Dynamic Characterization of Crystalline Supramolecular Rotors Assembled through Halogen Bonding

Article abstract of DOI:10.1021/jacs.5b10776

A modular molecular kit for the preparation of crystalline molecular rotors was devised from a set of stators and rotators to gain simple access to a large number of structures with different dynamic performance and physical properties. In this work, we have accomplished this with crystalline molecular rotors self-assembled by halogen bonding of diazabicyclo[2.2.2]octane, acting as a rotator, and a set of five fluorine-substituted iodobenzenes that take the role of the stator. Using variableerature ¹H T₁ spin-lattice relaxation measurements, we have shown that all structures display ultrafast Brownian rotation with activation energies of 2.4-4.9 kcal/mol and pre-exponential factors of the order of (1-9) × 10¹² s^-1. Line shape analysis of quadrupolar echo ²H NMR measurements in selected examples indicated rotational trajectories consistent with the 3-fold or 6-fold symmetric potential of the rotator.

Full text of DOI:10.1021/jacs.5b10776

Communication
pubs.acs.org/JACS
Dynamic Characterization of Crystalline Supramolecular Rotors
Assembled through Halogen Bonding
Luca Catalano,†,‡ Salvador Perez-Estrada,‡ Giancarlo Terraneo,*,† Tullio Pilati,† Giuseppe Resnati,†
́
Pierangelo Metrangolo,*,†,§ and Miguel A. Garcia-Garibay*,‡
†Laboratory of Nanostructured Fluorinated Materials (NFMLab), Department of Chemistry, Materials, and Chemical Engineering
“Giulio Natta”, Politecnico di Milano, via L. Mancinelli 7, 20131 Milano, Italy
‡Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States
§VTT-Technical Research Centre of Finland, P.O. Box 1000, Espoo FI-02044, Finland
S
* Supporting Information
hydrogen bonding (HB), and π−π stacking, halogen bonding
ABSTRACT: A modular molecular kit for the preparation
of crystalline molecular rotors was devised from a set of
stators and rotators to gain simple access to a large number
of structures with different dynamic performance and
physical properties. In this work, we have accomplished
this with crystalline molecular rotors self-assembled by
halogen bonding of diazabicyclo[2.2.2]octane, acting as a
rotator, and a set of five fluorine-substituted iodobenzenes
that take the role of the stator. Using variable-temperature
1H T1 spin−lattice relaxation measurements, we have
shown that all structures display ultrafast Brownian
rotation with activation energies of 2.4−4.9 kcal/mol and
pre-exponential factors of the order of (1−9) × 1012 s−1.
Line shape analysis of quadrupolar echo 2H NMR
measurements in selected examples indicated rotational
trajectories consistent with the 3-fold or 6-fold symmetric
potential of the rotator.
(XB) has only been used once for the design of amphidynamic
crystals.9,10 In particular, it was demonstrated recently that
bis(1,4-iodoethynyl)bicyclo[2.2.2]octane showed ultrafast rota-
tion when self-assembled in crystalline XB networks through
iodine−acetylene interactions at the two ends of each rotator.11
Under the hypothesis that the halogen bond, thanks to its
strength, directionality, selectivity, and tunability, may provide
an extra value in the design of co-crystals showing dynamic
components, we studied molecular motions of a series of XB
co-crystals involving 1,4-diazabicyclo[2.2.2]octane
(DABCO),12 a well-known C3-symmetric and cylindrically
shaped rotator that packs in a manner that does not generate
high rotational barriers.11,13
The commercially available fluoro-substituted iodobenzenes
(FnPheI) 1a−e were chosen as powerful XB donors that can
play the role of stators when assembled with the DABCO
rotator via FnPheI···NR3 interactions (Figure 1). We anticipated
that compounds 1a−e would simultaneously offer a suitable
static platform, a non-covalent axle, the halogen bond, and a
frame of reference for the rotation of DABCO.
mphidynamic crystals are materials specifically designed to
Apossess rapidly moving components (rotators) in the solid
state.1 It is expected that these materials may provide a suitable
platform for the development of future functional materials and
artificial molecular machines.1,2 Typically, molecular design in
this field has taken inspiration from structures akin to those of
macroscopic compasses and gyroscopes to explore and control
rotation in the solid state.3 Rotation in solids has been studied
in molecular crystals,3 inclusion compounds,4 metal−organic
frameworks,5 porous molecularly ordered silicates,6 and
amorphous solids.7
Self-assembly experiments were carried out by mixing in
acetone solutions 2 equiv of 1a−e per equivalent of DABCO,
Here we take advantage of the principles of crystal
engineering8 to assemble stators and rotators into co-crystals,
which results in the high-yield synthesis of active supra-
molecular rotor arrays. Advantages of this strategy originate
from the intrinsic flexibility and its modular design, thanks to
the wide variety of supramolecular synthons8 available in the
crystal engineering toolbox. Furthermore, the dynamic
performance of the supramolecular rotor may be tuned by
controlling the non-covalent interactions involving stators and
rotators in the crystal structures.
Figure 1. Self-assembly of the fluoro-substituted iodobenzenes
(FnPheI) 1a−e and DABCO, resulting in crystalline halogen-bonded
molecular rotors 2a−e.
Among the wide variety of non-covalent interactions
available, such as metal coordination, Coulombic interactions,
Received: October 14, 2015
Published: November 19, 2015
© 2015 American Chemical Society
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DOI: 10.1021/jacs.5b10776
J. Am. Chem. Soc. 2015, 137, 15386−15389

Journal of the American Chemical Society
Communication
taking into account the mono- and bidentate nature of 1a−e
and DABCO, respectively, as far as the XB donor/acceptor sites
are concerned. Upon slow solvent evaporation, crystalline solids
were obtained in all cases with melting points different from
those of the starting compounds and ranging from 311 to 378
K (Table 1). Peak integration of the 1H NMR spectra
The halogen bonds observed in complexes 2a−e are
characterized by short I···N distances (2.79 Å on average)
and almost linear C−I···N angles (176.7° on average), as shown
in Table 2. The XB becomes shorter, and thus stronger, with
Table 2. Halogen Bonds and Angles in 2a−e at 103 K
complex
N···I (Å)
N···IC (deg)
Table 1. Melting Points, Space Groups, and Arrhenius
Parameters for Rotational Dynamics from the Kubo−Tomita
Fit of the T1 Data for DABCO and the Related Halogen-
Bonded Co-crystals 2a−e
a
a
2a
2b
2c
2d
2e
2.971(4), 2.955(4)
176.32(2), 175.51(2)
a
a
2.857(2), 2.732(2)
173.86(8), 179.12(8)
2.832(1)
2.815(6)
175.19(4)
175.8(2)
Ea
2.713(1), 2.682(1), 2.681(1), 177.04(5), 178.64(5), 177.96(5),
compd/co-crystal
(mp, K)
space group (kcal/mol)
C (×108
s−2
a
a
2.701(1)
177.19(5)
a
(Z)
τ0 (s−1
−1
)
)
a
Independent halogen bonds.
b
c
c
DABCO (429)
2a (318−321)
2b (334−336)
2c (344−345)
2d (311−314)
2e (377−378)
P63/m (2)
8.2
4.8
2.7
2.4
2.8
3.6
1.3 × 1014
9.9 × 1012
2.0 × 1012
1.1 × 1012
2.3 × 1012
2.3 × 1012
n.a.
5.2
7.4
6.5
5.0
7.5
C2/c (4)
P21/n (4)
P21/c (4)
P21/c (2)
P21/c (2)
the increase of the fluorination degree of the stator, and hence
the magnitude of the iodine σ-hole.16 Similarly, m-fluorination
may explain why 1c behaves as a HB donor, while 1b does not.
No other driving non-covalent interactions characterize the
crystal packing of these complexes.15 Importantly, NMR, IR,
thermal, and powder XRD analyses confirmed the above-
described co-crystals were obtained in quantitative bulk.
To explore structural changes and the dynamics of DABCO
in the complexes 2a−e when the temperature was changed, we
a
Data obtained at 297 K; Z is the number of molecules per asymmetric
b
c
unit. Reference 14a. Reference 14b.
1
confirmed that all of the complexes except 2c showed the
expected 2:1 ratios of the starting compounds; 2c, instead,
showed a 1:1 ratio between 1c and DABCO.
performed variable-temperature (VT) single-crystal XRD, H
T1 NMR, and wide-line quadrupolar echo 2H NMR analyses. It
is known that changes in the size and orientation of the
anisotropic thermal parameters may be related to the potential
energy that determines dynamic processes in crystals.16 On
heating single crystals of the complexes 2a and 2e from ∼100 K
to room temperature, reversible phase transitions without
substantial modifications of the crystal packing were recorded.
These occurred at ∼280 and 230 K, respectively, as shown in
detail in the Supporting Information (SI). For 2e, XRD data
were collected in the range 103−298 K, following five
intermediate steps (140, 180, 200, 220, and 250 K). On
increasing the temperature of the XRD acquisition, increasing
rotational disorder of the DABCO −CH2CH2− alkyl bridges
was observed, which has to be expected from a thermally
activated rotational process (Figure 3). Conversely, the
Good-quality single crystals of the obtained co-crystals 2a−e
were submitted to X-ray diffraction (XRD) analysis, which was
in all cases carried out at 103 K. Structural analysis confirmed
the starting design of the desired XB motif with both the N-
atoms of DABCO working as XB acceptors toward the I-atoms
of two different FnPheI modules, resulting in trimeric
complexes. In contrast, the structure of complex 2c consisted
of 1D infinite chains in which 1c and DABCO alternated as a
consequence of the ditopic nature of 1c, which also behaved as
a HB donor through the H-atom in the para-position to the I-
atom in the benzene ring (Figure 2b). This explains the 1:1
1
ratio observed for this complex through H NMR analysis,
regardless of starting from either a 1:1 or 2:1 mixing ratio.
Figure 3. Views along the N−N axis of the DABCO component in co-
crystal 2e, which show increasing disorder of the alkyl bridges, in terms
of larger thermal ellipsoids and splitting over two positions as a
function of temperature from 103 to 298 K.
pentafluoroiodobenzene module only showed disorder in the
plane of the aromatic ring (see SI). Similar motion and disorder
were also observed for the co-crystals 2a−d.
Spin−lattice relaxation measurements have been used to
document dynamic processes in the vicinity of the Larmor
Figure 2. (a) Halogen-bonded trimer in the crystal structure of 2e. (b)
1D infinite chain in 2c due to the simultaneous XB and HB donor
behavior of 1c when mixed with DABCO. HB geometrical parameters
are C···N = 3.348(3) Å and C−H···N = 174.0(1)°. Color code: C,
gray; H, white; N, sky blue; F, light green; I, magenta. XB and HB are
represented as black and orange dashed lines, respectively.
frequency18 (300 MHz for H T1 measurements in this case).
1
This method is well suited for crystalline solids, in which the
main mechanism of relaxation is due to the motion-mediated
fluctuation of magnetic fields arising from dipolar coupling
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DOI: 10.1021/jacs.5b10776
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Journal of the American Chemical Society
Communication
among H-atoms. In the case of the co-crystals 2a−e, the
rotation of DABCO is the predominant thermally activated
process, with dynamics characterized by a correlation time (τc),
which follows an Arrhenius-type behavior (eq 1). Under these
conditions, it is possible to obtain its activation energy (Ea) and
pre-exponential factor (τ0−1) by fitting the measured T1 data as
a function of temperature to the Kubo−Tomita (KT)
relaxation expression (eq 2), by substituting the τc term with
eq 1.
τ −1 = τ0−1 exp(−Ea/RT)
Figure 5. Views along the b crystallographic axis (a) and along the N−
N molecular axis (b) of the crystal structure of 2e, showing the
isolation of a DABCO rotator (red) by neighboring pentafluoroiodo-
benzene modules in blue.
(1)
(2)
c
2
2
T−1 = C[τ (1 + ω0 τ 2)−1 + 4τ (1 + 4ω0 τ 2)−1]
1
c
c
c
c
The parameter C in eq 2 is a constant related to the strength
of the homonuclear dipolar interactions, which, as expected, are
similar for all of the studied co-crystals (see Table 1), and ω0 is
the Larmor frequency (for further details, please refer to the
SI). Good KT fitting of the experimental data were obtained,
allowing the determination of the Arrhenius parameters
reported in Table 1.
The higher activation energy for rotation of pure DABCO
may be associated with the closer rotator−rotator distances and
steric interactions in the crystal. The pre-exponential factor is
related to the frequency of oscillations within the local energy
well and can be associated with an attempt frequency for
rotational jumps that is determined by both thermal energy and
the size of the activation barrier (−Ea/RT). The attempt
frequency corresponds to the rotational exchange frequency
that would occur in the absence of a barrier, i.e., when Ea = 0.
As illustrated in Figure 4, with a ln(T1−1) vs 1000/T (K) plot
of the experimental data obtained on co-crystal 2e, there is a
−1
Notably, the τ0 values obtained for the XB co-crystals 2a−e
are comparable to those determined for other amphidynamic
crystals with analogous DABCO-based molecular rotators
linked to the stators by covalent bonds.11,13 This suggests
that the XB molecular rotors reported in this Communication
may be as efficient as similar rotors based fully on covalent
bonds. However, in co-crystal 2c, it was not possible to discern
the different contributions of XB and HB to the rotational
dynamics of the supramolecular rotor.
To elucidate the trajectories of motion, three supramolecular
rotors, 2a, 2b,and 2e, were taken as models and their dynamics
studied by wide-line quadrupolar echo 2H NMR.18 The sample
co-crystals were prepared by using perdeuterated DABCO
obtained by H/D exchange reaction using Raney nickel and
D2O.19 For the three studied systems, the wide-line spectra
collected between 140 and 295 K did not show any appreciable
changes, in good agreement with the high rotational
Figure 4. 1H spin−lattice relaxation times (1H T1) measured for 2e on
cooling from 295 to 141 K at 300 MHz (solid blue circles). The plot
ln(T1−1) vs 1000T−1 shows a maximum at ∼195 K. The red dotted
line corresponds to the Kubo−Tomita fit of the experimental data.
−1
frequencies expected from the low Ea and high τ0 values.
single maximum at ∼195 K, with an excellent KT fit for a single
dynamic process with an exceptionally low Ea ≈ 3.6 kcal/mol
and τ0−1 = 2.3 × 1012 s−1. Similar behaviors were obtained also
for 2a−d, with Ea = 2.4−4.9 kcal/mol (Table 1). Notably, data
obtained from co-crystal 2a showed a discontinuity that
correlates with a phase transition that occurs in the range
285−310 K (Figure S17). While it was not possible to fit the
experimental data in the high-temperature phase due to the
small number of experimental points in the proximity of the
crystal melting, the dynamics of the low-temperature phase was
fully characterized, with Ea = 4.9 kcal/mol and τ0−1 = 9.9 × 1012
s−1.
The simulated spectra20 suggested a dynamical process in
which DABCO rotates around its N−N axis in the fast-
exchange regime. The experimental data coupled with the
simulations are consistent with either a 3-site or 6-site potential
of the rotator; see example in Figure 6.
In conclusion, a series of two-component amphidynamic co-
crystals with very efficient rotational dynamics were readily
prepared from simple and commercially available building
blocks by taking advantage of a crystal engineering strategy
based on the use of XB. The reported systems are the first
examples of this new class of materials. Rotational dynamics
1
based on VT H T1 NMR spin−lattice relaxation revealed pre-
The activation energies measured for co-crystals 2a−e are
considerably lower than the activation energy for rotation
reported in the case of pure crystalline DABCO, which has Ea =
exponential factors comparable to those found in rotors with
covalent rotational axles, suggesting that the XB, and the HB in
the specific case of 2c, constitute new robust axles for molecular
rotation and a new design principle in the field of molecular
rotors. The discovery of these new crystalline multicomponent
systems thus opens up new avenues in the development of new
smart materials and molecular machines.
8.2 kcal/mol and τ0 = 1.3 × 1014 s−1 (Table 1). This may
−1
probably be due to the fact that rotator molecules in the
reported XB co-crystals are partially isolated from neighboring
rotators by the XB donors 1a−e, which thus function as
bearings and stators (Figure 5).
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DOI: 10.1021/jacs.5b10776
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Journal of the American Chemical Society
Communication
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b10776.
■
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S
Experimental procedures, further characterization, and
spectra (PDF)
X-ray data for 2e at 220 K (CIF)
X-ray data for 2e at 250 K (CIF)
X-ray data for 2e at rt (CIF)
X-ray data for 2e at 103 K (CIF)
X-ray data for 2e at 140 K (CIF)
X-ray data for 2e at 180 K (CIF)
X-ray data for 2e at 200 K (CIF)
X-ray data for 2a at 103 K (CIF)
X-ray data for 2a at 297 K (CIF)
X-ray data for 2b at 103 K (CIF)
X-ray data for 2b at 253 K (CIF)
X-ray data for 2b at rt (CIF)
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X-ray data for 2c at 103 K (CIF)
X-ray data for 2c at 233 K (CIF)
X-ray data for 2c at rt (CIF)
X-ray data for 2d at 100 K (CIF)
X-ray data for 2d at rt (CIF)
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AUTHOR INFORMATION
■
Corresponding Authors
*giancarlo.terraneo@polimi.it
*pierangelo.metrangolo@polimi.it
*mgg@chem.ucla.edu
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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M.A.G.-G. acknowledges support from the National Science
Foundation (grant DMR1402682). P.M. and G.T. acknowledge
support from the MIUR, PRIN 2010-2011 (grant
2010CX2TLM “InfoChem”).
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DOI: 10.1021/jacs.5b10776
J. Am. Chem. Soc. 2015, 137, 15386−15389