Concerted ligand exchange and the roles of counter anions in the reversible structural switching of crystalline peptide metallo-macrocycles

Article abstract of DOI:10.1021/ic500478p

To understand reversible structural switching in crystalline materials, we studied the mechanism of reversible crystal-to-crystal transformation of a tetranuclear Ni^II macrocycle consisting of artificial β-dipeptides. On the basis of detailed structural analyses and thermodynamic measurements made in a comparison of pseudo-isostructural crystals (NO₃ and BF₄ salts), we herein discuss how ligand-exchange reactions take place in the crystal due to changes in water content and temperature. Observations of the structural transformation of NO₃ salt indicated that a pseudo crystalline phase transformation takes place through concerted ligand-exchange reactions at the four Ni ^II centers of the macrocycle with hydrogen bond switching. A mechanism for this ligand exchange was supported by IR spectroscopy. Thermodynamic measurements suggested that the favorable compensation relationship of the enthalpy changes due to water uptake and structural changes are keys to the reversible structural transformation. On the basis of a comparison with the pseudo-isostructural crystals, it is apparent that the crystal packing structure and the types of counter anions are important factors for facilitating reversible ligand exchange with single crystallinity.

Full text of DOI:10.1021/ic500478p

Article
pubs.acs.org/IC
Concerted Ligand Exchange and the Roles of Counter Anions in the
Reversible Structural Switching of Crystalline Peptide Metallo-
Macrocycles
Ryosuke Miyake*,† and Mitsuhiko Shionoya‡
†Department of Chemistry and Biochemistry, Graduate School of Humanities and Sciences, Ochanomizu University, 2-1-1 Otsuka,
Bunkyo-ku, Tokyo 112-8610, Japan
‡Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
S
* Supporting Information
ABSTRACT: To understand reversible structural switching in
crystalline materials, we studied the mechanism of reversible
crystal-to-crystal transformation of a tetranuclear NiII macro-
cycle consisting of artificial β-dipeptides. On the basis of
detailed structural analyses and thermodynamic measurements
made in a comparison of pseudo-isostructural crystals (NO3
and BF4 salts), we herein discuss how ligand-exchange
reactions take place in the crystal due to changes in water
content and temperature. Observations of the structural
transformation of NO3 salt indicated that a pseudo crystalline
phase transformation takes place through concerted ligand-
exchange reactions at the four NiII centers of the macrocycle
with hydrogen bond switching. A mechanism for this ligand exchange was supported by IR spectroscopy. Thermodynamic
measurements suggested that the favorable compensation relationship of the enthalpy changes due to water uptake and structural
changes are keys to the reversible structural transformation. On the basis of a comparison with the pseudo-isostructural crystals, it
is apparent that the crystal packing structure and the types of counter anions are important factors for facilitating reversible ligand
exchange with single crystallinity.
accompanies the opening and closing of nano-cavities with
rearrangement of the functional groups in the cavity (Figure
1c). Furthermore, the structural changes take place stepwise
with changes in water content, as revealed by single-crystal X-
ray diffraction and thermodynamic analyses.13,15
Herein, we describe a detailed mechanism for this stepwise
structural transformation of the crystalline peptide metallo-
macrocycle by using X-ray analyses and IR spectroscopy. This
study revealed that the structural transformation took place
through concerted ligand-exchange reactions within one
macrocycle. This transformation was caused by the compensa-
tion of enthalpy changes due to structural changes in the
macrocycles and by stabilization due to water uptake. The
ligand exchange-triggered structural transformation process was
well-explained by the effect of crystal packing structure and the
different behaviors of counter anions as well as hydrogen bond
switching.13
INTRODUCTION
■
Crystalline materials have great potential for the development
of designer functions based on their structural regularity and
phase transition characteristics.1−3 Reversible structural changes
of metal-containing crystalline materials caused by external
stimuli (light, temperature, molecular adsorption, etc., in
particular) are known to enable functional switching with
respect to molecular adsorption,4 spin-state transition,5 electron
conductivity,6 catalytic reactions,7 and so on. Metal-centered
structural changes in crystal states, such as coordinated
geometrical changes4 and ligand-exchange reactions,4,8−10 can
potentially become keys to such switching behaviors that are
observed in metal-containing crystals. However, it is rather
difficult to control reversible ligand exchange reactions in a
crystal-to-crystal manner under mild conditions.11 It is,
therefore, a challenge to study how to facilitate such structural
switching12 based on proper design.
Recently, we demonstrated that a tetranuclear NiII macro-
cycle formed from β-dipeptides (1) as NO3 salt in the crystal
state exhibited rapid and reversible crystal-to-crystal trans-
formation by ligand-exchange reactions responsive to non-
coordinating guest stimuli (Figure 1).13 The cooperative
structural switching takes place at −40 °C via hydrogen bond
switching within the peptide ligands.14 This transformation
RESULTS AND DISCUSSION
■
Counter Anion Effects on Structural Transformation.
To clarify the effects of counter anions on the structural
Received: March 1, 2014
Published: May 14, 2014
© 2014 American Chemical Society
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Table 1. Crystal Data for [Ni414]8+ Obtained at 93 K
BF4 salt
NO3 salt13
Ni414(BF4)8·12.05H2O
0.29 × 0.20 × 0.15
Ni414(NO3)8·11H2O
0.16 × 0.14 × 0.10
crystal size
(mm)
a
formula
C48H131.10B8F32N28Ni4O24.05
2416.03
C48H130N36Ni4O47
2198.72
a
M
crystal system
space group
a (Å)
tetragonal
tetragonal
I4
̅
I4
̅
15.7021(6)
15.7021(6)
19.7468(8)
90
15.4881(7)
15.4881(7)
19.3282(9)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
90
90
90
90
4868.7(3)
2
4636.5(4)
2
ρcalcd (g·cm−1
F(000)
)
1.648
1.575
2497
2316
μ (mm−1
)
0.902
0.912
θ range (deg)
1.66−35.68
1.042
2.11−30.03
1.060
Figure 1. (a) The structure of NiII macrocycles consisting of artificial
β-dipeptide 1, (b) structural changes at the NiII center, and (c) the
water content-dependent, reversible crystal-to-crystal structural trans-
formation of the macrocycle.13 Hydrogen atoms are omitted for clarity.
GOF
reflections
collected
29091
17487
independent
reflections
11191
6693
flack parameter
0.01(2)
0.097(15)
0.0189
changes, we first examined BF4− anions, which have almost the
same crystal packing structures as NO3 salt.13 As a result, we
found that the BF4 salt of the NiII macrocycle has almost the
same structure as the NO3 salt. The dipeptide ligand 1 was
mixed with an equimolar amount of Ni(BF4)2·6H2O in water,
and the solution was slowly evaporated at 3 °C16 to obtain
purple prismatic crystals in 50% yield. As shown in Figure 2 and
Table 1, the X-ray structural analysis of the resulting single
Rint
Rσ
0.0204
0.0415
0.0363
final R1 (I >
2σ(I)
(all data))
0.0428(0.0511)
0.0507(0.0546)
wR2 (I > 2σ(I)
(all data))
0.1208(0.1276)
973213
0.1476(0.1532)
856057
CCDC No.
a
For the formula and molecular weight (M), we included hydrogen
atoms of water molecules in the crystal, although we did not determine
them in the refined structures.
crystal revealed that the BF4 salt of the NiII macrocycle had an
I4 crystal system with one-fourth of the molecules being in the
̅
asymmetric units. The crystal packing structure of the BF4 salt
was almost the same as that of the NO3 salt in terms of the
positions of the counter anions (Figure 2). The only differences
were the disorder pattern of the counter anions and the size of
crystal units. Although the positions of the counter anions in
the two crystals were almost the same (Supporting Information,
−
Figures S2 and S3), BF4 ions were disordered more
−
−
significantly than NO3 ions, suggesting that the NO3 ions
interacted closely with the framework of the peptide metallo-
macrocycle through hydrogen bonding. In addition, the BF4 salt
was a little larger in the c-axis direction compared with the NO3
salt.
First, we compared the structural changes in these pseudo-
isostructural crystals by removing the included water molecules.
As shown in Figure 3, the water-release processes revealed by
X-ray single-crystal analysis were similar to each other and
included water molecules in contact with both counter anions
and the dipeptide ligand (Supporting Information, Table S1).
During the heating process to 40 °C, both the NO3 and BF4
salts exhibited almost the same structural changes, with similar
water-release processes involving the migration of neighboring
macrocycles. In contrast, the crystal structural changes from 40
to 60 °C were completely different from each other. In the BF4
Figure 2. Thermal ellipsoid representation showing the crystal packing
structure of [Ni414]8+ as (a) NO3 salt13 and (b) BF4 salt. Only a few
counter anions located near the NiII centers are shown. The other
counter anions, all included water molecules, and hydrogen atoms are
omitted for clarity. Thermal ellipsoids are set at 50% probability.
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Figure 5. Powder diffraction patterns of the NO3 salt: (a) a simulated
pattern of the hydrated form, (b) the as-synthesized sample, (c) after
being dried at 60 °C in vacuo, (d) after water uptake, and (e) a
simulated pattern of the dehydrated form. All diffraction patterns were
measured under dry Ar gas at 20 °C using a sealed cell. The broad
peaks observed at around 5° represent the scattering of the kapton film
of the cell.
Figure 3. Comparison of temperature-dependent changes in the single
crystal structures between (a) NO3 salt13 and (b) BF4 salt at 20, 40,
and 60 °C. Counter anions and hydrogen atoms are omitted for clarity.
salt crystal, one water molecule per macrocycle still remained
after heating to 60 °C. In contrast, all of the water molecules
were removed in the case of the NO3 salt, and the crystal-to-
crystal structural transformation was accompanied by ligand-
exchange motions between a terminal amide oxygen of 1 and
an oxygen atom of an NO3− anion. As a result of the structural
changes, the nanochannel structure in the hydrated form was
converted to a dense structure in the dehydrated form. Because
of a decrease in the single crystallinity above 100 °C, the BF4
salt was analyzed by X-ray powder diffraction to determine the
crystal structure after removing all of the included water
molecules. The analysis was performed at room temperature
under dry conditions after the sample was dried at 200 °C in
vacuo for 1 h.17 As shown in Figure 4, no peak shifts, although
identical up to ca. 60 °C, suggesting that the counter anions
slightly affected the water-release processes. As was also
suggested by the X-ray structural analysis, more than one
water molecule per one macrocycle of the BF4 salt still
remained at around 60 °C, but the dehydration was completed
at around 200 °C. However, a differential scanning calorimetry
(DSC) analysis showed that the BF4 salt exhibited only one
broad and small endothermic peak (Supporting Information,
Figures S5 and S6), whereas the NO3 salt showed one relatively
broad peak for the first step water weight loss and a sharp
endothermic peak (Figure 6).13 This result also suggests that
there were no significant structural changes in the BF4 salt.
Figure 4. Powder diffraction patterns of the BF4 salt. (a) A simulated
pattern of the hydrated form. (b) A pattern of the as-synthesized
sample. (c) A pattern of the sample dried at 200 °C in vacuo. All
diffraction patterns were measured under an Ar atmosphere at 20 °C
using a sealed cell. The broad peaks observed at around 5° represent
the scattering of the kapton film of the cell.
Figure 6. DSC curves for Ni414(NO3)8 measured under N2 gas
containing water vapor at a pressure of 8.8 hPa with a repetitive
process of heating and cooling.
they were slightly broadened, were observed for the dried BF4
salt, while the dried NO3 salt showed significant peak shifts as
shown in Figure 5. Thus, only a slight difference between the
BF4 and NO3 ions caused the different structural trans-
formation behaviors.
We then compared the dehydration (water release) processes
by thermogravimetric analysis (Supporting Information, Figure
S4). The BF4 salt showed a stepwise weight loss with the rising
temperature, as was also observed for the NO3 salt. It is
noteworthy that their water-release processes were almost
Structural Transformation by Thermodynamic Study.
Although coordination of BF4 ions to NiII ions is well-
−
known,18 no ligand-exchange reactions took place in the BF4
salt crystal as described above. To clarify the reasons for the
smooth and reversible ligand exchanges in the NO3 salt, we
studied the origin of the reversible structural transformation by
using thermodynamic parameters. Since the DSC peaks of the
BF4 salt were too broad to estimate the parameters, we then
tried to investigate the ligand-exchange reactions by further
−
−
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thermodynamic study of NO3 salt. We first examined the
reversibility of the NO3 salt by a repetitive process of heating
and cooling under N2 gas mixed with 8.8 hPa water vapor. As
shown in Figure 6, the repeated DSC curves are highly
overlapped with each other. The sharp peaks, which are
assignable to the peaks for the transformation between the
hydrated form and the dehydrated form, exhibited hysteresis.
This agreed well with previously reported results:13 the peaks
showed high dependency on water vapor pressure, which can
be explained by the Clausius−Clapeyron equation, and strongly
suggested that, from the viewpoint of the states of the water
molecules, this phenomenon can be regarded as a pseudo phase
transition between the inclusion phase and the gas phase of the
water molecules.19 The vaporization enthalpy of water (ΔHvap)
was estimated to be 57 kJ mol−1 per one water molecule by this
equation.13 On the other hand, the total peak area for the
heating process was equal to that of the cooling process. These
peak areas indicated the total enthalpy changes of the structural
transformation, including enthalpy changes due to the
structural changes (ΔHstr) and water release (uptake) which
was already estimated as a value per one water molecule. Since
the same number of water molecules (i.e., four molecules per
one macrocycle13) induced the structural transformation, we
can estimate ΔHstr approximately by comparing the two
thermodynamic parameters (DSC peak area and vaporization
enthalpy). The enthalpy changes during vaporization of water
molecules should be 2.3 × 102 kJ mol−1 (i.e., 4 × 57 kJ mol−1)
per one macrocycle.13 In contrast, the total enthalpy
determined by the DSC analysis (1.5 × 102 kJ mol−1) was
smaller than that.20 This strongly suggests that the change in
structural enthalpy (ΔHstr) for the structural transformation
accompanied by the water release is negative (ca. −8 × 10 kJ
mol−1); that is, the crystal lattice of the dehydrated form is
much more stable than that of the hydrated form. This result
also agrees well with the fact that the NO3 salt easily released all
included water molecules at a relatively low temperature (above
−40 °C13), although the large ΔHvap value suggested high
stabilization of the included water molecules. In contrast, the
transformation in the opposite direction, that is, transformation
from the dehydrated to the hydrated form, also readily occurred
on exposure to an ambient atmosphere that included a small
amount of water vapor. Since the enthalpy change during the
vaporization of one water molecule was large in this system, the
uptake of one or two water molecules per macrocycle was
needed to overcome the disadvantage of structural enthalpy for
the ligand exchange from the hydrated to the dehydrated form.
Thus, the favorable balance between the changes in structural
enthalpy (ΔHstr) and vaporization (adsorption) enthalpy of
water (ΔHvap) was important in facilitating the reversible
structural changes.
Observation of the Structural Transformation Pro-
cesses in the NO3 Salt. Then, we further analyzed the
structural transformation processes of the NO3 salt by time-
course powder X-ray diffraction (PXRD) and IR spectroscopy.
The powder patterns of the NO3 salt varied with humidity
(Figure 5). Diffraction patterns of as-synthesized NO3 salt
(hydrated form) and dried NO3 salt (dehydrated form) agreed
well with those calculated from their single-crystal structures.
The structural transformation process was successfully observed
as changes in the diffraction patterns with the slow increase in
humidity due to the air leak from a sealed cell (Figure 7 and
Figure 7. (a) Powder diffraction patterns of the NO salt (I4) showing
̅
3
a two-stage structural change in the water uptake process: (a) dried
sample measured under dry Ar gas, and time-course analyses of the
structural changes following a slow increase in humidity in the cell
after (b) 1 h, (c) 1.5 h, and (d) 3 d.
Supporting Information, Figure S8). With an increase in
humidity in the cell, the peak intensity of the dehydrated form
gradually decreased, and new peaks appeared in 1 h. Then, the
diffraction peaks of the dehydrated form eventually disappeared
within 1.5 h. After 3 d, a further peak shift was observed. The
final powder pattern was consistent with that of the as-
synthesized NO3 salt. Considering that the simulated powder
patterns of the NO3 salt at 20 and 40 °C were similar to each
other, the latter peak shift can be assigned to the first structural
change in Figure 3a (migration of neighboring macrocycles).
The changes in the two diffraction peaks during the
transformation with the ligand-exchange reactions were
observed without any apparent decrease in intensity. Thus,
this result strongly suggests that the transformation takes place
with cooperative ligand-exchange reactions between the two
states, the dehydrated form and the hydrated form, within the
macrocycle without forming any detectable intermediates
(Figure 8).
The thermodynamic parameters also support the reason for
the difference in the structural changes between the NO3 and
BF4 salts described above. Since they exhibited similar water-
release processes, the difference should be responsible for the
enthalpy changes in the structural changes (i.e., ΔHstr). The
most likely possible differences affecting structural enthalpy are
the coordination abilities of the counter anions and the crystal-
packing structures. Therefore, the negligibly small structural
change in the BF4 salt, even after removing all of the included
water molecules, was probably due to the weaker coordination
−
−
ability of the BF4 ions compared with the NO3 ions and to
the slightly larger size of the BF4− ions than the NO3− ions for
dense packing structures.
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stage structural change suggested by the single-crystal and
powder X-ray analyses.
To gain a better understanding of the details of the
transformation process, the changes in the absorbances at
3413 cm−1 (included water molecules), 1582 cm−1 (amide
carbonyl), 1428 cm−1 (coordinated NO3 ), and 1320 cm−1
−
−
(NO3 ) were plotted against time, as shown in Figure 10. For
Figure 8. Schematic representation of the structural transformation
process with ligand-exchange reactions.
The structural changes were further studied by IR spectros-
copy to verify the changes in the chemical environment of the
individual functional groups during the slow water-uptake
process as analyzed by the PXRD measurements.21 As shown in
Figure 9 and Supporting Information, Figure S9, apparent peak
Figure 9. IR spectral changes of dried NO3 salt undergoing a slow
water-uptake process. (a) OH vibration region (3800−2800 cm−1).
(b) CO vibration region (1700−1500 cm−1). (c) NO vibration region
(1500−1300 cm−1). (d) Extended figure around the isosbestic points.
IR spectra are shown with black lines (the first half part) and gray lines
(the latter half part).
Figure 10. Changes over time in IR absorbance for each peak in (a)
the water-release process (Supporting Information, Figure S9) and (b)
changes were observed in the peak area ascribable to ligand-
exchange reactions: the hydroxyl group of water molecules
(3600−3300 cm−1), the amide carbonyl groups of the ligands
(1650−1550 cm−1), and the nitrate groups (1400−1300 cm−1).
The sharp peak at 1428 cm−1 was observed only for the
the water-uptake process (Figure 9). 3413 cm−1: × , 1580 cm−1:
,
◊
−1
−1
□
○
1431 cm : , 1318 cm : . The observed pressure of the measured
cell is shown above for selected data.
−
comparison with Figure 9, the data are colored differently to
distinguish the two-stage process for ligand exchange and
migration. Since the structural changes were triggered by water
uptake/release, other vibration regions in the IR spectra also
follow the changes in the water (O−H) vibration region.
dehydrated form. This peak is attributable to the NO3 anion
bound to the NiII center.22 In the time-course measurement,
the stepwise structural transformation was also observed in the
IR spectra both in the water-uptake (Figure 9) and water-
release processes (Supporting Information, Figure S9). In the
region of the stretching vibrations of the hydroxyl groups
(3600−3300 cm−1), a broadened peak emerged at around 3400
cm−1 with water uptake. In contrast, for the peaks in the amide
and nitrate regions at around 1580 and 1300 cm−1, respectively,
several split peaks changed to one broadened peak (Figure 9
and Supporting Information, Figure S9). Interestingly, the
spectrum in the nitrate region (1400−1300 cm−1) changed,
with isosbestic points indicating a two-stage process for ligand
exchange and migration (black and gray lines were used for
each stage in Figure 9d). This result is consistent with the two-
However, the peak changes at 1428 cm−1 for coordinated NO3
−
ions were seen only at the beginning of the structural
transformation with ligand exchange. This implies that
concerted ligand exchange was involved at the NiII centers,
leading to the structural transformation.
CONCLUSION
■
In summary, we revealed the detailed crystal-to-crystal
structural changes that occur in crystalline NiII macrocycles
with β-dipeptide and the effects of counter anions on the
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process. The distinct reversible transformation was caused by
compensation for the large degree of stabilization due to water
uptake and changes in structural stability in the transformation.
The structural transformation process involved concerted
ligand-exchange reactions in the macrocyclic structure, which
led to a two-state transformation. The ligand-exchange
behaviors were strongly affected by the types of counter
anions, even in the same crystal-packing structures (NO3 vs BF4
salts). Both PXRD and IR analyses confirmed the two-stage
process, migration of neighboring macrocycles and ligand
exchange, between the hydrated form and the dehydrated form.
This dynamic structural transformation caused by the water
content in the crystal state was most likely realized by the
structurally flexible peptide and its ability to form a hydrogen
bonding network. Such indirect control of ligand-exchange
reactions in the crystal state by external stimuli provides a clue
to developing a new type of functional materials.23
transformation. This material is available free of charge via the
Internet at http://pubs.acs.org. Crystallographic data can be
obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif (CCDC
973213−973219).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: miyake.ryosuke@ocha.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was partly supported by a Grant-in-Aid for Scientific
Research (S) to M.S. (Grant No. 21225003) and a Grant-in-
Aid for Young Scientists (B) to R.M. (Grant No. 25810037)
from the Ministry of Education, Culture, Sports, Science, and
Technology of Japan. We thank Prof. Masanobu Uchiyama and
Mr. Kengo Yoshida (The Univ. of Tokyo) for IR measure-
ments, Ms. Chika Kuwata (Ochanomizu Univ.) for assistance
with complex preparation, and Prof. Yuichi Masuda and Dr.
Yukie Mori (Ochanomizu Univ.) for helpful discussions.
EXPERIMENTAL SECTION
■
Dipeptide ligand 1·(CF3CO2H) was synthesized according to a
previously reported method.15 All reagents and solvents were used
without further purification. Differential scanning calorimetry (DSC)
measurements were conducted using a Bruker DSC 3100SA under
ambient pressure of N2 gas at a scanning rate of 1 °C min−1.
Thermogravimetric (TG) analysis was performed using a Bruker TG-
DTA 2000SA under ambient pressure of N2 gas at a scanning rate of 1
°C min−1. Crystallographic data were collected using a Bruker APEXII
CCD detector with Mo Kα radiation (λ = 0.710 75 Å). The structures
were solved by direct methods using the program SHELXS. The
refinement (on F2) and graphical calculations were performed using
the SHELIXL program suite.24,25 For the measurements at normal
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−
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−
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Arai, K.; Sato, K.; Hashimoto, K. Nat. Mater. 2004, 3, 857−861.
(9) For examples of reversible singe-crystal to single-crystal structural
transformation induced by ligand exchange reactions with coordinating
small guest stimuli: (a) Huang, Z.; White, P. S.; Brookhart, M. Nature
2010, 465, 598−601. (b) Lturrospe, A.; Artetxe, B.; Reinoso, S.; San
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Chem. 2013, 52, 3084−3093. (c) Sharma, M. K.; Bharadwaj, P. K.
SYNTHESIS OF NI414(BF4)8·12H2O
■
1·(CF3CO2H)4 (309 mg, 400 μmol) was neutralized by anion
exchange column chromatography (IRA-400) to produce acid-
free β-dipeptide 1 as a colorless syrup. Ni(BF4)2·6H2O (136
mg, 400 μmol) in H2O (1 mL) was added to a solution of the
acid-free 1 in H2O (2 mL). After slow evaporation of water at 3
−
°C to prevent hydrolysis of BF4 ions, the resulting purple
prismatic crystals were collected by filtration and dried in air to
afford NiII complex (120 mg, 50%); melting point = 296.5−
297.1 °C (dec). IR 3356, 1660, 1588, 1019 cm−1. Anal. Calc for
C48H132B8F32N28Ni4O24 (Ni414(BF4)8(H2O)12): C, 23.87; H,
5.51; N, 16.24. Found: C, 24.25; H, 5.65; N, 16.61%.
ASSOCIATED CONTENT
* Supporting Information
Additional structural data, and spectral data for the peptide
macrocycles with additional discussion about the structural
■
S
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Inorganic Chemistry
Article
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(14) There are some examples of peptide-based structural switching
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(15) Miyake, R.; Tashiro, S.; Shiro, M.; Tanaka, K.; Shionoya, M. J.
Am. Chem. Soc. 2008, 130, 5646−5647.
−
(16) The solution should be kept at 3 °C to avoid hydrolysis of BF4
ions and generation of F− ions. In the presence of F− ions, the
resulting crystals of Ni(II) macrocycles include a F− ion in each hole.
(17) TG measurements of the BF4 salt suggest that the included
water molecules can be removed at 200 °C. For the details, see
Supporting Information (Figure S4).
(18) For examples: (a) Yeh, C.-Y.; Chiang, Y.-L.; Lee, G.-H.; Peng,
S.-M. Inorg. Chem. 2002, 41, 4096−4098. (b) Arakawa, M.; Suzuki, N.;
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(19) A similar discussion was described in ref 12d.
(20) Since the sample weight changed slightly (∼5%) due to the
release/uptake of water molecules, the estimated enthalpy from the
DSC peak area included up to ca. 10% error. The reference values are
shown in the manuscript to help understand this.
(21) Different cells were used for PXRD and IR measurements.
Therefore, the rates of increasing humidity inside the cells were
completely different in both experiments.
(22) Davis, A. R.; Irish, D. E. Inorg. Chem. 1968, 7, 1699−1704.
(23) For examples of studies on conformational dynamics using
crystals of simple peptides: (a) Anedda, R.; Soldatov, D. V.;
Moudrakovski, I. L.; Casu, M.; Ripmeester, J. A. Chem. Mater. 2008,
20, 2908−2920. (b) Rabone, J.; Yue, Y.-F.; Chong, S. Y.; Stylianou, K.
C.; Bacsa, J.; Bradshaw, D.; Darling, G. R.; Berry, N. G.; Khimyak, Y.
Z.; Gnin, A. Y.; Wiper, P.; Claridge, J. B.; Rosseinsky, M. J. Science
2010, 329, 1053−1057. (c) Martí-Gastaldo, C.; Warren, J. E.;
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(24) Sheldrick, G. M. SHELXL-97, Program for refinement of crystal
structures; University of Gottingen: Gottingen, Germany, 1997.
̈
(25) Sheldrick, G. M. SADABS, Program for scaling and correction of
area detector data; University of Gottingen: Gottingen, Germany,
̈
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