Induced Fitting and Polarization of a Bromine Molecule in an Electrophilic Inorganic Molecular Cavity and Its Bromination Reactivity

Article abstract of DOI:10.1002/anie.202007406

Dodecavanadate, [V₁₂O₃₂]^4? (V12), possesses a 4.4 ? cavity entrance, and the cavity shows unique electrophilicity. Owing to the high polarizability, Br₂ was inserted into V12, inducing the inversion of one of the VO₅ square pyramids to form [V₁₂O₃₂(Br₂)]^4? (V12(Br2)). The inserted Br₂ molecule was polarized and showed a peak at 185 cm^?1 in the IR spectrum. The reaction of V12(Br2) and toluene yielded bromination of toluene at the ring, showing the electrophilicity of the inserted Br₂ molecule. Compound V12(Br2) also reacted with propane, n-butane, and n-pentane to give brominated alkanes. Bromination with V12(Br2) showed high selectivity for 3-bromopentane (64 %) among the monobromopentane products and preferred threo isomer among 2-,3-dibromobutane and 2,3-dibromopenane. The unique inorganic cavity traps Br₂ leading the polarization of the diatomic molecule. Owing to its new reaction field, the trapped Br₂ shows selective functionalization of alkanes.

Full text of DOI:10.1002/anie.202007406

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Accepted Article
Title: Induced-Fitting and Polarization of Bromine Molecule in an
Electrophilic Inorganic Molecular Cavity and Its Bromination
Reactivity
Authors: Yuji Kikukawa, Kensuke Seto, Daiki Watanabe, Hiromasa
Kitajima, Misaki Katayama, Shohei Yamashita, Yasuhiro
Inada, and Yoshihito Hayashi
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Induced-Fitting and Polarization of Bromine Molecule in an
Electrophilic Inorganic Molecular Cavity and Its Bromination
Reactivity
Yuji Kikukawa,*[a,b] Kensuke Seto,[a] Daiki Watanabe,[a] Hiromasa Kitajima,[a] Misaki Katayama,[c] Shohei
Yamashita,[d] Yasuhiro Inada,[c] Yoshihito Hayashi[a]
Abstract: Dodecavanadate, [V12O32]4− (V12), possesses a 4.4 Å
cavity entrance, and the cavity shows unique electrophilicity. Due to
the high polarizability, Br2 was inserted into V12, inducing the
inversion of one of the VO5 square pyramids to form [V12O32(Br2)]4−
(V12(Br2)). The inserted Br2 molecule was polarized and showed a
peak at 185 cm−1 in the IR spectrum. The reaction of V12(Br2) and
toluene yielded bromination of toluene at the ring, showing the
electrophilicity of the inserted Br2 molecule. Compound V12(Br2) also
reacted with propane, n-butane, and n-pentane to give brominated
alkanes. Bromination with V12(Br2) showed high selectivity for 3-
bromopentane (64%) among the monobromopentane products and
preferred threo isomer among 2-,3-dibromobutane and 2,3-
dibromopenane. The unique inorganic cavity traps Br2 leading the
polarization of the diatomic molecule. Due to its new reaction field, the
trapped Br2 shows selective functionalization of alkanes.
of substituent on alkane framework. The direct conversion of
alkane to bromoalkane is usually carried out by free radical
processes.[5] On the other hand, the polarized bromination of
alkanes was reported by Olah and coworkers.[6] The cationic Brδ+,
acting as an electrophilic reagent, reacts with the alkane via a
pentacoordinate carbonium intermediate.[6] Particularly, polarized
halogenation of methane is expected to be highly selective for
monohalogenated methane.[7] because the halogen atom
increasingly dominates the donating ability of the σ(C−H) bond.[7]
In addition, the size of bromine affects the formation of
monohalogenated methane unlike that of chlorine.[8–11] To prepare
polarized Br2, the creation of a reaction field with a pair of Lewis
acid and base in the appropriate space is required.[12-14]
Polyoxometalates are a large family of early transition metal
oxide cluster anions that exhibit well-defined (sub)nano-sized
molecular structures.[15-19] Keplerate-type polyoxometalates
possess pore-like voids with a diameter ca. 3.6 Å on their surface
through which organic molecules can enter.[20,21] The bowl-type
structure of dodecavanadate ([V12O32]4−, V12) is composed of
twelve VO5 square pyramids and has a cavity diameter of 4.4 Å.[22]
The interior of the guest-occupied V12 is relatively cationic owing
to high-valent and coordinatively unsaturated vanadium atoms to
stabilize an electron-rich group or anion, while the cavity entrance
is relatively anionic owing to the eight oxygen atoms.[23-26] Among
the halogen atoms, bromine fits the V12 cavity the best.[26] This
specific feature of V12 inspired the idea of preparing an polarized
Br2 by inserting Br2 into the V12 bowl and using this polarized Br2
as the electrophilic reagent for alkane bromination.
Among several neutral-molecule-occupied V12, tetra-n-
butyl ammonium salts of V12 with 1,2-dichloroethane (DCE) ({n-
Bu4N}4[V12O32(DCE)], V12(DCE)) show high crystallinity.[28]
Single crystals of V12(DCE) in a 50 mL eggplant flask were
exposed to 1 equiv of Br2 gas with respect to V12(DCE).
Fortunately, the crystallinity remains high after the disappearance
of the brown color of Br2, and the product is suitable for single-
crystal X-ray analysis (Figure 1, Table S1). The V12 crystal
structure is maintained and one DCE is inserted into the V12 bowl
as a guest molecule. The arrangement of tetra-n-butyl ammonium
cations are almost the same as that in the original single crystals.
The crystalline DCE near the rim of the V12 bowl is partly
substituted by Br2. The calculated occupancy of Br2 from crystal
analysis (39%) is identical to that from elemental analysis. The
Br−Br bond length is 2.31 Å, and the distance between a bromine
atom and the nearest-neighbor oxygen atom of V12 is 2.57 Å,
showing the presence of a halogen bond interaction (Figure 1).[29]
There are some reports of single-crystal analysis of samples
containing Br2.[30-34]
Alkanes are the major constituents of natural gas and oil.
The chemical inertness of the sp3 C−H bond, which has high bond
dissociation energy and low polarity, makes it difficult to use
alkanes as the carbon source of several chemical products.
Although the conversion from sp3 C−H to C−X (X = C, N, O, S, Se,
halogen) bond has been reported by several energetic
researchers, an innovative methodology for the direct conversion
of abundant and simple alkanes into value-added products are
strongly required from practical and fundamental points of view.[1–
3]
Organobromine compounds are versatile intermediates.[4]
To establish a direct bromination reaction of alkanes is important,
because it provides a useful scaffold for the introduction of variety
[a]
Dr. Y. Kikukawa, H. Kitajima, K. Seto, D. Watanabe, Dr. Y. Hayashi
Department of Chemistry, Graduate School of Natural Science and
Technology
Kanazawa University
Kakuma, Kanazawa 920-1192, Ishikawa (Japan)
E-mail: kikukawa@se.kanazawa-u.ac.jp
Dr. Y. Kikukawa
JST, PRESTO,
4-1-8 Honcho, Kawaguchi 332-0012, Saitama (Japan)
Dr. M. Katayama, Dr. Y. Inada
[b]
[c]
Department of Applied Chemistry, Graduate School of Life Sciences
Ritsumeikan University
1-1-1 Noji-Higashi, Kusatsu 525-8577, Shiga (Japan)
Dr. S. Yamashita
[d]
Photon Factory, Institute of Materials Structure Science
High Energy Accelerator Research Organization
1-1 Oho, Tsukuba 305-0801, Ibaraki (Japan)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.202007406
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Figure 2. IR spectra of V12-free coated onto a Si disk (a) before and (b) after
exposure to 0.6 kPa of Br2 gas. The spectrum in red is measured after
evacuation of excess Br2 gas. The insertion is represented by the difference
spectrum between spectrum (b) and the spectrum in red.
Figure 1. Anion structures of (a) V12-free and (b) V12(DCE) with adsorbed Br2
based on X-ray crystallographic analysis. (c) Schematic representation of the
possible V12(Br2) structure and Br2 polarization. Orange and red square
pyramids represent VO5 units. Orange, red, black, green, and brown balls
represent vanadium, oxygen, carbon, chlorine, and bromine atoms, respectively.
Hydrogen atoms are omitted for clarity. The dotted line represents halogen bond
interaction.
peak at 185 cm−1 appears. This peak is not observed when the
experiment is performed with V12(DCE) instead of V12-free. The
peak at 185 cm−1 observed with V12-free, disappears upon re-
exposure to 0.6 kPa of Br2 gas and reappears upon evacuation of
the gas. These results indicate that dodecavanadate with a Br2
molecule as a guest ([V12O32(Br2)]4−, V12(Br2)) is obtained by
exposure of V12- free to Br2 (Figure 1). The peak at 185 cm−1 is
due to the polarized Br2.[36] Density functional theory (DFT)
calculation supports the spectrum change. The energy of the
optimized structure of V12(Br2) is 5.6 kJ/mol lower than the sum
of the energy of V12-free and free Br2. The Mulliken charge of
outer Br atom is higher than the other, showing the polarization of
Br2. The vibrational frequencies of calculated V12(Br2) is identical
to the guest-occupied V12 in the main region of V−O stretching
vibrations They also show the peak due to Br−Br in the lower
region of V−O stretching vibrations. The cause of the observation
of an IR-inactive Br−Br stretching band is the polarization of Br2
molecule in the cavity: the arrangement of vanadium atoms in the
cavity make the one of the Br atom partially electronegative and
oxygen atoms at the entrance make the outer Br atom
electropositive. The disappearance of the peak at 185 cm−1 in the
presence of excess Br2 is probably caused by intermolecular
bromine interaction, which reduces the dipole moment of the
guest. In the previous work, molecules with low- or non-polarity,
such as CO, O2, N2, H2, and CH4 under 0.1 kPa, showed no
interaction with the inside of V12.[35] The reason Br2 can be
inserted to V12 is the high polarizability of Br2. The insertion rate
of Br2 into V12 was determined by monitoring the peak at 856
cm−1 (Figure S4). The rates at 0.6 kPa of Br2 gas at −5, 10, and
20 ˚C are 0.025, 0.043, and 0.10 s−1, respectively, which yield an
activation energy of 35 kJ/mol.
The X-ray analysis reveals that DCE effectively caps the
cavity and it prevents insertion of Br2 in the cavity. After removal
of the crystalline solvents, Br2 adsorption on V12(DCE) was
carried out in a glass vessel of the prescribed volume with a
greaseless valve (Figure S1). With increasing pressure of Br2 gas,
the amount of adsorbed Br2 gradually increases, and 1 mol/mol
Br2 to V12(DCE) is adsorbed at 2.5 kPa of Br2 gas (Figure S2).
For the insertion of Br2 into the V12 structure, tetra-n-butyl
ammonium salts of guest-free V12 (V12-free) were prepared
(Figure 1).[35] In the previous work, it is revealed that one of the
VO5 square pyramids at the bottom of the bowl was flipped by
removing a guest molecule, and that the flipped VO5 unit was
retrieved by re-inserting the guest molecule. These structural
transformations were observed by monitoring the 850 cm−1 peak
due to the V−O stretching vibration of the bottom part of V12 by
IR. The Br2 adsorption isotherms for V12-free show that the
amount of Br2 adsorbed on V12-free at 2.5 kPa (ca. 2 mol/mol) is
larger by 1 mol/mol than that on V12(DCE) (Figure S2). The
difference between V12(DCE) and V12-free is the presence or
absence of a DCE guest. Therefore, the additional adsorption of
Br2 on V12-free indicates the insertion of Br2 as a guest of V12.
To clarify the insertion of Br2 into the V12 structure, in-situ IR
spectra were measured (Figure 2). The intensity of the peak at
around 850 cm−1 due to V−O stretching vibration of V12-free is
low compared with that of the guest-occupied V12, and the peak
at 952 cm−1 probably due to the terminal V−O stretching vibration
of flipped VO5 is observed only in the spectrum of V12-free. Under
0.6 kPa of Br2 gas at 20 ˚C, the peak at 856 cm−1 increases, while
that at 952 cm−1 decreases, with time, suggesting the insertion of
a guest into V12. After 20 min, the peak at 952 cm−1 almost
disappears. Upon evacuation of Br2 gas from the vessel, a new
Quantitative reaction between V12-free and Br2 gas was
carried out (Figure S5). The gas-phase UV-visible spectrum of 50
μmol of Br2 shows a peak at 415 nm. 50 μmol of Br2 exist as gas
in the 9.8 mL cell at 20 ˚C. While the condensation of Br2 was
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observed in the cell at −10 ˚C, 20 μmol of Br2 existed as gas with
showing 0.40 absorbance at 415 nm with an optical path length
of 1 cm.[37] On the other hand, in the presence of 50 μmol of the
solid of V12-free, no condensation of Br2 was observed at −10 ˚C,
showing the adsorption of Br2 by V12.[37] The absorption of the
gas phase UV spectrum at 415 nm was below 0.01, suggesting
the adsorption of over 90% of Br2 by V12 at −10 ˚C. Elemental
and IR analyses confirm the quantitative insertion of Br2 to form
V12(Br2). The X-ray absorption fine structure (XAFS)
measurements also support the formation of V12(Br2). The
Fourier transform magnitude of radial distribution functions for the
vanadium K-edge extended XAFS (EXAFS) spectra of bromide-
occupied V12 (V12(Br) and V12-free substantially reflects the
distribution of the bond distances in their crystal structures (Figure
S6). The result of V12(Br2) is similar to that of V12(Br) rather than
that of V12-free. The peak intensity ratio of second to third peaks
of V−O bonds of V12(Br) and V12(Br2) are larger than that of
V12-free. The fourth and fifth peaks of V−V distances of V12(Br)
and V12(Br2) are slightly shorter than those of V12-free. The
bromine K-edge EXAFS spectrum suggests a Br−Br distance of
2.33 Å, which is slightly longer than that in gas-phase Br2 (2.28 Å,
Figure S7).[38]
Release of Br2 from V12(Br2) was investigated. 51V NMR
spectra of V12(Br2) in acetone or N,N-dimethylformamide is
identical to that of V12-free, suggesting the weak affinity between
Br2 and the cavity of V12.[27] Thermogravimetry analysis (TGA) of
V12(Br2) shows 6.35% weight loss by 80 ˚C (Figure S8). This
value is 10% lower than the calculated value of the release of Br2
from V12(Br2) (7.09%). Due to the weak affinity, part of Br2 was
desorbed from V12(Br2) during the preparation under an ambient
condition. IR spectrum of the sample after heating to 80 ˚C with
TGA, shows the structure conversion to V12-free. This structure
conversion also occurred with vacuum treatment of V12(Br2) at
25 ˚C, and the released Br2 was collected in a cold trap.
the reaction (Figure S9). The strong intensity at 850 cm−1 shows
the insertion of the guest moiety into V12. The 51V NMR spectrum
of the sample in acetone after the reaction shows peaks at −583,
−587, and −599 ppm due to [V12O32(Br)]5−. (Figure S10). With time,
new peaks due to [HV12O32(Br)]4− increases, suggesting that the
structure conversion to [HV12O32(Br)]4− with the presence of
proton.[39,40] On the other hand, when V12(Br2) is dissolved in
acetone, 51V NMR spectrum shows the removal of Br2 to form
V12-free. These results suggest that HBr-occupied V12 is
produced by the bromination with V12(Br2).
The bromination of n-pentane with V12(Br2), V12(DCE)-
Br2, and Br2 gives monobromopentane and dibromopentane
(Table S2). Because of the difficulty of separating bromoalkane
isomers, the development of a selective bromination reaction is
significant.[41] The selectivity for the brominated products with
V12(Br2) is different from those of V12(DCE)-Br2 and Br2(Figure
3). 3,3-Dibromopentane, which is selectively obtained by the
reaction of n-pentane with {n-Bu4N}Br3, is observed in all cases.
As the bromination products from tetra-n-butylammonium, 0.7
μmol of dibromobutane is detected with V12(Br2). The
contributions of the inserted and external Br2 in the V12 structure
were evaluated by considering the selectivity for specific
bromination products with V12(Br2) and V12(DCE)-Br2 as
discussed below.
With V12(Br2), the ratio of 3-bromopentane (64%) among
the monobromopentane products is higher than those with
V12(DCE)-Br2 (21%). To explain this selectivity, the bromination
of monobromopentane compounds was investigated (Table S3).
After 15 h of competitive bromination of a 50 μmol mixture of 2-
and 3-bromopentane (55:45) with 50 μmol of V12(Br2) in
perfluorohexane, 30 μmol of monobromopentane remains with
the ratio of 2-bromopentane and 3-bromopentane being 22:78. In
the case of V12(DCE)-Br2, 27 μmol of monobromopentane
remains and the ratio of 2-bromopentane and 3-bromopentane
is same as that before the reaction. These results suggest that
the high ratio of the obtained 3-bromopentane with V12(Br2), is
due to the consumption of 2-brommopentane formed during the
reaction. The bromination of 2- and 3-bromopentane with
To confirm the polarization of Br2 in V12(Br2), the reaction
with toluene was investigated (Scheme 1). The suspension of 50
μmol of V12(Br2) in 2 mL of toluene gives 33 μmol of ring-
brominated products as the main products. Addition of liquid Br2
to toluene produced benzyl bromide as the main product via a
radical reaction mechanism. The pretreated V12(DCE) exposed
to 1 equiv of Br2 gas (V12(DCE)-Br2) gave benzyl bromide as the
main product. These results indicate that Br2 inserted into V12
acts as an electrophilic reagent, while the external Br2 participates
in a radical reaction.
After the reaction V12(Br2), the solid was filtered out. The
IR spectrum shows that the V12 framework is maintained during
Scheme 1. Bromination of toluene. The reaction is carried out with 50 μmol of
bromine source in 2 mL of toluene at 25 ˚C for 15 h..
Figure 3. Selectivity for brominated products in the reaction of n-pentane with
a bromine source. The reaction is carried out with 50 μmol of bromine source in
2 mL of n-pentane at 25 ˚C for 15 h.
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[2]
A. J. L. Pombeiro, M. F. C. Guedes da Silva in Alkane Functionalization,
J. Wiley & Sons, Hoboken, NJ, USA, 2019.
V12(Br2) gives 2,3-dibromopentane and 1,2-dibromopentane
(Table S3). The diastereoselectivity of 2,3-dibromopentane
obtained with V12(Br2) and the mixture of 2- and 3-bromopentane
is unique, showing 83% of the threo isomer.[42] In the radical
mechanism with V12(DCE)-Br2, 6.7 μmol of 2,3-dibromopentane,
with the ratio of the threo and erythro isomers being 30:70. The
reactivity of 1-bromopentane with V12(Br2) is low and no
dibromopentanes were obtained. Going back to the bromination
of pentane, the distribution of the selectivity for threo-2,3-
dibromopentane is 44%. This values indicate that ca. 50% of Br2
inserted into V12 and ca. 50% of Br2 external to V12 reacted. The
reaction of V12(Br2) with 2-methylbutane gave trace amount of
monobrominated products and 1.1 μmol of dibrominated products,
while Br2 gave 2-bromo-2-methylbutane as a main product.
In addition, V12(Br2) reacted with n-butane and propane to
give 2-bromobutane 2,3-dibromobutane, and 1,2-dibromobutane,
and 2-bromopropane and 1,2-dibromopropane, respectively
(Table S2). The diastereoselectivity of 2,3-dibromobutane is also
unique, showing 70% of the threo isomer. The reactivity of butane
and propane with V12(Br2) is higher than that of pentane due to
the size of alkanes.[43]
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This work was supported by JST PRESTO Grant (No.
JPMJPR1655), Nippon Sheet Glass Foundation for Materials
Science and Engineering, JSPS KAKENHI Grant (No.
JP18K14239), and Core-to-Core Program. The XAFS
measurements in this work have been performed under the
approval of the High Energy Accelerator Research Organization
in Japan (Proposal No. 2017G713).
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Keywords: Bromine Polarization, Polyoxometalates, Vanadium,
Alkane Bromination, Molecular Container
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Entry for the Table of Contents
To accept the insertion of Br2, the bowl type structure of dodecavanadate was rearranged. Due to the unique electrophilic feature of
the bowl inside, Br2 was polarized to be observed by IR. Polarized Br2 in dodecavanadate bowl, acted as an electrophilic reagent to
achieve selective bromination of propane, n-butane, and n-pentane.
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