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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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content of this Accepted Article.

To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201912195

Angew. Chem. 10.1002/ange.201912195

Link to VoR: http://dx.doi.org/10.1002/anie.201912195

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Angewandte Chemie International Edition 10.1002/anie.201912195

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Solvent-Vapor-Induced Reversible Single-Crystal-to-Single-

Crystal Transformation of a Triphosphaazatriangulene-based
Metal–Organic Framework
Soichiro Nakatsuka,[a] Yusuke Watanabe,[a] Yoshinobu Kamakura,[a] Satoshi Horike,[b] Daisuke
Tanaka,[a] and Takuji Hatakeyama*[a]
Abstract: A triphosphaazatriangulene (H3L) was synthesized by way show heterogeneous surface charge (Figure 1b). Negative electro
of an intramolecular triple phospha-Friedel–Crafts reaction. The H3L statistic potential localized on the oxygen atoms of phosphinate

Accepted Manuscript
triangulene contains three phosphinate groups and an extended π- groups, whereas the rest of H3L has a slightly positive
conjugated framework, which enables the stimuli-responsive electrostatic potential. The larger variation around the oxygen
reversible transformation of [Cu(HL)(DMSO)·(MeOH)]n, a 3D-MOF atoms suggest that the hydrophobic π-conjugated moiety is
that exhibits reversible sorption characteristics, into surrounded by hydrophilic phosphinate groups. Based on these
(H3L·0.5[Cu2(OH)4·6H2O]·4H2O), a 1D-columnar assembled proton- features, we envisaged a dual role for H3L, namely as a scaffold
conducting material. The hydrophilic nature of the latter resulted in a for the construction of 3D-MOFs through phosphinate-metal
proton conductivity of 5.5 × 10−3 S·cm–1 at 95% relative humidity and linkages, and 1D-columnar assemblies through non-covalentπ-π
60 °C. interactions (Figure 1c).
(a) (b) +40 kcal mol –1
O OH
Among main-group-element-containing π-conjugated materials, Me P Me

π-conjugated organophosphorus materials are among the most

N
attractive due to the pyramidal geometries and σ*–π* interactions HO P P O
of their phosphorus atoms.[1] Based on these attractive features, O OH
π-conjugated organophosphorus materials are used in a variety hydrophobic hydrophilic
Me
of applications, such as liquid crystals, [2] organic electronics,[3] p-conjugated phosphinate
H 3L moiety groups –40 kcal mol –1
electrochromic materials,[4] and bioimaging probes,[5] among
(c)
others.[6] Recently, the introduction of multiple phosphorus atoms
into polycyclic aromatic hydrocarbons (PAHs) has been used to
H 2O vapor
tune their three-dimensional (3D) structures and electronic
in single crystals
properties,[7] which realized the effective recognition of
fullerenes[7b] and cumulative anisotropies with large dipole DMSO/MeOH
vapor
moments.[7d] On the other hand, compounds containing multiple
phosphorus atoms have been used as efficient linkers in metal– 3D-MOF (1) 1D-columnar assembly (1')
reversible sorbent proton conductor
organic frameworks (MOFs).[8,9] In particular, phosphonate
monoester groups (–P(O)(OH)(OR)) offer carboxylate-like
Figure 1. (a) Structure of triphosphaazatriangulene H3L. (b) Molecular
coordination modes to form MOFs [10] that are substantially electrostatic potential surfaces of H3L calculated at the B3LYP/6-31G(d) level of
chemically and thermally stable; these MOFs have been used as theory (color code: red = –40 kcal mol–1, blue = +40 kcal mol–1). (c) Illustrating
ion-exchange materials, proton conductors, catalysts, and the packing motif of the 3D-MOF and the 1D-columnar assembly.
sorbents.
Herein, we describe the design and synthesis of a novel
triphosphaazatriangulene (H3L, Figure 1a) that consists of a non- After extensively screening metal salts and reaction
planar polycyclic aromatic skeleton with three peripheral conditions, we prepared [Cu(HL)(DMSO)·(MeOH)]n (1), a copper-
phosphinate groups.[11] Electrostatic-potential calculations clearly based 3D-MOF, that exhibits reversible MeOH-sorption behavior.
Notably 1 is transformed into H3L·0.5[Cu2(OH)4·6H2O]·4H2O (1’),
a 1D-columnar assembled material under high-humidity
[a] Dr. Soichiro Nakatsuka, Yusuke Watanabe, Yoshinobu Kamakura, conditions, with the reverse process, the formation of MOF 1 from
Prof. Daisuke Tanaka, Prof. Takuji Hatakeyama 1’, occurring in a DMSO/MeOH-vapor atmosphere. These stimuli-
Department of Chemistry responsive transformations were found to be reversible, even in a
School of Science and Technology
single crystal, due to the amphiphilic nature of H3L and the
Kwansei Gakuin University
2-1 Gakuen, Sanda moderate coordinating abilities of diaryl phosphinates. The 1D
Hyogo 669-1337, Japan structure 1’ has hydrophilic domains composed of Cu2(OH)4 and
E-mail: hatake@kwansei.ac.jp H2O, and exhibits a high proton conductivity of 5.5 × 10−3 S· cm–1
[b] Prof. Satoshi Horike
at 95% relative humidity (RH) and 60 °C.
Institute for Integrated Cell-Material Sciences
Institute for Advance Study Triangulene H3L was synthesized in five steps from tri-p-
Kyoto University, Yoshida-Honmachi
Sakyo-ku, Kyoto, 606-8501 Japan
tolylamine (2) by the route shown in Figure 2. Tri-p-tolylamine 2
was first brominated with 3.1 equiv. of N-bromosuccinimide (NBS)
Supporting information for this article is given via a link at the end of
to give the corresponding tribromide 3 in 94% yield. The initial
the document.

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Angewandte Chemie International Edition 10.1002/anie.201912195

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lithium–bromine exchange of 3 with 3.1 equiv. of butyllithium, interact with guest molecules. The X-ray crystallographic data
subsequent trapping of the resulting aryllithium with 3.5 equiv. of clearly reveal that MeOH molecules interact weakly through
bis(N,N-diethylamino)chlorophosphine, followed by sulfurization hydrogen bonds to the oxygen atoms of the DMSO molecules
with 4.0 equiv. of S8 afforded intermediate 4 in 65% yield. The (O[MeOH]•••O[DMSO] = ~2.9 Å).
triple intramolecular phospha-Friedel–Crafts reaction[12] of 4 with
2.0 equiv. of N,N-diisopropylethylamine in the presence of 3.0 (a) Cu O2
equiv. of AlCl3 at 140 °C for 12 h afforded 5 in 47% yield. Oxidation O

Å
Å O5

1.904(2)
with m-CPBA afforded the corresponding phosphine oxide 6 in P 3)
0(
91% yield, with subsequent hydrolysis providing the desired H3L 08
O 2.074(2) Å 2.
ligand in 81% yield. A solution of H3L in DMSO (1.0 × 10–2 M) was O5 S

1.
N O O1 Cu

98
mixed with CuSO4·5H2O in MeOH (1.0 × 10–2 M) at 40 °C for 2 d

1.925(3) Å
O1 O4

6(
O P

2)
Å
to afford the [Cu(HL)(DMSO)·(MeOH)]n MOF (1) in 46% yield as P Cu
O O2 O3
a yellow crystalline powder. Cu O3

Accepted Manuscript
O4

Cu MeOH DMSO trigonal bipyramidal

(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.), CH3CN, 0 °C, then room temperature, 19 h. (b) n-butyllithium (3.1
equiv.), toluene, 0 °C, then room temperature, 2 h; then bis(N,N- The methanol-sorption isotherms for 1 at 298 K shown in
diethylamino)chlorophosphine (3.5 equiv.), toluene/ether (1:3 v/v), –78 °C, then
Figure 4b and the powder X-ray diffraction (PXRD) patterns
0 °C, 2 h; then S8 (4.0 equiv.), toluene, 0 °C, then room temperature, 14 h. (c)
AlCl3 (3.0 equiv.), N,N-diisopropylethylamine (2.0 equiv.), o-dichlorobenzene, shown in Figures S4 and S5 indicate that the framework is
0 °C, then room temperature, 12 h, then 140 °C, 12 h. (d) m-CPBA (12.0 equiv.), sufficiently flexible around the hydrophilic domain to absorb
dichloromethane, –40 °C, 20 min. (e) 3 M aq. HCl/THF (1:1 v/v), 0 °C, then methanol vapor while maintaining its overall crystalline structure
60 °C, 8 h.
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
Single crystals suitable for X-ray crystallography were
change from the yellow of 1 to the blue-green of 1’, the 1D-
obtained by the slow-diffusion intermixing of a solution of columnar assembled material. This transformation was complete
CuSO4·5H2O in MeOH and H3L in DMSO.[13] Figure 3a shows the within 12 h when the MeOH in 1 was removed in vacuo.
copper-coordination environment in 1; the Cu2+ ions are trigonal- Surprisingly, these crystals were persistently single-crystalline
bipyramidally coordinated to the four O atoms of HL2– and the
following this transformation. The single-crystal X-ray structure of
oxygen atom of a DMSO. The Cu–O2 and Cu–O4 bond lengths 1’ shows that the cell volume of the material had increased from
are 1.904(2) and 1.925(3) Å, respectively, shorter than the Cu– 2583.84(9) to 2819.2 (1409.6(15) × 2) Å3. Full assignment of the
O1, Cu–O3, and Cu–O5 bond lengths (2.074(2), 1.986(2), and structure of 1’ shows that the DMSO in 1 has been removed and
2.080(3) Å, respectively). The framework of 1 is constructed from
that 1 has been completely converted into
the Cu2+ center, which acts as a four-connected node using HL2– H3L·0.5[Cu2(OH)4·6H2O]·4H2O, a 1D-columnar assembled
as a four-connected linker (Figure 3b). This arrangement material of H3L and Cu2(OH)4 in single crystals (Figure 4a). A
produces a large twelve-membered ring composed of six trigonal comparison of the structural features of 1 and 1’ helps to reveal
bipyramidal Cu2+ moieties linked together by six HL units. The
the mechanism of the single-crystal-to-single-crystal (SC−SC)
framework expands in three directions to form a 3D network transformation. We hypothesize that H2O molecules coordinate to
(Figure 3c). MOF 1 possesses straight 1D channels along the a the Cu2+ center and the phosphonate groups, with the DMSO in 1
axis that are occupied by MeOH molecules. In 1, 10.9% of the cleaved from the Cu2+ center. Three aquo and two hydroxo
total void volume per unit cell volume, Vvoid, is accessible to
ligands replace the cleaved coordinating phosphonate groups
solvent molecules.[14] DMSO molecules, which are highly ordered and DMSO to maintain a trigonal bipyramidal geometry. In
on the surfaces of the channels and coordinate to Cu 2+, can addition, the resulting hydrophilic domain of Cu(OH) 2·3H2O is

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Angewandte Chemie International Edition 10.1002/anie.201912195

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pushed outward to form straight 1D channels due to the To characterize its proton conductivity, 1 was subjected to AC
hydrophobic π-conjugated H3L moiety; the overall result of this impedance spectroscopy under conditions of controlled humidity
process is the formation of edge-sharing octahedral dimers. H3L (Figures S10) and temperature after the MeOH in 1 had been
is densely accumulated in 1’ through intermolecular non-covalent removed in vacuo (Figure S13). Prior to any impedance
contacts, and the 1D-columnar assembly of H3L is stabilized by experiment, the phase purity of the bulk sample was confirmed by
the hydrophilic 1D Cu2(OH)4 and H2O domains through powder X-ray diffractometry (PXRD, Figure S5). The proton
intermolecular hydrogen bonding. Moreover, 1 was reconstructed conductivity was found to be 5.9 × 10–8 S cm–1 at 25 °C and 55%
within the single crystal upon exposure of 1’ to DMSO/MeOH relative humidity (RH). Nyquist plots under a variety of humidity
vapor. Thus, 3D-MOF and 1D-columnar assembled materials are conditions (from 55% to 95% RH) at 25 °C show that conductivity
formed reversibly in response to changes in solvent vapor through increases with increasing RH (Figure S10 and S13a) because the
the dual role played by H3L. water molecules adsorbed in the voids of 1 assist with proton
diffusion. The Nyquist plot shown in Figure S13a reveals that a
(a) (b) higher proton conductivity of 7.4 × 10–4 S cm–1 is observed at

[MeOH mol/Cu mol]

Accepted Manuscript
Amount adsorbed
Amount adsorbed

c 25 °C and 95% RH. The color of the pellet and the PXRD pattern
[mL (STP) g –1]

b a' acquired at 95% RH (Figure S5) suggests the formation of 1',

which has proton-transport pathways with stabilized hydrophilic
b' domains. The conductivity was 5.5 × 10−3 S cm–1 at 60 °C and
b a 95% RH. The Arrhenius plot provided a linear fit, from which the
a
c' activation energy for proton conduction was determined to be 0.52
c P/P 0
eV on the basis of impedance spectra recorded between 25 and
in vacuo
rt, 3 h 60 °C at 95% RH (Figure S14). We found that H3L plays an
[Cu(HL)(DMSO)·(MeOH)] n [Cu(HL)(DMSO)]n
important role in creating proton-conducting pathways; that is, the
(1) MeOH
vapor, rt hydrophilic domain is stabilized by the peripheral hydrophilic
DMSO H 2O H 2O
/MeOH vapor, rt vapor phosphinate groups of the H3L hydrophobic core.
vapor 1 week rt, 12 h
rt, 39 h In summary, we successfully synthesized H3L, a new
H 3L·0.5[Cu 2(OH) 4·6H2O]·4H2O
(1')
triphosphaazatriangulene, through the use of a triple phospha-
a'
Friedel–Crafts reaction. H3L has a hydrophobic extended π-
c
conjugated framework surrounded by hydrophilic phosphinate
groups, which is an attractive feature. Based on the amphiphilic
1
2 b' nature of H3L, external stimuli trigger transformations between the
b
3 Cu 3D-MOF 1 with reversible MeOH-sorption characteristics and the
N 1D-columnar assembled material 1’ with high proton conductivity.
c'
b P Even though this transformation involves the cooperative
a b
O
a breakage/formation of metal−phosphinate coordination bonds,
c a
1: 2.69 Å 2: 2.73 Å 3: 2.59 Å these transformations are single-crystal-to-single-crystal
c
(SC−SC) processes due to the dual assembly state nature of H3L,
which will contribute to the design and synthesis of new smart
76

76

2.77
2.77
2.

2.

2.6
1
2.6 materials.
2

1
2. 8

2. 8

Cu
Acknowledgements
H
This study was supported by a Grant-in-Aid for Scientific
Research (JP18H02051) and Challenging Research (Exploratory,
Figure 4. (a) Reversible formation of (top-left) 1 showing a MeOH-filled channel JP19K22191) from JSPS, and CREST (JPMJCR18R3) programs
and (middle) 1’ showing H2O-filled channels in a single crystal. Enlarged view from JST. Preliminary measurements for the X-ray crystal
of the packing structure of 1’ (bottom). The blue arrows show H3L–to–H2O, H3L– structure analysis were performed at the BL40XU beamline in
to–Cu2(OH)4·6H2O, and H2O–to–Cu2(OH)4·6H2O interactions that create an
extended the H-bonding network along the 1D channels to build a potential
SPring-8 with the approval of JASRI (2017A1132, 2017B1073,
proton-transfer pathway. Thermal ellipsoids are shown at the 50% probability 2018A1114, 2018B1125, 2019A1142) and with the help of Dr.
level and H atoms are omitted for clarity. (b) Methanol-sorption isotherms at Nobuhiro Yasuda (JASRI). We thank Prof. Hirofumi Yoshikawa
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
Keywords: Metal-organic frameworks • Phosphorus •
H3L, Cu2(OH)4, and the water molecules that form 1D channels
Polycycles • Phospha-Friedel–Crafts reaction • Proton transport
reveal that 1’ is potentially a highly proton-conducting material.

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