 8H), 1.83−1.89 (m, 8H), 3.12 (s, 6H), 3.15 (s, 18H), 3.38−3.44 (m, 8H). IR (selected absorptions): ν: 3022, 2976, 2945, 2918, 2852, 2359, 2340, 1479, 1471, 1455, 1417, 1254, 1047, 971 945, 902, 863, 792, 725 cm−1. Synthesis of 2,2,9,9,16,16-Hexamethyl-2,9,16-triazoniaheptadecane Tri-iodide/bis-diiodine Adduct (HMTAHD3+·I73−, 3b). A solution of I2 (20.2 mg, 0.08 mmol) in 1.0 mL of CH3CN was added to a solution of HMTAHD3+·3I− (28.4 mg, 0.04 mmol) in 3.0 mL of CH3CN and 1.0 mL of MeOH. The resulting mixture was diluted with 7.0 mL of CHCl3 and the obtained solution was transferred to a test tube. 5.0 mL of n-hexane or Et2O were layered on the top. After one day, the formed brown crystals of 3b were filtered and briefly dried under a vacuum. Mp 156−159 °C from DSC analysis. PXRD are shown in the Supporting Information. IR (selected absorptions): ν: 3153, 3032, 3016, 2968, 2942, 2905, 2877, 2865, 1486, 1463, 1440, 1238, 1035, 972, 762, 548 cm−1. Synthesis of 2,2,9,9,16,16-Hexamethyl-2,9,16-triazoniaheptadecane Tri-iodide/tris-diiodine Adduct HMTAHD3+·3I3−, 3c: Diiodine (10 mg, 0.04 mmol) or (15 mg, 0.06 mmol) was added to a solution of HMTAHD3+·3I− (14.2 mg, 0.02 mmol) in 1:1 methanol/ water (4.0 mL). The resulting mixture was allowed to evaporate solvent at room temperature. After some days, the tris-triiodide 3c was obtained as reddish plate crystals. Mp 117−120 °C from DSC analysis. PXRD are shown in the Supporting Information. IR (selected absorptions): ν: 3107, 3016, 2968, 2936, 2931, 2910, 2881, 2855, 1483, 1462, 1436, 1421, 1309, 1275, 758, 529 cm−1. A crystalline phase of 3c is also obtained quantitatively (as shown PXRD analysis) if HMTAHD3+·3I− (14.2 mg, 0.02 mmol) and diiodine (15.2 mg, 0.06 mmol) are ball milled for 30 min. 5759
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Crystalline diammonium diiodides proved to be able to function as dynamically porous systems capable to adsorb dihalogens7,8 and perhaloalkanes21 in solid−gas and solid−solid processes thanks to the formation of strong X-bonds between iodide anions and entering halogenated hosts. We thus decided to verify if also crystalline triammonium trisiodide 3a shows the same behavior and we tried a solid state synthesis of 3b.22 HMTAHD3+·3I− and I2 (1:2 ratio) were ball milled for 30 min and the formed microcrystalline powder showed a melting point (DSC analysis) identical to that of 3b obtained from solution. More interesting, the powder X-ray analysis gave a pattern nicely overlapping with the experimental pattern of 3b prepared from solution and the simulated pattern from single crystal X-ray analysis (see Supporting Information). This proves trication HMTAHD3+ maintains its ability to identify the nature of the formed polyiodide species also in solid state reactions. In order to identify the limits of the ability of HMTAHD3+·3I− 3a to template the formation of the I73− anion, we tried crystallization conditions which are known to disfavor the formation of large polyiodides. Specifically, when iodide anions and diiodine are in aqueous or highly polar solvents, the formation of triiodide anion is favored over the formation of larger polyiodides while the opposite is true in solvents of low polarity.1,10 This is related to the fact that smaller anions are more solvatable than larger ones. We thus tried the crystallization of HMTAHD3+·3I− 3a and I2 (1:2 ratio) from methanol/water solution (1:1 ratio). Brown-red thin crystals melting at 117−120 °C (DSC analysis) are formed exclusively when crystallization is stopped at 50−55% yields. The crystals obtained are a new species 3c different from 3b, and the melting point suggests that its diiodine content is higher than in 3b (156−159 °C, an empirical rule indicates that the higher the diiodine content of a crystal, the lower its melting point).9b The single crystal X-ray analysis of 3c confirmed this expectation, showing that the HMTAHD3+·3I−/ I2 ratio is 1:3 and that three well-defined but interacting I3− anions are present in 3c instead of the I73_ anion (Figure 4). The tris-triiodine adduct 3c was obtained in nearly quantitative yields when 3a and I2 in 1:3 ratio were crystallized from the same solvent mixture or ball milled for 30 min. Under these conditions the solvent polarity prevails over the templating ability of the trication HMTAHD3+ in identifying the nature of the formed polyiodide species. Raman spectroscopy is a very effective and diagnostic technique in investigating the nature of polyiodides. It nicely complements single crystal X-ray analysis because it gives information on the bulk material. Raman spectra of the heptaiodide 3b and the tris-triiodide 3c were registered (Figure 5). In the region 500−100 cm−1, samples of 3b from solution and ball milling gave the very same spectra suggesting their
Figure 1. View (ball and stick, Mercury) of the asymmetric unit (dark colors) and its symmetric pair (light colors) of HMTAHD3+·I73− 3b. XB is reported as dashed lines. Hydrogen atoms have been omitted for clarity. Color code: Gray, carbon; blue, nitrogen; purple, iodine.
supramolecular anion can be described as two I−···I2···I− polyiodide united as one by sharing one iodide ending. This description is perfectly in line with our crystal engineering aimed at templating this process by appending two ωtrimethylammonium-hexyl chains to a dimethylammonium core. Four symmetry related HMTAHD3+ trications define a bent cavity encapsulating the V-shaped I73− polyiodide (Figure 2)
Figure 2. Representation (Mercury) of a I73− trianion (in space filling) encapsulated, in 3b, by the cavity defined by four HMTAHD3+ trications (in sticks). Color code: whitish, hydrogen; other colors as in Figure 1.
and isolating it from neighboring I73− polyiodides. The shortest distance between two I73− supramolecular anions is 5.3110(7) Å and involves two symmetry related I2 atoms (iodine van der Waals radius is 1.98 Å and Pauling iodide anion radius is 2.16 Å). To the best of our knowledge, this is the first example of a discrete I 7 3_ polyiodide species. 2 0 The polyiodide I−···I2···I−···I2···I− are pinned in their position by electrostatic interactions and a net of I···H short contacts (Figure 3, left). The overall crystal packing of 3b recalls that of BTMAH2+ ·I42− 1b (Figure 3, right) wherein the tetraiodide I−···I2···I− is kept in cavities formed by four BTMAH2+ units by electrostatic interactions and I···H short contacts.
Figure 3. Crystal packings (ball and stick representation, Mercury) viewed along the c crystallographic axis of the complexes HMTAHD3+·I73− 3b (left) and BTMAH2+ ·I42− 1b (right). X-bonds and I···H hydrogen bonds are black and red dashed lines, respectively. Colors as in Figure 2. 5760
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The Raman spectrum of tris-triiodide 3c (500−100 cm−1 region) shows two prominent bands at 142 and 118 cm−1: they are characteristic20 for the ν3 antisymmetrical and ν1 symmetrical stretching modes of discrete, linear, and asymmetric triiodide ions. The broad and complex peak at 231 cm−1 is unprecedented, especially in terms of the observed Raman intensity. It may be due to specific geometric properties (i.e., deformation of the perfect linear shape) of the three I3−, namely, I93−, units which might affect and change vibrational selection rules. This is yet to be ascertained, while the nonperfect linear shape of the polyiodide, which reduces the overall molecular symmetry, sheds some confidence on this hypothesis.
Figure 4. View (ball and stick, Mercury) of the asymmetric unit (dark colors) and its symmetric pair (light colors) of HMTAHD3+·3I3− 3c. The I3− anion close to the trimethylammonium group is slightly asymmetric and nearly linear (I5−I4 and I4−I3 distances are 2.9484(7) Å and 2.9022(7) Å, respectively; I5−I4−I3 angle: 177.38(2)°). The other I3−, located in the vicinity of dimethylammonium group, is symmetric and nearly linear (I2−I1 distance: 2.9105(7) Å; I2−I1−I2′ angle of 172.55(3)°). I3− are further involved in weak interactions (black dashed lines, I2···I3 distance: 3.7351(9) Å). The trication HMTAHD3+ adopts a bent conformation similar to that in 3b. Hydrogen atoms have been omitted for clarity. Color code as in Figure 1.
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CONCLUSIONS In this paper we have shown how, in the presence of diiodine, the triammonium triiodide salt HMTAHD3+·3I− 3a affords the heptaiodide HMTAHD3+·I73− 3b wherein there is a nice dimensional matching between the trication and the supramolecular trianion. The heptaiodide I73− is effectively caged by four cation molecules. The templating ability of 3a is robust enough to give pure batches of 3b from solution and solid state processes. The strength of the XB formed when diiodine and I− are the XB donor and acceptor, respectively, probably contributes to the efficacy of the protocol. The behavior is strictly parallel to that shown by the diammonium diiodide BTMAH2+·2I− 1a which templates the formation of the uncommon tetraiodide I42−.7a The ability of the cation−anion size matching principle to prevail over other numerous factors influencing the formation of polyiodides seems to be general and modular and can thus be expected to allow for the predictable obtainment of other uncommon polyhalide and mixed polyhalide species. This has the potential for significantly augmenting our knowledge of polyhalide species, as well as providing a novel approach to the field of their design and synthesis.
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ASSOCIATED CONTENT
* Supporting Information S
Structural formula scheme, X-ray powder diffraction patterns, Raman spectra and CheckCIF/PLATON for 3b and 3c. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +39 02 2399 3180; Tel: +39 02 2399 3041 (P.M.), +39 02 2399 3032 (G.R.). E-mail:
[email protected] (G.R.);
[email protected] (P.M.).
Figure 5. Raman spectra of HMTAHD3+·I73− 3b (top) and of HMTAHD3+·3I3− 3c (bottom).
Notes
The authors declare no competing financial interest.
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homogeneity and confirming the effectiveness of the trication HMTAHD3+ in templating the formation of I73−. The spectrum shows a single intense band at 163 cm−1, a frequency typical for the νI−I vibration of the iodine molecule perturbed by an Xbonded iodide anion on each side.7a,23 This assignment supports the description of I73− as two I−···I2···I− tetraiodide united by sharing one iodide ending. No spurious peaks are present, indicating the stability of 3b under Raman measurement conditions despite its intense brown color. Polyiodide salts are frequently unstable and easily lose iodine on heating; the stability of 3b can be related to the effective caging of I73− by the four neighboring HMTAHD3+ cations.
ACKNOWLEDGMENTS Authors thank Fondazione Cariplo for financial support under the projects 2009-2550 and 2010-1351. REFERENCES
(1) (a) Küpper, F. C.; Feiters, M. C.; Olofsson, B.; Kaiho, T.; Yanagida, S.; Zimmermann, M. B.; Carpenter, L. J.; Luther, G. W., III; Lu, Z.; Jonsson, M.; Kloo, L. Angew. Chem., Int. Ed. 2011, 50, 11598− 11620. (b) Yin, Z.; Wang, Q. X.; Zeng, M. H. J. Am. Chem. Soc. 2012, 134, 4857−4863. (c) Svensson, P. H.; Kloo, L. Chem. Rev. 2003, 103, 1648−1684. (d) Heeger, A. J. Angew. Chem., Int. Ed. Engl. 2001, 40, 5761
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Å range exist in both structures. The presence of discrete I73_ polyhalides has been claimed also in Co(NH3)6](I3)(I4) but the compound contains isolated I3¯ and I42¯ units. Tebbe, K.-F. Acta Crystallogr. 1983, C39, 154−159. (21) Metrangolo, P.; Carcenac, Y.; Lahtinen, M.; Pilati, T.; Rissanen, K.; Vij, A.; Resnati, G. Science 2009, 323, 1461−1464. (22) We were unable to obtain a good quality single crystal of 3a for X-ray diffraction analysis, but the structure of HMTAHD3+·3Br−·3H2O supports the expectation that 3a is a non porous material and has an overall crystal packing remarkably different from 3b. (23) Deplano, P.; Ferraro, J. R.; Mercuri, M. L.; Trogu, E. F. Coord. Chem. Rev. 1999, 188, 71−95.
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dx.doi.org/10.1021/cg301262k | Cryst. Growth Des. 2012, 12, 5757−5762
