Efficient diffusion-controlled ligand exchange crystal growth of isostructural inorganic-organic halogenidorhodates(III): The missing hexaiodidorhodate(iii) anion

Article abstract of DOI:10.1021/cg501694d

The monohydrates of piperazine-1,4-diium hexabromidorhodate(III) bromide and hexaiodidorhodate(III) iodide were obtained by a diffusion-controlled 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-1,4-diium 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.

Full text of DOI:10.1021/cg501694d

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 fur Anorganische Chemie und Strukturchemie, Lehrstuhl II: Material- und Strukturforschung, Heinrich-Heine-Universitat
̈
̈
Dusseldorf, Universitatsstraße 1, D-40225 Dusseldorf, 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 diffusion-
controlled 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-1,4-
diium 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.
are more weakly associated with the anionic substructures than
in the corresponding hexachloridorhodates(III).
INTRODUCTION
■
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
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
Received: November 20, 2014
Revised: January 11, 2015
© XXXX American Chemical Society
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
1
2
3
4
formula
C8H26Br7N4ORh
856.54
C8H26I7N4ORh
1185.54
C4H14Br2N2O
265.97
C4H12I4N2
595.76
formula weight
crystal system
space group
a (Å)
orthorhombic
Pnma
orthorhombic
Pnma
monoclinic
C2/c
triclinic
P1
̅
26.3712(9)
10.1433(5)
7.9013(3)
90.000
27.4238(9)
10.5512(4)
8.3875(3)
90.000
11.4831(5)
5.9049(2)
13.9367(6)
90.000
6.5186(3)
7.4950(3)
8.1312(4)
64.635(4)
68.060(4)
69.507(4)
324.30(3)
1
b (Å)
c (Å)
α (deg)
β (deg)
90.000
90.000
108.393(4)
90.000
γ (deg)
V (Å3)
90.000
90.000
2113.53(15)
4
2426.96(15)
4
896.72(7)
4
Z
reflns collected
reflns unique, Rint
R1, wR2, I > 2σ(I)
22021
25227
8295
12400
2091, 0.0395
0.0203, 0.0343
2379, 0.0342
0.0196, 0.0353
927, 0.0485
0.0201, 0.0471
1341, 0.0441
0.0226, 0.0549
materials without further purification for the synthesis of piperazine-
1,4-diium hexabromidorhodate(III) bromide monohydrate, 1, piper-
azine-1,4-diium hexaiodidorhodate(III) iodide monohydrate, 2, as well
as piperazine-1,4-diium dibromide monohydrate, 3, and piperazine-
1,4-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.
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 coordina-
tion 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 com-
pounds,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.
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%.
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). FT-
Raman 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 trihy-
drate (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
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
B
DOI: 10.1021/cg501694d
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
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%.
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 character-
ization, 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.
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 least-
squares 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 displace-
ment 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
RESULTS AND DISCUSSION
■
Syntheses and Characterization. The diffusion-con-
trolled 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 chlorido-
bromido- 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 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.
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
DOI: 10.1021/cg501694d
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Crystal Growth & Design
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
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
a
and 2
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.
1 (X = Br)
2 (X = I)
Rh1−X1
Rh1−X2
Rh1−X3
Rh1−X4
2.4787(4)
2.5095(4)
2.4883(5)
2.5062(5)
87.79(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)
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
91.802(12)
179.387(18)
90.384(13)
89.907(14)
88.61(2)
89.165(14)
90.545(14)
179.60(2)
a
Symmetry code: (I) x, −y + 1/2, z.
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-
D
DOI: 10.1021/cg501694d
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Crystal Growth & Design
Article
Table 3. Hydrogen Bonds Geometries (Å, deg) for 1, 2, 3,
a
and 4
atoms
D−H
H···A
D···A
D−H···A
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
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
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
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
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
O1−H1···Br1VI
N1−H11···Br1VII
N1−H11···Br1VIII
N1−H12···Br1
N1−H11···O1
4
0.85(1)
0.90
2.67(1)
2.66
3.481(2)
3.359(2)
3.442(2)
3.295(2)
3.063(3)
161(1)
135
0.90
2.98
114
0.90
2.41
166
0.90
2.54
118
N1−H11···I1IX
0.90
0.90
2.72
2.75
3.556(4)
3.508(4)
155
143
ranges characteristic for I4 ions.40,42−45
2−
N1−H12···I1IV
Interactions. The analysis of interactions and environment
of the basic inorganic [RhX6]3− and organic piperazine-1,4-
diium 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
a
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.
transition associated with the dynamics of the imidazolium
cations and distortion of I4 rods was reported.45
2−
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
2−
The linear I4 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,
2−
Figure 3). Recently the thermal stability of I4 anion in
imidazolium iodide that undergoes a low-temperature phase
E
DOI: 10.1021/cg501694d
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
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).
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
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 symmetry-
independent 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.
The Hirshfeld (interaction) surfaces highlight and addition-
ally confirm the observation that interactions in which the
piperazine-1,4-diium cations are involved are somewhat differ-
ent 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 bromide-
containing 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
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.
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 halogen-
containing 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 I4 structural components in
2−
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
2−
hydrogen bonded water molecules and I4 ions. The IR
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
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
F
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Article
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AUTHOR INFORMATION
Corresponding Author
*Phone: +48 77 452 7159. Fax: +48 77 452 7101. E-mail:
maciej.bujak@uni-duesseldorf.de; mbujak@uni.opole.pl.
Notes
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The author declares no competing financial interest.
ACKNOWLEDGMENTS
The author is grateful to Prof. Dr. W. Frank (Institut fur
Anorganische Chemie und Strukturchemie, Lehrstuhl II:
Material- und Strukturforschung, Heinrich-Heine-Universitat
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