Humidity control of isostructural dehydration and pressure-induced polymorphism in 1, 4-Diazabicyclo[2.2.2]octane Dihydrobromide monohydrate

Article abstract of DOI:10.1021/cg200743n

1, 4-Diazabicyclo[2.2.2]octane dihydrobromide (dabco2HBr, [C ₆H₁₄N₂]²⁺ · 2Br^-) is extremely sensitive to humidity levels, by absorbing or releasing water from its structure. The gradual changes of water contents, in turn, modifies the crystal symmetries, albeit all polar at ambient pressure. The full water contents control has been achieved for the sample sealed in a high-pressure chamber. Isobaric crystallization of dabco2HBr from ethanol solution at dry conditions yields anhydrate dabco2HBr, and in open air two polymorphic hemihydrates dabco2HBr · ¹/₂H₂O and an isostructural monohydrate α-dabco2HBr·H₂O.; three more monohydrate dabco2HBr· H₂O polymorphs have been obtained by isothermal and isochoric crystallizations in a diamond-anvil cell. All the structures have been determined in situ by single-crystal X-ray diffraction. The anhydrate dabco2HBr, its hemihydrates α and β, and monohydrates α, β, and γ all crystallize in polar space groups: Cmc2 ₁ (dabco2HBr, β-dabco2HBr · ¹/ ₂H₂O, β-dabco2HBr · H₂O), Pbc2 ₁ (α-dabco2HBr · ¹/₂H₂O and α-dabco2HBr · H₂O) and P2₁ (γ-dabco2HBr·H₂O). However, all these crystals exhibit clear isostructural relations. Only the highest-pressure δ- dabco2HBr·H₂O monohydrate is centrosymmetric, space group Pmmn. Systematic structural transformations accompanying the hydration and pressure increase have been monitored: pressure gradually increases number of H-bonds involving each of the ammonium groups, from one to three, and the H-acceptor capacity of Br anions from zero to three.

Full text of DOI:10.1021/cg200743n

ARTICLE
pubs.acs.org/crystal
Humidity Control of Isostructural Dehydration and Pressure-Induced
Polymorphism in 1,4-Diazabicyclo[2.2.2]octane Dihydrobromide
Monohydrate
Michaz Andrzejewski, Anna Olejniczak, and Andrzej Katrusiak*
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznaꢀn, Poland
S Supporting Information
b
ABSTRACT: 1,4-Diazabicyclo[2.2.2]octane dihydrobromide (dabco2HBr,
[C6H14N2]2+ 2Brꢀ) is extremely sensitive to humidity levels, by absorbing
3
or releasing water from its structure. The gradual changes of water contents,
in turn, modifies the crystal symmetries, albeit all polar at ambient pressure.
The full water contents control has been achieved for the sample sealed in a
high-pressure chamber. Isobaric crystallization of dabco2HBr from ethanol
solution at dry conditions yields anhydrate dabco2HBr, and in open air two
1
polymorphic hemihydrates dabco2HBr /2H2O and an isostructural
3
monohydrate α-dabco2HBr H2O; three more monohydrate dabco2HBr
3
3
H2O polymorphs have been obtained by isothermal and isochoric crystal-
lizations in a diamond-anvil cell. All the structures have been determined
in situ by single-crystal X-ray diffraction. The anhydrate dabco2HBr, its hemihydrates α and β, and monohydrates α, β, and γ all
1
1
crystallize in polar space groups: Cmc21 (dabco2HBr, β-dabco2HBr /2H2O, β-dabco2HBr H2O), Pbc21 (α-dabco2HBr /2H2O
3
3
3
and α-dabco2HBr H2O) and P21 (γ-dabco2HBr H2O). However, all these crystals exhibit clear isostructural relations. Only the
highest-pressure δ-dabco2HBr H2O monohydrate is centrosymmetric, space group Pmmn. Systematic structural transformations
3
3
3
accompanying the hydration and pressure increase have been monitored: pressure gradually increases number of H-bonds involving
each of the ammonium groups, from one to three, and the H-acceptor capacity of Brꢀ anions from zero to three.
dabcoHI, yield exclusively anhydrous crystals when crystallized
from aqueous or methanol solution at normal conditions. Above
0.5 GPa, dabcoHI crystallized from aqueous solution favorably
1. INTRODUCTION
NH+ N bonded complexes of 1,4-diazabicyclo[2.2.2]-
3 3 3
octane (dabco) with monohydrogen mineral acids exhibit intri-
guing dielectric properties. These complexes of dabco with acids
HClO4 and HBF4, dabcoHClO4 and dabcoHBF4 respectively,
forms a monohydrate, α-dabcoHI H2O, and above 1.4 GPa
3
another monohydrate, β-dabcoHI H2O is formed.7 Methanol
3
solution yields unsolvated dabcoHI crystals below 1.8 GPa, and
at this pressure only the dabco disalt (denoted dabco2HI)
precipitated from the monosalt solution in the form of
were the first NH+ N bonded substances for which ferro-
3 3 3
electric properties were reported.1 The spontaneous polarization
in the dabco complex with HReO4, dabcoHReO4,2 is the only
dabco2HI 3CH3OH solvate. Above 2.4 GPa the reaction of
known structure where the NH+ N bonds are parallel to the
3
3 3 3
N-methylation of dabco takes place. Hydrobromide of dabco,
dabcoHBr, is isostructural with dabcoHI polymorph V when
crystallized from aqueous solution at normal conditions8 and has
analogous relaxor properties.5 However, its phase diagram is very
different:5,9 there are only three dabcoHBr phases in the range of
temperature and pressure analogous to that of 10 dabcoHI
polymorphs. No methanol solvate of dabcoHBr, analogous to
spontaneous polarization (ps) in the ferroelectric phase, whereas
in most ferroelectrics of KDP-type (KDP is an abbreviation for
potassium dihydrogen phosphate KH2PO4) the OH O bonds
3 3 3
are nearly perpendicular or considerably inclined to ps.3 More
recently, anisotropic relaxor properties were found in the dabco
1:1 complexes with HI and HBr acids, dabcoHI4 and dabcoHBr,5
the first organic and anisotropic relaxors of stoichiometric
composition. It was established that these relaxor crystals are
composed of nanodomains, with disproportionate dabco2H+
dications and neutral dabco molecules formed at the domain
boundaries. Furthermore, dabcoHI exhibits a rich phase diagram
of 10 documented polymorphs, labeled IꢀX, of which relaxor
properties were evidenced in two polymorphs, V and X.6 It was
found that high-pressure crystallizations of dabcoHI from metha-
nol and aqueous solution proceed differently than at ambient
pressure. All dabco monosalts mentioned above, including
dabco2HI 3CH3OH, has been obtained at high pressure. In order
3
to better understand the hydration of dabco monosalts, we have
extended ourstudy to dabco dihydrobromide, dabco2HBr, crystal-
lized at varied thermodynamic conditions. It was aimed at
investigating the ambient-pressure crystallization of diprotonated
Received: June 13, 2011
Revised:
September 21, 2011
Published: October 03, 2011
r
2011 American Chemical Society
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Figure 2. Isochoric growth of a single-crystal γ-dabco2HBr H2O
Figure 1. Isothermal growth of dabco2HBr H2O monohydrate in
3
3
hydrate from aqueous solution: (a) one small grain after dissolving
polycrystalline mass at 376 K; (b) the same crystal at 360 K; (c) 343 K;
(d) the single-crystal at 0.78 GPa/296 K (cf. Figure 1).
phase β from aqueous solution at 296 K: (a) one seed at 0.12 GPa,
(b, c) a single-crystal growth, and (d) the sample at 0.48 GPa. The ruby
chip for pressure calibration lies between the sample and the bottom
edge of the gasket.
level, the water content was difficult to control during the experiments. It
appears that the water contents between 0 and 1/2 favored the β-type
structure, and those between 1/2 and 1 favoredthe α type. However, in the
discussion below the approximate hydration levels of 0, 1/2, and 1 will be
used for the structures determined for the bare samples (at 0.1 MPa).
Most importantly, the “breathing” of the samples with water considerably
affected the quality of structural data obtained from the diffraction experi-
ments. All disadvantages of changing humidity were circumvented in the
high-pressure experiments in confined experimental environments.
2.3. High-Pressure Crystallization. High-pressure crystalliza-
tions of dabco2HBr performed in situ in a diamond-anvil cell (DAC)10
provided a much more precise control of experimental conditions and
hence resulted in much better quality of samples and more precise
results. The gasket was made of steel foil, 0.1 mm thick, with a spark-
eroded hole, 0.4 mm in diameter. Pressure in the DAC was calibrated by
the ruby-fluorescence method11 using a Betsa PRL spectrometer, with
an accuracy of 0.05 GPa. In the first of high-pressure crystallizations, the
chamber of the DAC, equipped with steel disks supporting the diamond
anvils, was loaded with saturated aqueous solution of dabco2HBr. Dis-
tilled water was used. A single crystal was grown in isothermal conditions
at 296 K (Figure 1). The collected X-ray diffraction data revealed a
monohydrate stoichiometry and that the crystal is isostructural with the
β-type anhydrate and hemihydrate; hence, this monohydrate has been
dabco salt, the pressure effect on its hydrates types, and the range
of pressure and temperature destabilizing their structure.
2. EXPERIMENTAL METHODS
2.1. Synthesis. Five grams of crystalline 1,4-diazabicyclo[2.2.2]-
octane (dabco, Aldrich, analytical grade 99%) was dissolved in 12 mL of
ethanol. Then 5.03 mL (7.3 g) equimolar quantity with dabco of bromic
acid (HBr) was poured into this solution. Two kinds of crystals
precipitated: 0.79 g at once and of the remaining solution 6.02 g after
the solvent evaporated. Single-crystal and powder X-ray diffraction
measurements showed that dabcoHBr precipitated first, and dabco2HBr
later. dabcoHBr formed nice hexagonal single crystals, whereas practi-
cally all the dabco2HBr crystals had defects of different sorts.
2.2. Crystals Growth at Ambient Pressure. dabco2HBr crys-
tals obtained in the reaction were of low quality and recrystallizations
were undertaken. The best single crystals were obtained from ethanol
solution, yielding small rectangular plates. However, even samples that
appeared good gave diffraction patterns with split and diffused reflec-
tions, and many samples had to be tested before satisfactory single-
crystals were selected. The crystals were very sensitive to changing con-
ditions, stress, and contact with other solvents. Two types of crystals
occurred of very similar habit. The first type of crystal had the symmetry
of orthorhombic space group Pbc21, and it included two hydrates labeled
denoted as β-dabco2HBr H2O.
3
Subsequently, pressure was increased to 0.78 GPa; however the single
crystal was crushed and a new crystal was grown in isochoric conditions.
First, the DAC was heated until all crushed fragments but one dissolved
at 376 K, and then temperature was slowly lowered to 296 K (Figure 2).
The X-ray measurement revealed another monohydrate, denoted as γ-
as type α: hemihydrate α-dabco2HBr 1/2H2O and monohydrate α-
3
dabco2HBr H2O. The second β-type crystal of orthorhombic space-
3
group symmetry Cmc21 was initially found only for hemihydrate β-
dabco2HBr 1/2H2O, but later the samples of the same crystal-symmetry
3
dabco2HBr H2O.
of space group Cmc21 were obtained of ethanol solution in a drybox in a
form of anhydrate β-dabco2HBr samples. The crystals of both symmetry
types α and β have similar structures in this sense, that the unit cell of
type α is the doubled unit cell of type β along [100], and the positions of
dabco2H+ centroids and of the Brꢀ anions are approximately the same.
The main differences between these structures are in the orientation of
the dabco2H+ dications and in the presence and position of water
molecules. The low quality of many of the single crystals was caused by
their aggregation and high sensitivity to humidity, leading to transfor-
mations during the sample handling and measurements. It should be
stressed that the water contents determined at ambient pressure is only
approximate. Because of the high sensitivity of dabco2HBr to humidity
3
After the diffraction data was collected, the pressure was further
increased to 1.5 GPa. The single crystal was grown in isochoric
conditions of the nucleous obtained at 433 K and then cooled slowly
for 5 h. Several times defects appeared on the crystal faces and the
crystallization process had to be repeated until the single-crystal sample
was obtained (Figure 3). The X-ray diffraction measurement revealed
yet another monohydrate, labeled as phase δ.
2.4. X-ray Diffraction Analyses. Small rectangular crystals were
chosen for ambient-pressure measurements. Their quality was checked
by diffraction. The single-crystal data have been measured with a KUMA
KM4-CCD diffractometer; for crystals enclosed in the DAC a procedure
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Crystal Growth & Design
ARTICLE
for high-pressure experiments was applied.12 The CrysAlis software13
was used for the data collections and the preliminary reduction of the
data. In the high-pressure data after correcting intensities for the effects
of DAC and sample absorption and sample shadowing by the gasket,
the sample reflections overlapping with diamond reflections were
eliminated.14 All structures were solved straightforwardly by direct
methods and refined by full-matrix least-squares.15 Anisotropic tem-
perature factors were refined for all non-H atoms in the structure of
phase γ and only for bromide ions in the others phases. The ethylene
H-atoms, water hydrogens, and amine protons in the structures were
located from molecular geometry (dOꢀH = 0.95 Å, dCꢀH = 0.97 Å,
and dNꢀH = 0.86 Å) and their Uiso parameters were constrained to
1.2 times Ueq/Uiso values of the carrier atoms. Although several
β-dabco2HBr 1/2H2O crystals were studied, the obtained structural
3
parameters were inaccurate due to the low quality of the samples. Their
quality was affected by their susceptibility to humidity changes, and
subsequent transformations to α-dabco2HBr 1/2H2O, clearly observed
3
as the a-parameter doubling. The selected crystal data and the structure-
refinements details are listed in Table 1 and in Table S1 of Supporting
Information. Structural drawings were prepared using the X-Seed
interface of POV-Ray and Mercury.16,17
3. RESULTS AND DISCUSSION
3.1. Ambient-Pressure Hydration. Ambient-pressure isoba-
ric crystallization of dabco2HBr of ethanol solution, performed
in the open air, yields a mixture of hemihydrate and monohydrate
crystals, which are very similar in morphology. When exposed to
air, their faces deteriorate because dabco2HBr crystals are
hygroscopic, even in moderately humid air. All monohydrate
crystals obtained at ambient pressure from various solutions were
in the form of polymorph α-dabco2HBr H2O. Aqueous solution
3
was not used for crystallizations because dabco2HBr dissolves
very well in water. The best quality crystals were obtained from
ethanol solution with an excess of HBr. The attempts to crystal-
lize dabco2HBr by sublimation was unsuccessful. After 48 h,
crystals left in open vessels were overgrown by other small
1
crystals. Hemihydrated α-dabco2HBr /2H2O and monohy-
3
drated α-dabco2HBr H2O were obtained concomitantly in this
3
way. Our attempts to crystallize the pure monohydrate form
from various solutions led to the conclusion that in the initial
stages of crystallization the monohydrate is formed, and on
decreased H2O concentration the hemihydrate precipitates.
These hemihydrates and monohydrates can also interconvert
after removing the crystals from solution, depending on humidity
conditions. Subsequent crystallizations in a drybox from dry
ethanol solution yielded anhydrate dabco2HBr, of the β-type
structure, which in open air slowly acquired water within the
β-type structure and eventually, on sufficiently high water con-
tents, converted into the α-type hemihydrate and monohydrate
Figure 3. Stages of isochoric growth of δ-dabco2HBr H2O monohy-
3
drate from aqueous solution: (a) a nucleus at 430 K; (b) the single-
crystal at 375 K with another small crystal growing on its side ꢀ it was
dissolved by heating the sample to 415 K, after which the slow cooling
was resumed; (c) the single crystal at 345 K; and (d) at 1.50 GPa/296 K
(cf. Figures 1 and 2).
Table 1. Selected Crystal Data of dabco2HBr Anhydrate, dabco2HBr 1/2H2O Hemihydrates, and Monohydrate dabco2HBr
H2O Polymorphs α, β, γ, and δ
3
3
dabco2HBr
dabco2HBr 1/2H2O
dabco2HBr H2O
3
3
structure type
formula
β
α
β
α
β
γ
δ
C6H14N2Br2
0.0001
C6H15N2Br2O1/2
0.0001
C6H15N2Br2O1/2
0.0001
C6H16N2Br2O
0.0001
C6H16N2Br2O
0.48(5)
C6H16N2Br2O
0.78(5)
C6H16N2Br2O
1.50(5)
p (GPa)
T (K)
296(2)
296(2)
296(2)
296(2)
296(2)
296(2)
296(2)
crystal system
orthorhombic
Cmc21
orthorhombic
Pbc21
orthorhombic
Cmc21
orthorhombic
Pbc21
orthorhombic
Cmc21
monoclinic
P21
orthorhombic
Pmmn
space group
unit cell (Å,°)
a
7.0917(13)
12.280(3)
11.4693(18)
90
14.3848(18)
12.4619(12)
11.3029(11)
90
7.272(2)
12.467(3)
11.265(4)
90
15.015(3)
12.486(2)
11.2364(16)
90
7.185(3)
12.382(6)
11.119(3)
90
6.516(4)
12.037(3)
6.792(3)
113.06(5)
490.2(4)
2/1
7.2844(19)
10.918(4)
5.663(4)
90
b
c
β
V (Å3)
Z/Z0
998.8(3)
4/0.5
2026.2(4)
8/2
1021.3(5)
4/0.5
2106.5(6)
8/2
989.2(7)
4/0.5
450.4(4)
2/0.25
F (g/cm3)
completeness (%)
R1/wR2 (I > 2σ1)
R1/wR2 (all data)
1.822
1.856
1.841
1.842
1.961
1.979
2.153
96.2
99.9
96.7
95.3
36.4
19.2
41.3
0.1259/0.3407
0.1860/0.3557
0.0355/0.0572
0.0645/0.0595
ꢀ/ꢀ
ꢀ/ꢀ
0.1059/0.3029
0.1705/0.3203
0.0640/0.1349
0.0845/0.1446
0.0322/0.0577
0.0490/0.0617
0.0532/0.1308
0.0608/0.1396
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Figure 5. The crystal structure of β-dabco2HBr H2O at 0.48 GPa. The
3
dications are superimposed with anions Br(1). The water molecule is
disordered and located in two positions but with one common site for
one of the hydrogen atoms. Hydrogen atoms of the dications have been
omitted for clarity.
differences in the orientation of dabco2H+ cations can be easily
seen in Figure 4. It can be noted that the structures of α-hemi-
hydrate and α-monohydrate can be considered as built of two
1
types layers perpendicular to [x], one between ꢀ /4 < x < 1/4
and the second between 1/4 < x < 3/4, and that the latter layer in
the dabco2HBr 1/2H2O hemihydrate is devoid of water mol-
3
1
ecules. Within the first layer, ꢀ /4 < x < 1/4, in both hemi- and
monohydrate structures, the locations of the water molecules
are similar, but their orientation is clearly different.
The structure of the anhydrate β-dabco2HBr resembles those
1
of α-dabco2HBr H2O and α-dabco2HBr /2H2O. Apart from
3
3
the absence of water molecules, halved unit-cell parameter a,
halved unit-cell volume and halved Z number (number of formula
units in the unit cell) compared to the α-mono- and α-hemi-
hydrate structures, and despite different space-group symmetry
of α-monohydrate and β-anhydrate, the arrangement of ions is
similar (Figure 4) and the α and β-type structures exhibit a
considerable three-dimensional isostructurality.18 It may be due
to the dominant role of electrostatic interactions between ions
and a lesser role of the interactions involving water molecules in
these crystals.
3.2. Pressure-Induced Polymorphism of Monohydrates.
The isothermal crystallization at 0.48 GPa leads to the mono-
Figure 4. Ambient-conditions (0.1 MPa, 296 K) crystal structures of
(a) β-dabco2HBr; (b) α-dabco2HBr 1/2H2O; and (c) α-dabco2HBr
3
3
hydrate polymorph β-dabco2HBr H2O, isostructural with the
3
H2O, all projected down crystal direction [001]. In drawing (a), the
dications are superimposed with anions Br(1); in drawings (b) and (c)
dications A (with atoms labeled 1ꢀ6) are superimposed with anions
Br(2) and dications B (atomic labels 11ꢀ16) are superimposed with
anions Br(1). All hydrogen atoms of dications have been omitted for
clarity and only those in the water molecules have been shown.
ambient-pressure anhydrous β-dabco2HBr and hemihydrate β-
1
dabco2HBr /2H2O (Figures 4 and 5 and Table 1). The β-
3
anhydrate, β-hemihydrate, and β-monohydrate crystals are iso-
structural in this way that they have the same space-group
symmetry, similar unit-cell dimensions (Table 1), and similar
positions of the ions. The main difference between the β-type
anhydrate and monohydrate is the presence of disordered H2O
molecules. The β-type crystals are also isostructural with the
α-type analogues, as explained above (Figures 4 and 5). The
main differences between the α- and β-type structures are in the
orientation of the cations, which in the β-type structures are
located on the mirror planes, absent in the α-type structures.
Because of the misalignment of the cations in type α phases,
there are two symmetry-independent cations and the unit-cell
parameter a is doubled. It is plausible that a factor favoring the
α-dabco2HBr H2O, as evidenced by X-ray powder diffraction
3
and thermal gravimetry. In dry air a reverse dehydration into
the anhydrate β-type structure dabco2HBr takes place over a day
or two for small crystals, with a considerable deterioration of
1
their diffracting quality. Crystals α-dabco2HBr /2H2O and α-
3
dabco2HBr H2O are isostructural: their space group symmetry
3
is the same and unit-cell dimensions are similar (Table 1 and
Figure 4). Positions of ions are very similar too, although small
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Figure 6. Aggregation of ions and water molecules in β-dabco2HBr, α-dabco2HBr 1/2H2O, and dabco2HBr H2O α, β, γ, and δ phases.
3
3
The blue dotted lines mark symmetry-independent hydrogen bonds, and their symmetry-equivalent H-bonds are represented by red dotted
lines; black labels mark symmetry-independent atoms, whereas those in their symmetry-equivalent positions have red labels. Two half-occupied
sites of the disordered water molecule in β-dabco2HBr H2O have been shown. Short contacts involving CH2 groups have been omitted for
3
clarity.
Scheme 1. Hydrogen Bonds Formed by the dabco2H+
misorientation of the cations — and thus transformation the β-
type structures into type α — is the presence of water molecules
at ambient pressure. In α-type hydrates, the water molecules are
located on one side of the dication, away from its mirror plane,
and in this way they break the symmetry of β-type crystals, except
for the β-dabco2HBr hydrate where the water molecule is
disordered. In the α-type hemihydrate and hydrate, and β-
hemihydrate the water molecules form H-bonds to one of the
amine protons of the dications and mediates in the H-bonding
to a Brꢀ anion (Figure 6). In the α-type structures these H-
bonds force the cations to change their orientation by pulling the
nitrogen atom toward this side where the H-bonded H2O
molecule and Brꢀ anion are located. Consequently, the mirror-
plane symmetry is broken and the unit cell doubles along [x] in
the α-type crystals, compared to the β-type ones. In terms of
Cations in the Structures of β-dabco2HBr, Hemihydrate
1
α-dabco2HBr /2H2O, and Monohydrate dabco2HBr H2O
3
3
Phases α, β, γ, and δa
crystals symmetry, it is much higher for β-dabco2HBr H2O than
3
for the α-dabco2HBr H2O: in the α-structures there are two
3
symmetry-independent formula units (Z0 = 2) and the structures
are ordered, while in the β-monohydrate only half of the
formula unit is symmetry independent (Z0 = 1/2) and the water
a The right column lists the structures where the H-bonding patterns
are present (cf. Figure 6). Symmetry-independent dications of α-
dabco2HBr 1/2H2O and α-dabco2HBr H2O are labeled A and B.
3
3
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Scheme 2. Hydrogen-Bonding Coordination of the Brꢀ
Anions (Left Column) in β-dabco2HBr Anhydrate,
1
Hemihydrate α-dabco2HBr /2H2O and Monohydrate
3
dabco2HBr H2O phases α, β, γ, and δa
3
a The right column lists the Brꢀ anions H-bonded in the specified
manner: superscripts s, α, β, γ, δ indicate the α-hemihydrate and
monohydrates α, β, γ, and δ, respectively; Br(1,2) anions with no
superscript refer to the anhydrate structure.
the anhydrate β-dabco2HBr only NH+
Brꢀ bonds are
3 3 3
1
formed by the dications; in hemihydrate α-dabco2HBr /2H2O
3
every second dication forms one NH+
OH2 bond, and in
3 3 3
α-dabco2HBr H2O and β-dabco2HBr H2O all dications are
3
3
NH+ OH2 bonded on one side and NH+ Brꢀ bonded on
3 3 3
3 3 3
the other. In all these structures, α-hemihydrate, α-monohydrate,
and β-monohydrate, the water molecule mediates the hydro-
gen bonds between the protonated amine and the Brꢀ anion:
NH+ OH2 Brꢀ.
3 3 3
3 3 3
Polymorph γ-dabco2HBr H2O (at 0.78 GPa) reveals another
3
systematic change in the H-bonding pattern of dabco2H2+
dications: one of the amine protons becomes involved in a
(bifurcated) three centered hydrogen bond to Brꢀ and H2O
(Scheme 1).
3.3. H-Bonding Coordination. It is apparent from Figures 4
1
and 6 that even in the isostructural α-dabco2HBr /2H2O and
3
α-dabco2HBr H2O, despite similar positions of ions and water
3
molecules, the H-bonding patterns are considerably different. In
all monohydrates, the dications are each H-bonded at least to
one Brꢀ anion. There are unique or repeated H-bonding types
(Figures 6 and 7; Scheme 1; Table S2 in the Supporting
Information). For β-dabco2HBr and dication A of α-hemi-
hydrate both amines are H-bonded to two Brꢀ anions each
(Scheme 1). In all monohydrate polymorphs, both NH+ Brꢀ
3 3 3
and NH+ OH2 bonds are formed. In the lower range of
3 3 3
pressure (dication B of hemihydrate β-dabco2HBr H2O, both
3
dications in α-dabco2HBr H2O and one in β-dabco2HBr H2O),
3 ꢀ
3
one amine is NH+ Br bonded and the other amine is
3 3 3
NH+ OH2 bonded to a water molecule. In γ-dabco2HBr H2O,
3 3 3
3
one amine remains NH+ Brꢀ bonded and the other becomes
Figure 7. Autosterograms20 of the structures of dabco2HBr H2O
3 3 3
3
shared between Brꢀ and H2O in a bifurcated (three-centered)
hydrogen bond. Both protons form trifurcated H-bonds in the δ
phase. Along with the increasing number of H-bonds involving
NH+ group pressure systematically increases also the H-accept-
ing capacity of Brꢀ anions: in α-hemihydrate and α-mono-
hydrate there are Brꢀ anions accepting no H-bonds, and H-
accepting capacity of Brꢀ increases to three in β, γ, and δ
high-pressure monohydrates: (a) phase β at 0.48 GPa/296 K (two
NH+ OH2 bonds are to one disordered water molecule); (b) phase γ
3 3 3
at 0.78 GPa/296 K; and (c) phase δ at 1.50 GPa/296 K. Hydrogen
bonds involving N, O, and Br atoms are marked with dashed lines.
molecules are disordered. In the α- and β-type structures, on
hydratation of the anhydrate and hemihydrates, the water
molecules gradually replace the NH+ Brꢀ bonds on one side
of the dications, as illustrated in Figure 6 and Scheme 1. Thus, in
dabco2HBr H2O (Scheme 2). No similar systematic pressure
dependence has been noted for the H-bonding capacity of water
3 3 3
3
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Crystal Growth & Design
ARTICLE
Figure 8. Intermolecular and interionic contacts shorter than the sums
of van der Waals radii19 in β-dabco2HBr and α-dabco2HBr 1/2H2O at
3
ambient conditions. The square symbols of symmetry-independent
anions in α-dabco2HBr 1/2H2O have been discriminated by colors:
contacts of Br(1) are shown in red, those of Br(2) in blue, Br(3) in
yellow, and Br(4) in purple.
Figure 10. The crystal volume per formula unit in the function of
pressure for dabco2HBr anhydrate, hemihydrates, and monohydrates.
The blue arrows indicate the volume of one H2O molecule in liquid23
and ice VI,24 depending on the pressure range; empty triangles show
3
difference between the volume of dabco2HBr H2O formula units and
3
one H2O molecule. Much smaller volume differences are observed
between monohydrates and anhydrate β.
contacts involving methylene H-atoms is consistent with the
observed pressure-generated hydrogen bonds CH O and
3 3 3
CH N.21,22
3 3 3
4. CONCLUSIONS
At normal conditions dabco2HBr can absorb and its hydrates
release water depending on air humidity. The gradual absorption
of water leads to the transformations of crystals between
anhydrate, hemihydrates, and monohydrates with similar struc-
tures of isostructural types α and β. High pressure stabilizes the
monohydrate structures, and no higher hydrates have been
obtained. The high-pressure β-monohydrate at 0.48 GPa is
isostructural with the β-type anhydrate and more remotely similar
to the α-type monohydrate, obtained at 0.1 MPa. Apparently,
increased pressure favors more the higher-symmetry β-structure
(Z0 = 1/2) with disordered water molecules, than lower-sym-
metry α structure (Z0 = 2) with all atoms ordered. Thus, both
structures are stabilized due to a higher entropy contribution: in
the β-monohydrate it is connected with the water molecule
disorder, whereas in α-monohydrate there are four times more
configurations in the molecular/ionic arrangement. Higher pres-
sure naturally eliminates the disorder, and still higher pressure
leads to systematic transformations of the hydrogen-bonding
patterns in the structures of γ and δ monohydrates, with the
H-donor capacity of NH+ groups and H-acceptor capacity of Brꢀ
anions increasing. There are clear isostructural relations between
monohydrates α, β, and γ. In the highest-pressure δ-monohy-
drate, each NH+ proton is involved in trifurcated (six centered)
H-bond to Brꢀ and water molecule, and the Brꢀ anion is 3-fold
coordinated. It is remarkable that only monohydrate polymorphs
of dabco2HBr have been obtained in high pressure and that
high humidity at normal conditions lead to another monohydrate,
too. Furthermore, in the highest-pressure polymorphs the γ-
dabco2HBr H2O and δ-dabco2HBr H2O the bifurcated and
Figure 9. Intermolecular and interionic contacts involving H-atoms in
dabco2HBr H2O polymorphs α, β, γ, and δ shorter than the sums of
3
van der Waals radii.19 The square symbols of symmetry-independent
anions in phase α have been discriminated by colors: contacts of Br(1)
are shown in red, those of Br(2) blue, Br(3) yellow, and Br(4) purple.
molecules, each involved in three or four H-bonds in the
hemihydrate and monohydrate crystals (Scheme S1 in Support-
ing Information). The criteria applied for identifying H-bonds
have been based on the H acceptor distances compared with
3 3 3
the sums of appropriate van der Waals radii19 (Figures 8 and 9). It
can be observed that all the pressure-induced H-bonds are
considerably shorter than the van der Waals radii sums.
3.4. Intermolecular and Interionic Contacts. There is a
considerable number of short contacts involving CH2 groups in
the dabco2HBr structures. It can be observed from the plots in
Figures 8 and 9 that short NH+ O contacts are characteristic
3 3 3
both of hemihydrates and monohydrates, but CH O contacts
3 3 3
appear in the monohydrates only. It illustrates that the incor-
poration of water molecules between large ions is advantageous
for the close packing in dabco2HBr monohydrates and explains
the small volume increase of the hydrated dabco2HBr forms
compared to the anhydrate (Figure 10). The formation of short
3
3
trifurcated H-bonds formed by the NH+ group are similar to that
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Crystal Growth & Design
ARTICLE
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observed at 1.4 GPa in the β-dabco2HI H2O, to one water mol-
3
ecule and one Iꢀ anion, despite considerable differences between
all these structures.25 The formation of bifurcated and trifurcated
H-bonds involving the NH+, Brꢀ/Iꢀ, and H2O groups is con-
sistent with their increased H-donor and H-acceptor capacities in
high pressure. The polar crystals of α, β, and γ types and their
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)
2010, 1, 844–849.
(22) Olejniczak, A; Katrusiak, A. J. Phys. Chem. B 2008, 112, 7183–
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(25) Olejniczak, A.; Katrusiak, A. Cryst. Growth Des. 2011, DOI:
10.1021/cg101653x.
’ ASSOCIATED CONTENT
S
Supporting Information. Detailed crystallographic infor-
b
mation on high-pressure anhydrate, hemihydrates, and hydrates
of dabco2HBr (Table S1), the shortest intermolecular inter-
actions (Table S2). This material is available free of charge via the
Internet at http://pubs.acs.org. The CIF files can also be ob-
tained from the Cambridge Crystallographic Data Centre (CCDC)
1
as Nos. 818932 β-dabco2HBr, 818933 α-dabco2HBr /2H2O,
3
1
818934 β-dabco2HBr /2H2O, 818935 α-dabco2HBr H2O,
3
3
818936 β-dabco2HBr H2O, 818938 γ-dabco2HBr H2O and
3
3
818937 δ-dabco2HBr H2O.
3
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: katran@amu.edu.pl.
’ ACKNOWLEDGMENT
Authors are grateful to Prof. Zofia Dega-Szafran, of the Faculty
of Chemistry, Adam Mickiewicz University, for her help in
synthesis of dabco2HBr. This study was supported by the
Foundation for Polish Science, Team Program 2009-4/6. A.O.
acknowledges the reception of scholarship START from the
Foundation for Polish Science in 2011.
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