Halogen Bonding in Host-Guest Compounds: Structures and Kinetics of Enclathration and Desolvation

Article abstract of DOI:10.1021/acs.cgd.5b00402

(Graph Presented). The host compounds tetrakis(4-bromophenyl) ethylene and its iodo-analogue form inclusion compounds with a series of chloro- and iodo-methanes. Their structures have been elucidated, and their nonbonded halogen...halogen contacts have be

Full text of DOI:10.1021/acs.cgd.5b00402

Article
pubs.acs.org/crystal
Halogen Bonding in Host−Guest Compounds: Structures and
Kinetics of Enclathration and Desolvation
Francoise M. Amombo Noa, Susan A. Bourne, and Luigi R. Nassimbeni*
Centre for Supramolecular Chemistry Research, Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
S
* Supporting Information
ABSTRACT: The host compounds tetrakis(4-bromophenyl) ethylene and its iodo-analogue form inclusion compounds with a
series of chloro- and iodo-methanes. Their structures have been elucidated, and their nonbonded halogen···halogen contacts have been
analyzed and classified. Their kinetics of desolvation have been studied, and the concomitant activation energies have been established.
Five of the clathrates are isostructural and display similar activation energies of desolvation, thus correlating structure and function. The
velocity of the enclathration for the solid host−methyl iodide vapor reactions and associated rate law have been established.
The atomic numbering is given in Scheme 1. In this publica-
tion, the symbol for the element iodine is given as to distinguish
it from the Roman numeral I, which is used to denote “type I”.
INTRODUCTION
■
0
The topic of halogen bonding and its importance in various
aspects of crystal engineering is receiving considerable interest.
Desiraju reviewed it in his book Crystal Engineering1 and more
recently pointed out its effects in crystal plasticity2 and on the
packing of dihalogenated phenols.3 Halogen bonding has been
likened to hydrogen bonding, and its applications in synthetic
chemistry and materials science have been evaluated.4,5 The
nature of halogen bonding in the gas phase has been examined6
and its occurrence in biological molecules such as proteins and
nucleic acids has been studied.7 Recently, a complete issue of
Crystal Growth & Design was devoted to halogen bonding in
crystal engineering.8 This covered a variety of topics, ranging from
the theoretical to the practical applications of these secondary
interactions, and attests to the importance of this subject.
Scheme 1. Host and Guest Compounds
In this work, we present the results of the synthesis, structural
characterization, and dynamic behavior of a series of inclusion
compounds formed between two related hosts, tetrakis(4-
bromophenyl) ethylene (H1) and its iodo-analogue (H2),
with a series of halogenated methanes. The host compounds
were chosen because they are the most reactive in terms of
halogen bonding,9 and the guests were selected due to their low
boiling points, giving them high volatility. This feature is useful in
examining their kinetics by thermal gravimetry.
EXPERIMENTAL SECTION
H1 was synthesized by exposing tetraphenylethene to bromine vapor in
a desiccator for 24 h. The resulting product was recrystallized twice from
a 50/50 (v/v) mixture of ethanol and chloroform.
H2 was synthesized by mixing a suspension of tetraphenylethene (8.3 g,
25 mmol), iodine (12.7 g, 50.0 mmol), and bis[trifluoroacetoxy]phenyl
■
Inclusion compounds of H1 with acetone, tetrahydrofuran,10
and p-xylene11 have been reported. H2 formed clathrates with
1,1,2,2-tetrachloroethane9 and C60 fullerene/toluene solvate.12
Received: March 23, 2015
Revised: May 21, 2015
Published: May 26, 2015
© 2015 American Chemical Society
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Article
iodine (26.2 g, 60.0 mmol) in chloroform (30.0 mL) and stirring at room
temperature for 12 h. After the iodine color disappeared, the reaction
mixture was filtered, washed with a little hexane, dried, and recrystallized
from toluene to give colorless prisms.9
Scheme 2. Halogen···Halogen Interactions
Crystal Growth. The inclusion compounds were obtained by
dissolving H1 or H2 in the various liquid guests and allowing the solu-
tions to evaporate. The single crystals obtained were initially analyzed by
thermal gravimetry (TG), which detected the formation of the inclusion
compound and yielded the host/guest ratio.
Structure Analysis. Cell dimensions were established from the
intensity data measured on Nonius Kappa CCD13 and Bruker DUO
APEX II14 diffractometers using graphite-monochromated Mo Kα
radiation. The intensity data were collected by the standard φ scan and
ω scan techniques, scaled, and reduced using DENZO-SMN15 or
SAINT-Plus.16 The structures were solved by direct methods and
refined by full-matrix least-squares on F2 using SHELX-9717 program
packages. The program X-SEED18 was used as a graphic interface.
All the non-hydrogen atoms were refined anisotropically, while the guest
atoms were treated isotropically or anisotropically depending on the
occurrence of disorder. All hydrogen atoms were placed geometrically
and with a riding model for their isotropic temperature factors.
Crystallographic data for the structures in this paper have been
deposited with the Cambridge Crystallographic Centre as supplemen-
tary publication number 1055011−1055019. Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, U.K. (fax: (+44) 1223-336-033; e-mail: deposit@
ccdc.cam.ac.uk).
Thermal Gravimetry (TG). TG data were collected on a TGA
Q500 (TA Instruments) with a purge gas of dry nitrogen flowing at
60 mL min−1. The kinetics of desolvation was determined by the method
of Flynn and Wall,19 in which the mass loss of the compounds was
recorded at fixed heating rates β of 2, 4, 8, 16, and 32 °C min−1. Plots
of log β versus 1000/T yielded the values of the activation energies for
the reactions.
The kinetics of enclathration was determined in a specially con-
structed balance in which a powdered sample of the host was exposed to
the vapor of the volatile guest. The apparatus allows the sorption to be
carried out under controlled conditions of temperature and vapor
pressure. The mass gain is automatically recorded as a function of time,
and the data are converted to the extent of reaction α.
All halogen···halogen interactions are reported in Tables 2
and 4, which specify the X···X distance, angles θ1 and θ2, and the
type of bond as shown in Scheme 2.
We recorded X···X distances that were less than the sum of
the van der Waals radii +5% for interactions of type I and IIa and
type IIb. In general the type IIb interactions gave reasonable
nonbonded distances, but the angles θ1/θ2 sometimes deviated
significantly from the ideal values of 180°/120°.
Structure Analysis. Crystal data and experimental and
refinement parameters are given in Table 1 for the host−guest
compounds with H1.
Structure 1 is that of a new polymorph of the apohost, the
first structure of which was published by Tanaka et al.11 That
structure was obtained from meta-xylene and crystallizes in the
space group Pccn with Z = 8. By contrast, this new polymorph of
H1, obtained from acetonitrile, crystallizes in the space group
P21/n with Z = 8. The packing, shown as a projection viewed
along [010], is illustrated in Figure 1. There are four Br···Br
interactions, all of type IIa (Table 2).
Structure 2, H1·2CH2Cl2, crystallizes in P212121 with Z = 4.
The packing is shown in Figure 2, which displays channels
contiguous to the screw axis along [100] in which half the
dichloromethane guests are located. The other dichloromethane
guests reside in cavities. There are six X···X interactions, reported
in Table 2.
Hirshfeld Surface Analysis. In order to further understand
the nonbonded interactions that occur in these structures, the
program Crystal Explorer23−25 was employed, which calculates
the Hirshfeld surfaces of a molecule in the structure and depicts
all the molecular interactions of a targeted molecule with its
neighbors.
The results for structure 2, H1·(2CH2Cl2), are given as an
example. The host compound H1 was selected as the target
molecule, and the Hirshfed surface was calculated. The finger-
print plots for the various nonbonded interactions are shown
in Figure 3a−h, and the corresponding frequency of these
interactions is reported as a percentage in the legend, in which
the first atom is from the targeted host and the second from any
surrounding molecules.
α = (mt − mo)/(m∞ − mo)
where mo, mt, and m∞ are the masses at the start, during, and at the end
of the experiment, respectively. The α−time curves may then be fitted to
an appropriate rate law20 and the rate constant thus evaluated.
Differential Scanning Calorimetry. DSC was performed for all
inclusion compounds. The crystals were crushed and placed in crimped
and vented pans, then analyzed using a DSC Q200 series with a purge
gas of nitrogen at 60 mL min−1. Samples were analyzed between 303 and
600 K for H1 and 303−623 K for H2 at a heating rate of 10 K min−1.
RESULTS AND DISCUSSION
■
The character of halogen···halogen interactions has been dis-
cussed as a model derived from experimental charge density anal-
ysis.21 The results are based on the anisotropy of the electron
density distribution around the halogen nuclei and may be
understood in terms of a polarization, which is positive in the
polar region of the halogen atom and negative in the equatorial
region. The outcome of this study suggests three types of
geometries, shown in Scheme 2.
Thus, for example, Figure 3c shows the interactions between C
atoms in the targeted host and all H atoms in the surrounding
hosts and guests. It shows that the closest approach, shown as
peak 2, occurs at the sum of the internal and external distances
from the surface (di + de) = 2.95 Å, slightly longer than the sum of
the van der Waals radii.
What we glean from these maps is that the packing is
dominated by H···H interactions (28.1%), Figure 3b, with a
closest approach at peak 1 at (di + de) = 2.60 Å, somewhat longer
than the sum of the van der Waals radii (2.40 Å).
Type I interactions are of the van der Waals type, and their
X···X distances are generally greater than the sum of the van der
Waals radii. Type IIa and IIb interactions are considered as
attractive, and the X···X distances are usually shorter than those
of type I.
In this work, we have employed the van der Waals radii of
Bondi22 (radii in Å, H = 1.20, Cl = 1.75, Br = 1.85,
0
= 1.98).
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Table 1. Crystallographic Data and Structure Refinement Parameters of H1 and Its Inclusion Compounds
compound
1
2
3
4
code
H1
H1·(2CH2Cl2)
C26H16Br4·2CH2Cl2
817.84
H1·(2CH3
0
)
H1·(1.5CH202
)
structural formula
molecular mass (g mol−1
C26H16Br4
648.03
296(2)
0.10 × 0.13 × 0.15
P21/n
C26H16Br4·2CH3
0
C26H16Br4·1.5CH2
0
2
)
931.86
1237.92
data collection temp (K)
173(2)
173(2)
173(2)
crystal size (mm)
space group
a (Å)
0.13 × 0.14 × 0.15
P212121
9.3805(4)
15.3256(6)
21.0238(8)
90
0.1 × 0.11 × 0.12
P212121
9.3728(19)
14.7617(3)
21.8708(4)
90
0.04 × 0.07 × 0.12
P21/n
20.5195(6)
9.7865(3)
24.0043(8)
90
25.4217(18)
9.5847(6)
25.8099(17)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
vol (Å3)
Z
Dc, (g cm−3
absorp coeff (mm−1
91.598(1)
90
90
90
103.462(1)
90
90
90
4818.57(3)
8
3022.42(2)
4
3026.01(10)
4
6116.04(7)
8
)
1.787
1.797
2.045
2.280
)
6.692
5.697
7.375
8.308
θ range
1.29−26.46
69270
1.64−27.97
37644
1.66−26.37
12346
1.01−27.96
164831
10288
reflns collected
no. data I > 2σ(I)
7362
5969
5322
final R indices [I >2σ(I)]
R indices (all data)
GOF on F2
0.0425
0.0666
1.047
0.0326
0.0584
0.0709
1.056
0.0383
0.0477
0.0696
1.020
1.056
Table 2. X···X Interactions of H1 with Its Inclusion Compounds
d − ∑vdw
θ1/θ2
[deg]
compound
X···X
d [Å]
[Å]
%
type
1
Br1A···Br34 3.706(1)
Br1A···Br14 3.637(1)
Br4A···Br44 3.681(1)
Br24···Br34 3.755(1)
150/77
171/139
170/122
137/80
162/113
169/130
162/92
137/115
155/88
153/79
169/113
167/132
127/113
166/76
179/114
170/113
156/70
169/135
177/74
165/71
139/61
166/78
163/113
+0.006
−0.063
−0.019
+0.055
+0.078
−0.222
+0.166
+0.062
+0.047
+0.063
+0.017
−0.044
+0.053
+0.066
−0.104
−0.070
+0.084
−0.065
−0.062
−0.096
+0.200
−0.105
+0.161
+0.2
−1.7
−0.5
+1.5
+2.2
−6.0
+4.6
+1.7
+1.3
+1.8
+0.4
−1.2
+1.4
+1.7
−2.8
−1.8
+2.3
−1.8
−1.6
−2.5
+5.2
−2.7
+4.1
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
2
Br14···Cl2
Br24···Br34 3.478(1)
Br24···Cl2 3.766(2)
Br34···Br44 3.762(2)
3.678(2)
Cl1···Cl2
Cl2···Cl4
3.547(2)
3.563(2)
3.847(2)
3
4
Br14···
0
2
Br24···Br34 3.656(7)
Br34···Br44 3.753(8)
4.026(8)
0 0
1···
2
Br1A···Br34 3.596(3)
3.760(3)
Br1A···0
5
Br2A···Br14 3.784(3)
Br4A···Br14 3.635(3)
3.768(3)
3.734(3)
4.030(3)
3.725(3)
4.121(3)
Br24···
Br44···
Br44···
0
4
1
0
0
2
Br44···0
0 ··· 0 6
Figure 1. Packing diagram of 1 along [010] with hydrogen atoms omitted.
3
5
A striking feature is that the Br···Br interactions (6.2%),
Figure 3g, and Br···Cl interactions (3.6%), Figure 3h, are buried
within the fingerprint plots, peaking at approximately the sum of
the van der Waals radii, as previously reported in Table 2. This
puts the halogen···halogen interactions in context of the packing
as a whole.
The same calculations were performed on the other struc-
tures, and similar results were obtained. The resulting percentage
frequencies of the interactions are reported as tables in the
Supporting Information.
Structure 3 is homeotypic with 2 in that the unit cell dimen-
sions are similar, space group and host/guest ratios are the same,
and the host atoms occupy the same positions. The only dif-
ference is that the CH2Cl2 guests are replaced with CH3 , and the
0
subsequent halogen···halogen contacts are reported in Table 2.
When the methyl iodide guest was replaced by methylene
iodide, CH20 , structure 4, which crystallized in P21/n, was ob-
2
tained. This structure contains two host and three guest mole-
cules in the asymmetric unit, and the packing is shown in Figure 4.
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Figure 2. Packing diagram of 2 along [100] illustrating the channels and cavities in the structure.
Figure 3. Two-dimensional plots for the host molecule in structure 2: (a) overall fingerprint with H1 as target; (b) H···H = 28.1%; (c) C···H = 10.9%;
(d) H···Br = 9.9%; (e) H···Cl = 10.5%; (f) H···C = 5.3%; (g) Br···Br = 6.2%; (h) Br···Cl = 3.6%.
The guest molecules are located along constricted hourglass
channels running along [010].
Structures 5, 6, and 7 where the Br atoms of the host have been
are located in cavities shown in Figure 6. There are 11 halogen···
halogen interactions, whereby seven of these interactions are
between host···guest molecules and four between host···host
molecules. Three of the halogen···halogen interactions are of
type I and eight of type IIa listed in Table 4.
replaced by
0
, and in which the guests are CH2Cl2, CH3 , and
0
CHCl3 respectively, are also homeotypic with structure 2. Their
crystal data and experimental and refinement parameters are
reported in Table 3. The halogen···halogen interactions of these
homeotypic structures are listed in Table 4.
Kinetics of Decomposition, Adsorption, and DSC. The
decomposition of all the inclusion compounds was analyzed, and
the kinetics were studied by recording the mass loss as a function
of temperature at known heating rates. A typical result, that of the
decomposition of H2·(2CH30 ) is shown in Figure 7a, which
illustrates the desolvation curves at rates varying geometrically
from 2 to 32 °C min−1. The corresponding semilogarithmic plot
(Figure 7b) yielded straight lines for decomposition correspond-
ing to 8%, 24%, 40%, and 56% of the extent of reaction. The con-
comitant activation energies varied from 122.4 to 130.9 kJ mol−1.
The differential scanning calorimeter (DSC) result recorded at a
heating rate of 10 °C min−1 is shown in Figure 7c.
Structure 8 crystallizes in Pbcn with Z = 4. The host atoms are
located on the dyads at Wyckoff position c, while the guests lie in
channels running along [010], which are shown in Figure 5a and
with van der Waals radii in Figure5b. It is interesting that there
are halogen···halogen contacts between host···host, host···guest,
and guest···guest molecules shown in Table 4.
The CCl4 molecules are in restricted channels. The diagram
displays the molecules with van der Waals radii. In the projection,
one notes the apparent overlap of host and guest atoms.
Structure 9 crystallizes in P21/c with Z = 8. The asymmetric
This displays the first endotherm at 359.4 K, corresponding to
unit contains two host and two guest molecules. CH20 molecules
the loss of the CH3
0
guest. The second sharper endotherm,
2
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Figure 4. Packing diagram of 4 along [010] showing halogen···halogen interactions.
Table 3. Crystallographic Data and Structure Refinement Parameters of H2 Inclusion Compounds
compound
5
6
7
8
9
code
H2·(2CH2Cl2)
H2·(2CHCl3)
H2·(2CCl4)
H2·(2CH3
0
)
H2·CH202
structural formula
molecular mass (g mol−1
C26H1604·2CH2Cl2
C26H1604·2CH3
0
C26H1604·2CHCl3
C26H1604·2CCl4
C26H16
0
4·CH2
0
2
)
1005.84
1119.86
1073.71
1143.61
1103.82
data collection temp (K)
173(2)
173(2)
173(2)
173(2)
173(2)
crystal size (mm)
space group
a (Å)
0.06 × 0.14 × 0.15
P212121
9.6916(19)
15.450(3)
22.047(4)
90
0.07 × 0.10 × 0.12
P212121
9.5601(19)
15.1117(3)
22.5968(5)
90
0.07 × 0.12 × 0.13
P212121
9.7376(19)
15.3250(3)
22.9939(5)
90
0.09 × 0.10 × 0.12
Pbcn
0.02 × 0.06 × 0.10
P21/c
18.949(13)
10.3422(7)
17.9428(12)
90
21.2266(42)
9.8811(2)
29.6534(59)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
vol (Å3)
Z
90
90
90
90
103.043(3)
90
90
90
90
90
3301.18(11)
4
3264.55(12)
4
3431.35(12)
4
3516.3(4)
4
6059.11(2)
8
Dc, (g cm−3
)
2.024
2.278
2.078
2.160
2.420
absorp coeff (mm−1
)
4.116
5.724
4.117
4.172
6.166
θ range
1.61−27.48
141283
5965
3.44−27.46
86357
1.60−27.43
107385
7835
2.15−28.43
65710
1.41−27.52
148904
13923
reflns collected
no. data I > 2σ(I)
6191
3290
final R indices [I > 2σ(I)]
R indices (all data)
GOF on F2
0.0499
0.0784
1.147
0.0335
0.0427
0.0310
0.0506
1.007
0.0653
0.0490
0.0658
0.1512
1.045
1.083
1.074
112−138 kJ mol−1. This is to be expected since they are
which occurs at 572.1 K, is due to the melting of the host. The
peak temperatures were recorded as opposed to onset temper-
atures because not all the DSC profiles gave a steady baseline.
The activation energies and the temperatures for the guest de-
solvation are given in Table 5. The results of the decomposition
of the other inclusion compounds have been deposited as
Supporting Information.
isostructural and the three guests have normal boiling points in a
small range of ∼20 K. (boiling point (K), CH2Cl2 = 312.8, CH3 =
0
315.5, and CHCl3 = 334.4).
For compound 8, the activation energy is higher; this is
because the CCl4 molecules lie in restricted channels (Figure 4)
and the boiling point of CCl4 is higher at 349.9 K.
It is noteworthy that the activation energies of the desolvation
reactions for compounds 2, 3, 5, 6, and 7 fall in a tight range of
The only compound that did not follow this decomposition
pattern, namely, loss of guest followed by melting of the host, is
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Table 4. X···X Interactions of H2 Inclusion Compounds
compound
X···X
d [Å]
θ1/θ2 [deg]
d − ∑vdw [Å]
%
type
5
4.028(8)
3.794(7)
4.163(7)
3.670(7)
3.621(7)
4.032(8)
4.159(8)
3.944(8)
4.127(8)
3.857(8)
4.017(8)
3.995(8)
4.156(8)
4.023(9)
3.988(9)
3.930(9)
3.561(8)
3.666(8)
3.128(7)
3.748(3)
3.952(3)
3.830(3)
3.871(3)
3.592(2)
4.056(8)
3.937(8)
3.942(8)
4.011(8)
4.070(8)
3.831(8)
3.910(8)
3.682(7)
4.142(8)
3.870(8)
3.777(8)
129/118
167/132
173/88
160/84
157/89
171/107
148/122
168/128
176/91
166/132
126/113
166/75
157/73
177/87
164/133
133/114
141/101
160/122
174/160
173/95
162/108
142/69
135/63
110/103
138/75
179/88
150/146
141/93
132/111
136/136
173/113
167/79
139/91
165/98
174/79
+0.068
−0.166
+0.203
+0.170
+0.121
+0.072
+0.199
−0.016
+0.167
−0.103
+0.057
+0.035
+0.196
+0.063
+0.028
+0.200
+0.061
+0.166
−0.372
−0.212
−0.008
+0.100
+0.140
+0.092
+0.096
−0.023
−0.018
+0.051
+0.110
−0.129
−0.050
−0.278
+0.182
−0.009
−0.183
+1.7
−4.2
+5.1
+4.9
+3.5
+1.8
+5.0
−0.4
+4.2
−2.6
+1.4
+0.9
+4.9
+1.6
+0.7
+5.4
+1.7
+4.7
−10.6
−5.4
−0.2
+2.7
+3.8
+2.6
+2.4
−0.6
−0.5
+1.3
+2.8
−3.3
−1.3
−7.0
+4.6
−2.3
−4.6
IIb
IIb
IIb
IIa
IIa
IIa
I
0 ··· 0
14
24
0 ··· 0
14
44
0 ··· 0
24
44
Cl1···Cl7
Cl1···Cl2
0 ··· 0
6
14
1
0 ··· 0
14
24
IIa
IIb
IIb
IIb
IIa
IIa
IIa
IIa
I
0 ··· 0
14
2
0 ··· 0
24
44
0 ··· 0
24
34
0 ··· 0
34
0 ··· 0 44
1
4
0 ··· 0
2
4
7
0 ··· 0
14
34
0 ··· 0
24
34
0
34···Cl4
Cl1···Cl9
Cl5···Cl9
Cl2···Cl5
0 ··· 0
I
IIa
I
8
9
IIa
IIa
IIa
IIa
I
14
24
0 ··· 0
14
24
014···Cl2
0 ···Cl4
24
Cl2···Cl4
IIa
IIa
I
0 ··· 0
14
34A
0 ··· 0
14
4
0 ··· 0
24
34A
IIa
I
0 ··· 0
34
3
0 ··· 0
34
4
I
0 ··· 0
34
34
IIa
IIa
IIa
IIa
IIa
0 ··· 0
44
44A
0 ··· 0
44
3
0
14A ··· 0
1
0
14A ··· 0
2
024A ··· 0
1
Figure 5. Packing diagram of 8 along [010] showing (a) carbon tetrachloride molecules in channels and (b) van der Waals radii crystal packing
down [010].
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Table 5. Activation Energies and DSC Endotherm of
H1 and H2 Inclusion Compounds
DSC guest
release, K DSC host melt, K
compounds structures Ea, kJ mol−1
H1·(2CH2Cl2)
H1·(2CH3
2
3
4
5
6
7
8
9
112.5−137.6 315.4
126.7−144.5 342.4
72.8−83.3 336.4
115.5−122.9 342.0
122.4−130.9 359.4
526.1
526.7, 531.1
decomposes
573.8
0
)
H1·(1.5CH202
)
H2·(2CH2Cl2)
572.1
H2·(2CH3 )
0
H2·(2CHCl3)
H2·(2CCl4)
H2·CH202
127.1−137.9 347.7, 369.9 573.5
149.0−156.7 365.6
368.3
561.9
a
450.4 (exotherm
peak)
a
Experiment not performed due to paucity of material.
This is followed by a sharp exotherm peak at 412.4 K, which
implies a reaction between the host and the CH20 . We surmise
2
that this is a reaction at the double bond. The same reaction is
observed in structure 9, and we did not pursue this further.
Compound 4, H1·(1.5CH2 2) has a significantly lower
0
activation energy of desolvation. It was noted that the crystals
obtained, very thin needles (0.04 × 0.07 × 0.12 mm3), were very
unstable and began to decompose immediately upon removal
from the mother liquor. They were therefore quickly covered in
paratone oil, mounted in a plastic loop, and put in the stream of
cold nitrogen to minimize decomposition. The crystal instability
matches the DSC result, which has the endotherm due to
guest loss at 336.4 K, in comparison to the high boiling point
Figure 6. Packing diagram of 9 down [010] with the guest molecules
located in cavities.
compound H1·(1.5CH2
exhibits an endotherm at 336.4 K due to the loss of CH20 guest.
0
2). The DSC trace for this compound
of CH20 (454.4 K).
2
2
Figure 7. (a) Nonisothermal TG curves for 6, (b) plot of log β vs the reciprocal of the temperature for 6, and (c) DSC curve for 6.
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Figure 8. Kinetic curves for (a) 3 and (b) 6 at 25 °C.
The kinetics of enclathration was determined on a specially
constructed balance, which consists of a Precisa four place
balance the weighing mechanism of which has been separated
from its electronics component and placed under an inverted
desiccator. This allows the sorption of guest vapor to be carried
out under controlled conditions of temperature and vapor pres-
sure of volatile guest.
a corresponding guest-release endotherm. The average energies
of activation of their decomposition lie in a narrow range
of ∼119−135 kJ mol−1, thus correlating structure and thermal
behavior. The compound containing the less volatile guest
carbon tetrachloride has significantly higher activation energy of
desolvation. The compounds with diiodomethane form unstable
crystals, yield low activation energy of desolvation, and react with
the host compounds when heated.
The results of the kinetics of enclathration of H1 with CH3
vapor at 25 °C is shown in Figure 8a, which displays the extent
of reaction α versus time. The mass of CH3 absorbed cor-
responded to a host/guest ratio of 1:2 as was found in the crystal
structure derived by the crystallization of H1 in pure CH3 . The
under
0
ASSOCIATED CONTENT
* Supporting Information
0
■
S
0
Triangular halogen···halogen interaction, DSC traces, kinetics of
decomposition, plot of log β vs the reciprocal of the temperature,
Hirshfeld surface analysis, and crystallographic data in cif format.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.5b00402.
powdered host was exposed to the vapor of liquid CH3
atmospheric conditions.
0
The enclathration reaction displays a long induction period of
140 min, after which the absorption curve has a sigmoidal profile
that fits the Avrami−Erofe’ev equation:
AUTHOR INFORMATION
Corresponding Author
f(α) = [−ln(1 − α)]1/4 = kt
■
*Tel: +27 21 650 5893. Fax: +27 21 650 2569. E-mail:
luigi.nassimbeni@uct.ac.za.
where k is the rate constant, which has a value of 6.64 ×
10−3 min−1 (r = 0.998) at 25 °C.
Notes
The kinetic curve for the reaction of H2 with CH3
0
vapor
at 25 °C is shown in Figure 8b. This reaction has no induction
period, follows the same kinetic mechanism as the previous case,
and is much faster with k = 1.91 × 10−1 min−1 (r = 0.997). In
contrast to the previous reaction, this host only absorbs 1 mol of
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the University of Cape Town and the National
Research Foundation (South Africa) for funding.
CH3
0
per mole of host, although the host/guest ratio of the
inclusion compound derived from solution is 1:2.
REFERENCES
■
CONCLUSION
■
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