New layered copper 1,3,5-benzenetriphosphonates pillared with N-donor ligands: their synthesis, crystal structures, and adsorption properties

Article abstract of DOI:10.1039/c5dt01137j

The synthesis, crystal structures, and adsorption properties of a series of pillared layered copper phosphonates are reported herein. A hydrothermal reaction of copper oxide, 1,3,5-benzenetriphosphonic acid (BTP = H₆bpt), and N-donor ligands wi

Full text of DOI:10.1039/c5dt01137j

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ARTICLE
New layered copper 1,3,5-
benzenetriphosphonates pillared with N-donor
ligands: Synthesis, crystal structures, and
adsorption properties
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
Atsushi Kondo, Tokuya Satomi, Kanami Azuma, Rie Takeda, Kazuyuki Maeda*,
www.rsc.org/
Synthesis, crystal structures, and adsorption properties of a series of pillared layered copper
phosphonates are reported. Hydrothermal reaction of copper oxide, 1,3,5-benzenetriphosphonic acid
(
BTP = H
organophosphonates. X-ray crystal structural analyses revealed that compound
pyr = pyrazine), forms a dense 3D network structure without open pores, and on other hand compounds
, [Cu (H btp)(H O) (L)] (L = bipyridine for , 1,2-bis(4-pyridyl)ethylene for , and 1,3-bis(4-
pyridyl)propane for ), show topologically identical pillared layered frameworks with micropores. The
6
bpt), and N-donor ligands with different lengths gives three new and one reported copper
1
, [Cu (H btp) (pyr)]
2
4
2
(
2
-
4
2
2
2
2
2
3
4
framework structures are composed of copper phosphonate layers and N-donor ligands expanding the
interlayer distances. Guest water molecules occupy the pores and are removable by drying treatment.
Compounds
2
-
4
adsorb and desorb water molecules reversibly with structural transformation.
adsorbs CO and ethanol molecules in spite of no adsorption of N molecules,
Interestingly, compound
3
2
2
indicating selective adsorption properties based not on molecular size but on the molecular affinities to
the surfaces.
Introduction
Reaction of metal cations and monophosphonic acids
generally tends to form layered materials covered with inert
Recently, metalꢀorganic frameworks (MOFs), which are organic groups, resulting in stable layered materials with poor
composed of metal ions or metal clusters and organic ligands, reactivities.34–38 Similarly, use of diphosphonic acids with metal
have emerged as a new porous materials with structural sources often forms dense pillared layered structures, which are
1
–10
diversities and designability.
Especially, MOFs provide composed of alternating inorganic metal phosphonate layers
finely controlled structures by selecting and combining building and organic pillar moieties.39–43 Use of triꢀfunctional or higher
units, and therefore the functionalities of the MOFs have been functional organophosphonic acids has been explored to avoid
explored in academic and practical points of view. Metal formation of such conventional dense pillared layered
organophosphonates are important class of MOFꢀrelated structures and several groups including us successfully
materials and have received much attention because of their synthesized metal organophosphonates with unique framework
4
4–52
potential applications in such areas as molecular structures and properties.
For example, by using triꢀ
functional organophosphonic acid 1,3,5ꢀbenzenetriphosphonic
They often exhibit acid (BTP = H btp), we synthesized a layered zinc phosphonate
1
1–15
16–19
20–22
adsorption/separation,
magnetism,
catalysis,
and proton conductivity.
relatively high thermal and chemical stability because of stable Zn [C H (PO ) PO H]•0.5H bpy•H O (ZnBPꢀbpy), which is
ion exchange,
29–33
2
3–28
6
2
6
3
3 2
3
2
2
PꢀC bonds and perhaps the presence of stable metal composed of anionic metal phosphonate layers and protonated
4
9
phosphonate clusters or networks in their frameworks. They 4,4'ꢀbipyridine cations, by
a
hydrothermal reaction.
also show various connection networks with different number Interestingly, ZnBPꢀbpy shows a topotactic transformation to a
of PꢀOꢀM bonds between metals and phosphonate groups, zeoliteꢀlike openꢀframework with selective cation exchange
5
0
resulting in a variety of framework structures like chains, properties. Shimizu et al. reported proton conduction via
layers, and three dimensional openꢀframeworks.
ordered water molecules in a 2D phosphonate MOF with a
layer structure closely resembling that of ZnBPꢀbpy.
3
0
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However, such metal organophosphonates with multiꢀ optimizing the degassing condition, samples were also
functional phosphonates are still limited and adsorption evacuated at room temperature. SEM observation was
properties of them are rarely investigated with several kinds of performed on a JEOL JSMꢀ6510 electron microscope at an
1
5,48,52
gases.
Here, we show synthesis, crystal structures, and gas shows blockꢀshaped crystals except compound
adsorption properties of a series of copper phosphonates with the crystals are in several tens of micrometer to several
Nꢀdonor ligands as pillar molecules. Four Nꢀdonor ligands of millimeters. Meanwhile, compound shows aggregation of
pyrazine (pyr), 4,4ʹꢀbipyridine (bpy), transꢀ1,2ꢀbis(4ꢀ plateꢀshaped crystals in smaller crystal size. Crystals of
accelerating voltage of 15 kV (Figure 1). All the compounds
2
. The size of
2
pyridyl)ethylene (bpe), and 1,3ꢀbis(4ꢀpyridyl)propane (bpp) compound
were selected as the pillar ligands. It is well known that these
pillar molecules have potential to expand interlayer distances of
layered materials. Actually, the obtained copper phosphonates
have interlayer spaces corresponding to the size of each pillar
molecules used and the interlayer distances are in 0.9 nm – 1.8
4 have many cracks on the surfaces and are
nm based on the crystal structures. Although compound
1
,
[
Cu (H btp) (pyr)], is a nonporous material because of the
2
4
2
small
molecular
size
)] (
of
pyr,
compounds
2
ꢀ
4
4
)
,
[
Cu (H btp)(H O) (
L
L
= bpy for
2
, bpe for
3
, and bpp for
2
2
2
2
show porous structures. Compounds
2ꢀ4 adsorb and desorb
water molecules reversibly with structural transformation.
Interestingly, compounds 2-4 adsorb CO and ethanol vapor in
2
spite of no adsorption of N molecules, indicating selective
2
adsorption properties based not on molecular size but on the
molecular affinities to the surfaces.
Experimental
Synthesis and characterization
General Remarks
confirmed to have stacking plates, implying a layered structure
Supporting information, Figure 1S).
(
All chemicals except BTP were used as purchased without
further purification. BTP was prepared from arylphosphonate
ester 1,3,5ꢀC H [PO(OiPr) ] (BTPP). BTPP was prepared by a
Synthesis of the compounds 1-4
All the compounds were prepared through hydrothermal
reaction of aqueous mixtures of copper oxides, BTP, and Nꢀ
donor ligands. Powder CuO (7.96 mg, 1.0 mmol), BTP (159
mg, 0.50 mmol), Nꢀdonor ligand (0.50 mmol), and water (2.7
ml) was put in a Teflon vessel. A typical molar ratio of CuO,
1ꢀ4
6
3
2 3
Niꢀcatalyzed variant of the Arbusov reaction starting from
4
7,53
1
,3,5ꢀtribromobenzene according to the literature.
After
purification by reducedꢀpressure distillation of the BTPP, BTP
was obtained by a simple hydrolysis reaction of BTPP with 1 M
hydrochloric acid as white powder.
BTP, Nꢀdonor ligands, and H O was 2 : 1 : 1 : 300. The mixture
2
was first stirred at an ambient temperature for 1 h, and the
resulting mixture was hydrothermally reacted in a 23 mL
Teflonꢀlined stainless steel autoclave for 48 h. The material was
obtained after filtration, washing by water, and drying. As Nꢀ
Characterization
CHN elemental analysis was performed on a PerkinꢀElmer
Series II CHNS/O analyzer 2400. Fourier transform infrared
donor
pyridyl)ethylene, and 1,3ꢀdi(4ꢀpyridly)propane were used in
each synthesis for compounds , and . The reaction
temperature was 100°C for compounds and , and 140°C for
compounds and . The compounds have adsorbed water
ligands,
pyrazine,
4,4ʹꢀbipyridine,
1,2ꢀdi(4ꢀ
(
FTꢀIR) spectra were recorded on a JEOL JIRꢀWINSPEC50
ꢀ
1
over the range 4000ꢀ600 cm by averaging 128 scans. TGꢀDTA
was measured in the temperature range of 25 – 1000 ºC on a
Rigaku Thermo Plus 2 at a heating rate of 10 K min under an
1
,
2
,
3
4
1
4
ꢀ
1
2
3
1ꢀ4
ꢀ
1
air flow at 100 – 120 ml min . Powder Xꢀray diffraction (XRD)
patterns except the high resolution data used for the structure
solution and indexing were measured on a Rigaku RINTꢀ2100S
diffractometer using monochromated CuꢀKα radiation.
Nitrogen gas adsorption isotherms were obtained by using an
automatic volumetric gas adsorption instrument Bel Japan
molecules on the surface and inside of the crystals and desorbed
all or part of the water. Therefore, the calculated CHN values
are based on the chemical compositions including/excluding
guest water molecules. The compound
calcd. C 22.4, H 1.6, N 3.3 %; found C 21.5, H 1.4, N 3.1 %.
IR: = 3125 (vw), 3071 (vw), 1417 (w), 1402 (vw), 1204 (w),
155 (vw), 1120 (s), 1087 (m), 1065 (m), 1001(w), 945 (w),
1, C H Cu N P O :
16 14 2 2 6 19
ν
Belsorp 28 at 77 K. Adsorption isotherms of CO gas at 273 K,
2
1
9
2
and water and ethanol vapor at 303 K were recorded on Bel
Japan Belsorp 18. As a standard pretreatment, samples were
ꢀ
1
30 (m), 913 (m), 885 (w), 806(s), 695 (s) cm . The compound
, C H Cu N P O : calcd. C 30.4, H 2.7, N 4.4 %; found C
ꢀ
2
ꢀ3
16 17
2
2
3
11
treated under high vacuum (10 – 10 Pa) at 423 K for 3 h. By
2
| J. Name., 2012, 00, 1-3
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A
0
R
11
T
3
I
7
C
J
LE
3
47
3
1
0.3, H 2.2, N 4.0 %. IR:
ν
= 3078 (vw), 1610 (w), 1601 (w), Å , and
Z
= 4). The differences of each unit cell parameter are
538 (vw), 1488 (w), 1409 (w), 1241 (vw), 1227 (vw), 1214 less than 2 % despite the different measurement temperatures
and 110 K for the MOF
m), 931 (m), 893 (m), 815 (s), 760 (w), 726 (w), 697 (m), 641 reported by Clearfield et al.); for parameter , and , the
, C H Cu N P O : calcd. C,29.6, H differences are 0.05 %, 1.39 %, 0.54 %, and 1.90 %,
(
(
(
vw), 1186 (vw), 1107 (s), 1075 (w), 1037 (s), 996 (m), 980 (room temperature for compound 4
a
,
b
,
c
V
ꢀ
1
s) cm . The compound
3
1
8
27
2
2
3
15
3
1
.7, N 3.8 %; found C, 28.9, H 3.4, N 3.7 %. IR:
616 (m), 1506 (vw), 1429 (vw), 1406 (w), 1251 (vw), 1210 et al. is composed of copper(II) ion, H bpt ion, water, and bpp
ν = 3099 (vw), respectively. The framework of the MOF reported by Clearfield
4
ꢀ
2
(
8
vw), 1117 (s), 1066 (w), 1024 (s), 995 (m), 950 (m), 915 (w), molecules, which are contained in the starting mixture of
4
. So,
ꢀ
1
95 (w), 838 (m), 822 (w), 698 (m) cm . The compound
4
,
we concluded that compound 4 was an essentially identical
C H Cu N P O : calcd. C 30.4, H 2.8, N 3.9; found C 30.3, material to the reported one, which was obtained through a
1
8
27
2
2
3
13
H 2.8, N 3.7 %. IR:
ν
= 3013 (vw), 1618 (m), 1502 (vw), 1428 solvothermal synthesis in DMF with a different metal source
(
w), 1405 (w), 1249 (vw), 1221 (vw), 1209 (vw), 1127 (m), and a different mixing ratio. The results of the following FTꢀIR,
1
9
186 (vw), 1127 (m), 1090 (w), 1064 (w), 1036 (s), 1023 (s), TG, and elemental analysis are consistent with the conclusion
ꢀ
1
96 (w), 953 (s), 8952(w), 821 (m), 757 (vw) cm .
in spite of different guest solvent. The discussion concerning
compound below is, therefore, based on the reported crystal
structure. Details of crystallographic parameters of the
were obtained as large single crystals compounds are summarized in Table 1.
4
Structural analysis
Compounds and
1
3
1ꢀ3
suitable for Xꢀray single crystal analysis and evaluated
according to the procedure of single crystal structural analysis.
Suitable single crystals of the compounds
mounted in a nylon cryoloop for Xꢀray data collection. The
diffraction data were collected at 100 K ( ) and 173 K ( ) on a
Rigaku RꢀAXIS Rapid imagingꢀplateꢀmounted twoꢀ
dimensional Xꢀray diffractometer with graphiteꢀ
monochromated CuꢀKα radiation (λ = 0.154186 nm), using an
ꢀscan technique. Numerical absorption correction was applied
1 and 3 were
Table 1. Crystal structure data of compounds 1 - 3.
1
3
1
2
3
Formula
C
16
H
2
10Cu N
2 18
O P
6
C
16
H
2
25Cu N
O
2 15
P
3
18 2 2 15 3
C H19Cu N O P
M
w
831.16
100
705.39
300
723.21
173
T [K]
ω
by using fully automatic data collection software Rapid AUTO. λ [Å]
1.54187
0.99752
-
1.54187
The structure was solved by the direct method using SHELXS
Crystal
dimensio
ns [mm]
0.30 × 0.30 × 0.05
0.25 × 0.20 × 0.15
5
4
2
013 , and all nonꢀhydrogen atoms were refined
anisotropically with fullꢀmatrix least square technique based on
2
Crystal
system
Monoclinic
Orthorhombic
Orthorhombic
F
value. Hydrogen atoms were added geometrically.
On the other hand, compounds and were obtained as
polycrystalline solids. Then, powder Xꢀray crystal structure
2
4
Space
group
1
P2 /c
Cmca
Cmca
analyses were performed. For precise structural analysis, high a [Å]
9.08250(10)
18.5934(3)
7.86290(19)
109.3520(19)
1252.82(3)
4
28.4774(5)
9.4694(1)
18.8921(3)
90
33.7186(11)
7.9481(3)
20.5910(5)
90
resolution powder Xꢀray diffraction data were collected at 300
b [Å]
K on a transmission geometry diffractometer with a Debye–
Scherrer camera having an imaging plate (IP) at the BL02B2 of
c [Å]
β [°]
SPringꢀ8 (λ = 0.99752(2) Å) (Hyogo, Japan) using the powder
3
samples loaded in 0.3 mm glass capillaries. The unit cells were V [Å ]
5094.5(1)
8
5518.4(3)
8
5
5
56
found in NTREOR and refined in the Le Bail method using
Z
5
7
EXPO2009 software . For compound 2, the initial structural
D
calcd
.
[g
2.203
1.894
1.741
-
1
model was obtained in the direct method in EXPO2009 and the mol ]
missing parts were modelled by using Accelrys Materials
F(000)
824
2864
2911
Studio software. The model was refined with soft geometrical
[
a]
R1
or
or
0.0323
0.0318 (R
B
)
0.0464
[
b]
restraints excluding a few diffraction peaks from tiny amount of
an impurity phase and the analysis reached convergence in the
R
B
[c]
wR2
wp
0.0916
0.01706 (Rwp
)
0.1164
[
d]
5
8
R
Rietveld method using RietanꢀFP. (Supporting information,
Figure 2S) The final model of compound is in the space group
Cmca (no. 64), = 28.4774(5), = 9.4694(1), = 18.8921(3)
Å, = 5094.5(1) Å , and = 8. Compound was indexed in
orthorhombic space group Pbam with unit cell parameters of
2
2
o
[
a] R1 = Σ||F
o
| − |F
) ]} . [d] ] Rwp = {Σ{w(y
c
||/Σ|F
o
|. [b] R
B
= Σ|I
o
− I ||/Σ|I |. [c] wR
2 1/2
c
o
2
= {Σ{(F
–
a
b
c
2
2
2
o
2
1/2
2
2
F
c
) /Σ[w(F
o
–y
c o
) /Σ[w(y ) ]} .
3
V
Z
4
a
3
=
20.377(2),
b
= 8.0272(2),
c = 17.873(1) Å, V = 2923.7(3) Å ,
and
Z = 4. The space group is exactly the same as that of a
Results and Discussion
MOF reported by Clearfield et al, and both the unit cell
parameters are similar to each other (Pbam (no. 55),
0.3854(16), = 7.9172(6), = 17.7768(14) Å, = 2869.1(4)
a
=
Crystal structures of compounds 1-4
2
b
c
V
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The crystal structure of compound
4
was previously reported by framework structures, in which the Nꢀdonor ligands are aligned
crystalizes in orthorhombic parallel to each other along axis with the distance of 3.4ꢀ3.5 Å
space group Pbam, and the asymmetric unit of the crystal ) and 3.2ꢀ3.6 Å ( ) (Figure 3e and 4). As one can see in the
4
7
Clearfield et al
.
Compound
4
b
(
2
3
4
ꢀ
structure contains one copper ion, half of a H bpt ligand, and figure 3e and 4, both the compounds has preudoꢀrectangular
2
half of bpp ligand. Copper ion is in a fiveꢀcoordination
4
ꢀ
geometry and surrounded by three H bpt ligands, one bpp
2
(a)
(b)
ligand, and one water molecule. Compound
4 has layer motifs
represented as [Cu (H btp)(H O) ] and the layers are pillared
2
2
2
2
with bpp ligands to form quasiꢀrectangular pore windows of
0.3 × 14.0 Å estimated from the distances between copper
1
ions. The pores are separated each other by the bpp molecules
and the 2D layers, resulting in the formation of 1D channels
along
b
axis. The estimated total accessible pore space is 791.9
3
Å which is 27.6 % of the unit cell volume calculated by
(c)
5
9
PLATON software.
Both compounds
space group Cmca, which is a supergroup of the space group
Pbam of compound . The framework structures of compounds
and are topologically identical to that of compound . All
copper (II) ions in compounds and are crystallographically
equivalent and are in fiveꢀcoordination geometry similar to the
copper(II) ions in compound . The copper (II) ions in
compounds and are coordinated by one nitrogen atom from
2
and 3 are crystallized in orthorhombic
4
2
3
4
2
3
4
2
3
the Nꢀdonor ligands, and four oxygen atoms from three
different phosphonates and from a water molecule that
4
ꢀ
terminate framework connectivity. Each H bpt ion bonds to
2
six copper(II) ions through three phosphonate groups (Figure
2
). In detail, one phosphonate group has four PꢀOꢀCu bonds
(d)
with four copper(II) ions and the other two have one PꢀOꢀCu
4
ꢀ
bond, respectively, in one H bpt ion. The former phosphonate
2
group should be doubly deprotonated to have two negative
charges, and the latter two phosphonate groups should have one
negative charge to keep the charge balance. The single crystal
structure analysis of compound
3 supports the deprotonation of
the phosphonate groups, because of the averaged shorter PꢀO
bond (1.53 Å) in the former phosphonate groups and longer one
(e)
(
1.55 Å) in the latter phosphonate groups. As a result, the
chemical composition of the layer is shown as
Cu (H btp)(H O) ]. There are metal phosphonate clusters
[
2
2
2
2
composed of four copper(II) ions and six phosphonate groups
Figure 3a), and these clusters are bridged by aromatic rings of
(
4
ꢀ
H bpt to form a distorted rhombic shaped network (Figure
2
3b). The network expands in bꢀc plane to form a 2D layer
structure (Figure 3c and d). The 2D sheets are pillared with Nꢀ
donor ligands through copper(II) ions with different interlayer
distances of 14.2 Å (2) and 16.9 Å (3) to construct 3D open
4
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(a)
(
a)
b)
(
(c)
(d)
shaped pores along the
As mentioned above, the crystal structures of compounds
are topologically identical and are recognized as isostructural
pillared layered materials with different interlayer distances of
4.2 Å ( ), 16.9 Å ( ), and 17.8 Å ( ). The layer structures in
compounds , and are closely resemble but slightly
different with each other. In compounds , the network
b axis.
2
ꢀ
4
1
2
3
4
2
,
3
4
2ꢀ4
topology in the layers is exactly the same, and however, the
orientation of the clusters is different due to the different
orientation of the phosphonate groups. The difference of the
layer structures appears in the occupied area of the layers. The
calculated area per [Cu (H btp) ] unit, which correspond to the
8
2
4
unit cell area in
plane for compound
), respectively, indicating that the layer structures of
compounds and are more similar to each other and more
dense than the layer in compound . Compound and also
b
ꢀ
c
plane for compounds
2
and
3
, and in
aꢀb
2
2
2
4
, are 179 Å (2), 164 Å (3), and 161.3 Å
(
4
3
4
space group
from compounds
P2 /c, which is essentially a different structure
1
2
3
4
2ꢀ4. All copper(II) ions in compound 1 are
have pseudoꢀrectangular shaped 1D pores. Using the atomic
centers of copper(II) ions as points of the rectangular, we
crystallographically equivalent and the local coordination
geometry around the copper(II) ions is described as a distorted
squareꢀpyramidal (Figure 5a). The copper(II) ions are
coordinated by one nitrogen atom and four oxygen atoms,
which is the same first coordination environment as those of the
measured the edge of the rectangular as ca. 9.8 × 11.2 Å (
2
),
1
1
0.4 × 13.4 Å (
3
), and 10.3 × 14.0 Å (
217 Å (23.9 %) for compound , 1280.2 Å (23.2 %) for
, and 791.9Å (27.6 %) for compound
4). The accessible void is
3
3
2
3
compound
respectively, calculated by PLATON software. Because of the
half value of = 4) in compound compared with that in
compounds = 8), the void in compound is doubled
to be 1583.8 Å for easy comparison. As a results, it is clear
that the void volume is increased by using longer Nꢀdonor
ligands.
3
4,
compound
2ꢀ4. Each copper(II) ion is coordinated by four
5
9
different phosphonate groups through oxygen atoms and one
pyr ligand through a nitrogen atom. Among the bonds, one
bond between Cu(1) and O(6) is much longer than the other
Z
(
Z
4
2
and
3
(
Z
4
3
2
ꢀ
bonds (Table 2). The H bpt ligands are parallelꢀaligned along
4
the
c
axis with the average distance of 4.1 Å and bridged by
axis to form 2D copper
copper(II) ions in the direction of
a
Single crystal Xꢀray analysis revealed that compound
1,
phosphonate sheets (figure 5b and c). The 2D sheets are
pillared with pyr ligands and furthermore bridged through the
[
Cu (H btp) (pyr)], crystallizes to form a dense structure in the
2
4
2
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Table 2. Selected bond length [Å] and angles [°] in compound 1.
Cu1 – O1
Cu1 – O4
Cu1 – N1
Cu1 – O7
Cu1 – O6
1.920(2)
1.938(2)
2.041(2)
1.937(2)
2.525(2)
O1 - Cu1 – O7
O1 – Cu1 – O4
O1 – Cu1 – N1
O7– Cu1– O4
O7 – Cu1 – N1
O4– Cu1 – N1
90.66(8)
167.57(9)
88.74(9)
92.07(8)
175.57(9)
89.44(9)
The values in parenthis mean estimated standard deviations.
long Cu(1)ꢀO(6) bonds to form a 3D framework structure with
interlayer distance of 9.3 Å. Because of the short interlayer
distance and well developed bridging networks between the
layers, compound
which is estimated by PLATON software using the single
1 proves to have no accessible void space,
5
9
crystal structural data of compound
1
.
Thermal and structural investigation
TGA curves of compounds
TG analysis indicates that compound
molecules at ca. 100 ºC and a part of bpy ligands at ca. 250 ºC
with the help of a NMR experiment. Accordingly compound
was heated at 100 ºC and 250 ºC under vacuum, and the XRD
patterns of the samples were measured after exposure to air
2 releases water
1
ꢀ
4
are shown in Figure 3S
(
supporting information). All compounds show weight losses
2
between room temperature to 200 ºC, which correspond to
removal of water molecules inside and/or outside of the
crystals. The weight loss percentages of compounds
much higher than that of compound , indicating larger amount
of adsorbed water in the 1D pores. In compound , the weight
loss of 9.3 wt% between 200 ꢀ 400 ºC corresponds to the
combustion of pyrazine ligand (calcd. 9.5 wt%). There is some
large weight losses at higher temperature of 400 – 800 ºC,
which should be owing to decomposition of H bpt and
condensation of hydroxyl groups of phosphonate groups. The
expected final products at 1000 ºC are Cu P O and P O ,
which correspond to 53.7 wt% of compound
wt%). The crystalline phase of Cu P O was confirmed by
2ꢀ4 are
(
Figure 6). Through the removal of adsorbed water molecules,
compound kept the crystallinity with slight structural changes
Figure 6a to 6b). The partial removal of bpy ligands resulted in
1
2
1
(
a different crystalline phase. Because of the broadening of
diffraction peaks in the XRD patterns of the samples treated at
250 ºC, the vacuumꢀheating treatment should also induce
decrease of crystallite size and/or crystal distortion (Figure 6a
to 6c). Water was again dropped on the treated samples,
resulting in recovery of XRD patterns similar to that of the
2
ꢀ
4
2
2
7
2
5
1 (calcd. 53.3
pristine sample. These results suggest that compound
2 keeps
2
2
7
the pillared layered network and shows structural reversibility
during water accommodation/removal. It is interesting that the
samples heated at 100 ºC and 250 ºC and with following water
treatment show very similar XRD patterns, although the sample
treated at 250 ºC has less amount of pillaring bpy molecules.
Further heat treatment at 380 ºC gave almost amorphous and
weak diffraction peaks recognized as Cu P O phase, indicating
XRD pattern of the residue after the TG measurement of
compounds , and . In compound , there are threeꢀstep
weight losses of 30.2 wt% between 200 – 800 ºC, which should
1
,
3
4
2
4
ꢀ
be due to removal of bpy ligands, decomposition of H bpt ,
2
and condensation of hydroxyl groups of phosphonates (calcd.
2
9.9 wt%). The residue at 800 ºC was 52.8 wt% and estimated
2
2
7
as Cu P O and partially remaining P O (calcd. 54.3 wt%).
2
2
7
2
5
collapse of the pillared layered structure. Compounds
3 and 4
Compound
3 shows the weight loss of 3.0 wt% between 200 –
release water molecules below 150 ºC and also showed similar
structural change through accommodation/removal of guest
water molecules (Supporting information, Figure 4S).
Therefore, it should be noted that these pillared layered
compounds have potential to show flexible nature through
molecular accommodation depending on molecular species.
4
00 ºC, which should stem from condensation of the hydroxyl
4
ꢀ
groups of H bpt ligand (calculated at 2.5 %). The weight loss
of 32.1 wt% of compound
decomposition of bpe and H bpt ligand (calculated at 34.4 %).
The TG curve of compound
compound and similar decomposition process should occur in
the heating process. The weight percentage of the residue of
compound at 800 ºC is 60.2 wt%, which is well matched with
the calculated percentage of Cu P O and 0.9P O (60.3 wt%).
2
3 in 400 – 800 ºC corresponds to
4
ꢀ
2
4 is very similar to that of
3
Gas adsorption analysis
4
Nitrogen adsorption isotherms were measured on compounds
at 77 K. Although all compounds have accessible void
space with pore entrances large enough to adsorb N molecules
2ꢀ
2
2
7
2
5
4
2ꢀ4
In order to investigate the structural stability of the
framework, pillared layered compounds were examined.
2
2ꢀ4
6
0
(
minimum dimension; 3.0 Å) based on the crystal structures,
6
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all compounds showed almost no uptake at low relative the water adsorption isotherm. By optimizing the degassing
6
1
2
pressure. The BET surface area of compounds are 1.1 m /g condition under vacuum at room temperature, compound
3
2
2
(
2
), 2.8 m /g (
3
), and 3.0 m /g (
4
), respectively. These results shows increase of CO uptake up to 76 mg/g, indicating that
2
indicate that these compounds change their structures from compound
3 can essentially adsorb CO molecules. In addition,
2
open forms to nonꢀporous forms through the removal of guest all compounds
2ꢀ4 also adsorb ethanol vapor at 303 K below at
molecules by the pretreatment.
P
/P
0
= 1 with hysteresis loops in all isotherms (Supporting
As suggested by the results of TG, compounds
2ꢀ4
information, Figure 5S and 6S). These adsorption results with
accommodate guest water molecules in the asꢀsynthesized different kind of molecular species clearly indicate that these
states and, the reversible structural changes of these compounds compounds show moleculeꢀdependent gas adsorptivities.
were observed by the XRD experiments with water drop Because the molecular size of ethanol is larger than those of N
2
treatment mentioned above. Therefore, compounds
2
ꢀ
4
and CO , the adsorption properties of compounds 2ꢀ4 are not
2
6
0
potentially adsorb water vapor at ambient temperature. Figure 7 explained by a common molecular sieving effect.
As
shows water vapor adsorption isotherms at 303 K on indicated by the XRD experiments with the water drop
compounds
. As expected, all compounds show apparent treatments, compounds have framework flexibility, and
adsorption uptakes at low relative pressure and large hysteresis after the pretreatment they should be nonporous forms as
loops, which are often observed in the adsorption isotherms of suggested by the N adsorption experiments. These adsorption
The pore volumes calculated by selectivities should appear as a result of the molecular affinities
2
ꢀ
4
2ꢀ4
2
6
2–65
the flexible MOFs.
DubininꢀRadushkevich (DR) analysis are 0.047 mL/g (
.095 mL/g ( ), and 0.083 mL/g (
), respectively. Compared flexible frameworks.
with the void volumes estimated from the crystal structures, the
pore filling percentages are 32 % ( ), 61 % ( ), and 46 % (
6
6
2
), to the surfaces, inducing structural transformation in their
6
7–70
0
3
4
2
3
4
), Conclusions
respectively. It is not straightforward that the use of longer
pillar ligands does not give higher adsorption uptakes. It should
be noted that the percentages are in a considerably wide range
and far from a complete filling of 100 %. These adsorption
results suggest that compounds shows different adsorptivities of
small molecules with different structural responses based on the
molecular species and accommodation conditions.
In this paper, the synthesis, crystal structure, and gas adsorption
properties of a series of copper phosphonates with different Nꢀ
donor ligands were reported. As pillar molecules, four Nꢀdonor
ligands of pyrazine (pyr), 4,4ʹꢀbipyridine (bpy), transꢀ1,2ꢀbis(4ꢀ
pyridyl)ethylene (bpe), and 1,3ꢀbis(4ꢀpyridyl)propane (bpp)
were used. Compound 1, [Cu (H btp) (pyr)] (pyr = pyrazine), is
2 4 2
a dense nonporous 3D framework compound composed of 2D
layers bridged through the Nꢀdonor ligands pyrazine and
Further investigation of molecular adsorptivities on
compounds
adsorption experiments. Figure 8 shows CO2 adsorption
isotherms on compounds at 273 K. Interestingly,
has almost no uptake, and however,
shows a steep uptake at low pressure region like
2ꢀ4 were performed through CO2 and ethanol
phosphonate groups. Compounds
pillared layered materials, in which the 2D layers are expressed
as [Cu (H btp)(H O) ] and different from that in compound
2ꢀ4 are topologically identical
2ꢀ4
1
.
2
2
2
2
compounds
compound
2 and 4
The layers are pillared by Nꢀdonor ligands giving 3D open
framework structures with quasiꢀrectangular shaped pores.
3
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Journal Name
According to the crystal structures, both the pore size and the 15 M. Taddei, F. Costantino, A. Ienco, A. Comotti, P. V Dau and S. M.
pore volume of compounds become larger with longer Nꢀdonor
ligands. Compounds show the reversible structural
Cohen, Chemical communications (Cambridge, England), 2013, 49,
2
ꢀ
4
1315–7.
transformation accompanied with inclusion/removal of guest 16 S. B. Acids, Z. Wang, J. M. Heising and A. Clearfield, 2003, 10375–
water molecules. Compounds
2
ꢀ
4
do not show N adsorption at
10383.
2
7
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that compounds 2-4 also adsorb ethanol which is larger in
Chemical, 2004, 215, 177–186.
molecular size than that of N and water. In addition, compound 18 L. Ma, C. Abney and W. Lin, Chemical Society Reviews, 2009, 38
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3
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1248–1256.
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properties of these compounds should be based not on 19 S. Chessa, N. J. Clayden, M. Bochmann and J. a Wright, Chemical
molecular size but on the molecular affinities to the surfaces.
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A graphical and textual abstract
New layered copper phosphonates with N-donor pillaring ligands were synthesized
and adsorption properties of them were investigated.