Efficient Diffusion-Controlled Ligand Exchange Crystal Growth of

Article pubs.acs.org/crystal

Efficient Diffusion-Controlled Ligand Exchange Crystal Growth of Isostructural Inorganic−Organic Halogenidorhodates(III): The Missing Hexaiodidorhodate(III) Anion Maciej Bujak* Institut für Anorganische Chemie und Strukturchemie, Lehrstuhl II: Material- und Strukturforschung, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, D-40225 Düsseldorf, Germany Faculty of Chemistry, University of Opole, Oleska 48, 45-052 Opole, Poland S Supporting Information *

ABSTRACT: The monohydrates of piperazine-1,4-diium hexabromidorhodate(III) bromide and hexaiodidorhodate(III) iodide were obtained by a diffusioncontrolled ligand exchange crystal growth method using a hydrochloric acid solution of rhodium(III) chloride trihydrate and piperazine, dissolved in hydrobromic and hydroiodic acid, respectively, separated by a layer of hydrohalic acid. Both inorganic−organic hybrids are defined by the general formula (C4H12N2)2[RhX6]X·H2O (X = Br, 1 or I, 2). They both crystallize in the orthorhombic Pnma space group, and they are isostructural with an isostructurality index above 95%. The cationic building blockspiperazine-1,4diium ions and the inorganic componentsslightly distorted octahedral [RhX6]3− complexes, isolated X− anions and water of crystallization molecules are connected by related but different systems of interactions. The comparison of packing arrangements and interactions in the crystals of 1 and 2 with those in metal-free (C4H12N2)Br2·H2O, 3, and (C4H12N2)I2·I2, 4, clearly illustrates the occurrence and hierarchy of specific interactions: the bromide-containing structures are dominated by the O/N/C−H···X hydrogen bonds that are less pronounced or exchanged by the X···X halogen bonds in the corresponding iodide-containing structures. The structural features derived from the X-ray diffraction studies are confirmed by the solid-state IR and Raman spectroscopic results supported by the thermoanalytical analyses.

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INTRODUCTION

are more weakly associated with the anionic substructures than in the corresponding hexachloridorhodates(III). We have recently reported structural, spectroscopic, and thermoanalytical properties of the [4,4′-H2bipy][H7O3][RhBr6] hybrid that exhibits enhanced thermal and chemical stability. This inorganic−organic compound was also obtained by the slow diffusion of reactants using rhodium(III) chloride trihydrate as a starting material.11 The aim of introducing the relatively large aromatic 4,4′-bipyridindiium cations was to create a specific pattern with appropriate cavities for inclusion of the [H7O3]+ aquahydrogen guest ions. The results of those studies showed that the physical, chemical, and thermal properties of the framework are controlled, to a large extent, by the relatively rigid components of the structure that form appropriate voids for the embedded [H7O3]+ ions. Further, the weakly associated aquahydrogen ions leave the sample in a controlled manner under specific thermal conditionsupon heating the polycrystalline sample up to 395 K first water is lost, and then starting at 426 K [H3O]Br is lost. Therefore, this relatively simple synthesized hybrid was found to be a

Open-framework hybrid materials have been a focus of chemical research for numerous applications, including gas storage and separation, because of the opportunity to control their electronic and catalytic properties by tailored synthesis to incorporate specific functional groups into the frameworks.1−6 There are several methods and techniques used to synthesize, control the final composition, and grow single crystals of those hybrids. These include establishing the synthesis and crystallization conditions together with controlling the composition and properties of starting materials. Ligand exchange reactions were found to be a convenient way to obtain new materials and, which is more important, easily modify, redesign, and improve materials using preplanned synthesis that can be carried out at relatively mild conditions.6,7 Two α,ω-diammonioalkane hexabromidorhodates(III) [NH3(CH2)xNH3]2[H5O2][RhBr6]Br2 (x = 3, 4) were the first rhodium(III) compounds obtained in the ligand exchange preparation procedure using rhodium(III) chloride trihydrate, amine, and hydrobromic acid as the starting materials.8 The bromido by chlorido ligand replacement did not apply any significant changes, 9,10 but clearly in the crystals of hexabromidorhodates(III) the diaquahydrogen [H5O2]+ cations © XXXX American Chemical Society

Received: November 20, 2014 Revised: January 11, 2015

A

DOI: 10.1021/cg501694d Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Table 1. Selected Crystal Data for 1, 2, 3, and 4 formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z reflns collected reflns unique, Rint R1, wR2, I > 2σ(I)

1

2

3

4

C8H26Br7N4ORh 856.54 orthorhombic Pnma 26.3712(9) 10.1433(5) 7.9013(3) 90.000 90.000 90.000 2113.53(15) 4 22021 2091, 0.0395 0.0203, 0.0343

C8H26I7N4ORh 1185.54 orthorhombic Pnma 27.4238(9) 10.5512(4) 8.3875(3) 90.000 90.000 90.000 2426.96(15) 4 25227 2379, 0.0342 0.0196, 0.0353

C4H14Br2N2O 265.97 monoclinic C2/c 11.4831(5) 5.9049(2) 13.9367(6) 90.000 108.393(4) 90.000 896.72(7) 4 8295 927, 0.0485 0.0201, 0.0471

C4H12I4N2 595.76 triclinic P1̅ 6.5186(3) 7.4950(3) 8.1312(4) 64.635(4) 68.060(4) 69.507(4) 324.30(3) 1 12400 1341, 0.0441 0.0226, 0.0549

materials without further purification for the synthesis of piperazine1,4-diium hexabromidorhodate(III) bromide monohydrate, 1, piperazine-1,4-diium hexaiodidorhodate(III) iodide monohydrate, 2, as well as piperazine-1,4-diium dibromide monohydrate, 3, and piperazine1,4-diium diiodide diiodine, 4. Single crystals of 1 and 2 were prepared according to a previously reported procedure.8,11 Piperazine (94.2 mg, 1.09 mmol) was added to 3.5 (8.0) mL of preheated concentrated hydrobromic (hydroiodic, the mixture was filtered) acid. This solution was carefully transferred into a test tube containing the 0.10 mL solution of RhCl3·3H2O (0.15 mmol RhCl3) covered by a 0.5 mL layer of concentrated hydrobromic (hydroiodic) acid. The system was left at room temperature and subsequently formed needle-shaped dark red and dark brown crystals of 1 and 2, respectively. 1, IR, νmax, cm−1: 3345 (s, br), 3065 (vs), 3032 (vs), 3003 (vs), 2959 (vs), 2902 (s), 2879 (s), 2820 (s), 2782 (s), 2768 (s), 2712 (s), 2671 (s), 2647 (s), 2610 (m), 2565 (m), 2532 (m), 2479 (m), 2444 (m), 1708 (w), 1595 (s), 1574 (m), 1551 (m), 1478 (w), 1455 (m), 1440 (m), 1433 (m), 1409 (m), 1378 (m), 1311 (m), 1296 (w), 1285 (w), 1208 (w), 1186 (w), 1078 (m), 1055 (m), 1042 (w), 1008 (w), 1000 (w), 944 (w), 932 (w), 861 (m), 836 (vw), 810 (vw), 668 (vw), 641 (vw), 626 (vw), 582 (w), 546 (vw). Raman, νmax, cm−1: 3033 (w), 2994 (m), 2958 (m), 2904 (w), 1594 (vw), 1565 (vw), 1463 (w), 1404 (w), 1375 (w), 1323 (vw), 1308 (m), 1290 (w), 1204 (w), 1037 (m), 807 (m), 579 (w), 446 (w), 245 (m), 197 (s), 181 (vs), 168 (vs), 114 (s). Anal. Found: C 10.65, H 3.04, N 6.24. C8H26Br7N4ORh requires: C 11.22, H 3.06, N 6.54%. 2, IR, νmax, cm−1: 3331 (s, br), 3013 (vs), 2984 (vs), 2957 (vs), 2946 (vs), 2914 (vs), 2871 (s), 2821 (s), 2777 (vs), 2763 (vs), 2704 (s), 2666 (m), 2598 (m), 2550 (m), 2512 (m), 2487 (w), 2461 (w), 2447 (w), 2421 (w), 2371 (w), 1710 (vw), 1586 (s), 1540 (m), 1454 (m), 1427 (m), 1403 (m), 1373 (m), 1316 (w), 1304 (w), 1280 (w), 1198 (w), 1181 (w), 1073 (w), 1047 (w), 1038 (w), 992 (w), 934 (w), 920 (vw), 856 (w), 841 (w), 828 (vw), 803 (w), 648 (w), 566 (w). Raman, νmax, cm−1: 2981 (m), 2946 (m), 2896 (vw), 1607 (vw), 1582 (vw), 1546 (vw), 1193 (w), 1034 (w), 931 (w), 837 (vw), 800 (w), 568 (vw), 439 (w), 240 (vw), 202 (m), 168 (w), 130 (vs), 115 (vs). Anal. Found: C 7.95, H 2.30, N 4.73. C8H26I7N4ORh requires: C 8.10, H 2.21, N 4.73%. The single crystals of 3 and 4 were grown by slow evaporation at room temperature from solutions obtained by dissolving 0.50 g (5.8 mmol) and 0.25 g (2.9 mmol) of piperazine in preheated ca. 12 mL of concentrated hydrobromic and ca. 40 mL of concentrated hydroiodic acid, respectively. 3, IR, νmax, cm−1: 3512 (s), 3451 (s), 3232 (w), 3096 (vs), 3012 (s), 3001 (vs), 2975 (vs), 2953 (vs), 2914 (vs), 2787 (vs), 2723 (vs), 2590 (m), 2556 (s), 2446 (m), 2358 (m), 2159 (vw), 2100 (vw), 1933 (vw), 1626 (s), 1547 (s), 1468 (w), 1450 (m), 1422 (s), 1414 (vs), 1375 (m), 1318 (w), 1302 (m), 1183 (w), 1080 (s), 1052 (s), 1014

convenient and weighable source of strong acid reagent that can be used in acid-catalyzed reactions. Here the application of ligand exchange chemistry to the RhCl3·3H2O/HX-piperazine/HX (X = Br or I) system with a focus on the ligand exchange process together with its structural consequences is described. This offers an opportunity to further explore the generality of a ligand exchange approach in the “predictable” formation of hydrogen bonded coordination networks of [RhX6]3− using simple nitrogen-containing organic linkers. Taking into account the relatively small spatial dimensions of the piperazine-1,4-diium cation and the fact that a large number of inorganic−organic hybrids with that cation crystallize as simple hydrates or [H3O]+-containing compounds,12,13 one expects to find the water of crystallization molecules and aquahydrogen cations embedded in the crystal lattice. It was also interesting to study changes of the structural arrangements of ions and especially determine what effect varying the identity of the halide replacement has on both the inorganic and organic substructures and their interactions. To better understand all of those processes, in particular, the changes in different interactions and their hierarchy, we have studied the simple nonmetal-containing compounds formed as the products of piperazine and hydrobromic (C4H12N2)Br2· H2O and hydroiodic acid (C4H12N2)I2·I2 reactions.

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EXPERIMENTAL SECTION

Elemental analyses (C, H, and N) were performed with a Euro EA3000 elemental analyzer. The X-ray fluorescence analyses were made using an EDAX Eagle-II μ-Probe spectrometer. The thermoanalytical TG/DTA studies were done on powder samples using a NETZSCH STA 449C apparatus. The temperature ranged from 298 to 973 K with a heating rate of 5 K·min−1. IR spectra were recorded with a Biorad Excalibur FTS 3500 FT-IR spectrometer in the range 4000−550 cm−1 using a DTGS detector (resolution 4 cm−1). The crystalline samples were placed on a MIRacle single refraction ATR sample plate (ZnSe for 1, 2, and 3, and diamond for 4). FTRaman spectra, for crystalline samples, were recorded between 4000 and 50 cm−1 with the FT-Raman III subsystem attached to the Biorad Excalibur FTS 3500 spectrometer at a resolution of 4 and 8 cm−1 using the 1064 nm line of an Nd:YAG laser (power 59−1688 mW). Syntheses and Characterization. Rhodium(III) chloride trihydrate (6 mol/L hydrochloric acid solution of RhCl3·3H2O containing 20% of RhCl3, Degussa AG, Germany), concentrated hydrobromic acid (47%, Ferak, Germany), concentrated hydroiodic acid (67%, p.a., Merck-Schuchardt, Germany), and piperazine anhydrous (≥98%, purum, Fluka Chemika, Switzerland) were used as the starting B



Article

(vw), 918 (s), 867 (m), 563 (s). Raman, νmax, cm−1: 3105 (w), 3012 (s), 3000 (vs), 2967 (s), 2906 (m), 2844 (vw), 2787 (w), 2742 (vw), 1627 (vw), 1549 (w), 1535 (m), 1472 (m), 1462 (m), 1426 (vw), 1402 (m), 1376 (w), 1303 (vs), 1202 (s), 1150 (m), 1054 (s), 1038 (vs), 826 (m), 812 (vs), 462 (m), 438 (m), 387 (s), 320 (vw), 248 (w), 195 (vw), 141 (vw). Anal. Found: C 18.03, H 5.84, N 10.29. C4H14Br2N2O requires: C 18.06, H 5.31, N 10.53%. 4, IR, νmax, cm−1: 3406 (s, br), 3071 (s), 2999 (vs), 2985 (vs), 2968 (vs), 2946 (vs), 2926 (vs), 2795 (m), 2771 (m), 2746 (vs), 2727 (s), 2714 (s), 2682 (m), 2581 (m), 2565 (m), 2503 (w), 2484 (w), 2449 (m), 2357 (m), 2096 (w), 1928 (vw), 1742 (vw), 1539 (vs), 1451 (m), 1443 (w), 1427 (s), 1383 (s), 1373 (vs), 1315 (w), 1292 (m), 1192 (m), 1069 (m), 1046 (m), 1019 (vw), 987 (m), 916 (s), 855 (m), 556 (w). Raman, νmax, cm−1: 2999 (vw), 2984 (vw), 2947 (w), 1535 (w), 1456 (vw), 1439 (vw), 1407 (vw), 1390 (vw), 1376 (vw), 1305 (w), 1294 (w), 1132 (vw), 1040 (vw), 1032 (vw), 826 (vw), 816 (vw), 798 (vw), 456 (vw), 434 (vw), 390 (vw), 217 (w), 199 (m), 167 (vs), 139 (m). Anal. Found: C 8.14, H 2.01, N 4.59. C4H12I4N2 requires: C 8.06, H 2.03, N 4.70%. Crystal Structure Determinations. Intensity data were collected at 295(2) K on STADI4 CCD and Xcalibur Eos four circle area detector single crystal diffractometers, with graphite monochromated MoKα radiation from crystals of all four compounds. The KUMA Diffraction Instruments and Oxford Diffraction software was used during the data collection, unit cell parameters determinations, and data-reduction processes. All data were corrected for Lorentz, polarization, and absorption corrections.14,15 All the structures were solved by the Patterson method and refined by the full-matrix leastsquares method against F2 using SHELX.16 In all of the structures the non-hydrogen atoms were refined using anisotropic displacement parameters. All of the hydrogen atom positions were located in subsequent difference Fourier maps. The riding model was applied to the hydrogen atoms attached to nitrogen and carbon atoms, in all structures, whereas the hydrogen atoms bonded to oxygen atoms were refined using appropriate geometrical restraints (DFIX command of SHELXL).16 The isotropic displacement parameters of hydrogen atoms were taken with coefficients being 1.2 and 1.5 (for hydrogen atoms bonded to carbon, nitrogen, and oxygen atoms, respectively) times larger than the respective parameters of their parent atoms. Selected crystal data and the structure determination details for all structures are listed in Table 1 (Table S1, Supporting Information). The structure drawings were prepared using Mercury.17 The interactions were compared using the Hirshfeld surface analysis provided by CrystalExplorer.18−21

The crystals of 1 and 2, stored at room temperature, were reinvestigated a few months after the first elemental analysis investigations and were found to be unchanged. This suggests that they are stable at ambient conditions over long time scales, in contrast to the other compounds of this class.23 Further, compound 1 was found to easily and repeatedly crystallize at different experimental conditions involving different (i) molar ratios of starting materials (piperazine to rhodium(III) chloride trihydrate molar ratios of 1:10, 6:1, and 10:1), (ii) concentrations of hydrobromic acid (ca. 9, 4.5, and 3 mol/L HBr for the same 6:1 molar ratio of starting materials), and (iii) method/temperature of crystallization−hydrothermal24 and diffusion-controlled8,11 processes (for the substrates molar ratio of 2:1). The single crystal products always show the same dark red color and the shape of flat needles. Some very few tiny red plates, unfortunately not suitable for the further characterization, e.g., by X-ray diffraction analysis, were also found (in particular, in the process of decreasing the concentration of HBr). Thermogravimetric analyses show a stepwise thermal decomposition for all four compounds (Figure S1, Supporting Information). The thermal decomposition of 1 and 3 proceeds, in general, in two main steps, whereas in the case of 2 and 4, in principle, three steps were found suggesting the lower thermal stability of the iodide-containing compounds. The first steps, thermolyses, for 1−3, are associated with dehydration starting at ca. 365 K for 1 and 2, and at ca. 340 K for 3. Then the loss of HX (X = Br or I) followed by the pyrolysis of organic components occurs. The loss of 4 and 2 equivalents of HI at ca. 585 and 520 K for 2 and 4, respectively, is indicated. This suggests the formation, at this stage, of a relatively stable [RhI2(C4H10N2)2]I semiproduct in the case of 2. Structures of 1 and 2. The compounds 1 and 2 are clearly isostructural; i.e., they crystallize in the same orthorhombic centrosymmetric Pnma space group, their unit cell parameters are similar, the positions of corresponding atoms are approximately the same, and they are characterized by similar crystal packing (Tables 1, S1 and S2, Figure 1). The high degree of isostructurality is further confirmed by the calculated unit cell similarity factor of 0.0438 and the isostructurality index of 98.8%25,26 and 95.3%.27 It is interesting to note that the axial dimensions of 2 are almost proportionally larger than those of 1; however the largest relative change (5.8%) was found for the smallest unit cell c parameter. Examination of the molecular packing, shown in Figure 1, indicates that the larger [RhI6]3− ions together with the longer distances between the structural components in 2 and a greater volume of voids17 seem to be responsible for the larger lattice expansion in the c direction. As stated above, the principal arrangement of the structural components in both crystals (the [RhX6]3− octahedral complexes, isolated X− ions, piperazine-1,4-diium cations, and H2O molecules) that all have crystallographically imposed mirror symmetry is similar in 1 and 2. However, the inspection of the system of noncovalent interactions shows clear differences between them. The geometry of [RhX6]3− octahedra, in both structures, slightly deviates from the ideal octahedron reflecting the different radii of Br and I28−30 and the differences in their structural environment (Figure 2). It is of note that the structural environment should be considered as a decisive factor, especially in the case of 2, responsible for formation of the thermodynamically stable species that tend to be lost of heating.

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RESULTS AND DISCUSSION Syntheses and Characterization. The diffusion-controlled ligand exchange crystal growth procedure, using the same basic “parent” inorganic starting material and an organic linker, i.e., rhodium(III) chloride trihydrate dissolved in hydrochloric acid and solutions of piperazine in hydrobromic and hydroiodic acid, that were additionally separated by a “neutral” layer of the hydrohalic acid, yielded the needle-shaped single crystals of 1 and 2, respectively. No mixed chloridobromido- nor chlorido-iodiodorhodate(III) ions were found in the crystal samples. The efficiency of ligand exchange processes along with the purity of compounds 1 and 2 were confirmed by both energy dispersive X-ray fluorescence analysis investigating selected single crystals and FT-Raman spectroscopy using the crystalline samples. The present work is the first report of the synthesis and crystal growth of a compound containing isolated [RhI6]3− octahedra. In contrast to the previous multistep complicated procedure used for obtaining Cs3[IrI6],22 the liquid−liquid diffusion controlled ligand exchange crystal growth method was found to be very simple and efficient in terms of time, yield, and purity of the single product of 2. C



Article

Figure 1. Packing diagrams of 1 (a) and 2 (b) showing the intermolecular space accessible to a probing sphere of radius 0.55 Å indicated in yellow. The void volume is 3.2 and 5.8% for 1 and 2, respectively. Displacement ellipsoids are plotted at the 25% probability level.

The linear distortions of the [RhX6]3− complexes increase with increasing radius of the X ligands. The lengths of the four crystallographically independent Rh−X bonds of the [RhX6]3− ions vary from 2.4787(4) to 2.5095(4) Å (difference of 0.0308(6) Å) for 1, and from 2.6700(4) to 2.7116(5) Å (difference of 0.0416(6) Å) for 2 giving the mean Rh−X distances of 2.4957 and 2.6940 Å for 1 and 2, respectively. The cis-X−Rh−X angles range from 87.79(2) to 91.802(12)° and from 87.901(17) to 91.787(9)° for 1 and 2, respectively (Table 2). The geometric parameters for 1 are similar to those found in other hexabromidorhodates(III).8,11,31 Also the Rh−I distances in 2 are consistent with those found in related compounds.32,33 It should be noted that the average cis-X−Rh− X angles for both compounds 1 and 2 are 89.74°. All geometric parameters of the piperazine-1,4-diium cations in 1 and 2, that adopt the energetically preferred chair conformation,34 are as expected (Table S3, Supporting Information), and they are also in agreement with those reported for the structure of piperazine and other compounds containing this ion.35−40 Structures of 3 and 4. The structures of products obtained in the reactions of piperazine and hydrobromic and hydroiodic acid as the “substrate” compounds of 1 and 2 were investigated for the purpose of comparing and better understanding the structural properties and interactions in more complex 1 and 2 halogenidorhodates(III). The crystal structures of both 3 and 4 are built up from piperazine-1,4-diium cations located at the inversion centers and X− ions that occupy general positions. The X− anions are isolated in 3 and are engaged with iodine I2 molecules by I···I halogen bonds in 4. In addition, in the structure of 3 the water molecules located on the 2-fold axes are present. Although both compounds have different compositions and they crystallize in two different crystal systems, they show some structural similarities in the location of H2O/I2 molecules and directionality of some interactions that are depicted in Figure 3. As in the structures of 1 and 2, N−C and C−C bond lengths as well as N−C−C and C−N−C angles within the piperazine-

Figure 2. [RhX6]3− octahedral complexes with their environment in 1 (a) and 2 (b). The broken green and red lines represent the O/N− H···X hydrogen and I···I halogen bonds, respectively. Displacement ellipsoids are plotted at the 25% probability level. Symmetry codes: (I) x, −y + 1/2, z; (II) −x + 1/2, −y + 1, z − 1/2; (III) −x + 1/2, −y, z − 1/2; (IV) x, y, z − 1; (V) −x + 1/2, −y + 1, z + 1/2; (VI) −x + 1/2, −y, z + 1/2.

Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1 and 2a Rh1−X1 Rh1−X2 Rh1−X3 Rh1−X4 X1−Rh1−X1I X1−Rh1−X2 X1−Rh1−X2I X1−Rh1−X3 X1−Rh1−X4 X2−Rh1−X2I X2−Rh1−X3 X2−Rh1−X4 X3−Rh1−X4 a

D

1 (X = Br)

2 (X = I)

2.4787(4) 2.5095(4) 2.4883(5) 2.5062(5) 87.79(2) 91.802(12) 179.387(18) 90.384(13) 89.907(14) 88.61(2) 89.165(14) 90.545(14) 179.60(2)

2.6700(4) 2.7066(4) 2.6876(5) 2.7116(5) 87.901(17) 91.787(9) 179.338(18) 90.224(12) 89.877(13) 88.518(18) 89.193(13) 90.706(12) 179.86(2)

Symmetry code: (I) x, −y + 1/2, z.



Article

Table 3. Hydrogen Bonds Geometries (Å, deg) for 1, 2, 3, and 4a atoms 1 O1−H1···Br2I O1−H1···Br5 N1−H11···Br1 N1−H12···Br2II N1−H11···Br4 N4−H42···Br5II N5−H51···Br1 N5−H52···Br2II N5−H52···Br3II N8−H81···Br5III C3−H31···Br1I C3−H32···Br4II C7−H71···Br1IV C7−H72···Br3 N4−H41···O1 N8−H82···O1V 2 O1−H1···I2I N1−H11···I1 N1−H12···I2II N1−H11···I4 N4−H42···I5II N5−H52···I3II N8−H81···I5III C3−H31···I1I C7−H71···I1IV N4−H41···O1 N8−H82···O1V 3 O1−H1···Br1VI N1−H11···Br1VII N1−H11···Br1VIII N1−H12···Br1 N1−H11···O1 4 N1−H11···I1IX N1−H12···I1IV

Figure 3. Packing diagrams of 3 (a) and 4 (b). The broken green and red lines represent the O/N−H···X/O hydrogen and I···I halogen bonds, respectively. Displacement ellipsoids are plotted at the 25% probability level.

1,4-diium cations in 3 and 4, showing the same chair conformation, are in the expected ranges35−40 (Table S3). The I2 molecule in 4 shows an I−I distance of 2.7703(7) Å that is significantly longer than that in the structure of solid iodine 2.715(6) Å.41 The elongation is caused by the I···I interactions with two isolated I− ions at a distance of 3.3805(6) Å, with which each I2 molecule is engaged. Both the I−I and the I···I distances and the respective angles are within the ranges characteristic for I42− ions.40,42−45 Interactions. The analysis of interactions and environment of the basic inorganic [RhX6]3− and organic piperazine-1,4diium ions in the structures of 1 and 2 together with their comparison to the structures of the “substrate” compounds 3 and 4 and also to piperazine-1,4-diium dichloride hydrates36−38 clarifies the understanding of the interactions by showing their hierarchy and changes occurring with the halogen replacement. The structures of 1, 2, 3, and piperazine-1,4-diium dichloride hemihydrate38 show the important stabilizing role played by water molecules that serve as both donors and acceptors of hydrogen bonding to the other structural components (Table 3, Figures 2 and 3). In the structure of 4 the water molecules are “replaced” by the I2 molecules and as a consequence there are no O−H···I and N−H···O hydrogen bonds that are, in part, exchanged by the characteristic I···I halogen bonds that reflect the tendency of iodine to concatenate.42,43,46 The linear I42− ion has been found to be unstable in theoretical investigations.47 Therefore, it is not surprising that, in the stable structure of 4, it is engaged in the N−H···I hydrogen bonds with its surrounded organic cations (Table 3, Figure 3). Recently the thermal stability of I42− anion in imidazolium iodide that undergoes a low-temperature phase

D−H

H···A

D···A

D−H···A

0.84(1) 0.84(1) 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.97 0.97 0.97 0.97 0.90 0.90

2.66(1) 2.86(1) 2.76 2.78 2.80 2.40 2.83 2.84 2.67 2.43 2.78 2.90 2.82 2.90 1.92 1.91

3.370(1) 3.293(3) 3.474(3) 3.519(3) 3.411(4) 3.287(4) 3.581(3) 3.518(3) 3.345(4) 3.331(4) 3.657(4) 3.756(3) 3.668(3) 3.714(3) 2.818(5) 2.809(5)

142(1) 114(1) 137 141 127 171 142 134 133 175 151 148 146 143 178 172

0.85(1) 0.90 0.90 0.90 0.90 0.90 0.90 0.97 0.97 0.90 0.90

2.76(1) 2.99 3.04 2.96 2.68 2.84 2.77 2.97 2.98 1.95 1.93

3.518(1) 3.681(4) 3.786(4) 3.604(4) 3.563(5) 3.546(5) 3.662(5) 3.845(4) 3.833(4) 2.847(7) 2.822(7)

150(1) 135 142 130 168 136 169 151 148 176 173

0.85(1) 0.90 0.90 0.90 0.90

2.67(1) 2.66 2.98 2.41 2.54

3.481(2) 3.359(2) 3.442(2) 3.295(2) 3.063(3)

161(1) 135 114 166 118

0.90 0.90

2.72 2.75

3.556(4) 3.508(4)

155 143

Symmetry codes: (I) −x + 1/2, −y, z + 1/2; (II) x, y, z + 1; (III) x + 1/2, −y + 1/2, −z +1/2; (IV) −x + 1, −y, −z + 1; (V) x + 1/2, −y + 1/2, −z + 3/2; (VI) x + 1/2, y + 1/2, z; (VII) x + 1/2, y − 1/2, z; (VIII) −x + 1/2, y − 1/2, −z + 3/2; (IX) −x, −y, −z + 1. a

transition associated with the dynamics of the imidazolium cations and distortion of I42− rods was reported.45 A similar hydrogen versus halogen bonding behavior was found in the structures of 1 and 2. The water molecules in 1 are hydrogen bonded to piperazine-1,4-diium and isolated Br− ions showing no dihalogen bonding, whereas in the structure of 2 there are no water molecule···isolated I− ion interactions, but instead Rh−I···I···I−Rh contacts of 3.9164(4) Å commensurate with the sum of the van der Waals radii of two I atoms were found.28−30 These contacts, occurring between the I1 ligands belonging to the [RhI6]3− complexes and isolated I− anions along with the 3.3805(6) Å contacts found in 4 between the I2 molecules and isolated I− ions, can be classified as type I interactions (both the Rh/I−I···I, θ1 and I···I−Rh/I, θ2 angles are equal and close to 163 and 178° in 2 and 4, respectively).48−52 E



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features that can be attributed to the water of crystallization molecules. The appearance of infrared active lines for 1, 2, 3, and 4 are consistent with the spectra of other similar compounds.55−57 However, the strong νN−H band at ca. 3210 cm−1 characteristic of non- and monoprotonated piperazine is absent from the spectra of all compounds.58 The spectral lines in the Raman spectra of 1 and 2 could be clearly assigned to the piperazine-1,4-diium ions and water of crystallization molecules by comparison with the spectra of 3, 4, piperazine, and other related compounds.55,57,59,60 The isolated [RhX6]3− ion with octahedral symmetry exhibits three fundamental vibrational modes in the Raman spectrum: the total symmetric stretching mode ν1 (A1g), the asymmetric stretching mode ν2 (Eg), and the deformation mode ν5 (F2g). Those modes are attributed to the strong bands at 181, 168, 114, and at 130, 115 cm−1 (the deformation mode ν5 (F2g) in 2 is not clearly observed) in the Raman spectrum of 1 and 2, respectively.61 They are somewhat shifted toward lower wavenumbers than observed in the related compounds.8,11 The appearance of a very strong band at 167 cm−1 (νI−I) in the Raman spectrum of 4, in which the I42− ion is considered as an iodine I2 molecule coordinated by two iodide I− ions,40 is in agreement with the X-ray diffraction results and also with the Raman studies of similar compounds.43,62,63

In order to further understand and compare all the interactions with the aim of quantifying their similarities and differences, a Hirshfeld surface analysis19−21,53 was performed on the piperazine-1,4-diium cations, as the common organic hydrogen bonded linkers occurring in all the structures (Figure 4).

Figure 4. Hirshfeld surfaces for the piperazine-1,4-diium cations in the structures of 1 (a, b), 3 (c), 2 (d, e), and 4 (f). Note two symmetryindependent cations (N1N4 (a) and (d), and N5N8 (b) and (e)) in the structures of 1 and 2. The color scheme describes distances shorter (red), equal (white), and longer (navy-blue) than the respective van der Waals radii.

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The Hirshfeld (interaction) surfaces highlight and additionally confirm the observation that interactions in which the piperazine-1,4-diium cations are involved are somewhat different for the rhodium(III)-containing compounds 1 and 2, and also for their “substrates” 3 and 4. The piperazine-1,4-diium cations are involved in more hydrogen bonds in the bromidecontaining structures of 1 and 3 (Table 3; Figure 4, first row) than in the corresponding iodide-containing analogues of 2 and 4 (Table 3; Figure 4, second row). However, the new I···I halogen contacts are formed in both 2 and 4, but they do not fully fill the interaction gap of the “lost” hydrogen bonds. There are also, in all the structures, much weaker and less important X···H interactions, mostly with the distances slightly below the respective van der Waals separation distances, that may assist in the packing and molecular association. On the other hand the distances between halogen and carbon atoms to which the H atoms are bonded are clearly longer than the sum of the X and C van der Waals radii.28−30 The observation of the increasing role played by X···X contacts in relation to N/C−H···X hydrogen bonds in the iodide-containing structures compared to other halogencontaining compounds is consistent with the largest, among all halogens, polarizability of iodine50,52 and confirmed by interactions occurring in the analogous systematically studied compounds, e.g., simple monohalomethanes.54 IR and Raman Spectra. The ATR-FTIR and FT-Raman spectra of the crystalline samples of 1, 2 and 3, 4 show individual differences mainly derived from the presence of [RhX6]3−, (C4H12N2)2+, and I42− structural components in those structures. Therefore, the observed bands can be divided into those which arise from the inorganic [RhX6]3− octahedral complexes, internal vibrations of the organic cations, the hydrogen bonded water molecules and I42− ions. The IR spectra are dominated by bands arising from internal modes of the piperazine-1,4-diium ions and water molecules, while Raman spectra show strong bands of hexahalogenidorhodate(III) and I42− components in 1, 2, and 4, respectively, bands of the piperazine-1,4-diium cations in 3, and some additional

CONCLUSIONS The key advantage of inorganic−organic hybrid materials, in comparison to their pure organic and inorganic counterparts, is the possibility to relatively easily tune their composition through a change of all of the structural components not only the central metal atom, but also the ligand and the organic linker. Further, knowing the structure and properties of crystallized hybrids, it is possible to go back to the design stage and improve it by tailoring synthesis so as to incorporate specific components into a framework or to change the pattern of specific interactions so as to stabilize the solid. In most cases inorganic−organic hybrids are synthesized by means of solvothermal or hydrothermal methods. Here a simple and efficient ligand exchange crystal growth preparation procedure that provides a convenient way for the halide replacement in the rhodium(III) compounds has been reported. It is also emphasized that this style of synthesis has allowed the preparation and characterization for the first time of a hybrid compound containing isolated [RhI6]3− octahedra. The diffraction data revealed slight changes within the inorganic and organic substructures of both isostructural (C4H12N2)2[RhX6]X·H2O (X = Br or I) rhodium(III) compounds, but the ligand replacement clearly effects the formation and strength of intermolecular interactions. The “close-packed” bromido-ligand crystal structure is dominated by hydrogen bonds that are not present or are partly exchanged by the I···I interactions in the “loose-packed” iodido-ligand crystal. A similar behavior was found in their “substrate” (C4H12N2)Br2· H2O and (C4H12N2)I2·I2 crystals, where the even more drastic changes associated with the replacement of the water by iodine molecules occur. This implies the importance of competition between the various interactions in two aspects. First with respect to the accommodation of the water or iodine molecules in a crystal lattice, in the case of piperazine and hydroiodic acid reaction (C4H12N2)I2·I2 was crystallized instead of expected (C4H12N2)I2·H2O. The stabilization of the structures is also dependent on F



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this competition between the specific interactions: (i) the neutral water molecules can serve as both donors and acceptors of hydrogen bonds, whereas the iodine molecules show the preferences to form halogen bonds, (ii) in both bromidecontaining crystals the piperazine-1,4-diium cations are involved in a large number and stronger hydrogen bond interactions than in the corresponding iodide-containing crystals. The observation that the increase of polarizability and halogen radius, in general, elongates some of the existing interactions and is responsible for the changes in formation of the new interactions that then stabilize the arrangement of quite unstable species opens the way to build predictable assemblies that incorporate in their lattices neutral, weakly associated molecules and to gain control over the properties of inorganic−organic multifunctional materials that depend on the specific system of hierarchical interactions.64

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ASSOCIATED CONTENT

S Supporting Information *

Tables containing crystallographic data (Tables S1−S3), TG curves as well as CIF files for 1−4. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic information files are also available from Cambridge Crystallographic Data Center (CCDC) upon request (http:// www.ccdc.cam.ac.uk, CCDC deposition numbers 1034591− 1034594).

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AUTHOR INFORMATION

Corresponding Author

*Phone: +48 77 452 7159. Fax: +48 77 452 7101. E-mail: [email protected]; [email protected]. Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS The author is grateful to Prof. Dr. W. Frank (Institut für Anorganische Chemie und Strukturchemie, Lehrstuhl II: Material- und Strukturforschung, Heinrich-Heine-Universität Düsseldorf, Germany) for providing starting materials, the opportunity to prepare and study crystals, and for valuable discussions and manuscript corrections. The support from the Fonds der Chemischen Industrie and the former Degussa AG is acknowledged. The author also thanks Ms. E. Hammes, Ms. K. Skierakowska, and Mr. P. Roloff (Institut für Anorganische Chemie und Strukturchemie, Lehrstuhl II: Material- und Strukturforschung, Heinrich-Heine-Universität Düsseldorf, Germany) for recording the vibrational, X-ray fluorescence spectra, and performing elemental and thermoanalytical analyses. The author is also grateful to Dr. R. J. Angel (Dipartimento di Geoscienze, Università di Padova, Italy) for his comments and suggestions.

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Efficient Diffusion-Controlled Ligand Exchange Crystal Growth of

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