Solvent-Vapor-Induced Reversible Single-Crystal-to-Single-Crystal Transformation of a Triphosphaazatriangulene-Based Metal–Organic Framework

Article abstract of DOI:10.1002/anie.201912195

A triphosphaazatriangulene (H₃L) was synthesized through an intramolecular triple phospha-Friedel–Crafts reaction. The H₃L triangulene contains three phosphinate groups and an extended π-conjugated framework, which enables the stimul

Full text of DOI:10.1002/anie.201912195

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Accepted Article
Title: Solvent-Vapor-Induced Reversible Single-Crystal-to-Single-
Crystal Transformation of a Triphosphaazatriangulene-based
Metal–Organic Framework
Authors: Soichiro Nakatsuka, Yusuke Watanabe, Yoshinobu
Kamakura, Satoshi Horike, Daisuke Tanaka, and Takuji
Hatakeyama
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201912195
Angew. Chem. 10.1002/ange.201912195
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http://dx.doi.org/10.1002/ange.201912195

Angewandte Chemie International Edition
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COMMUNICATION
Solvent-Vapor-Induced Reversible Single-Crystal-to-Single-
Crystal Transformation of a Triphosphaazatriangulene-based
Metal–Organic Framework
[
a]
[a]
[a]
[b]
Soichiro Nakatsuka, Yusuke Watanabe, Yoshinobu Kamakura, Satoshi Horike, Daisuke
[
a]
[a]
Tanaka, and Takuji Hatakeyama*
Abstract: A triphosphaazatriangulene (H
3
L) was synthesized by way
show heterogeneous surface charge (Figure 1b). Negative electro
statistic potential localized on the oxygen atoms of phosphinate
of an intramolecular triple phospha-Friedel–Crafts reaction. The H
3
L
triangulene contains three phosphinate groups and an extended π-
3
groups, whereas the rest of H L has a slightly positive
conjugated framework, which enables the stimuli-responsive
electrostatic potential. The larger variation around the oxygen
atoms suggest that the hydrophobic π-conjugated moiety is
surrounded by hydrophilic phosphinate groups. Based on these
reversible transformation of [Cu(HL)(DMSO)·(MeOH)]
that exhibits reversible sorption characteristics,
L·0.5[Cu (OH) ·6H O]·4H O), a 1D-columnar assembled proton-
conducting material. The hydrophilic nature of the latter resulted in a
n
, a 3D-MOF
into
(H
3
2
4
2
2
3
features, we envisaged a dual role for H L, namely as a scaffold
for the construction of 3D-MOFs through phosphinate-metal
linkages, and 1D-columnar assemblies through non-covalentπ-π
interactions (Figure 1c).
−
3
–1
proton conductivity of 5.5 × 10 S·cm at 95% relative humidity and
0 °C.
6
+40 kcal mol–1
(a)
(b)
O
OH
Me
P
Me
Among main-group-element-containing π-conjugated materials,
π-conjugated organophosphorus materials are among the most
attractive due to the pyramidal geometries and σ*–π* interactions
of their phosphorus atoms.[1] Based on these attractive features,
π-conjugated organophosphorus materials are used in a variety
N
HO
P
P
O
OH
O
hydrophobic
p-conjugated
moiety
hydrophilic
phosphinate
groups
[2]
[3]
Me
of applications, such as liquid crystals, organic electronics,
H3L
–40 kcal mol–1
electrochromic materials,[4] and bioimaging probes,[5] among
others.[6] Recently, the introduction of multiple phosphorus atoms
into polycyclic aromatic hydrocarbons (PAHs) has been used to
tune their three-dimensional (3D) structures and electronic
properties,[7] which realized the effective recognition of
fullerenes[7b] and cumulative anisotropies with large dipole
moments.[7d] On the other hand, compounds containing multiple
phosphorus atoms have been used as efficient linkers in metal–
organic frameworks (MOFs).[8,9] In particular, phosphonate
monoester groups (–P(O)(OH)(OR)) offer carboxylate-like
coordination modes to form MOFs[10] that are substantially
chemically and thermally stable; these MOFs have been used as
ion-exchange materials, proton conductors, catalysts, and
sorbents.
(
c)
H2O vapor
in single crystals
DMSO/MeOH
vapor
3D-MOF (1)
reversible sorbent
1D-columnar assembly (1')
proton conductor
Figure 1. (a) Structure of triphosphaazatriangulene
L calculated at the B3LYP/6-31G(d) level of
theory (color code: red = –40 kcal mol–1, blue = +40 kcal mol ). (c) Illustrating
3
H L. (b) Molecular
electrostatic potential surfaces of H
3
–1
the packing motif of the 3D-MOF and the 1D-columnar assembly.
Herein, we describe the design and synthesis of a novel
After extensively screening metal salts and reaction
3
triphosphaazatriangulene (H L, Figure 1a) that consists of a non-
conditions, we prepared [Cu(HL)(DMSO)·(MeOH)]
based 3D-MOF, that exhibits reversible MeOH-sorption behavior.
Notably 1 is transformed into H L·0.5[Cu (OH) ·6H O]·4H O (1’),
1D-columnar assembled material under high-humidity
n
(1), a copper-
planar polycyclic aromatic skeleton with three peripheral
phosphinate groups. Electrostatic-potential calculations clearly
[
11]
3
2
4
2
2
a
conditions, with the reverse process, the formation of MOF 1 from
1’, occurring in a DMSO/MeOH-vapor atmosphere. These stimuli-
responsive transformations were found to be reversible, even in a
[
a]
Dr. Soichiro Nakatsuka, Yusuke Watanabe, Yoshinobu Kamakura,
Prof. Daisuke Tanaka, Prof. Takuji Hatakeyama
Department of Chemistry
School of Science and Technology
Kwansei Gakuin University
single crystal, due to the amphiphilic nature of H
3
L and the
moderate coordinating abilities of diaryl phosphinates. The 1D
2-1 Gakuen, Sanda
Hyogo 669-1337, Japan
E-mail: hatake@kwansei.ac.jp
Prof. Satoshi Horike
Institute for Integrated Cell-Material Sciences
Institute for Advance Study
Kyoto University, Yoshida-Honmachi
Sakyo-ku, Kyoto, 606-8501 Japan
structure 1’ has hydrophilic domains composed of Cu
2
(OH)
4
and
−
3
–1
H
2
O, and exhibits a high proton conductivity of 5.5 × 10 S· cm
[
b]
at 95% relative humidity (RH) and 60 °C.
Triangulene H L was synthesized in five steps from tri-p-
3
tolylamine (2) by the route shown in Figure 2. Tri-p-tolylamine 2
was first brominated with 3.1 equiv. of N-bromosuccinimide (NBS)
to give the corresponding tribromide 3 in 94% yield. The initial
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
10.1002/anie.201912195
COMMUNICATION
lithium–bromine exchange of 3 with 3.1 equiv. of butyllithium,
subsequent trapping of the resulting aryllithium with 3.5 equiv. of
bis(N,N-diethylamino)chlorophosphine, followed by sulfurization
interact with guest molecules. The X-ray crystallographic data
clearly reveal that MeOH molecules interact weakly through
hydrogen bonds to the oxygen atoms of the DMSO molecules
(O[MeOH]•••O[DMSO] = ~2.9 Å).
with 4.0 equiv. of S
triple intramolecular phospha-Friedel–Crafts reaction[12] of 4 with
.0 equiv. of N,N-diisopropylethylamine in the presence of 3.0
equiv. of AlCl at 140 °C for 12 h afforded 5 in 47% yield. Oxidation
with m-CPBA afforded the corresponding phosphine oxide 6 in
1% yield, with subsequent hydrolysis providing the desired H
ligand in 81% yield. A solution of H
L in DMSO (1.0 × 10–2 M) was
mixed with CuSO ·5H
O in MeOH (1.0 × 10–2 M) at 40 °C for 2 d
8
afforded intermediate 4 in 65% yield. The
2
(a)
Cu
O2
3
O
Å
)
Å
2
)
O5
P
N
(
0(3
8
4
0
.9
2.0
1.9
9
3
L
O
2.074(2)
1
Å
O5
S
3
O1
8
O
Cu
6(2
Å
O
P
O1
O4
Cu
O3
)
)
Å
4
2
3
(
P
5
to afford the [Cu(HL)(DMSO)·(MeOH)]
n
MOF (1) in 46% yield as
2
O
O2
9
.
O3
Cu
1
a yellow crystalline powder.
O4
trigonal bipyramidal
Cu
MeOH
DMSO
(b)
(c)
b
b
c
c
a
a
Figure 3. (a) The crystallographic environment around the Cu2+. (b, c) Two
views of the packing structure of 1. Hydrogen atoms and guests in the pores are
omitted for clarity. Selected bond angles (°): O1–Cu–O5 = 120.48(11), O1–Cu–
O3 = 111.59(10), O3–Cu–O5 = 127.93(11), O2–Cu–O4 = 170.20(12), O1–Cu–
O2 = 91.81(10), O2–Cu–O5 = 85.64(11).
Figure 2. The six-step synthesis of 1. Conditions: (a) N-bromosuccinimide (3.1
equiv.), CH CN, 0 °C, then room temperature, 19 h. (b) n-butyllithium (3.1
equiv.), toluene, °C, then room temperature, h; then bis(N,N-
diethylamino)chlorophosphine (3.5 equiv.), toluene/ether (1:3 v/v), –78 °C, then
°C, 2 h; then S (4.0 equiv.), toluene, 0 °C, then room temperature, 14 h. (c)
AlCl (3.0 equiv.), N,N-diisopropylethylamine (2.0 equiv.), o-dichlorobenzene,
°C, then room temperature, 12 h, then 140 °C, 12 h. (d) m-CPBA (12.0 equiv.),
dichloromethane, –40 °C, 20 min. (e) 3 M aq. HCl/THF (1:1 v/v), 0 °C, then
0 °C, 8 h.
3
0
2
The methanol-sorption isotherms for 1 at 298 K shown in
Figure 4b and the powder X-ray diffraction (PXRD) patterns
shown in Figures S4 and S5 indicate that the framework is
sufficiently flexible around the hydrophilic domain to absorb
methanol vapor while maintaining its overall crystalline structure
during the process. In order to examine its stimuli-responsive
transformation, 1 was exposed to water vapor in air for 1 week,
during which time the color of the crystal was observed to slowly
change from the yellow of 1 to the blue-green of 1’, the 1D-
columnar assembled material. This transformation was complete
within 12 h when the MeOH in 1 was removed in vacuo.
Surprisingly, these crystals were persistently single-crystalline
following this transformation. The single-crystal X-ray structure of
0
8
3
0
6
Single crystals suitable for X-ray crystallography were
obtained by the slow-diffusion intermixing of a solution of
CuSO ·5H O in MeOH and H
L in DMSO.[13] Figure 3a shows the
4
2
3
2
+
copper-coordination environment in 1; the Cu ions are trigonal-
bipyramidally coordinated to the four O atoms of HL2– and the
oxygen atom of a DMSO. The Cu–O2 and Cu–O4 bond lengths
are 1.904(2) and 1.925(3) Å, respectively, shorter than the Cu–
O1, Cu–O3, and Cu–O5 bond lengths (2.074(2), 1.986(2), and
1
’ shows that the cell volume of the material had increased from
3
2583.84(9) to 2819.2 (1409.6(15) × 2) Å . Full assignment of the
structure of 1’ shows that the DMSO in 1 has been removed and
that has been completely converted into
L·0.5[Cu (OH) O]·4H 1D-columnar assembled
material of H L and Cu (OH) in single crystals (Figure 4a). A
2
.080(3) Å, respectively). The framework of 1 is constructed from
1
the Cu center, which acts as a four-connected node using HL2–
as a four-connected linker (Figure 3b). This arrangement
produces a large twelve-membered ring composed of six trigonal
2+
H
3
2
4
·6H
2
2
O,
a
3
2
4
comparison of the structural features of 1 and 1’ helps to reveal
the mechanism of the single-crystal-to-single-crystal (SC−SC)
transformation. We hypothesize that H O molecules coordinate to
2
the Cu2+ center and the phosphonate groups, with the DMSO in 1
cleaved from the Cu2+ center. Three aquo and two hydroxo
ligands replace the cleaved coordinating phosphonate groups
and DMSO to maintain a trigonal bipyramidal geometry. In
2
+
bipyramidal Cu moieties linked together by six HL units. The
framework expands in three directions to form a 3D network
(
Figure 3c). MOF 1 possesses straight 1D channels along the a
axis that are occupied by MeOH molecules. In 1, 10.9% of the
total void volume per unit cell volume, Vvoid, is accessible to
solvent molecules.[14] DMSO molecules, which are highly ordered
2
+
on the surfaces of the channels and coordinate to Cu , can
2 2
addition, the resulting hydrophilic domain of Cu(OH) ·3H O is
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Angewandte Chemie International Edition
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COMMUNICATION
pushed outward to form straight 1D channels due to the
To characterize its proton conductivity, 1 was subjected to AC
impedance spectroscopy under conditions of controlled humidity
(Figures S10) and temperature after the MeOH in 1 had been
removed in vacuo (Figure S13). Prior to any impedance
experiment, the phase purity of the bulk sample was confirmed by
powder X-ray diffractometry (PXRD, Figure S5). The proton
conductivity was found to be 5.9 × 10–8 S cm–1 at 25 °C and 55%
relative humidity (RH). Nyquist plots under a variety of humidity
conditions (from 55% to 95% RH) at 25 °C show that conductivity
increases with increasing RH (Figure S10 and S13a) because the
water molecules adsorbed in the voids of 1 assist with proton
hydrophobic π-conjugated H
process is the formation of edge-sharing octahedral dimers. H
is densely accumulated in 1’ through intermolecular non-covalent
contacts, and the 1D-columnar assembly of H L is stabilized by
the hydrophilic 1D Cu (OH) and domains through
3
L moiety; the overall result of this
3
L
3
2
4
2
H O
intermolecular hydrogen bonding. Moreover, 1 was reconstructed
within the single crystal upon exposure of 1’ to DMSO/MeOH
vapor. Thus, 3D-MOF and 1D-columnar assembled materials are
formed reversibly in response to changes in solvent vapor through
the dual role played by H
3
L.
diffusion. The Nyquist plot shown in Figure S13a reveals that a
higher proton conductivity of 7.4 × 10–4 S cm–1 is observed at
(a)
(b)
[
M
A
d
e
m
c
e
]
O
1
o
25 °C and 95% RH. The color of the pellet and the PXRD pattern
acquired at 95% RH (Figure S5) suggests the formation of 1',
which has proton-transport pathways with stabilized hydrophilic
domains. The conductivity was 5.5 × 10−3 S cm–1 at 60 °C and
b
–
H
r
u
b
o
g
n
a'
s
)
m
t
d
P
o
a
a
T
l
d
t
/
s
S
n
(
u
o
u
r
L
m
b
o
m
o
e
b'
m
[
d
A
]
l
9
5% RH. The Arrhenius plot provided a linear fit, from which the
b
a
a
c'
activation energy for proton conduction was determined to be 0.52
eV on the basis of impedance spectra recorded between 25 and
P/P0
c
in vacuo
3
60 °C at 95% RH (Figure S14). We found that H L plays an
rt, 3 h
[
Cu(HL)(DMSO)·(MeOH)]n
1)
[Cu(HL)(DMSO)]n
important role in creating proton-conducting pathways; that is, the
hydrophilic domain is stabilized by the peripheral hydrophilic
phosphinate groups of the H L hydrophobic core.
3
(
MeOH
vapor, rt
DMSO
H O
H2O
vapor
rt, 12 h
2
/
MeOH
vapor
vapor, rt
1 week
rt, 39 h
In summary, we successfully synthesized
triphosphaazatriangulene, through the use of a triple phospha-
Friedel–Crafts reaction. H L has a hydrophobic extended π-
3
H L, a new
H3L·0.5[Cu2(OH)4·6H2O]·4H2O
1')
(
3
a'
conjugated framework surrounded by hydrophilic phosphinate
groups, which is an attractive feature. Based on the amphiphilic
c
1
3
2
b'
3
nature of H L, external stimuli trigger transformations between the
b
3D-MOF 1 with reversible MeOH-sorption characteristics and the
Cu
N
1D-columnar assembled material 1’ with high proton conductivity.
Even though this transformation involves the cooperative
breakage/formation of metal−phosphinate coordination bonds,
c'
b
a
P
a
b
O
c
a
these
transformations
are
single-crystal-to-single-crystal
L,
1
: 2.69 Å 2: 2.73 Å 3: 2.59 Å
c
(
SC−SC) processes due to the dual assembly state nature of H
3
6
2
.76
2
which will contribute to the design and synthesis of new smart
materials.
.
7
.
7
.
7
2
7
2
7
2
2
.6
.
6
2
2
.8
2
1
1
.
2
8
Cu
Acknowledgements
H
This study was supported by a Grant-in-Aid for Scientific
Research (JP18H02051) and Challenging Research (Exploratory,
JP19K22191) from JSPS, and CREST (JPMJCR18R3) programs
from JST. Preliminary measurements for the X-ray crystal
structure analysis were performed at the BL40XU beamline in
SPring-8 with the approval of JASRI (2017A1132, 2017B1073,
2018A1114, 2018B1125, 2019A1142) and with the help of Dr.
Nobuhiro Yasuda (JASRI). We thank Prof. Hirofumi Yoshikawa
Figure 4. (a) Reversible formation of (top-left) 1 showing a MeOH-filled channel
and (middle) 1’ showing H O-filled channels in a single crystal. Enlarged view
of the packing structure of 1’ (bottom). The blue arrows show H L–to–H O, H L–
to–Cu (OH) ·6H O, and H O–to–Cu (OH) ·6H O interactions that create an
extended the H-bonding network along the 1D channels to build a potential
proton-transfer pathway. Thermal ellipsoids are shown at the 50% probability
level and H atoms are omitted for clarity. (b) Methanol-sorption isotherms at
2
3
2
3
2
4
2
2
2
4
2
25 °C for 1.
(
Kwansei Gakuin University) for allowing us to use gas adsorption
analyzer and Ms. Nanae Shimanaka (Kyoto University) for
supporting conductivity measurement.
The presence of an extended hydrogen-bonded network and
the additional protons from the phosphinic acid (–P(O)(OH)) in
L, Cu (OH) , and the water molecules that form 1D channels
reveal that 1’ is potentially a highly proton-conducting material.
Keywords: Metal-organic frameworks • Phosphorus •
H
3
2
4
Polycycles • Phospha-Friedel–Crafts reaction • Proton transport
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