Hydrogen-bonding motifs and thermotropic polymorphism in redetermined halide salts of hexamethylenediamine

Article abstract of DOI:10.1107/S0108270108022919

The redetermined crystal structures of hexane-1,6-diammonium dichloride, C₆H₁₈N₂²⁺·2Cl^-, (I), hexane-1,6-diammonium dibromide, C₆H₁₈N ₂²⁺·2Br^-, (II), and hexane-1,6- diammonium diiodide, C₆H₁₈N₂ ²⁺·2I^-, (III), are described, focusing on their hydrogen-bonding motifs. The chloride and bromide salts are isomorphous, with both demonstrating a small deviation from planarity [173.89 (10) and 173.0 (2)°, respectively] in the central C - C - C - C torsion angle of the hydro-carbon backbone. The chloride and bromide salts also show marked similarities in their hydrogen-bonding inter-actions, with subtle differences evident in the hydrogen-bond lengths reported. Bifurcated inter-actions are exhibited between the N-donor atoms and the halide acceptors in the chloride and bromide salts. The iodide salt is very different in mol-ecular structure, packing and inter-molecular inter-actions. The hydro-carbon chain of the iodide straddles an inversion centre and the ammonium groups on the diammonium cation of the iodide salt are offset from the planar hydro-carbon backbone by a torsion angle of 69.6 (4)°. All three salts exhibit thermotropic polymorphism, as is evident from differential scanning calorimetry analysis and variable-temperature powder X-ray diffraction studies.

Full text of DOI:10.1107/S0108270108022919

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
bromide salt, (II), were initially determined almost 60 years
ago (Binnie & Robertson, 1949a,b), and the chloride salt was
redetermined 30 years ago (Borkakoti et al., 1978). Some
hydrogen-bonding details were published in Borkakoti’s
paper and our work expands further on the motifs and
networks visible in the chloride salt. No further work on the
structure of the bromide has been published since 1949 and no
discussion of the hydrogen-bonding patterns was presented by
Binnie and Robertson. The structure of the iodide salt, (III),
was initially determined 45 years ago (Han, 1963) and no
further work has since been reported for this material. We
present in this paper the redetermined structures of all three
halide salts, (I)–(III), and compare their hydrogen-bonding
networks, motifs and interactions.
ISSN 0108-2701
Hydrogen-bonding motifs and
thermotropic polymorphism in
redetermined halide salts of
hexamethylenediamine
Charmaine van Blerk* and Gert J. Kruger
Department of Chemistry, University of Johannesburg, PO Box 524, Auckland Park,
Johannesburg 2006, South Africa
Correspondence e-mail: cvanblerk@uj.ac.za
Received 18 June 2008
Accepted 21 July 2008
Online 20 September 2008
The redetermined crystal structures of hexane-1,6-diammo-
2+
nium dichloride, C6H18N2 ꢀ2Clꢁ, (I), hexane-1,6-diammo-
2+
nium dibromide, C6H18N2 ꢀ2Brꢁ, (II), and hexane-1,6-
2+
diammonium diiodide, C6H18N2 ꢀ2Iꢁ, (III), are described,
focusing on their hydrogen-bonding motifs. The chloride and
bromide salts are isomorphous, with both demonstrating a
small deviation from planarity [173.89 (10) and 173.0 (2)ꢂ,
respectively] in the central C—C—C—C torsion angle of the
hydrocarbon backbone. The chloride and bromide salts also
show marked similarities in their hydrogen-bonding inter-
actions, with subtle differences evident in the hydrogen-bond
lengths reported. Bifurcated interactions are exhibited
between the N-donor atoms and the halide acceptors in the
chloride and bromide salts. The iodide salt is very different in
molecular structure, packing and intermolecular interactions.
The hydrocarbon chain of the iodide straddles an inversion
centre and the ammonium groups on the diammonium cation
of the iodide salt are offset from the planar hydrocarbon
backbone by a torsion angle of 69.6 (4)ꢂ. All three salts exhibit
thermotropic polymorphism, as is evident from differential
scanning calorimetry analysis and variable-temperature
powder X-ray diffraction studies.
Fig. 1 depicts the molecular structures of all three
compounds. The chloride and bromide salts are isomorphous,
and a nonstandard unit cell (with a ꢀ angle less than 90ꢂ) was
selected for the former so that the two salts would have
compatible coordinates and so could be directly compared
with each other. The asymmetric units of the chloride and
bromide salts each consist of one hexane-1,6-diammonium
cation and two halide anions. The diammonium cation chains
deviate slightly from planarity, as can be seen from the torsion
angles across C1—C2—C3—C4 (Tables 1 and 3). The iodide
salt is markedly different from the other two halides in that its
asymmetric unit consists of one-half of the hexane-1,6-
diammonium cation and an iodide anion, with the hydro-
carbon chain of the former straddling a crystallographic
inversion centre. The ammonium groups of the iodide salt are
offset from the planar hydrocarbon chain, as can be seen from
the N1—C1—C2—C3 torsion angle (Table 5).
All three structures are hydrogen-bonded three-dimen-
sional lattices. Figs. 2 and 3 depict the packing and the
hydrogen-bonding motifs for both the chloride and bromide
salts. Hydrogen-bond geometries for the chloride and bromide
salts appear in Tables 2 and 4, respectively. There is evidence
of bifurcated hydrogen-bonding interactions involving atoms
H1E and H2D of the chloride salt (Table 2) and of the
bromide salt (Table 4). Two of these contacts in both chloride
and bromide salts (N1—H1Cꢀ ꢀ ꢀCl1ii, N2—H2Dꢀ ꢀ ꢀCl1v, N1—
H1Cꢀ ꢀ ꢀBr1ii and N2—H2Dꢀ ꢀ ꢀBr1v) are almost out of the
range of generally accepted hydrogen-bond distances and may
be the consequences of stronger hydrogen-bonding inter-
actions between the ammonium cation and the halide anion.
The packing diagram and hydrogen-bonding motifs for the
Comment
Applications and structure–property relationships of n-alkyl
diammonium salts are of continued interest and form the basis
of our investigations. Our research focuses specifically on
these materials as they are precursor ligands in transition
metal complexes that have applications in propellants,
explosives and pyrotechnic compositions (Singh et al., 2005,
2006), and they have structure-directing properties in the
synthesis of a number of nanoparticles (Chen et al., 2007;
Takami et al., 2007).
The halide salts of hexamethylenediamine form the focus of
this work. The structures of both the chloride salt, (I), and the
Acta Cryst. (2008). C64, o537–o542
doi:10.1107/S0108270108022919
# 2008 International Union of Crystallography o537

organic compounds
Figure 1
The molecular structures of (I) (left), (II) (centre) and (III) (right), showing the atomic numbering schemes. Displacement ellipsoids are drawn at the
50% probability level and H atoms are shown as small spheres of arbitrary radii. [Symmetry code: (i) 1 ꢁ x, ꢁy, 2 ꢁ z.]
Figure 3
A packing diagram for (II), viewed down the a axis, showing the
hydrogen-bonding network and motifs (top). A magnified view of the ring
motifs (slightly offset from the a axis) is shown at the bottom left; selected
cation chains have been truncated for clarity. A close-up view of the
individual hydrogen-bonding contacts is shown at the bottom right.
(Symmetry codes as in Table 2.)
Figure 2
A packing diagram for (I), viewed down the a axis, showing the hydrogen-
bonding network and motifs (top). A magnified view of the ring motifs
(slightly offset from the a axis) is shown at the bottom left; selected cation
chains have been truncated for clarity. A close-up view of the individual
hydrogen-bonding contacts is shown at the bottom right. (Symmetry
codes as in Table 2.)
Examination of the packing diagram of the chloride salt in
Fig. 2 shows, at first glance, six different hydrogen-bonding
ring motifs (top of Fig. 2, within the solid circle). All hydrogen-
bonding motifs are described using graph-set notation
iodide salt can be seen in Fig. 4, which is clearly different from
the chloride and bromide salts. Details of the hydrogen-bond
geometry for the iodide salt are given in Table 6.
o538 Blerk and Kruger C6H18N2 ꢀ2Clꢁ, C6H18N2 ꢀ2Brꢁ and C6H18N2 ꢀ2Iꢁ
Acta Cryst. (2008). C64, o537–o542
2+
2+
2+
ꢃ

organic compounds
Figure 4
A packing diagram for (III), viewed down the a axis (top left) and down
the c axis (top right), showing the hydrogen-bonding network and motifs.
A magnified view of the 30-membered ring motif (slightly offset from the
a axis) is shown at the bottom left. A close-up view of the asymmetric unit
of (III) is given at the bottom right and shows the individual hydrogen-
bonding contacts. (Symmetry codes as in Table 6.)
Figure 6
(Bernstein, 2002). There is one 19-membered ring (on the left
within the solid circle) involving three diammonium cations
and three chloride anions, with graph-set notation R34(19), one
22-membered ring (centre of the solid circle) involving two
diammonium cations and two chloride anions, R24(22), and
another 19-membered ring (on the right within the solid circle)
involving three diammonium cations and three chloride
anions, R34(19). The remaining three motifs appear to be an
overlaid square, and diamond-shaped rings and multiple
triangular rings, that when viewed slightly offset from the a
axis and magnified (bottom left of Fig. 2) are actually inter-
actions that link the packing sheets together. Further magni-
fication of one diammonium cation clearly shows the eight
individual hydrogen-bonding contacts (bottom right of Fig. 2)
and demonstrates the bifurcation mentioned above.
Comparative powder diffraction patterns of the chloride salt. The pattern
at the top is of the material at the starting temperature of 303 K. The
pattern in the middle is of the salt at 453 K after undergoing the phase
change. The pattern at the bottom is of the salt after it has been cooled to
the starting temperature.
ring motifs (two 19-membered rings and one 22-membered
ring) that are described for the chloride salt also exist in the
bromide salt. The remaining three motifs (overlaid square and
diamond-shaped rings plus multiple triangular rings), when
also viewed slightly offset from the a axis and magnified
(bottom left of Fig. 3), are actually the interactions that link
the packing sheets together. As with the chloride salt, further
magnification of one diammonium cation clearly shows the
eight individual hydrogen-bonding contacts (bottom right of
Fig. 3).
Fig. 4 shows the packing diagram and hydrogen-bonding
interactions for the iodide salt. Aview of the packing down the
a axis shows, at first glance, two different hydrogen-bonding
ring motifs (top left of Fig. 4, within the solid circle). There is
one large 30-membered ring involving four diammonium
cations and four iodide anions, graph-set motif R46(30). The
remaining motif is triangular and, when viewed slightly offset
from the a axis and magnified (bottom left of Fig. 4), these are
the interactions that link the packing layers together. A view
of the packing down the c axis (top right of Fig. 4) reveals
further 22-membered ring motifs that are shaped as elongated
hexagons with the graph-set R24(22). The individual hydrogen-
bonding contacts that link the layers are shown as a magnified
view at the bottom right of Fig. 4, where the three hydrogen-
bonding contacts in the asymmetric unit of the iodide salt are
clearly evident.
An identical pattern exists for the bromide salt in Fig. 3 that
shows, at first glance, six different hydrogen-bonding ring
motifs (top of Fig. 3, within the solid circle). The same three
The thermal properties of these salts were investigated by
differential scanning calorimetry (DSC) and hot-stage micro-
Figure 5
DSC traces of the three halide salts.
C6H18N2 ꢀ2Clꢁ, C6H18N2 ꢀ2Brꢁ and C6H18N2 ꢀ2Iꢁ o539
2+
2+
2+
ꢃ
Blerk and Kruger
Acta Cryst. (2008). C64, o537–o542

organic compounds
scopy. Fig. 5 shows the DSC scans for all three halide salts. The
chloride shows two endothermic events, while the bromide
and iodide each show one. Hot-stage microscopy showed that
the first endothermic event exhibits a change in morphology of
the crystals, while the second, seen only in the chloride salt, is
a sublimation at 493 K. Based on these observations, we
decided to carry out variable-temperature powder diffraction
(VT-PXRD) studies to establish if the changes in crystal
morphologies are thermotropic phase changes. The VT-PXRD
results for the chloride, bromide and iodide salts are shown in
Figs. 6, 7 and 8, respectively. Each figure shows a powder
pattern of the starting material at 303 K (top), a powder
pattern of the material after the phase change at the respective
high-temperature value (middle) and a powder pattern of the
material on cooling to the starting temperature of 303 K
(bottom). The results allow us to conclude that the endo-
thermic events evident from the DSC data are in fact ther-
motropic phase changes. It is evident from the powder data
that the chloride and bromide exhibit irreversible phase
changes, which was also confirmed by the hot-stage micros-
copy. The iodide, however, exhibits a reversible phase change,
as the two powder patterns at 303 K show the same peak
positions. Further investigation is required in order to char-
acterize the different phases.
Figure 7
Comparative powder diffraction patterns of the bromide salt. The pattern
at the top is of the material at the starting temperature of 303 K. The
pattern in the middle is of the salt at 403 K after undergoing the phase
change. The pattern at the bottom is of the salt after it has been cooled to
the starting temperature.
Experimental
For the preparation of (I), concentrated hydrochloric acid (HCl, 2 ml,
63.6 mmol; Merck) was added to hexane-1,6-diamine (0.50 g,
4.30 mmol; Aldrich) in a sample vial. For the preparation of (II),
concentrated hydrobromic acid (HBr, 2 ml, 37.07 mmol; Merck) was
added to hexane-1,6-diamine (0.50 g, 4.30 mmol; Aldrich) in a sample
vial. For the preparation of (III), concentrated hydriodic acid (HI,
2 ml, 26.58 mmol; Merck) was added to hexane-1,6-diamine (0.50 g,
4.30 mmol; Aldrich) in a sample vial. The three individual sample
mixtures were refluxed at 363 K for 2 h. The three solutions were
cooled to room temperature at a rate of 2 K hꢁ1 and colourless
crystals of hexane-1,6-diammonium dichloride, (I), hexane-1,6-
diammonium dibromide, (II), and hexane-1,6-diammonium diiodide,
(III), were formed. Suitable crystals of each salt were selected for
study by single-crystal X-ray diffraction. The polycrystalline powders
used for the variable-temperature powder diffraction studies were
obtained by hand-milling each salt using an agate mortar and pestle.
The powder patterns for the three halide salts were collected on a
PANalytical X’Pert PRO powder diffractometer (Cu Kꢁ) fitted with
an Anton Paar HTK1200 variable-temperature oven with an alumina
sample cup. The powder patterns were collected at selected
temperatures within the angular range 5 < 2ꢂ < 60ꢂ.
Compound (I)
Crystal data
2+
3
C6H18N2 ꢀ2Clꢁ
˚
V = 1020.77 (4) A
Z = 4
Mo Kꢁ radiation
ꢃ = 0.58 mmꢁ1
T = 295 (2) K
Figure 8
Mr = 189.12
Monoclinic, P21=c
˚
a = 4.6042 (1) A
b = 14.1570 (3) A
˚
c = 15.6614 (4) A
ꢀ = 89.327 (1)ꢂ
Comparative powder diffraction patterns of the iodide salt. The pattern at
the top is of the material at the starting temperature of 303 K. The pattern
in the middle is of the salt at 343 K after undergoing the phase change.
The pattern at the bottom is of the salt after it has been cooled to the
starting temperature.
˚
0.48 ꢄ 0.20 ꢄ 0.18 mm
o540 Blerk and Kruger C6H18N2 ꢀ2Clꢁ, C6H18N2 ꢀ2Brꢁ and C6H18N2 ꢀ2Iꢁ
Acta Cryst. (2008). C64, o537–o542
2+
2+
2+
ꢃ

organic compounds
Table 4
Hydrogen-bond and contact geometry (A, ) for (II).
Data collection
ꢂ
˚
Bruker SMART CCD
diffractometer
Absorption correction: multi-scan
(APEX2 AXScale; Bruker, 2008)
Tmin = 0.769, Tmax = 0.903
10699 measured reflections
2540 independent reflections
2195 reflections with I > 2ꢄ(I)
Rint = 0.021
D—Hꢀ ꢀ ꢀA
D—H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D—Hꢀ ꢀ ꢀA
N1—H1Cꢀ ꢀ ꢀBr1i
N1—H1Cꢀ ꢀ ꢀBr1ii
N1—H1Dꢀ ꢀ ꢀBr1iii
N1—H1Eꢀ ꢀ ꢀBr2iii
N2—H2Cꢀ ꢀ ꢀBr1
N2—H2Dꢀ ꢀ ꢀBr2iv
N2—H2Dꢀ ꢀ ꢀBr1v
N2—H2Eꢀ ꢀ ꢀBr2vi
0.89
0.89
0.89
0.89
0.89
0.89
0.89
0.89
2.56
3.11
2.47
2.43
2.50
2.59
3.09
2.47
3.408 (2)
3.529 (2)
3.3466 (19)
3.313 (2)
3.330 (2)
3.299 (2)
3.505 (2)
3.310 (2)
159
111
168
174
156
137
111
158
Refinement
R[F2 > 2ꢄ(F2)] = 0.026
wR(F2) = 0.075
S = 1.05
93 parameters
H-atom parameteꢁrs3 constrained
˚
Áꢅmax = 0.37 e A
Áꢅmin = ꢁ0.25 e A
ꢁ3
˚
2540 reflections
Symmetry codes: as in Table 2.
Table 1
Selected torsion angles (ꢂ) for (I).
Compound (III)
N1—C1—C2—C3
C1—C2—C3—C4
C2—C3—C4—C5
176.12 (10)
ꢁ179.62 (10)
173.90 (10)
C3—C4—C5—C6
C4—C5—C6—N2
ꢁ176.27 (11)
Crystal data
C6H18N2 ꢀ2Iꢁ
2+
177.22 (10)
3
˚
V = 613.58 (3) A
Z = 2
Mr = 372.02
Monoclinic, P21=c
Mo Kꢁ radiation
ꢃ = 5.08 mmꢁ1
T = 295 (2) K
0.48 ꢄ 0.18 ꢄ 0.18 mm
˚
a = 4.8884 (1) A
Table 2
Hydrogen-bond and contact geometry (A, ) for (I).
˚
b = 12.8756 (4) A
ꢂ
˚
˚
c = 9.7488 (3) A
ꢀ = 90.423 (2)ꢂ
D—Hꢀ ꢀ ꢀA
D—H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D—Hꢀ ꢀ ꢀA
Data collection
N1—H1Cꢀ ꢀ ꢀCl1i
N1—H1Cꢀ ꢀ ꢀCl1ii
N1—H1Dꢀ ꢀ ꢀCl1iii
N1—H1Eꢀ ꢀ ꢀCl2iii
N2—H2Cꢀ ꢀ ꢀCl1
N2—H2Dꢀ ꢀ ꢀCl2iv
N2—H2Dꢀ ꢀ ꢀCl1v
N2—H2Eꢀ ꢀ ꢀCl2vi
0.89
0.89
0.89
0.89
0.89
0.89
0.89
0.89
2.43
2.94
2.32
2.29
2.33
2.43
3.18
2.31
3.2599 (11)
3.3754 (10)
3.2000 (10)
3.1695 (10)
3.1765 (10)
3.1616 (10)
3.5361 (11)
3.1511 (11)
156
112
168
172
158
139
107
157
Bruker SMART CCD
diffractometer
Absorption correction: integration
(XPREP; Bruker, 2008)
Tmin = 0.194, Tmax = 0.462
5135 measured reflections
1551 independent reflections
1452 reflections with I > 2ꢄ(I)
Rint = 0.028
Refinement
1
2
3
2
1
1
1
1
Symmetry codes: (i) ꢁx þ 1; y þ ; ꢁz þ ; (ii) x þ 1; ꢁy þ ; z ꢁ ; (iii) x; ꢁy þ ; z ꢁ ;
R[F2 > 2ꢄ(F2)] = 0.024
wR(F2) = 0.059
S = 1.09
48 parameters
H-atom parameteꢁrs3 constrained
2
2
3
2
2
1
2
3
2
1
(iv) ꢁx þ 2; y ꢁ ; ꢁz þ ; (v) x þ 1; y; z; (vi) ꢁx þ 1; y ꢁ ; ꢁz þ .
2
2
˚
Áꢅmax = 1.24 e A
ꢁ3
˚
1551 reflections
Áꢅmin = ꢁ0.51 e A
Compound (II)
Crystal data
2+
3
C6H18N2 ꢀ2Brꢁ
V = 1104.92 (4) A
Z = 4
˚
Table 5
Selected torsion angles (ꢂ) for (III).
Mr = 278.04
Monoclinic, P21=c
˚
a = 4.7044 (1) A
Mo Kꢁ radiation
ꢃ = 7.28 mmꢁ1
T = 295 (2) K
N1—C1—C2—C3
69.6 (4)
C1—C2—C3—C3i
ꢁ178.3 (4)
˚
b = 14.4462 (3) A
˚
c = 16.2582 (4) A
0.44 ꢄ 0.26 ꢄ 0.10 mm
Symmetry code: (i) ꢁx; ꢁy; ꢁz þ 2.
ꢀ = 90.115 (1)ꢂ
Table 6
Hydrogen-bond geometry (A, ) for (III).
ꢂ
˚
Data collection
Bruker SMART CCD
diffractometer
19693 measured reflections
2753 independent reflections
D—Hꢀ ꢀ ꢀA
D—H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D—Hꢀ ꢀ ꢀA
Absorption correction: multi-scan
(APEX2 AXScale; Bruker, 2008)
Tmin = 0.117, Tmax = 0.483
2364 reflections with I > 2ꢄ(I)
Rint = 0.042
N1—H1Cꢀ ꢀ ꢀI1ii
N1—H1Dꢀ ꢀ ꢀI1iii
N1—H1Eꢀ ꢀ ꢀI1
0.89
0.89
0.89
2.76
2.79
2.91
3.617 (3)
3.586 (3)
3.666 (3)
163
149
144
1
2
1
2
Symmetry codes: (ii) x ꢁ 1; y; z; (iii) x ꢁ 1; ꢁy þ ; z ꢁ .
Refinement
R[F2 > 2ꢄ(F2)] = 0.025
wR(F2) = 0.060
S = 1.05
94 parameters
H-atom parameteꢁrs3 constrained
˚
H atoms were positioned geometrically and refined in the riding-
Áꢅmax = 0.84 e A
ꢁ3
˚
˚
˚
model approximation, with C—H = 0.97 A and N—H = 0.89 A, and
2753 reflections
Áꢅmin = ꢁ0.65 e A
with Uiso(H) = 1.2Ueq(C) or 1.5Ueq(N). For (III), the highest peak of
ꢁ3
˚
˚
1.24 e A in the final difference map lies 0.75 A from atom I1.
For all compounds, data collection: SMART-NT (Bruker, 1999);
cell refinement: SAINT (Bruker, 2008); data reduction: SAINT
(Bruker, 2008); program(s) used to solve structure: SHELXS97
(Sheldrick, 2008); program(s) used to refine structure: SHELXL97
(Sheldrick, 2008); molecular graphics: X-SEED (Barbour, 2001) and
Table 3
Selected torsion angles (ꢂ) for (II).
N1—C1—C2—C3
C1—C2—C3—C4
C2—C3—C4—C5
175.8 (2)
ꢁ179.7 (2)
173.0 (2)
C3—C4—C5—C6
C4—C5—C6—N2
ꢁ175.4 (2)
176.6 (2)
C6H18N2 ꢀ2Clꢁ, C6H18N2 ꢀ2Brꢁ and C6H18N2 ꢀ2Iꢁ o541
2+
2+
2+
ꢃ
Acta Cryst. (2008). C64, o537–o542
Blerk and Kruger

organic compounds
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o542 Blerk and Kruger C6H18N2 ꢀ2Clꢁ, C6H18N2 ꢀ2Brꢁ and C6H18N2 ꢀ2Iꢁ
Acta Cryst. (2008). C64, o537–o542
2+
2+
2+
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