Binding of halogens by a Cr8 metallacrown

Article abstract of DOI:10.1039/c8dt03172j

A Cr₈ metallacrown binds halogens X₂ (Cl₂, Br₂ and I₂) without loss of crystallinity; the binding has been studied by X-ray diffraction and thermodynamic techniques.

Full text of DOI:10.1039/c8dt03172j

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Binding of halogens by a Cr8 metallacrown†
Daniel Florin Sava,
Nan Zheng, Iñigo J. Vitórica-Yrezábal, Grigore A. Timco
Cite this: DOI: 10.1039/c8dt03172j
and Richard E. P. Winpenny
*
Received 3rd August 2018,
Accepted 7th September 2018
DOI: 10.1039/c8dt03172j
rsc.li/dalton
A Cr8 metallacrown binds halogens X2 (Cl2, Br2 and I2) without loss ordered Cl2 molecules with occupancies of 0.419(5) and 0.324(5).
of crystallinity; the binding has been studied by X-ray diffraction The shortest Cl⋯F distance is 2.852(3) Å and the Cl–Cl–F
and thermodynamic techniques.
angle is 174.2(3) (Table 1). The Cl⋯Cl bond is found to be
1.889(5) in the first position and 1.907(6) Å in the second.
These are both shorter than the value found for Cl2 in the gas
phase which is 1.99.9,10
The crystal structure of Br2@Cr8 presents four disordered
Br2 molecules with occupancies varying from 0.3901(18) to
0.0614(18) (Fig. S9,† the main component is shown in Fig. 2).
Halogens play a major role in industry, despite being toxic,
corrosive and volatile.1 Designing new materials for safe hand-
ling of halogens would have a great impact and, for this
reason, research groups are working to develop ionic liquids,2
MOFs3,4 and zeolites5 as a way to tackle these problems. The
background for our study is our observation that a metalla-
crown, [CrF(O2C-tBu)2]8 Cr8, binds CO2.6 We also noticed the
use of fluorides for MOF ligands that confer better adsorption
and interaction properties.7,8 Here we present the behaviour of
Cr8 when exposed to Cl2, Br2 and I2.
The synthesis of Cr8 followed literature procedures.6 Slow
cooling of a Cr8 saturated solution in 1-bromodecane produces
the metallacrown in an unsolvated form. The structure of Cr8
is an octagon of CrIII ions held together by bridging fluorides
and pivalates. The bridging fluorides are alternately slightly
above and below the mean plane of the CrIII ions on the inside
of the octagon, while on each edge one pivalate is close to the
plane of the chromium sites (equatorial) and the other is alter-
nately above or below the plane (axial).
In separate experiments, crystals of Cr8 were exposed to an
atmosphere of Cl2 and Br2 for 16 h at room temperature yield-
ing Cl2@Cr8 and Br2@Cr8 complexes. An increment in
exposure time resulted in the degradation of Cl2@Cr8 and
Br2@Cr8 crystals. Single crystal X-ray diffraction studies on
complexes Cl2@Cr8 and Br2@Cr8 show no significant change
in the Cr8 metallacrown structure. A region of electron density
was found at the centre of the cavity corresponding to a dis-
ordered molecule of X2 (X = Cl and Br). As shown in Fig. 1
(Fig. S8†), the crystal structure of Cl2@Cr8 presents two dis-
School of Chemistry, The University of Manchester, Oxford Road,
Manchester M13 9PL, UK. E-mail: richard.winpenny@manchester.ac.uk
†Electronic supplementary information (ESI) available. CCDC 1859929–1859947.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8dt03172j
Fig. 1 Structure of Cl2@Cr8 at 150 K – showing position 1 of the Cl2
which is the disordered form with the highest occupancy at this tem-
perature. Top – front view, bottom – side view. Methyl groups of piva-
lates have been omitted for clarity. Cr, green; F, yellow; O, red; C, grey;
and Cl, light green.
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Table 1 Structural parameters of the halogen–metallacrown complex of Cr8
Position &
occupancy
X–F
contact/Å
Sum of X–F van der
Difference X–F vdW
vs. X–F exp./Å
X–X
bond/Å
X–X Gas phase
bond dist./Å
X–X–F
angle/°
X2
Waals radii14/Å
Cl2
1 – 0.419(5)
2 – 0.324(5)
2.852(3)
2.873(5)
2.782(2)
2.796(2)
2.78(1)
3.22
3.33
0.368
0.347
0.548
0.534
0.55
1.889(5)
1.907(6)
2.237(2)
2.255(2)
2.19(2)
1.99
174.2(3)
177.0(4)
173.13(9)
171.3(1)
170.7(7)
176.2(7)
165.02(9)
173.9(2)
174.3(5)
Br2
1 – 0.3901(18)
2 – 0.2829(18)
3 – 0.0673(18)
4 – 0.0614(18)
1 – 0.2143(11)
2 – 0.1245(12)
3 – 0.0404(11)
2.28
2.67
2.78(2)
0.55
2.21(2)
I2
2.999(2)
3.054(3)
2.84(1)
3.49
0.491
0.436
0.65
2.656(3)
2.576(4)
2.626(13)
Fig. 2 Structure of Br2@Cr8 at 150 K – showing position 1 of the Br2
which is the disordered form with the highest occupancy at this temp-
erature. Top – front view, bottom – side view. Methyl groups of pivalates
have been omitted for clarity. Cr, green; F, yellow; O, red; C, grey; and
Br, brown.
Fig. 3 Structure of I2@Cr8 at 150 K – showing position 1 of the I2 which
is the disordered form with the highest occupancy at this temperature.
Top – front view, bottom – side view. Methyl groups of pivalates have
been omitted for clarity. Cr, green; F, yellow; O, red; C, grey; and I,
purple.
The four positions involve halogen bonds formed between the of the three disorder models lies between 2.84(1) and 3.054(3)
Br and four fluorides on one side of the ring (Fig. S9†). The Å. The three sites have site occupancies of 0.2143(11),
shortest Br⋯F contact in the four disorder models lies 0.1245(12) and 0.0404(11). The I⋯I bond was found to be
between 2.78(1) and 2.796(2) Å. The Br⋯Br bond is found to between 2.576(4) and 2.656(3) Å which is shorter than the gas
be between 2.185(10) and 2.255(2) Å which is shorter than the phase I2 bond length of 2.67 Å.10
experimental gas phase Br2 bond length of 2.28 Å.10
The Cr8 metallacrown has an asymmetric central void, in
Crystals of Cr8 were exposed to an atmosphere of I2 for one which one of the sides is more densely packed due to the
week at 50 °C, yielding I2@Cr8. The iodine molecule was found pivalate ligand conformation, being effectively closed to gas
disordered over three positions (Fig. S10†), and the main com- uptake. The Cr8 molecules pack in the lattice with all the
ponent is presented in Fig. 3. The shortest I⋯F contact in each open sites stacked in the [100] plane. The Cr8 molecules are
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slightly tilted with respect to the [100] plane (Fig. S11†), creat-
ing a preferential access route for the gas molecules through
non-porous crystals. Four fluorides (F2, F4, F6 and F8) lie
above the plane of eight Cr(III) ions offering four possible
binding sites for halogen capture through halogen bonding.
The fact that two out of the four possible positions for
halogen bonding are favoured is linked to the tilt of metalla-
crown molecules with respect to the [100] plane. The rotation
and structural disorder of the pivalate groups allows the
transport of the gases from the pockets between metalla-
crowns towards the cavity. The cooperative transport of guest
molecules through disordered moieties is characteristic of
effectively non-porous materials that are able to absorb guest
molecules.11,12
Fig. 4 Site occupancy of the guest molecules inside Cr8 vs. heating
time at 350 K. In the case of I2, exponential regression was performed.
The presence of halogens in the structure is confirmed by
Raman spectroscopy (Fig. S4†). The presence of chlorine in
Cl2@Cr8 is shown by a peak at 495 cm−1 which is red-shifted
from the ν1 540 cm−1 vibration for gaseous Cl2.13 In Br2@Cr8, integrates to a binding energy of 39.1 kJ mol−1 (Fig. S6†). We
we do not observe the ν1 318 cm−1 vibration as it overlaps with assume that the first peak corresponds to the intermolecular
vibrational bands from Cr8. A peak at 554 cm−1 is assigned as Br2 and the second peak corresponds to the Br2 inside the
the 2ν1 vibration, red-shifted from 611 cm−1 for gaseous Br2.14 cavity.
In I2@Cr8 we observe a peak at 212 cm−1, slightly red-shifted
In the case of I2@Cr8 the DSC shows a sharp endothermic
from the ν1 214.5 cm−1 vibration of gaseous I2. We also peak at 157 °C that integrates to a binding energy of 18.2
observe other overtones of the iodine stretching vibrations at kJ mol−1 (Fig. S7†). The binding energy appears to decrease in
422, 628 and 835 cm−1 15 All the peaks in the Raman spectra the following order Br2@Cr8 39.1 kJ mol−1 (second DSC peak –
.
assigned to the halogens are shifted to lower wavenumbers corresponding to the Br2 bound to the Cr8 cavity) > I2@Cr8
compared to the Raman shifts from the gaseous 18.2 kJ mol−1 > Br2@Cr8 11.6 kJ mol−1 (first peak DSC – corres-
dihalogens.13–15 The red shift is a consequence of the donation ponding to the Br2 in the intermolecular pockets) > Cl2@Cr8
of electron density from the fluorides to the halogen when 10.6 kJ mol−1
forming the halogen bond.
.
The halide release of X2@Cr8 (X = Cl2, Br2 and I2) was inves-
Thermogravimetric analysis (TGA) of the freshly prepared tigated using single crystal X-ray diffraction. Crystals of
samples of X2@Cr8 was performed under N2. The TGA of X2@Cr8 (X = Cl2, Br2 and I2) were heated in 1 hour intervals at
Cl2@Cr8 shows a loss of 3.6% between 45 and 284 °C which 350 K, while the data sets were measured at 200 K. In Fig. 4 we
corresponds to 1.1 Cl2 molecules per formula unit (Fig. S1†). show the change in the site occupancy of the X2 molecules
The TGA of Br2@Cr8 shows a loss of 7.1% between 37 and over 6 cycles. X2@Cr8 (X = Cl and Br) crystals fully released the
104 °C and 4.3% between 109 and 166 °C which corresponds halide guest molecules after 1 (X = Cl2) and 2 (X = Br2) hours at
to a total of 1.766 Br2 molecules per formula unit (Fig. S2†). 350 K.
The TGA of the freshly prepared samples of compound I2@Cr8
Crystals of I2@Cr8 did not fully release I2 after 6 hours at
shows a loss of 3.8% between 74 and 273 °C which corres- 350 K. Initial I2 occupancy 0.4550(18) falls to 0.178(1) in six
ponds to 0.34 I2 molecules (Fig. S3†). This is in very good cycles. The decrease in site occupancy for I2 was fitted by an
agreement with the X-ray structure.
exponential decay with a correlation of 0.99, consistent with
The TGA for compounds Cl2@Cr8 and Br2@Cr8 shows a a first order rate constant. The calculated release rate coeffi-
greater mass loss than can be accounted for from the di- cient is equal to 0.149 h−1. A second I2 release experiment on
halogen molecules found in the cavity of the ring by single crystals of I2@Cr8 was monitored by single crystal X-ray diffrac-
crystal X-ray diffraction. The crystal lattice of Cr8 has signifi- tion. The crystals were heated at one hour intervals at 400 K,
cant voids (≈286 Å3) between the large metallacrown units and while crystal structure determination was performed at 200 K.
we believe that additional dihalogen molecules are located in Full release of I2 guest molecules was accomplished after three
these voids in the crystal lattice, disrupting the packing of the cycles.
Cr8 molecules and thus causing the observed degradation of
Fitting the I2 release occupancy at 400 K with the same
the crystals.
exponential decay allows us to calculate the release rate coeffi-
Differential scanning calorimetry (DSC) of the freshly pre- cient to be equal to 0.466 h−1 (Fig. S8†). These two rate con-
pared samples of Cl2@Cr8 shows a sharp endothermic peak at stants allow us to calculate an activation energy for the release
159 °C that integrates to a binding energy of 10.6 kJ mol−1 of I2 of 26.5 kJ mol−1. This contrasts with the value from DSC
(Fig. S5†). For Br2@Cr8 the DSC shows two broad endothermic of 18.2 kJ mol−1. The release rate of the dihalide molecules is
peaks, the first one at 108 °C which integrates to a binding dominated by the size of the guest molecules, being Cl2@Cr8 >
energy of 11.7 kJ mol−1, and the second one at 129 °C which Br2@Cr8 > I2@Cr8.
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There are many examples of materials with high uptake of
I2,16–22 but fewer studies in terms of the heat of adsorption
(binding energy). Hughes et al. published a study comparing
the binding energy of I2 to different substrates and the energy
ranges from 5.4 kJ mol−1 in the case of the perylene-I2 complex
to 41.5 kJ mol−1 in the case of the ZIF-8 cage-I2 complex.23
In the case of Cl2 and Br2 there are fewer studies on their
encapsulation,24–28 and, to our knowledge, the only studies
that present binding energies are derived from theoretical
calculations of binding towards graphene29 and ethylene.30
The exception to this are the studies of the physisorption
energies of Br2 towards graphene (33.8 kJ mol−1) deter-
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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This work was supported by the University of Manchester by
the award of a President’s Doctoral Scholarship to DFS. We
also thank the EPSRC (UK) for support (EP/R011079/1) and for
funding of an X-ray diffractometer (EP/P001386/1).
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